Fundamentals of Vacuum Technology (Leybold)

Fundamentals of Vacuum Technology (Leybold)
Fundamentals of
Vacuum Technology
00.200.02
Kat.-Nr. 199 90
Preface
Oerlikon Leybold Vacuum, a member of the globally active
industrial Oerlikon Group of companies has developed into
the world market leader in the area of vacuum technology.
In this leading position, we recognize that our customers
around the world count on Oerlikon Leybold Vacuum to
deliver technical superiority and maximum value for all our
products and services. Today, Oerlikon Leybold Vacuum is
strengthening that well-deserved reputation by offering a
wide array of vacuum pumps and aftermarket services to
meet your needs.
This brochure is meant to provide an easy to read overview
covering the entire range of vacuum technology and is independent of the current Oerlikon Leybold Vacuum product
portfolio. The presented product diagrams and data are provided to help promote a more comprehensive understanding
of vacuum technology and are not offered as an implied warranty. Content has been enhanced through the addition of
new topic areas with an emphasis on physical principles
affecting vacuum technology.
To us, partnership-like customer relationships are a fundamental component of our corporate culture as well as the
continued investments we are making in research and
development for our next generation of innovative vacuum
technology products. In the course of our over 150 year-long
corporate history, Oerlikon Leybold Vacuum developed a
comprehensive understanding of process and application
know-how in the field of vacuum technology. Jointly with our
partner customers, we plan to continue our efforts to open up
further markets, implement new ideas and develop pioneering products.
Cologne, June 2007
1
Fundamentals of Vacuum Technology
2
Preface
Fundamentals
of Vacuum
Technology
revised and compiled by
Dr. Walter Umrath
with contributions from
Dr. Hermann Adam †, Alfred Bolz, Hermann Boy,
Heinz Dohmen, Karl Gogol, Dr. Wolfgang Jorisch,
Walter Mönning, Dr. Hans-Jürgen Mundinger,
Hans-Dieter Otten, Willi Scheer, Helmut Seiger,
Dr. Wolfgang Schwarz, Klaus Stepputat, Dieter Urban,
Heinz-Josef Wirtzfeld, Heinz-Joachim Zenker
3
4
Table of Contents
1.
1.1
1.2
1.3
1.3.1
1.3.2
1.4
1.5
1.5.1
1.5.2
1.5.3
1.5.4
2.
2.1
2.1.1
2.1.1.1
2.1.2
2.1.2.1
2.1.2.2
2.1.2.2.1
2.1.2.2.2
2.1.2.2.3
2.1.2.2.4
2.1.3
2.1.3.1
2.1.3.2
2.1.3.2.1
2.1.3.2.2
2.1.4
2.1.5
2.1.6
2.1.6.1
2.1.6.2
2.1.6.3
2.1.6.4
2.1.6.5
2.1.7
2.1.8
2.1.8.1
2.1.8.2
2.1.8.3
2.1.8.4
2.1.9
2.1.9.1
2.1.9.2
2.1.9.3
2.1.9.4
2.1.9.5
2.1.9.6
Vacuum physics Quantities, their symbols,
units of measure and definitions . . . . . . . . . . . . . . . . . . . . . . .9
Basic terms and concepts in vacuum technology . . . . . . . . . . . .9
Atmospheric air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Gas laws and models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Continuum theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Kinetic gas theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
The pressure ranges in vacuum technology and
their characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Types of flow and conductance . . . . . . . . . . . . . . . . . . . . . . . . .15
Types of flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Calculating conductance values . . . . . . . . . . . . . . . . . . . . . . . .16
Conductance for piping and openings . . . . . . . . . . . . . . . . . . . .16
Conductance values for other elements . . . . . . . . . . . . . . . . . .18
Vacuum Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Vacuum pumps: A survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Oscillation displacement vacuum pumps . . . . . . . . . . . . . . . . . .20
Diaphragm pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Liquid sealed rotary displacement pumps . . . . . . . . . . . . . . . . .20
Liquid ring pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Oil sealed rotary displacement pumps . . . . . . . . . . . . . . . . . . . .21
Rotary vane pumps
(TRIVAC A, TRIVAC B, TRIVAC E, SOGEVAC) . . . . . . . . . . . .21
Rotary plunger pumps (E Pumps) . . . . . . . . . . . . . . . . . . . . . . .23
Trochoid pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
The gas ballast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
Dry compressing rotary displacement pumps . . . . . . . . . . . . . .27
Roots pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
Claw pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Claw pumps with internal compression for the
semiconductor industry (“DRYVAC Series”) . . . . . . . . . . . . . . .33
Claw pump without internal compression for chemistry
applications (“ALL·ex”) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
Accessories for oil-sealed rotary displacement pumps . . . . . . .38
Condensers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
Fluid-entrainment pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40
(Oil) Diffusion pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
Oil vapor ejector pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
Pump fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Pump fluid backstreaming and its suppression
(Vapor barriers, baffles) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Water jet pumps and steam ejectors . . . . . . . . . . . . . . . . . . . . .45
Turbomolecular pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46
Sorption pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
Adsorption pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
Sublimation pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51
Sputter-ion pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51
Non evaporable getter pumps (NEG pumps) . . . . . . . . . . . . . .53
Cryopumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54
Types of cryopump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54
The cold head and its operating principle . . . . . . . . . . . . . . . . .55
The refrigerator cryopump . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56
Bonding of gases to cold surfaces . . . . . . . . . . . . . . . . . . . . . . .56
Pumping speed and position of the cryopanels . . . . . . . . . . . . .57
Characteristic quantities of a cryopump . . . . . . . . . . . . . . . . . . .57
2.2
2.2.1
2.2.2
2.2.3
2.2.4
2.2.5
2.2.6
2.3
2.3.1
2.3.1.1
2.3.1.2
2.3.1.3
2.3.2
2.3.3
2.3.4
2.3.5
2.3.6
2.3.7
2.3.8
3.
3.1
3.2
3.2.1
3.2.2
3.2.2.1
3.2.2.2
3.2.2.3
3.2.2.4
3.2.3
3.2.3.1
3.2.3.2
3.3
3.3.1
3.3.2
3.3.3
3.3.3.1
3.3.3.2
3.4
3.4.1
3.5
3.5.1
3.5.2
3.5.3
3.5.4
3.5.5
Choice of pumping process . . . . . . . . . . . . . . . . . . . . . . . . . . . .60
Survey of the most usual pumping processes . . . . . . . . . . . . . .60
Pumping of gases (dry processes) . . . . . . . . . . . . . . . . . . . . . .62
Pumping of gases and vapors (wet processes) . . . . . . . . . . . . .62
Drying processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64
Production of an oil-free (hydrocarbon-free) vacuum . . . . . . . .65
Ultrahigh vacuum working Techniques . . . . . . . . . . . . . . . . . . .65
Evacuation of a vacuum chamber and determination of
pump sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66
Evacuation of a vacuum chamber
(without additional sources of gas or vapor) . . . . . . . . . . . . . . .66
Evacuation of a chamber in the rough vacuum region . . . . . . .67
Evacuation of a chamber in the high vacuum region . . . . . . . .68
Evacuation of a chamber in the medium vacuum region . . . . .68
Determination of a suitable backing pump . . . . . . . . . . . . . . . . .69
Determination of pump-down time from nomograms . . . . . . . .70
Evacuation of a chamber where gases and
vapors are evolved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71
Selection of pumps for drying processes . . . . . . . . . . . . . . . . . .71
Flanges and their seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73
Choice of suitable valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73
Gas locks and seal-off fittings . . . . . . . . . . . . . . . . . . . . . . . . . .75
Vacuum measurement, monitoring, control
and regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76
Fundamentals of low-pressure measurement . . . . . . . . . . . . . .76
Vacuum gauges with pressure reading that is independent
of the type of gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
Bourdon vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
Diaphragm vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
Capsule vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
DIAVAC diaphragm vacuum gauge . . . . . . . . . . . . . . . . . . . . . .78
Precision diaphragm vacuum gauges . . . . . . . . . . . . . . . . . . . .78
Capacitance diaphragm gauges . . . . . . . . . . . . . . . . . . . . . . . .78
Liquid-filled (mercury) vacuum gauges . . . . . . . . . . . . . . . . . . .79
U-tube vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79
Compression vacuum gauges (according to McLeod) . . . . . . .79
Vacuum gauges with gas-dependent pressure reading . . . . . . .81
Spinning rotor gauge (SRG) (VISCOVAC) . . . . . . . . . . . . . . . . .81
Thermal conductivity vacuum gauges . . . . . . . . . . . . . . . . . . . .82
Ionization vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83
Cold-cathode ionization vacuum gauges
(Penning vacuum gauges) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83
Hot-cathode ionization vacuum gauges . . . . . . . . . . . . . . . . . . .84
Adjustment and calibration; DKD, PTB national standards . . . .86
Examples of fundamental pressure measurement methods
(as standard methods for calibrating vacuum gauges . . . . . . . .87
Pressure monitoring,control and regulation in
vacuum systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88
Fundamentals of pressure monitoring and control . . . . . . . . . .88
Automatic protection, monitoring and control
of vacuum systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89
Pressure regulation and control in rough and medium
vacuum systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .90
Pressure regulation in high and ultrahigh vacuum systems . . .92
Examples of applications with diaphragm controllers . . . . . . . .93
5
Table of Contents
4.
4.1
4.2
4.3
4.3.1
4.3.1.1
4.3.1.2
4.3.1.3
4.4
4.4.1
4.4.2
4.4.3
4.4.4
4.5
4.5.1
4.5.2
4.5.3
4.5.4
4.5.5
4.5.6
4.5.7
4.6
4.6.1
4.6.2
4.6.3
4.6.4
4.7
4.7.1
4.7.2
4.7.3
4.7.4
4.8
4.9
5
5.1
5.2
5.2.1
5.2.2
5.3
5.4
5.4.1
5.4.2
5.4.3
5.4.4
5.4.5
5.4.6
5.4.7
5.4.8
5.4.9
5.5
6
Analysis of gas at low pressures using
mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95
A historical review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95
The quadrupole mass spectrometer (TRANSPECTOR) . . . . . .96
Design of the sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96
The normal (open) ion source . . . . . . . . . . . . . . . . . . . . . . . . . .96
The quadrupole separation system . . . . . . . . . . . . . . . . . . . . . .97
The measurement system (detector) . . . . . . . . . . . . . . . . . . . . .98
Gas admission and pressure adaptation . . . . . . . . . . . . . . . . . .99
Metering valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99
Pressure converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99
Closed ion source (CIS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99
Aggressive gas monitor (AGM) . . . . . . . . . . . . . . . . . . . . . . . . .99
Descriptive values in mass spectrometry (specifications) . . . .101
Line width (resolution) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
Mass range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
Smallest detectable partial pressure . . . . . . . . . . . . . . . . . . . .101
Smallest detectable partial pressure ratio (concentration) . . . .101
Linearity range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102
Information on surfaces and amenability to bake-out . . . . . . .102
Evaluating spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102
Ionization and fundamental problems in gas analysis . . . . . . .102
Partial pressure measurement . . . . . . . . . . . . . . . . . . . . . . . . .106
Qualitative gas analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106
Quantitative gas analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .107
Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .108
Standard SQX software (DOS) for stand-alone operation
(1 MS plus, 1 PC, RS 232) . . . . . . . . . . . . . . . . . . . . . . . . . . .108
Multiplex/DOS software MQX
(1 to 8 MS plus 1 PC, RS 485) . . . . . . . . . . . . . . . . . . . . . . . .108
Process-oriented software –
Transpector-Ware for Windows . . . . . . . . . . . . . . . . . . . . . . . .108
Development software TranspectorView . . . . . . . . . . . . . . . . .109
Partial pressure regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . .109
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109
Leaks and their detection . . . . . . . . . . . . . . . . . . . . . . . . . . .110
Types of leaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110
Leak rate, leak size, mass flow . . . . . . . . . . . . . . . . . . . . . . . .110
The standard helium leak rate . . . . . . . . . . . . . . . . . . . . . . . . .112
Conversion equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Terms and definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Leak detection methods without a leak detector unit . . . . . . . .113
Pressure rise test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113
Pressure drop test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .114
Leak test using vacuum gauges which are sensitive
to the type of gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .114
Bubble immersion test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115
Foam-spray test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115
Vacuum box check bubble . . . . . . . . . . . . . . . . . . . . . . . . . . . .115
Krypton 85 test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115
High-frequency vacuum test . . . . . . . . . . . . . . . . . . . . . . . . . .115
Testing with chemical reactions and dye penetration . . . . . . .115
Leak detectors and how they work . . . . . . . . . . . . . . . . . . . . .116
5.5.1
5.5.2
5.5.2.1
5.5.2.2
5.5.2.3
5.5.2.4
5.5.2.5
5.5.2.6
5.5.2.7
5.5.2.8
5.5.2.9
5.6
5.7
5.7.1
5.7.2
5.7.3
5.7.3.1
5.7.3.2
5.7.4
5.8
6
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
7
7.1
7.2
7.2.1
7.2.2
7.2.3
7.2.4
7.3
7.3.1
7.3.2
Halogen leak detectors (HLD 4000, D-Tek) . . . . . . . . . . . . . .116
Leak detectors with mass spectrometers (MS) . . . . . . . . . . . .116
The operating principle for a MSLD . . . . . . . . . . . . . . . . . . . . .117
Detection limit, background, gas storage in oil
(gas ballast), floating zero-point suppression . . . . . . . . . . . . .117
Calibrating leak detectors; test leaks . . . . . . . . . . . . . . . . . . . .118
Leak detectors with quadrupole
mass spectrometer (ECOTEC II) . . . . . . . . . . . . . . . . . . . . . . .119
Helium leak detectors with 180° sector mass spectrometer
(UL 200, UL 500) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .119
Direct-flow and counter-flow leak detectors . . . . . . . . . . . . . . .120
Partial flow operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120
Connection to vacuum systems . . . . . . . . . . . . . . . . . . . . . . . .121
Time constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .121
Limit values / Specifications for the leak detector . . . . . . . . . .122
Leak detection techniques using helium leak detectors . . . . .122
Spray technique (local leak test) . . . . . . . . . . . . . . . . . . . . . . .122
Sniffer technology (local leak testing using the
positive pressure method) . . . . . . . . . . . . . . . . . . . . . . . . . . . .123
Vacuum envelope test (integral leak test) . . . . . . . . . . . . . . . .123
Envelope test – test specimen pressurized with helium . . . . .123
a) Envelope test with concentration measurement and
subsequent leak rate calculation . . . . . . . . . . . . . . . . . . . .123
b) Direct measurement of the leak rate with the leak
detector (rigid envelope) . . . . . . . . . . . . . . . . . . . . . . . . . . .123
Envelope test with test specimen evacuated . . . . . . . . . . . . . .123
a) Envelope = “plastic tent” . . . . . . . . . . . . . . . . . . . . . . . . . . .123
b) Rigid envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .123
“Bombing” test, “Storage under pressure” . . . . . . . . . . . . . . . .123
Industrial leak testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .124
Thin film controllers and control units with quartz
oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .125
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .125
Basic principles of coating thickness measurement
with quartz oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .125
The shape of quartz oscillator crystals . . . . . . . . . . . . . . . . . .126
Period measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127
The Z match technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127
The active oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127
The mode-lock oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . .128
Auto Z match technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .129
Coating thickness regulation . . . . . . . . . . . . . . . . . . . . . . . . . .130
INFICON instrument variants . . . . . . . . . . . . . . . . . . . . . . . . . .131
Application of vacuum technology for
coating techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133
Vacuum coating technique . . . . . . . . . . . . . . . . . . . . . . . . . . . .133
Coating sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133
Thermal evaporators (boats, wires etc.) . . . . . . . . . . . . . . . . .133
Electron beam evaporators (electron guns) . . . . . . . . . . . . . . .134
Cathode sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .134
Chemical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . .134
Vacuum coating technology/coating systems . . . . . . . . . . . . .135
Coating of parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .135
Web coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .135
Table of Contents
7.3.3
7.3.4
7.3.5
Optical coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .136
Glass coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .137
Systems for producing data storage disks . . . . . . . . . . . . . . . .137
8
8.1
Instructions for vacuum equipment operation . . . . . . . . . .139
Causes of faults where the desired ultimate pressure is
not achieved or is achieved too slowly . . . . . . . . . . . . . . . . . .139
Contamination of vacuum vessels and eliminating
contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .139
General operating information for vacuum pumps . . . . . . . . . .139
Oil-sealed rotary vacuum pumps
(Rotary vane pumps and rotary piston pumps) . . . . . . . . . . . .140
Oil consumption, oil contamination, oil change . . . . . . . . . . . .140
Selection of the pump oil when handling aggressive vapors . .140
Measures when pumping various chemical substances . . . . .141
Operating defects while pumping with gas ballast –
Potential sources of error where the required ultimate
pressure is not achieved . . . . . . . . . . . . . . . . . . . . . . . . . . . . .142
Roots pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .142
General operating instructions,
installation and commissioning . . . . . . . . . . . . . . . . . . . . . . . .142
Oil change, maintenance work . . . . . . . . . . . . . . . . . . . . . . . . .142
Actions in case of operational disturbances . . . . . . . . . . . . . .143
Turbomolecular pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .143
General operating instructions . . . . . . . . . . . . . . . . . . . . . . . . .143
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .143
Diffusion and vapor-jet vacuum pumps . . . . . . . . . . . . . . . . . .144
Changing the pump fluid and cleaning the pump . . . . . . . . . .144
Operating errors with diffusion and vapor-jet pumps . . . . . . .144
Adsorption pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .144
Reduction of adsorption capacity . . . . . . . . . . . . . . . . . . . . . . .144
Changing the molecular sieve . . . . . . . . . . . . . . . . . . . . . . . . .144
Titanium sublimation pumps . . . . . . . . . . . . . . . . . . . . . . . . . .145
Sputter-ion pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .145
Information on working with vacuum gauges . . . . . . . . . . . . . .145
Information on installing vacuum sensors . . . . . . . . . . . . . . . .145
Contamination at the measurement system and its removal . .146
The influence of magnetic and electrical fields . . . . . . . . . . . .146
Connectors, power pack, measurement systems . . . . . . . . . .146
8.2
8.3
8.3.1
8.3.1.1
8.3.1.2
8.3.1.3
8.3.1.4
8.3.2
8.3.2.1
8.3.2.2
8.3.2.3
8.3.3
8.3.3.1
8.3.3.2
8.3.4
8.3.4.1
8.3.4.2
8.3.5
8.3.5.1
8.3.5.2
8.3.6
8.3.7
8.4
8.4.1
8.4.2
8.4.3
8.4.4
9.
Tab I
Tables, formulas, nomograms, diagrams and symbols . . .147
Permissible pressure units including the torr and its
conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .147
Tab II
Conversion of pressure units . . . . . . . . . . . . . . . . . . . . . . . . . .147
Tab III Mean free path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .147
Tab IV Compilation of important formulas pertaining to the
kinetic theory of gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .148
Tab V
Important values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .148
Tab VI Conversion of pumping speed (volume flow rate) units . . . . . .149
Tab VII Conversion of throughput
(a,b)
QpV units; leak rate units . . . . . . . . . . . . . . . . . . . . . . . . . . . . .149
Tab VIII Composition of atmospheric air . . . . . . . . . . . . . . . . . . . . . . . .150
Tab IX Pressure ranges used in vacuum technology and their
characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .150
Tab X
Outgassing rate of materials . . . . . . . . . . . . . . . . . . . . . . . . . .150
Tab XI
Nominal internal diameters (DN) and internal diameters
of tubes, pipes and apertures with circular cross-section
(according to PNEUROP). . . . . . . . . . . . . . . . . . . . . . . . . . . . .151
Tab XII Important data for common solvents . . . . . . . . . . . . . . . . . . . .151
Tab XIII Saturation pressure and density of water . . . . . . . . . . . . . . . .152
Tab XIV Hazard classificationof fluids . . . . . . . . . . . . . . . . . . . . . . . . . .153
Tab XV Chemical resistance of commonly used elastomer
gaskets and sealing materials . . . . . . . . . . . . . . . . . . . . . . . . .155
Tab XVI Symbols used invacuum technology . . . . . . . . . . . . . . . . . . . .157
Tab XVII Temperature comparison and conversion table . . . . . . . . . . . .160
Fig. 9.1 Variation of mean free path λ (cm) with pressure
for various gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .160
Fig. 9.2 Diagram of kinetics of gases for air at 20°C . . . . . . . . . . . . . .160
Fig. 9.3 Decrease in air pressure and change in temperature
as a function of altitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .161
Fig. 9.4 Change in gas composition of the atmosphere as a
function of altitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .161
Fig. 9.5 Conductance values for piping of commonly used
nominal internal diameters with circular crosssection for molecular flow . . . . . . . . . . . . . . . . . . . . . . . . . . . .161
Fig. 9.6 Conductance values for piping of commonly used
nominal internal diameters with circular crosssection for molecular flow . . . . . . . . . . . . . . . . . . . . . . . . . . . .161
Fig. 9.7 Nomogram for determination of pump-down time tp
of a vessel in the rough vacuum pressure range . . . . . . . . . .162
Fig. 9.8 Nomogram for determination of the conductance of
tubes with a circular cross-section for air at 20°C
in the region of molecular flow . . . . . . . . . . . . . . . . . . . . . . . . .163
Fig. 9.9 Nomogram for determination of conductance of
tubes in the entire pressure range . . . . . . . . . . . . . . . . . . . . . .164
Fig. 9.10 Determination of pump-down time in the medium vacuum
range taking into account the evolution of gas
from the walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .165
Fig.9.11 Saturation vapor pressure of various substances . . . . . . . . . .166
Fig. 9.12 Saturation vapor pressure of pump fluids for oil and
mercury fluid entrainment pumps . . . . . . . . . . . . . . . . . . . . . . .166
Fig. 9.13 Saturation vapor pressure of major metals used in
vacuum technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .166
Fig. 9.14 Vapor pressure of nonmetallic sealing materials
(the vapor pressure curve for fluoro rubber lies between
the curves for silicone rubber and Teflon). . . . . . . . . . . . . . . . .167
Fig. 9.15 Saturation vapor pressure ps of various substances
relevant for cryogenic technology in a
temperaturerange of T = 2 – 80 K. . . . . . . . . . . . . . . . . . . . . .167
Fig. 9.16 Common working ranges of vacuum pumps . . . . . . . . . . . . . .167
Fig. 9.16a Measurement ranges of common vacuum gauges . . . . . . . .168
Fig. 9.17 Specific volume of saturated water vapor . . . . . . . . . . . . . . . .169
Fig. 9.18 Breakdown voltage between electrodes for air
(Paschen curve) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .169
Fig 9.19 Phase diagram of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .170
10.
10.1
10.2
10.3
The statutory units used in vacuum technology . . . . . . . .171
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .171
Alphabetical list of variables, symbols and units frequently
used in vacuum technology and its applications . . . . . . . . . . .171
Remarks on alphabetical list in Section 10.2 . . . . . . . . . . . . . .175
7
Table of Contents
10.4
10.4.1
10.4.2
10.4.3
10.4.4
Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .176
Basic SI units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .176
Derived coherent SI units with special names andsymbols . .177
Atomic units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .177
Derived noncoherent SI units with special names
and symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .177
11.
National and international standards and
recommendations particularly relevant
to vacuum technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .178
National and international standards and
recommendations of special relevance to
vacuum technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .178
11.1
12.
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .182
13.
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .194
8
Vacuum physics
1.
Quantities, their symbols, units of measure
and definitions
(cf. DIN 28 400, Part 1, 1990, DIN 1314 and DIN 28 402)
1.1
Basic terms and concepts in vacuum technology
Pressure p (mbar)
of fluids (gases and liquids). (Quantity: pressure; symbol: p; unit of measure: millibar; abbreviation: mbar.) Pressure is defined in DIN Standard
1314 as the quotient of standardized force applied to a surface and the
extent of this surface (force referenced to the surface area). Even though
the Torr is no longer used as a unit for measuring pressure (see Section
10), it is nonetheless useful in the interest of “transparency” to mention this
pressure unit: 1 Torr is that gas pressure which is able to raise a column of
mercury by 1 mm at 0 °C. (Standard atmospheric pressure is 760 Torr or
760 mm Hg.) Pressure p can be more closely defined by way of subscripts:
Absolute pressure pabs
Absolute pressure is always specified in vacuum technology so that the
“abs” index can normally be omitted.
Total pressure pt
The total pressure in a vessel is the sum of the partial pressures for all the
gases and vapors within the vessel.
Partial pressure pi
The partial pressure of a certain gas or vapor is the pressure which that gas
or vapor would exert if it alone were present in the vessel.
Important note: Particularly in rough vacuum technology, partial pressure in
a mix of gas and vapor is often understood to be the sum of the partial
pressures for all the non-condensable components present in the mix – in
case of the “partial ultimate pressure” at a rotary vane pump, for example.
Ultimate pressure pend
The lowest pressure which can be achieved in a vacuum vessel. The
socalled ultimate pressure pend depends not only on the pump’s suction
speed but also upon the vapor pressure pd for the lubricants, sealants and
propellants used in the pump. If a container is evacuated simply with an
oil-sealed rotary (positive displacement) vacuum pump, then the ultimate
pressure which can be attained will be determined primarily by the vapor
pressure of the pump oil being used and, depending on the cleanliness of
the vessel, also on the vapors released from the vessel walls and, of
course, on the leak tightness of the vacuum vessel itself.
Ambient pressure pamb
or (absolute) atmospheric pressure
Overpressure pe or gauge pressure
(Index symbol from “excess”)
pe = pabs – pamb
Here positive values for pe will indicate overpressure or gauge pressure;
negative values will characterize a vacuum.
Working pressure pW
During evacuation the gases and/or vapors are removed from a vessel.
Gases are understood to be matter in a gaseous state which will not, however, condense at working or operating temperature. Vapor is also matter in
a gaseous state but it may be liquefied at prevailing temperatures by
increasing pressure. Finally, saturated vapor is matter which at the prevailing temperature is gas in equilibrium with the liquid phase of the same substance. A strict differentiation between gases and vapors will be made in the
comments which follow only where necessary for complete understanding.
Particle number density n (cm-3)
According to the kinetic gas theory the number n of the gas molecules, referenced to the volume, is dependent on pressure p and thermodynamic
temperature T as expressed in the following:
p=n·k·T
Saturation vapor pressure ps
The pressure of the saturated vapor is referred to as saturation vapor pressure ps. ps will be a function of temperature for any given substance.
Vapor pressure pd
Partial pressure of those vapors which can be liquefied at the temperature
of liquid nitrogen (LN2).
Standard pressure pn
Standard pressure pn is defined in DIN 1343 as a pressure of
pn = 1013.25 mbar.
(1.1)
n = particle number density
k = Boltzmann’s constant
At a certain temperature, therefore, the pressure exerted by a gas depends
only on the particle number density and not on the nature of the gas. The
nature of a gaseous particle is characterized, among other factors, by its
mass mT.
Gas density ρ (kg · m-3, g · cm-3)
The product of the particle number density n and the particle mass mT is
the gas density ρ:
ρ = n · mT
(1.2)
The ideal gas law
The relationship between the mass mT of a gas molecule and the molar
mass M of this gas is as follows:
M = N A · mT
(1.3)
9
Vacuum physics
Avogadro’s number (or constant) NA indicates how many gas particles will
be contained in a mole of gas. In addition to this, it is the proportionality
factor between the gas constant R and Boltzmann’s constant k:
R = NA · k
(1.4)
Derivable directly from the above equations (1.1) to (1.4) is the correlation
between the pressure p and the gas density ρ of an ideal gas.
p=ρ⋅ R·T
(1.5)
M
In practice we will often consider a certain enclosed volume V in which the
gas is present at a certain pressure p. If m is the mass of the gas present
within that volume, then
m
ρ = −−−
V
(1.6)
The ideal gas law then follows directly from equation (1.5):
m
p ⋅ V = −−− ⋅ R ⋅ T = ν ⋅ R ⋅ T
M
(1.7)
Here the quotient m / M is the number of moles υ present in volume V.
The simpler form applies for m / M = 1, i.e. for 1 mole:
p·V=R·T
(1.7a)
The following numerical example is intended to illustrate the correlation
between the mass of the gas and pressure for gases with differing molar
masses, drawing here on the numerical values in Table IV (Chapter 9).
Contained in a 10-liter volume, at 20 °C, will be
a) 1g of helium
b) 1g of nitrogen
When using the equation (1.7) there results then at V = 10 l , m = 1 g,
p=
1·g · 83.14 · mbar · · mol – 1· K– 1 · 293 · K
10 · · K · 4 · g · mol –1
= 609 mbar
=
1·g · 83.14 · mbar · · mol – 1· K– 1 · 293 · K
10 · · K · 28 · g · mol –1
The following important terms and concepts are often used in vacuum
technology:
Volume V (l, m3, cm3)
The term volume is used to designate
a) the purely geometric, usually predetermined, volumetric content of a
vacuum chamber or a complete vacuum system including all the piping
and connecting spaces (this volume can be calculated);
b) the pressure-dependent volume of a gas or vapor which, for example, is
moved by a pump or absorbed by an adsorption agent.
Volumetric flow (flow volume) qv
(l/s, m3/h, cm3/s )
The term “flow volume” designates the volume of the gas which flows
through a piping element within a unit of time, at the pressure and temperature prevailing at the particular moment. Here one must realize that,
although volumetric flow may be identical, the number of molecules moved
may differ, depending on the pressure and temperature.
Pumping speed S (l/s, m3/h, cm3/s )
The pumping speed is the volumetric flow through the pump’s intake port.
dV
S = −−−
dt
=
(1.8a)
If S remains constant during the pumping process, then one can use the
difference quotient instead of the differential quotient:
ΔV
S = −−−
Δt
R = 83.14 mbar · l · mol–1 · K–1, T = 293 K (20 °C)
p=
The main task of vacuum technology is to reduce the particle number density n inside a given volume V. At constant temperature this is always
equivalent to reducing the gas pressure p. Explicit attention must at this
point be drawn to the fact that a reduction in pressure (maintaining the
volume) can be achieved not only by reducing the particle number density
n but also (in accordance with Equation 1.5) by reducing temperature T at
constant gas density. This important phenomenon will always have to be
taken into account where the temperature is not uniform throughout
volume V.
(1.8b)
(A conversion table for the various units of measure used in conjunction
with pumping speed is provided in Section 9, Table VI).
= 87 mbar
In case a) where M = 4 g · mole-1 (monatomic gas):
In case b), with M = 28 ≠ g mole-1 (diatomic gas):
The result, though appearing to be paradoxical, is that a certain mass of a
light gas exerts a greater pressure than the same mass of a heavier gas. If
one takes into account, however, that at the same gas density (see
Equation 1.2) more particles of a lighter gas (large n, small m) will be present than for the heavier gas (small n, large m), the results become more
understandable since only the particle number density n is determinant for
the pressure level, assuming equal temperature (see Equation 1.1).
10
Quantity of gas (pV value), (mbar ⋅ l)
The quantity of a gas can be indicated by way of its mass or its weight in
the units of measure normally used for mass or weight. In practice, however, the product of p · V is often more interesting in vacuum technology than
the mass or weight of a quantity of gas. The value embraces an energy
dimension and is specified in millibar · liters (mbar · l) (Equation 1.7).
Where the nature of the gas and its temperature are known, it is possible to
use Equation 1.7b to calculate the mass m for the quantity of gas on the
basis of the product of p · V:
p ·V = m · R · T
M
(1.7)
Vacuum physics
m=
p· V ·M
R ·T
(1.7b)
qpV = p · S
(1.10a)
where S is the pumping speed of the pump at intake pressure of p.
Although it is not absolutely correct, reference is often made in practice to
the “quantity of gas” p · V for a certain gas. This specification is incomplete;
the temperature of the gas T, usually room temperature (293 K), is normally
implicitly assumed to be known.
Example: The mass of 100 mbar · l of nitrogen (N2) at room temperature
(approx. 300 K) is:
−
m=
=
100 mbar · · 28 g · mol 1
=
−
−
83 mbar · · mol 1 · K 1 · 300 K
Conductance C (l · s–1)
The pV flow through any desired piping element, i.e. pipe or hose, valves,
nozzles, openings in a wall between two vessels, etc., is indicated with
Analogous to this, at T = 300 K:
1 mbar · l O2 = 1.28 · 10-3 g O2
70 mbar · l Ar = 1.31 · 10-1 g Ar
qpV = C(p1 – p2) = Δp · C
The quantity of gas flowing through a piping element during a unit of time –
in accordance with the two concepts for gas quantity described above – can
be indicated in either of two ways, these being:
Mass flow qm (kg/h, g/s),
this is the quantity of a gas which flows through a piping element, referenced to
time
or as
pV flow qpV (mbar · l · s–1).
pV flow is the product of the pressure and volume of a quantity of gas flowing through a piping element, divided by time, i.e.:
p·V
d (p · V)
qpV = ⎯⎯⎯ = ⎯⎯⎯⎯
t
dt
pV flow is a measure of the mass flow of the gas; the temperature to be
indicated here.
Pump throughput qpV
The pumping capacity (throughput) for a pump is equal either to the mass
flow through the pump intake port:
m
qm = ⎯⎯
t
The concept of pump throughput is of major significance in practice and
should not be confused with the pumping speed! The pump throughput is
the quantity of gas moved by the pump over a unit of time, expressed in
mbar ≠ l/s; the pumping speed is the “transportation capacity” which the
pump makes available within a specific unit of time, measured in m3/h
or l/s.
The throughput value is important in determining the size of the backing
pump in relationship to the size of a high vacuum pump with which it is connected in series in order to ensure that the backing pump will be able to
“take off” the gas moved by the high vacuum pump (see Section 2.32).
2800
g = 0.113 g
300 · 83
m
qm = −−−
t
(The throughput of a pump is often indicated with Q, as well.)
(1.9)
or to the pV flow through the pump’s intake port:
p·V
qpV = ⎯⎯⎯
t
It is normally specified in mbar · l · s–1. Here p is the pressure on the intake
side of the pump. If p and V are constant at the intake side of the pump, the
throughput of this pump can be expressed with the simple equation
(1.11)
Here Δp = (p1 – p2) is the differential between the pressures at the inlet and
outlet ends of the piping element. The proportionality factor C is designated
as the conductance value or simply “conductance”. It is affected by the
geometry of the piping element and can even be calculated for some simpler configurations (see Section 1.5).
In the high and ultrahigh vacuum ranges, C is a constant which is independent of pressure; in the rough and medium-high regimes it is, by contrast,
dependent on pressure. As a consequence, the calculation of C for the piping elements must be carried out separately for the individual pressure
ranges (see Section 1.5 for more detailed information).
From the definition of the volumetric flow it is also possible to state that:
The conductance value C is the flow volume through a piping element. The
equation (1.11) could be thought of as “Ohm’s law for vacuum technology”,
in which qpV corresponds to current, Δp the voltage and C the electrical
conductance value. Analogous to Ohm’s law in the science of electricity, the
resistance to flow
1
R = −−−
C
has been introduced as the reciprocal value to the conductance value. The
equation (1.11) can then be re-written as:
1
qpV = —— · Δp
R
(1.12)
The following applies directly for connection in series:
R∑ = R1 + R2 + R3 . . .
(1.13)
When connected in parallel, the following applies:
1
1
1
1
------ = −−− + −−− + −−− + ⋅ ⋅ ⋅ ⋅
R∑
R1
R2
R3
(1.13a)
11
Vacuum physics
Leak rate qL (mbar · l · s–1)
According to the definition formulated above it is easy to understand that
the size of a gas leak, i.e. movement through undesired passages or “pipe”
elements, will also be given in mbar · l · s–1. A leak rate is often measured
or indicated with atmospheric pressure prevailing on the one side of the
barrier and a vacuum at the other side (p < 1 mbar). If helium (which may
be used as a tracer gas, for example) is passed through the leak under
exactly these conditions, then one refers to “standard helium conditions”.
--C
Z = ----λ
where
and
Outgassing (mbar · l)
The term outgassing refers to the liberation of gases and vapors from the
walls of a vacuum chamber or other components on the inside of a vacuum
system. This quantity of gas is also characterized by the product of p · V,
where V is the volume of the vessel into which the gases are liberated, and
by p, or better Δp, the increase in pressure resulting from the introduction
of gases into this volume.
Outgassing rate (mbar · l · s–1)
This is the outgassing through a period of time, expressed in mbar · l · s–1.
Outgassing rate (mbar · l · s–1 · cm–2)
(referenced to surface area)
In order to estimate the amount of gas which will have to be extracted,
knowledge of the size of the interior surface area, its material and the surface characteristics, their outgassing rate referenced to the surface area
and their progress through time are important.
Mean free path of the molecules λ (cm) and collision rate z (s-1)
The concept that a gas comprises a large number of distinct particles
between which – aside from the collisions – there are no effective forces,
has led to a number of theoretical considerations which we summarize
today under the designation “kinetic theory of gases”.
One of the first and at the same time most beneficial results of this theory
was the calculation of gas pressure p as a function of gas density and the
mean square of velocity c2 for the individual gas molecules in the mass of
molecules mT:
__ 1
__
1
p = --- ρ ⋅ c2 = ---- ⋅ n ⋅ mT ⋅ c2
3
3
(1.15)
The gas molecules fly about and among each other, at every possible
velocity, and bombard both the vessel walls and collide (elastically) with
each other. This motion of the gas molecules is described numerically with
the assistance of the kinetic theory of gases. A molecule’s average number
of collisions over a given period of time, the so-called collision index z, and
the mean path distance which each gas molecule covers between two collisions with other molecules, the so-called mean free path length λ, are
described as shown below as a function of the mean molecule velocity -c
the molecule diameter 2r and the particle number density molecules n – as
a very good approximation:
12
λ=
8· k ·T
=
π · mT
1
π · 2 · n · (2r)2
8· R ·T
π ·M
(1.17)
(1.18)
Thus the mean free path length λ for the particle number density n is, in
accordance with equation (1.1), inversely proportional to pressure p. Thus
the following relationship holds, at constant temperature T, for every gas
λ ⋅ p = const
(1.19)
Used to calculate the mean free path length λ for any arbitrary pressures and
various gases are Table III and Fig. 9.1 in Chapter 9. The equations in gas
kinetics which are most important for vacuum technology are also summarized (Table IV) in chapter 9.
Impingement rate zA (cm–2 ⋅ s–1) and
monolayer formation time τ (s)
A technique frequently used to characterize the pressure state in the high
vacuum regime is the calculation of the time required to form a monomolecular or monoatomic layer on a gas-free surface, on the assumption that
every molecule will stick to the surface. This monolayer formation time is
closely related with the so-called impingement rate zA. With a gas at rest the
impingement rate will indicate the number of molecules which collide with
the surface inside the vacuum vessel per unit of time and surface area:
zA =
n· c
4
(1.20)
If a is the number of spaces, per unit of surface area, which can accept a
specific gas, then the monolayer formation time is
τ=
a 4 ·a
=
zA n · c
(1.21)
(1.14)
where
__
k·T
c2 = 3 ⋅ -----mT
c=
(1.16)
Collision frequency zv (cm–3 · s–1)
This is the product of the collision rate z and the half of the particle number
density n, since the collision of two molecules is to be counted as only one
collision:
zV = n ·z
2
(1.21a)
Vacuum physics
1.2
Atmospheric air
Prior to evacuation, every vacuum system on earth contains air and it will
always be surrounded by air during operation. This makes it necessary to
be familiar with the physical and chemical properties of atmospheric air.
The atmosphere is made up of a number of gases and, near the earth’s
surface, water vapor as well. The pressure exerted by atmospheric air is
referenced to sea level. Average atmospheric pressure is 1013 mbar
(equivalent to the “atmosphere”, a unit of measure used earlier). Table VIII
in Chapter 9 shows the composition of the standard atmosphere at relative
humidity of 50 % and temperature of 20 °C. In terms of vacuum technology
the following points should be noted in regard to the composition of the air:
a) The water vapor contained in the air, varying according to the humidity
level, plays an important part when evacuating a vacuum plant (see
Section 2.2.3).
b) The considerable amount of the inert gas argon should be taken into
account in evacuation procedures using sorption pumps (see Section
2.1.8).
c) In spite of the very low content of helium in the atmosphere, only about
5 ppm (parts per million), this inert gas makes itself particularly obvious
in ultrahigh vacuum systems which are sealed with Viton or which incorporate glass or quartz components. Helium is able to permeate these
substances to a measurable extent.
The pressure of atmospheric air falls with rising altitude above the earth’s
surface (see Fig. 9.3 in Chapter 9). High vacuum prevails at an altitude of
about 100 km and ultrahigh vacuum above 400 km. The composition of the
air also changes with the distance to the surface of the earth (see Fig. 9.4
in Chapter 9).
Gay-Lussac’s Law (Charles’ Law)
V = V0 (1 + β · t )
for p = constant (isobar)
Amonton’s Law
p = p0 (1 + γ · t )
for V = constant (isochor)
Dalton’s Law
∑ pi = p total
i
Poisson’s Law
p ⋅ Vκ = const
(adiabatic)
Avogadro’s Law
m1 m 2
:
= M1 : M 2
V1 V2
Ideal gas Law
p ·V =
m
· R · T = ν · R ·T
M
Also: Equation of state for ideal gases (from the continuum theory)
van der Waals’ Equation
(p +
1.3
a
) · ( Vm − b) = R · T
Vm2
Gas laws and models
1.3.1 Continuum theory
Model concept: Gas is “pourable” (fluid) and flows in a way similar to a liquid. The continuum theory and the summarization of the gas laws which follows are based on experience and can explain all the processes in gases
near atmospheric pressure. Only after it became possible using ever better
vacuum pumps to dilute the air to the extent that the mean free path rose
far beyond the dimensions of the vessel were more far-reaching assumptions necessary; these culminated in the kinetic gas theory. The kinetic gas
theory applies throughout the entire pressure range; the continuum theory
represents the (historically older) special case in the gas laws where atmospheric conditions prevail.
Summary of the most important gas laws (continuum theory)
Boyle-Mariotte Law
p ⋅ V = const.
for T = constant (isotherm)
a, b = constants (internal pressure, covolumes)
Vm = Molar volume
also: Equation of state for real gases
Clausius-Clapeyron Equation
L =T·
dp
· ( V − Vm, l )
dT m, v
L = Enthalpy of evaporation,
T = Evaporation temperature,
Vm,v, Vm,l = Molar volumes of vapor or liquid
1.3.2 Kinetic gas theory
With the acceptance of the atomic view of the world – accompanied by the
necessity to explain reactions in extremely dilute gases (where the continuum theory fails) – the ”kinetic gas theory” was developed. Using this it is
possible not only to derive the ideal gas law in another manner but also to
calculate many other quantities involved with the kinetics of gases – such
as collision rates, mean free path lengths, monolayer formation time, diffusion constants and many other quantities.
13
Vacuum physics
Model concepts and basic assumptions:
1. Atoms/molecules are points.
2. Forces are transmitted from one to another only by collision.
3. The collisions are elastic.
4. Molecular disorder (randomness) prevails.
A very much simplified model was developed by Krönig. Located in a cube
are N particles, one-sixth of which are moving toward any given surface of
the cube. If the edge of the cube is 1 cm long, then it will contain n particles
(particle number density); within a unit of time n · c · Δt/6 molecules will
reach each wall where the change of pulse per molecule, due to the
change of direction through 180 °, will be equal to 2 · mT · c. The sum of
the pulse changes for all the molecules impinging on the wall will result in a
force effective on this wall or the pressure acting on the wall, per unit of
surface area.
1
n
· c · 2 · m T · c = · n · c2 · m T = p
3
6
n=
where
The mass of the molecules is
mT = M = Mass / mol
NA Molecules / mol
where NA is Avogadro’s number (previously: Loschmidt number).
Avogadro constant
NA = 6.022 ⋅ 1023 mol–1
For 1 mole,
and
V = Vm = 22.414 l (molar volume);
Thus from the ideal gas law at standard conditions
(Tn = 273.15 K and pn = 1013.25 mbar):
p ·V =
N
V
mT
=1
M
m
· R ·T
M
For the general gas constant:
Derived from this is
p ·V =
R=
1
· N · mT · c2
3
1013.25 mbar · 22.4 · mol −1
=
273.15 K
= 83.14
mbar · mol · K
Ideal gas law (derived from the kinetic gas theory)
–
If one replaces c2 with c2 then a comparison of these two “general” gas
equations will show:
p ·V =
m
1
· R · T = · N · m T · c2
M
3
p·V = N · (
mT · R
M
) ·T =
or
m · c2
2
·N·( T
)
2
3
The expression in brackets on the left-hand side is the Boltzmann constant
k; that on the right-hand side a measure of the molecules’ mean kinetic
energy:
Boltzmann constant
k=
mT · R
M
= 1.38 · 10 −23
J
K
Mean kinetic energy of the molecules
E kin =
thus
mT · c 2
2
p · V = N· k · T =
2
· N · E kin
3
In this form the gas equation provides a gas-kinetic indication of the temperature!
14
1.4
The pressure ranges in vacuum technology
and their characterization
(See also Table IX in Chapter 9.) It is common in vacuum technology to
subdivide its wide overall pressure range – which spans more than 16 powers of ten – into smaller individual regimes. These are generally defined as
follows:
Rough vacuum (RV)
Medium vacuum (MV)
High vacuum (HV)
Ultrahigh vacuum (UHV)
1000 – 1
1 – 10–3
10–3 – 10–7
10–7 – (10–14)
mbar
mbar
mbar
mbar
This division is, naturally, somewhat arbitrary. Chemists in particular may
refer to the spectrum of greatest interest to them, lying between 100 and
1 mbar, as “intermediate vacuum”. Some engineers may not refer to vacuum at all but instead speak of “low pressure” or even “negative pressure”.
The pressure regimes listed above can, however, be delineated quite satisfactorily from an observation of the gas-kinetic situation and the nature of
gas flow. The operating technologies in the various ranges will differ, as
well.
Vacuum physics
1.5
Types of flow and conductance
Δp
mbar
Three types of flow are mainly encountered in vacuum technology: viscous
or continuous flow, molecular flow and – at the transition between these two
– the so-called Knudsen flow.
qm
%
1
100
2
1.5.1 Types of flow
Laminar flow in circular tubes with parabolic velocity distribution is known
as Poiseuille flow. This special case is found frequently in vacuum technology. Viscous flow will generally be found where the molecules’ mean free
path is considerably shorter than the diameter of the pipe: λ « d.
A characteristic quantity describing the viscous flow state is the dimensionless Reynolds number Re.
Re is the product of the pipe diameter, flow velocity, density and reciprocal
value of the viscosity (internal friction) of the gas which is flowing. Flow is
turbulent where Re > 2200, laminar where Re < 2200.
The phenomenon of choked flow may also be observed in the viscous flow
situation. It plays a part when venting and evacuating a vacuum vessel and
where there are leaks.
Gas will always flow where there is a difference in pressure
Δp = (p1 – p2) > 0. The intensity of the gas flow, i.e. the quantity of gas
flowing over a period of time, rises with the pressure differential. In the case
of viscous flow, however, this will be the case only until the flow velocity,
which also rises, reaches the speed of sound. This is always the case at a
certain pressure differential and this value may be characterized as “critical”:
⎛ p2 ⎞
.
⎜ ⎟ = 0528
⎝ p1 ⎠ crit
50
470
25
0
s
venting time (t)
1 – Gas flow rate qm choked = constant (maximum value)
2 – Gas flow not impeded, qm drops to Δp = 0
Fig. 1.1
Knudsen flow
The transitional range between viscous flow and molecular flow is known as
Knudsen flow. It is prevalent in the medium vacuum range: λ ≈ d.
The product of pressure p and pipe diameter d for a particular gas at a certain temperature can serve as a characterizing quantity for the various
types of flow. Using the numerical values provided in Table III, Chapter 9,
the following equivalent relationships exist for air at 20 °C:
Rough vacuum – Viscous flow
(1.22)
(1.23)
Schematic representation of venting an evacuated vessel
Molecular flow
Molecular flow prevails in the high and ultrahigh vacuum ranges. In these
regimes the molecules can move freely, without any mutual interference.
Molecular flow is present where the mean free path length for a particle is
very much larger than the diameter of the pipe: λ >> d.
λ<
A further rise in Δp > Δpcrit would not result in any further rise in gas flow;
any increase is inhibited. For air at 20,°C the gas dynamics theory reveals
a critical value of
qm
Δp
Viscous or continuum flow
This will be found almost exclusively in the rough vacuum range. The character of this type of flow is determined by the interaction of the molecules.
Consequently internal friction, the viscosity of the flowing substance, is a
major factor. If vortex motion appears in the streaming process, one speaks
of turbulent flow. If various layers of the flowing medium slide one over the
other, then the term laminar flow or layer flux may be applied.
⎡ ⎛p ⎞ ⎤
Δpcrit = p1 ⎢1− ⎜ 2⎟ ⎥
⎣ ⎝ p1⎠ crit ⎦
75
1000
d
⇔ p ⋅ d > 6.0 ⋅ 10–1 mbar ⋅ cm
100
Medium vacuum – Knudsen flow
d
d
<λ<
2
100
⇔
⇔ 6 ⋅ 10–1 > p ⋅ d > 1.3 ⋅ 10–2 mbar ⋅ cm
High and ultrahigh vacuum – Molecular flow
The chart in Fig. 1.1 represents schematically the venting (or airing) of an
evacuated container through an opening in the envelope (venting valve),
allowing ambient air at p = 1000 mbar to enter. In accordance with the information given above, the resultant critical pressure is
Δpcrit = 1000 ⋅ (1– 0.528) mbar ≈ 470 mbar; i.e. where Δp > 470 mbar the
flow rate will be choked; where Δp < 470 mbar the gas flow will decline.
λ>
d
2
⇔ p ⋅ d < 1.3 ⋅ 10–2 mbar ≠ cm
In the viscous flow range the preferred speed direction for all the gas molecules will be identical to the macroscopic direction of flow for the gas. This
alignment is compelled by the fact that the gas particles are densely packed
and will collide with one another far more often than with the boundary
walls of the apparatus. The macroscopic speed of the gas is a
15
Vacuum physics
“group velocity” and is not identical with the “thermal velocity” of the gas
molecules.
In the molecular flow range, on the other hand, impact of the particles with
the walls predominates. As a result of reflection (but also of desorption following a certain residence period on the container walls) a gas particle can
move in any arbitrary direction in a high vacuum; it is no longer possible to
speak of ”flow” in the macroscopic sense.
It would make little sense to attempt to determine the vacuum pressure
ranges as a function of the geometric operating situation in each case. The
limits for the individual pressure regimes (see Table IX in Chapter 9) were
selected in such a way that when working with normal-sized laboratory
equipment the collisions of the gas particles among each other will predominate in the rough vacuum range whereas in the high and ultrahigh vacuum
ranges impact of the gas particles on the container walls will predominate.
In the high and ultrahigh vacuum ranges the properties of the vacuum container wall will be of decisive importance since below 10–3 mbar there will
be more gas molecules on the surfaces than in the chamber itself. If one
assumes a monomolecular adsorbed layer on the inside wall of an evacuated sphere with 1 l volume, then the ratio of the number of
adsorbed particles to the number of free molecules in the space will be as
follows:
at 1
at 10–6
at 10–11
mbar
mbar
mbar
10–2
10+4
10+9
For this reason the monolayer formation time τ (see Section 1.1) is used to
characterize ultrahigh vacuum and to distinguish this regime from the high
vacuum range. The monolayer formation time τ is only a fraction of a second in the high vacuum range while in the ultrahigh vacuum range it
extends over a period of minutes or hours. Surfaces free of gases can
therefore be achieved (and maintained over longer periods of time) only
under ultrahigh vacuum conditions.
Further physical properties change as pressure changes. For example, the
thermal conductivity and the internal friction of gases in the medium vacuum range are highly sensitive to pressure. In the rough and high vacuum
regimes, in contrast, these two properties are virtually independent of pressure.
Thus, not only will the pumps needed to achieve these pressures in the
various vacuum ranges differ, but also different vacuum gauges will be
required. A clear arrangement of pumps and measurement instruments for
the individual pressure ranges is shown in Figures 9.16 and 9.16a in
Chapter 9.
represents an resistance to flow, the consequence of which is that the
effective pumping speed Seff is always less than the pumping speed S of
the pump or the pumping system alone. Thus to ensure a certain effective
pumping speed at the vacuum vessel it is necessary to select a pump with
greater pumping speed. The correlation between S and Seff is indicated by
the following basic equation:
1
1 1
= +
Seff S C
Here C is the total conductance value for the pipe system, made up of the
individual values for the various components which are connected in series
(valves, baffles, separators, etc.):
1 1 1 1
1
= + + + . . .
C C1 C2 C3
Cn
The effective pumping speed required to evacuate a vessel or to carry out
a process inside a vacuum system will correspond to the inlet speed of a
particular pump (or the pump system) only if the pump is joined directly to
the vessel or system. Practically speaking, this is possible only in rare situations. It is almost always necessary to include an intermediate piping system comprising valves, separators, cold traps and the like. All this
16
(1.25)
Equation (1.24) tells us that only in the situation where C = ∞ (meaning
that the flow resistance is equal to 0) will S = Seff. A number of helpful
equations is available to the vacuum technologist for calculating the conductance value C for piping sections. The conductance values for valves,
cold traps, separators and vapor barriers will, as a rule, have to be
determined empirically.
It should be noted that in general that the conductance in a vacuum component is not a constant value which is independent of prevailing vacuum
levels, but rather depends strongly on the nature of the flow (continuum or
molecular flow; see below) and thus on pressure. When using conductance
indices in vacuum technology calculations, therefore, it is always necessary
to pay attention to the fact that only the conductance values applicable to a
certain pressure regime may be applied in that regime.
1.5.3 Conductance for piping and orifices
Conductance values will depend not only on the pressure and the nature of
the gas which is flowing, but also on the sectional shape of the conducting
element (e.g. circular or elliptical cross section). Other factors are the
length and whether the element is straight or curved. The result is that various equations are required to take into account practical situations. Each of
these equations is valid only for a particular pressure range. This is always
to be considered in calculations.
a) Conductance for a straight pipe, which is not too short, of length l, with
a circular cross section of diameter d for the laminar, Knudsen and molecular flow ranges, valid for air at 20 °C (Knudsen equation):
C = 135
1.5.2 Calculating conductance values
(1.24)
d4
d 3 1 + 192 · d · p
p +12.1 ·
/s
l 1 + 237 · d · p
l
(1.26)
where
p=
p1 + p2
2
d
l
p1
p2
Pipe inside diameter in cm
Pipe length in cm (l ≥ 10 d)
Pressure at start of pipe (along the direction of flow) in mbar
Pressure at end of pipe (along the direction of flow) in mbar
=
=
=
=
Vacuum physics
for δ ≥ 0.528
If one rewrites the second term in (1.26) in the following form
3
C = 12.1·
d
· f (d · p )
l
(1.26a)
with
1 + 203 · d · p + 2.78 ·10 3 · d 2 · p 2
f (d · p ) =
1 + 237 · d · p
(1.27)
A 1− δ s
and for δ ≤ 0,03
(1.29b)
s
δ = 0.528 is the critical pressure situation for air
(1.28a)
Limit for molecular flow
(d · –p < 10–2 mbar · cm) :
d3
C = 12.1 ·
/s
l
C visc = 20 ·
(1.29a)
Cvisc = 20 · A
Limit for laminar flow
(d · –p > 6 · 10–1 mbar · cm):
d4
· p / s
l
Cvisc = 76.6 · δ 0.712 · 1 − δ 0.288
for δ ≤ 0.528
it is possible to derive the two important limits from the course of the function f (d · –p):
C = 135 ·
(1.29)
A ·
1− δ s
(1.28b)
In the molecular flow region the conductance value is independent of
pressure!
The complete Knudsen equation (1.26) will have to be used in the transitional area 10–2 < d · –p < 6 · 10–1 mbar · cm. Conductance values for
straight pipes of standard nominal diameters are shown in Figure 9.5 (laminar flow) and Figure 9.6 (molecular flow) in Chapter 9. Additional nomograms for conductance determination will also be found in Chapter 9
(Figures 9.8 and 9.9).
⎛p ⎞
2
⎜ ⎟
⎝ p1 ⎠ crit
Flow is choked at δ < 0.528; gas flow is thus constant. In the case of molecular flow (high vacuum) the following will apply for air:
Cmol = 11,6 · A · l · s-1 (A in cm2)
(1.30)
Given in addition in Figure 1.3 are the pumping speeds S*visc and S*mol
refer-enced to the area A of the opening and as a function of δ = p2/p1.
The equations given apply to air at 20 °C. The molar masses for the flowing
gas are taken into consideration in the general equations, not shown here.
l · s–1 · cm–2
b) Conductance value C for an orifice A
(A in cm2): For continuum flow (viscous flow) the following equations
(after Prandtl) apply to air at 20 °C where p2/p1 = δ:
Fig. 1.2
Flow of a gas through an opening (A) at high pressures (viscous flow)
Fig. 1.3 Conductance values relative to the area, C*visc, C*mol, and pumping speed S*visc and
S*mol for an orifice A, depending on the pressure relationship p2/p1 for air at 20 °C.
17
Vacuum physics
Gas (20 °C)
Molecular flow
Laminar flow
Air
1.00
1.00
Oxygen
0.947
0.91
Neon
1.013
1.05
Helium
2.64
0.92
Hydrogen
3.77
2.07
Carbon dioxide
0.808
1.26
Water vapor
1.263
1.73
Axial length
The technical data in the Leybold catalog states the conductance values for
vapor barriers, cold traps, adsorption traps and valves for the molecular
flow range. At higher pressures, e.g. in the Knudsen and laminar flow
ranges, valves will have about the same conductance values as pipes of
corresponding nominal diameters and axial lengths. In regard to right-angle
valves the conductance calculation for an elbow must be applied.
Table 1.1 Conversion factors (see text)
When working with other gases it will be necessary to multiply the conductance values specified for air by the factors shown in Table 1.1.
Nomographic determination of conductance values
The conductance values for piping and openings through which air and
other gases pass can be determined with nomographic methods. It is possible not only to determine the conductance value for piping at specified
values for diameter, length and pressure, but also the size of the pipe diameter required when a pumping set is to achieve a certain effective pumping
speed at a given pressure and given length of the line. It is also possible to
establish the maximum permissible pipe length where the other parameters
are known. The values obtained naturally do not apply to turbulent flows. In
doubtful situations, the Reynolds number Re (see Section 1.5.) should be
estimated using the relationship which is approximated below
Re = 15 ·
qpV
d
(1.31)
Here qpV = S · p is the flow output in mbar l/s, d the diameter of the pipe
in cm.
A compilation of nomograms which have proved to be useful in practice will
be found in Chapter 9.
1.5.4 Conductance values for other elements
Where the line contains elbows or other curves (such as in right-angle
valves), these can be taken into account by assuming a greater effective
length leff of the line. This can be estimated as follows:
leff = laxial +133
. ·
θ
·d
180°
Where
laxial : axial length of the line (in cm)
leff
: Effective length of the line (in cm)
d
: Inside diameter of the line (in cm)
θ
: Angle of the elbow (degrees of angle)
18
(1.32)
In the case of dust filters which are used to protect gas ballast pumps and
roots pumps, the percentage restriction value for the various pressure levels are listed in the catalog. Other components, namely the condensate
separators and condensers, are designed so that they will not reduce
pumping speed to any appreciable extent.
The following may be used as a rule of thumb for dimensioning vacuum
lines: The lines should be as short and as wide as possible. They must
exhibit at least the same cross-section as the intake port at the pump. If
particular circumstances prevent shortening the suction line, then it is
advisable, whenever this is justifiable from the engineering and economic
points of view, to include a roots pump in the suction line. This then acts as
a gas entrainment pump which reduces line impedance.
Vacuum generation
2.
2.1.
Vacuum generation
which the first three classes belong to the compression pumps and where
the two remaining classes belong to the condensation and getter pumps:
Vacuum pumps: A survey
1. Pumps which operate with periodically increasing and decreasing pump
cham-ber volumes (rotary vane and rotary plunger pumps; also trochoid
pumps)
Vacuum pumps are used to reduce the gas pressure in a certain volume
and thus the gas density (see equation 1.5). Consequently consider the gas
particles need to be removed from the volume. Basically differentiation is
made between two classes of vacuum pumps:
a) Vacuum pumps where – via one or several compression stages – the
gas particles are removed from the volume which is to be pumped and
ejected into the atmosphere (compression pumps). The gas particles are
pumped by means of displacement or pulse transfer.
b) Vacuum pumps where the gas particles which are to be removed
condense on or are bonded by other means (e.g. chemically) to a solid
surface, which often is part of the boundary forming volume itself.
A classification which is more in line with the state-of-the-art and practical
applications makes a difference between the following types of pumps, of
2. Pumps which transport quantities of gas from the low pressure side to
the high pressure side without changing the volume of the pumping
chamber (Roots pumps, turbomolecular pumps)
3. Pumps where the pumping effect is based mainly on the diffusion of
gases into a gas-free high speed vapor jet (vapor pumps)
4. Pumps which pump vapors by means of condensation (condensers) and
pumps which pump permanent gases by way of condensation at very
low temperatures (cryopumps)
5. Pumps which bond or incorporate gases by adsorption or absorption to
surfaces which are substantially free of gases (sorption pumps).
A survey on these classes of vacuum pumps is given in the diagram of
Table 2.1.
Vacuum pump
(Operating principle)
Gas transfer
vacuum pump
Entrapment
vacuum pump
Positive displacement
vacuum pump
Reciprocating
positive displacement
vacuum pump
Diaphragm
vacuum pump
Piston
vacuum pump
Kinetic
vacuum pump
Rotary
vacuum pump
Drag
vacuum pump
Liquid sealed
vacuum pump
Liquid ring
vacuum pump
Rotary vane
vacuum pump
Multiple vane
vacuum pump
Gaseous ring
vacuum pump
Turbine
vacuum pump
Fluid entrainment
vacuum pump
Ion transfer
vacuum pump
Ejector
vacuum pump
Adsorption
pump
Getter pump
Liquid jet
vacuum pump
Bulk getter pump
Axial flow
vacuum pump
Gas jet
vacuum pump
Sublimation
pump
Radial flow
vacuum pump
Vapor jet
vacuum pump
Getter ion pump
Rotary piston
vacuum pump
Molecular drag
vacuum pump
Diffusion pump
Evaporation ion pump
Rotary plunger
vacuum pump
Turbomolecular pump
Self-purifying
diffusion pump
Sputter-ion pump
Dry compressing
vacuum pump
Fractionating
diffusion pump
Cryopump
Roots
vacuum pump
Claw
vacuum pump
Diffusion ejector
pump
Condenser
Scroll pump
Table 2.1 Classification of vacuum pumps
19
Vacuum generation
2.1.1 Oscillation displacement vacuum pumps
2.1.1.1 Diaphragm pumps
Recently, diaphragm pumps have becoming ever more important, mainly
for environmental reasons. They are alternatives to water jet vacuum
pumps, since diaphragm pumps do not produce any waste water. Overall, a
diaphragm vacuum pump can save up to 90 % of the operating costs
compared to a water jet pump. Compared to rotary vane pumps, the
pumping chamber of diaphragm pumps are entirely free of oil. By design,
no oil immersed shaft seals are required. Diaphragm vacuum pumps are
single or multi-stage dry compressing vacuum pumps (diaphragm pumps
having up to four stages are being manufactured). Here the circumference
of a diaphragm is tensioned between a pump head and the casing wall
(Fig. 2.1). It is moved in an oscillating way by means of a connecting rod
and an eccentric. The pumping or compression chamber, the volume of
which increases and decreases periodically, effects the pumping action.
The valves are arranged in such a way that during the phase where the
volume of the pumping chamber increases it is open to the intake line.
During compression, the pumping chamber is linked to the exhaust line.
The diaphragm provides a hermetic seal between the gear chamber and
the pumping chamber so that it remains free of oil and lubricants (dry
compressing vacuum pump). Diaphragm and valves are the only
components in contact with the medium which is to be pumped. When
coating the diaphragm with PTFE (Teflon) and when manufacturing the inlet
and exhaust valves of a highly fluorinated elastomer as in the case of the
DIVAC from LEYBOLD, it is then possible to pump aggressive vapors and
gases. It is thus well suited for vacuum applications in the chemistry lab.
Due to the limited elastic deformability of the diaphragm only a
comparatively low pumping speed is obtained. In the case of this pumping
principle a volume remains at the upper dead center – the so called “dead
space” – from where the gases can not be moved to the exhaust line. The
quantity of gas which remains at the exhaust pressure expands into the
expanding pumping chamber during the subsequent suction stroke thereby
filling it, so that as the intake pressure reduces the quantity of inflowing new
gas reduces more and more. Thus volumetric efficiency worsens
continuously for this reason. Diaphragm vacuum pumps are not capable of
attaining a higher compression ratio than the ratio between “dead space”
and maximum volume of the pumping chamber. In the case of single-stage
diaphragm vacuum pumps the attainable ultimate pressure amounts to
approximately 80 mbar. Two-stage pumps such as the DIVAC from
LEYBOLD can attain about 10 mbar (see Fig. 2.2), three-stage pumps can
attain about 2 mbar and four-stage diaphragm pumps can reach about
5·10-1 mbar.
Diaphragm pumps offering such a low ultimate pressure are suited as
backing pumps for turbomolecular pumps with fully integrated Scroll stages
(compound or wide range turbomolecular pumps, such as the
TURBOVAC 55 from LEYBOLD). In this way a pump system is obtained
which is absolutely free of oil, this being of great importance to
measurement arrangements involving mass spectrometer systems and leak
detectors. In contrast to rotary vane pumps this combination of pumps for
leak detectors offers the advantage that naturally no helium is dissolved in
the diaphragm pump thereby entirely avoiding a possible build up of a
helium background.
2.1.2 Liquid sealed rotary displacement pumps
2.1.2.1 Liquid ring pumps
Due to the pumping principle and the simple design, liquid ring vacuum
pumps are particularly suited to pumping gases and vapors which may also
contain small amounts of liquid. Air, saturated with water vapors or other
gases containing condensable constituents, may be pumped without
problems. By design, liquid ring pumps are insensitive to any contamination
EX
IN
a)
b)
c)
d)
(1)
(2)
(3)
(4)
Fig. 2.1
20
Casing lid
Valves
Lid
Diaphragm disk
(5)
(6)
(7)
(8)
1st stage
Diaphragm
Diaphragm support disk
Connecting rod
Eccentric disk
Schematic on the design of a diaphragm pump stage (Vacuubrand)
2nd stage
Opening and closing of the valves, path and pumping mechanism during four
subsequent phases of a turn of the connecting rod (a-d)
Fig. 2.2
Principle of operation for a two-stage diaphragm pump (Vacuubrand)
Vacuum generation
Constant, minimum clearance a for the entire sealing passage b
1 Rotor
2 Rotor shaft
Fig. 2.3
3 Casing
4 Intake channel
5 Liquid ring
6 Flex. discharge channel
Liquid ring vacuum pump, schematic (Siemens)
which may be present in the gas flow. The attainable intake pressures are
in the region between atmospheric pressure and the vapor pressure of the
operating liquid used. For water at 15 °C it is possible to attain an operating
pressure of 33 mbar. A typical application of water ring vacuum pumps is
venting of steam turbines in power plants. Liquid ring vacuum pumps
(Fig. 2.3) are rotary displacement pumps which require an operating liquid
which rotates during operation to pump the gas. The blade wheel is
arranged eccentrically in a cylindrical casing. When not in operation,
approximately half of the pump is filled with the operating fluid. In the axial
direction the cells formed by the blade wheel are limited and sealed off by
“control discs”. These control discs are equipped with suction and ejection
slots which lead to the corresponding ports of the pump. After having
switched on such a pump the blade wheel runs eccentrically within the
casing; thus a concentrically rotating liquid ring is created which at the
narrowest point fully fills the space between the casing and the blade wheel
and which retracts from the chambers as the rotation continues. The gas is
sucked in as the chambers empty and compression is obtained by
subsequent filling. The limits for the intake or discharge process are set by
the geometry of the openings in the control discs.
Fig. 2.4
Arrangement of the sealing passage in rotary vane pumps
also known as “duo seal”
As shown in the classification Table 2.1, the oil sealed displacement pumps
include rotary vane and rotary plunger pumps of single and two-stage design
as well as single-stage trochoid pumps which today are only of historic
interest. Such pumps are all equipped with a gas ballast facility which was
described in detail (for details see 2.1.2.2.4) for the first time by Gaede
(1935). Within specified engineering limits, the gas ballast facility permits
pumping of vapors (water vapor in particular) without condensation of the
vapors in the pump.
2.1.2.2.1
Rotary vane pumps (TRIVAC A, TRIVAC B,
TRIVAC E, SOGEVAC)
Rotary vane pumps (see also Figs. 2.5 and 2.6) consist of a cylindrical
housing (pump-ing ring) (1) in which an eccentrically suspended and slotted
rotor (2) turns in the direction of the arrow. The rotor has vanes (16) which
are forced outwards usually by centrifugal force but also by springs so that
In addition to the task of compression, the operating fluid fulfills three
further important tasks:
1. Removal of the heat produced by the compression process.
2. Uptake of liquids and vapors (condensate).
3. Providing the seal between the blade wheel and the casing.
2.1.2.2
Oil sealed rotary displacement pumps
A displacement vacuum pump is generally a vacuum pump in which the gas
which is to be pumped is sucked in with the aid of pistons, rotors, vanes
and valves or similar, possibly compressed and then discharged. The
pumping process is effected by the rotary motion of the piston inside the
pump. Differentiation should be made between oiled and dry compressing
displacement pumps. By the use of sealing oil it is possible to attain in a
single-stage high compression ratios of up to about 105. Without oil, “inner
leakiness” is considerably greater and the attainable compression ratio is
correspondingly less, about 10.
1
2
3
4
5
6
7
8
Fig. 2.5
Pump housing
Rotor
Oil-level sight glass
Suction duct
Anti-suckback valve
Dirt trap
Intake port
Lid of gas ballast valve
9
10
11
12
13
14
15
16
Exhaust port
Air inlet silencer
Oil filter
Exhaust valve
Exhaust duct
Gas ballast duct
Oil injection
Vane
Cross section of a single-stage rotary vane pump (TRIVAC A)
21
Vacuum generation
1
2
3
4
5
6
7
Fig. 2.6
Intake port
Dirt trap
Anti-suckback valve
Intake duct
Vane
Pumping chamber
Rotor
8 Orifice, connection for
inert gas ballast
9 Exhaust duct
10 Exhaust valve
11 Demister
12 Spring
13 Orifice; connection for
oil filter
Cross section of a single-stage rotary vane pump (TRIVAC B)
the vanes slide inside the housing. Gas entering through the intake (4) is
pushed along by the vanes and is finally ejected from the pump by the oil
sealed exhaust valve (12).
The older range of TRIVAC A pumps (Fig. 2.5) from LEYBOLD has three
TRIVAC A
TRIVAC B
TRIVAC BCS
TRIVAC E
SOGEVAC
Vanes per stage
3
2
2
2
3 (tangential)
Pumping speed
[m3/h]
1 – 1.5
2–4
8 – 16
30 – 60
1.6
4–8
16 – 25
40 – 65
–
16 – 25
40 – 65
–
–
2.5
–
–
16 – 25
40 – 100
180 – 280
585 – 1200
Sealing passage
yes
yes
yes
yes
no
–
< 5 · 10–1
Ultimate pressure,
single-stage [mbar]
<2·
10–2
<2·
10–2
<2·
10–2
Ultimate pressure
two-stage [mbar]
< 2.5 · 10–4
< 1 · 10–4
< 1 · 10–4
< 1 · 10–4
–
Oil supply
Pressure difference
Gear pump
Gear pump
Eccentric pump
Pressure difference
Slots
Comparable for all types: about 0.01 to 0.05 mm
Bearing/lubrication
Axial face / oil
Axial face / oil
Axial face / oil
Ball / grease
Ball / oil
Special
characteristics
–
Hydropneumatic
anti-suckback valve
Coated parts in
contact with medium
Many
accessories
Cost-effective
Media
No ammonia
Clean to
light particles
Aggressive and
corrosive
Clean to
light particles
Clean
Main areas of
application
Multipurpose
Multipurpose
Semiconductor
industry
Multipurpose
Packaging
industry
Table 2.2 Rotary vacuum pump ranges
22
radial vanes offset by 120°. The TRIVAC B range (Fig. 2.6) has only two
vanes offset by 180°. In both cases the vanes are forced outwards by the
centrifugal forces without the use of springs. At low ambient temperatures
this possibly requires the use of a thinner oil. The A-Series is lubricated
through the arising pressure difference whereas the B-Series pumps have
a geared oil pump for pressure lubrication. The TRIVAC B-Series is
equipped with a particularly reliable anti-suckback valve; a horizontal or
vertical arrangement for the intake and exhaust ports. The oil level sight
glass and the gas ballast actuator are all on the same side of the oil box
(user friendly design). In combination with the TRIVAC BCS system it may
be equipped with a very comprehensive range of accessories, designed
chiefly for semiconductor applications. The oil reservoir of the rotary vane
pump and also that of the other oil sealed displacement pumps serves the
purpose of lubrication and sealing, and also to fill dead spaces and slots. It
removes the heat of gas compression, i.e. for cooling purposes. The oil
provides a seal between rotor and pump ring. These parts are “almost” in
contact along a straight line (cylinder jacket line). In order to increase the
oil sealed surface area a so-called sealing passage is integrated into the
pumping ring (see Fig. 2.4). This provides a better seal and allows a higher
compression ratio or a lower ultimate pressure. LEYBOLD manufactures
three different ranges of rotary vane pumps which are specially adapted to
different applications such as high intake pressure, low ultimate pressure or
applications in the semiconductor industry. A summary of the more
important characteristics of these ranges is given in Table 2.2. The TRIVAC
rotary vane pumps are produced as single-stage (TRIVAC S) and twostage (TRIVAC D) pumps (see Fig. 2.7). With the two-stage oil sealed
pumps it is possible to attain lower operating and ultimate pressures
Vacuum generation
Valve stop
Leaf
spring
of the
valve
I High vacuum stage
II Second forevacuum stage
Fig. 2.7
Cross section of a two-stage rotary vane pump, schematic
compared to the corresponding single-stage pumps. The reason for this is
that in the case of single-stage pumps, oil is unavoidably in contact with the
atmosphere outside, from where gas is taken up which partially escapes to
the vacuum side thereby restricting the attainable ultimate pressure. In the
oil sealed two-stage displacement pumps manufactured by LEYBOLD, oil
which has already been degassed is supplied to the stage on the side of
the vacuum (stage 1 in Fig. 2.7): the ultimate pressure lies almost in the
high vacuum range, the lowest operating pressures lie in the range
between medium vacuum / high vacuum. Note: operating the so called high
vacuum stage (stage 1) with only very little oil or no oil at all will – in spite
of the very low ultimate pressure – in practice lead to considerable
difficulties and will significantly impair operation of the pump.
2.1.2.2.2
Fig. 2.8a Cross section of a two-stage rotary vane pump (TRIVAC E)
through the intake channel of the slide valve (12) into the pumping chamber
(14). The slide valve forms a unit with the piston and slides to and fro
between the rotatable valve guide in the casing (hinge bar 13). The gas
drawn into the pump finally enters the compression chamber (4). While
turning, the piston compresses this quantity of gas until it is ejected through
the oil sealed valve (5). As in the case of rotary vane pumps, the oil reservoir
is used for lubrication, sealing, filling of dead spaces and cooling. Since the
pumping chamber is divided by the piston into two spaces, each turn
completes an operating cycle (see Fig. 2.10). Rotary plunger pumps are
manufactured as single and two-stage pumps. In many vacuum processes
combining a Roots pump with a single-stage rotary plunger pump may offer
Rotary plunger pumps (E Pumps)
Shown in Fig. 2.9 is a sectional view of a rotary plunger pump of the single
block type. Here a piston (2) which is moved along by an eccentric (3) turning
in the direction of the arrow moves along the chamber wall. The gas which is
to be pumped flows into the pump through the intake port (11), passes
1
2
3
4
5
6
7
8
Fig. 2.8b SOGEVAC pump SV 300 with three tangential vanes
Fig. 2.9
Casing
Cylindrical piston
Eccentric
Compression chamber
Oil sealed pressure valve
Oil-level sight glass
Gas ballast channel
Exhaust pot
9
10
11
12
13
14
Gas ballast valve
Dirt trap
Intake port
Slide valve
Hinge bar
Pumping chamber
(air is flowing in).
Cross section of a single-stage rotary plunger pump (monoblock design)
23
Vacuum generation
2.1.2.2.3
1 Upper dead point
2 Slot in suction channel of slide valve
is freed – beginning of suction period
3 Lower dead point – slot in suction
channel is quite free, and pumped-in
gas (arrow) enters freely into the
pumping chamber (shown shaded)
4 Slot in suction channel is closed again
by swivelling hinge bar – end of suction
period
5 Upper dead point – maximum space
between rotating piston and stator
6 Shortly before beginning of
compression period, the front surface
of the rotating plunger frees gas ballast
opening – commencement of gas
ballast inlet
7 Gas ballast opening is quite free
8 End of gas ballast inlet
9 End of pumping period.
Trochoid pumps
Trochoid pumps belong to the class of so called rotary piston pumps, which
(see overview of Table 2.1) in turn belong to the group of rotary pumps.
With rotary piston pumps, the piston’s center of gravity runs along a circular
path about the rotational axis (hence rotary piston machines). A rotary
piston pump can – in contrast to the rotary plunger pump – be completely
balanced dynamically. This offers the advantage that larger pumps can
operate without vibration so that they can be installed without needing
foundations. Moreover, such pumps may be operated at higher speed,
compared to rotary plunger pumps (see below). The volume of the pumping
chamber with respect to the volume of the entire pump – the so called
specific volume – is, in the case of trochoid pumps, approximately twice of
that of rotary plunger pumps. Larger rotary plunger pumps run at speeds of
500 rpm. The trochoid pump may run at 1000 rpm and this applies also to
larger designs. It is thus about four times smaller compared to a rotary
plunger pump having the same pumping speed and runs without producing
any vibrations. Unfortunately the advantages in the area of engineering are
combined with great disadvantages in the area of manufacturing, so that
today LEYBOLD does not produce trochoid pumps any more. Operation of
such a pump is shown in the sectional diagram of Fig. 2.12.
Fig. 2.10 Operating cycle of a rotary plunger pump (for positions 1 to 9 of the plunger)
more advantages than a two-stage rotary plunger pump alone. If such a
combination or a two-stage pump is inadequate, the use of a Roots pump in
connection with a two-stage pump is recommended. This does not apply to
combinations involving rotary vane pumps and Roots pumps.
2.1.2.2.4
The gas ballast
Motor power
The motors supplied with the rotary vane and rotary plunger pumps deliver
enough power at ambient temperatures of 12 °C and when using our
special oils to cover the maximum power requirement (at about 400 mbar).
Within the actual operating range of the pump, the drive system of the
warmed up pump needs to supply only about one third of the installed
motor power (see Fig. 2.11).
The gas ballast facility (see Fig. 2.13) prevents condensation of vapors in
the pump chamber of the pump. When pumping vapors these may only be
compressed up to their saturation vapor pressure at the temperature of the
pump. If pumping water vapor, for example, at a pump temperature of
70 °C, the vapor may only be compressed to 312 mbar (saturation vapor
pressure of water at 70 °C (see Table XIII in Section 9)). When
compressing further, the water vapor condenses without increasing the
Power of the drive motor [Watt]
The gas ballast facility as used in the rotary vane, rotary plunger and
trochoid pumps permits not only pumping of permanent gases but also
even larger quantities of condensable gases.
Pressure [mbar]
1
2
3
4
Operating temp. curve 1 32°C,
Operating temp. curve 2 40°C,
Operating temp. curve 3 60°C,
Operating temp. curve 4 90°C,
5 Theoretic curve for adiabatic
compression
6 Theoretic curve for isotherm compression
Fig. 2.11 Motor power of a rotary plunger pump (pumping speed 60 m3/h) as a function of
intake pressure and operating temperature. The curves for gas ballast pumps of other
sizes are similar.
24
1
2
3
4
5
6
Toothed wheel fixed to the driving shaft
Toothed wheel fixed to the piston
Elliptic piston
Inner surface of the pump
Driving shaft
Eccentric
Fig. 2.12 Cross section of a trochoid pump
inle
t
Di
sc
ha
rg
e
Ga
sb
alla
st
Com
pre
ss
ion
Vacuum generation
Gas ballast
inlet
1–2 Suction
2–5 Compression
3–4 Gas ballast inlet
5–6 Discharge
Position
of the
leading
vane
on
Sucti
Fig. 2.13 Working process within a rotary vane pump with gas ballast
pressure. No overpressure is created in the pump and the exhaust valve is
not opened. Instead the water vapor remains as water in the pump and
emulsifies with the pump’s oil. This very rapidly impairs the lubricating
properties of the oil and the pump may even seize when it has taken up too
much water. The gas ballast facility developed in 1935 by Wolfgang Gaede
inhibits the occurrence of condensation of the vapor in the pump as follows. Before the actual compression process begins (see Fig. 2.13), a
precisely defined quantity of air (“the gas ballast”) is admitted into the
pumping chamber of the pump. The quantity is such that the compression
ratio of the pump is reduced to 10:1 max. Now vapors which have been
taken in by the pump may be compressed together with the gas ballast,
before reaching their condensation point and ejected from the pump. The
partial pressure of the vapors which are taken in may however not exceed
a certain value. It must be so low that in the case of a compression by a
factor of 10, the vapors can not condense at the operating temperature of
the pump. When pumping water vapor this critical value is termed the
“water vapor tolerance”.
Shown schematically in Fig. 2.14 is the pumping process with and without
gas ballast as it takes place in a rotary vane pump when pumping
condensable vapors.
a) Without gas ballast
1) Pump is connected to the vessel, which
is already almost empty of air (70 mbar) – it
must thus transport mostly vapor particles
2) Pump chamber is separated from the
vessel – compression begins
3) Content of pump chamber is already so
far compressed that the vapor condenses to
form droplets – overpressure is not yet
reached
4) Residual air only now produces the
required overpressure and opens the discharge valve, but the vapor has already
condensed and the droplets are precipitated
in the pump.
b) With gas ballast
1) Pump is connected to the vessel, which
is already almost empty of air (70 mbar) – it
must thus transport mostly vapor particles
2) Pump chamber is separated from the
vessel – now the gas ballast valve, through
which the pump chamber is filled with additional air from outside, opens – this additional air is called gas ballast
3) Discharge valve is pressed open, and
particles of vapor and gas are pushed out –
the overpressure required for this to occur is
reached very early because of the supplementary gas ballast air, as at the beginning
the entire pumping process condensation
cannot occur
4) The pump discharges further air and
vapor
Two requirements must be met when pumping vapors:
1) the pump must be at operating temperature.
Fig. 2.14 Diagram of pumping process in a rotary vane pump without (left) and with (right) gas
ballast device when pumping condensable substances.
2) the gas ballast valve must be open.
Where:
(With the gas ballast valve open the temperature of the pump increases by
about 10 °C. Before pumping vapors the pump should be operated for half
an hour with the gas ballast valve open).
pvapor
= is the partial pressure of vapor at the intake of
the pump
pperm
= is the total pressure of all pumped permanent
gases at the intake of the pump
pvapor,sat
= is the saturation pressure of the pumped
vapor, depending on temperature (see Fig. 2.15)
psum
= pexhaust + Δpvalve + Δpexhaust filter
Δpvalve
= is the pressure difference across the exhaust valve
which amounts depending on type of pump and
operating conditions to 0.2 ... 0.4 bar
Simultaneous pumping of gases and vapors
When simultaneously pumping permanent gases and condensable vapors
from a vacuum system, the quantity of permanent gas will often suffice to
prevent any condensation of the vapors inside the pump. The quantity of
vapor which may be pumped without condensation in the pump can be
calculated as follows:
p
pvapour
< vapour, sat
pvapour + pperm
p sum
(2.1)
Δpexhaust filter = is the pressure difference across the exhaust filter
amounting to 0 ... 0.5 bar
25
Vacuum generation
Example 1:
With a rotary vane pump with an external oil mist filter in series, a mixture
of water vapor and air is being pumped. The following values are used for
applying eq. (2.1):
ratio 3 parts of permanent gases to 1 part of water vapor, as indicated in
example 1, can be for guidance only. In actual practice it is recommended
to run up a rotary pump of the types described hitherto always with the gas
ballast valve open, because it takes some time until the pump has reached
its final working temperature.
pexhaust = 1 bar
From eq. (2.1) follows for the permissible partial pressure pvapor of the
pumped vapor the relation
Δpvalve + Δpexhaust filter = 0.35 bar,
temperature of the pump 70 °C
Hence:
psum = 1.35 bar; pvapor sat (H2O)
= 312 mbar
(see Table XIII in chapter 9)
pvapour ≤
pvapour, sat
·p
psum − pvapour, sat perm
(2.2)
This relation shows that with pperm = 0 no vapors can be pumped without
condensation in the pump, unless the gas-ballast concept is applied. The
corresponding formula is:
Using eq. (2.1) follows:
pvapour, H O
312
= 0. 23
<
pvapour, H O + pair 1350
2
2
The pressure of the water vapor in the air/water vapor mixture must not
exceed 23 % of the total pressure of the mixture.
(p
)
−p
pvapour ≤ B psum · Vapour, sat vapour, g.b. +
psum − pvapour, sat
S
p
sat
pperm
+ p vapour
sum − pvapour sat
(2.3)
Where:
Example 2:
Ethanoic acid is to be pumped with a rotary plunger pump.
pexhaust
Δpvalve
Δpexhaust filter
Hence:
psum
= 1.1 bar (taking into consideration the flow resistance
of the pipes)
= 0.25 bar
= 0.15 bar
(pressure loss in the oil mist trap)
= 1.5 bar.
By controlled cooling the pump and oil temperature is set at 100 °C. The
saturation pressure of the acid therefore is – see
Fig. 2.15 – pvapor, sat = 500 mbar.
From eq. (2.1) follows:
pvapour, acid
0.5 1
=<
=
pvapour, acid + pair
1.5 3
Saturation vapor pressure [Torr]
Saturation vapor pressure [mbar]
Returning to the question of pumping water vapor in a mixture with air, the
Temperature [° C]
B
= is the volume of air at 1013 mbarwhich is admitted to the
pump chamber per unit time, called in brief the “gas ballast”
S
= is the nominal speed of the pump (volume flow rate)
psum
= is the pressure at the discharge outlet of the pump, assumed
to be a maximum at 1330 mbar
pvapor, sat
= Saturation vapor pressure of the vapor at the pump’s exhaust port
pvapor, g.b. = is the partial pressure of any vapor that might be present in
the gas used as gas ballast (e.g. water vapor contained in
the atmospheric air when used as gas ballast)
pperm
= is the total pressure of all permanent gases at the inlet port
of the pump
Eq. (2.3) shows that when using gas ballast (B ≠ 0) vapors can also be
pumped without condensation if no gas is present at the intake of the
pump. The gas ballast may also be a mixture of non-condensable gas and
condensable vapor as long as the partial pressure of this vapor
(pvapor, g.b.) is less than the saturation pressure pvapor, sat of the pumped
vapor at the temperature of the pump.
Water vapor tolerance
An important special case in the general considerations made above
relating to the topic of vapor tolerance is that of pumping water vapor.
According to PNEUROP water vapor tolerance is defined as follows:
“Water vapor tolerance is the highest pressure at which a vacuum pump,
under normal ambient temperatures and pressure conditions (20 °C,
1013 mbar), can continuously take in and transport pure water vapor. It is
quoted in mbar”. It is designated as pW,O.
Applying eq. (2.3) to this special case means:
pperm = 0 and pvapor, sat = ps (H2O), thus:
Fig. 2.15 Saturation vapor pressures
26
Vacuum generation
p W, O =
p ( H O ) − pvapour, g.b.
B
p sum s 2
psum − ps ( H 2O )
S
(2.4)
If for the gas ballast gas atmospheric air of 50 % humidity is used, then pvapor, g.b. = 13 mbar; with B/S = 0.10 – a usual figure in practice – and psum
(total exhaust pressure) = 1330 mbar, the water vapor tolerance pW,0 as
function of the temperature of the pump is represented by the lowest curve
in diagram Fig. 2.16. The other curves correspond to the pumping of water
vapor-air mixtures, hence pperm = pair ≠ O), indicated by the symbol pL in
millibar. In these cases a higher amount of water vapor partial pressure pw
pW [mbar]
pL [mbar]
2.1.3 Dry compressing rotary displacement
pumps
2.1.3.1
Roots pumps
The design principle of the Roots pumps was already invented in 1848 by
Isaiah Davies, but it was 20 years later before it was implemented in
practice by the Americans Francis and Philander Roots. Initially such
pumps were used as blowers for combustion motors. Later, by inverting the
drive arrangement, the principle was employed in gas meters. Only since
1954 has this principle been employed in vacuum engineering. Roots
pumps are used in pump combinations together with backing pumps (rotary
vane- and rotary plunger pumps) and extend their operating range well into
the medium vacuum range. With two stage Roots pumps this extends into
the high vacuum range. The operating principle of Roots pumps permits the
assembly of units having very high pumping speeds (over 100,000 m3/h)
which often are more economical to operate than steam ejector pumps
running in the same operating range.
Temperature of the pump
Fig. 2.16 Partial pressure pW of water vapor that can be pumped with the gas ballast valve
open without condensation in the pump, as a function of the pump temperature for
various partial pressures pL of air. The lowest curve corresponds to the water vapor
tolerance pW,O of the pump.
can be pumped as shown in the diagram. The figures for pW,0 given in the
catalogue therefore refer to the lower limit and are on the safe side.
According to equation 2.4 an increase in the gas ballast B would result in
an increased water vapor tolerance pW,0. In practice, an increase in B,
especially in the case of single-stage gas ballast pumps is restricted by the
fact that the attainable ultimate vacuum for a gas ballast pump operated
with the gas ballast valve open becomes worse as the gas ballast B
increases. Similar considerations also apply to the general equation 2.3 for
the vapor tolerance pvapor.
At the beginning of a pump down process, the gas ballast pump should
always be operated with the gas ballast valve open. In almost all cases a
thin layer of water will be present on the wall of a vessel, which only
evaporates gradually. In order to attain low ultimate pressures the gas
ballast valve should only be closed after the vapor has been pumped out.
LEYBOLD pumps generally offer a water vapor tolerance of between 33
and 66 mbar. Two-stage pumps may offer other levels of water vapor
tolerance corresponding to the compression ratio between their stages –
provided they have pumping chamber of different sizes.
Other gases as ballast
Generally atmospheric air is used as the gas ballast medium. In special
cases, when pumping explosive or toxic gases, for example, other
permanent gases like noble gases or nitrogen, may be used
(see Section 8.3.1.3).
1 Intake flange
2 Rotors
3 Chamber
4 Exhaust flange
5 Casing
Fig. 2.17 Schematic cross section of a Roots pump
A Roots vacuum pump (see Fig. 2.17) is a rotary positive-displacement type
of pump where two symmetrically-shaped impellers rotate inside the pump
casing past each other in close proximity. The two rotors have a cross
section resembling approximately the shape of a figure 8 and are
synchronized by a toothed gear. The clearance between the rotors and the
casing wall as well as between the rotors themselves amounts only to a few
tenths of a millimeter. For this reason Roots pumps may be operated at
high speeds without mechanical wear. In contrast to rotary vane and rotary
plunger pumps, Roots pumps are not oil sealed, so that the internal leakage
of dry compressing pumps by design results in the fact that compression
ratios only in the range 10 – 100 can be attained. The internal leakage of
Roots pumps, and also other dry compressing pumps for that matter, is
mainly based on the fact that owing to the operating principle certain
surface areas of the pump chamber are assigned to the intake side and the
compression side of the pump in alternating fashion. During the
compression phase these surface areas (rotors and casing) are loaded with
gas (boundary layer); during the suction phase this gas is released. The
thickness of the traveling gas layer depends on the clearance between the
two rotors and between the rotors and the casing wall. Due to the relatively
complex thermal conditions within the Roots pump it is not possible to base
27
Vacuum generation
one’s consideration on the cold state. The smallest clearances and thus the
lowest back flows are attained at operating pressures in the region of
1 mbar. Subsequently it is possible to attain in this region the highest
compression ratios, but this pressure range is also most critical in view of
contacts between the rotors and the casing.
Characteristic quantities of roots pumps
The quantity of gas Qeff effectively pumped by a Roots pump is calculated
from the theoretically pumped quantity of gas Qth and the internal leakage
QiR (as the quantity of gas which is lost) as:
Qeff = Qth – QiR
(2.5)
The following applies to the theoretically pumped quantity of gas:
Qth = pa · Sth
(2.6)
where pa is the intake pressure and Sth is the theoretical pumping speed.
This in turn is the product of the pumping volume VS and the speed n:
Sth = n · VS
(2.7)
Similarly the internal leakage QiR is calculated as:
QiR = n · ViR
(2.8)
where pV is the forevacuum pressure (pressure on the forevacuum side)
and SiR is a (notional) “reflow” pumping speed with
SiR = n · ViR
(2.10)
By using equations 2.5, 2.6, 2.7 and 2.8 one obtains
η = 1−
pV SiR
·
pa Sth
(2.11)
SiR
Sth
(2.11a)
Maximum compression is attained at zero throughput (see PNEUROP and
DIN 28 426, Part 2). It is designated as k0:
k0 = (
Sth
) =
SiR η 0
(2.12)
k0 is a characteristic quantity for the Roots pump which usually is stated as
a function of the forevacuum pressure pV (see Fig. 2.18). k0 also depends
(slightly) on the type of gas.
For the efficiency of the Roots pump, the generally valid equation applies:
η = 1− k
ko
28
(2.14)
From this
k=
S
pV
= η · th
pa
SV
k = η · kth
(2.15)
(2.16)
Equation (2.16) implies that the compression k attainable with a Roots
pump must always be less than the grading kth between Roots pump and
backing pump since volumetric efficiency is always < 1. When combining
equations (2.13) and (2.16) one obtains for the efficiency the well known
expression
η=
When designating the compression pv/pa as k one obtains
η = 1− k
SV · pV = Seff · pa = η · Sth · pa
The ratio Sth/SV (theoretical pumping speed of the Roots pump / pumping
speed of the backing pump) is termed the gradation kth. From (2.15) one
obtains
Volumetric efficiency of a Roots pumps is given by
Q eff
Q th
Normally a Roots pump will be operated in connection with a downstream
rough vacuum pump having a nominal pumping speed SV. The continuity
equation gives:
(2.9)
i.e. the product of speed n and internal leakage volume ViR.
η=
Fig. 2.18 Maximum compression k0 of the Roots pump RUVAC WA 2001 as a function of fore
vacuum pressure pV
k0
ko + k th
(2.17)
The characteristic quantities to be found in equation 2.17 are only for the
combination of the Roots pump and the backing pump, namely maximum
compression k0 of the Roots pump and gradation kth between Roots pump
and backing pump.
With the aid of the above equations the pumping speed curve of a given
combination of Roots pump and backing pump may be calculated. For this
the following must be known:
a) the theoretical pumping speed of the Roots pump: Sth
b) the max. compression as a function of the fore
vacuum pressure: k0 (pV)
c) the pumping speed characteristic of the backing pump SV (pV)
(2.13)
The way in which the calculation is carried out can be seen in Table 2.3
giving the data for the combination of a Roots pump RUVAC WA 2001 /
E 250 (single-stage rotary plunger pump, operated without gas ballast).
Vacuum generation
The power losses summarized in NV are – as shown by experience –
approximately proportional to Sth, i.e.:
In this the following is taken for Sth:
Sth = 2050 – 2.5 % = 2000
m3/h
The method outlined above may also be applied to arrangements which
consist of a rotary pump as the backing pump and several Roots pumps
connected in series, for example. Initially one determines – in line with an
iteration method – the pumping characteristic of the backing pump plus the
first Roots pump and then considers this combination as the backing pump
for the second Roots pump and so on. Of course it is required that the
theoretical pumping speed of all pumps of the arrangement be known and
that the compression at zero throughput k0 as a function of the backing
pressure is also known. As already stated, it depends on the vacuum
process which grading will be most suitable. It may be an advantage when
backing pump and Roots pump both have the same pumping speed in the
rough vacuum range.
(2.20)
Depending on the type of pump and its design the value of the constant
ranges between 0.5 and 2 Wh / m3 .
The total power is thus:
Ntot = Sth (pv – pa + const.)
The corresponding numerical value equation which is useful for calculations
is:
Ntot = Sth (pv – pa + const.) · 3 · 10-2 Watt
(2.21)
with pv, pa in mbar, Sth in m3 / h and the constant “const.” being between
18 and 72 mbar.
Power requirement of a roots pump
Compression in a Roots pump is performed by way of external
compression and is termed as isochoric compression. Experience shows
that the following equation holds approximately:
Ncompression = Sth (pv – pa)
∑ Nv = const · Sth
(2.18)
In order to determine the total power (so-called shaft output) of the pump,
mechanical power losses NV (for example in the bearing seals) must be
considered:
Ntot = Ncompression + ∑ NV
(2.19)
Forevacuum
pressure
Pv
Pumping speed
Sv of the
E 250
kth = Sth / Sv
= 2001/Sv
k0 (pv) of
the RUVAC
WA 2001
η = k0 / k0+kth
(Volumetric.
efficiency)
Seff = η Sth
(equation 2.14)
100
250
8.0
12.5
0.61
1.220
21
40
250
8.0
18
0.69
1.380
7.2
10
250
8.0
33
0.8
1.600
1.6
5
250
8.0
42
0.84
1.680
0.75
1
250
8.0
41
0.84
1.680
0.15
5 · 10–1
220
9.1
35
0.79
1.580
7 · 10–2
1 · 10–1
120
16.6
23
0.6
1.200
1 · 10–2
4 · 10–2
30
67
18
0.21
420
3 · 10–3
↓
↓
The values taken from the two right-hand columns give point by point the pumping speed curve for the combination WA 2001/E250 (see Fig. 2.19, topmost curve)
Intake pressure
pa = pv · Sv / Seff
Pumping speed characteristic
for the combination
WA 2001 / E250
Table 2.3
29
Pumping speed S
Vacuum generation
Intake pressure pa →
Fig. 2.19 Pumping speed curves for different pump combinations with the corresponding backing pumps
Load rating of a roots pump
The amount of power drawn by the pump determines its temperature. If the
temperature increases over a certain level, determined by the maximum
permissible pressure difference pV – pa, the danger exists that the rotors
may seize in the casing due to their thermal expansion. The maximum
permissible pressure difference Δpmax is influenced by the following factors:
forevacuum or compression pressure pV, pumping speed of the backing
pump SV, speed of the Roots pump n, gradation kth and the adiabatic
exponent κ of the pumped gas. Δpmax increases when pV and SV increase
and decreases when n and kth increase. Thus the maximum difference
between forevacuum pressure and intake pressure, pV – pa must – during
continuous operation – not exceed a certain value depending on the type of
pump. Such values are in the range between 130 and 50 mbar. However,
the maximum permissible pressure difference for continuous operation may
be exceed for brief periods. In the case of special designs, which use gas
cooling, for example, high pressure differences are also permissible during
continuous operation.
Maintaining the allowed pressure difference
In the case of standard Roots pumps, measures must be introduced to
ensure that the maximum permissible pressure difference between intake
and exhaust port due to design constraints is not exceeded. This is done
either by a pressure switch, which cuts the Roots pump in and out
depending on the intake pressure, or by using a pressure difference or
overflow valve in the bypass of the Roots pumps (Fig. 2.20 and 2.21). The
use of an overflow valve in the bypass of the Roots pump is the better and
more reliable solution. The weight and spring loaded valve is set to the
maximum permissible pressure difference of the particular pump. This
ensures that the Roots pump is not overloaded and that it may be operated
in any pressure range. In practice this means that the Roots pump can be
switched on, together with the backing pump, at atmospheric pressure. In
the process any pressure increases will not adversely affect combined
operation, i.e. the Roots pump is not switched off in such circumstances.
Types of motors used with roots pumps
Standard flange-mounted motors are used as the drive. The shaft
feedthroughs are sealed by two oil sealed radial shaft seals running on a
wear resistant bushing in order to protect the drive shaft. Flange motors of
any protection class, voltage or frequency may be used.
Integral leak tightness of this version is < 10-4 mbar · l · s-1.
In the case of better leak tightness requirements of < 10-5 mbar · l · s-1 the
Roots pump is equipped with a canned motor. The rotor is seated in the
vacuum on the drive shaft of the pump and is separated from the stator by
a vacuum-tight non-magnetic tube. The stator coils are cooled by a fan
having its own drive motor. Thus shaft seals which might be subject to wear
are no longer required. The use of Roots pumps equipped with canned
motors is especially recommended when pumping high purity-, toxic- or
radioactive gases and vapors.
Fig. 2.20 Cross section of a Roots pumps with bypass line
30
Vacuum generation
flows into the pumping chamber via the pre-admission channel. Finally the
rotors eject the pumped medium via the discharge port. The cooled gas,
which in the case of single-stage compression is taken from the
atmosphere and admitted from the pre-admission cooler, and which in the
case of multi-stage pump systems is taken from downstream gas coolers,
performs a pre-compression and removes by “inner cooling” the heat of
compression at the point of time it occurs.
WAU 2001
2.1.3.2
SOGEVAC SV 1200
Fig. 2.21 Vacuum diagram – Roots pump with integrated bypass line and backing pump
Pre-admission cooling (Fig. 2.22)
In the case of Roots pumps with pre-admission cooling, the compression
process basically is the same as that of a normal Roots pump. Since
greater pressure differences are allowed more installed power is needed,
which at the given speed and the pressure difference between inlet and
discharge port is directly proportional and is composed of the theoretical
work done on compression and various power losses. The compression
process ends normally after opening of the pumping chamber in the
direction of the discharge port. At this moment warmed gas at higher
pressure flows into the pumping chamber and compresses the transported
volume of gas. This compression process is performed in advance in the
case of pre-admission cooling. Before the rotor opens the pumping
chamber in the direction of the discharge port, compressed and cooled gas
Claw pumps
Like Roots pumps, claw pumps belong to the group of dry compressing
rotary piston vacuum pumps (or rotary vacuum pumps). These pumps may
have several stages; their rotors have the shape of claws.
The design principle of a claw pump is explained by first using an
example of a four-stage design. The cross section inside the pump’s casing
has the shape of two partly overlapping cylinders (Fig. 2.23). Within these
cylinders there are two freely rotating rotors in each pump stage: (1) with
their claws and the matching recesses rotating in opposing directions about
their vertical axes. The rotors are synchronized by a gear just like a Roots
pump. In order to attain an optimum seal, the clearance between the rotor
at the center of the casing and the amount of clearance with respect to the
inside casing wall is very small; both are in the order of magnitude of a few
0.01 mm. The rotors periodically open and close the intake and discharge
slots (5) and (4). At the beginning of the work cycle in position a, the right
rotor just opens the intake slot (5). Gas now flows into the continually
increasing intake space (3) in position b until the right rotor seals off the
intake slot (5) in position c. After both claws have passed through the
center position, the gas which has entered is then compressed in the
a
1
b
4
4
c
3
2
1 Intake port
2 Discharge port
3 Gas cooler
Fig. 2.22 Diagram of a Roots pump with pre-admission cooling
4 Flow of cold gas
1
2
3
4
Rotors
Compression chamber
Intake space
Exhaust slot
5 Intake slot
6 Intermediate stage purge
gas
Fig. 2.23 Principle of operation
31
Vacuum generation
IN
OUT
1 mbar
Stage 1
3 mbar
P
IN
3 mbar
OUT 15 mbar
Stage 2
Z
IN
15 mbar
OUT 150
Stage 3
P
Z
IN
150
OUT
Stage 4
1000
P
Z
P
Fig. 2.25 Pressures in pump stages 1 to 4
Fig. 2.24 Arrangement of the pumps and guiding of the gas flow. P = Pump stage Z =
Intermediate ring
compression chamber (2) (position a) so long until the left rotor releases
the discharge slot (4) (position b) thereby discharging the gas. Immediately
after the compression process has started (position a) the intake slot (5) is
opened simultaneously and gas again flows into the forming intake space
(3) (position b). Influx and discharge of the gas is performed during two half
periods. Each rotor turns twice during a full work cycle. Located between
the pumping stages are intermediate discs with flow channels which run
from the discharge side of the upper stage to the intake side of the next
stage, so that all inlet or exhaust sides are arranged vertically above each
other (Fig. 2.24). Whereas in a Roots pump the incoming gas is pumped
through the pump at a constant volume and compression is only performed
in the forevacuum line (see Section 2.1.3.1), the claw pump compresses
the gas already within the pumping chamber until the rotor releases the
discharge slot. Shown in Fig. 2.25 are the average pressure conditions in
the individual pumping stages of a DRYVAC at an intake pressure of 1
mbar. In order to meet widely differing requirements LEYBOLD
manufactures two different series of claw pumps, which chiefly differ in the
type of compression process used:
1) Pumps with internal compression, multi-stage for the semiconductor
industry (DRYVAC Series) and
2) Pumps without internal compression, two-stage for chemistry
applications (“ALL·ex”).
Figs. 2.26 and 2.27 demonstrate the differences in design. Shown is the
course of the pressure as a function of the volume of the pumping chamber
by way of a pV diagram.
Fig. 2.26 shows the (polytropic) course of the compression for pumps with
P
P
1
2
W Compr.
1
W Compr.
2
3
4
3
V
Fig. 2.26 Compression curve for a claw pump with internal compression
32
V
Fig. 2.27 Compression curve for a claw pump without internal compression (“isochoric compression”)
Vacuum generation
1
Casing
suction
Intake
line
2
Cooling water
3
Exhaust line
1 Intake port
2 Front panel / electronics
3 Main switch
Fig. 2.28 DRYVAC pump
internal compression. The pressure increases until the discharge slot is
opened. If at that point the exhaust back pressure has not been reached,
the compression space is suddenly vented with hot exhaust gas. As the
volume is reduced further, the gas at exhaust pressure is ejected. The work
done on compression is represented by the area under the p-v curve
1-2-3-4. It is almost completely converted into heat. In the case of dry
compressing vacuum pumps not much of this heat can be lost to the cooled
casing due to the low density of the gas. This results in high gas
temperatures within the pump. Experiences with claw pumps show that the
highest temperatures occur at the rotors.
Shown in Fig. 2.27 is the principle of isochoric compression in a p-v diagram. Here the compression is not performed by reducing the volume of the
pumping chamber, but by venting with cold gas which is applied from the
outside after completion of the intake process. This is similar to the
admission of a gas ballast when opening the gas ballast valve after
completion of the intake phase. From the diagram it is apparent that in the
case of isochoric compression the work done on compression must be
increased, but cold gas instead of hot exhaust gas is used for venting. This
method of direct gas cooling results in considerably reduced rotor
temperatures. Pumps of this kind are discussed as “ALL·ex” in Section
2.1.3.2.2.
2.1.3.2.1
Fig. 2.29a Vacuum diagram for the DRYVAC B
with perfluoropolyether (PFPE). The gear box is virtually hermetically sealed
from the pumping chamber by piston rings and a radial shaft seal. The
bearings in the upper end disk are lubricated with PFPE grease. In order to
protect the bearings and shaft seals against aggressive substances, a
barrier gas facility is provided. A controlled water cooling system allows the
control of the casing temperature over a wide range as the pump is
subjected to differing gas loads coming from the process. The four stage
design is available in several pumping speed and equipment grades of 25,
50 and 100 m3/h DRYVAC pumps:
a) as the basic version for non-aggressive clean processes:
DRYVAC 25 B, 50 B and 100 B (Fig. 2.29a)
b) as a version for semiconductor processes: DRYVAC 25 P, 50 P and
100 P (Fig. 2.29b)
c) as a system version with integrated self monitoring: DRYVAC 50 S and
100 S
Casing
suction
Intake
line
100 P
Cooling water
Claw pumps with internal compression for the
semiconductor industry (“DRYVAC series”)
Exhaust line
Design of DRYVAC Pumps
Due to the work done on compression in the individual pumping stages,
multi-stage claw pumps require water cooling for the four stages to remove
the compression heat. Whereas the pumping chamber of the pump is free
of sealants and lubricants, the gear and the lower pump shaft are lubricated
Inert gas
Fig. 2.29b
Vacuum diagram for the DRYVAC P
33
Vacuum generation
To be provided by the customer
TSH
Temperature switch
PSL
Pressure switch
PSH
Pressure switch
FSL
Flow switch
MPS
Motor protection switch
PT 100 Temperature sensor
For DRYVAC with LIMS
CS
Current sensor
EPS
Exhaust pressure sensor
Fig. 2.29c Vacuum diagram for the DRYVAC S
Fig. 2.30 Key to Figures 2.29a – 2.29c
d) as a system version with integrated self monitoring offering an
increased pumping speed in the lower pressure range: DRYVAC 251 S
and 501 S (Fig. 2.29c)
thus the formation of layers within the pump and reduces the risk that the
claw rotor may seize. Care must be taken to ensure that the velocity of the
gas flow within the individual pumping stages is at all times greater than the
settling speed of the particles entrained in the gas flow. As can be seen in
Fig. 2.31, the settling speed of the particles depends strongly on their size.
The mean velocity (VGas) of the flowing gas during the compression phase
is given by the following equation:
The ultimate pressure attainable with the DRYVAC 251 S or 501 S is –
compared to the versions without integrated Roots pump – by
approximately one order of magnitude lower (from 2 · 10-2 mbar to
3 · 10-3 mbar) and the attainable throughput is also considerably increased.
It is of course possible to directly flange mount LEYBOLD RUVAC pumps
on to the DRYVAC models (in the case of semiconductor processes also
mostly with a PFPE oil filling for the bearing chambers).
The pumps of the DRYVAC family are the classic dry compressing claw
vacuum pumps that are preferably used in the semiconductor industry,
whereby the pumps need to meet a variety of special requirements. In
semiconductor processes, as in many other vacuum applications, the
formation of particles and dusts during the process and/or in the course of
compressing the pumped substances to atmospheric pressure within the
pump, is unavoidable. In the case of vacuum pumps operating on the claw
principle it is possible to convey particles through the pump by means of so
called “pneumatic conveying”. This prevents the deposition of particles and
v
Gas
=
q pV mbar · · s −1
·
=
p · A mbar · cm2
=
10 · q pV m
·
p· A s
(2.22)
qpV = gas throughput
p = pressure
A = surface area
One can see that with increasing pressure the velocity of the pumped gas
flow slows down and attains the order of magnitude of the settling speed of
the particles in the gas flow (Fig. 2.32). This means that the risk of
Stage 2
2 500 mbar · /s
Stage 3
Stage 4
8 300 mbar · /s 20 000 mbar · /s
Limit speed
Gas velocity
Particle size
Throughput in mbar · /s
Pressure
Throughput cross section 6.5 cm2, constant
Pressure
Fig. 2.31 Settling speed as a function of pressure p. Parameter: particle size
34
Fig. 2.32 Mean gas velocity vg during compression without purge gas (left) and with purge gas
(right) in stages 2, 3 and 4
Vacuum generation
• Losses in pumping speed and a reduction in ultimate pressure can be
kept very small due to the special way in which the gas is made to pass
through the pump.
Ultimate pressure
mbar
2.1.3.2.2
Purge gas flow
Fig. 2.33 Ultimate pressure of the DRYVAC 100S as a function of pure gas flow in stages 2 – 4
depositing particles in the operating chamber of the pump and the resulting
impairment increases with increasing pressure. In parallel to this the
potential for the formation of particles from the gaseous phase increases at
increasing compression levels. In order to keep the size of the forming
particles small and thus their settling speed low and to maintain a high
velocity for the gas, an additional quantity of gas is supplied into the pump
via the individual intermediate discs (purge gas). For this, the inflowing
quantity of purge gas is matched to the pressure conditions prevailing in the
individual pumping stages (see top right part of Fig. 2.32). This keeps the
velocity of the gas flow high enough within the entire pump by so-called
pneumatic pumping. Through the way in which the gas is lead within the
pump, i.e. from the intake through the four pumping stages with the related
intermediate discs to the exhaust, it is possible to reduce the influence of
the purge gas on the ultimate pressure to a minimum. Test results (Fig.
2.33) indicate that the influence of purge gas in the fourth stage is – as to
be expected – of the lowest level since there are located between this
stage and the intake side the three other pumping stages. The admission of
purge gas via the second and third stages (Fig. 2.33) has a comparatively
small influence on the ultimate pressure as can be seen from the pumping
speed curve in Fig. 2.34. Finally it can be said that the formation of particles
is to be expected in most CVD processes. When using dry compressing
claws vacuum pumps, the controlled admission of purge gas via the
individual intermediate discs is the best approach to avoid the formation of
layers. When applying this method several effects can be noted:
• The admitted purge gas dilutes the pumped mixture of substances,
particle-forming reactions will not occur, or are at least delayed.
Claw pumps without internal compression
for chemistry applications (“ALL·ex” )
The chemical industry requires vacuum pumps which are highly reliable and
which do not produce waste materials such as contaminated waste oil or
waste water. If this can be done, the operating costs of such a vacuum
pump are low in view of the measures otherwise required for protecting the
environment (disposal of waste oil and water, for example). For operation of
the simple and rugged “ALL·ex” pump from LEYBOLD there are no
restrictions as to the vapor flow or the pressure range during continuous
operation. The “ALL·ex” may be operated within the entire pressure range
from 5 to 1000 mbar without restrictions.
Design of the “ALL·ex” pump
The design of the two-stage ALL·ex is shown schematically in Fig. 2.35.
The gas flows from top to bottom through the vertically arranged pumping
stages in order to facilitate the ejection of condensates and rinsing liquids
which may have formed. The casing of the pump is water cooled and
permits cooling of the first stage. There is no sealed link between gas
chamber and cooling channel so that the entry of cooling water into the
pumping chamber can be excluded. The pressure-burst resistant design of
the entire unit underlines the safety concept in view of protection against
internal explosions, something which was also taken into account by direct
cooling with cold gas (see operating principle). A special feature of the
“ALL·ex” is that both shafts have their bearings exclusively in the gear. On
the pumping side, the shafts are free (cantilevered). This simple design
allows the user to quickly disassemble the pump for cleaning and servicing
without the need for special tools.
In order to ensure a proper seal against the process medium in the
pumping chamber the shaft seal is of the axial face seal type – a sealing
• The risk of an explosion through self-igniting substances is significantly
reduced.
• Particles which have formed are conveyed pneumatically through the
pump
Intake port
Motor
1st stage
Claws
Pumping speed
2nd stage
Coupling
Axial face
seals
mbar · l/s
Stage 2
Stage 3
Stage 4
Gear, complete
with shafts and
bearings
mbar · l/s
mbar · l/s
Pressure
Fig. 2.34 Pumping speed with and without purge gas
Fig. 2.35 Simple arrangement of the dry compression “ALL·ex” pump
35
Vacuum generation
1
3
4
2
5
6
7
1 Motor
2 Pump
3 Intake port
4 Exhaust port
5 Exhaust cooler
6 Cooling water connection
7 Cooling receiver
Fig. 2.36 “ALL·ex” pump
concept well proven in chemistry applications. This type of seal is capable
of sealing liquids against liquids, so that the pump becomes rinseable and
insensitive to forming condensate. Fig. 2.36 shows the components
supplied with the ALL.ex, together with a gas cooler and a receiver.
Operating principle
Isochoric compression, which also serves the purpose of limiting the
temperature ultimately attained during compression, especially in the stage
on the side of the atmosphere, and which ensures protection against
internal explosions, is performed by venting the pumping chamber with cold
gas from a closed refrigerating gas cycle (Fig. 2.37). Fig. 2.38-1 indicates
the start of the intake process by opening the intake slot through the control
edge of the right rotor. The process gas then flows into the intake chamber
which increases in size. The intake process is caused by the pressure
gradient produced by increasing the volume of the pumping chamber. The
maximum volume is attained after 3/4 of a revolution of the rotors (Fig.
2.38-2). After the end of the intake process, the control edge of the left
rotor opens the cold gas inlet and at the same time the control edge of the
right rotor opens the intake slot (Fig. 2.38-3) once more. In Fig. 2.38-4 the
control edge of the left rotor terminates the discharge of the gas which has
been compressed to 1000 mbar with the cold gas; at the same time the
control edge of the right rotor completes an intake process again.
The total emissions from the system are not increased by the large
quantities of cold gas, since a closed refrigerating cycle is maintained by
way of an externally arranged gas cooler and condenser (Fig. 2.37). The
hot exhaust gas is made to pass through the cooler and is partly returned
in the form of cold gas for pre-admission cooling into the pump. The pump
36
takes in the quantity of cold process gas needed for venting the pumping
chamber back into the compression space on its own. This process,
however, has no influence on the pumping speed of the “ALL·ex” because
the intake process has already ended when the venting process starts.
Designing the cooler as a condenser allows for simple solvent recovery.
The method of direct gas cooling, i.e. venting of the pumping chamber with
cold gas supplied from outside (instead of hot exhaust gas) results in the
case of the “ALL·ex” in rotor temperatures which are so low that mixtures
of substances rated as ExT3 can be pumped reliably under all operating
conditions. The “ALL·ex” thus fully meets the requirements of the chemical
industry concerning the protection against internal explosions. A certain
degree of liquid compatibility makes the “ALL·ex” rinseable, thus avoiding
the formation of layers in the pump, for example, or the capability of
dissolving layers which may already have formed respectively. The rinsing
liquids are usually applied to the pump after completion of the connected
process (batch operation) or while the process is in progress during brief
blocking phases. Even while the “ALL·ex” is at standstill and while the
pumping chamber is completely filled with liquid it is possible to start this
pump up. Shown in Fig. 2.39 is the pumping speed characteristic of an
“ALL·ex” 250. This pump has a nominal pumping speed of 250 m3/h and
an ultimate pressure of < 10 mbar. At 10 mbar it still has a pumping speed
of 100 m3/h. The continuous operating pressure of the pump may be as
high as 1000 mbar; it consumes 13.5 kW of electric power.
Vacuum generation
1000
m
. h-1
Saugvermˆgen
Pumping speed
Cooling gas
3
Exhaust gas
100
10
8
6
4
2
1
1
2
4
6
8
100
10
Ansaugdruck
Intake
pressure
Fig. 2.37 Circulation of the cold gas in the “ALL·ex” with cooler / condenser
mbar
1000
Fig. 2.39 Pumping speed characteristic of an ALL·ex 250
Vmax
Exhaust slot
1
Volume of the pump
chamber starts to increase
Intake
slot
Suction
Cold gas inlet
Vmin
100
(10)
1000 Pmbar
(100)
100
(10)
1000 Pmbar
(100)
Vmax
2
Volume of the pump
chamber at maximum
End of suction
Vmin
Cold gas inlet
Vmax
3
Volume of the pump chamber
stars to decrease (without
compression). Pressure
increase to 1000 mbar
only by admitting cold gas.
Vmin
Beginning of admitting cold gas
100
(10)
1000 Pmbar
(100)
100
(10)
1000 Pmbar
(100)
Vmax
4
Ejection of the mixture
composed of sucked
in gas and cold gas.
Exhaust slot
Cold gas inlet
Vmin
Fig. 2.38 Diagrams illustrating the pumping principle of the ALL·ex pump (claw pump without inner compression)
37
Vacuum generation
2.1.4 Accessories for oil-sealed rotary
displacement pumps
During a vacuum process, substances harmful to rotary pumps can be
present in a vacuum chamber.
Elimination of water vapor
Water vapor arises in wet vacuum processes. This can cause water to be
deposited in the inlet line. If this condensate reaches the inlet port of the
pump, contamination of the pump oil can result. The pumping performance
of oil-sealed pumps can be significantly impaired in this way. Moreover,
water vapor discharged through the outlet valve of the pump can condense
in the discharge outlet line. The condensate can, if the outlet line is not
correctly arranged, run down and reach the interior of the pump through the
discharge outlet valve. Therefore, in the presence of water vapor and other
vapors, the use of condensate traps is strongly recommended. If no
discharge outlet line is connected to the gas ballast pump (e.g., with
smaller rotary vane pumps), the use of discharge filters is recommended.
These catch the oil mist discharged from the pump.
Some pumps have easily exchangeable filter cartridges that not only hold
back oil mist, but clean the circulating pump oil. Whenever the amount of
water vapor present is greater than the water vapor tolerance of the pump,
a condenser should always be installed between the vessel and the pump.
(For further details, see Section 2.1.5)
Elimination of dust
Solid impurities, such as dust and grit, significantly increase the wear on
the pistons and the surfaces in the interior of the pump housing. If there is
a danger that such impurities can enter the pump, a dust separator or a
dust filter should be installed in the inlet line of the pump. Today not only
conventional filters having fairly large casings and matching filter inserts
are available, but also fine mesh filters which are mounted in the centering
ring of the small flange. If required, it is recommended to widen the cross
section with KF adaptors.
Elimination of oil vapor
The attainable ultimate pressure with oil-sealed rotary pumps is strongly
influenced by water vapor and hydrocarbons from the pump oil. Even with
two-stage rotary vane pumps, a small amount of back-streaming of these
molecules from the pump interior into the vacuum chamber cannot be
avoided. For the production of hydrocarbon-free high and ultrahigh vacuum,
for example, with sputter-ion or turbomolecular pumps, a vacuum as free
as possible of oil is also necessary on the forevacuum side of these
pumps. To obtain this, medium vacuum adsorption traps (see Fig. 2.40)
filled with a suitable adsorption material (e.g., LINDE molecular sieve 13X)
are installed in the inlet line of such oil-sealed forepumps. The mode of
action of a sorption trap is similar to that of an adsorption pump. For further
details, see Section 2.1.8. If foreline adsorption traps are installed in the
inlet line of oil-sealed rotary vane pumps in continuous operation, two
adsorption traps in parallel are recommended, each separated by valves.
Experience shows that the zeolite used as the adsorption material loses
much of its adsorption capacity after about 10 – 14 days of running time,
after which the other, now-regenerated, adsorption trap can be utilized;
hence the process can continue uninterrupted. By heating the adsorption
trap, which is now not connected in the pumping line, the vapors escaping
38
1
2
3
4
5
Housing
Basket holding the sieve
Molecular sieve (filling)
Sealing flanges
Intake port with small flange
6 Upper section
7 Vessel for the heater or
refrigerant
8 Connection on the side of the
pump with small flange
Fig. 2.40 Cross section of a medium vacuum adsorption trap
from the surface of the zeolite can be most conveniently pumped away with
an auxiliary pump. In operation, pumping by the gas ballast pump generally
leads to a covering of the zeolite in the other, unheated adsorption trap and
thus to a premature reduction of the adsorption capacity of this trap.
Reduction of the effective pumping speed
All filters, separators, condensers, and valves in the inlet line reduce the
effective pumping speed of the pump. On the basis of the values of the
conductances or resistances normally supplied by manufacturers, the
actual pumping speed of the pump can be calculated. For further details,
see Section 1.5.2.
2.1.5 Condensers
For pumping larger quantities of water vapor, the condenser is the most
economical pump. As a rule, the condenser is cooled with water of such
temperature that the condenser temperature lies sufficiently below the dew
point of the water vapor and an economical condensation or pumping
action is guaranteed. For cooling, however, media such as brine and
refrigerants (NH3, Freon ) can also be used.
When pumping water vapor in a large industrial plant, a certain quantity of
Vacuum generation
For a mathematical evaluation of the combination of condenser and gas
ballast pump, it can be assumed that no loss of pressure occurs in the
condenser, that the total pressure at the condenser entrance ptot 1, is equal
to the total pressure at the condenser exit, ptot 2:
ptot 1 = ptot 2
(2.23)
The total pressure consists of the sum of the partial pressure portions of the
air pp and the water vapor pv:
Fig. 2.41 Condenser (I) with downstream gas ballast pump (II) for pumping of large quantities
of water vapor in the rough vacuum range (III) – adjustable throttle
air is always involved, which is either contained in the vapor or originates
from leaks in the plant (the following considerations for air and water vapor
obviously apply also in general for vapors other than water vapor).
Therefore, the condenser must be backed by a gas ballast pump (see Fig.
2.41) and hence always works – like the Roots pump – in a combination.
The gas ballast pump has the function of pumping the fraction of air, which
is often only a small part of the water-vapor mixture concerned, without
simultaneously pumping much water vapor. It is, therefore, understandable
that, within the combination of condenser and gas ballast pump in the
stationary condition, the ratios of flow, which occur in the region of rough
vacuum, are not easily assessed without further consideration. The simple
application of the continuity equation is not adequate because one is no
longer concerned with a source or sink-free field of flow (the condenser is,
on the basis of condensation processes, a sink). This is emphasized
especially at this point. In a practical case of “non-functioning” of the
condenser – gas ballast pump combination, it might be unjustifiable to blame
the condenser for the failure.
In sizing the combination of condenser and gas ballast pump, the following
points must be considered:
a) the fraction of permanent gases (air) pumped simultaneously with the
water vapor should not be too great. At partial pressures of air that are
more than about 5 % of the total pressure at the exit of the condenser, a
marked accumulation of air is produced in front of the condenser
surfaces. The condenser then cannot reach its full capacity (See also the
account in Section 2.2.3 on the simultaneous pumping of gases and
vapors).
b) the water vapor pressure at the condenser exit – that is, at the inlet side of
the gas ballast pump – should not (when the quantity of permanent gas
described in more detail in Section 2.2.3 is not pumped simultaneously)
be greater than the water vapor tolerance for the gas ballast pump
involved. If – as cannot always be avoided in practice – a higher water
vapor partial pressure is to be expected at the condenser exit, it is convenient to insert a throttle between the condenser exit and the inlet port of
the gas ballast pump. The conductance of this throttle should be variable
and regulated (see Section 1.5.2) so that, with full throttling, the pressure
at the inlet port of the gas ballast pump cannot become higher than the
water vapor tolerance. Also, the use of other refrigerants or a decrease of
the cooling water temperature may often permit the water vapor pressure
to fall below the required value.
pp1 + pv1 = pp2 + pv2
(2.23a)
As a consequence of the action of the condenser, the water vapor pressure
pD2 at the exit of the condenser is always lower than that at the entrance;
for (2.23) to be fulfilled, the partial pressure of air pp2 at the exit must be
higher than at the entrance pp1, (see Fig. 2.43), even when no throttle is
present.
The higher air partial pressure pp2 at the condenser exit is produced by an
accumulation of air, which, as long as it is present at the exit, results in a
stationary flow equilibrium. From this accumulation of air, the (eventually
throttled) gas ballast pump in equilibrium removes just so much as streams
from the entrance (1) through the condenser.
All calculations are based on (2.23a) for which, however, information on the
quantity of pumped vapors and permanent gases, the composition, and the
pressure should be available. The size of the condenser and gas ballast
pump can be calculated, where these two quantities are, indeed, not
mutually independent. Fig. 2.42 represents the result of such a calculation
as an example of a condenser having a condensation surface of 1 m2, and
at an inlet pressure pv1, of 40 mbar, a condensation capacity that amounts
to 15 kg / h of pure water vapor if the fraction of the permanent gases is
very small. 1 m3 of cooling water is used per hour, at a line overpressure of
3 bar and a temperature of 12 °C. The necessary pumping speed of the
gas ballast pump depends on the existing operating conditions, particularly
the size of the condenser. Depending on the efficiency of the condenser,
the water vapor partial pressure pv2 lies more or less above the saturation
pressure pS which corresponds to the temperature of the refrigerant. (By
cooling with water at 12 °C, pS, would be 15 mbar (see Table XIII in Section
9)). Correspondingly, the partial air pressure pp2 that prevails at the
condenser exit also varies. With a large condenser, pv2 ≈ pS, the air partial
pressure pp,2 is thus large, and because pp · V = const, the volume of air
involved is small. Therefore, only a relatively small gas ballast pump is
Condensation capacity [kg · h-1]
1 Inlet of the condenser
2 Discharge of the condenser
3 See text
Intake pressure pD1
Fig. 2.42 Condensation capacity of the condenser (surface area available to condensation 1
m2) as a function of intake pressure pD1 of the water vapor. Curve a: Cooling water
temperature 12°C. Curve b: Temperature 25 °C. Consumption in both cases 1 m3/h at
3 bar overpressure.
39
Vacuum generation
two following cases:
Pv2
P’v2
Pv1
Pp2
P’p2
Pp1
Fig. 2.43 Schematic representation of the pressure distribution in the condenser. The full lines
correspond to the conditions in a condenser in which a small pressure drop takes
place (ptot 2 < ptot 1). The dashed lines are those for an ideal condenser (ptot 2 ≈ ptot 1).
pD: Partial pressure of the water vapor, pL: Partial pressure of the air
necessary. However, if the condenser is small, the opposite case arises:
pv2 > pS · pp2, is small. Here a relatively large gas ballast pump is required.
Since the quantity of air involved during a pumping process that uses
condensers is not necessarily constant but alternates within more or less
wide limits, the considerations to be made are more difficult. Therefore, it is
necessary that the pumping speed of the gas ballast pump effective at the
condenser can be regulated within certain limits.
1. Pumping of permanent gases with small amounts of water vapor. Here
the size of the condenser – gas ballast pump combination is decided on the
basis of the pumped-off permanent gas quantity. The condenser function is
merely to reduce the water vapor pressure at the inlet port of the gas
ballast pump to a value below the water vapor tolerance.
2. Pumping of water vapor with small amounts of permanent gases. Here,
to make the condenser highly effective, as small as possible a partial
pressure of the permanent gases in the condenser is sought. Even if the
water vapor partial pressure in the condenser should be greater than the
water vapor tolerance of the gas ballast pump, a relatively small gas ballast
pump is, in general, sufficient with the then required throttling to pump away
the prevailing permanent gases.
Important note: During the process, if the pressure in the condenser drops
below the saturation vapor pressure of the condensate (dependent on the
cooling water temperature), the condenser must be blocked out or at least
the collected condensate isolated. If this is not done, the gas ballast pump
again will pump out the vapor previously condensed in the condenser
2.1.6 Fluid-entrainment pumps
b) Next to the large pump for rough pumping a holding pump with low
speed is installed, which is of a size corresponding to the minimum
prevailing gas quantity. The objective of this holding pump is merely to
maintain optimum operating pressure during the process.
Basically, a distinction is made between ejector pumps such as water jet
pumps (17 mbar < p < 1013 mbar), vapor ejector vacuum pumps
(10-3 mbar < p < 10-1 mbar) and diffusion pumps (p < 10-3 mbar). Ejector
vacuum pumps are used mainly for the production of medium vacuum.
Diffusion pumps produce high and ultrahigh vacuum. Both types operate
with a fast-moving stream of pump fluid in vapor or liquid form (water jet as
well as water vapor, oil or mercury vapor). The pumping mechanism of all
fluid-entrainment pumps is basically the same. The pumped gas molecules
are removed from the vessel and enter into the pump fluid stream which
expands after passing through a nozzle. The molecules of the pump fluid
stream transfer by way of impact impulses to the gas molecules in the
direction of the flow. Thus the gas which is to be pumped is moved to a
space having a higher pressure.
c) The necessary quantity of air is admitted into the inlet line of the pump
through a variable-leak valve. This additional quantity of air acts like an
enlarged gas ballast, increasing the water vapor tolerance of the pump.
However, this measure usually results in reduced condenser capacity.
Moreover, the additional admitted quantity of air means additional power
consumption and (see Section 8.3.1.1) increased oil consumption. As
the efficiency of the condenser deteriorates with too great a partial
pressure of air in the condenser, the admission of air should not be in
front, but generally only behind the condenser.
In fluid-entrainment pumps corresponding vapor pressures arise during
operation depending on the type of pump fluid and the temperature as well
as the design of the nozzle. In the case of oil diffusion pumps this may
amount to 1 mbar in the boiling chamber. The backing pressure in the
pump must be low enough to allow the vapor to flow out. To ensure this,
such pumps require corresponding backing pumps, mostly of the
mechanical type. The vapor jet cannot enter the vessel since it condenses
at the cooled outer walls of the pump after having been ejected through the
nozzle.
If the starting time of a process is shorter than the total running time,
technically the simplest method – the roughing and the holding pump – is
used. Processes with strongly varying conditions require an adjustable
throttle section and, if needed, an adjustable air admittance.
Wolfgang Gaede was the first to realize that gases at comparatively low
pressure can be pumped with the aid of a pump fluid stream of essentially
higher pressure and that, therefore, the gas molecules from a region of low
total pressure move into a region of high total pressure. This apparently
paradoxical state of affairs develops as the vapor stream is initially entirely
free of gas, so that the gases from a region of higher partial gas pressure
(the vessel) can diffuse into a region of lower partial gas pressure (the
vapor stream). This basic Gaede concept was used by Langmuir (1915) in
the construction of the first modern diffusion pump. The first diffusion
pumps were mercury diffusion pumps made of glass, later of metal. In the
In practice, the following measures are usual:
a) A throttle section is placed between the gas ballast pump and the
condenser, which can be short-circuited during rough pumping. The flow
resistance of the throttle section must be adjustable so that the effective
speed of the pump can be reduced to the required value. This value can
be calculated using the equations given in Section 2.2.3.
On the inlet side of the gas ballast pump a water vapor partial pressure pv2
is always present, which is at least as large as the saturation vapor
pressure of water at the coolant temperature. This ideal case is realizable in
practice only with a very large condenser (see above).
With a view to practice and from the stated fundamental rules, consider the
40
Vacuum generation
Sixties, mercury as the medium was almost completely replaced by oil. To
obtain as high a vapor stream velocity as possible, he allowed the vapor
stream to emanate from a nozzle with supersonic speed. The pump fluid
vapor, which constitutes the vapor jet, is condensed at the cooled wall of
the pump housing, whereas the transported gas is further compressed,
usually in one or more succeeding stages, before it is removed by the
backing pump. The compression ratios, which can be obtained with fluid
entrainment pumps, are very high: if there is a pressure of 10-9 mbar at the
inlet port of the fluid-entrainment pump and a backing pressure of
10-2 mbar, the pumped gas is compressed by a factor of 107!
Basically the ultimate pressure of fluid entrainment pumps is restricted by
the value for the partial pressure of the fluid used at the operating
temperature of the pump. In practice one tries to improve this by introducing
baffles or cold traps. These are “condensers” between fluid entrainment
pump and vacuum chamber, so that the ultimate pressure which can be
attained in the vacuum chamber is now only limited by the partial pressure
of the fluid at the temperature of the baffle.
The various types of fluid entrainment pumps are essentially distinguished
by the density of the pump fluid at the exit of the top nozzle facing the high
vacuum side of the pump:
1. Low vapor density:
Diffusion pumps
2.1.6.1 (Oil) Diffusion pumps
These pumps consist basically (see Fig. 2.44) of a pump body (3) with a
cooled wall (4) and a three- or four-stage nozzle system (A – D). The oil
serving as pump fluid is in the boiler (2) and is vaporized from here by
electrical heating (1). The pump fluid vapor streams through the riser tubes
and emerges with supersonic speed from the ring-shaped nozzles (A – D).
Thereafter the jet so-formed widens like an umbrella and reaches the wall
where condensation of the pump fluid occurs. The liquid condensate flows
downward as a thin film along the wall and finally returns into the boiler.
Because of this spreading of the jet, the vapor density is relatively low. The
diffusion of air or any pumped gases (or vapors) into the jet is so rapid that
despite its high velocity the jet becomes virtually completely saturated with
the pumped medium. Therefore, over a wide pressure range diffusion
pumps have a high pumping speed. This is practically constant over the
entire working region of the diffusion pump (≤ 10-3 mbar) because the air at
these low pressures cannot influence the jet, so its course remains
undisturbed. At higher inlet pressures, the course of the jet is altered. As a
result, the pumping speed decreases until, at about 10-1 mbar, it becomes
immeasurably small.
The forevacuum pressure also influences the vapor jet and becomes
detrimental if its value exceeds a certain critical limit. This limit is called
maximum backing pressure or critical forepressure. The capacity of the
Oil diffusion pumps
(Series: LEYBODIFF, DI and DIP)
Mercury diffusion pumps
2. High vapor density:
Vapor jet pumps
Water vapor pumps
Oil vapor jet pumps
Mercury vapor jet pumps
3. Combined
oil diffusion/ vapor jet pumps
4. Water jet pumps
Cooling of fluid entrainment pumps
The heater power that is continuously supplied for vaporizing the pump fluid
in fluid-entrainment pumps must be dissipated by efficient cooling. The
energy required for pumping the gases and vapors is minimal. The outside
walls of the casing of diffusion pumps are cooled, generally with water.
Smaller oil diffusion pumps can, however, also be cooled with an air stream
because a low wall temperature is not so decisive to the efficiency as for
mercury diffusion pumps. Oil diffusion pumps can operate well with wall
temperatures of 30 °C, whereas the walls of mercury diffusion pumps must
be cooled to 15 °C. To protect the pumps from the danger of failure of the
cooling water – insofar as the cooling-water coil is not controlled by
thermally operated protective switching – a water circulation monitor should
be installed in the cooling water circuit; hence, evaporation of the pump
fluid from the pump walls is avoided.
1
2
3
4
5
Heater
Boiler
Pump body
Cooling coil
High vacuum flange
6 Gas molecules
7 Vapor jet
8 Backing vacuum connection
A
B
C
D
펂
Nozzles
Fig. 2.44 Mode of operation of a diffusion pump
41
Vacuum generation
chosen backing pump must be such (see 2.3.2) that the amount of gas
discharged from the diffusion pump is pumped off without building up a
backing pressure that is near the maximum backing pressure or even
exceeding it.
The attainable ultimate pressure depends on the construction of the pump,
the vapor pressure of the pump fluid used, the maximum possible
condensation of the pump fluid, and the cleanliness of the vessel.
Moreover, backstreaming of the pump fluid into the vessel should be
reduced as far as possible by suitable baffles or cold traps (see Section
2.1.6.4).
Degassing of the pump oil
In oil diffusion pumps it is necessary for the pump fluid to be degassed
before it is returned to the boiler. On heating of the pump oil, decomposition
products can arise in the pump. Contamination from the vessel can get into
the pump or be contained in the pump in the first place. These constituents
of the pump fluid can significantly worsen the ultimate pressure attainable
by a diffusion pump, if they are not kept away from the vessel. Therefore,
the pump fluid must be freed of these impurities and from absorbed gases.
This is the function of the degassing section, through which the circulating
oil passes shortly before re-entry into the boiler. In the degassing section,
the most volatile impurities escape. Degassing is obtained by the carefully
controlled temperature distribution in the pump. The condensed pump fluid,
which runs down the cooled walls as a thin film, is raised to a temperature
of about 130 °C below the lowest diffusion stage, to allow the volatile
components to evaporate and be removed by the backing pump. Therefore,
the re-evaporating pump fluid consists of only the less volatile components
of the pump oil.
Pumping speed
The magnitude of the specific pumping speed S of a diffusion pump – that
is, the pumping speed per unit of area of the actual inlet surface – depends
on several parameters, including the position and dimensions of the high
vacuum stage, the velocity of the pump fluid vapor, and the mean
molecular velocity -c of the gas being pumped (see equation 1.17 in Section
1.1). With the aid of the kinetic theory of gases, the maximum attainable
specific pumping speed at room temperature on pumping air is calculated
to Smax = 11.6 l · s-1 · cm-2. This is the specific (molecular) flow
conductance of the intake area of the pump, resembling an aperture of the
same surface area (see equation 1.30 in Section 1.5.3). Quite generally,
diffusion pumps have a higher pumping speed for lighter gases compared
to heavier gases.
To characterize the effectiveness of a diffusion pump, the so called HO
factor is defined. This is the ratio of the actually obtained specific pumping
speed to the theoretical maximum possible specific pumping speed. In the
case of diffusion pumps from LEYBOLD optimum values are attained (of
0.3 for the smallest and up to 0.55 for the larger pumps).
The various oil diffusion pumps manufactured by LEYBOLD differ in the
following design features (see Fig. 2.45 a and b).
42
1
2
a) LEYBODIFF-Pump with fractionating
facility
1 Center
2 Middle section
3 Outer part of the boiler (fractionation)
b) DI pump; side view on to the internal
heater
1 Thermostat sensor
2 Heating cartridge
Fig. 2.45 Diagram showing the basic differences in LEYBOLD oil diffusion pumps
a) LEYBODIFF series
This series of pumps is equipped with a fractionating device. The various
constituents of the pump fluid are selected so that the high vacuum nozzle
is supplied only by the fraction of the pump fluid that has the lowest vapor
pressure. This assures a particularly low ultimate pressure. Fractionating
occurs because the degassed oil first enters the outer part of the boiler,
which serves the nozzle on the backing vacuum side. Here a part of the
more volatile constituents evaporates. Hence the already purified pump
fluid reaches the intermediate part of the boiler, which serves the
intermediate nozzle. Here the lighter constituents are evaporated in greater
quantities than the heavier constituents. When the oil enters the central
region of the boiler, which serves the high vacuum nozzle, it has already
been freed of the light volatile constituents.
b) DI series
In these pumps an evaporation process for the pump fluid which is
essentially free of bursts is attained by the exceptional heater design
resulting in a highly constant pumping speed over time. The heater is of the
internal type and consists of heating cartridges into which tubes with
soldered on thermal conductivity panels are introduced. The tubes made of
stainless steel are welded horizontally into the pump’s body and are located
above the oil level. The thermal conductivity panels made of copper are
only in part immersed in the pump fluid. Those parts of the thermal
conductivity panels are so rated that the pump fluid can evaporate
intensively but without any retardation of boiling. Those parts of the thermal
conductivity panels above the oil level supply additional energy to the
vapor. Owing to the special design of the heating system, the heater
cartridges may be exchanged also while the pump is still hot.
Vacuum generation
1 Nozzle (Laval)
2 Diffuser nozzle (Venturi)
3 Mixing chamber
4 Connection to the vacuum chamber
Fig. 2.46 Operation of a vapor jet pump
2.1.6.2 Oil vapor ejector pumps
1 High vacuum port
Fig. 2.47
2 Diffusion stages
3 Ejector stages
Diagram of an oil jet (booster) pump
Pumping speed [mbar ·l · s–1]
not remain constant toward low inlet pressures. As a consequence of the
high vapor stream velocity and density, oil vapor ejector pumps can
transport gases against a relatively high backing pressure. Their critical
backing pressure lies at a few millibars. The oil vapor ejector pumps used in
present-day vacuum technology have, in general, one or more diffusion
stages and several subsequent ejector stages. The nozzle system of the
booster is constructed from two diffusion stages and two ejector stages in
cascade (see Fig. 2.47). The diffusion stages provide the high pumping
speed between 10-4 and 10-3 mbar (see Fig. 2.48), the ejector stages, the
high gas throughput at high pressures (see Fig. 2.49) and the high critical
backing pressure. Insensitivity to dust and vapors dissolved in the pump
fluid is obtained by a spacious boiler and a large pump fluid reservoir. Large
quantities of impurities can be contained in the boiler without deterioration
of the pumping characteristics.
Pumping speed [l · s–1]
Pumping speed [Torr · l · s–1]
The pumping action of a vapor ejector stage is explained with the aid of
Fig. 2.46. The pump fluid enters under high pressure p1 the nozzle (1),
constructed as a Laval nozzle. There it is expanded to the inlet pressure
p2. On this expansion, the sudden change of energy is accompanied by an
increase of the velocity. The consequently accelerated pump fluid vapor jet
streams through the mixer region (3), which is connected to the vessel (4)
being evacuated. Here the gas molecules emerging from the vessel are
dragged along with the vapor jet. The mixture, pump fluid vapor – gas, now
enters the diffuser nozzle constructed as a Venturi nozzle (2). Here the
vapor – gas mixture is compressed to the backing pressure p3 with
simultaneous diminution of the velocity. The pump fluid vapor is then
condensed at the pump walls, whereas the entrained gas is removed by the
backing pump. Oil vapor ejector pumps are ideally suited for the pumping of
larger quantities of gas or vapor in the pressure region between 1 and
10-3 mbar. The higher density of the vapor stream in the nozzles ensures
that the diffusion of the pumped gas in the vapor stream takes place much
more slowly than in diffusion pumps, so that only the outer layers of the
vapor stream are permeated by gas. Moreover, the surface through which
the diffusion occurs is much smaller because of the special construction of
the nozzles. The specific pumping speed of the vapor ejector pumps is,
therefore, smaller than that of the diffusion pumps. As the pumped gas in
the neighborhood of the jet under the essentially higher inlet pressure
decisively influences the course of the flow lines, optimum conditions are
obtained only at certain inlet pressures. Therefore, the pumping speed does
Intake pressure pa
Fig. 2.48 Pumping speed of various vapor pumps as a function of intake pressure related to a
nominal pumping speed of 1000 l/s.
End of the working range of oil vapor ejector pumps (A) and diffusion pumps (B)
Intake pressure pa
Fig. 2.49 Pumping speed of various vapor pumps (derived from Fig. 2.48)
43
Vacuum generation
2.1.6.3
Pump fluids
possible, various measures must be undertaken simultaneously:
a) Oils
The suitable pump fluids for oil diffusion pumps are mineral oils, silicone
oils, and oils based on the polyphenyl ethers. Severe demands are placed
on such oils which are met only by special fluids. The properties of these,
such as vapor pressure, thermal and chemical resistance, particularly
against air, determine the choice of oil to be used in a given type of pump
or to attain a given ultimate vacuum. The vapor pressure of the oils used in
vapor pumps is lower than that of mercury. Organic pump fluids are more
sensitive in operation than mercury, because the oils can be decomposed
by long-term admission of air. Silicone oils, however, withstand longer
lasting frequent admissions of air into the operational pump.
a) the high vacuum-side nozzle and the shape of the part of the pump
body surrounding this nozzle must be constructed so that as few as
possible vapor molecules emerge sideways in the path of the vapor
stream from the nozzle exit to the cooled pump wall.
Typical mineral oils are DIFFELEN light, normal and ultra. The different
types of DIFFELEN are close tolerance fractions of a high quality base
product (see our catalog).
Two chief requirements must be met in the construction of baffles or cold
traps for oil diffusion pumps. First, as far as possible, all backstreaming
pump-fluid vapor molecules should remain attached to (condensed at) the
inner cooled surfaces of these devices. Second, the condensation surfaces
must be so constructed and geometrically arranged that the flow
conductance of the baffles or cold traps is as large as possible for the
pumped gas. These two requirements are summarized by the term
“optically opaque”. This means that the particles cannot enter the baffle
without hitting the wall, although the baffle has a high conductance. The
implementation of this idea has resulted in a variety of designs that take
into account one or the other requirement.
Silicone oils (DC 704, DC 705, for example) are uniform chemical
compounds (organic polymers). They are highly resistant to oxidation in the
case of air inrushes and offer special thermal stability characteristics.
DC 705 has an extremely low vapor pressure and is thus suited for use in
diffusion pumps which are used to attain extremely low ultimate pressures
of < 10-10 mbar.
ULTRALEN is a polyphenylether. This fluid is recommended in all those
cases where a particularly oxidation-resistant pump fluid must be used and
where silicone oils would interfere with the process.
APIEZON AP 201 is an oil of exceptional thermal and chemical resistance
capable of delivering the required high pumping speed in connection with
oil vapor ejector pumps operating in the medium vacuum range. The
attainable ultimate total pressure amounts to about 10-4 mbar.
b) Mercury
Mercury is a very suitable pump fluid. It is a chemical element that during
vaporization neither decomposes nor becomes strongly oxidized when air
is admitted. However, at room temperature it has a comparatively high
vapor pressure of 10-3 mbar. If lower ultimate total pressures are to be
reached, cold traps with liquid nitrogen are needed. With their aid, ultimate
total pressures of 10-10 mbar can be obtained with mercury diffusion
pumps. Because mercury is toxic, as already mentioned, and because it
presents a hazard to the environment, it is nowadays hardly ever used as a
pump fluid. LEYBOLD supplies pumps with mercury as the pump fluid only
on request. The vapor pressure curves of pump fluids are given in Fig.
9.12, Section 9.
2.1.6.4
Pump fluid backstreaming and its suppression
(vapor barriers, baffles)
In the vapor stream from the topmost nozzle of a diffusion pump, pump
fluid molecules not only travel in the direction of streaming to the cooled
pump wall, but also have backward components of velocity because of
intermolecular collisions. They can thus stream in the direction of the
vessel. In the case of LEYBODIFF and DI pumps, the oil-backstreaming
amounts to a few micrograms per minute for each square centimeter of
inlet cross-sectional area. To reduce this backstreaming as much as
44
b) the method for cooling the pump wall must allow as complete as
possible condensation of the pump fluid vapor and, after condensation,
the fluid must be able to drain away readily.
c) one or more pump-fluid traps, baffles, or cold traps must be inserted
between the pump and the vessel, depending on the ultimate pressure
that is required.
A cold cap baffle is constructed so that it can be mounted immediately
above the high vacuum nozzle. The cold cap baffle is made of metal of high
thermal conductivity in good thermal contact with the cooled pump wall, so
that in practice it is maintained at the cooling-water temperature or, with aircooled diffusion pumps, at ambient temperature. In larger types of pumps,
the cold cap baffle is water cooled and permanently attached to the pump
body. The effective pumping speed of a diffusion pump is reduced by about
10 % on installation of the cold cap baffle, but the oil backstreaming is
reduced by about 90 to 95 %.
Shell baffles consist of concentrically arranged shells and a central baffle
plate. With appropriate cooling by water or refrigeration, almost entirely oil
vapor-free vacua can be produced by this means. The effective pumping
speed of the diffusion pump remains at least at 50 %, although shell baffles
are optically opaque. This type of baffle has been developed by LEYBOLD
in two different forms: with a stainless-steel cooling coil or – in the so-called
Astrotorus baffles – with cooling inserts of copper. The casing of the former
type is made entirely of stainless steel.
For the smaller air-cooled, oil diffusion pumps, plate baffles are used. The
air-cooled arrangement consists of a copper plate with copper webs to the
housing wall. The temperature of the plate baffle remains nearly ambient
during the operation of the diffusion pump.
Hydrocarbon-free vacuum
If extreme demands are made on freedom from oil vapor with vacuum
produced by oil diffusion pumps, cold traps should be used that are cooled
with liquid nitrogen so that they are maintained at a temperature
of - 196 °C.
Low-temperature baffles or cold traps should always be used with a cold
cap in place. On this the greatest part of the backstreaming oil is
Vacuum generation
few highly volatile components of the pump fluid that do not remain
attached to the very low temperature surfaces. The temperature and the
vapor molecules adsorbed at the surface of the vessel determine exactly
the pressure in the vessel. If, the surfaces are not fully covered with
adsorbed pump-fluid molecules after a bake-out process, their vapor
pressure contributes only insignificantly to the pressure in the vessel.
After a certain time, the “stay-down time”, a continuous layer of oil
molecules builds up, and the ultimate pressure is practically determined by
the vapor pressure of the pump fluid at the temperature of the vessel walls.
This “stay-down” time can even amount to several hours, indeed even to
days, with the use of low-temperature baffles.
Oil can reach the vessel not only as vapor, but also as a liquid film,
because oil wets readily and thus creeps up the wall.
By installation of an anticreep barrier (see Fig. 2.50) made of Teflon
polymer, a material that is not wetted by oil and can stand a bake-out
temperature up to 200 °C, further creeping of the oil can be effectively
prevented. It is most appropriate to arrange the anticreep barrier above the
upper baffle (see Fig. 2.50).
1 Diffusion pump
with cold cap baffle (cooled
by contact),
2 Shell or chevron baffle
3 Anticreep barrier
4
5
6
7
Sealing gasket
Bearing ring
LN2 cold trap,
Vacuum chamber
Fig. 2.50 Schematic arrangement of baffle, anticreep barrier and cold trap above a diffusion
pump
condensed, so that the inevitable loss of pump fluid from the condensation
of the pump fluid on the low-temperature surface is kept at a minimum.
With longer-term operation it is always advisable to install, in place of the
cold cap, a water-cooled shell or chevron baffle between the diffusion pump
and the low-temperature baffle or cold trap (see Fig. 2.50).
LEYBOLD manufactures cold traps made of metal so called LN2 cold
traps. These cold traps are to be used in those cases where a cold trap is
to be operated for prolonged periods of time without requiring a filling facility
for liquid nitrogen. The temperature increase at the vessel containing the
refrigerant is so slight over the operating time that – as the liquid level
drops – no significant desorption of the condensate takes place. Located on
the pumping side is an impact panel made of copper. The low temperature
of this panel ensures that the greater part of the condensed pump fluid
remains in the liquid state and may drip back into the pump. Today the oils
used to operate diffusion pumps have a very low vapor pressure at room
temperature (for example DIFFELEN light, 2 · 10-8 mbar;
DC 705, 4 · 10-10 mbar). The specified provisions with a liquid-nitrogencooled baffle or cold trap would enable an absolutely oil-free vacuum to be
produced.
In practice, however, complete suppression of oil-backstreaming is never
attained. There are always a few pump-fluid molecules that, as a result of
collisions with one another, reach the vessel without having hit one of the
cooled surfaces of the baffle or the cold trap. Moreover, there are always a
Note:
It must be noted that data on backstreaming as specified in catalogs apply
only to continuously-operated oil diffusion pumps. Shortly after starting a
pump the uppermost nozzle will not eject a well directed vapor jet. Instead
oil vapor spreads in all directions for several seconds and the
backstreaming effect is strong. When switching a diffusion pump on and off
frequently the degree of oil brackstreaming will be greater.
2.1.6.5
Water jet pumps and steam ejectors
Included in the class of fluid-entrainment pumps are not only pumps that
use a fast-streaming vapor as the pump fluid, but also liquid jet pumps. The
simplest and cheapest vacuum pumps are water jet pumps. As in a vapor
pump (see Fig. 2.46 or 2.51), the liquid stream is first released from a
nozzle and then, because of turbulence, mixes with the pumped gas in the
mixing chamber. Finally, the movement of the water – gas mixture is slowed
down in a Venturi tube. The ultimate total pressure in a container that is
pumped by a water jet pump is determined by the vapor pressure of the
water and, for example, at a water temperature of 15 °C amounts to about
17 mbar.
Essentially higher pumping speeds and lower ultimate pressures are
produced by steam ejector pumps. The section through one stage is
shown in Fig. 2.51. The markings correspond with those shown in Fig. 2.46.
In practice, several pumping stages are usually mounted in cascade. For
laboratory work, two-stage pump combinations are suitable and consist of a
steam ejector stage and a water jet (backing) stage, both made of glass.
The water jet backing stage enables operation without other backing
pumps. With the help of a vapor stream at overpressure, the vacuum
chamber can be evacuated to an ultimate pressure of about 3 mbar. The
condensate from the steam is led off through the drain attachment. The
water jet stage of this pump is cooled with water to increase its efficiency.
Steam ejector pumps are especially suitable for work in laboratories,
particularly if very aggressive vapors are to be pumped. Steam ejector
pumps, which will operate at a pressure of a few millibars, are especially
recommended for pumping laboratory distillation apparatus and similar
45
Vacuum generation
rotor blades reaches the order of magnitude of the average thermal velocity
of the molecules which are to be pumped. Kinetic gas theory supplies for -c
othe equation 1.17:
c=
8· R ·T
π ·M
in which the dependency on the type of gas as a function of molar mass M
is contained. The calculation involving cgs-units (where R = 83.14 · 106
mbar · cm3 / mol · K) results in the following Table:
Gas
H2
He
H2O
Ne
CO
N2
Air
O2
Ar
CO2
CC13F (F11)
1 Steam inlet
2 Jet nozzle
3 Diffuser
4 Mixing region
5 Connection to the vacuum chamber
Fig. 2.51 Schematic representation of the operation of a steam ejector pump
plants when the pressure from a simple water jet pump is insufficient. In
this instance, the use of rotary pumps would not be economical.
Even in spite of their low investment costs water jet pumps and steam
ejectors are being replaced in the laboratories more and more by
diaphragm pumps because of the environmental problems of using water
as the pump fluid. Solvent entering the water can only be removed again
through complex cleaning methods (distillation).
Table 2.4
Molar
Mass M
2
4
18
20
28
28
28.96
32
40
44
134.78
Mean thermal
velocity (m/s)
1761
1245
587
557
471
471
463
440
394
375
68
-c as a function of molar mass M
Whereas _
the dependence
__ of the pumping speed on the type of gas is fairly
low (S ~ c ~ 1 / 앀 M ), the dependence of the compression k0 at zero
throughput and thus also__
the compression k, because
of k0 ~ e앀M log k0 ~ 앀 M, is greater as shown by the experimentallydetermined relationship in Fig. 2.55.
Example:
from theory it follows that
log k0(He)
4
1
1
=
=
=
28
7 2.65
log k0(N2)
⇒ log k0(N 2) = 2.65 · log k0(He)
2.1.7 Turbomolecular pumps
this with k0 (He) = 3 · 103 from Fig. 2.55 results in:
The principle of the molecular pump – well known since 1913 – is that the
gas particles to be pumped receive, through impact with the rapidly moving
surfaces of a rotor, an impulse in a required flow direction. The surfaces of
the rotor – usually disk-shaped – form, with the stationary surfaces of a
stator, intervening spaces in which the gas is transported to the backing
port. In the original Gaede molecular pump and its modifications, the
intervening spaces (transport channels) were very narrow, which led to
constructional difficulties and a high degree of susceptibility to mechanical
contamination.
This agrees – as expected – well (order of magnitude) with the
experimentally determined value for k0 (N2) = 2.0 · 108 from Fig. 2.55. In
view of the optimizations for the individual rotor stages common today, this
consideration is no longer correct for the entire pump. Shown in Fig. 2.56
are the values as measured for a modern TURBOVAC 340 M.
At the end of the Fifties, it became possible – through a turbine-like design
and by modification of the ideas of Gaede – to produce a technically viable
pump the socalled “turbomolecular pump”. The spaces between the
stator and the rotor disks were made in the order of millimeters, so that
essentially larger tolerances could be obtained. Thereby, greater security in
operation was achieved. However, a pumping effect of any significance is
only attained when the circumferential velocity (at the outside rim) of the
In order to meet the condition,_ a circumferential velocity for the rotor of the
same order of magnitude as c high rotor speeds are required for
turbomolecular pumps. They range from about 36,000 rpm for pumps
having a large diameter rotor (TURBOVAC 1000) to 72,000 rpm in the case
of smaller rotor diameters (TURBOVAC 35 / 55). Such high speeds
naturally raise questions as to a reliable bearing concept. LEYBOLD offers
three concepts, the advantages and disadvantages of which are detailed in
46
log k0 (N2) = 2.65 · log (3 · 103) = 9.21
or k0 (N2) = 1.6 · 109.
Vacuum generation
the following:
• Oil lubrication / steel ball bearings
+ Good compatibility with particles by circulating oil lubricant
- Can only be installed vertically
+ Low maintenance
• Grease lubrication / hybrid bearings
+ Installation in any orientation
+ Suited for mobile systems
± Air cooling will do for many applications
+ Lubricated for life (of the bearings)
• Free of lubricants / magnetic suspension
+ No wear
+ No maintenance
+ Absolutely free of hydrocarbons
+ Low noise and vibration levels
+ Installation in any orientation
Steel ball bearings / hybrid ball bearings (ceramic ball bearings): Even
a brief tear in the thin lubricating film between the balls and the races can –
if the same type of material is used – result in microwelding at the points
of contact. This severely reduces the service life of the bearings. By using
dissimilar materials in so called hybrid bearings (races: steel, balls:
ceramics) the effect of microwelding is avoided.
The most elegant bearing concept is that of the magnetic suspension. As
early as 1976 LEYBOLD delivered magnetically suspended turbomolecular
pumps – the legendary series 550M and 560M. At that time a purely active
magnetic suspension (i.e. with electromagnets) was used. Advances in -
electronics and the use of permanent magnets (passive magnetic
suspension) based on the “System KFA Jülich” permitted the magnetic
suspension concept to spread widely. In this system the rotor is maintained
in a stable position without contact during operation, by magnetic forces.
Absolutely no lubricants are required. So-called touch down bearings are
integrated for shutdown.
Fig. 2.52 shows a sectional drawing of a typical turbomolecular pump. The
pump is an axial flow compressor of vertical design, the active or pumping
part of which consists of a rotor (6) and a stator (2). Turbine blades are
located around the circumferences of the stator and the rotor. Each rotor –
stator pair of circular blade rows forms one stage, so that the assembly is
composed of a multitude of stages mounted in series. The gas to be
pumped arrives directly through the aperture of the inlet flange (1), that is,
without any loss of conductance, at the active pumping area of the top
blades of the rotor – stator assembly. This is equipped with blades of
especially large radial span to allow a large annular inlet area. The gas
captured by these stages is transferred to the lower compression stages,
whose blades have shorter radial spans, where the gas is compressed to
backing pressure or rough vacuum pressure. The turbine rotor (6) is
mounted on the drive shaft, which is supported by two precision ball
bearings (8 and 11), accommodated in the motor housing. The rotor shaft is
directly driven by a medium-frequency motor housed in the forevacuum
space within the rotor, so that no rotary shaft lead-through to the outside
atmosphere is necessary. This motor is powered and automatically
controlled by an external frequency converter, normally a solid-state
frequency converter that ensures a very low noise level. For special
applications, for example, in areas exposed to radiation, motor generator
frequency converters are used.
2
Turbomolecular
pump stage
3
4
Siegbahn stage
5
6
7
8
1
1
2
3
4
High vacuum inlet flange
Stator pack
Venting flange
Forevacuum flange
Fig. 2.52
5
6
7
8
Splinter guard
Rotor
Pump casing
Ball bearings
9 Cooling water connection
10 3-phase motor
11 Ball bearings
Schematic diagram of a grease lubricated TURBOVAC 151 turbomolecular pump
1 Vacuum port
2 High vacuum flange
3 Rotor
4 Stator
5 Bearing
6 Motor
7 Fan
8 Bearing
Fig. 2.52a Cross section of a HY.CONE turbomolecular pump
47
Vacuum generation
10000
l · s–1
l · s–1
1000/1000 MC
600
1000
500
S
340M
200
361
151
100
50/55
10
ñ6
2
10
4
6 8
ñ5
10
ñ4
10
p
ñ3
ñ2
10
10
mbar
ñ1
10
Fig. 2.53 Pumping speed for air of different turbomolecular pumps
Fig. 2.54 Pumping speed curves of a TURBOVAC 600 for H2, He, N2 and Ar
The vertical rotor – stator configuration provides optimum flow conditions of
the gas at the inlet.
component at the forevacuum flange of the pump and that at the high vacuum flange: maximum compression k0 is to be found at zero throughput. For
physical reasons, the compression ratio of turbomolecular pumps is very
high for heavy molecules but considerably lower for light molecules. The
relationship between compression and molecular mass is shown in Fig.
2.55. Shown in Fig. 2.56 are the compression curves of a TURBOVAC 340
M for N2, He and H2 as a function of the backing pressure. Because of the
high compression ratio for heavy hydrocarbon molecules, turbomolecular
pumps can be directly connected to a vacuum chamber without the aid of
one or more cooled baffles or traps and without the risk of a measurable
partial pressure for hydrocarbons in the vacuum chamber (hydrocarbon-free
vacuum! – see also Fig. 2.57: residual gas spectrum above a TURBOVAC
361). As the hydrogen partial pressure attained by the rotary backing pump
To ensure vibration-free running at high rotational speeds, the turbine is
dynamically balanced at two levels during its assembly.
The pumping speed (volume flow rate) characteristics of turbomolecular
pumps are shown in Fig. 2.53. The pumping speed remains constant over
the entire working pressure range. It decreases at intake pressures above
10-3 mbar, as this threshold value marks the transition from the region of
molecular flow to the region of laminar viscous flow of gases. Fig. 2.54
shows also that the pumping speed depends on the type of gas.
The compression ratio (often also simply termed compression) of
turbomolecular pumps is the ratio between the partial pressure of one gas
k0
⎛
M
⎟
Fig. 2.55 TURBOVAC 450 – Maximum compression k0 as a function of molar mass M
48
Fig. 2.56 Maximum compression k0 of a turbomolecular pump TURBOVAC 340 M for H2, He
and N2 as a function of backing pressure
Vacuum generation
M = Mass number = Relative molar mass at an ionization 1
I = Ion current
Fig. 2.57 Spectrum above a TURBOVAC 361
Fig. 2.58 Determination of the cut-in pressure for turbomolecular pumps when evacuating large
is very low, the turbomolecular pump is capable of attaining ultimate
pressures in the 10-11 mbar range in spite of its rather moderate
compression for H2. To produce such extremely low pressures, it will, of
course, be necessary to strictly observe the general rules of UHV
technology: the vacuum chamber and the upper part of the turbomolecular
pump must be baked out, and metal seals must be used. At very low
pressures the residual gas is composed mainly of H2 originating from the
metal walls of the chamber. The spectrum in Fig. 2.57 shows the residual
gas composition in front of the inlet of a turbomolecular pump at an
ultimate pressure of 7 · 10-10 mbar nitrogen equivalent. It appears that the
portion of H2 in the total quantity of gas amounts to approximately 90 to 95
% . The fraction of “heavier” molecules is considerably reduced and masses
greater than 44 were not detected. An important criterion in the assessment
of the quality of a residual gas spectrum are the measurable hydrocarbons
from the lubricants used in the vacuum pump system. Of course an
“absolutely hydrocarbon-free vacuum” can only be produced with pump
systems which are free of lubricants, i.e. for example with magneticallysuspended turbomolecular pumps and dry compressing backing pumps.
When operated correctly (venting at any kind of standstill) no hydrocarbons
are detectable also in the spectrum of normal turbomolecular pumps.
Information on the operation of turbomolecular pumps
Starting
As a rule turbomolecular pumps should generally be started together with
the backing pump in order to reduce any backstreaming of oil from the
backing pump into the vacuum chamber. A delayed start of the
turbomolecular pump, makes sense in the case of rather small backing
pump sets and large vacuum chambers. At a known pumping speed for the
backing pump SV (m3/h) and a known volume for the vacuum chamber (m3)
it is possible to estimate the cut-in pressure for the turbomolecular pump:
A further development of the turbomolecular pump is the hybrid or
compound turbomolecular pump. This is actually two pumps on a common
shaft in a single casing. The high vacuum stage for the molecular flow
region is a classic turbomolecular pump, the second pump for the viscous
flow range is a molecular drag or friction pump.
LEYBOLD manufactures pumps such as the TURBOVAC 55 with an
integrated Holweck stage (screw-type compressor) and, for example, the
HY.CONE 60 or HY.CONE 200 with an integrated Siegbahn stage (spiral
compressor). The required backing pressure then amounts to a few mbar
so that the backing pump is only required to compress from about 5 to 10
mbar to atmospheric pressure. A sectional view of a HY.CONE is shown in
Fig. 2.52a.
Simultaneous start when
Sv
> 40 h−1
V
and delayed start when
Sv
< 40 h−1
V
at a cut-in pressure of:
⎛ SV ⎞
⎜
⎟
pV, Start = e ⎝ 6 · V ⎠ mbar
(2.24)
When evacuating larger volumes the cut-in pressure for turbomolecular
pumps may also be determined with the aid of the diagram of Fig. 2.58.
Venting
After switching off or in the event of a power failure, turbomolecular
pumps should always be vented in order to prevent any backdiffusion of
hydrocarbons from the forevacuum side into the vacuum chamber. After
switching off the pump the cooling water supply should also be switched off
to prevent the possible condensation of water vapor. In order to protect the
rotor it is recommended to comply with the (minimum) venting times stated
in the operating instructions. The pump should be vented (except in the
case of operation with a barrier gas) via the venting flange which already
contains a sintered metal throttle, so that venting may be performed using a
normal valve or a power failure venting valve.
49
Vacuum generation
Barrier gas operation
In the case of pumps equipped with a barrier gas facility, inert gas – such
as dry nitrogen – may be applied through a special flange so as to protect
the motor space and the bearings against aggressive media. A special
barrier gas and venting valve meters the necessary quantity of barrier gas
and may also serve as a venting valve.
Decoupling of vibrations
TURBOVAC pumps are precisely balanced and may generally be
connected directly to the apparatus. Only in the case of highly sensitive
instruments, such as electron microscopes, is it recommended to install
vibration absorbers which reduce the present vibrations to a minimum. For
magnetically suspended pumps a direct connection to the vacuum
apparatus will usually do because of the extremely low vibrations produced
by such pumps.
For special applications such as operation in strong magnetic fields,
radiation hazard areas or in a tritium atmosphere, please contact our
Technical Sales Department which has the necessary experience and
which is available to you at any time.
1
2
3
4
Inlet port
Degassing port
Support
Pump body
5 Thermal conducting
vanes
6 Adsorption material
(e.g. Zeolith)
Fig. 2.59 Cross section of an adsorption pump
2.1.8 Sorption pumps
2.1.8.1
Adsorption pumps
Adsorption pumps (see Fig. 2.59) work according to the principle of the
physical adsorption of gases at the surface of molecular sieves or other
adsorption materials (e.g. activated Al2O3). Zeolite 13X is frequently used
as an adsorption material. This alkali aluminosilicate possesses for a mass
of the material an extraordinarily large surface area, about 1000 m2/g of
solid substance. Correspondingly, its ability to take up gas is considerable.
The pore diameter of zeolite 13X is about 13 Å, which is within the order of
size of water vapor, oil vapor, and larger gas molecules (about 10 Å).
Assuming that the mean molecular diameter is half this value, 5 · 10-8 cm,
about 5 · 1018 molecules are adsorbed in a monolayer on a surface of
50
1 m2 . For nitrogen molecules with a relative molecular mass Mr = 28 that
corresponds to about 2 · 10-4 g or 0.20 mbar · l (see also Section 1.1).
Therefore, an adsorption surface of 1000 m2 is capable of adsorbing a
monomolecular layer in which more than 133 mbar · l of gas is bound.
Hydrogen and light noble gases, such as helium and neon, have a
relatively small particle diameter compared with the pore size of 13 Å for
zeolite 13X. These gases are, therefore, very poorly adsorbed.
The adsorption of gases at surfaces is dependent not only on the
temperature, but more importantly on the pressure above the adsorption
surface. The dependence is represented graphically for a few gases by the
adsorption isotherms given in Fig. 2.60. In practice, adsorption pumps are
Pressure [Torr]
Adsorbed quantity of gas per quantity of adsorbent [mbar ·l · g–1]
The term “sorption pumps” includes all arrangements for the removal of
gases and vapors from a space by sorption means. The pumped gas
particles are thereby bound at the surfaces or in the interior of these
agents, by either physical temperature-dependent adsorption forces (van
der Waals forces), chemisorption, absorption, or by becoming embedded
during the course of the continuous formation of new sorbing surfaces. By
comparing their operating principles, we can distinguish between
adsorption pumps, in which the sorption of gases takes place simply by
temperature-controlled adsorption processes, and getter pumps, in which
the sorption and retention of gases are essentially caused by the formation
of chemical compounds. Gettering is the bonding of gases to pure, mostly
metallic surfaces, which are not covered by oxide or carbide layers. Such
surfaces always form during manufacture, installation or while venting the
system. The mostly metallic highest purity getter surfaces are continuously
generated either directly in the vacuum by evaporation (evaporator
pumps) or by sputtering (sputter pumps) or the passivating surface layer
of the getter (metal) is removed by degassing the vacuum, so that the pure
material is exposed to the vacuum. This step is called activation (NEG
pumps NEG = Non Evaporable Getter).
Pressure [mbar]
Fig. 2.60 Adsorption isotherms of zeolite 13X for nitrogen at –195 °C and 20 °C, as well as for
helium and neon at –195 °C
Vacuum generation
connected through a valve to the vessel to be evacuated. It is on immersing
the body of the pump in liquid nitrogen that the sorption effect is made
technically useful. Because of the different adsorption properties, the
pumping speed and ultimate pressure of an adsorption pump are different
for the various gas molecules: the best values are achieved for nitrogen,
carbon dioxide, water vapor, and hydrocarbon vapors. Light noble gases
are hardly pumped at all because the diameter of the particles is small
compared to the pores of the zeolite. As the sorption effect decreases with
increased coverage of the zeolite surfaces, the pumping speed falls off with
an increasing number of the particles already adsorbed. The pumping
speed of an adsorption pump is, therefore, dependent on the quantity of
gas already pumped and so is not constant with time.
The ultimate pressure attainable with adsorption pumps is determined in the
first instance by those gases that prevail in the vessel at the beginning of the
pumping process and are poorly or not at all adsorbed (e.g. neon or helium)
at the zeolite surface. In atmospheric air, a few parts per million of these
gases are present. Therefore, pressures < 10-2 mbar can be obtained.
If pressures below 10-3 mbar exclusively are to be produced with adsorption
pumps, as far as possible no neon or helium should be present in the gas
mixture.
After a pumping process, the pump must be warmed only to room
temperature for the adsorbed gases to be given off and the zeolite is
regenerated for reuse. If air (or damp gas) containing a great deal of water
vapor has been pumped, it is recommended to bake out the pump
completely dry for a few hours at 200 °C or above.
To pump out larger vessels, several adsorption pumps are used in parallel
or in series. First, the pressure is reduced from atmospheric pressure to a
few millibars by the first stage in order to “capture” many noble gas
molecules of helium and neon. After the pumps of this stage have been
saturated, the valves to these pumps are closed and a previously closed
valve to a further adsorption pump still containing clean adsorbent is
opened so that this pump may pump down the vacuum chamber to the next
lower pressure level. This procedure can be continued until the ultimate
pressure cannot be further improved by adding further clean adsorption
pumps.
2.1.8.2
2.1.8.3
Sputter-ion pumps
The pumping action of sputter-ion pumps is based on sorption processes
that are initiated by ionized gas particles in a Penning discharge (cold
cathode discharge). By means of “paralleling many individual Penning cells”
the sputter ion pump attains a sufficiently high pumping speed for the
individual gases.
Operation of sputter-ion pumps
The ions impinge upon the cathode of the cold cathode discharge electrode
system and sputter the cathode material (titanium). The titanium deposited
at other locations acts as a getter film and adsorbs reactive gas particles
(e.g., nitrogen, oxygen, hydrogen). The energy of the ionized gas particles
is not only high enough to sputter the cathode material but also to let the
impinging ions penetrate deeply into the cathode material (ion implantation).
This sorption process “pumps” ions of all types, including ions of gases
which do not chemically react with the sputtered titanium film, i.e. mainly
noble gases.
The following arrangement is used to produce the ions: stainless-steel,
cylindrical anodes are closely arranged between, with their axes
perpendicular to, two parallel cathodes (see Fig. 2.61). The cathodes are at
negative potential (a few kilovolts) against the anode. The entire electrode
system is maintained in a strong, homogeneous magnetic field of a flux
density of B = 0.1 T, (T = Tesla = 104 Gauss) produced by a permanent
magnet attached to the outside of the pump’s casing. The gas discharge
produced by the high tension contains electrons and ions. Under the
influence of the magnetic field the electrons travel along long spiral tracks
(see Fig. 2.61) until they impinge on the anode cylinder of the
corresponding cell. The long track increases ion yield, which even at low
gas densities (pressures) is sufficient to maintain a self-sustained gas
discharge. A supply of electrons from a hot cathode is not required.
Because of their great mass, the movement of the ions is unaffected by the
magnetic field of the given order of magnitude; they flow off along the
Sublimation pumps
Sublimation pumps are sorption pumps in which a getter material is
evaporated and deposited on a cold inner wall as a getter film. On the
surface of such a getter film the gas molecules form stable compounds,
which have an immeasurably low vapor pressure. The active getter film is
renewed by subsequent evaporations. Generally titanium is used in
sublimation pumps as the getter. The titanium is evaporated from a wire
made of a special alloy of a high titanium content which is heated by an
electric current. Although the optimum sorption capacity (about one nitrogen
atom for each evaporated titanium atom) can scarcely be obtained in
practice, titanium sublimation pumps have an extraordinarily high pumping
speed for active gases, which, particularly on starting processes or on the
sudden evolution of greater quantities of gas, can be rapidly pumped away.
As sublimation pumps function as auxiliary pumps (boosters) to sputter-ion
pumps and turbomolecular pumps, their installation is often indispensable
(like the “boosters” in vapor ejector pumps; see Section 2.1.6.2).
PZ
← ⊕Direction of motion of the ionized gas
molecules
• → Direction of motion of the sputtered
titanium
-––Spiral tracks of the electrons
PZ Penning cells
Fig. 2.61 Operating principle of a sputter-ion pump
51
Vacuum generation
ü
atoms
•Ö Titanium
Gas molecules
⊕ Ions
Electrons
B Magnetic field
Fig. 2.62 Electrode configuration in a diode sputter-ion pump
shortest path and bombard the cathode.
The discharge current i is proportional to the number density of neutral
particles n0, the electron density n-, and the length l of the total discharge
path:
i = n0 · n– · σ · l
(2.25)
The effective cross section s for ionizing collisions depends on the type of
gas. According to (2.25), the discharge current i is a function of the number
particle density n0, as in a Penning gauge, and it can be used as a
measure of the pressure in the range from 10-4 to 10-8 mbar. At lower
pressures the measurements are not reproducible due to interferences
from field emission effects.
In diode-type, sputter-ion pumps, with an electrode system configuration
as shown in Fig. 2.62, the getter films are formed on the anode surfaces
and between the sputtering regions of the opposite cathode. The ions are
buried in the cathode surfaces. As cathode sputtering proceeds, the buried
gas particles are set free again. Therefore, the pumping action for noble
gases that can be pumped only by ion burial will vanish after some time
and a “memory effect” will occur.
Unlike diode-type pumps, triode sputter-ion pumps exhibit excellent
stability in their pumping speed for noble gases because sputtering and film
forming surfaces are separated. Fig. 2.63 shows the electrode configuration
of triode sputter-ion pumps. Their greater efficiency for pumping noble
gases is explained as follows: the geometry of the system favors grazing
incidence of the ions on the titanium bars of the cathode grid, whereby the
sputtering rate is considerably higher than with perpendicular incidence.
The sputtered titanium moves in about the same direction as the incident
atoms
•Ö Titanium
Gas molecules
ü
ä Ions
Electrons
A Anode cylinder
(same as in the diode
pump)
B Magnetic field
F Target plate
(pump housing)
as the third electrode
K Cathode grid
Fig. 2.63 Electrode configuration in a triode sputter-ion pump
52
ions. The getter films form preferentially on the third electrode, the target
plate, which is the actual wall of the pump housing. There is an increasing
yield of ionized particles that are grazingly incident on the cathode grid
where they are neutralized and reflected and from which they travel to the
target plate at an energy still considerably higher than the thermal energy
1/2 · k · T of the gas particles. The energetic neutral particles can penetrate
into the target surface layer, but their sputtering effect is only negligible.
These buried or implanted particles are finally covered by fresh titanium
layers. As the target is at positive potential, any positive ions arriving there
are repelled and cannot sputter the target layers. Hence the buried noble
gas atoms are not set free again. The pumping speed of triode sputterion pumps for noble gases does not decrease during the operation of
the pump.
The pumping speed of sputter-ion pumps depends on the pressure and the
type of gas. It is measured according to the methods stated in DIN 28 429
and PNEUROP 5615. The pumping speed curve S(p) has a maximum. The
nominal pumping speed Sn is given by the maximum of the pumping speed
curve for air whereby the corresponding pressure must be stated.
For air, nitrogen, carbon dioxide and water vapor, the pumping speed is
practically the same. Compared with the pumping speed for air, the
pumping speeds of sputter-ion pumps for other gases amount to
approximately:
Hydrogen
Methane
Other light
hydrocarbons
Oxygen
Argon
Helium
150 to 200 %
100 %
80 to 120 %
80 %
30 %
28 %
Sputter-ion pumps of the triode type excel in contrast to the diode-type
pumps in high-noble gas stability. Argon is pumped stably even at an inlet
pressure of 1 · 10-5 mbar. The pumps can be started without difficulties at
pressures higher than 1 · 10-2 mbar and can operate continuously at an air
inlet producing a constant air pressure of 5 · 10-5 mbar. A new kind of
design for the electrodes extends the service life of the cathodes by 50 %.
Influence on processes in the vacuum chamber by magnetic stray
fields and stray ions from the sputter-ion pump.
The high-magnetic-field strength required for the pumping action leads
inevitably to stray magnetic fields in the neighborhood of the magnets. As a
result, processes in the vacuum chamber can be disturbed in some cases,
so the sputter-ion pump concerned should be provided with a screening
arrangement. The forms and kinds of such a screening arrangement can be
regarded as at an optimum if the processes taking place in the vacuum
chamber are disturbed by no more than the earth’s magnetic field which is
present in any case.
Fig. 2.64 shows the magnetic stray field at the plane of the intake flange of
a sputter-ion pump IZ 270 and also at a parallel plane 150 mm above. If
stray ions from the discharge region are to be prevented from reaching the
vacuum chamber, a suitable screen can be set up by a metal sieve at
opposite potential in the inlet opening of the sputter-ion pump (ion barrier).
This, however, reduces the pumping speed of the sputter-ion pump
depending on the mesh size of the selected metal sieve.
Vacuum generation
since hydrogen contributes mostly to the ultimate pressure in an UHV
system, and for which NEG pumps have a particularly high pumping speed,
whereas the pumping effect for H2 of other pumps is low. Some typical
examples for applications in which NEG pumps are used are particle
accelerators and similar research systems, surface analysis instruments,
SEM columns and sputtering systems. NEG pumps are manufactured
offering pumping speeds of several `/s to about 1000 l/s. Custom pumps
are capable of attaining a pumping speed for hydrogen which is by several
orders of magnitude higher.
Fig. 2.64 Stray magnetic field of a sputter-ion pump in two places parallel to the inlet flange
(inserts) curves show lines of constant magnetic induction B in Gauss.1 Gauss = 1 ·
10–4 Tesla
2.1.8.4
Non evaporable getter pumps (NEG Pumps)
The non evaporable getter pump operates with a non evaporable, compact
getter material, the structure of which is porous at the atomic level so that it
can take up large quantities of gas. The gas molecules adsorbed on the
surface of the getter material diffuse rapidly inside the material thereby
making place for further gas molecules impinging on the surface. The non
evaporable getter pump contains a heating element which is used to heat
the getter material to an optimum temperature depending on the type of gas
which is preferably to be pumped. At a higher temperature the getter
material which has been saturated with the gas is regenerated (activated).
As the getter material, mostly zirconium-aluminum alloys are used in the
form of strips. The special properties of NEG pumps are:
• constant pumping speed in the HV and UHV ranges
• no pressure restrictions up to about 12 mbar
• particularly high pumping speed for hydrogen and its isotopes
• after activation the pump can often operate at room temperature and will
then need no electrical energy
• no interference by magnetic fields
• hydrocarbon-free vacuum
• free of vibrations
• low weight
NEG pumps are mostly used in combination with other UHV pumps
(turbomolecular and cryopumps). Such combinations are especially useful
when wanting to further reduce the ultimate pressure of UHV systems,
53
Vacuum generation
2.1.9 Cryopumps
As you may have observed water condenses on cold water mains or
windows and ice forms on the evaporator unit in your refrigerator. This
effect of condensation of gases and vapors on cold surfaces, water vapor in
particular, as it is known in every day life, occurs not only at atmospheric
pressure but also in vacuum.
This effect has been utilized for a long time in condensers (see 2.1.5)
mainly in connection with chemical processes; previously the baffle on
diffusion pumps used to be cooled with refrigerating machines. Also in a
sealed space (vacuum chamber) the formation of condensate on a cold
surface means that a large number of gas molecules are removed from the
volume: they remain located on the cold surface and do not take part any
longer in the hectic gas atmosphere within the vacuum chamber. We then
say that the particles have been pumped and talk of cryopumps when the
“pumping effect” is attained by means of cold surfaces.
Cryo engineering differs from refrigeration engineering in that the
temperatures involved in cryo engineering are in the range below 120 K
(< -153 °C). Here we are dealing with two questions:
•
•
•
•
Gifford-McMahon process
Stirling process
Brayton process
Claude process
The Gifford-McMahon process is mostly used today and this process is that
which has been developed furthest. It offers the possibility of separating the
locations for the large compressor unit and the expansion unit in which the
refrigeration process takes place. Thus a compact and low vibration cold
source can be designed. The cryopumps series-manufactured by
LEYBOLD operate with two-stage cold heads according to the GiffordMcMahon process which is discussed in detail in the following.
The entire scope of a refrigerator cryopump is shown in Fig. 2.65 and
consists of the compressor unit (1) which is linked via flexible pressure lines
(2) – and thus vibration-free – to the cryopump (3). The cryopump itself
consists of the pump casing and the cold head within. Helium is used as
the refrigerant which circulates in a closed cycle with the aid of the
compressor.
a) What cooling principle is used in cryo engineering or in cryopumps and
how is the thermal load of the cold surface lead away or reduced?
b) What are the operating principles of the cryopumps?
2.1.9.1 Types of cryopump
Depending on the cooling principle a difference is made between
• Bath cryostats
• Continuous flow cryopumps
• Refrigerator cryopumps
In the case of bath cryostats – in the most simple case a cold trap filled
with LN2 (liquid nitrogen) – the pumping surface is cooled by direct contact
with a liquefied gas. On a surface cooled with LN2 (T ≈ 77 K) H2O and CO2
are able to condense. On a surface cooled to ≈ 10 K all gases except He
and Ne may be pumped by way of condensation. A surface cooled with
liquid helium (T ≈ 4.2 K) is capable of condensing all gases except helium.
1
3
2
In continuous flow cryopumps the cold surface is designed to operate as
a heat exchanger. Liquid helium in sufficient quantity is pumped by an
auxiliary pump from a reservoir into the evaporator in order to attain a
sufficiently low temperature at the cold surface (cryopanel).
The liquid helium evaporates in the heat exchanger and thus cools down
the cryopanel. The waste gas which is generated (He) is used in a second
heat exchanger to cool the baffle of a thermal radiation shield which
protects the system from thermal radiation coming from the outside. The
cold helium exhaust gas ejected by the helium pump is supplied to a helium
recovery unit. The temperature at the cryopanels can be controlled by
controlling the helium flow.
Today refrigerator cryopumps are being used almost exclusively (cold
upon demand). These pumps operate basically much in the same way as a
common household refrigerator, whereby the following thermodynamic
cycles using helium as the refrigerant may be employed:
54
1 Compressor unit
2 Flexible pressure lines
3 Cold head (without
condensation surfaces)
Fig. 2.65 All items of a refrigerator cryopump
Vacuum generation
2.1.9.2
The cold head and its operating principle
(Fig. 2.66)
Within the cold head, a cylinder is divided into two working spaces V1 and
V2 by a displacer. During operation the right space V1 is warm and the left
space V2 is cold. At a displacer frequency f the refrigerating power W of the
refrigerator is:
W = (V2,max – V2,min) · (pH – pN) · f
(2.26)
The displacer is moved to and fro pneumatically so that the gas is forced
through the displacer and thus through the regenerator located inside the
V2 (cold)
Regenerator
V2 (cold)
Regenerator
V2 (cold)
displacer. The regenerator is a heat accumulator having a large heat
exchanging surface and capacity, which operates as a heat exchanger
within the cycle. Outlined in Fig. 2.66 are the four phases of refrigeration in
a single-stage refrigerator cold head operating according to the GiffordMcMahon principle.
The two-stage cold head
The series-manufactured refrigerator cryopumps from LEYBOLD use a twostage cold head operating according to the Gifford-McMahon principle (see
Fig. 2.67). In two series connected stages the temperature of the helium is
reduced to about 30 K in the first stage and further to about 10 K in the
V1 (warm)
Displacer
V1 (warm)
Displacer
Phase 1:
The displacer is at the left dead center; V2
where the cold is produced has its minimum
size. Valve N remains closed, H is opened. Gas
at the pressure pH flows through the regenerator into V2. There the gas warms up by the
pressure increase in V1.
Phase 2:
Valve H remains open, valve N closed: the displacer moves to the right and ejects the gas
from V1 through the regenerator to V2 where it
cools down at the cold regenerator.; V2 has its
maximum volume.
V1 (warm)
Phase 3:
Regenerator
V2 (cold)
Displacer
Valve H is closed and the valve N to the low
pressure reservoir is opened. The gas expands
from pH to pN and thereby cools down. This
removes heat from the vicinity and it is transported with the expanding gas to the compressor.
V1 (warm)
Phase 4:
Regenerator
Displacer
With valve N open the displacer moves to the
left; the gas from V2,max flows through the
regenerator, cooling it down and then flows into
the volume V1 and into the low pressure reservoir. This completes the cycle.
Fig. 2.66 Refrigerating phases using a single-stage cold head operating according to the Gifford-McMahon process
55
Vacuum generation
1 Electric connections and current
feedthrough for the motor in the cold
head
2 He high pressure connection
3 He low pressure connection
4 Cylinder, 1st stage
5 Displacer, 1st stage
6 Regenerator, 1st stage
7 Expansion volume, 1st stage
8 1st (cooling) stage
(copper flange)
9 Cylinder, 2nd stage
10 Displacer, 2nd stage
11 Regenerator, 2nd stage
12 Expansion volume, 2nd stage
13 2nd (cooling) stage
(copper flange)
14 Measurement chamber for the
vapor pressure
15 Control piston
16 Control volume
17 Control disk
18 Control valve
19 Gauge for the hydrogen vapor pressure thermometer
20 Motor in the cold head
Fig. 2.67 Two-stage cold head
second stage. The attainable low temperatures depend among other things
on the type of regenerator. Commonly copperbronze is used in the
regenerator of the first stage and lead in the second stage. Other materials
are available as regenerators for special applications like cryostats for
extremely low temperatures (T < 10 K). The design of a two-stage cold
head is shown schematically in Fig. 2.67. By means of a control
mechanism with a motor driven control valve (18) with control disk (17) and
control holes first the pressure in the control volume (16) is changed which
causes the displacers (6) of the first stage and the second stage (11) to
move; immediately thereafter the pressure in the entire volume of the
cylinder is equalized by the control mechanism. The cold head is linked via
flexible pressure lines to the compressor.
1
2
3
4
5
6
7
8
High vacuum flange
Pump casing
Forevacuum flange
Safety valve for gas discharge
Thermal radiation shield
Baffle
2nd stage of the cold head (≈10 K);
Cryopanels
Fig. 2.68 Design of a refrigerator cryopump (schematic)
inside and polished as well as nickel plated on the outside. Under no-load
conditions the baffle and the thermal radiation shield (first stage) attain a
temperature ranging between 50 to 80 K at the cryopanels and about 10 K
at the second stage. The surface temperatures of these cryopanels are
decisive to the actual pumping process. These surface temperatures
depend on the refrigerating power supplied by the cold head, and the
thermal conduction properties in the direction of the pump’s casing. During
operation of the cryopump, loading caused by the gas and the heat of
condensation results in further warming of the cryopanels. The surface
temperature does not only depend on the temperature of the cryopanel, but
also on the temperature of the gas which has already been frozen on to the
cryopanel. The cryopanels (8) attached to the second stage (7) of the cold
head are coated with activated charcoal on the inside in order to be able to
pump gases which do not easily condense and which can only be pumped
by cryosorption (see 2.1.9.4).
2.1.9.4
2.1.9.3
The refrigerator cryopump
Fig. 2.68 shows the design of a cryopump. It is cooled by a two-stage cold
head. The thermal radiation shield (5) with the baffle (6) is closely linked
thermally to the first stage (9) of the cold head. For pressures below
10-3 mbar the thermal load is caused mostly by thermal radiation. For this
reason the second stage (7) with the condensation and cryosorption panels
(8) is surrounded by the thermal radiation shield (5) which is black on the
56
9 1st stage of the cold head
(≈ 50–80 K)
10 Gauge for the hydrogen vapor pressure thermometer
11 Helium gas connections
12 Motor of the cold head with casing
and electric connections
Bonding of gases to cold surfaces
The thermal conductivity of the condensed (solid) gases depends very
much on their structure and thus on the way in which the condensate is
produced. Variations in thermal conductivity over several orders of
magnitude are possible! As the condensate increases in thickness, thermal
resistance and thus the surface temperature increase subsequently
reducing the pumping speed. The maximum pumping speed of a newly
regenerated pump is stated as its nominal pumping speed. The bonding
process for the various gases in the cryopump is performed in three steps:
first the mixture of different gases and vapors meets the baffle which is at
Vacuum generation
a temperature of about 80 K. Here mostly H2O and CO2 are condensed.
The remaining gases penetrate the baffle and impinge in the outside of the
cryopanel of the second stage which is cooled to about 10 K. Here gases
like N2, O2 or Ar will condense. Only H2, He and Ne will remain. These
gases can not be pumped by the cryopanels and these pass after several
impacts with the thermal radiation shield to the inside of these panels which
are coated with an adsorbent (cryosorption panels) where they are bonded
by cryosorption. Thus for the purpose of considering a cryopump the gases
are divided into three groups depending at which temperatures within the
cryopump their partial pressure drops below 10-9 mbar:
1st group:
ps < 10-9 mbar at T ≈ 77K (LN2): H2O, CO2
2nd group:
ps < 10-9 mbar at T ≈ 20K: N2, O2, Ar
3rd group:
ps < 10-9 mbar at T < 4.2K: H2, He, Ne
A difference is made between the different bonding mechanisms as follows:
Cryocondensation is the physical and reversible bonding of gas molecules
through Van der Waals forces on sufficiently cold surfaces of the same
material. The bond energy is equal to the energy of vaporization of the solid
gas bonded to the surface and thus decreases as the thickness of the
condensate increases as does the vapor pressure. Cryosorption is the
physical and reversible bonding of gas molecules through Van der Waals
forces on sufficiently cold surfaces of other materials. The bond energy is
equal to the heat of adsorption which is greater than the heat of
vaporization. As soon as a monolayer has been formed, the following
molecules impinge on a surface of the same kind (sorbent) and the process
transforms into cryocondensation. The higher bond energy for
cryocondensation prevents the further growth of the condensate layer
thereby restricting the capacity for the adsorbed gases. However, the
adsorbents used, like activated charcoal, silica gel, alumina gel and
molecular sieve, have a porous structure with very large specific surface
areas of about 106 m2/kg. Cryotrapping is understood as the inclusion of a
low boiling point gas which is difficult to pump such as hydrogen, in the
matrix of a gas having a higher boiling point and which can be pumped
easily such as Ar, CH4 or CO2. At the same temperature the condensate
mixture has a saturation vapor pressure which is by several orders of
magnitude lower than the pure condensate of the gas with the lower boiling
point.
2.1.9.5
Pumping speed and position of the cryopanels
Considering the position of the cryopanels in the cryopump, the
conductance from the vacuum flange to this surface and also the
subtractive pumping sequence (what has already condensed at the baffle
can not arrive at the second stage and consume capacity there), the
situation arises as shown in Fig. 2.69.
The gas molecules entering the pump produce the area related theoretical
pumping speed according the equation 2.29a with T = 293 K. The different
pumping speeds have been combined for three representative gases H2, N2
and H20 taken from each of the aforementioned groups. Since water vapor
is pumped on the entire entry area of the cryopump, the pumping speed
Hydrogen
Water vapor
Nitrogen
Area related conductance of the intake flange in l / s · cm2:
43.9
14.7
11.8
Area related pumping speed of the cryopump in l / s · cm2:
13.2
14.6
7.1
Ratio between pumping speed and conductance:
30 %
99 %
60 %
Fig. 2.69 Cryopanels – temperature and position define the efficiency in the cryopump
measured for water vapor corresponds almost exactly to the theoretical
pumping speed calculated for the intake flange of the cryopump. N2 on the
other hand must first overcome the baffle before it can be bonded on to the
cryocondensation panel. Depending on the design of the baffle, 30 to 50
percent of all N2 molecules are reflected.
H2 arrives at the cryosorption panels after further collisions and thus cooling
of the gas. In the case of optimally designed cryopanels and a good contact
with the active charcoal up to 50 percent of the H2 which has overcome the
baffle can be bonded. Due to the restrictions regarding access to the
pumping surfaces and cooling of the gas by collisions with the walls inside
the pump before the gas reaches the pumping surface, the measured
pumping speed for these two gases amounts only to a fraction of the
theoretical pumping speed. The part which is not pumped is reflected
chiefly by the baffle. Moreover, the adsorption probability for H2 differs
between the various adsorbents and is < 1, whereas the probabilities for
the condensation of water vapor and N2 ≈ 1.
Three differing capacities of a pump for the gases which can be pumped
result from the size of the three surfaces (baffle, condensation surface at
the outside of the second stage and sorption surface at the inside of the
second stage). In the design of a cryopump, a mean gas composition (air)
is assumed which naturally does not apply to all vacuum processes
(sputtering processes, for example. See 2.1.9.6 “Partial Regeneration”).
2.1.9.6
Characteristic quantities of a cryopump
The characteristic quantities of a cryopump are as follows (in no particular
order):
•
•
•
•
•
•
•
•
•
•
Cooldown time
Crossover value
Ultimate pressure
Capacity
Refrigerating power and net refrigerating power
Regeneration time
Throughput and maximum pV flow
Pumping speed
Service life / duration of operation
Starting pressure
57
Vacuum generation
Cooldown time: The cooldown time of cryopumps is the time span from
start-up until the pumping effect sets in. In the case of refrigerator
cryopumps the cooldown time is stated as the time it takes for the second
stage of the cold head to cool down from 293 K to 20 K.
Crossover value: The crossover value is a characteristic quantity of an
already cold refrigerator cryopump. It is of significance when the pump is
connected to a vacuum chamber via an HV / UHV valve. The crossover
value is that quantity of gas with respect to Tn=293 K which the vacuum
chamber may maximally contain so that the temperature of the cryopanels
does not increase above 20 K due to the gas burst when opening the
valve. The crossover value is usually stated as a pV value in in mbar · l.
The crossover value and the chamber volume V result in the crossover
pressure pc to which the vacuum chamber must be evacuated first before
opening the valve leading to the cryopump. The following may serve as a
guide:
pc ≤
35 ·
· Q 2 (20 K ) mbar
V
(2.27)
V = Volume of the vacuum chamber (l),
.
Q.2(20K) = Net refrigerating capacity in Watts, available at the second stage
of the cold head at 20 K.
Ultimate pressure pend: For the case of cryocondensation (see Section
2.1.9.4) the ultimate pressure can be calculated by:
pend = ps( TK ) ·
TG
TK
(2.28)
pS is the saturation vapor pressure of the gas or gases which are to be
pumped at the temperature TK of the cryopanel and TG is the gas
temperature (wall temperature in the vicinity of the cryopanel).
Example: With the aid of the vapor pressure curves in Fig. 9.15 for H2 and
N2 the ultimate pressures summarized in Table 2.6 at TG = 300 K result.
The Table shows that for hydrogen at temperatures T < 3 K at a gas
TK (K)
Ultimate pressure
Ult. press. (mbar)
(according to equ. 2.28)
H2
Ult. press.(mbar)
N2
10.95 · ps
3.28 · 10–14
immeasurably low
4.2
8.66 · ps
4.33 ·
10–9
immeasurably low
20
3.87 · ps
3.87 · 10+3
2.5
3.87 · 10–11
Table 2.6 Ultimate temperatures at a wall temperature of 300 K
temperature of TG= 300 K (i.e. when the cryopanel is exposed to the
thermal radiation of the wall) sufficiently low ultimate pressures can be
attained. Due to a number of interfering factors like desorption from the wall
and leaks, the theoretical ultimate pressures are not attained in practice.
Capacity C (mbar · l): The capacity of a cryopump for a certain gas is that
quantity of gas (pV value at Tn = 293 K) which can be bonded by the
cryopanels before the pumping speed for this type of gas G drops to below
50 % of its initial value.
The capacity for gases which are pumped by means of cryosorption
depends on the quantity and properties of the sorption agent; it is pressure
dependent and generally by several orders of magnitude lower compared
58
to the pressure independent capacity for gases which are pumped by
means of cryocondensation.
.
Refrigerating power Q (W): The refrigerating power of a refrigeration
source at a temperature T gives the amount of heat that can be extracted
by the refrigerating source whilst still maintaining this temperature. In the
case of refrigerators it has been agreed to state for single-stage cold heads
the refrigerating power at 80 K and for two-stage cold heads the
refrigerating power for the first stage at 80 K and for the second stage at 20
K when simultaneously loading both stages thermally. During the
measurement of refrigerating power the thermal load is generated by
electric heaters. The refrigerating power is greatest at room temperature
and is lowest (Zero) at ultimate temperature.
.
Net refrigerating power Q (W): In the case of refrigerator cryopumps the
net refrigerating power available at the usual operating temperatures
(T1 < 80 K, T2 < 20 K) substantially defines the throughput and the
crossover value. The net. refrigerating power is – depending on the
configuration of the pump – much lower than the refrigerating power of the
cold head used without the pump.
pV flow see 1.1
Regeneration time: As a gas trapping device, the cryopump must be
regenerated after a certain period of operation. Regeneration involves the
removal of condensed and adsorbed gases from the cryopanels by heating.
The regeneration can be run fully or only partially and mainly differs by the
way in which the cryopanels are heated.
In the case of total regeneration a difference is made between:
1. Natural warming: after switching off the compressor, the cryopanels at
first warm up only very slowly by thermal conduction and then in
addition through the released gases.
2. Purge gas method: the cryopump is warmed up by admitting warm
purging gas.
3. Electric heaters: the cryopanels of the cryopump are warmed up by
heaters at the first and second stages. The released gases are
discharged either through an overpressure valve (purge gas method) or
by mechanical backing pumps. Depending on the size of the pump one
will have to expect a regeneration time of several hours.
Partial regeneration: Since the limitation in the service life of a cryopump
depends in most applications on the capacity limit for the gases nitrogen,
argon and hydrogen pumped by the second stage, it will often be required
to regenerate only this stage. Water vapor is retained during partial
regeneration by the baffle. For this, the temperature of the first stage must
be maintained below 140 K or otherwise the partial pressure of the water
vapor would become so high that water molecules would contaminate the
adsorbent on the second stage.
In 1992, LEYBOLD was the first manufacturer of cryopumps to develop a
method permitting such a partial regeneration. This fast regeneration
process is microprocessor controlled and permits a partial regeneration of
the cryopump in about 40 minutes compared to 6 hours needed for a total
regeneration based on the purge gas method. A comparison between the
typical cycles for total and partial regeneration is shown in Fig. 2.70. The
time saved by the Fast Regeneration System is apparent. In a production
environment for typical sputtering processes one will have to expect one
Vacuum generation
higher pV flow is permissible (see crossover value).
Pumping speed Sth: The following applies to the (theoretical) pumping
speed of a cryopump:
⎛
Temperature (K)
S th = A K · SA · α · ⎜1 −
⎝
2
(2.29)
1
AK
SA
(1)
(2)
40 minutes
Fig. 2.70
pend⎞
⎟
p ⎠
Time
about 6 hours
α
pend
p
Size of the cryopanels
Surface area related pumping speed (area related impact rate
according to equations 1.17 and 1.20, proportional to the mean
velocity of the gas molecules in the direction of the cryopanel)
Probability of condensation (pumping)
Ultimate pressure (see above)
Pressure in the vacuum chamber
Comparison between total (1) and partial (2) regeneration
total regeneration after 24 partial regenerations.
Throughput and maximum pV flow: (mbar l/s): the throughput of a
cryopump for a certain gas depends on the pV flow of the gas G through
the intake opening of the pump:
QG = qpV,G;
the following equation applies
QG = pG · SG
with
p >> pend > Ps so that:
Sth = AK · SA
The maximum pV flow at which the cryopanels are warmed up to T ≈ 20 K
in the case of continuous operation, depends on the net refrigerating
power of the pump at this temperature and the type of gas. For refrigerator
cryopumps and condensable gases the following may be taken as a guide:
.
Qmax = 2.3 Q 2 (20 K) mbar · l/s
.
Q 2 (20 K) is the net refrigerating power in Watts available at the second
stage of the cold heat at 20 K. In the case of intermittent operation, a
Gas
(2.29a)
with
pG = intake pressure,
SG = pumping capacity for the gas G
Symbol
The equation (2.29) applies to a cryopanel built into the vacuum chamber,
the surface area of which is small compared to the surface of the vacuum
chamber. At sufficiently low temperatures α = 1 for all gases. The equation
(2.29) shows that for p >> pend the expression in brackets approaches 1 so
that in the oversaturated case
M
Molar
mass
g/mol
SA
at 293 K
gas temp.
l/s · cm2
R · TG
TG
SA = c =
= 365
.
/ s · cm2
4
2· π · M
M
TG
M
Gas temperature in K
Molar mass
Given in Table 2.7 is the surface arearelated pumping speed SA in
l · s-1 · cm-2 for some gases at two different gas temperatures TG in K
determined according to equation 2.29a. The values stated in the Table are
limit values. In practice the condition of an almost undisturbed equilibrium
SA
at 80 K
gas temp.
l/s · cm2
TS
Boiling point
1013 mbar
K
Triple point
(= melting point)
Tt
K
pt
mbar
H2
Hydrogen
2.016
43.88
22.93
20.27
13.80
70.4
He
Helium
4.003
31.14
16.27
4.222
2.173
50.52
CH4
Methane
4.003
15.56
8.13
111.67
90.67
116.7
H2O
Water
18.015
14.68
–
373.15
273.16
6.09
Ne
Neon
20.183
13.87
7.25
27.102
24.559
433.0
CO
Carbon monoxide
28.000
11.77
6.15
81.67
68.09
153.7
N2
Nitrogen
28.013
11.77
6.15
77.348
63.148
126.1
Air
28.96
11.58
6.05
≈ 80.5
≈ 58.5
–
O2
Oxygen
31.999
11.01
5.76
90.188
54.361
1.52
Ar
Argon
39.948
9.86
5.15
87.26
83.82
687.5
Kr
Krypton
83.80
6.81
3.56
119.4
115.94
713.9
Xe
Xenon
131.3
5.44
2.84
165.2
161.4
Table 2.7
Surface-related pumping speeds for some gases
59
Vacuum generation
(small cryopanels compared to a large wall surface) is often not true,
because large cryopanels are required to attain short pumpdown times and
a good end vacuum. Deviations also result when the cryopanels are
surrounded by a cooled baffle at which the velocity of the penetrating
molecules is already reduced by cooling.
Service life / duration of operation top (s): The duration of operation of
the cryopump for a particular gas depends on the equation:
2.2
Choice of pumping process
2.2.1 Survey of the most usual pumping
processes
Vacuum technology has undergone rapid development since the Fifties. In
research and in most branches of industry today, it is indispensable.
t op, G
CG =
∫Q
0
G
with
(t)dt
CG = Capacity of the cryopump for the gas G
QG(t) = Throughput of the cryopump for the gas at the point of time t
__
If the constant mean over time for the throughput QG is known, the
following applies:
t op, G =
CG
C
= G
QG pG · SG
(2.30)
After the period of operation top,G has elapsed the cryopump must be
regenerated with respect to the type of gas G.
Starting pressure po: Basically it is possible to start a cryopump at
atmospheric pressure. However, this is not desirable for several reasons.
As long as the mean free path of the gas molecules is smaller than the
dimensions of the vacuum chamber (p > 10-3 mbar), thermal conductivity of
the gas is so high that an unacceptably large amount of heat is transferred
to the cryopanels. Further, a relatively thick layer of condensate would form
on the cryopanel during starting. This would markedly reduce the capacity
of the cryopump available to the actual operating phase. Gas (usually air)
would be bonded to the adsorbent, since the bonding energy for this is
lower than that for the condensation surfaces. This would further reduce
the already limited capacity for hydrogen. It is recommended that
cryopumps in the high vacuum or ultrahigh vacuum range are started with
the aid of a backing pump at pressures of p < 5 · 10-2 mbar. As soon as the
starting pressure has been attained the backing pump may be switched off.
Corresponding to the many areas of application, the number of technical
procedures in vacuum processes is extraordinarily large. These cannot be
described within the scope of this section, because the basic calculations in
this section cover mainly the pumping process, not the process taking place
in the vessel. A survey of the most important processes in vacuum
technology and the pressure regions in which these processes are chiefly
carried out is given in the diagrams (Figures 2.71 and 2.72).
Generally, the pumping operation for these processes can be divided into
two categories – dry- and wet – vacuum procedures, that is, into processes
in which no significant amounts of vapor have to be pumped and those in
which vapors (mostly water or organic) arise.
Distinctions between the two categories are described briefly:
Dry processes work primarily in a narrow and limited pressure region.
The system is usually evacuated to a suitable characteristic pressure
before the actual working process begins. This happens, for example, in
plants for evaporative coating, electron-beam welding, and crystal pulling; in
particle accelerators, mass spectrometers, electron microscopes; and
others.
Further, there are dry processes in which degassing in vacuum is the actual
technical process. These include work in induction- and arc furnaces, steel
degassing plants, and plants for the manufacture of pure metals and
electron tubes.
Wet processes are undertaken primarily in a prescribed working
operation that covers a wider pressure region. This is especially
important in the drying of solid materials. If, for instance, work is undertaken
prematurely at too low a pressure, the outer surfaces dry out too quickly. As
a result, the thermal contact to the moisture to be evaporated is impaired
and the drying time is considerably increased. Predominantly processes
that are carried out in drying, impregnating, and freeze-drying plants belong
in this category.
In the removal of water vapor from liquids or in their distillation, particularly
in degassing columns, vacuum filling, and resin-casting plants, as well as in
molecular distillation, the production of as large a liquid surface as possible
is important. In all wet processes the provision of the necessary heat
for evaporation of the moisture is of great importance.
Basic pumping procedures are given in the following paragraphs.
If you have specific questions, you should get in touch with a specialist
department in LEYBOLD where experts are available to you who can draw
on many years of experience.
60
Vacuum generation
Ultrahigh vacuum
High vacuum
Medium vacuum
Rough vacuum
Mass spectrometers
Molecular beam apparatus
Ion sources
Particle accelerators
Electron microscopes
Electron diffraction apparatus
Vacuum spectographs
Low-temperature research
Production of thin films
Surface physics
Plasma research
Nuclear fusion apparatus
Space simulation
Material research
Preparations for
electron microscopy
10–13
10–10
10–7
10–3
100
103
Pressure [mbar]
Fig. 2.71 Pressure ranges (p < 1000 mbar) of physical and chemical analytical methods
Classifications of typical vacuum processes and plants according to
the pressure regions
Rough vacuum 1013 mbar – 1 mbar
• Drying, distillation, and steel degassing.
Medium vacuum 1 -10-3 mbar
• Molecular distillation, freeze-drying, impregnation, melting and casting
furnaces, and arc furnaces.
High vacuum 10-3 – 10-7 mbar
Ultrahigh vacuum
• Evaporative coating, crystal pulling, mass spectrometers, tube
production, electron microscopes, electron beam plants, and particle
accelerators.
Ultrahigh vacuum: < 10-7 mbar
• Nuclear fusion, storage rings for accelerators, space research, and
surface physics.
High vacuum
Medium vacuum
Rough vacuum
Annealing of metals
Melting of metals
Degassing of molten metals
Steel degassing
Electron-beam melting
Electron-beam welding
Evaporation coating
Sputtering of metals
Zone melting and crystal growing in high-vacuum
Molecular distillation
Degassing of liquids
Sublimation
Casting of resins and lacquers
Drying of plastics
Drying of insulating papers
Freeze-drying of mass materials
Freeze-drying of pharmaceutical products
Production of incandescent lamps
Production of electron tubes
Production of gas-discharge tubes
10–10
10–7
10–3
100
103
Pressure [mbar]
Fig. 2.72 Pressure ranges of industrial vacuum processes
61
Vacuum generation
2.2.2 Pumping of gases (dry processes)
For dry processes in which a non condensable gas mixture (e.g., air) is to
be pumped, the pump to be used is clearly characterized by the required
working pressure and the quantity of gas to be pumped away. The choice
of the required working pressure is considered in this section. The choice of
the required pump is dealt with in Section 2.3.
Each of the various pumps has a characteristic working range in which it
has a particularly high efficiency. Therefore, the most suitable pumps for
use in the following individual pressure regions are described. For every
dry-vacuum process, the vessel must first be evacuated. It is quite possible
that the pumps used for this may be different from those that are the
optimum choices for a process that is undertaken at definite working
pressures. In every case the choice should be made with particular
consideration for the pressure region in which the working process
predominantly occurs.
a) Rough vacuum (1013 – 1 mbar)
The usual working region of the rotary pumps described in Section 2 lies
below 80 mbar. At higher pressures these pumps have a very high power
consumption (see Fig. 2.11) and a high oil consumption (see Section
8.3.1.1). Therefore, if gases are to be pumped above 80 mbar over long
periods, one should use, particularly on economic grounds, jet pumps,
water ring pumps or dry running, multi-vane pumps. Rotary vane and rotary
piston pumps are especially suitable for pumping down vessels from
atmospheric pressure to pressures below 80 mbar, so that they can work
continuously at low pressures. If large quantities of gas arise at inlet
pressures below 40 mbar, the connection in series of a Roots pump is
recommended. Then, for the backing pump speed required for the process
concerned, a much smaller rotary vane or piston pump can be used.
b) Medium vacuum (1 – 10-3 mbar)
If a vacuum vessel is merely to be evacuated to pressures in the medium
vacuum region, perhaps to that of the required backing pressure for
diffusion or sputter-ion pumps, single- and two-stage rotary pumps are
adequate for pressures down to 10-1 and 10-3 mbar, respectively. It is
essentially more difficult to select the suitable type of pump if medium vacuum processes are concerned in which gases or vapors are evolved
continuously and must be pumped away. An important hint may be given at
this point. Close to the attainable ultimate pressure, the pumping speed of
all rotary pumps falls off rapidly. Therefore, the lowest limit for the normal
working pressure region of these pumps should be that at which the
pumping speed still amounts to about 50 % of the nominal pumping speed.
Between 1 and 10-2 mbar at the onset of large quantities of gas, Roots
pumps with rotary pumps as backing pumps have optimum pumping
properties (see Section 2.1.3.1). For this pressure range, a single-stage
rotary pump is sufficient, if the chief working region lies above 10-1 mbar. If
it lies between 10-1 and 10-2 mbar, a two-stage backing pump is
recommended. Below 10-2 mbar the pumping speed of single-stage Roots
pumps in combination with two-stage rotary pumps as backing pumps
decreases. However, between 10-2 and 10-4 mbar, two-stage Roots pumps
(or two single-stage Roots pumps in series) with two-stage rotary pumps as
backing pumps still have a very high pumping speed. Conversely, this
pressure region is the usual working region for vapor ejector pumps. For
work in this pressure region, they are the most economical pumps to
purchase. As backing pumps, single-stage rotary positive displacement
62
pumps are suitable. If very little maintenance and valveless operation are
convenient (i.e., small vessels in short operation cycles are to be pumped
to about 10-4 mbar or large vessels are to be maintained at this pressure
unattended for weeks), the previously mentioned two-stage Roots pumps
with two-stage rotary pumps as backing pumps are the suitable
combinations. Allthough, such a combination does not work as
economically as the corresponding vapor ejector pump, it can operate for a
much longer time without maintenance.
c) High vacuum (10-3 to 10-7 mbar)
Diffusion, sputter-ion, and turbomolecular pumps typically operate in the
pressure region below 10-3 mbar. If the working region varies during a
process, different pumping systems must be fitted to the vessel. There are
also special diffusion pumps that combine the typical properties of a
diffusion pump (low ultimate pressure, high pumping speed in the high vacuum region) with the outstanding properties of a vapor ejector pump (high
throughput in the medium vacuum region, high critical backing pressure). If
the working region lies between 10-2 and 10-6 mbar, these diffusion pumps
are, in general, specially recommended.
d) Ultrahigh vacuum (< 10-7 mbar)
For the production of pressures in the ultrahigh vacuum region, sputter-ion,
and sublimation pumps, as well as turbomolecular pumps and cryopumps,
are used in combination with suitable forepumps. The pump best suited to a
particular UHV process depends on various conditions (for further details,
see Section 2.5).
2.2.3 Pumping of gases and vapors (wet
processes)
When vapors must be pumped, in addition to the factors working pressure
and pumping speed, a third determining factor is added namely the vapor
partial pressure – which may vary considerably in the course of a process.
This factor is decisive in determining the pumping arrangement to be
installed. In this regard, the condensers described in Section 2.15 are very
important adjuncts to rotary displacement pumps. They have a particularly
high pumping speed when pumping vapors. The next section covers
pumping of water vapor (the most frequent case). The considerations apply
similarly to other non-aggressive vapors.
Pumping of Water Vapor
Water vapor is frequently removed by pumps that operate with water or
steam as a pump fluid, for example, water ring pumps or steam ejector
pumps. This depends considerably on circumstances, however, because
the economy of steam ejector pumps at low pressures is generally far
inferior to that of rotary pumps. For pumping a vapor – gas mixture in which
the vapor portion is large but the air portion is small, the vapor can be
pumped by condensers and the permanent gases, by relatively small gas
ballast pumps (see Section 2.1.5).
Comparatively, then, a pump set consisting of a Roots pump, condenser,
and backing pump, which can transport 100 kg/h of vapor and 18 kg/h of air
at an inlet pressure of 50 mbar, has a power requirement of 4 – 10 kW
(depending on the quantity of air involved). A steam ejector pump of the
same performance requires about 60 kW without altering the quantity of air
involved.
Vacuum generation
Section 2.1.5. At a water vapor tolerance of 60 mbar, the lower limit of this
region is
pv > 6O + 0.46 pp mbar
Water vapor partial pressure pv
6
=
/pp
0.4
pv
Air partial pressure pp
Fig. 2.73 Areas of application for gas ballast pumps and condensers pumping water vapor
(o.G. = without gas ballast)
For the pumping of water vapor, gas ballast pumps and combinations of
gas ballast pumps, Roots pumps, and condensers are especially suitable.
Pumping of water vapor with gas ballast pumps
The ratio of vapor partial pressure pv to air partial pressure pp is decisive in
the evaluation of the correct arrangement of gas ballast pumps, as shown
previously by equations 2.2 and 2.3. Therefore, if the water vapor tolerance
of the gas ballast pump is known, graphs may be obtained that clearly give
the correct use of gas ballast pumps for pumping water vapor (see Fig.
2.73). Large single-stage rotary plunger pumps have, in general, an
operating temperature of about 77 °C and hence a water vapor tolerance of
about 60 mbar. This value is used to determine the different operating
regions in Fig. 2.73. In addition it is assumed that the pressure at the
discharge outlet port of the gas ballast pump can increase to a maximum of
1330 mbar until the discharge outlet valve opens.
Region A: Single-stage, rotary plunger pumps without gas ballast
inlet.
At a saturation vapor pressure pS of 419 mbar at 77 °C, according to
equation 2.2, the requirement is given that pv < 0.46 pp, where
pv is the water vapor partial pressure
pp is the partial pressure of air
pv + pp = ptot total pressure
This requirement is valid in the whole working region of the single-stage
rotary plunger pump – hence, at total pressures between 10-1 and 1013
mbar.
Region B: Single-stage rotary plunger pumps with gas ballast and an
inlet condenser.
In this region the water vapor pressure exceeds the admissible partial
pressure at the inlet. The gas ballast pump must, therefore, have a
condenser inserted at the inlet, which is so rated that the water vapor
partial pressure at the inlet port of the rotary pump does not exceed the
admissible value. The correct dimensions of the condenser are selected
depending on the quantity of water vapor involved. For further details, see
Region C: Single-stage rotary plunger pumps with a gas ballast.
The lower limit of region C is characterized by the lower limit of the working
region of this pump. It lies, therefore, at about ptot = 1 mbar. If large
quantities of vapor arise in this region, it is often more economical to insert
a condenser: 20 kg of vapor at 28 mbar results in a volume of about
1000 m3. It is not sensible to pump this volume with a rotary pump. As a
rule of thumb:
A condenser should always be inserted at the pump’s inlet if saturated
water vapor arises for a considerable time.
As a precaution, therefore, a Roots pump should always be inserted in front
of the condenser at low inlet pressures so that the condensation capacity is
essentially enhanced. The condensation capacity does not depend only on
the vapor pressure, but also on the refrigerant temperature. At low vapor
pressures, therefore, effective condensation can be obtained only if the
refrigerant temperature is correspondingly low. At vapor pressures below
6.5 mbar, for example, the insertion of a condenser is sensible only if the
refrigerant temperature is less than 0 °C. Often at low pressures a gas –
vapor mixture with unsaturated water vapor is pumped (for further details,
see Section 2.1.5). In general, then, one can dispense with the condenser.
Region D: Two-stage gas ballast pumps, Roots pumps, and vapor
ejector pumps, always according to the total pressure concerned in
the process.
It must again be noted that the water vapor tolerance of two-stage gas
ballast pumps is frequently lower than that of corresponding single-stage
pumps.
Pumping of water vapor with roots pumps
Normally, Roots pumps are not as economical as gas ballast pumps for
continuous operation at pressures above 40 mbar. With very large pump
sets, which work with very specialized gear ratios and are provided with
bypass lines, however, the specific energy consumption is indeed more
favorable. If Roots pumps are installed to pump vapors, as in the case of
gas ballast pumps, a chart can be given that includes all possible cases
(see Fig. 2.74).
Region A: A Roots pump with a single-stage rotary plunger pump
without gas ballast.
As there is merely a compression between the Roots pump and the rotary
plunger pump, the following applies here too:
pv < 0.46 pp
The requirement is valid over the entire working region of the pump
combination and, therefore, for total pressures between 10-2 and 40 mbar
(or 1013 mbar for Roots pumps with a bypass line).
Region B: A main condenser, a Roots pump with a bypass line, an
intermediate condenser, and a gas ballast pump.
This combination is economical only if large water vapor quantities are to
be pumped continuously at inlet pressures above about 40 mbar. The size
63
of the main condenser depends on the quantity of vapor involved. The
intermediate condenser must decrease the vapor partial pressure below 60
mbar. Hence, the gas ballast pump should be large enough only to prevent
the air partial pressure behind the intermediate condenser from exceeding
a certain value; for example, if the total pressure behind the Roots pump
(which is always equal to the total pressure behind the intermediate
condenser) is 133 mbar, the gas ballast pump must pump at least at a
partial air pressure of 73 mbar, the quantity of air transported to it by the
Roots pump. Otherwise, it must take in more water vapor than it can
tolerate. This is a basic requirement: the use of gas ballast pumps is
wise only if air is also pumped!
With an ideally leak-free vessel, the gas ballast pump should be isolated
after the required operating pressure is reached and pumping continued
with the condenser only. Section 2.1.5 explains the best possible
combination of pumps and condensers.
Region C: A Roots pump, an intermediate condenser, and a gas
ballast pump.
The lower limit of the water vapor partial pressure is determined through
the compression ratio of the Roots pump at the backing pressure, which is
determined by the saturation vapor pressure of the condensed water. Also,
in this region the intermediate condenser must be able to reduce the vapor
partial pressure to at least 60 mbar. The stated arrangement is suitable –
when cooling the condenser with water at 15 °C – for water vapor
pressures between about 4 and 40 mbar.
Region D: A roots pump and a gas ballast pump.
In this region D the limits also depend essentially on the stages and ratios
of sizes of the pumps. In general, however, this combination can always be
used between the previously discussed limits – therefore, between 10-2 and
4 mbar.
2.2.4 Drying processes
Often a vacuum process covers several of the regions quoted here. In
batch drying the process can, for example (see Fig. 2.74), begin in region A
(evacuation of the empty vessel) and then move through regions B, C, and
D in steps. Then the course of the process would be as follows:
A. Evacuating the vessel by a gas ballast pump and a Roots pump
with a bypass line.
B. Connecting the two condensers because of the increasing vapor
pressure produced by heating the material.
The choice of the pumping System is decided by the highest vapor partial
pressure occurring and the lowest air partial pressure at the inlet.
C. Bypassing the main condenser
It will now not have an effect. Instead it would only be pumped empty by
the pumping system with a further drop in vapor pressure.
D. Bypassing the intermediate condenser.
Roots pumps and gas ballast pumps alone can now continue pumping.
With short-term drying, the separation of the condenser filled with
condensed water is particularly important, because the gas ballast pump
would continue to pump from the condenser the previously condensed
64
Water vapor partial pressure pv
Vacuum generation
6
=
0.4
/p p
pv
Air
Air partial
partialpressure
pressurepppp
Fig. 2.74 Areas of application for Roots pumps and condensers pumping water vapor (o.G. =
without gas ballast)
water vapor at the saturation vapor pressure of water.
With longer-term drying processes, it suffices to shut off the condensate
collector from the condenser. Then only the remaining condensate film on
the cooling tubes can reevaporate. Depending on the size of the gas ballast
pump, this reevaporation ensues in 30 – 60 min.
E. If the drying process should terminate at still lower pressures
When a pressure below 10-2 mbar is reached a previously bypassed oil
vapor ejector pump should be switched on in addition.
Drying of solid substances
As previously indicated, the drying of solid substances brings about a series
of further problems. It no longer suffices that one simply pumps out a
vessel and then waits until the water vapor diffuses from the solid
substance. This method is indeed technically possible, but it would
intolerably increase the drying time.
It is not a simple technical procedure to keep the drying time as short as
possible. Both the water content and the layer thickness of the drying
substance are important. Only the principles can be stated here. In case of
special questions we advise you to contact our experts in our Cologne
factory.
The moisture content E of a material to be dried of which the diffusion
coefficient depends on the moisture content (e.g. with plastics) as a
function of the drying time t is given in close approximation by the following
equation:
E=
E0
(1 + K · t )q
%
(2.31)
E0 where E is the moisture content before drying
q is the temperature-dependent coefficient. Thus equation (2.31) serves
only for the temperature at which q was determined
K is a factor that depends on the temperature, the water vapor partial
pressure in the vicinity of the material, the dimensions, and the
properties of the material.
Vacuum generation
With the aid of this approximate equation, the drying characteristics of
many substances can be assessed. If K and q have been determined for
various temperatures and water vapor partial pressures, the values for
other temperatures are easily interpolated, so that the course of the drying
process can be calculated under all operating conditions. With the aid of a
similarity transformation, one can further compare the course of drying
process of a material with known properties with that of a material with
different properties.
Absolutely oil-free vacua may be produced in the medium vacuum region
with adsorption pumps. Since the pumping action of these pumps for the
light noble gases is only small, vessels initially filled with air can only be
evacuated by them to about 10-2 mbar. Pressures of 10-3 mbar or lower can
then be produced with adsorption pumps only if neither neon nor helium is
present in the gas mixture to be pumped. In such cases it can be useful to
expel the air in the vessel by first flooding with nitrogen and then pumping it
away.
Fundamentally, in the drying of a material, a few rules are noteworthy:
Experience has shown that shorter drying times are obtained if the water
vapor partial pressure at the surface of the material is relatively high, that
is, if the surface of the material to be dried is not yet fully free of moisture.
This is possible because the heat conduction between the source of heat
and the material is greater at higher pressures and the resistance to
diffusion in a moist surface layer is smaller than in a dry one. To fulfill the
conditions of a moist surface, the pressure in the drying chamber is
controlled. If the necessary relatively high water vapor partial pressure
cannot be maintained permanently, the operation of the condenser is
temporarily discontinued. The pressure in the chamber then increases and
the surface of the material becomes moist again. To reduce the water vapor
partial pressure in the vessel in a controlled way, it may be possible to
regulate the refrigerant temperature in the condenser. In this way, the
condenser temperature attains preset values, and the water vapor partial
pressure can be reduced in a controlled manner.
c) High- and ultrahigh vacuum region (< 10-3 mbar)
When there is significant evolution of gas in the pressure regions that must
be pumped, turbomolecular pumps, or cryopumps should be used. A
sputter-ion pump is especially suitable for maintaining the lowest possible
pressure for long periods in a sealed system where the process does not
release large quantities of gas. Magnetically-suspended turbomolecular
pumps also guarantee hydrocarbon-free vacua. However, while these
pumps are switched off, oil vapors can enter the vessel through the pump.
By suitable means (e.g., using an isolating valve or venting the vessel with
argon), contamination of the vessel walls can be impeded when the pump
is stationary. If the emphasis is on generating a “hydrocarbon-free vacuum”
with turbomolecular pumps, then hybrid turbomolecular pumps with
diaphragm pumps or classic turbomolecular pumps combined with scroll
pumps should be used as oil-free backing pumps.
2.2.6 Ultrahigh vacuum working Techniques
2.2.5 Production of an oil-free
(hydrocarbon-free) vacuum
Backstreaming vapor pump fluids, vapors of oils, rotary pump lubricants,
and their cracking products can significantly disturb various working
processes in vacuum. Therefore, it is recommended that certain
applications use pumps and devices that reliably exclude the presence of
hydrocarbon vapors.
a) Rough vacuum region (1013 to 1 mbar)
Instead of rotary pumps, large water jet, steam ejector, or water ring pumps
can be used. For batch evacuation, and the production of hydrocarbon-free
fore vacuum for sputter-ion pumps, adsorption pumps (see Section 2.1.8.1)
are suitable. If the use of oil-sealed rotary vane pumps cannot be avoided,
basically two-stage rotary vane pumps should be used. The small amount
of oil vapor that backstreams out of the inlet ports of these pumps can be
almost completely removed by a sorption trap (see Section 2.1.4) inserted
in the pumping line.
b) Medium vacuum region (1 to 10-3 mbar)
For the pumping of large quantities of gas in this pressure region, vapor
ejector pumps are by far the most suitable. With mercury vapor ejector
pumps, completely oil-free vacua can be produced. As a precaution, the
insertion of a cold trap chilled with liquid nitrogen is recommended so that
the harmful mercury vapor does not enter the vessel. With the medium vacuum sorption traps described under a), it is possible with two-stage rotary
vane pumps to produce almost oil-free vacua down to below
10-4 mbar.
The boundary between the high and ultrahigh vacuum region cannot be
precisely defined with regard to the working methods. In practice, a border
between the two regions is brought about because pressures in the high
vacuum region may be obtained by the usual pumps, valves, seals, and
other components, whereas for pressures in the UHV region, another
technology and differently constructed components are generally required.
The “border” lies at a few 10-8 mbar. Therefore, pressures below 10-7 mbar
should generally be associated with the UHV region.
The gas density is very small in the UHV region and is significantly
influenced by outgasing rate of the vessel walls and by the tiniest leakages
at joints. Moreover, in connection with a series of important technical
applications to characterize the UHV region, generally the monolayer time
(see also equation 1.21) has become important. This is understood as the
time τ that elapses before a monomolecular or monatomic layer forms on
an initially ideally cleaned surface that is exposed to the gas particles.
Assuming that every gas particle that arrives at the surface finds a free
place and remains there, a convenient formula for τ is
τ=
3.2 – 6
· 10 s
p
(p in mbar)
Therefore, in UHV (p < 10-7 mbar) the monolayer formation time is of the
order of minutes to hours or longer and thus of the same length of time as
that needed for experiments and processes in vacuum. The practical
requirements that arise have become particularly significant in solid-state
physics, such as for the study of thin films or electron tube technology. A
UHV system is different from the usual high vacuum system for the
following reasons:
65
Vacuum generation
a) the leak rate is extremely small (use of metallic seals),
b) the gas evolution of the inner surfaces of the vacuum vessel and of the
attached components (e.g., connecting tubulation; valves, seals) can be
made extremely small,
c) suitable means (cold traps, baffles) are provided to prevent gases or
vapors or their reaction products that have originated from the pumps
used from reaching the vacuum vessel (no backstreaming).
To fulfill these conditions, the individual components used in UHV
apparatus must be bakeable and extremely leaktight. Stainless steel is the
preferred material for UHV components.
The construction, start-up, and operation of an UHV system also demands
special care, cleanliness, and, above all, time. The assembly must be
appropriate; that is, the individual components must not be in the least
damaged (i.e. by scratches on precision-worked sealing surfaces).
Fundamentally, every newly-assembled UHV apparatus must be tested for
leaks with a helium leak detector before it is operated. Especially important
here is the testing of demountable joints (flange connections), glass seals,
and welded or brazed joints. After testing, the UHV apparatus must be
baked out. This is necessary for glass as well as for metal apparatus. The
bake-out extends not only over the vacuum vessel, but frequently also to
the attached parts, particularly the vacuum gauges. The individual stages of
the bake-out, which can last many hours for a larger system, and the bakeout temperature are arranged according to the kind of plant and the
ultimate pressure required. If, after the apparatus has been cooled and the
other necessary measures undertaken (e.g., cooling down cold traps or
baffles), the ultimate pressure is apparently not obtained, a repeated leak
test with a helium leak detector is recommended. Details on the
components, sealing methods and vacuum gauges are provided in our
catalog.
2.3.
Evacuation of a vacuum chamber and
determination of pump sizes
Basically, two independent questions arise concerning the size of a
vacuum system:
1. What effective pumping speed must the pump arrangement maintain to
reduce the pressure in a given vessel over a given time to a desired
value?
2. What effective pumping speed must the pump arrangement reach during
a vacuum process so that gases and vapors released into the vessel
can be quickly pumped away while a given pressure (the operating
pressure) in the vessel, is maintained and not exceeded?
During the pumping-out procedure of certain processes (e.g., drying and
heating), vapors are produced that were not originally present in the
vacuum chamber, so that a third question arises:
3. What effective pumping speed must the pump arrangement reach so
that the process can be completed within a certain time?
The effective pumping speed of a pump arrangement is understood as
the actual pumping speed of the entire pump arrangement that prevails
at the vessel. The nominal pumping speed of the pump can then be
determined from the effective pumping speed if the flow resistance
(conductances) of the baffles, cold traps, filters, valves, and tubulations
installed between the pump and the vessel are known (see Sections 1.5.2
to 1.5.4). In the determination of the required nominal pumping speed it is
further assumed that the vacuum system is leaktight; therefore, the leak
rate must be so small that gases flowing in from outside are immediately
removed by the connected pump arrangement and the pressure in the
vessel does not alter (for further details, see Section 5). The questions
listed above under 1., 2. and 3. are characteristic for the three most
essential exercises of vacuum technology
1. Evacuation of the vessel to reach a specified pressure.
2. Pumping of continuously evolving quantities of gas and vapor at a
certain pressure.
3. Pumping of the gases and vapors produced during a process by
variation of temperature and pressure.
Initial evacuation of a vacuum chamber is influenced in the medium-, high-,
and ultrahigh vacuum regions by continually evolving quantities of gas,
because in these regions the escape of gases and vapors from the walls of
the vessel is so significant that they alone determine the dimensions and
layout of the vacuum system.
2.3.1 Evacuation of a vacuum chamber
(without additional sources of gas
or vapor)
Because of the factors described above, an assessment of the pump-down
time must be basically different for the evacuation of a container in the
rough vacuum region from evacuation in the medium- and high vacuum
regions.
66
Vacuum generation
2.3.1.1
Evacuation of a chamber in the rough vacuum
region
In this case the required effective pumping speed Seff, of a vacuum pump
assembly is dependent only on the required pressure p, the volume V of
the container, and the pump-down time t.
With constant pumping speed Seff and assuming that the ultimate pressure
pend attainable with the pump arrangement is such that pend << p, the
decrease with time of the pressure p(t) in a chamber is given by the
equation:
−
dp Seff
=
·p
V
dt
Example: A vacuum chamber having a volume of 500 l shall be pumped
down to 1 mbar within 10 minutes. What effective pumping speed is
required?
500 l = 0.5 m3; 10 min = 1/6 h
According to equation (2.34) it follows that:
0.5
1013
· 2.3 · log
1
1/ 6
= 3 · 2.3 · 3.01 = 20.8 m3/h
Seff =
(2.32)
Beginning at 1013 mbar at time t = 0, the effective pumping speed is
calculated depending on the pump-down time t from equation (2.32) as
follows:
p dp
Seff
·t
∫ p =−
V
1013
n
orders of magnitude lower than the desired pressure.
S
p
= − eff · t
1013
V
V
1013 V
1013
Seff = · n
= · 2.3 · log
t
p
p
t
(2.33a)
(2.33b)
(2.34)
For the example given above one reads off the value of 7 from the straight
line in Fig. 2.75. However, from the broken line a value of 8 is read off.
According to equation (2.35) the following is obtained:
0.5
· 7 = 21 m 3/ h or
1
6
0.5
·8 = 24 m 3/ h
Seff =
1
6
Seff =
under consideration of the fact that the pumping speed reduces below
10 mbar. The required effective pumping speed thus amounts to about
24 m3/h.
Introducing the dimensionless factor
1013
1013
σ = n p = 2.3 · log p
(2.34a)
into equation (2.34), the relationship between the effective pumping speed
Seff, and the pump-down time t is given by
Seff =
V
·σ
t
(2.35)
The ratio V/Seff is generally designated as a time constant τ. Thus the
pump-down time of a vacuum chamber from atmospheric pressure to a
pressure p is given by:
t=τ⋅σ
Pressure →
(2.36)
with
τ= V
and
σ = n 1013
Seff
p
The dependence of the factor from the desired pressure is shown in Fig.
2.75. It should be noted that the pumping speed of single-stage rotary vane
and rotary piston pumps decreases below 10 mbar with gas ballast and
below 1 mbar without gas ballast. This fundamental behavior is different for
pumps of various sizes and types but should not be ignored in the
determination of the dependence of the pump-down time on pump size. It
must be pointed out that the equations (2.32 to 2.36) as well Fig. 2.75 only
apply when the ultimate pressure attained with the pump used is by several
Dimensionless factor σ
Fig. 2.75 Dependency of the dimensionless factor s for calculation of pumpdown time t according to equation 2.36. The broken line applies to single-stage pumps where the pumping speed decreases below 10 mbar.
67
Vacuum generation
2.3.1.2 Evacuation of a chamber in the high vacuum region
It is considerably more difficult to give general formulas for use in the high
vacuum region. Since the pumping time to reach a given high vacuum
pressure depends essentially on the gas evolution from the chamber’s
inner surfaces, the condition and pre-treatment of these surfaces are of
great significance in vacuum technology. Under no circumstances should
the material used exhibit porous regions or – particularly with regard to
bake-out – contain cavities; the inner surfaces must be as smooth as
possible (true surface = geometric surface) and thoroughly cleaned (and
degreased). Gas evolution varies greatly with the choice of material and the
surface condition. Useful data are collected in Table X (Section 9). The gas
evolution can be determined experimentally only from case to case by the
pressure-rise method: the system is evacuated as thoroughly as possible,
and finally the pump and the chamber are isolated by a valve. Now the
time is measured for the pressure within the chamber (volume V) to rise by
a certain amount, for example, a power of 10. The gas quantity Q that
arises per unit time is calculated from:
forevacuum pressure pV instead. Then equation (2.34) transforms into:
p
Seff = V · n V = V · n K
t
t
p
At a backing pressure of pV = 2 · 10-3 mbar “compression” K is in our
example:
K=
2 ⋅10 – 3
= 200
1⋅10 – 5
In order to attain an ultimate pressure of 1 · 10-5 mbar within 5 minutes
after starting to pump with the diffusion pump an effective pumping speed
of
Seff =
500
· 2.3 · log 200 ≈ 9 s
5 · 60
(Δp = measured pressure rise )
is required. This is much less compared to the effective pumping speed
needed to maintain the ultimate pressure. Pumpdown time and ultimate
vacuum in the high vacuum and ultrahigh vacuum ranges depends mostly
on the gas evolution rate and the leak rates. The underlying mathematical
rules can not be covered here. For these please refer to books specializing
on that topic.
The gas quantity Q consists of the sum of all the gas evolution and all
leaks possibly present. Whether it is from gas evolution or leakage may be
determined by the following method:
2.3.1.3
Q=
Δp · V
t
(2.37)
The gas quantity arising from gas evolution must become smaller with time,
the quantity of gas entering the system from leakage remains constant with
time. Experimentally, this distinction is not always easily made, since it
often takes a considerable length of time – with pure gas evolution – before
the measured pressure-time curve approaches a constant (or almost a
constant) final value; thus the beginning of this curve follows a straight line
for long times and so simulates leakage (see Section 5, Leaks and Leak
Detection).
If the gas evolution Q and the required pressure pend are known, it is easy
to determine the necessary effective pumping speed:
Seff =
Q
pend
(2.38)
Example: A vacuum chamber of 500 l may have a total surface area
(including all systems) of about 5 m2. A steady gas evolution of
2 · 10-4 mbar · l/s is assumed per m2 of surface area. This is a level which
is to be expected when valves or rotary feedthroughs, for example are
connected to the vacuum chamber. In order to maintain in the system a
pressure of 1 · 10-5 mbar, the pump must have a pumping speed of
5 · 2 ·10 – 4 mbar · / s
Seff =
= 100 / s
1·10 – 5 mbar
A pumping speed of 100 l/s alone is required to continuously pump away
the quantity of gas flowing in through the leaks or evolving from the
chamber walls. Here the evacuation process is similar to the examples
given in Sections 2.3.1.1. However, in the case of a diffusion pump the
pumping process does not begin at atmospheric pressure but at the
68
Evacuation of a chamber in the medium vacuum
region
In the rough vacuum region, the volume of the vessel is decisive for the
time involved in the pumping process. In the high and ultrahigh vacuum
regions, however, the gas evolution from the walls plays a significant role.
In the medium vacuum region, the pumping process is influenced by both
quantities. Moreover, in the medium vacuum region, particularly with rotary
pumps, the ultimate pressure pend attainable is no longer negligible. If the
quantity of gas entering the chamber is known to be at a rate Q (in millibars
liter per second) from gas evolution from the walls and leakage, the
differential equation (2.32) for the pumping process becomes
dp
=−
dt
S eff ⎛⎝ p − p
⎞
end ⎠
−Q
V
(2.39)
Integration of this equation leads to
⎛p − p ⎞
⎜ o
⎟
end ⎠ − Q / Seff
⎝
V
t=
n
S
⎛p− p ⎞ − Q/S
⎜
⎟
eff
eff
⎝
(2.40)
end ⎠
where
p0 is the pressure at the beginning of the pumping process
p is the desired pressure
In contrast to equation 2.33b this equation does not permit a definite
solution for Seff, therefore, the effective pumping speed for a known gas
evolution cannot be determined from the time – pressure curve without
further information.
In practice, therefore, the following method will determine a pump with
sufficiently high pumping speed:
Vacuum generation
a) The pumping speed is calculated from equation 2.34 as a result of the
volume of the chamber without gas evolution and the desired pumpdown time.
b) The quotient of the gas evolution rate and this pumping speed is found.
This quotient must be smaller than the required pressure; for safety, it
must be about ten times lower. If this condition is not fulfilled, a pump
with correspondingly higher pumping speed must be chosen.
2.3.2 Determination of a suitable backing pump
The gas or vapor quantity transported through a high vacuum pump must
also be handled by the backing pump. Moreover, in the operation of the
high vacuum pump (diffusion pump, turbomolecular pump), the maximum
permissible backing pressure must never, even for a short time, be
exceeded. If Q is the effective quantity of gas or vapor, which is pumped by
the high vacuum pump with an effective pumping speed Seff at an inlet
pressure pA, this gas quantity must certainly be transported by the backing
pump at a pumping speed of SV at the backing pressure pV. For the
effective throughput Q, the continuity equation applies:
Q = pA · Seff = pv · SV
(2.41)
The required pumping speed of the backing pump is calculated from:
SV =
pA
·S
pV eff
(2.41a)
Example: In the case of a diffusion pump having a pumping speed of
400 l/s the effective pumping speed is 50 % of the value stated in the
catalog when using a shell baffle. The max. permissible backing pressure is
2 · 10-1 mbar. The pumping speed required as a minimum for the backing
pump depends on the intake pressure pA according to equation 2.41a.
At an intake pressure of pA = 1 · 10-2 mbar the pumping speed for the high
vacuum pump as stated in the catalog is about 100 l/s, subsequently 50 %
of this is 50 l/s. Therefore the pumping speed of the backing pump must
amount to at least
SV =
1·10 – 2
· 50 = 2.5 /s = 9 m 3/ h
2 ·10 – 1
At an intake pressure of pA = 1 · 10-3 mbar the pump has already reached
its nominal pumping speed of 400 l/s; the effective pumping speed is now
Seff = 200 l/s; thus the required pumping speed for the backing pump
amounts to
SV =
1·10 – 3
· 200 = 1/s = 3. 6 m 3/h
2 ·10 – 1
If the high vacuum pump is to be used for pumping of vapors between 10-3
and 10-2 mbar, then a backing pump offering a nominal pumping speed of
12 m3/h must be used, which in any case must have a pumping speed of
9 m3/h at a pressure of 2 · 10-1 mbar. If no vapors are to be pumped, a
single-stage rotary vane pump operated without gas ballast will do in most
cases. If (even slight) components of vapor are also to be pumped, one
should in any case use a two-stage gas ballast pump as the backing pump
which offers – also with gas ballast – the required pumping speed at
2 · 10-1 mbar.
If the high vacuum pump is only to be used at intake pressures below
10-3 mbar, a smaller backing pump will do; in the case of the example given
this will be a pump offering a pumping speed of 6 m3/h. If the continuous
intake pressures are even lower, below 10-4 mbar, for example, the required
pumping speed for the backing pump can be calculated from equation
2.41a as:
SV =
1·10 – 4
· 200 = 0.1 /s = 0. 36 m 3/h
2 ·10 – 1
Theoretically in this case a smaller backing pump having a pumping speed
of about 1 m3/h could be used. But in practice a larger backing pump
should be installed because, especially when starting up a vacuum system,
large amounts of gas may occur for brief periods. Operation of the high
vacuum pump is endangered if the quantities of gas can not be pumped
away immediately by the backing pump. If one works permanently at very
low inlet pressures, the installation of a ballast volume (backing-line vessel
or surge vessel) between the high vacuum pump and the backing pump is
recommended. The backing pump then should be operated for short times
only. The maximum admissible backing pressure, however, must never be
exceeded.
The size of the ballast volume depends on the total quantity of gas to be
pumped per unit of time. If this rate is very low, the rule of thumb indicates
that 0.5 l of ballast volume allows 1 min of pumping time with the backing
pump isolated.
For finding the most adequate size of backing pump, a graphical method
may be used in many cases. In this case the starting point is the pumping
speed characteristic of the pumps according to equation 2.41.
The pumping speed characteristic of a pump is easily derived from the
measured pumping speed (volume flow rate) characteristic of the pump as
shown for a 6000 l/s diffusion pump (see curve S in Fig. 2.76). To arrive at
the throughput characteristic (curve Q in Fig. 2.76), one must multiply each
ordinate value of S by its corresponding pA value and plotted against this
value. If it is assumed that the inlet pressure of the diffusion pump does not
exceed 10-2 mbar, the maximum throughput is 9.5 mbar · l/s
Hence, the size of the backing pump must be such that this throughput can
be handled by the pump at an intake pressure (of the backing pump) that is
equal to or preferably lower than the maximum permissible backing
pressure of the diffusion pump; that is, 4 · 10-1 mbar for the 6000 l/s
diffusion pump.
After accounting for the pumping speed characteristics of commercially
available two-stage rotary plunger pumps, the throughput characteristic for
each pump is calculated in a manner similar to that used to find the Q curve
for the diffusion pump in Fig. 2.76 a. The result is the group of Q curves
numbered 1 – 4 in Fig. 2.76 b, whereby four 2-stage rotary-plunger pumps
were considered, whose nominal speeds were 200, 100, 50, and 25 m3/h,
respectively. The critical backing pressure of the 6000 l/s diffusion pump is
marked as V.B. (p = 4 · 10-1 mbar). Now the maximum throughput
Q = 9.5 mbar · l/s is shown as horizontal line a. This line intersects the four
throughput curves. Counting from right to left, the first point of intersection
that corresponds to an intake pressure below the critical backing pressure
69
Q [mbar · l · s–1]
Throughput Q [mbar · l · s–1]
Vacuum generation
Intake pressure pa [mbar]
a) Pumping speed characteristic of a 6000 l/s diffusion pump
b) Series of throughput curves for two-stage rotary
plunger pumps (V.B. = Critical forevacuum pressure)
Fig. 2.76 Diagram for graphically determining a suitable backing pump
of 4 · 10-1 mbar is made with throughput characteristic 2. This corresponds
to the two-stage rotary plunger pump with a nominal pumping speed of
100 m3/h. Therefore, this pump is the correct backing pump for the 6000 l/s
diffusion pump under the preceding assumption.
However, if the pumping process is such that the maximum throughput of
9.5 mbar · l/s is unlikely, a smaller backing pump can, of course, be used.
This is self-explanatory, for example, from line b in Fig. 2.76 b, which
corresponds to a maximum throughput of only 2 mbar l/s. In this case a
25 m3/h two-stage rotary-plunger pump would be sufficient.
2.3.3 Determination of pump-down time from
nomograms
In practice, for instance, when estimating the cost of a planned vacuum
plant, calculation of the pump-down time from the effective pumping speed
Seff, the required pressure p, and the chamber volume V by formulas
presented would be too troublesome and time-consuming. Nomograms are
very helpful here. By using the nomogram in Fig. 9.7 in Section 9, one can
quickly estimate the pump-down time for vacuum plants evacuated with
rotary pumps, if the pumping speed of the pump concerned is fairly
constant through the pressure region involved. By studying the examples
presented, one can easily understand the application of the nomogram.
The pump-down times of rotary vane and rotary piston pumps, insofar as
the pumping speed of the pump concerned is constant down to the
required pressure, can be determined by reference to example 1.
In general, Roots pumps do not have constant pumping speeds in the
working region involved. For the evaluation of the pump-down time, it
usually suffices to assume the mean pumping speed. Examples 2 and 3 of
the nomogram show, in this context, that for Roots pumps, the compression
70
ratio K refers not to the atmospheric pressure (1013 mbar), but to the
pressure at which the Roots pump is switched on.
In the medium vacuum region, the gas evolution or the leak rate becomes
significantly evident. From the nomogram 9.10 in Section 9, the
corresponding calculations of the pump-down time in this vacuum region
can be approximated.
In many applications it is expedient to relate the attainable pressures at any
given time to the pump-down time. This is easily possible with reference to
the nomogram 9.7 in Section 9.
As a first example the pump-down characteristic – that is, the relationship
pressure p (denoted as desired pressure pend) versus pumping time tp – is
derived from the nomogram for evacuating a vessel of 5 m3 volume by the
single-stage rotary plunger pump E 250 with an effective pumping speed of
Seff = 250 m3/h and an ultimate pressure pend,p = 3 · 10-1 mbar when
operated with a gas ballast and at pend,p = 3 · 10-2 mbar without a gas
ballast. The time constant τ = V / Seff (see equation 2.36) is the same in
both cases and amounts as per nomogram 9.7 to about 70 s (column 3).
For any given value of pend > pend,p the straight line connecting the “70 s
point” on column 3 with the (pend – pend,p) value on the right-hand scale of
column 5 gives the corresponding tp value. The results of this procedure
are shown as curves a and b in Fig. 2.77.
It is somewhat more tedious to determine the (pend,tp) relationship for a
combination of pumps. The second example discussed in the following
deals with evacuating a vessel of 5 m3 volume by the pump combination
Roots pump WA 1001 and the backing pump E 250 (as in the preceding
example). Pumping starts with the E 250 pump operated without gas ballast
alone, until the Roots pump is switched on at the pressure of 10 mbar. As
the pumping speed characteristic of the combination WA 1001/ E 250 – in
contrast to the characteristic of the E 250 – is no longer a horizontal
Vacuum generation
straight line over the best part of the pressure range (compare this to the
corresponding course of the characteristic for the combination WA 2001 / E
250 in Fig. 2.19), one introduces, as an approximation, average values of
Seff, related to defined pressure ranges. In the case of the WA 1001/ E 250
combination the following average figures apply:
Seff = 800 m3/h in the range 10 – 1 mbar,
Seff = 900 m3/h in the range 1 mbar to
5 · 10-2 mbar,
Seff = 500 m3/h in the range 5 · 10-2 to
5 · 10-3 mbar
The ultimate pressure of the combination WA 1001 / E 250 is:
Pend,p = 3 · 10-3 mbar. From these figures the corresponding time constants
in the nomogram can be determined; from there, the pump-down time tp
can be found by calculating the pressure reduction R on the left side of
column 5. The result is curve c in Fig. 2.77.
Computer aided calculations at LEYBOLD
Of course calculations for our industrial systems are performed by computer
programs. These require high performance computers and are thus usually
not available for simple initial calculations.
2.3.4 Evacuation of a chamber where gases and
vapors are evolved
The preceding observations about the pump-down time are significantly
altered if vapors and gases arise during the evacuation process. With bakeout processes particularly, large quantities of vapor can arise when the
surfaces of the chamber are cleared of contamination. The resulting
necessary pump-down time depends on very different parameters.
Increased heating of the chamber walls is accompanied by increased
desorption of gases and vapors from the walls. However, because the
higher temperatures result in an accelerated escape of gases and vapors
from the walls, the rate at which they can be removed from the chamber is
also increased.
The magnitude of the allowable temperature for the bake-out process in
question will, indeed, be determined essentially by the material in the
chamber. Precise pump-down times can then be estimated by calculation
only if the quantity of the evolving and pumped vapors is known. However,
since this is seldom the case except with drying processes, a quantitative
consideration of this question is abandoned within the scope of this
publication.
2.3.5 Selection of pumps for drying processes
Fundamentally, we must distinguish between short-term drying and drying
processes that can require several hours or even days. Independently of
the duration of drying, all drying processes proceed approximately as in
Section 2.24
As an example of an application, the drying of salt (short-term drying) is
described, this being an already well-proven drying process.
Drying of salt
First, 400 kg of finely divided salt with a water content of about 8 % by
mass is to be dried in the shortest possible time (about 1 h) until the water
content is less than 1 % by mass. The expected water evolution amounts to
about 28 kg. The salt in the chamber is continuously agitated during the
drying process and heated to about 80 °C. The vacuum system is
schematically drawn in Fig. 2.78.
Fig. 2.77 Pumpdown time, tp, of a 5 m3 vessel using a rotary plunger pump E 250 having a
nominal pumping speed of 250 m3/h with (a) and without (b) gas ballast, as well as
Roots/rotary plunger pump combination WA 1001 / E250 for a cut-in pressure of 10
mbar for the WA 1001 (e).
During the first quarter of drying time far more than half the quantity of
water vapor is evolved. Then the condenser is the actual main pump.
Because of the high water vapor temperature and, at the beginning of the
drying, the very high water vapor pressure, the condensation efficiency of
the condenser is significantly increased. In Fig. 2.78 it is understood that
two parallel condensers each of 1 m2 condensation surface can together
condense about 15 l of water at an inlet pressure of 100 mbar in 15 min.
However, during this initial process, it must be ensured that the water vapor
pressure at the inlet port of the rotary piston pump does not exceed 60
71
Vacuum generation
2. Predrying
During predrying – depending on the pressure region in which the work is
carried out – about 75 % of the moisture is drawn off. This predrying should
occupy the first third of the drying time. The rate at which predrying
proceeds depends almost exclusively on the sufficiency of the heat supply.
For predrying 1 ton of paper in 5 h, 60 kg of water must be evaporated; that
is, an energy expenditure of about 40 kWh is needed to evaporate water.
Since the paper must be heated to a temperature of about 120 °C at the
same time, an average of about 20 kW must be provided. The mean vapor
evolution per hour amounts to 12 kg. Therefore, a condenser with a
capacity of 15 kg/h should be sufficient. If the paper is sufficiently preheated
(perhaps by air-circulation drying) before evacuation, in the first hour of
drying, double vapor evolution must be anticipated.
1 Vacuum chamber with salt
filling
2 RUVAC WA 501
3 Condensers
4 Throttle valve
5 Rotary plunger pump
Fig. 2.78 Vacuum diagram for drying of salt. Pump combination consisting of Roots pump, condenser and rotary plunger pump for stepwise switching of the pumping process (see
text)
mbar (see Section 2.15 for further details). Since the backing pump has
only to pump away the small part of the noncondensable gases at this
stage, a single-stage rotary piston pump TRIVAC S 65 B will suffice. With
increasing process time, the water vapor evolution decreases, as does the
water vapor pressure in the condenser. After the water pressure in the
chamber falls below 27 mbar, the Roots pump (say, a Roots pump RUVAC
WA 501) is switched in. Thereby the water vapor is pumped more rapidly
out of the chamber, the pressure increases in the condensers, and their
condensation efficiency again increases. The condensers are isolated by a
valve when their water vapor reaches its saturation vapor pressure. At this
point, there is a water vapor pressure in the chamber of only about 4 mbar,
and pumping is accomplished by the Roots pump with a gas ballast
backing pump until the water vapor pressure reaches about 0.65 mbar.
From experience it can be assumed that the salt has now reached the
desired degree of dryness.
Drying of paper
If the pumps are to be of the correct size for a longer process run, it is
expedient to break down the process run into characteristic sections. As an
example, paper drying is explained in the following where the paper has
an initial moisture content of 8 %, and the vessel has the volume V.
1. Evacuation
The backing pump must be suitably rated with regard to the volume of the
vessel and the desired pump-down time. This pump-down time is arranged
according to the desired process duration: if the process is to be finished
after 12 – 15 h, the pump-down time should not last longer than 1 h. The
size of the backing pump may be easily calculated according to Section
2.3.1.
3. Main drying
If, in the second stage, the pressure in a further 5 h is to fall from 20 to
about 5.3 mbar and 75 % of the total moisture (i.e., 19 % of the total
moisture of 15 kg) is to be drawn off, the pump must, according to
equations (2.37) and (2.38), have a pumping speed of
Seff =
V · Δp
t·p
According to equation 1.7, 15 kg of water vapor corresponds at 15 °C to a
quantity of water vapor of
V · Δp =
m · R · T 15 ·83.14 · 288 ≈
=
M
18
≈ 20000 mbar · m3
Seff =
subsequently
20000
= 750 m 3/h
5 · 5. 3
Hence the Roots pump RUVAC WA 1001 would be the suitable pump. The
permissible remaining moisture in the product determines the attainable
ultimate pressure. The relationship between the ultimate pressure and the
remaining moisture is fixed for every product but different from product to
product. LEYBOLD has many years of experience to its record regarding
applications in this area. Assume that a 0.1 % residual moisture content is
required, for which the necessary ultimate pressure is 6 · 10-2 mbar. During
the last 5 h the remaining 6 % of the moisture content, or 5 kg of water, is
removed. At a mean pressure of about 0.65 mbar, 2000 m3/h of vapor is
evolved. Two possibilities are offered:
a) One continues working with the above-mentioned Roots pump
WA 1001. The ultimate total pressure settles at a value according to the
water vapor quantity evolving. One waits until a pressure of about
6.5 · 10-2 mbar is reached, which naturally takes a longer time.
b From the beginning, a somewhat larger Roots pump is chosen (e.g., the
RUVAC WA 2001 with a pumping speed of 2000 m3/h is suitable). For
larger quantities of paper (5000 kg, for example) such a pumping system will be suitable which at a pumping speed for water vapor of up to
20,000 m3/h automatically lowers the pressure from 27 to 10-2 mbar.
The entire time need for drying is significantly reduced when using such
pumps.
2.3.6 Flanges and their seals
72
Vacuum generation
In general, demountable joints in metallic vacuum components, pumps,
valves, tubulations, and so on are provided with flanges. Vacuum
components for rough, medium, and high vacuum from LEYBOLD are
equipped with the following standardized flange systems:
range requires a basically different approach and the use of CF flanges
fitted with metallic sealing rings.
• Small flanges (KF) (quick-action connections to DIN 28 403) of nominal
widths 10, 16, 20, 25, 32, 40 and 50 mm. The values 10, 16, 25 and 40
are preferred widths according to the PNEUROP recommendations and
the ISO recommendations of the technical committee ISO/TC 112 (see
also Section 11). For a complete connection of two identical flanges one
clamping ring and one centering ring are required.
2.3.7 Choice of suitable valves
• Clamp flanges (ISO-K) of nominal widths 65, 100, 160, 250, 320, 400,
500 and 630 mm. Also, these flanges correspond to the nominal widths
and construction of the PNEUROP and ISO/TC 112 recommendations.
Clamp flanges are joined together by clamps or collar rings. Centering
rings or gaskets are needed for sealing.
• Bolted flanges (ISO-F) for the same nominal widths as above
(according to PNEUROP and ISO/TC 112). In special cases bolted
flanges having a smaller nominal width are used. Clamp flanges and
bolted flanges are in accordance with DIN 28 404.
The nominal width is approximately equal to the free inner diameter of the
flange in millimeters; greater deviations are exceptions, so the clamp flange
DN 63 has an inner diameter of 70 mm. See also Table XI in Section 9).
High vacuum components are made of aluminum or stainless steel.
Stainless steel is slightly more expensive but offers a variety of advantages:
lower degassing rate, corrosion resistant, can be degassed at temperatures
up to 200 °C, metal seals are possible and stainless steel is much more
resistant to scratching compared to aluminum.
Vacuum technology puts great demands on the functioning and reliability of
the valves, which are often needed in large numbers in a plant. The
demands are fulfilled only if correct shut-off devices are installed for each
application, depending on the method of construction, method of operation,
and size. Moreover, in the construction and operation of vacuum plants,
factors such as the flow conductance and leak-tightness of valves are of
great importance.
Valves are constructed so that they will not throttle pumping speed. Hence,
when opened fully, their conductance in the rough and medium vacuum
regions equals that of corresponding tube components. For example, the
conductance of a right-angle valve will equal the conductance of a bent
tube of the same nominal bore and angle. Similarly, the conductance of the
valve for molecular flow (i. e., in the high and ultrahigh vacuum regions), is
so high that no significant throttling occurs. Actual values for the
conductance of various components are given in the catalog.
To meet stringent leak-tightness demands, high-quality vacuum valves are
designed so that gas molecules adhering to the surface of the valve shaft
are not transferred from the outer atmosphere into the vacuum during
operation. Such valves are therefore equipped with metal bellows for
isolating the valve shaft from the atmosphere, or alternatively, they are fully
encapsulated, that is, only static seals exist between atmosphere and
vacuum. This group is comprised of all medium and high vacuum valves
from LEYBOLD that are operated either manually or electropneumatically
(Fig. 2.80) and (Fig. 2.79). The leak rate of these valves is less than
10-9 mbar · l/s.
Ultrahigh vacuum components are made of stainless steel and have CF
flanges bakeable to a high temperature. These components, including the
flanges, are manufactured in a series production, starting with a nominal
width of 16 up to 250 mm. CF flanges are available as fixed flanges or also
with rotatable collar flanges. They may be linked with CONFLAT flanges
from almost all manufacturers. Copper gaskets are used for sealing
purposes.
Basically, the flanges should not be smaller than the connecting tubes and
the components that are joined to them. When no aggressive gases and
vapors are pumped and the vacuum system is not exposed to a
temperature above 80 °C, sealing with NBR (Perbunan) or CR (Neoprene)
flange O-rings is satisfactory for work in the rough-, medium-, and high vacuum regions. This is often the case when testing the operation of vacuum
systems before they are finally assembled.
All stainless steel flanges may be degassed at temperatures up to 200 °C
without impairment. However, then Perbunan sealing material is not
suitable as a flange sealant. Rather, VITILAN® (a special FPM) sealing
rings and also aluminum seals, which allow heating processes up to
150 °C and 200 °C respectively, should be used. After such degassing,
pressures down to 10-8 mbar, i.e. down to the UHV range, can be attained
in vacuum systems.
Generating pressures below 10-8 mbar requires higher bake-out
temperatures. As explained above (see Section 2.2.6) work in the UHV
1 Casing
2 Valve disk
3 Compression spring
4 Solenoid coil
Fig. 2.79 Right angle vacuum valve with solenoid actuator
73
Vacuum generation
and motor driven, such as variable-leak valves. The variety is even more
enhanced by the various housing designs. In addition to the various
materials used, right-angle and straight-through valves are required.
Depending on their nominal width and intended application, flanges fitted to
valves may be small (KF), clamp (ISO-K), bolted (ISO-F), or UHV (CF).
In addition to the vacuum valves, which perform solely an isolation function
(fully open – fully closed position), special valves are needed for special
functions. Typical are variable leak valves, which cover the leakage range
from 10-10 cm3/s (NTP) up to 1.6 · 103 cm3/s (NTP). These valves are
usually motor driven and suitable for remote control and when they are
connected to a pressure gauge, the process pressures can be set and
maintained. Other special valves fulfill safety functions, such as rapid,
automatic cut-off of diffusion pumps or vacuum systems in the event of a
power failure. For example, SECUVAC valves belong to this group. In the
event of a power failure, they cut off the vacuum system from the pumping
system and vent the forevacuum system. The vacuum system is enabled
only after a certain minimum pressure (about 200 mbar) has been attained
once the power has been restored.
1 Casing
2 Valve disk
3 Bellows
4 Compressed air supply
5 Piston
Fig. 2.80 Right angle vacuum valve with electropneumatic actuator
Valves sealed with oil or grease can be used for highly stringent demands.
Their leakage rate is also about 10-9 mbar · l/s. However, a special case is
the pendulum-type gate valve. Despite its grease-covered seal, the leak
rate between vacuum and external atmosphere is virtually the same as for
bellows-sealed valves because when the valve is in operation the shaft
carries out only a rotary motion so that no gas molecules are transferred
into the vacuum. Pendulum-type gate valves are not manufactured by
LEYBOLD.
For working pressures down to 10-7 mbar, valves of standard design suffice
because their seals and the housing materials are such that permeation
and outgassing are insignificant to the actual process. If pressures down to
10-9 mbar are required, baking up to 200 °C is usually necessary, which
requires heat resistant sealing materials (e.g., VITILANh) and materials of
high mechanical strength, with prepared (inner) surfaces and a low
outgassing rate. Such valves are usually made of stainless steel. Flange
connections are sealed with aluminum gaskets, so permeation problems of
elastomer seals are avoided. In the UHV range these issues are of special
significance so that mainly metallic seals must be used. The gas molecules
bonded to the surface of the materials have, at pressures below 10-9 mbar,
a very great influence. They can only be pumped away within a reasonable
period of time by simultaneous degassing. Degassing temperatures up to
500 °C required in UHV systems, pose special requirements on the sealing
materials and the entire sealing geometry. Gaskets made of gold or copper
must be used.
The various applications require valves with different drives, that is, valves
that are manually operated, electropneumatically- or magnetically-operated,
74
When aggressive gases or vapors have to be pumped, valves made of
stainless steel and sealed with VITILAN® sealant are usually used. For
nuclear technology, valves have been developed that are sealed with
special elastomer or metal gaskets. We will be pleased to provide further
design information for your area of application upon request.
Vacuum generation
2.3.8 Gas locks and seal-off fittings
In many cases it is desirable not only to be able to seal off gas-filled or
evacuated vessels, but also to be in a position to check the pressure or the
vacuum in these vessels at some later time and to post-evacuate or
supplement or exchange the gas filling.
This can be done quite easily with a seal-off fitting from LEYBOLD which is
actuated via a corresponding gas lock. The small flange connection of the
evacuated or gas-filled vessel is hermetically sealed off within the tube by a
small closure piece which forms the actual valve. The gas lock required for
actuation is removed after evacuation or filling with gas. Thus one gas lock
will do to actuate any number of seal-off fittings. Shown in Fig. 2.81 is a
sectional view of such an arrangement. Gas locks and seal-off fittings are
manufactured by LEYBOLD having a nominal width of DN 16 KF, DN 25 KF
and DN 40 KF. They are made of stainless steel. The leak rate of the sealoff fittings is less than 1 · 10-9 mbar l/s. They can sustain overpressures up
to 2.5 bar, are temperature resistant up to 150 °C and may be protected
against dirt by a standard blank flange.
Typical application examples are double-walled vessels with an insulating
vacuum, like Dewar vessels, liquid gas vessels (tanks) or long distance
energy pipelines and many more. They are also used for evacuation or
post-evacuation of reference and support vacua in scientific instruments
seal-off fittings with gas locks are often used. Previously it was necessary
to have a pump permanently connected in order to post-evacuate as
required. Through the use of gas locks with seal-off fittings a vacuum-tight
seal is provided for the vessel and the pump is only required from time to
time for checking or post-evacuation.
h
DN
a
a
h1
h2
d
Fig. 2.81 Gas lock with centering ring and seal-off fitting, sectional view
75
Vacuum measurement
3.
Vacuum measurement, monitoring, control
and regulation
The pressures measured in vacuum technology today cover a range from
1013 mbar to 10-12 mbar, i.e. over 15 orders of magnitude. The enormous
dynamics involved here can be shown through an analogy analysis of vacuum pressure measurement and length measurement, as depicted in Table
3.1.
Analogy analysis
Determination by
means of
empirical world
of human beings
simple measuring
methods
mechanical
measuring
methods
indirect
methods
extreme indirect
methods
Absolute
pressure
Length
1 bar
1m
> 1 mbar
> 1 mm
> 10–3 mbar
> 1 mm
10–9 mbar
≈ 1/100 atom∅
≈ 0.18
electron∅
10–12 mbar
Relative measurement uncertainty (%)
Measuring instruments designated as vacuum gauges are used for measurement in this broad pressure range. Since it is impossible for physical
reasons to build a vacuum gauge which can carry out quantitative measurements in the entire vacuum range, a series of vacuum gauges is available,
each of which has a characteristic measuring range that usually extends over
several orders of magnitude (see Fig. 9.16a). In order to be able to allocate
the largest possible measuring ranges to the individual types of vacuum
gauges, one accepts the fact that the measurement uncertainty rises very
rapidly, by up to 100 % in some cases, at the upper and lower range limits.
This interrelationship is shown in Fig. 3.1 using the example of the VISCOVAC. Therefore, a distinction must be made between the measuring range
as stated in the catalogue and the measuring range for “precise” measurement. The measuring ranges of the individual vacuum gauges are limited in
the upper and lower range by physical effects.
Vacuum gauges are devices for measuring gas pressures below atmospheric pressure (DIN 28 400, Part 3, 1992 issue). In many cases the pressure indication depends on the nature of the gas. With compression vacuum gauges it should be noted that if vapors are present, condensation may
occur due to the compression, as a result of which the pressure indication
is falsified. Compression vacuum gauges measure the sum of the partial
pressures of all gas components that do not condense during the measurement procedure. In the case of mechanically compressing pumps, the final
partial pressure can be measured in this way (see 1.1). Another way of
measuring this pressure, is to freeze out the condensable components in
an LN2 cold trap. Exact measurement of partial pressures of
certain gases or vapors is carried out with the aid of partial pressure measuring instruments which operate on the mass spectrometer principle (see
section 4).
1. Instruments that by definition measure the pressure as the force which
acts on an area, the so-called direct or absolute vacuum gauges.
According to the kinetic theory of gases, this force, which the particles
exert through their impact on the wall, depends only on the number of
gas molecules per unit volume (number density of molecules n) and
their temperature, but not on their molar mass. The reading of the measuring instrument is independent of the type of gas. Such units
include liquid-filled vacuum gauges and mechanical vacuum gauges.
2. Instruments with indirect pressure measurement. In this case, the
pressure is determined as a function of a pressure-dependent (or more
accurately, density-dependent) property (thermal conductivity, ionization
probability, electrical conductivity) of the gas. These properties are
dependent on the molar mass as well as on the pressure. The pressure
reading of the measuring instrument depends on the type of gas.
The scales of these pressure measuring instruments are always based on
air or nitrogen as the test gas. For other gases or vapors correction factors,
usually based on air or nitrogen, must be given (see Table 3.2). For precise
pressure measurement with indirectly measuring vacuum gauges that
determine the number density through the application of electrical energy
(indirect pressure measurement), it is important to know the gas composition. In practice, the gas composition is known only as a rough approximation. In many cases, however, it is sufficient to know whether light or heavy
molecules predominate in the gas mixture whose pressure is to be measured (e.g. hydrogen or pump fluid vapor molecules).
20
Example: If the pressure of a gas essentially consisting of pump fluid molecules is measured with an ionization vacuum gauge, then the pressure
reading (applying to air or N2), as shown in Table 3.2, is too high by a factor
of about 10.
“favorable measuring range”
15
(relative measurement uncertainty < 5%)
10
5
1
–6
10
76
Fundamentals of low-pressure measurement
Dependence of the pressure indication on the type of gas
A distinction must be made between the following vacuum gauges:
Table 3.1
Fig. 3.1
3.1
10–5
10–4
10–3
Pressure (mbar)
10–2
10–1
Measurement uncertainty distribution over the measuring range: VISCOVAC
1
Measurement of pressures in the rough vacuum range can be carried out
relatively precisely by means of vacuum gauges with direct pressure measurement. Measurement of lower pressures, on the other hand, is almost
always subject to a number of fundamental errors that limit the measuring
accuracy right from the start so that it is not comparable at all to the degree
Vacuum measurement
of accuracy usually achieved with measuring instruments. In order to measure pressure in the medium and high vacuum ranges with a measurement
uncertainty of less than 50 %, the person conducting the experiment must
proceed with extreme care. Pressure measure-ments that need to be accurate to a few percent require great effort and, in general, the deployment of
special measuring instruments. This applies particularly to all pressure measurements in the ultrahigh vacuum range
(p < 10-7 mbar).
To be able to make a meaningful statement about a pressure indicated by
a vacuum gauge, one first has to take into account at what location and in
what way the measuring system is connected. In all pressure areas where
laminar flows prevail (1013 > p > 10-1 mbar), note must be taken of pressure gradients caused by pumping. Immediately in front of the pump (as
seen from the vessel), a lower pressure is created than in the vessel. Even
components having a high conductance may create such a pressure gradient. Finally, the conductance of the connecting line between the vacuum
system and the measuring system must not be too small because the line
will otherwise be evacuated too slowly in the pressure region of laminar
flow so that the indicated pressure is too high.
The situation is more complicated in the case of high and ultrahigh vacuum.
According to the specific installation features, an excessively high pressure
or, in the case of well-degassed measuring tubes, an excessively low pressure may be recorded due to outgassing of the walls of the vacuum gauge
or inadequate degassing of the measuring system. In high and ultrahigh
vacuum, pressure equalization between the vacuum system and the measuring tubes may take a long time. If possible, so-called nude gauges are
used. The latter are inserted directly in the vacuum system, flange-mounted, without a connecting line or an envelope. Special consideration must
always be given to the influence of the measuring process itself on the
pressure measurement. For example, in ionization vacuum gauges that
work with a hot cathode, gas particles, especially those of the higher hydrocarbons, are thermally broken down. This alters the gas composition. Such
effects play a role in connection with pressure measurement in the ultrahigh
vacuum range. The same applies to gas clean-up in ionization vacuum
gauges, in particular Penning gauges (of the order of 10-2 to 10-1 l/s).
Contamination of the measuring system, interfering electrical and magnetic
fields, insulation errors and inadmissibly high ambient temperatures falsify
pressure measurement. The consequences of these avoidable errors and
the necessary remedies are indicated in the discussion of the individual
measuring systems and in summary form in section 8.4.
3.2
Vacuum gauges with pressure reading that
is independent of the type of gas
Mechanical vacuum gauges measure the pressure directly by recording the
force which the particles (molecules and atoms) in a gas-filled space exert
on a surface by virtue of their thermal velocity.
3.2.1 Bourdon vacuum gauges
The interior of a tube bent into a circular arc (so-called Bourdon tube) (3) is
connected to the vessel to be evacuated (Fig. 3.2). Through the effect of
the external air pressure the end of the tube is deflected to a greater or
lesser extent during evacuation and the attached pointer mechanism (4)
and (2) is actuated. Since the pressure reading depends on the external
atmospheric pressure, it is accurate only to approximately 10 mbar, provided that the change in the ambient atmospheric pressure is not corrected.
3.2.2 Diaphragm vacuum gauges
3.2.2.1 Capsule vacuum gauges
The best-known design of a diaphragm vacuum gauge is a barometer with
an aneroid capsule as the measuring system. It contains a hermetically
sealed, evacuated, thin-walled diaphragm capsule made of a copper-beryllium alloy. As the pressure drops, the capsule diaphragm expands. This
movement is transmitted to a point by a lever system. The capsule vacuum gauge, designed according to this principle, indicates the pressure on a
linear scale, independent of the external atmospheric pressure.
Selection of vacuum gauges
The desired pressure range is not the only factor considered when selecting a suitable measuring instrument. The operating conditions under which
the gauge works also play an important role. If measurements are to be
carried out under difficult operating conditions, i.e. if there is a high risk of
contamination, vibrations in the tubes cannot be ruled out, air bursts can be
expected, etc., then the measuring instrument must be robust. In industrial
facilities, Bourdon gauges, diaphragm vacuum gauges, thermal conductivity
vacuum gauges, hot cathode ionization vacuum gauges and Penning vacuum gauges are used. Some of these measuring instruments are sensitive to
adverse operating conditions. They should and can only be used successfully if the above mentioned sources of errors are excluded as far as possible and the operating instructions are followed.
1 Connecting tube to
connection flange
2 Pointer
Fig. 3.2
3 Bourdon tube
4 Lever system
Cross-section of a Bourdon gauge
77
Vacuum measurement
3.2.2.2
DIAVAC diaphragm vacuum gauge
The most accurate pressure reading possible is frequently required for levels below 50 mbar. In this case, a different diaphragm vacuum gauge is
more suitable, i.e. the DIAVAC, whose pressure scale is considerably
extended between 1 and 100 mbar. The section of the interior in which the
lever system (2) of the gauge head is located (see Fig. 3.3) is evacuated to
a reference pressure pref of less than 10-3 mbar. The closure to the vessel
is in the form of a corrugated diaphragm (4) of special steel. As long as the
vessel is not evacuated, this diaphragm is pressed firmly against the wall
(1). As evacuation increases, the difference between the pressure to be
measured px and the reference pressure decreases. The diaphragm bends
only slightly at first, but then below 100 mbar to a greater degree. With the
DIAVAC the diaphragm deflection is again transmitted to a pointer (9). In
particular the measuring range between 1 and 20 mbar is considerably
extended so that the pressure can be read quite accurately (to about 0.3
mbar). The sensitivity to vibration of this instrument is somewhat higher
than for the capsule vacuum gauge.
3.2.2.3
Precision diaphragm vacuum gauges
A significantly higher measuring accuracy than that of the capsule vacuum
gauge and the DIAVAC is achieved by the precision diaphragm vacuum
gauge. The design of these vacuum gauges resembles that of capsule vacuum gauges. The scale is linear. The obtainable degree of precision is the
maximum possible with present-day state-of-the-art equipment. These
instruments permit measurement of 10-1 mbar with a full-scale deflection of
20 mbar. The greater degree of precision also means a higher sensitivity to
vibration.
Capsule vacuum gauges measure pressure accurately to 10 mbar (due to
the linear scale, they are least accurate at the low pressure end of the
scale). If only pressures below 30 mbar are to be measured, the DIAVAC
Fig. 3.4
Piezoelectric sensor (basic diagram)
is recommended because its reading (see above) is considerably more
accurate. For extremely precise measuring accuracy requirements precision
diaphragm vacuum gauges should be used. If low pressures have to be
measured accurately and for this reason a measuring range of, for example, up to 20 mbar is selected, higher pressures can no longer be measured since these gauges have a linear scale. All mechanical vacuum
gauges are sensitive to vibration to some extent. Small vibrations, such as
those that arise in the case of direct connection to a backing pump, are
generally not detrimental.
3.2.2.4
Capacitance diaphragm gauges
Deflection of a diaphragm can also be electrically measured as “strain” or
as a change in capacitance. In the past, four strain gauges, which change
their resistance when the diaphragm is deflected, i.e. under tensile load,
were mounted on a metallic diaphragm in a bridge circuit. At LEYBOLD
such instruments have been given a special designation, i.e. MEMBRANOVAC. Later, silicon diaphragms that contained four such “strain resistances” directly on their surface were used. The electrical arrangement
again consisted of a bridge circuit, and a constant current was fed in at two
opposite corner points while a linear voltage signal proportional to the pressure was picked up at the two other corner points. Fig. 3.4 illustrates the
principle of this arrangement. Such instruments were designated as PIEZOVAC units and are still in use in many cases. Today the deflection of the
diaphragm is measured as the change in capacitance of a plate capacitor:
one electrode is fixed, the other is formed by the diaphragm. When the
1
2
C1
1
2
3
4
5
Base plate
Lever system
Connecting flange
Diaphragm
Reference pressure
pref
6 Pinch-off end
78
7
8
9
10
11
12
Mirror sheet
Plexiglass sheet
Pointer
Glass bett
Mounting plate
Housing
C2
p1
p2
Vacuum measurement
C ~ A/d
C = capacitance
A = area
d = distance
Diaphragm
(Inconel)
Reference chamber closure
Grid + reference chamber closure
Amplifier
+ 15 V DC
– 15 V DC
C ~ A/d
C = capacitance
A = area
d = distance
Capacitor plates
Diaphragm
(ceramic)
Sensor body
(ceramic)
Signal converter
+ amplifier
0 – 10 V
24 V DC, 4 – 20 mA
Electrode
left: CAPACITRON (Inconel diaphragm)
Fig. 3.6
Capacitive sensors (basic diagram)
diaphragm is deflected, the distance between the electrodes and thus
capacitance of the capacitor is altered. Fig. 3.5 illustrates the principle of
this arrangement. A distinction is made between sensors with metallic and
those with ceramic diaphragms. The structure of the two types is similar
and is shown on the basis of two examples in Fig. 3.6. Capacitance
diaphragm gauges are used from atmospheric pressure to 1·10-3 mbar
(from 10-4 mbar the measurement uncertainty rises rapidly). To ensure sufficient deflection of the diaphragms at such low pressures, diaphragms of
varying thicknesses are used for the various pressure levels. In each case,
the pressure can be measured with the sensors to an accuracy of 3 powers
of ten:
1013 to 1 mbar
100 to 10–1 mbar
10
to 10–2 mbar und
1
to 10–3 mbar.
If the pressures to be measured exceed these range limits, it is recommended that a multichannel unit with two or three sensors be used, possibly with automatic channel changeover. The capacitance diaphragm gauge
thus represents, for all practical purposes, the only absolute pressure measuring instrument that is independent of the type of gas and designed for
pressures under 1 mbar. Today two types of capacitive sensors are available:
1) Sensors DI 200 and DI 2000 with aluminum oxide diaphragms, which
are particularly overload-free, with the MEMBRANOVAC DM 11 and
DM 12 units.
2) Sensors with Inconel diaphragms CM 1, DM 10, CM 100, CM 1000
with extremely high resolution, with the DM 21 and DM 22 units.
3.2.3 Liquid-filled (mercury) vacuum gauges
3.2.3.1
right: MEMBRANOVAC (aluminum oxide diaphragm)
U-tube vacuum gauges
U-tube vacuum gauges filled with mercury are the simplest and most exact
instruments for measuring pressure in the rough vacuum range (1013 to a
few mbar). Unfortunately their use in technical plants is limited because of
their size and proneness to breakage (see 3.4.1a).
In the evacuated limb of the U-tube vacuum gauge a constant pressure is
maintained equal to the vapor pressure of mercury at room temperature
(about 10-3 mbar). The other limb is connected to the volume in which the
pressure is to be measured. From the difference in the levels of the two
columns, the pressure to be measured can be determined on the mbar
scale provided. The reading is independent of the atmospheric pressure.
3.2.3.2
Compression vacuum gauges (according to
McLeod)
The compression vacuum gauge developed by McLeod in 1874 is a very
rarely used type of vacuum gauge today. In its refined form the instrument
can be used for absolute pressure measurement in the high vacuum range
down to 10-5 mbar. In the past it was frequently used as a reference instrument for the calibration of medium and sometimes also of high vacuum
gauges. For such measurements, however, numerous precautionary rules
had to be taken into account before it was possible to assess the measuring accuracy. The pressure is measured by compressing a quantity of gas
that initially occupies a large volume into a smaller volume by raising a mercury level. The increased pressure obtained in this manner can be measured in the same way as in a U-tube manometer and from it the original
pressure is calculated (see equations below).
According to the type of scale division, a distinction is made between two
forms of compression vacuum gauges: those with a linear scale (see Fig.
3.7) and those with a square-law scale (see Fig. 3.8). In the case of the
compression vacuum gauges of the McLeod linear-scale type, the ratio of
the enclosed residual volume Vc to the total volume V must be known for
each height of the mercury level in the measurement capillary; this ratio is
shown on the scale provided with the instrument. In the case of compression vacuum gauges with a square-law scale, the total volume and the capillary diameter d must be known.
Nowadays a “shortened” McLeod type compression vacuum gauge according to Kammerer is used to measure the “partial final pressure” of
mechanically compressing pumps. Through the high degree of compres-
79
Vacuum measurement
Upper limit for:
d = 2.5 mm
measuring
range
Upper limit for:
Vcmax. = 0.1 cm3
hmax. = 100 mm
Lower limit for:
d = 1 mm
Volume V [cm3]
Volume V [cm3]
Fig. 3.8
McLeod compression vacuum gauge with linear scale (equation 3.1b)
sion the condensable gas components (vapors) are discharged as liquid
(the volume of the same mass is then smaller by a factor of around 105 and
can be neglected in the measurement) so that only the pressure of the permanently gaseous components is measured (this is where the expression
permanent gases comes from).
Principle of measurement with compression vacuum gauges
If h is the difference in the mercury level between the measurement capillary and the reference capillary (measured in mm), then it follows
from the Boyle-Mariotte law:
p · V = (p + h) · Vc
p = h⋅
(3.1)
Vc
V − Vc
(3.1a)
Vc
V
(3.1b)
Vc and V must be known, h is read off (linear scale).
These relationships remain unchanged if the difference in level is read off a
scale with mbar division. The pressure is then obtained in mbar:
p = 4 ⋅ h ⋅ Vc
3 V
h in mm
(3.1c)
If during measurement the mercury level in the measurement capillary is
always set so that the mercury level in the reference capillary corresponds
to the upper end of the measurement capillary (see Figs. 3.7 and 3.8), the
volume Vc is always given by:
Vc = h ⋅ π ⋅ d 2
4
h ....difference in level, see Fig. 3.5
d ....inside diameter of measurement capillary
If this term is substituted for Vc in equation (3.1b), the result is:
80
McLeod compression vacuum gauge with square-law scale (equation 3.1f)
p = h2 ⋅ π ⋅ d
4 V
2
(3.1e)
that is, a square-law scale in mm (torr) if d and V are measured in mm or
mm3. If the scale is to be divided into mbar, then the equation is:
2
p = h2 ⋅ π ⋅ d
3 V
(3.1f)
where h in mm
d in mm
and
V in mm3
Compression vacuum gauges ensure a reading of the sum of all partial
pressures of the permanent gases, provided that no vapors are present that
condense during the compression procedure.
p measured in mm of mercury (= torr). If Vc << V, then:
p = h⋅
measuring
range
Lower limit for:
Vcmin. = 5 · 10–3
cm3hmin. = 1 mm
Fig. 3.7
Upper limit for:
d = 1 mm
Lower limit for:
d = 2.5 mm
measuring
range
Pressure p
measuring
range
Upper limit for:
Vcmax. = 1 cm3
hmax. = 100 mm
(3.1d)
The measuring range between the top and bottom ends is limited by the
maximum and minimum ratios of the capillary volume to the total volume
(see Figs. 3.7 and 3.8). The accuracy of the pressure measurement
depends to a great extent on the reading accuracy. By using a vernier and
mirror, pressure measurements with an accuracy of ± 2 % can be
achieved. In the low pressure range, where h is very small, this accuracy is
no longer attainable, chiefly because small geometric deviations have a
very noticeable effect at the closed end of the capillary (systematic error).
The presence of vapors that may condense during compression influences
the measurement, often in an indefinite manner. One can easily determine
whether vapors having a pressure that is not negligible are present. This
can be done by setting different heights h in the measurement capillary
under constant pressure while using the linear scale and then calculating p
according to equation 3.1b. If no vapors are present, or only those whose
pressure is negligible at room temperature (such as mercury), then the
same value of p must result for each h.
The scale of compression vacuum gauges can be calculated from the geometric dimensions. This is why they were used in the past by official calibration stations as normal pressure (see equation 3.4.1a).
Vacuum measurement
3.3
Vacuum gauges with gas-dependent pressure reading
This type of vacuum gauge does not measure the pressure directly as an
area-related force, but indirectly by means of other physical variables that
are proportional to the number density of particles and thus to the pressure.
The vacuum gauges with gas-dependent pressure reading include: the
decrement gauge (3.3.1), the thermal conductivity vacuum gauge (3.3.2)
and the ionization vacuum gauge having different designs (3.3.3).
The instruments consist of the actual sensor (gauge head, sensor) and the
control unit required to operate it. The pressure scales or digital displays
are usually based on nitrogen pressures; if the true pressure pT of a gas (or
vapor) has to be determined, the indicated pressure pI must be multiplied
by a factor that is characteristic for this gas. These factors differ, depending
on the type of instrument, and are either given in tabular form as factors
independent of pressure (see Table 3.2) or, if they depend on the pressure,
must be determined on the basis of a diagram (see Fig. 3.11).
1 Ball
2 Measuring tube,
closed at one end, welded
into
connection flange 7
3
4
5
6
7
Permanent magnets
Stabilization coils
4 drive coils
Bubble level
Connection flange
In general, the following applies:
Fig. 3.9
True pressure pT = indicated pressure pI · correction factor
If the pressure is read off a “nitrogen scale” but not corrected, one refers to
“nitrogen equivalent” values.
In all electrical vacuum gauges (they include vacuum gauges that are
dependent on the type of gas) the increasing use of computers has led to
the wish to display the pressure directly on the screen, e.g. to insert it at the
appropriate place in a process flow diagram. To be able to use the most
standardized computer interfaces possible, so-called transmitters (signal
converters with standardized current outputs) are built instead of a sensor
and display unit (e.g. THERMOVAC transmitter, Penning transmitter, IONIVAC transmitter). Transmitters require a supply voltage (e.g. +24 volts) and
deliver a pressure-dependent current signal that is linear over the entire
measuring range from 4 to 20 mA or 0 – 10 V. The pressure reading is not
provided until after supply of this signal to the computer and processing by
the appropriate software and is then displayed directly on the screen.
3.3.1 Spinning rotor gauge (SRG) (VISCOVAC)
Pressure-dependent gas friction at low gas pressures can be utilized to
measure pressures in the medium and high vacuum range. In technical
instruments of this kind a steel ball that has a diameter of several millimeters and is suspended without contact in a magnetic field (see Fig. 3.9) is
used as the measuring element. The ball is set into rotation through an
electromagnetic rotating field: after reaching a starting speed (around 425
Hz), the ball is left to itself. The speed then declines at a rate that depends
on the prevailing pressure under the influence of the pressure-dependent
gas friction. The gas pressure is derived from the relative decline of the
speed f (slowing down) using the following equation:
−f ⋅
df 10 p ⋅ σ
= ⋅
dt π c ⋅ r ⋅ ρ
Cross-section of the gauge head of a VISCOVAC VM 212 spinning rotor gauge (SRG)
p = gas pressure
r = radius of the ball
ρ = density of the ball material
-c = mean speed of the gas particles,
dependent on type of gas
σ = coefficient of friction of the ball, independent of the
type of gas, nearly 1.
As long as a measurement uncertainty of 3 % is sufficient, which is usually
the case, one can apply σ = 1 so that the sensitivity of the spinning rotor
gauge (SRG) with rotating steel ball is given by the calculable physical size
of the ball, i.e. the product radius x density r · ρ (see equation 3.2). Once a
ball has been “calibrated”, it is suitable for use as a “transfer standard”, i.e.
as a reference device for calibrating another vacuum gauge through comparison, and is characterized by high long-term stability. Measurements with
the VISCOVAC are not limited to measurement of the pressure, however.
Other variables involved in the kinetic theory of gases, such as mean free
path, monolayer time, particle number density and impingement rate, can
also be measured. The instrument permits storage of 10 programs and
easy changeover between these programs. The measuring time per slowing-down operation is between 5 seconds for high pressures and about 40
seconds for lower pressures. The measurement sequence of the instrument
is controlled fully automatically by a microprocessor so that a new value is
displayed after every measurement (slowing-down procedure). The programs additionally enable calculation of a number of statistical variables
(arithmetic mean, standard deviation) after a previously specified number of
measurements.
While in the case of the kinetic theory of gases with VISCOVAC the counting of particles directly represents the measuring principle (transferring the
particle pulses to the rotating ball, which is thus slowed down).
(3.2)
81
Vacuum measurement
With other electrical measuring methods that are dependent on the type
of gas, the particle number density is measured indirectly by means of
the amount of heat lost through the particles (thermal conductivity vacuum
gauge) or by means of the number of ions formed (ionization vacuum
gauge).
3.3.2 Thermal conductivity vacuum gauges
Classical physics teaches and provides experimental confirmation that the
thermal conductivity of a static gas is independent of the pressure at higher
pressures (particle number density), p > 1 mbar. At lower pressures,
p < 1 mbar, however, the thermal conductivity is pressure-dependent
(approximately proportional 1 / 冑앙
M). It decreases in the medium vacuum
range starting from approx. 1 mbar proportionally to the pressure and
reaches a value of zero in the high vacuum range. This pressure dependence is utilized in the thermal conductivity vacuum gauge and enables
precise measurement (dependent on the type of gas) of pressures in the
medium vacuum range.
The most widespread measuring instrument of this kind is the Pirani vacuum gauge. A current-carrying filament with a radius of r1 heated up to
around 100 to 150 °C (Fig. 3.10) gives off the heat generated in it to the
gas surrounding it through radiation and thermal conduction (as well as, of
course, to the supports at the filament ends). In the rough vacuum range
the thermal conduction through gas convection is virtually independent of
pressure (see Fig. 3.10). If, however, at a few mbar, the mean free path of
the gas is of the same order of magnitude as the filament diameter, this
type of heat transfer declines more and more, becoming dependent on the
density and thus on the pressure. Below 10-3 mbar the mean free path of a
gas roughly corresponds to the size of radius r2 of the measuring tubes.
The sensing filament in the gauge head forms a branch of a Wheatstone
bridge. In the THERMOTRON thermal conductivity gauges with variable
resistance which were commonly used in the past, the sensing filament
was heated with a constant current. As gas pressure increases, the temperature of the filament decreases because of the greater thermal
conduction through the gas so that the bridge becomes out of balance. The
bridge current serves as a measure for the gas pressure, which is indicated
on a measuring scale. In the THERMOVAC thermal conductivity gauges
with constant resistance which are almost exclusively built today, the
sensing filament is also a branch of a Wheatstone bridge. The heating voltage applied to this bridge is regulated so that the resistance and therefore
the temperature of the filament remain constant, regardless of the heat
loss. This means that the bridge is always balanced. This mode of regulation involves a time constant of a few milliseconds so that such instruments,
in contrast to those with variable resistance, respond very quickly to pressure changes. The voltage applied to the bridge is a measure of the pressure. The measuring voltage is corrected electronically such that an approximately logarithmic scale is obtained over the entire measuring range.
Thermal conductivity vacuum gauges with constant resistance have a measuring range from 10-4 to 1013 mbar. Due to the very short response time,
they are particularly suitable for controlling and pressure monitoring applications (see section 3.5). The measurement uncertainty varies in the different
pressure ranges. The maximum error at full-scale deflection is about 1 to 2
%. In the most sensitive range, i.e. between 10-3 and 1 mbar, this corresponds to around 10 % of the pressure reading. The measurement uncertainty is significantly greater outside this range.
As in all vacuum gauges dependent on the type of gas, the scales of the
indicating instruments and digital displays in the case of thermal conductivity vacuum gauges also apply to nitrogen and air. Within the limits of error,
the pressure of gases with similar molecular masses, i.e. O2, CO and others, can be read off directly. Calibration curves for a series of gases are
shown in Fig. 3.11.
An extreme example of the discrepancy between true pressure pT and indicated pressure pI in pressure measurement is the admission of air to a vacuum system with argon from a pressure cylinder to avoid moisture (pumping time). According to Fig. 3.11, one would obtain a pI reading of only 40
mbar on reaching an “Ar atmospheric pressure” pT with a THERMOVAC as
N2, O2 air – pI = pT
II
III
r2
r1
l ⱕ r2
l ‹‹ r – r
- 2 1
l ‹‹ r1
True pressure [mbar] pW
Heat loss
Abgeführter
Wärmefluß
I
l ›› r1
pI < p T
pI > p T
10–5
10–4
I Thermal dissipation due
to radiation and conduction
in the metallic ends
II Thermal dissipation due
10–3
10–2
10–1
1
Druck
[mbar][mbar]
Pressure
10
100
to the gas, pressure-dependent
III Thermal dissipation due
to radiation and convection
Fig. 3.10 Dependence of the amount heat dissipated by a heated filament (radius r1) in a tube
(radius r2) at a constant temperature difference on the gas pressure (schematic diagram).
82
Indicated pressure [mbar] pI
Fig. 3.11 Calibration curves of THERMOVAC gauges for various gases, based on nitrogen
equivalent reading
Vacuum measurement
a pressure measuring instrument. Argon might escape from the vessel
(cover opens, bell jar rises). For such and similar applications, pressure
switches or vacuum gauges that are independent of the type of gas must
be used (see section 3.2).
3.3.3 Ionization vacuum gauges
Ionization vacuum gauges are the most important instruments for measuring gas pressures in the high and ultrahigh vacuum ranges. They measure
the pressure in terms of the number density of particles proportional to the
pressure. The gas whose pressure is to be measured enters the gauge
heads of the instruments and is partially ionized with the help of an electric
field. Ionization takes place when electrons are accelerated in the electric
field and attain sufficient energy to form positive ions on impact with gas
molecules. These ions transmit their charge to a measuring electrode (ion
collector) in the system. The ion current, generated in this manner (or, more
precisely, the electron current in the feed line of the measuring electrode
that is required to neutralize these ions) is a measure of the pressure
because the ion yield is proportional to the particle number density and thus
to the pressure.
The formation of ions is a consequence of either a discharge at a high electric field strength (so-called cold-cathode or Penning discharge, see 3.3.3.1)
or the impact of electrons that are emitted from a hot cathode (see 3.3.3.2).
Under otherwise constant conditions, the ion yield and thus the ion current
depend on the type of gas since some gases are easier to ionize than others. As all vacuum gauges with a pressure reading that is dependent on the
type of gas, ionization vacuum gauges are calibrated with nitrogen as the
reference gas (nitrogen equivalent pressure, see 3.3). To obtain the true
pressure for gases other than nitrogen, the read-off pressure must be multiplied by the correction factor given in Table 3.2 for the gas concerned. The
factors stated in Table 3.2 are assumed to be independent of the pressure,
though they depend somewhat on the geometry of the electrode system.
Therefore, they are to be regarded as average values for various types of
ionization vacuum gauges (see Fig. 3.16).
Given the presence
of predominantly
(type of gas)
He
Ne
Ar
Kr
Xe
Hg
H2
CO
CO2
CH4
higher
hydrocarbons
Table 3.2 Correction factors
Correction factor
based on N2
(nitrogen = 1)
6.9
4.35
0.83
0.59
0.33
0.303
2.4
0.92
0.69
0.8
3.3.3.1
Cold-cathode ionization vacuum gauges
(Penning vacuum gauges)
Ionization vacuum gauges which operate with cold discharge are called
cold-cathode- or Penning vacuum gauges. The discharge process in a
measuring tube is, in principle, the same as in the electrode system of a
sputter ion pump (see section 2.1.8.3). A common feature of all types of
cold-cathode ionization vacuum gauges is that they contain just two unheated electrodes, a cathode and an anode, between which a so-called cold
discharge is initiated and maintained by means of a d.c. voltage (of around
2 kV) so that the discharge continues at very low pressures. This is
achieved by using a magnetic field to make the paths of the electrons long
enough so that the rate of their collision with gas molecules is sufficiently
large to form the number of charge carriers required to maintain the discharge. The magnetic field (see Fig. 3.12) is arranged such that the magnetic field lines of force cross the electric field lines. In this way the electrons are confined to a spiral path. The positive and negative charge carriers produced by collision move to the corresponding electrodes and form
the pressure-dependent discharge current, which is indicated on the meter.
The reading in mbar depends on the type of gas. The upper limit of the
measuring range is given by the fact that above a level of several
10-2 mbar the Penning discharge changes to a glow discharge with intense
light output in which the current (at constant voltage) depends only to a
small extent on the pressure and is therefore not suitable for measurement
purposes. In all Penning gauges there is considerably higher gas sorption
than in ionization vacuum gauges that operate with a hot cathode. A
Penning measuring tube pumps gases similarly to a sputter ion pump
(S ≈ 10-2 l/s). Here again the ions produced in the discharge are accelerated towards the cathode where they are partly retained and partly cause
sputtering of the cathode material. The sputtered cathode material forms a
1 Small flange DN 25 KF;
DN 40 KF
2 Housing
3 Ring anode with ignition pin
4 Ceramic washer
5
6
7
8
Current leadthrough
Connecting bush
Anode pin
Cathode plate
0.1 – 0.4
Fig. 3.12 Cross-section of PENNINGVAC PR 35 gauge
83
Vacuum measurement
gettering surface film on the walls of the gauge tube. In spite of these disadvantages, which result in a relatively high degree of inaccuracy in the
pressure reading (up to around 50 %), the cold-cathode ionization gauge
has three very outstanding advantages. First, it is the least expensive of all
high vacuum measuring instruments. Second, the measuring system is
insensitive to the sudden admission of air and to vibrations; and third, the
instrument is easy to operate.
3.3.3.2
Hot-cathode ionization vacuum gauges
Generally speaking, such gauges refer to measuring systems consisting of
three electrodes (cathode, anode and ion collector) where the cathode is a
hot cathode. Cathodes used to be made of tungsten but are now usually
made of oxide-coated iridium (Th2O3, Y2O3) to reduce the electron output
work and make them more resistant to oxygen. Ionization vacuum gauges
of this type work with low voltages and without an external magnetic field.
The hot cathode is a very high-yield source of electrons. The electrons are
accelerated in the electric field (see Fig. 3.13) and receive sufficient energy
from the field to ionize the gas in which the electrode system is located. The
positive gas ions formed are transported to the ion collector, which is negative with respect to the cathode, and give up their charge there. The ion current thereby generated is a measure of the gas density and thus of the gas
pressure. If i- is the electron current emitted by the hot cathode, the pressure-proportional current i+ produced in the measuring system is defined by:
i+ = C · i– · p und
p=
For nitrogen this variable is generally around 10 mbar-1. With a constant
electron current the sensitivity S of a gauge head is defined as the quotient
of the ion current and the pressure. For an electron current of 1 mA and C
= 10 mbar-1, therefore, the sensitivity S of the gauge head is:
S = i+ / p = C · i- = 10 mbar-1 · 1 mA = 10 mbar-1 · 10-3 A = 1 · 10-2 A/mbar.
Hot-cathode ionization vacuum gauges also exhibit gas sorption (pumping
action), which, however, is considerably smaller than with Penning systems,
i.e. approx. 10-3 l/s. Essentially this gas sorption takes place on the glass
wall of the gauge head and, to a lesser extent, at the ion collector. Here use
is made of nude gauges that are easy to operate because an external magnet is not needed. The upper limit of the measuring range of the hot-cathode ionization gauge is around 10-2 mbar (with the exception of special
designs). It is basically defined by the scatter processes of ions at gas molecules due to the shorter free path at higher pressures (the ions no longer
reach the ion collector = lower ion yield). Moreover, uncontrollable glow or
arc discharges may form at higher pressures and electrostatic discharges
can occur in glass tubes. In these cases the indicated pressure pI may
deviate substantially from the true pressure pT.
At low pressures the measuring range is limited by two effects: by the X-ray
effect and by the ion desorption effect. These effects results in loss of the
strict proportionality between the pressure and the ion current and produce
a low pressure threshold that apparently cannot be crossed (see Fig. 3.14).
(3.3)
+
i
i− ⋅ C
(3.3a)
The variable C is the vacuum gauge constant of the measuring system.
Anode
UC
UA
(+ 50V) (+ 200V)
Indicated pressure mbar
Ion collector
Cathode
UA
Actual pressure mbar
UC
I Pressure reading without X-ray effect
II Apparent low pressure
limit due to X-ray effect
III Sum of I and II
i+: ion current
i-: electron current
Fig. 3.13 Schematic diagram and potential curve in a hot-cathode ionization vacuum gauge
84
Fig. 3.14 Apparent low pressure limit due to X-ray effect in a normal ionization vacuum gauge
Vacuum measurement
The X-ray effect (see Fig. 3.15)
The electrons emitted from the cathode impinge on the anode, releasing
photons (soft X-rays). These photons, in turn, trigger photoelectrons from
surfaces they strike. The photoelectrons released from the ion collector flow
to the anode, i.e. the ion collector emits an electron current, which is indicated in the same manner as a positive ion current flowing to the ion collector. This photocurrent simulates a pressure. This effect is called the positive
X-ray effect, and it depends on the anode voltage as well as on the size of
the surface of the ion collector.
Under certain circumstances, however, there is also a negative X-ray effect.
Photons which impinge on the wall surrounding the gauge head release
photoelectrons there, which again flow towards the anode, and since the
anode is a grid structure, they also flow into the space within the anode. If
the surrounding wall has the same potential as the ion collector, e.g. ground
potential, a portion of the electrons released at the wall can reach the ion
collector. This results in the flow of an electron current to the ion collector,
i.e. a negative current flows which can compensate the positive ion current.
This negative X-ray effect depends on the potential of the outer wall of the
gauge head.
The ion desorption effect
Adsorbed gases can be desorbed from a surface by electron impact. For an
ionization gauge this means that, if there is a layer of adsorbed gas on the
anode, these gases are partly desorbed as ions by the impinging electrons.
The ions reach the ion collector and lead to a pressure indication that is initially independent of the pressure but rises as the electron current increases. If such a small electron current is used so that the number of electrons
incident at the surface is small compared to the number of adsorbed gas
particles, every electron will be able to desorb positive ions. If the electron
current is then increased, desorption will initially increase because more
electrons impinge on the surface. This finally leads to a reduction in
adsorbed gas particles at the surface. The reading falls again and generally
reaches values that may be considerably lower than the pressure reading
observed with a small electron current. As a consequence of this effect in
practice, one must ascertain whether the pressure reading has been influenced by a desorption current. This can be done most simply by temporarily altering the electron current by a factor of 10 or 100. The reading for the
larger electron current is the more precise pressure value.
In addition to the conventional ionization gauge, whose electrode structure
resembles that of a common triode, there are various ionization vacuum
gauge systems (Bayard-Alpert system, Bayard-Alpert system with modulator, extractor system) which more or less suppress the two effects, depending on the design, and are therefore used for measurement in the high and
ultrahigh vacuum range. Today the Bayard-Alpert system is usually the
standard system.
a) The conventional ionization vacuum gauge
A triode of conventional design (see Fig. 3.16 a) is used as the gauge head,
but it is slightly modified so that the outer electrode serves as the ion collec-
a)
Conventional
ionization vacuum gauge system
b)
ionization vacuum
gauge system for
higher pressures (up
to 1 mbar)
c)
Bayard-Alpert
ionization vacuum gauge system
d)
Bayard-Alpert
ionization vacuum
gauge system with
modulator
e)
extractor ionization
vacuum gauge system
C Cathode
A Anode
I Ion collector
Fig. 3.15 Explanation of the X-ray effect in a conventional ionization gauge. The electrons eemitted by the cathode C collide with anode A and trigger a soft X-ray radiation (photons) there. This radiation strikes, in part, the ion collector and generates photoelec–
trons e s there.
I ion collector
Sc screen
M modulator
A anode
C cathode
R reflector
Fig. 3.16 Schematic drawing of the electrode arrangement of various ionization vacuum gauge
measuring systems
85
Vacuum measurement
tor and the grid within it as the anode. With this arrangement the electrons
are forced to take very long paths (oscillating around the grid wires of the
anode) so that the probability of ionizing collisions and thus the sensitivity of
the gauge are relatively high. Because the triode system can generally only
be used in high vacuum on account of its strong X-ray effect, the gas sorption (pumping) effect and the gas content of the electrode system have only
a slight effect on the pressure measurement.
b) The high-pressure ionization vacuum gauge (up to 1 mbar)
A triode is again used as the electrode system (see Fig. 3.16 b), but this
time with an unmodified conventional design. Since the gauge is designed
to allow pressure measurements up to 1 mbar, the cathode must be resistant to relatively high oxygen pressure. Therefore, it is designed as a socalled non-burnout cathode, consisting of an yttria-coated iridium ribbon. To
obtain a rectilinear characteristic (ion current as a linear function of the
pressure) up to a pressure of 1 mbar, a high-ohmic resistor is installed in
the anode circuit.
c) Bayard-Alpert ionization vacuum gauge (the standard measuring
system used today)
To ensure linearity between the gas pressure and the ion current over as
large a pressure range as possible, the X-ray effect must be suppressed as
far as possible. In the electrode arrangement developed by Bayard and
Alpert, this is achieved by virtue of the fact that the hot cathode is located
outside the anode and the ion collector is a thin wire forming the axis of the
electrode system (see Fig. 3.16 c). The X-ray effect is reduced by two to
three orders of magnitude due to the great reduction in the surface area of
the ion collector. When pressures in the ultrahigh vacuum range are measured, the inner surfaces of the gauge head and the connections to the vessel affect the pressure reading. The various effects of adsorption, desorption, dissociation and flow phenomena cannot be dealt with in this context.
By using Bayard-Alpert systems as nude gauge systems that are placed
directly in the vessel, errors in measurement can be extensively avoided
because of the above mentioned effects.
d) Bayard-Alpert ionization vacuum gauge with modulator
The Bayard-Alpert system with modulator (see Fig. 3.16 d), introduced by
Redhead, offers pressure measurement in which errors due to X-ray and
ion desorption effects can be quantitatively taken into account. In this
arrangement there is a second thin wire, the modulator, near the anode in
addition to the ion collector inside the anode. If this modulator is set at the
anode potential, it does not influence the measurement. If, on the other
hand, the same potential is applied to the modulator as that on the ion collector, part of the ion current formed flows to the modulator and the current
that flows to the ion collector becomes smaller. The indicated pressure pI of
the ionization gauge with modulator set to the anode potential consists of
the portion due to the gas pressure pg and that due to the X-ray effect pg:
p A = pg + pγ
(3.4)
After switching the modulator from the anode potential over to the ion collector potential, the modulated pressure reading pM is lower than the pI
reading because a portion of the ions now reaches the modulator. Hence:
pM = α · pg + pγ
with
86
α < 1.
The pg share of the X-ray effect is the same in both cases. After determining the difference between (3.4) and (3.5), we obtain the equation for the
gas pressure pg:
pg =
p −p
A
M
1− α
(3.6)
α can immediately be determined by experiment at a higher pressure
(around 10-6 mbar) at which the X-ray effect and thus pγ are negligible. The
pressure corresponding to the two modulator potentials are read off and
their ratio is formed. This modulation method has the additional advantage
that the ion desorption effect is determined in this way. It permits pressure
measurements up to the 10-11 mbar range with relatively little effort.
e) Extractor ionization vacuum gauge
Disruptive effects that influence pressure measurement can also be extensively eliminated by means of an ion-optical system first suggested by
Redhead. With this extractor system (see Fig. 3.16 e) the ions from the
anode cylinder are focused on a very thin and short ion collector. The ion
collector is set up in a space, the rear wall of which is formed by a cupshaped electrode that is maintained at the anode potential so that it cannot
be reached by ions emanating from the gas space. Due to the geometry of
the system as well as the potential of the of individual electrodes, the disruptive influences through X-ray effects and ion desorption are almost completely excluded without the need of a modulator. The extractor system
measures pressures between 10-4 and 10-12 mbar. Another advantage is
that the measuring system is designed as a nude gauge with a diameter of
only 35 mm so that it can be installed in small apparatus.
3.4
Adjustment and calibration;
DKD, PTB national standards
Definition of terms: Since these terms are often confused in daily usage, a
clear definition of them will first be provided:
Adjustment or tuning refers to the correct setting of an instrument. For
example, setting 0 and 100 % in THERMOVACs or setting the mass spectrometer to mass 4 in the helium leak detector.
Calibration inspection refers to comparison with a standard in accordance
with certain statutory regulations by specially authorized personnel (Bureau
of Standards). If the outcome of this regular inspection is positive, an operating permit for the next operation period (e.g. three years) is made visible
for outsiders by means of a sticker or lead seal. If the outcome is negative,
the instrument is withdrawn from operation.
Calibration refers to comparison with a standard in accordance with certain
statutory regulations by specially authorized personnel (calibration facility).
The result of this procedure is a calibration certificate which contains the
deviations of the readings of the instrument being calibrated from the standard.
(3.5)
Calibration facilities carry out this calibration work. One problem that arises
Vacuum measurement
is the question of how good the standards are and where they are calibrated. Such standards are calibrated in calibration facilities of the German
Calibration Service (DKD). The German Calibration Service is managed
by the Federal Physical-Technical Institute (PTB). Its function is to ensure
that measuring and testing equipment used for industrial measurement purposes is subjected to official standards. Calibration of vacuum gauges and
test leaks within the framework of the DKD has been assigned to LEYBOLD, as well as other companies, by the PTB. The required calibration
pump bench was set up in accordance with DIN 28 418 (see Table 11.1)
and then inspected and accepted by the PTB. The standards of the DKD
facilities, so-called transfer standards (reference vacuum gauges), are calibrated directly by the PTB at regular intervals. Vacuum gauges of all
makes are calibrated on an impartial basis by LEYBOLD in Cologne
according to customer order. A DKD calibration certificate is issued with all
characteristic data on the calibration. The standards of the Federal
Physical-Technical Institute are the so-called national standards. To be
able to guarantee adequate measuring accuracy or as little measurement
uncertainty as possible in its calibrations, the PTB largely carries out its
measurements through the application of fundamental methods. This
means, for example, that one attempts to describe the calibration pressures
through the measurement of force and area or by thinning the gases in
strict accordance with physical laws. The chain of the recalibration of standard instruments carried out once a year at the next higher qualified calibration facility up to the PTB is called “resetting to national standards”. In other
countries as well, similar methods are carried out by the national standards
institutes as those applied by the Federal Physical-Technical Institute (PTB)
in Germany. Fig. 3.17 shows the pressure scale of the PTB. Calibration
guidelines are specified in DIN standards (DIN 28 416) and ISO proposals.
3.4.1 Examples of fundamental pressure measurement methods (as standard methods
for calibrating vacuum gauges)
a) Measuring pressure with a reference gauge
An example of such an instrument is the U-tube vacuum gauge, with which
the measurement of the pressure in the measurement capillary is based on
a measurement of the weight over the length of the mercury column.
In the past the McLeod vacuum gauge was also used for calibration purposes. With a precision-made McLeod and carefully executed measurements, taking into account all possible sources of error, pressures down to
10-4 mbar can be measured with considerable accuracy by means of such
an instrument.
Another reference gauge is the VISCOVAC decrement gauge with rotating
ball (see 3.3.1) as well as the capacitance diaphragm gauge (see 3.2.2.4).
b) Generation of a known pressure; static expansion method
On the basis of a certain quantity of gas whose parameters p, V and T are
known exactly – p lies within the measuring range of a reference gauge
such as a U-tube or McLeod vacuum gauge – a lower pressure within the
working range of ionization gauges is reached via expansion in several
stages.
If the gas having volume V1 is expanded to a volume (V1 + V2), and from
V2 to (V2 + V3), etc., one obtains, after n stages of expansion:
pn = p ⋅
1
Vn −1
V1
V2
⋅
⋅⋅ ⋅⋅
V1 + V2 V2 + V3
Vn −1 + Vn
(3.7)
p1 = initial pressure measured directly in mbar
pn = calibration pressure
The volumes here must be known as precisely as possible (see Fig. 3.18)
and the temperature has to remain constant. This method requires that the
apparatus used be kept very clean and reaches its limit at pressures where
the gas quantity can be altered by desorption or adsorption effects beyond
Dynamic
expansion
IM
10
+
p1
V2 = 1000 cm
V1 = 25 cm
p3
+
3
p2
+
Static expansion
1
+
+
Molecular
beam
IM
3
3
+
V3 = 25 cm
3
V4 =
13000 cm3
p4
U-Tube
+
Relative uncertainly of the pressure determination [%]
30
0.3
0.1
–12
10
–9
10
–6
10
–3
10
0
10
3
10
Pressure [mbar]
Fig. 3.17 Pressure scale of Federal Physical-Technical Institute (PTB), Berlin, (status as at
August 1984) for inert gases, nitrogen and methane
Fig. 3.18 Generation of low pressures through static expansion
87
Vacuum measurement
and thus
p2 = p ⋅
1
L1
S
=p⋅
1
L1
L2
⋅ (1 +
L2
Sp
)
(3.9)
This method has the advantage that, after reaching a state of equilibrium,
sorption effects can be ignored and this procedure can therefore be used
for calibrating gauges at very low pressures.
p2 = p1
1
2
3
4
5
6
7
8
Volume 1
Volume 2
Inlet valve (conductance L1)
Aperture with conductance L2
Valve
to pump system
Valve
to gas reservoir
9
10
11
12
13
14
15
L1
S
3.5
(Sp >> L2)
Valve
LN2 cold trap
to pump system
U-tube vacuum gauge
McLeod vacuum gauge
Valve
Calibrated ionization gauge
tube
16 to pump
(pumping speed PSp)
17 Gas inlet
18 Mass spectrometer
19,20 Gauges to be calibrated
21 Nude gauge to be calibrated
22 Bake-out furnace
Fig. 3.19 Apparatus for calibration according to the dynamic expansion method
the permissible limits of error. According to experience, this lower limit is
around 5 · 10-7 mbar. This method is called the static expansion method
because the pressure and volume of the gas at rest are the decisive variables.
c) Dynamic expansion method (see Fig. 3.19)
According to this method, the calibration pressure p is produced by admitting gas at a constant throughput rate Q into a vacuum chamber while gas
is simultaneously pumped out of the chamber by a pump unit with a constant pumping speed S. At equilibrium the following applies according to
equation 1.10 a:
Q is obtained either from the quantity of gas that flows into the calibration
chamber from a supply vessel in which constant pressure prevails or from
the quantity of gas flowing into the calibration chamber at a measured pressure through a known conductance. The pressure in front of the inlet valve
must be high enough so that it can be measured with a reference gauge.
The inlet apertures of the valve (small capillaries, sintered bodies) must be
so small that the condition d << λ is met, i.e. a molecular flow and hence a
constant conductance of the inlet valve are obtained (see Section 1.5). The
quantity of gas is then defined by p1 · L1, where p1 = pressure in front of
the inlet valve and L1 = conductance of the valve. The pumping system
consists of a precisely measured aperture with the conductance L2 in a wall
that is as thin as possible (screen conductance) and a pump with a pumping speed of PSp:
L 2 ⋅ Sp
L + Sp
2
88
3.5.1 Fundamentals of pressure monitoring and
control
In all vacuum processes the pressure in the system must be constantly
checked and, if necessary, regulated. Modern plant control additionally
requires that all measured values which are important for monitoring a plant
are transmitted to central stations, monitoring and control centers and compiled in a clear manner. Pressure changes are frequently recorded over
time by recording equipment. This means that additional demands are
placed on vacuum gauges:
a) continuous indication of measured values, analog and digital as far as
possible
b) clear and convenient reading of the measured values
c) recorder output to connect a recording instrument or control or regulation equipment
d) built-in computer interface (e.g. RS 232)
e) facility for triggering switching operations through built-in trigger points
p = Q/S
S=
Pressure monitoring, control and regulation in vacuum systems
=
L2
1+
L2
Sp
(3.8)
These demands are generally met by all vacuum gauges that have an electric measured value display, with the exception of Bourdon, precision
diaphragm and liquid-filled vacuum gauges. The respective control units are
equipped with recorder outputs that supply continuous voltages between 0
and 10 V, depending on the pressure reading on the meter scale, so that
the pressure values can be recorded over time by means of a recording
instrument. If a pressure switching unit is connected to the recorder output
of the gauge, switching operations can be triggered when the values go
over or below specified setpoints. The setpoints or switch threshold values
for triggering switching operations directly in the gauges are called trigger
values. Apart from vacuum gauges, there are diaphragm pressure switches
that trigger a switching operation (without display of a measured value) via
a contact amplifier when a certain pressure is reached. Valves, for example,
can also be controlled through such switching operations.
Vacuum measurement
3.5.2 Automatic protection, monitoring and control of vacuum systems
g) pressure rise above a maximum backing pressure (critical forepressure
of the diffusion pump)
Protection of a vacuum system against malfunctions is extremely important.
In the event of failure, very high material values may be at risk, whether
through loss of the entire system or major components of it, due to loss of
the batch of material to be processed or due to further production down
time. Adequate operational control and protection should therefore be provided for, particularly in the case of large production plants. The individual
factors to be taken into account in this connection are best illustrated on the
basis of an example: Fig. 3.20 shows the schematic diagram of a high vacuum pump system. The vessel (11) can be evacuated by means of a Roots
pump (14) or a diffusion pump (15), both of which operate in conjunction
with the backing pump (1). The Roots pump is used in the medium vacuum
range and the diffusion pump in the high vacuum range. The valves (3), (8)
and (16) are operated electropneumatically. The individual components are
actuated from a control panel with pushbuttons. The pump system is to be
protected against the following malfunctions:
The measures to be taken in order to forestall such malfunctions will be discussed in the same order:
a) power failure
a) Measures in the event of power failure: All valves are closed so as to
prevent admission of air to the vacuum vessel and protect the diffusion
pump against damage.
b) Protection in the event of a drop in pressure in the compressed air network: The compressed air is monitored by a pressure monitoring device
(5). If the pressure falls under a specified value, a signal can initially be
emitted or the valves can be automatically closed. In this case, a sufficient reserve supply of compressed air is necessary (not shown in Fig.
3.20), which allows all valves to be actuated at least once.
c) Measures in the event of failure of cooling water to the diffusion pump:
The cooling water is monitored by a flow or temperature monitoring
device (6) and (7). If the flow of cooling water is inadequate, the heater
of the diffusion pump is switched off and a signal is given; the valve (8)
closes.
b) drop in pressure in the compressed air network
c) failure of cooling water to the diffusion pump
d) fault in diffusion pump heating system
e) failure of backing pump
f) pressure rise in the vessel above a maximum permissible value
d) Protection against failure of the diffusion pump heater: Interruption of the
diffusion pump heating system can be monitored by a relay. If the
temperature rises above a maximum permissible value, a temperature
monitoring device (6) responds. In both cases the valve (8) closes and a
signal is given.
e) Protection in the event of backing pump failure: Belt-driven backing
pumps must have a centrifugal switch which shuts down the entire system in the event of belt breakage or another malfunction. Monoblock
pumps for which the drive is mounted directly on the shaft can be monitored by current relays and the like.
f) Protection against a pressure rise in the vessel above a certain limit
value: The high vacuum monitoring device (10) emits a signal when a
specified pressure is exceeded.
1 Backing pump
2 Backing pressure monitoring
device
3 Electropneumatic valve
4 Compressed air connection
5 Pressure monitoring device
6 Temperature monitoring
device
7 Cooling water monitoring
device
8
9
10
11
12
13
14
15
16
17
Electropneumatic valve
Recorder
High-vacuum monitoring device
Vessel
High-vacuum gauge
Limit switches
Roots pump
Diffusion pump
Electropneumatic valve
Venting valve
g) Ensuring the critical forepressure of the diffusion pump: When a certain
backing pressure is exceeded, all valves are closed by the backing pressure monitoring device (2), the pumps are switched off and again a signal is given. The position of the valves (3), (8) and (16) is indicated on
the control panel by means of limit switches (13). The pressure in the
vessel is measured with a high vacuum gauge (12) and recorded with a
recorder (9). Protection against operating errors can be provided by
interlocking the individual switches so that they can only be actuated in
a predetermined sequence. The diffusion pump, for example, may not
be switched on when the backing pump is not running or the required
backing pressure is not maintained or the cooling water circulation is not
functioning.
In principle, it is not a big step from a system protected against all malfunctions to a fully automatic, program-controlled plant, though the complexity of
the electrical circuits, of course, increases significantly.
Fig. 3.20 Schematic diagram of a high vacuum pump system with optional operation of a Roots
pump or a diffusion pump
89
Vacuum measurement
3.5.3 Pressure regulation and control in rough
and medium vacuum systems
Control and regulation have the function of giving a physical variable – in
this case the pressure in the vacuum system – a certain value. The common feature is the actuator which changes the energy supply to the physical variable and thus the variable itself. Control refers to influencing a system or unit through commands. In this case the actuator and hence the
actual value of the physical variable is changed directly with a manipulated
variable. Example: Actuation of a valve by means of a pressure-dependent
switch. The actual value may change in an undesirable way due to additional external influences. The controlled unit cannot react to the control
unit. For this reason control systems are said to have an open operating
sequence. In the case of regulation, the actual value of the physical variable is constantly compared to the specified setpoint and regulated if there
is any deviation so that it completely approximates the setpoint as far as
possible. For all practical purposes regulation always requires control. The
main difference is the controller in which the setpoint and the actual value
are compared. The totality of all elements involved in the control process
forms the control circuit. The terms and characteristic variables for describing control processes are stipulated in DIN 19226.
Generally a distinction is made between discontinuous control (e.g. twostep or three-step control) with specification of a pressure window, within
which the pressure may vary, and continuous control (e.g. PID control) with
a specified pressure setpoint, which should be maintained as precisely as
possible. We have two possible ways of adjusting the pressure in a vacuum
system: first, by changing the pumping speed (altering the speed of the
pump or throttling by closing a valve); second, through admission of gas
(opening a valve). This results in a total of 4 procedures.
Discontinuous pressure regulation
Although continuous regulation undoubtedly represents the more elegant
procedure, in many cases two-step or three-step regulation is fully adequate in all vacuum ranges. To specify the pressure window, two or three
variable, pressure-dependent switch contacts are necessary. It does not
matter here whether the switch contacts are installed in a gauge with display or in a downstream unit or whether it is a pressure switch without display. Fig. 3.21 illustrates the difference between two-step regulation through
pumping speed throttling, two-point regulation through gas admission and
three-point regulation through a combination of pumping speed throttling
and gas admission. Figures 3.22 and 3.23 show the circuit and structure of
Two-step regulation through
pumping speed throttling
¿
¡
¬
√
ƒ
Gauge with two switching points
Throttle valve
Vacuum pump
Pump valve
Vacuum vessel
Fu
R, Mp
Smax
Smin
PV
R1
K1
M
Fuse
Mains connection 220 V/50 Hz
Switching point for maximum value
Switching point for minimum value
Pump valve
Auxiliary relay for pump valve
Relay contact of R1
Measuring and switching device
Fig. 3.22 Two-step regulation through pumping speed throttling
the two two-step regulation systems. In the case of two-step regulation
through pumping speed throttling (Fig. 3.22), voltage is supplied to pump
valve 4, i.e. it is open when the relay contacts are in the release condition.
At a level below the upper switching point the valve remains open because
of the self-holding function of the auxiliary relay. Only at a level below the
lower switching point is the relay latching released. If the pressure subsequently rises, the valve is opened again at the upper switching point.
In the case of two-step regulation through gas admission, the inlet valve is
initially closed. If the upper pressure switching point is not reached, nothing
changes; only when the pressure falls below the lower switching point, do
the “make contacts” open the gas inlet valve and actuate the auxiliary relay
with self-holding function simultaneously. Return to the idle state with closing of the gas inlet valve is not effected until after the upper switching point
is exceeded due to the release of the relay self-holding function.
Fig. 3.24 shows the corresponding three-step regulation system which was
Three-step regulation through
gas admission
pumping speed throttling
and gas admission
Pressure
Pressure
Pressure
pAtm
pAtm
pAtm
pmax
pmitte
pmax
pmin
pmin
Time
pmin
Time
Fig. 3.21 Schematic diagram of two-step and three-step regulation
90
Time
¿
¡
¬
√
ƒ
≈
Δ
Gauge with two switching points
Variable-leak valve
Inlet valve
Gas supply
Throttle valve
Vacuum pump
Vacuum vessel
Fu Fuse
R, Mp Mains connection 220 V/50 Hz
Smax Switching point for maximum value
Smin Switching point for minimum value
EV
Inlet valve
R2
Auxiliary relay for inlet valve
K2
Relay contact of R2
M
Measuring and switching device
Fig. 3.23 Two-step regulation through gas admission
Vacuum measurement
¿ Gauge with three switching
points
¡ Variable-leak valve
¬ Variable-leak valve
√ Inlet valve
ƒ Gas supply
≈ Throttle valve
Δ Vacuum pump
« Pump valve
» Vacuum vessel
Fu
R, Mp
Smax
Smitte
Smin
T
PV
EV
R1
R2
K1
K2
M
Fuse
Mains connection 220 V/50 Hz
Switching point for maximum value
Switching point for mean value
Switching point for minimum value
TORROSTAT® S 020
Pump valve
Inlet valve
Auxiliary relay for pump interval
Auxiliary relay for inlet interval
Relay contact of R1
Relay contact of R2
Measuring and switching device
Fig. 3.24 Three-step regulation system
Fig. 3.26 LEYBOLD-A series, equipment with level and interval triggers
created with the two components just described. As the name indicates, two
switching points, the lower switching point of the regulation system through
pumping speed throttling and the upper switching point of the gas inlet regulation system, were combined.
Continuous pressure regulation
We have to make a distinction here between electric controllers (e.g. PID
controllers) with a proportional valve as actuator and mechanical diaphragm
controllers. In a regulation system with electric controllers the coordination
between controller and actuator (piezoelectric gas inlet valve, inlet valve
with motor drive, butterfly control valve, throttle valve) is difficult because of
the very different boundary conditions (volume of the vessel, effective
pumping speed at the vessel, pressure control range). Such control circuits
tend to vibrate easily when process malfunctions occur. It is virtually impossible to specify generally valid standard values.
To avoid the complicated installation with auxiliary relays, many units offer a
facility for changing the type of function of the built-in trigger values via software. Initially one can choose between individual switching points (so-called
“level triggers”) and interlinked switching points (“interval triggers”). These
functions are explained in Fig. 3.25. With interval triggers one can also
select the size of the hysteresis and the type of setpoint specification, i.e.
either fixed setting in the unit or specification through an external voltage,
e.g. from 0 – 10 volts. A three-step regulation system (without auxiliary
relay), for example, can be set up with the LEYBOLD MEMBRANOVAC of
the A series. Fig. 3.26 shows different units of the new LEYBOLD A series,
which, although they function according to different measuring methods, all
display a uniform appearance.
Many control problems can be better solved with a diaphragm controller.
The function of the diaphragm controller (see Fig. 3.27) can be easily
derived from that of a diaphragm vacuum gauge: the blunt end of a tube or
pipe is either closed off by means of an elastic rubber diaphragm (for reference pressure > process pressure) or released (for reference pressure <
process pressure) so that in the latter case, a connection is established
between the process side and the vacuum pump. This elegant and more or
less “automatic” regulation system has excellent control characteristics (see
Fig. 3.28).
process chamber
connection
measuring connection
for process pressure
reference chamber
control chamber
diaphragm
controller seat
measuring connection
for reference chamber
reference pressure
adjustment valve
Fig. 3.25 Diagram of level triggers and interval triggers
pump connection
Fig. 3.27 Principle of a diaphragm controller
91
Vacuum measurement
P1 = process pressure, P2 = pressure in pump, Pref = reference pressure
Fig. 3.28 Control characteristics of a diaphragm controller
To achieve higher flow rates, several diaphragm controllers can be connected in parallel. This means that the process chambers and the reference
chambers are also connected in parallel. Fig. 3.29 shows such a connection
of 3 MR 50 diaphragm controllers.
To control a vacuum process, it is frequently necessary to modify the pressure in individual process steps. With a diaphragm controller this can be
done either manually or via electric control of the reference pressure.
Electric control of the reference pressure of a diaphragm controller is relatively easy because of the small reference volume that always remains constant. Fig. 3.31 shows such an arrangement on the left as a picture and on
the right schematically, see 3.5.5 for application examples with
diaphragm controllers.
To be able to change the reference pressure and thus the process pressure
towards higher pressures, a gas inlet valve must additionally be installed at
the process chamber. This valve is opened by means of a differential pressure switch (not shown in Fig. 3.31) when the desired higher process pressure exceeds the current process pressure by more than the pressure difference set on the differential pressure switch.
Fig. 3.29 Triple connection of diaphragm controllers
92
DC Diaphragm controller
P Vacuum pump
M Measuring and switching device
PS Pressure sensor
V1 Pump valve
V2 Gas inlet valve
TH Throttle
RC Reference chamber
PC Process chamber
CV Internal reference pressure control valve
Fig. 3.30 Control of vacuum drying processes by regulation of the intake pressure of the vacuum pump according to the water vapor tolerance
3.5.4 Pressure regulation in high and ultrahigh
vacuum systems
If the pressure is to be kept constant within certain limits, an equilibrium
must be established between the gas admitted to the vacuum vessel and
the gas simultaneously removed by the pump with the aid of valves or throttling devices. This is not very difficult in rough and medium vacuum systems because desorption of adsorbed gases from the walls is generally
negligible in comparison to the quantity of gas flowing through the system.
Pressure regulation can be carried out through gas inlet or pumping speed
regulation. However, the use of diaphragm controllers is only possible
between atmospheric pressure and about 10 mbar.
In the high and ultrahigh vacuum range, on the other hand, the gas evolution from the vessel walls has a decisive influence on the pressure. Setting
of specific pressure values in the high and ultrahigh vacuum range, therefore, is only possible if the gas evolution from the walls is negligible in relation to the controlled admission of gas by means of the pressure-regulating
unit. For this reason, pressure regulation in this range is usually effected as
gas admission regulation with an electric PID controller. Piezoelectric or
servomotor-controlled variable-leak valves are used as actuators. Only
bakeable all-metal gas inlet valves should be used for pressure regulation
below 10-6 mbar.
Vacuum measurement
3.5.5 Examples of applications with diaphragm
controllers
1) Regulation of a drying/distillation process, taking into account the maximum water vapor tolerance of a vane type rotary pump
In a drying process it is frequently desirable to carry out drying solely by
means of vacuum pumps without inserting condensers. In view of the limited water vapor tolerance of vacuum pumps – approx. 30 mbar as a rule –
this would result in condensation of the vapors produced within the vacuum
pump, given non-throttled or non-regulated pumping speed. One can avoid
this through process-dependent remote control of a diaphragm controller
with auxiliary control valves and a measuring and switching device with a
pressure sensor at the inlet connection of the vacuum pump if the intake
pressure is adapted to the pumps water vapor tolerance through automatic
monitoring of the intake pressure of the vacuum pump and by throttling the
pumping speed. Fig. 3.30 shows the principle of this arrangement.
Mode of operation: Starting from atmospheric pressure with the process
heating switched off, valve V1 is initially open (maximum switching point
exceeded) so that atmospheric pressure also prevails in the reference
chamber.
The diaphragm controller is therefore closed. When the system is started
up, the connecting line between the vacuum pump and pump valve V2 is
first evacuated. As soon as the pressure drops below the maximum switching point, valve V1 closes. When the pressure falls below the minimum
switching point, valve V2 opens.
In this manner the pressure in the reference chamber is slowly lowered, the
throttling of the diaphragm controller is reduced accordingly and thus the
process pressure is lowered until the quantity of process gas is greater than
the quantity conveyed by the pump so that the minimum switching point is
again exceeded. Valve V2 closes again. This interaction repeats itself until
the pressure in the process chamber has dropped below the minimum
switching point. After that, valve V2 remains open so that the process can
be brought down to the required final pressure with a completely open
diaphragm controller.
The material to be dried is usually heated to intensify and speed up the drying process. If a certain amount of water vapor is produced, the intake pressure rises above the two switching points. As a result, valve V2 first closes
and V1 opens. Through incoming air or protective gas the pressure in the
reference chamber is raised and the throughput at the diaphragm controller
thus throttled until the intake pressure of the vacuum pump has dropped
below the set maximum switching point again. Then valve V1 closes.
Depending on the quantity of vapor that accumulates, the throughput of the
diaphragm controller is set by increasing or decreasing the reference pressure in each case so that the maximum permissible partial water vapor
pressure at the inlet connection of the vacuum pump is never exceeded.
As soon as the pressure in the process chamber drops below the set minimum switching point towards the end of the drying process, valve V2 opens
and remains open. In this way the unthrottled cross-section of the
diaphragm controller is available again for rapid final drying. At the same
time the final drying procedure can be monitored by means of the pressure
sensor PS.
2) Pressure regulation by means of diaphragm controller with external
automatic reference pressure adjustment (see Fig. 3.31)
For automatic vacuum processes with regulated process pressure, presetting of the desired set pressure must often function and be monitored automatically. If a diaphragm controller is used, this can be done by equipping
the reference chamber with a measuring and switching device and a control
valve block at the reference chamber. The principle of this arrangement is
DC Diaphragm controller
PS Process pressure sensor
RS Reference pressure sensor
V1 Gas inlet valve
V2 Pump valve
V3 Gas inlet variable-leak valve
TH Throttle
M Measuring and switching device
PP Process pump
RC Reference chamber
PC Process chamber
AP Auxiliary pump
CV Internal reference pressure control
valve
Fig. 3.31 Diaphragm controller with external automatic reference pressure regulation
93
Vacuum measurement
shown in Fig. 3.31.
Mode of operation: Starting with atmospheric pressure, gas inlet valve V1 is
closed at the beginning of the process. Pump valve V2 opens. The process
chamber is now evacuated until the set pressure, which is preset at the
measuring and switching device, is reached in the process chamber and in
the reference chamber. When the pressure falls below the set switching
threshold, pump valve V2 closes. As a result, the pressure value attained is
“caught” as the reference pressure in the reference chamber (RC) of the
diaphragm controller (DC). Now the process pressure is automatically
maintained at a constant level according to the set reference pressure by
means of the diaphragm controller (DC). If the reference pressure should
rise in the course of the process due to a leak, this is automatically detected by the measuring and switching device and corrected by briefly opening
pump valve V2. This additional control function enhances the operational
reliability and extends the range of application. Correcting the increased
reference pressure to the originally set value is of special interest for regulated helium circuits because the pressure rise in the reference chamber
(RC) of the diaphragm controller can be compensated for through this
arrangement as a consequence of the unavoidable helium permeability of
the controller diaphragm of FPM.
To be able to change the reference pressure and thus increase the process
pressure to higher pressures, a gas inlet valve must be additionally
installed at the process chamber. This valve is opened by means of a differential pressure switch (not shown in Fig. 3.31) when the desired higher
process pressure exceeds the current process pressure by more than the
pressure differential set at the differential pressure switch.
94
Mass spectrometry
4.
Analysis of gas at low pressures using
mass spectrometry
4.1
General
Analyses of gases at low pressures are useful not only when analyzing the
residual gases from a vacuum pump, leak testing at a flange connection or
for supply lines (compressed air, water) in a vacuum. They are also essential in the broader fields of vacuum technology applications and processes.
For example in the analysis of process gases used in applying thin layers of
coatings to substrates. The equipment used for qualitative and/or quantitative analyses of gases includes specially developed mass spectrometers
with extremely small dimensions which, like any other vacuum gauge, can
be connected directly to the vacuum system. Their size distinguishes these
measurement instruments from other mass spectrometers such as those
used for the chemical analyses of gases. The latter devices are poorly suited, for example, for use as partial pressure measurement units since they
are too large, require a long connector line to the vacuum chamber and
cannot be baked out with the vacuum chamber itself. The investment for an
analytical mass spectrometer would be unjustifiably great since, for example, the requirements as to resolution are far less stringent for partial pressure measurements. Partial pressure is understood to be that pressure
exerted by a certain type of gas within a mix of gases. The total of the partial pressures for all the types of gas gives the total pressure. The distinction among the various types of gases is essentially on the basis of their
molar masses. The primary purpose of analysis is therefore to register
qualitatively the proportions of gas within a system as regards the molar
masses and determine quantitatively the amount of the individual types of
gases associated with the various atomic numbers.
Partial pressure measurement devices which are in common use comprise
the measurement system proper (the sensor) and the control device
required for its operation. The sensor contains the ion source, the separation system and the ion trap. The separation of ions differing in masses and
charges is often effected by utilizing phenomena which cause the ions to
resonate in electrical and magnetic fields.
Initially, the control units were quite clumsy and offered uncountable
manipulation options. It was often the case that only physicists were able to
handle and use them. With the introduction of PCs the requirements in
regard to the control units became ever greater. At first, they were fitted
with interfaces for linkage to the computer. Attempts were made later to
equip a PC with an additional measurement circuit board for sensor operation. Today’s sensors are in fact transmitters equipped with an electrical
power supply unit attached direct at the atmosphere side; communication
with a PC from that point is via the standard computer ports (RS 232, RS
485). Operating convenience is achieved by the software which runs on the
PC.
c
a
b
a: High-performance sensor with Channeltron
b: Compact sensor with Micro-Channelplate
c: High-performance sensor with Faraday cup
Fig. 4.1a TRANSPECTOR sensors
4.2
A historical review
Following Thomson’s first attempt in 1897 to determine the ratio of charge
to mass e/m for the electron, it was quite some time (into the 1950s) before
a large number and variety of analysis systems came into use in vacuum
technology. These included the Omegatron, the Topatron and ultimately the
quadrupole mass spectrometer proposed by Paul and Steinwedel in 1958,
available from INFICON in its standard version as the TRANSPECTOR
(see Fig. 4.1). The first uses of mass spectrometry in vacuum-assisted
process technology applications presumably date back to Backus’ work in
the years 1943 / 44. He carried out studies at the Radiographic Laboratories at the University of California. Seeking to separate uranium isotopes,
he used a 180° sector field spectrometer after Dempster (1918), which he
referred to as a “vacuum analyzer”. Even today a similar term, namely the
“residual gas analyzer” (RGA), is frequently used in the U.S.A. and the U.K.
instead of “mass spectrometer”. Today’s applications in process monitoring
are found above all in the production of semiconductor components.
Fig. 4.1b TRANSPECTOR XPR sensor
95
Mass spectrometry
4.3
The quadrupole mass spectrometer
(TRANSPECTOR)
The ion beam extracted from the electron impact ion source is diverted into
a quadrupole separation system containing four rod-shaped electrodes. The
cross sections of the four rods form the circle of curvature for a hyperbola
so that the surrounding electrical field is nearly hyperbolic. Each of the two
opposing rods exhibits equal potential, this being a DC voltage and a superimposed high-frequency AC voltage (Fig. 4.2). The voltages applied induce
transverse oscillations in the ions traversing the center, between the rods.
The amplitudes of almost all oscillations escalate so that ultimately the ions
will make contact with the rods; only in the case of ions with a certain ratio
of mass to charge m/e is the resonance condition which allows passage
through the system satisfied. Once they have escaped from the separation
system the ions move to the ion trap (detector, a Faraday cup) which may
also take the form of a secondary electron multiplier pick-up (SEMP).
The length of the sensor and the separation system is about 15 cm. To
ensure that the ions can travel unhindered from the ion source to the ion
trap, the mean free path length inside the sensor must be considerably
greater than 15 cm. For air and nitrogen, the value is about
p · λ = 6 · 10–3 mbar · cm. At p = 1 · 10-4 bar this corresponds to a mean
free path length of λ = 60 cm. This pressure is generally taken to be the
minimum vacuum for mass spectrometers. The emergency shut-down feature for the cathode (responding to excessive pressure) is almost always
set for about 5 · 10-4 mbar. The desire to be able to use quadrupole spectrometers at higher pressures too, without special pressure convertors, led
to the development of the XPR sensor at INFICON (XPR standing for
extended pressure range). To enable direct measurement in the range of
about 2 · 10-2 mbar, so important for sputter processes, the rod system was
reduced from 12 cm to a length of 2 cm. To ensure that the ions can execute the number of transverse oscillations required for sharp mass separation, this number being about 100, the frequency of the current in the XPR
sensor had to be raised from about 2 MHz to approximately 6 times that
value, namely to 13 MHz. In spite of the reduction in the length of the rod
system, ion yield is still reduced due to dispersion processes at such high
pressures. Additional electronic correction is required to achieve perfect
depiction of the spectrum. The dimensions of the XPR sensor are so small
that it can “hide” entirely inside the tubulation of the connection flange (DN
40, CF) and thus occupies no space in the vacuum chamber proper. Fig.
4.1a shows the size comparison for the normal high-performance sensors
with and without the Channeltron SEMP, the normal sensor with channelplate SEMP. Fig. 4.1b shows the XPR sensor. The high vacuum required for
the sensor is often generated with a TURBOVAC 50 turbomolecular pump
and a D 1.6 B rotary vane pump. With its great compression capacity, a further advantage of the turbomolecular pump when handling high molar mass
gases is that the sensor and its cathode are ideally protected from contamination from the direction of the forepump.
4.3.1 Design of the sensor
One can think of the sensor as having been derived from an extractor measurement system (see Fig. 4.3), whereby the
separation system was inserted between the ion source and the
ion trap.
4.3.1.1
The normal (open) ion source
The ion source comprises an arrangement of the cathode, anode and several baffles. The electron emission, kept constant, causes partial ionization
of the residual gas, into which the ion source is “immersed” as completely
as possible. The vacuum in the vicinity of the sensor will naturally be influenced by baking the walls or the cathode. The ions will be extracted
through the baffles along the direction of the separation system. One of the
baffles is connected to a separate amplifier and – entirely independent of
ion separation – provides continuous total pressure measurement (see Fig.
4.4). The cathodes are made of iridium wire and have a thorium oxide coating to reduce the work associated with electron discharge. (For some time
now the thorium oxide has gradually been replaced by yttrium oxide.)
These coatings reduce the electron discharge work function so that the
desired emission flow will be achieved even at lower cathode temperatures.
Available for special applications are tungsten
cathodes (insensitive to hydrocarbons but sensitive to oxygen) or rhenium
cathodes (insensitive to oxygen and hydrocarbons but evaporate slowly
during operation due to the high vapor pressure).
Anode
Cathode
Focussing plate
(extractor diaphragm) Ion source exit
diaphragm
(total pressure measurement)
Cathode
Anode
Quadrupole exit
diaphragm
Reflector
Ion trap
Shielding
Extractor measurement system
Ion source
Fig. 4.2
96
Quadrupole separation system
Schematic for quadrupole mass spectrometer
Ion detector
Transpector measurement head
Fig. 4.3
Quadrupole mass spectrometer – Extractor ionization vacuum gauge
Mass spectrometry
Cathode
xz plane
1
-
+
-
+
Rod:
+U+V, cos ω
Transmission:
low-pass
+
Shielding
+
Anode
2
yz plane
Rod:
+U
Transmission:
full
+
+
i+
+
Rod:
–U
Transmission:
none
+
Rod:
–U–V · cos ω
Transmission:
high-pass
-
i+
3
Extractor diaphragm
V1
Total pressure diaphragm
V
V1
i+
V
i+
4
M1
Fig. 4.4
i+
5
The quadrupole separation system
Here the ions are separated on the basis of their mass-to-charge ratio. We
know from physics that the deflection of electrically charged particles (ions)
from their trajectory is possible only in accordance with their ratio of mass
to charge, since the attraction of the particles is proportional to the charge
while the inertia (which resists change) is proportional to its mass. The separation system comprises four cylindrical metal rods, set up in parallel and
isolated one from the other; the two opposing rods are charged with identical potential. Fig. 4.2 shows schematically the arrangement of the rods and
their power supply. The electrical field Φ inside the separation system is
generated by superimposing a DC voltage and a high-frequency AC voltage:
Φ = (U + V ◊ cos ωt) · (x2 – y2) / r02
r0 = radius of the cylinder which can be inscribed inside the system of rods
Exerting an effect on a single charged ion moving near and parallel to the
center line inside the separation system and perpendicular to its movement
are the forces
Fx = − 2e ⋅ x ⋅ cos (ω ⋅ t )
r02
Fy = − 2e2 ⋅ y ⋅ cos (ω ⋅ t )
r0
Fz = 0
The mathematical treatment of these equations of motion uses Mathieu’s
differential equations. It is demonstrated that there are stable and unstable
ion paths. With the stable paths, the distance of the ions from the separation system center line always remains less than ro (passage condition).
With unstable paths, the distance from the axis will grow until the ion ultimately collides with a rod surface. The ion will be discharged (neutralized),
thus becoming unavailable to the detector (blocking condition).
Even without solving the differential equation, it is possible to arrive at a
purely phenomenological explanation which leads to an understanding of
the most important characteristics of the quadrupole separation system.
If we imagine that we cut open the separation system and observe the
deflection of a singly ionized, positive ion with atomic number M, moving in
two planes, which are perpendicular one to the other and each passing
through the centers of two opposing rods. We proceed step-by-step and
first observe the xz plane (Fig. 4.5, left) and then the yz plane (Fig.4.5,
M
M1
Superimposition of the xy and yz planes
Open ion source
4.3.1.2
M
yz
xz
I
II
U .. Selectivity (resolution)
V
Fig. 4.5
( UV fixed)
III
M
Sensitivity
Phenomenological explanation of the separation system
right):
1. Only DC potential U at the rods:
xz plane (left): Positive potential of +U at the rod, with a repellant effect
on the ion, keeping it centered; it reaches the collector (→ passage).
yz plane (right): Negative potential on the rod -U, meaning that at even
the tiniest deviations from the center axis the ion will be drawn toward
the nearest rod and neutralized there; it does not reach the collector
(→ blocking).
2. Superimposition of high-frequency voltage V · cos ω t:
xz plane (left): Rod potential +U + V · cos ω t. With rising AC voltage
amplitude V the ion will be excited to execute transverse oscillations
with ever greater amplitudes until it makes contact with a rod and is neutralized. The separation system remains blocked for very large values of
V.
yz plane (right): Rod potential -U -V · cos ω t. Here again superimposition induces an additional force so that as of a certain value for V the
amplitude of the transverse oscillations will be smaller than the clearance between the rods and the ion can pass to the collector at very
large V.
3. Ion emission i+ = i+ (V) for a fixed mass of M:
xz plane (left): For voltages of V < V1 the deflection which leads to an
escalation of the oscillations is smaller than V1, i.e. still in the “pass”
range. Where V > V1 the deflection will be sufficient to induce escalation
and thus blockage.
yz plane (right): For voltages of V < V1 the deflection which leads to the
damping of the oscillations is smaller than V1, i.e. still in the “block”
range. Where V > V1 the damping will be sufficient to settle oscillations,
allowing passage.
4. Ion flow i+ = i+ (M) for a fixed ratio of U / V:
Here the relationships are exactly opposite to those for i+ = i+ (V) since
the influence of V on light masses is greater than on heavy masses.
97
Mass spectrometry
xz plane: For masses of M < M1 the deflection which results in escalation of the oscillations is greater than at M1, which means that the ions
will be blocked. At M > M1 the deflection is no longer sufficient for escalation, so that the ion can pass.
yz plane: For masses of M < M1 the deflection which results in damping
of the oscillations is greater than at M1, which means that the ion will
pass. At M > M1 the damping is not sufficient to calm the system and so
the ion is blocked.
5. Combination of the xz and yz planes. In the superimposition of the ion
currents i+ = i+ (M) for both pairs of rods (U / V being fixed) there are
three important ranges:
Range I: No passage for M due to the blocking behavior of the xz pair of
rods.
Range II: The pass factor of the rod systems for mass M is determined
by the U/V ratio (other ions will not pass). We see that great permeability
(corre- sponding to high sensitivity) is bought at the price of low selectivity (= resolution, see Section 4.5). Ideal adjustment of the separation
system thus requires a compromise between these two properties. To
achieve constant resolution, the U/V ratio will remain constant over the
entire measurement range. The “atomic number” M (see 4.6.1) of the
ions which can pass through the separation system must satisfy this
condition:
Once they have left the separation system the ions will meet the ion trap or
detector which, in the simplest instance, will be in the form of a Faraday
cage (Faraday cup). In any case the ions which impinge on the detector will
be neutralized by electrons from the ion trap. Shown, after electrical amplification, as the measurement signal itself is the corresponding “ion emission
stream”. To achieve greater sensitivity, a secondary electron multiplier pickup (SEMP) can be employed in place of the Faraday cup.
Channeltrons or Channelplates can be used as SEMPs. SEMPs are virtually inertia-free amplifiers with gain of about 10+6 at the outset; this will
indeed drop off during the initial use phase but will then become virtually
constant over a long period of time. Fig. 4.6 shows at the left the basic configuration of a Faraday ion trap and, on the right, a section through a
Channeltron. When recording spectra the scanning period per mass line t0
and the time constants of the amplifier t should satisfy the condition that
t0 = 10 τ. In modern devices such as the TRANSPECTOR the otherwise
unlimited selection of the scanning period and the amplifier time constants
will be restricted by microprocessor control to logical pairs of values.
m
V
≈M=
e
14.438 ⋅ f 2 ⋅ ro2
V = High-frequency amplitude,
rO = Quadrupole inscribed radius
f = High-frequency
As a result of this linear dependency there results a mass spectrum with
linear mass scale due to simultaneous, proportional modification of U
and V.
Range III: M cannot pass, due to the blocking characteristics of the yz
pair of rods.
4.3.1.3
The measurement system (detector)
Separation system output
Positive ion
Collector
Electron suppressor
Faraday cup
Connection
to front end
of the inside
surface
Amplifier
Amplifier
Resistance of the inner surface
Resistance ≈ 108 Ω
Negative
. high voltage
6
R ≈ 4 · 10 Ω
Fig. 4.6
98
Left: Principle of the Faraday cup; Right: Configuration of the Channeltron
Mass spectrometry
4.4
Gas admission and pressure adaptation
4.4.1 Metering valve
The simplest way to adapt a classical mass spectrometer to pressures
exceeding 1 · 10-4 mbar is by way of a metering valve. The inherent disadvantage is, however, that since the flow properties are not unequivocally
defined, a deviation from the original gas composition might result.
4.4.2 Pressure converter
In order to examine a gas mix at total pressure exceeding 1 · 10-4 mbar it is
necessary to use pressure converters which will not segregate the gases.
Figure 4.7 is used to help explain how such a pressure converter works:
a. Process pressure < 1 mbar: Single-stage pressure converter. Gas is
allowed to pass out of the vacuum vessel in molecular flow, through a
diaphragm with conduc- tance value L2 and into the “sensor chamber” (with
its own high vacuum system). Molecular flow causes segregation but this
will be independent of the pressure level (see Section 1.5). A second
diaphragm with molecular flow, located between the sensor chamber and
the turbomolecular pump, will compensate for the segregation occurring at
L 2.
b. Process pressure > 1 mbar: Two-stage pressure converter. Using a small
(rotary vane) pump a laminar stream of gas is diverted from the rough vacuum area through a capillary or diaphragm (conductance value L3). Prior to
entry into the pump, at a pressure of about 1 mbar, a small part of this flow
is again allowed to enter the sensor chamber through the diaphragm with
conductance value L2, again as molecular flow.
A falsification of the gas composition resulting from adsorption and condensation can be avoided by heating the pressure converter and the capillary.
To evaluate the influence on the gas composition by the measurement unit
itself, information on the heating temperature, the materials and surface
areas for the metallic, glass and ceramic components will be needed along
with specifications on the material and dimensions of the cathode (and ultimately regarding the electron impact energy for the ion source as well).
4.4.3 Closed ion source (CIS)
In order to curb – or avoid entirely – influences which could stem from the
sensor chamber or the cathode (e.g. disturbance of the CO-CO2 equilibrium
by heating the cathode) a closed ion source (CIS) will be used in many
cases.
The CIS is divided into two sections: a cathode chamber where the electrons are emitted, and an impact chamber, where the impact ionization of
the gas particles takes place. The two chambers are pumped differentially:
the pressure in the cathode chamber comes to about
10-5 mbar, that in the impact room about 10-3 mbar. The gas from the vacuum chamber is allowed to pass into the impact chamber by way of a metalsealed, bakeable valve (pressure converter, ultrahigh vacuum technology).
There high-yield ionization takes place at about 10-3 mbar. The electrons
exerting the impact are emitted in the cathode chamber at about 10-5 mbar
and pass through small openings from there into the impact chamber. The
signal-to-noise ratio (residual gas) via à vis the open ion source will be
increased overall by a factor of 10+3 or more. Figure 4.8 shows the fundamental difference between the configurations for open and closed ion
sources for a typical application in sputter technology. With the modified
design of the CIS compared with the open ion source in regard to both the
geometry and the electron energy (open ion source 102 eV, CIS 75 or 35
eV), different fragment distribution patterns may be found where a lower
electron energy level is selected. For example, the argon36++ isotope at
mass of 18 cannot be detected at electron energy of less than 43.5 eV and
can therefore not falsify the detection of H2O+ at mass 18 in the sputter
processes using argon as the working gas – processes which are of great
importance in industry.
4.4.4 Aggressive gas monitor (AGM)
Non-segregating gas inlet system
Stage B
p ≤ 10 –4 mbar
p = 1 ... 10 mbar
L2
Mass
spectrometer
Seff
Seff L1 → Seff ~
QPumping
Laminar flow
2
L1
L3
QHV
L1
L2 molecular → λ dL
1
~ 았앙
M
1
~ 았앙
M
p = 10 ... 1000 mbar
Capillary
Molecular flow
L2
Stage A
In many cases the process gas to be examined is so aggressive that the
cathode would survive for only a short period of time. The AGM uses the
property of laminar flow by way of which there is no “reverse” flow of any
kind. Controlled with a separate AGM valve, a part of the working gas fed to
the processes is introduced as “purging gas”, ahead of the pressure converter, to the TRANSPECTOR; this sets up a flow toward the vacuum
chamber. Thus process gas can reach the TRANSPECTOR only with the
AGM valve closed. When the valve is open the TRANSPECTOR sees only
pure working gas. Fig. 4.9 shows the AGM principle.
No
segregation
QHV QPumping
(Transition laminar/molecular)
1
M
았앙
Seff compensates for L2 → No segregation
S
Fig. 4.7
Principle of the pressure converter (stage B only in the single-stage version and
stages A and B in two-stage units)
99
Mass spectrometry
Process: 10 -2 mbar
Process: 10 -2 mbar
(Elastomer)
10
-5
10
Valve
Inlet diaphragm
-5
(Metal)
10
-5
10
-3
Example of the sputter
process
To be detected is 1 ppm N2 as
contamination in argon,
the working gas
10 -5
10 -5
10 -5
Impact chamber
Cathode chamber
„Exit diaphragm“
10 -5
1 ppm N 2 at the inlet:
1 ppm N 2 at the inlet:
10 -6 ·10 -5 mbar = 10 -11 mbar
10 -6 · 10 -3 mbar = 10 -9 mbar
Exit diaphragm
Background:
Residual gas (valve closed) 10 -6 mbar total
total, including 1% by mass 28 : 10 -8 mbar
Fig. 4.8
Pump
Background:
Residual gas (valve closed) 10 -7 mbar total
total, including 1% by mass 28 : 10 -9 mbar
Pump
Background noise
1 ppm
Signal at 1‰ of background
cannot be detected
Background noise
Signal twice the background noise
amplitude; can just be clearly detected
Open ion source (left) and closed ion source (right)
Impact chamber
Working gas for the process (Ar)
Cathode chamber
“Exit diaphragms”
AGM protective gas valve
10 -5
Process:
e. g. 50 mbar
10 -3
Diaphragm
Fig. 4.9
100
Principle behind the aggressive gas monitor (AGM)
10 -5
10 -5
Pump
Mass spectrometry
4.5
Descriptive values in mass spectrometry
(specifications)
A partial pressure measurement unit is characterized essentially by the following properties (DIN 28 410):
4.5.4 Smallest detectable partial pressure
The smallest detectable partial pressure is defined as a ratio of noise amplitude to sensitivity:
Pmin =
Δ ⋅iR+
E
(mbar)
Δ · i+ = Noise amplitude
4.5.1 Line width (resolution)
R
The line width is a measure of the degree to which differentiation can be
made between two adjacent lines of the same height. The resolution is normally indicated. It is defined as: R = M / ΔM and is constant for the quadrupole spectrometer across the entire mass range, slightly greater than 1 or
ΔM < 1.
Often an expression such as “unit resolution with 15% valley” is used. This
means that the “bottom of the valley” between two adjacent peaks of identical height comes to 15 % of the height of the peak or, put another way, at
7.5 % of its peak height the line width DM measured across an individual
peak equals 1 amu (atomic mass unit); see in this context the schematic
drawing in Fig. 4.10.
Example (from Fig. 4.11):
Sensitivity E =
1⋅ 10 – 4
Noise amplitude Δ · i+ = 4 · 10–14 A
R
pmin(FC) =
4 ⋅ 10 – 14A
= 4 ⋅10 – 10 mbar
1⋅10 – 4A / mbar
4.5.5 Smallest detectable partial pressure ratio
(concentration)
4.5.2 Mass range
The definition is:
The mass range is characterized by the atomic numbers for the lightest and
heaviest ions with a single charge which are detected with the unit.
SDPPR = pmin / pΣ (ppm)
4.5.3 Sensitivity
Sensitivity E is the quotient of the measured ion flow and the associated
partial pressure; it is normally specified for argon or nitrogen:
A
mbar
This definition, which is somewhat “clumsy” for practical use, is to be
explained using the detection of argon36 in the air as the example: Air contains 0.93 % argon by volume; the relative isotope frequency of Ar40 to Ar36
is 99.6 % to 0.337 %. Thus the share of Ar36 in the air can be calculated as
follows:
0.93 · 10–2 · 0.337 · 10–2 = 3.13 · 10–5 = 31.3 ppm
+
E=
i ⎛ A ⎞
⎜
⎟
pG ⎝ mbar⎠
(4.1)
Typical values are:
Faraday cup: E = 1⋅10 – 4
+2
SEMP: E = 1⋅10
A
mbar
A
mbar
Figure 4.11 shows the screen print-out for the measurement. The peak
height for Ar36 in the illustration is determined to be 1.5 · 10-13 A and noise
amplitude Δ · i+ to be 4 · 10-14 A. The minimum detectable concentration is
R
that concentration at which the height of the peak is equal to the noise
amplitude. This results in the smallest measurable peak height being
1.5 · 10-13 A/2.4 · 10-14 A = 1.875. The smallest detectable concentration is
then derived from this by calculation to arrive at:
31.3 · 10–6 / 1.875 = 16.69 · 10–6 = 16.69 ppm.
100%
i
+
15%
7,5%
1 amu
M
M+1
Atomic number
ΔM
Fig. 4.10 Line width – 15 % valley
Fig. 4.11 Detection of Argon36
101
Mass spectrometry
4.5.6 Linearity range
4.6
The linearity range is that pressure range for the reference gas (N2, Ar) in
which sensitivity remains constant within limits which are to be specified
(± 10 % for partial pressure measurement devices).
4.6.1 Ionization and fundamental problems in
gas analysis
In the range below 1 · 10-6 mbar the relationship between the ion flow and
partial pressure is strictly linear. Between 1 · 10-6 mbar and 1 · 10-4 mbar
there are minor deviations from linear characteristics. Above 1 · 10-4 mbar
these deviations grow until, ultimately, in a range above 10-2 mbar the ions
for the dense gas atmosphere will no longer be able to reach the ion trap.
The emergency shut-down for the cathode (at excessive pressure) is
almost always set for 5 · 10-4 mbar. Depending on the information required,
there will be differing upper limits for use.
Evaluating spectra
Continuous change in the voltages applied to the electrodes in the separation syssem (“scanning”) gives rise to a relationship between the ion flow I+
and the “atomic number” which is proportional to the m/e ratio and
expressed as:
M=
Mr
ne
(4.2)
(Mr = relative molar mass, ne = number of elementary charges e)
10-6
In analytical applications, 1 ·
mbar should not be exceeded if at all possible. The range from 1 · 10-6 mbar to 1 · 10-4 mbar is still suitable for clear
depictions of the gas composition and partial pressure regulation (see Fig.
4.12).
4.5.7 Information on surfaces and amenability
to bake-out
Additional information required to evaluate a sensor includes specifications
on the bake-out temperature (during measurement or with the cathode or
SEMP switched off), materials used and surface areas of the metal, glass
and ceramic components and the material and dimensions for the cathode;
data is also needed on the electron impact energy at the ion source (and on
whether it is adjustable). These values are critical to uninterrupted operation
and to any influence on the gas composition by the sensor itself.
log i
This is the so-called mass spectrum, i+ = i+(M). The spectrum thus shows
the peaks i+ as ordinates, plotted against the atomic number M along the
abscissa. One of the difficulties in interpreting a mass spectrum such as
this is due to the fact that one and the same mass as per the equation (4.2)
may be associated with various ions. Typical examples, among many oth+
++
ers, are: The atomic number M = 16 corresponds to CH4 and O2 ;
+
M = 28 for CO+, N2 and C2H+! Particular attention must therefore be paid
to the following points when evaluating spectra:
1) In the case of isotopes we are dealing with differing positron counts in
the nucleus (mass) of the ion at identical nuclear charge numbers (gas
type). Some values for relative isotope frequency are compiled in Table 4.2.
Element
Ordinalnumber
Atomic
number
Relative
frequency
H
1
1
2
99.985
0.015
He
2
3
4
0.00013
≈ 100.0
B
5
10
11
19.78
80.22
C
6
12
13
98.892
1.108
N
7
14
15
99.63
0.37
O
8
16
17
18
99.759
0.0374
0.2039
F
9
19
100.0
Ne
10
20
90.92
21
0.257
22
8.82
23
100.0
+
Regulation
Exact measurement
range
Automatic shut-down:
≈ 5 · 10 –4
10–8
10–7
Fig. 4.12 Qualitative linearity curve
102
10–6
10–5
10–4
10–3
log P
Na
11
Table 4.2 Relative frequency of isotopes
Mass spectrometry
Al
Si
Ordinalnumber
Atomic
number
Relative
frequency
12
13
27
100.0
10
14
28
29
30
92.27
4.68
3.05
P
15
31
100.0
S
16
32
33
34
36
95.06
0.74
4.18
0.016
Ions formed per cm · mbar
Element
Ar
+
8
6
4
2
0
100
200
300
400
500
Electron energy (eV)
Cl
Ar
Kr
Xe
17
18
36
54
35
37
75.4
24.6
36
38
40
0.337
0.063
99.60
78
80
82
83
84
86
0.354
2.27
11.56
11.55
56.90
17.37
124
126
128
129
130
131
132
134
136
0.096
0.090
1.919
26.44
4.08
21.18
26.89
10.44
8.87
Threshold
energy
for argon ions
Ar+ 15,7 eV
Ar++ 43,5 eV
Ar3+ 85,0 eV
Ar4+ 200 eV
Fig. 4.13 Number of the various Ar ions produced, as a factor of electron energy level
tion. The fragment distribution patterns thus created are the so-called
characteristic spectra (fingerprint, cracking pattern). Important: In the
tables the individual fragments specified are standardized either against the
maximum peak (in % or ‰ of the highest peak) or against the total of all
peaks (see the examples in Table 4.4).
Both the nature of the fragments created and the possibility for multiple ionization will depend on the geometry (differing ion number, depending on the
length of the ionization path) and on the energy of the impacting electrons
(threshold energy for certain types of ions). Table values are always referenced to a certain ion source with a certain electron energy level. This is
why it is difficult to compare the results obtained using devices made by different manufacturers.
Often the probable partial pressure for one of the masses involved will be
estimated through critical analysis of the spectrum. Thus the presence of air
in the vacuum vessel (which may indicate a leak) is manifested by the
detection of a quantity of O2+ (with mass of 32) which is about one-quarter
of the share of N2+ with its mass of 28. If, on the other hand, no oxygen is
2) Depending on the energy of the impacting electrons (equalling the
potential differential, cathode – anode), ions may be either singly or multiply
ionized. For example, one finds Ar+ at mass of 40, Ar++ at mass of 20 and
Ar+++ at mass of 13.3. At mass of 20 one will, however, also find neon, Ne+.
There are threshold energy levels for the impacting electrons for all ionization states for every type of gas, i.e., each type of ion can be formed only
above the associated energy threshold. This is shown for Ar in Fig. 4.13.
3) Specific ionization of the various gases Sgas, this being the number of
ions formed, per cm and mbar, by collisions with electrons; this will vary
from one type of gas to the next. For most gases the ion yield is greatest at
an electron energy level between about 80 and 110 eV; see Fig. 4.14.
In practice the differing ionization rates for the individual gases will be taken
into account by standardization against nitrogen; relative ionization
probabilities (RIP) in relationship to nitrogen will be indicated (Table 4.3).
4) Finally, gas molecules are often broken down into fragments by ioniza-
Ions formed per cm · mbar
Table 4.2 Relative frequency of isotopes
Electron energy (eV)
Fig. 4.14 Specific ionization S for various gases by electrons exhibiting energy level E
103
Mass spectrometry
Type of gas
Acetone (Propanone)
Air
Ammonia
Argon
Benzene
Benzoic acid
Bromine
Butane
Carbon dioxide
Carbon disulfide
Carbon monoxide
Carbon tetrachloride
Chlorobenzene
Chloroethane
Chloroform
Chlormethane
Cyclohexene
Deuterium
Dichlorodifluoromethane
Dichloromethane
Dinitrobenzene
Ethane
Ethanol
Ethylene oxide
Helium
Hexane
Hydrogen
Symbol
(CH3)2CO
NH3
Ar
C6H6
C6H5COOH
Br
C4H10
CO2
CS2
CO
CCl4
C6H4Cl
C2H3Cl
CHCl3
CH3Cl
C6H12
D2
CCl2F2
CH2Cl2
C6H4(NO2)2
C2H6
C2H5OH
(CH2)2O
He
C6H14
H2
RIP
3.6
1.0
1.3
1.2
5.9
5.5
3.8
4.9
1.4
4.8
1.05
6.0
7.0
4.0
4.8
3.1
6.4
0.35
2.7
7.8
7.8
2.6
3.6
2.5
0.14
6.6
0.44
Type of gas
Hydrogen chloride
Hydrogen fluoride
Hydrogen iodide
Hydrogen sulfide
Iodine
Krypton
Lithium
Methane
Methanol
Neon
Nitrogen
Nitrogen oxide
Nitrogen dioxide
Oxygen
n-pentane
Phenol
Phosphine
Propane
Silver perchlorate
Tin iodide
Sulfur dioxide
Sulfur hexafluoride
Toluene
Trinitrobenzene
Water vapor
Xenon
Xylols
Symbol
HCl
HF
HI
H 2S
I2
Kr
Li
CH4
CH3OH
Ne
N2
NO
N 2O
O2
C5H17
C6H5OH
PH3
C3H8
AgClO4
Snl4
SO2
SF6
C6H5CH3
C6H3(NO2)3
H 2O
Xe
C6H4(CH3)2
RIP
1.6
1.4
3.1
2.2
1.7
1.9
1.6
1.8
0.23
1.0
1.2
1.7
1.0
6.0
6.2
2.6
3.7
3.6
6.7
2.1
2.3
6.8
9.0
11.0
3.0
7.8
Table 4.3 Relative ionization probabilities (RIP) vis à vis nitrogen, electron energy 102 eV
Electron energy :
Gas
Argon
Symbol
Ar
Carbon dioxide
CO2
Carbon monoxide
CO
Neon
Ne
Oxygen
O2
Nitrogen
N2
Water vapor
H2O
Mass
40
20
36
45
44
28
16
12
29
28
16
14
12
22
20
10
34
32
16
29
28
14
19
18
17
16
2
1
75 eV (PGA 100)
Σ = 100 %
Greatest peak = 100 %
74.9
100
24.7
33.1
0.95
72.7
8.3
11.7
6.15
1.89
91.3
1.1
1.7
3.5
9.2
89.6
0.84
0.45
84.2
15.0
0.7
86.3
12.8
1.4
60
16.1
1.9
5.0
15.5
Table 4.4 Fragment distribution for certain gases at 75 eV and 102 eV
104
1.3
100
11.5
16.1
8.4
2.0
100
1.2
1.9
3.8
10.2
100
0.93
0.53
100
17.8
0.8
100
15
2.3
100
27
3.2
8.4
20
102 eV (Transpector)
Σ = 100 %
Greatest peak = 100 %
90.9
100
9.1
10
0.3
0.8
1
84
100
9.2
11
7.6
9
5
6
0.9
1
92.6
100
1.9
2
0.8
4.6
5
0.9
11
90.1
100
9
4
90.1
9.9
0.9
92.6
6.5
100
11
1
100
12
74.1
18.5
1.5
1.5
4.4
100
25
2
2
6
Mass spectrometry
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
Gas
Acetone
Air
Ammonia
Argon
Benzene
Carbon dioxide
Carbon monoxide
Carbon tetrachloride
Carbon tetrafluoride
Diff. pump oil, DC 705
Diff. pump oil, Fomblin
Diff. pump oil, PPE
Ethanol
Halocarbon 11
Halocarbon 12
Halocarbon 13
Halocarbon 14
Halocarbon 23
Halocarbon 113
Helium
Heptane
Hexane
Hydrogen
Hydrogen sulfide
Isopropyl alcohol
Krypton
Methane
Mehtyl alcohol
Methyl ethyl ketone
Mechanical pump oil
Neon
Nitrogen
Oxygen
Perfluorokerosene
Perfluor-tributylamine
Silane
Silicon tetrafluoride
Toluene
Trichloroethane
Trichloroethylene
Trifluoromethane
Turbomolecular pump oil
Water vapor
Xenon
Symbol
(CH3)2CO
NH3
Ar
C6H6
CO2
CO
CCl4
CF4
CH3CH2OH
CCl3F
CCl2F2
CClF3
CF4
CHF3
C2C13F3
He
C7H16
C6H14
H2
H 2S
C3H8O
Kr
CH4
CH3OH
C4H8O
Ne
N2
O2
C12F27N
SiH4
SiF4
C6H5CH3
C2H3Cl3
C2HCl3
CHF3
H 2O
Xe
1 = 100
43/100
28/100
17/100
40/100
78/100
44/100
28/100
117/100
69/100
78/100
69/100
50/100
31/100
101/100
85/100
69/100
69/100
51/100
101/100
4/100
43/100
41/100
2/100
34/100
45/100
84/100
16/100
31/100
43/100
43/100
20/100
28/100
32/100
69/100
69/100
30/100
85/100
91/100
97/100
95/100
69/100
43/100
18/100
132/100
2
15/42
32/27
16/80
20/10
77/22
28/11
12/5
119/91
50/12
76/83
20/28
77/89
45/34
103/60
87/32
85/15
12/7
31/58
103/62
41/62
43/92
1/5
32/44
43/16
86/31
15/85
29/74
29/25
41/91
22/10
14/7
16/11
119/17
131/18
31/80
87/12
92/62
61/87
130/90
51/91
57/88
17/25
129/98
3
58/20
14/6
15/8
51/18
16/9
16/2
47/51
31/5
39/73
16/16
63/29
27/24
35/16
50/16
50/14
19/6
69/40
85/55
29/49
57/85
33/42
27/16
83/20
14/16
32/67
72/16
57/73
10/1
29/1
51/12
31/6
29/31
28/12
39/12
99/61
132/85
31/49
41/76
1/6
131/79
4
14/10
16/3
14/2
50/17
12/6
29/1
82/42
19/4
43/59
31/9
62/27
29/23
66/15
35/12
31/9
31/5
50/19
31/50
27/40
29/84
36/4
29/10
82/20
13/8
15/50
27/16
55/64
131/11
51/5
28/28
33/10
65/6
26/43
97/64
50/42
55/73
16/2
134/39
5
27/19
40/1
52/15
45/1
35/39
91/32
97/8
64/21
46/17
47/12
35/7
50/8
52/1
151/41
57/34
27/65
35/2
41/7
80/4
1/4
28/16
57/6
71/20
100/5
50/3
32/8
86/5
45.5/4
27/31
60/57
12/4
71/52
2/2
136/33
6
42/8
39/10
22/1
121/29
47/8
38/7
26/8
31/10
87/5
21/1
153/25
71/28
56/50
39/6
12/2
2/16
42/5
39/19
31/4
114/2
33/2
47/5
51/4
63/27
35/31
69/49
130/15
Table 4.5 Spectrum library of the 6 highest peaks for the TRANSPECTOR
detected in the spectrum, then the peak at atomic number 28 would indicate carbon monoxide. In so far as the peak at atomic number 28 reflects
the CO+ fragment of CO2 (atomic number 44), this share is 11 % of the
value measured for atomic number 44 (Table 4.5). On the other hand, in all
cases where nitrogen is present, atomic number 14 (N++
2 ) will always be
found in the spectrum in addition to the atomic number 28 (N2+); in the case
of carbon monoxide, on the other hand, there will always appear – in addi-
tion to CO+ – the fragmentary masses of 12 (C+) and 16 (O++
2 )).
Figure 4.15 uses a simplified example of a “model spectrum” with superimpositions of hydrogen, nitrogen, oxygen, water vapor, carbon monoxide,
carbon dioxide, neon and argon to demonstrate the difficulties involved in
evaluating spectra.
105
Mass spectrometry
CO+
O+
O+
H2+
Ar++
CO+
H 2 O+
H2+
O2 +
Ar+
CO2+
O+
N+
H+
Ne+
OH+
C+
H3O+
13C+
+
Ne++ C
H+
N2+
O+
13CO+
22Ne +
16O18O+
13C16O +
2
36Ar+
14N15N+
0
5
10
15
Hydrogen
Nitrogen
20
25
30
Oxygen
Water
35
40
Carbon dioxide
Neon
45
50
Argon
Carbon monoxide
Evaluation problems: The peak at atomic number 16 may, for example, be due to oxygen fragments resulting from O2, H2O, CO2 and CO; the peak at atomic number 28 from contributions
by N2 as well as by CO and CO as a fragment of CO2; the peak at atomic number 20 could result from singly ionized Ne and double-ionized Ar.
Fig. 4.15 Model spectrum
4.6.2 Partial pressure measurement
Thus
+
igas
The number of ions
produced from a gas in the ion source is proportional to the emission current i–, to the specific ionization Sgas, to a geometry factor f representing the ionization path inside the ionization source, to
the relative ionization probability RIPgas, and to the partial pressure pgas.
This number of ions produced is, by definition, made equal to the sensitivity
Egas times the partial pressure pgas:
+
⇒ pgas =
+
igas
, m2(produced)
BFgas , m2 ⋅ E gas
+
igas , m (measured )
2
BFgas , m ⋅ E gas ⋅TF (m)
2
and with
igas (produced) = i- · Sgas · f · RIPgas · pgas
a
= Egas · pgas
the ultimate result is:
due to
RIPN2 = 1
EN2 is equal to i– · SN2 · f
and Egas is equal to EN2 · RIPgas
Almost all gases form fragments during ionization. To achieve quantitative
evaluation one must either add the ion flows at the appropriate peaks or
measure (with a known fragment factor [FF]) one peak and calculate the
overall ion flow on that basis:
+
igas
,m1
+
+
+
igas
(produced )= igas
,m 1 + igas ,m2 +.... =
BFgas,m 1
+
igas,m2
=
=.... = Egas ⋅ pgas
BFgas,m2
In order to maintain the number of ions arriving at the ion trap, it is necessary to multiply the number above with the transmission factor TF(m), which
will be dependent on mass, in order to take into account the permeability of
the separation system for atomic number m (analogous to this, there is the
detection factor for the SEMP; it, however, is often already
contained in TF). The transmission factor (also: ion-optical transmission) is
thus the quotient of the ions measured and the ions produced.
106
pgas =
Egas = EN2 · RIPgas
+
pgas = igas
, m 2 (measured ) ⋅
1
1
⋅
E N2 BFgas , m2 ⋅ RIPgas ⋅TF (m)
(4.3)
The partial pressure is calculated from the ion flow measured for a certain
fragment by multiplication with two factors. The first factor will depend only
on the nitrogen sensitivity of the detector and thus is a constant for the
device. The second will depend only on the specific ion properties.
These factors will have to be entered separately for units with direct partial
pressure indication (at least for less common types of ions).
4.6.3 Qualitative gas analysis
The analysis of spectra assumes certain working hypotheses:
1. Every type of molecule produces a certain, constant mass spectrum or
fragment spectrum which is characteristic for this type of molecule (fingerprint, cracking pattern).
2. The spectrum of every mixture of gases is the same as would be found
Mass spectrometry
through linear superimposition of the spectra of the individual gases.
The height of the peaks will depend on the gas pressure.
3. The ion flow for each peak is proportional to the partial pressure of each
component responsible for the peak. Since the ion flow is proportional to
the partial pressure, the constant of proportionality (sensitivity) varies
from one gas to the next.
Although these assumptions are not always correct (see Robertson: Mass
Spectrometry) they do represent a useful working hypothesis.
In qualitative analysis, the unknown spectrum is compared with a known
spectrum in a library. Each gas is “definitively determined” by its spectrum.
The comparison with library data is a simple pattern recognition process.
Depending on the availability, the comparison may be made using any of a
number ancillary aids. So, for example, in accordance with the position,
size and sequence of the five or ten highest peaks. Naturally, comparison is
possible only after the spectrum has been standardized, by setting the
height of the highest line equal to 100 or 1000 (see Table 4.5 as an example).
many gas components; here a qualitative analysis will have to be made
before attempting quantitative analysis. The degree of difficulty encountered
will depend on the number of superimpositions (individual / a few / many).
In the case of individual superimpositions, mutual, balancing of the ion
flows during measurement of one and the same type of gas for several
atomic numbers can often be productive.
Where there is a larger number of superimpositions and a limited number of
gases overall, tabular evaluation using correction factors vis à vis the spectrum of a calibration gas of known composition can often be helpful.
Parent spectrum
A
The comparison can be made manually on the basis of collections of tables
(for example, A. Cornu & R. Massot: Compilation of Mass Spectral Data) or
may be effected with computer assistance; large databases can be used
(e.g. Mass Spectral Data Base, Royal Society of Chemistry, Cambridge).
When making comparisons with library information, it is necessary to pay
attention to whether identical ion sources or at least identical electron
impact energies were used.
All these capabilities are, however, generally too elaborate for the problems
encountered in vacuum technology. Many commercial mass spectrometers
can show a number of library spectra in the screen so that the user can see
immediately whether the “library substance” might be contained in the substance measured. Usually the measured spectrum was the result of a mix
of gases and it is particularly convenient if the screen offers the capacity for
subtracting (by way of trial) the spectra of individual (or several) gases from
the measured spectrum. The gas can be present only when the subtraction
does not yield any negative values for the major peaks. Figure 4.16 shows
such a step-by-step subtraction procedure using the Transpector-Ware software.
Regardless of how the qualitative analysis is prepared, the result is always
just a “suggestion”, i.e. an assumption as to which gases the mixture might
contain. This suggestion will have still to be examined, e.g. by considering
the likelihood that a certain substance would be contained in the spectrum.
In addition, recording a new spectrum for this substance can help to
achieve clarity.
4.6.4 Quantitative gas analysis
Particular difficulties are encountered when interpreting the spectrum of an
unknown mixture of gases. The proportions of ion flow from various sources
can be offset one against the other only after all the sources have been
identified. In many applications in vacuum technology one will be dealing
with mixtures of a few simple gases of known identity, with atomic numbers
less of than 50 (whereby the process-related gases can represent
exceptions). In the normal, more complicated case there will be a spectrum
with a multitude of superimpositions in a completely unknown mixture of
Parent spectrum
2
1
A = Parent
range
Assumption:
Groups
1 = Krypton+
2 = Krypton++
Library spectrum:
Krypton
Parent spectrum without krypton
4
3
Assumption:
3
Argon+
4
Argon++
Library spectrum:
Argon
Parent spectrum without argon
5
Assumption:
5
Neon+
Library spectrum:
Neon
Parent spectrum after detection
of krypton, argon and neon
Fig. 4.16 Subtracting spectra contained in libraries
107
Mass spectrometry
In the most general case a plurality of gases will make a greater or lesser
contribution to the ion flow for all the masses. The share of a gas g in each
case for the atomic number m will be expressed by the fragment factor
Ffm,g. In order to simplify calculation, the fragment factor Ffm,g will also contain the transmission factor TF and the detection factor DF. Then the ion
current to mass m, as a function of the overall ion currents of all the gases
involved, in matrix notation, is:
⎡i j+ ⎤ ⎡ BF j, k
⎢ ⎥ ⎢
⋅
⎢⋅⎥ ⎢
⎢⋅⎥ ⎢ ⋅
⎢i + ⎥ ⎢ ⋅
=
⎢ m⎥ ⎢
⎢⋅⎥ ⎢ ⋅
⎢⋅⎥ ⎢ ⋅
⎢ +⎥ ⎢
⎣iu ⎦ ⎣ BFu, k
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅
FFm, g ⋅
⋅
⋅
⋅
⋅
⋅
⋅
⋅ BF j,o ⎤ ⎡ I k+ ⎤
⎢ ⎥
⋅
⋅ ⎥ ⋅
⎥ ⎢ ⎥
⋅
⋅ ⎥ ⎢⋅⎥
⋅
⋅ ⎥ · ⎢ I g+ ⎥
⎥ ⎢ ⎥
⋅
⋅ ⎥ ⎢⋅⎥
⋅
⋅ ⎥ ⎢⋅⎥
⎥ ⎢ ⎥
⋅ BFu,o⎦ ⎣ I o+ ⎦
The ion current vector for the atomic numbers m (resulting from the contributions by the fragments of the individual gases) is equal to the fragment
matrix times the vector of the sum of the flows for the individual gases.
0
im+ = ∑ BFm, g · I g+
or:
g=k
(in simplified notation: i = FF · I)
+
where im = ion flow vector for the atomic numbers, resulting from contributions of fragments of various individual gases
0
∑ BFm, g
g=k
= fragment matrix
I+g = Vector of the sum of the flows for the individual gases or:
Ff m, g
6444474448
im+ = ∑ pg ⋅ E N2 ⋅ RIPg ⋅ FFm ⋅ TFm
Transmission factor
for the mass m
Fragment factor
for the gas to mass m
Relative ionization probability
for the gas
Nitrogen sensitivity (equipment constant)
FFT · i = FFT · FF · I
which can be evaluated direct by the computer. The ion current vector for
the individual gases is then:
–1
I=
4.7
[FF T ⋅ i] ⋅ [FF T ⋅ BF ]
det[FF T ⋅ BF ]
Software
4.7.1 Standard SQX software (DOS) for standalone operation (1 MS plus 1 PC, RS 232)
The conventional software package (SQX) contains the standard routines
for the operation of the mass spectrometer (MS)– various spectra depictions, queries of individual channels with the corresponding screen displays
as tables or bar charts, partial pressure conversion, trend displays, comparison with spectra libraries (with the capability for trial subtraction of library
spectra), leak testing mode etc. – and for sensor balancing, as well. Using
PCs as the computer and display unit naturally makes available all the
usual functions including storing and retrieving data, printing, etc.
Characteristic of the conventional software package is that specific individual spectra will be measured, even though the measurement is fully automated and takes place at a point in time which is specified in advance. A
spectrum of this type can thus be only a “snapshot” of a process in
progress.
4.7.2 Multiplex/DOS software MQX
(1 to 8 MS plus 1 PC, RS 485)
The first step toward process-oriented software by LEYBOLD is the MQX. It
makes possible simultaneous monitoring of a maximum of eight sensors
and you can apply all the SQX functions at each sensor.
4.7.3 Process-oriented software –
Transpector-Ware for Windows
Partial pressure of the gas
Ion current for atomic number m
One sees that the ion flow caused by a gas is proportional to the partial
pressure. The linear equation system can be solved only for the special
instance where m = g (square matrix); it is over-identified for m > g. Due to
unavoidable measurement error (noise, etc.) there is no set of overall ion
flow I+g (partial pressures or concentrations) which satisfies the equation
system exactly. Among all the conceivable solutions it is now necessary to
+
identify set Ig+* which after inverse calculation to the partial ion flows Im
*
will exhibit the smallest squared deviation from the partial ion currents i+m
actually measured. Thus:
∑(im − im*) = min
+
+
2
This minimization problem is mathematically identical to the solution of
another equation system
108
Transpector-Ware is based on an entirely new philosophy. During the
course of the process (and using settings – the “recipe” – determined
beforehand) data will be recorded continuously – like the individual frames
in a video. These data can be stored or otherwise evaluated. It is possible
in particular to analyze interesting process sections exactly, both during the
process and retroactively, once the process has run to completion, without
having to interrupt the measurement operations which are running in the
background. Where ongoing monitoring of identical processes is undertaken the program can generate statistics (calculating mean values and standard deviations) from which a bandwidth for “favorable process operation”
can be derived. Error reports are issued where limit values are exceeded.
Mass spectrometry
4.7.4 Development software –
TranspectorView
This software used to for develop custom software versions for special situations. It is based on the LabView development package and includes the
drivers required to operate the Transpector.
4.8
b and c only the integration period was raised, from 0.1 to 1.0 and 10 seconds (thus by an overall factor of 100), respectively. By comparison, in the
sequence a-d-e-f, at constant integration time, the total pressure was raised
in three steps, from 7.2 · 10-6 mbar to 7.2 · 10-5 mbar (or by a factor of just
10 overall).
4.9
Partial pressure regulation
Some processes, such as reactive sputter processes, require the most constant possible incidence rates for the reacting gas molecules on the substrate being coated.
The “incidence rate” is the same as the “impingement rate” discussed in
Chapter 1; it is directly proportional to the partial pressure. The simplest
attempt to keep the partial pressure for a gas component constant is
throughput by regulating with a flow controller; it does have the disadvantage that the regulator cannot determine whether, when and where the gas
consumption or the composition of the gas in the vacuum chamber
changes. The far superior and more effective option is partial pressure control using a mass spectrometer via gas inlet valves. Here the significant
peaks of the gases being considered are assigned to channels in the mass
spectrometer. Suitable regulators compare the analog output signals for
these channels with set-point values and derive from the difference
between the target and actual values for each channel the appropriate actuation signal for the gas inlet valve for the channel. A configuration of this
kind has been realized to control six channels in the QUADREX PPC. Gas
inlet valves matching the unit can also be delivered.
The gas used to measure the impingement rate (partial pressure) must naturally be drawn from a representative point in the vacuum chamber. When
evaluating the time constant for a regulation circuit of this type it is important to take into account all the time aspects and not just the electrical signal propagation and the processing in the mass spectrometer, but also the
vacuum-technology time constants and flow velocities, as illustrated in
Figure 4.17. Pressure converters or unfavorably installed gas inlet lines
joining the control valve and the vacuum vessel will make particularly large
contributions to the overall time constant. It is generally better to establish a
favorable S/N ratio with a large signal (i.e. through an inlet diaphragm with
a large opening) rather than with long integration periods at the individual
channels. Contrasted in Figure 4.18 are the effects of boosting pressure
and lengthening the integration time on signal detectability. In depictions a,
Maintenance
(Cathode service life, sensor balancing, cleaning the ion source and rod
system)
The service life of the cathode will depend greatly on the nature of the
loading. Experience has shown that the product of operating period multiplied by the operating pressure can serve as a measure for the loading.
Higher operating pressures (in a range of 1 · 10-4 to 1 · 10-3 mbar) have a
particularly deleterious effect on service life, as do certain chemical influences such as refrigerants, for example. Changing out the cathode is quite
easy, thanks to the simple design of the sensor. It is advisable, however, to
take this opportunity to change out or at least clean the entire ion source.
Sensor balancing at the mass axis (often erroneously referred to as calibration) is done today in a very easy fashion with the software (e.g. SQX,
Transpector-Ware) and can be observed directly in the screen. Naturally,
not only the arrangement along the mass axis will be determined here, but
also the shape of the lines, i.e. resolution and sensitivity (see Section 4.5).
It will be necessary to clean the sensor only in exceptional cases where it
is heavily soiled. It is usually entirely sufficient to clean the ion source,
which can be easily dismantled and cleaned. The rod system can be
cleaned in an ultrasonic bath once it has been removed from the configuration. If dismantling the system is unavoidable due to particularly stubborn grime, then the adjustment of the rods which will be required afterwards will have to be carried out at the factory.
5.
Leaks and their detection
c
b
a
t4
t5
Mass spectrometer
d
Pressure stage
Regulation valve
t6
컄
컄
컄컅
t1
Vacuum vessel
t2
컄컅
t3
e
f
Sensor
TMP50CF
Fig. 4.17 Partial shares for overall time constants
Fig. 4.18 Improving the signal-to-noise ratio by increasing the pressure or extending the integration time
109
Leak detection
Apart from the vacuum systems themselves and the individual components
used in their construction (vacuum chambers, piping, valves, detachable
[flange] connections, measurement instruments, etc.), there are large numbers of other systems and products found in industry and research which
must meet stringent requirements in regard to leaks or creating a so-called
“hermetic” seal. Among these are many assemblies and processes in the
automotive and refrigeration industries in particular but also in many other
branches of industry. Working pressure in this case is often above ambient
pressure. Here “hermetically sealed” is defined only as a relative “absence
of leaks”. Generalized statements often made, such as “no detectable
leaks” or “leak rate zero”, do not represent an adequate basis for acceptance testing. Every experienced engineer knows that properly formulated
acceptance specifications will indicate a certain leak rate (see Section 5.2)
under defined conditions. Which leak rate is acceptable is also determined
by the application itself.
5.1
Types of leaks
Differentiation is made among the following leaks, depending on the nature
of the material or joining fault:
5.2
Leak rate, leak size, mass flow
No vacuum device or system can ever be absolutely vacuum-tight and it
does not actually need to be. The simple essential is that the leak rate be
low enough that the required operating pressure, gas balance and ultimate
pressure in the vacuum container are not influenced. It follows that the
requirements in regard to the gas-tightness of an apparatus are the more
stringent the lower the required pressure level is. In order to be able to register leaks quantitatively, the concept of the “leak rate” with the symbol QL
was introduced; it is measured with mbar · l/s or cm3/s (STP) as the unit of
measure. A leak rate of QL = 1 mbar · l/s is present when in an enclosed,
evacuated vessel with a volume of 1 l the pressure rises by 1 mbar per second or, where there is positive pressure in the container, pressure drops by
1 mbar. The leak rate QL defined as a measure of leakiness is normally
specified in the unit of measure mbar · l/s. With the assistance of the status
equation (1.7) one can calculate QL when giving the temperature T and the
type of gas M, registering this quantitatively as mass flow, e.g. in the g/s
unit of measure. The appropriate relationship is then:
QL =
Δ(p ⋅ V ) R ⋅ T Δm
⋅
=
Δt
M Δt
(5.1)
• Leaks in detachable connections:
Flanges, ground mating surfaces, covers
where R = 83.14 mbar · l/mol · K, T = temperature in K; M = molar mass in
g/mole; Δm for the mass in g; Δt is the time period in seconds. Equation
5.1 is then used
• Leaks in permanent connections:
Solder and welding seams, glued joints
a) to determine the mass flow Δm / Δt at a known pV gas flow of Δp · V/Δt
(see in this context the example at 5.4.1) or
• Leaks due to porosity: particularly following mechanical deformation
(bending!) or thermal processing of polycrystalline materials and cast
components
b) to determine the pV leak gas flow where the mass flow is known (see
the following example).
• Thermal leaks (reversible): opening up at extreme temperature loading
(heat/ cold), above all at solder joints
• Apparent (virtual) leaks: quantities of gas will be liberated from hollows
and cavities inside cast parts, blind holes and joints (also due to the
evaporation of liquids)
• Indirect leaks: leaking supply lines in vacuum systems or furnaces
(water, compressed air, brine)
Example for case b) above:
A refrigeration system using Freon (R 12) exhibits refrigerant loss of 1 g of
Freon per year (at 25 °C). How large is the leak gas flow QL? According to
equation 5.1 for M(R12) = 121 g/mole:
QL =
Δ( p ⋅V )
8314
. mbar ⋅ ⋅ 298K ⋅ 1g
=
Δt
mol ⋅ K ⋅ 121g ⋅ mol –1⋅ 1year
=
• “Serial leaks”: this is the leak at the end of several “spaces connected in
series”, e.g. a leak in the oil-filled section of the oil pan in a rotary vane
pump
. ⋅ 2.98 ⋅102⋅ 1 mbar ⋅ 8314
⋅
121⋅1
. ⋅107s
315
=
8314
. ⋅ 2.98⋅102 –7 mbar ⋅ ⋅ 10 ⋅
s
1.21⋅102⋅ 315
.
• “One-way leaks”: these will allow gas to pass in one direction but are
tight in the other direction (very seldom)
= 65 ⋅10–7⋅
An area which is not gas-tight but which is not leaky in the sense that a
defect is present would be the
• Permeation (naturally permeability) of gas through materials such as
rubber hoses, elastomer seals, etc. (unless these parts have become
brittle and thus “leaky”).
mbar ⋅ s
Thus the Freon loss comes to QL = 6.5 · 10–6 mbar · l/s. According to the
“rule of thumb” for high vacuum systems given below, the refrigeration system mentioned in this example may be deemed to be very tight. Additional
conversions for QL are shown in Tables VIIa and VIIb in Chapter 9.
The following rule of thumb for quantitative characterization of high vacuum
equipment may be applied:
Total leak rate < 10-6 mbar · l/s:
Equipment is very tight
Total leak rate 10-5 mbar · l/s:
110
Leak detection
Equipment is sufficiently tight
10-4
Total leak rate >
mbar · l/s:
Equipment is leaky
A leak can in fact be “overcome” by a pump of sufficient capacity because it
is true that (for example at ultimate pressure pend and disregarding the gas
liberated from the interior surfaces):
p
end
=
QL
(5.2)
S
eff
(QL Leak rate, Seff the effective pumping speed at the pressure vessel)
Where Seff is sufficiently great it is possible – regardless of the value for the
leak rate QL – always to achieve a pre-determined ultimate pressure of
pend. In practice, however, an infinite increase of Seff will run up against
economic and engineering limitations (such as the space required by the
system).
Whenever it is not possible to achieve the desired ultimate pressure in an
apparatus there are usually two causes which can be cited: The presence
of leaks and/or gas being liberated from the container walls and sealants.
Partial pressure analysis using a mass spectrometer or the pressure rise
method may be used to differentiate between these two causes. Since the
pressure rise method will only prove the presence of a leak without indicating its location in the apparatus, it is advisable to use a helium leak detector
with which leaks can, in general, also be located much more quickly.
In order to achieve an overview of the correlation between the geometric
size of the hole and the associated leak rate it is possible to operate on the
basis of the following, rough estimate: A circular hole 1 cm in diameter in
the wall of a vacuum vessel is closed with a gate valve. Atmospheric pressure prevails outside, a vacuum inside. When the valve is suddenly opened
all the air molecules in a cylinder 1 cm in diameter and 330 m high would
within a 1-second period of time “fall into” the hole at the speed of sound
(330 m/s). The quantity flowing into the vessel each second will be 1013
mbar times the cylinder volume (see Fig. 5.1). The result is that for a hole 1
cm in diameter QL (air) will be 2.6 · 104 mbar · l/s. If all other conditions are
kept identical and helium is allowed to flow into the hole at its speed of
Δp = 1013 mbar, Hole diameter d = 1 cm
m
Gas speed = Speed of sound = 330 s
Volume/second:
Quantity/second:
3
2
+3 cm
12 · π
330 m
s = 25.95 s
s · 4 · cm = 25.95 · 10
+4
+4
mbar · 1013 mbar · 25.95 s = 2.63 · 10 앒 10
s
Diameter cm
10–2 m=
10–3 m=
10–4 m=
10–5 m=
10–6 m=
10–7 m=
–8
10 m=
10–9 m=
10–10 m=
Fig. 5.1
1.0 cm
1.0 mm
0.1 mm
0.01 mm
1.0 μm
0.1 μm
0.01 μm
1.0 nm
1.0 Angstrom
mbar · Leak rate
s
10+4
10+2
100 (= 1)
10–2
10–4
10–6
–8
10
10–10
10–12 (Detection limit, He leak detector)
Correlation between leak rate and hole size
sound of 970 m/s, then in analogous fashion the QL (helium) will come to
7.7 · 10+4 mbar · l/s, or a pV leaking gas current which is larger by a factor
of 970 / 330 = 2.94. This greater “sensitivity” for helium is used in leak
detection practice and has resulted in the development and mass production of highly sensitive helium-based leak detectors (see Section 5.5.2).
Shown in Figure 5.1 is the correlation between the leak rate and hole size
for air, with the approximation value of QL (air) of 10+4 mbar · l/s for the
“1 cm hole”. The table shows that when the hole diameter is reduced to
1 µm (= 0.001 mm) the leak rate will come to 10-4 mbar · l/s, a value which
in vacuum technology already represents a major leak (see the rule of
thumb above). A leak rate of 10-12 mbar · l/s corresponds to hole diameter
of 1 Å; this is the lower detection limit for modern helium leak detectors.
Since the grid constants for many solids amount to several Å and the diameter of smaller molecules and atoms (H2, He) are about 1 Å, inherent permeation by solids can be registered metrologically using helium leak detectors. This has led to the development of calibrated reference leaks with very
small leak rates (see Section 5.5.2.3). This is a measurable “lack of tightness” but not a “leak” in the sense of being a defect in the material or joint.
Estimates or measurements of the sizes of atoms, molecules, viruses, bacteria, etc. have often given rise to everyday terms such as “watertight” or
“bacteria-tight”; see Table 5.1.
Compiled in Figure 5.2 are the nature and detection limits of frequently
used leak detection methods.
Concept / criterion
Comment
Water-tight*)
Droplets
Vapor-tight
“Sweating”
QL [mbar · l/s]
QL <
QL < 10–3
Bacteria-tight*)
(cocci)
(rod-shaped)
QL < 10–4
Oil-tight
QL < 10–5
Virus-tight*)
(vaccines such as pox)
(smallest viruses,
bacteriophages)
(viroids, RNA)
QL < 10–8
QL < 10–10
Gas-tight
QL < 10–7
“Absolutely tight”
QL < 10–6
Technical
Relevant particle size
10–2
Avg. ≈ 1 µm
Avg. ≈ 0.5-1 µm, 2–10 µm long
Ø ≈ 3 · 10–7 m
Ø ≈ 3 · 10–8 m
Ø ª≈ 1 · 10–9 m (thread-like)
QL < 10–10
*) As opposed to vapor, it is necessary to differentiate between hydrophilic and
hydrophobic solids. This also applies to bacteria and viruses since they are
transported primarily in solutions.
Table 5.1 Estimating borderline leak rates
111
Leak detection
Helium leak detector ULTRATEST UL 200 dry/UL 500
Helium standard leak rate:
p1 = 1 bar, p2 < 1 mbar (Δp = 1 bar)
Test gas = Helium
Familiar leaks:
Dripping water faucet
4 mm diam., 1 Hz, Δp = 4 bar
➔
➔
Helium leak detector ULTRATEST UL 200/UL 500 dry/Modul 200/LDS 1000
Substance quantity trhough hole per unit of time
Δ (p · V)
Definition: Q =
Δt
➔
Contura Z
Vacuum method
Leak <----> Hole
Q ... Leak rate,
In short: Leak
Quantity escaping:
冢
mg
34 s Water
Standard He leak rate:
mbar · 艎
= 6.45
Air
s
冣
0.17
mbar · 艎
He Std
s
Pressure rise
Hair on a gasket
103................100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 mbar · l · s-1
Overpressure method
Ecotec II / Protec
ULTRATEST with helium sniffer
Halogen sniffer HLD4000A
Bubble test
10 –2
mbar · 艎
Air
s
Bicycle tube in water
(bubble test)
3
–3 Ncm
2 mm diam., 1 Hz, Δp = 0.1 bar 4.19 · 10
s
0.9 · 10 –2
冢 = 4.24 · 10
–3
mbar · 艎
Air
s
Car tire loses air
25 l, 6 Mo: 1.8 --> 1.6 bar
3.18 · 10 –4
Small refrigerant cylinder
empties in 1 year
430 g refrigerant R12, 25°C
mbar · 艎
g
F12
430 a Frigen = 2.8 · 10 –3
s
冣
mbar · 艎
Air
s
冢
1.88 · 10 –2
4.3 · 10 –5
冣
4.33 · 10 –5
mbar · 艎
He Std
s
mbar · 艎
He Std
s
mbar · 艎
He Std
s
mbar · 艎
He Std
s
Pressure drop test
Fig. 5.2
Leak rate ranges for various leak detection processes and devices
5.2.1 The standard helium leak rate
Required for unequivocal definition of a leak are, first, specifications for the
pressures prevailing on either side of the partition and, secondly, the nature
of the medium passing through that partition (viscosity) or its molar mass.
The designation “helium standard leak” (He Std) has become customary to
designate a situation frequently found in practice, where testing is carried
out using helium at 1 bar differential between (external) atmospheric pressure and the vacuum inside a system (internal, p < 1 mbar), the designation “helium standard leak rate” has become customary. In order to indicate
the rejection rate for a test using helium under standard helium conditions it
is necessary first to convert the real conditions of use to helium standard
conditions (see Section 5.2.2). Some examples of such conversions are
shown in Figure 5.3.
5.2.2 Conversion equations
When calculating pressure relationships and types of gas (viscosity) it is
necessary to keep in mind that different equations are applicable to laminar
and molecular flow; the boundary between these areas is very difficult to
ascertain. As a guideline one may assume that laminar flow is present at
leak rates where QL > 10-5 mbar · l/s and molecular flow at leak rates
where QL < 10-7 mbar · l/s. In the intermediate range the manufacturer
(who is liable under the guarantee terms) must assume values on the safe
side. The equations are listed in Table 5.2.
Range
Laminar
Molecular
Pressure
QI ⋅ (p12 − p22)II = Q II ⋅ (p12 − p22)I
QI ⋅ (p1 − p2)II = QII ⋅ (p1 − p2)I
Gas
Qgas A · ηgas A = Qgas B · ηgas B
Qgas A ⋅ Mgas A = Qgas B ⋅ Mgas B
Table 5.2 Conversion formulae for changes of pressure and gas type
112
Fig. 5.3
Examples for conversion into helium standard leak rates
Here indices “I” and “II” refer to the one or the other pressure ratio and
indices “1” and “2” reference the inside and outside of the leak point,
respectively.
5.3
Terms and definitions
When searching for leaks one will generally have to distinguish between
two tasks:
1. Locating leaks and
2. Measuring the leak rate.
In addition, we distinguish, based on the direction of flow for the fluid,
between the
a. vacuum method (sometimes known as an “outside-in leak”), where the
direction of flow is into the test specimen (pressure inside the specimen
being less than ambient pressure), and the
b. positive pressure method (often referred to as the “inside-out leak”),
where the fluid passes from inside the test specimen outward (pressure
inside the specimen being greater than ambient pressure).
The specimens should wherever possible be examined in a configuration
corresponding to their later application – components for vacuum applications using the vacuum method and using the positive pressure method for
parts which will be pressurized on the inside.
When measuring leak rates we differentiate between registering
a. individual leaks (local measurement) – sketches b and d in Figure 5.4,
and registering
b. the total of all leaks in the test specimen (integral measurement) –
sketches a and c in Figure 5.4.
Leak detection
Helium
detector. This procedure can be carried out using either helium or refrigerants or SF6 as the test gas.
5.4
a: Integral leak detection; vacuum inside
specimen
c: Integral leak detection (test gas enrichment inside the enclosure); pressurized test
gas inside specimen
Helium
Helium
b: Local leak detection; vacuum inside specimen
Fig. 5.4
d: Local leak detection; pressurized test gas
inside the specimen
Leak test techniques and terminology
The leak rate which is no longer tolerable in accordance with the acceptance specifications is known as the rejection rate. Its calculation is based
on the condition that the test specimen may not fail during its planned utilization period due to faults caused by leaks, and this to a certain degree of
certainty. Often it is not the leak rate for the test specimen under normal
operating conditions which is determined, but rather the throughput rate of
a test gas – primarily helium – under test conditions. The values thus found
will have to be converted to correspond to the actual application situation in
regard to the pressures inside and outside the test specimen and the type
of gas (or liquid) being handled.
Where a vacuum is present inside the test specimen (p < 1 mbar), atmospheric pressure outside, and helium is used at the test gas, one refers to
standard helium conditions. Standard helium conditions are always present during helium leak detection for a high vacuum system when the system is connected to a leak detector and is sprayed with helium (spray
technique). If the specimen is evacuated solely by the leak detector, then
one would say that the leak detector is operating in the direct-flow mode.
If the specimen is itself a complete vacuum system with its own vacuum
pump and if the leak detector is operated in parallel to the system’s pumps,
then one refers to partial-flow mode. One also refers to partial stream
mode when a separate auxiliary pump is used parallel to the leak detector.
When using the positive pressure method it is sometimes either impractical
or in fact impossible to measure the leakage rate directly while it could certainly be sensed in an envelope which encloses the test specimen. The
measurement can be made by connecting that envelope to the leak detector or by accumulation (increasing the concentration) of the test gas inside
the envelope. The “bombing test” is a special version of the accumulation
test (see Section 5.7.4). In the so-called sniffer technique, another variation of the of the positive pressure technique, the (test) gas issuing from
leaks is collected (extracted) by a special apparatus and fed to the leak
Leak detection methods without a leak
detector unit
The most sensible differentiation between the test methods used is differentiation as to whether or not special leak detection equipment is used.
In the simplest case a leak can be determined qualitatively and, when using
certain test techniques, quantitatively as well (this being the leak rate) without the assistance of a special leak detector. Thus the quantity of water
dripping from a leaking water faucet can be determined, through a certain
period of time, using a graduated cylinder but one would hardly refer to this
as a leak detector unit. In those cases where the leak rate can be determined during the course of the search for the leak without using a leak
detector (see, for example, Sect. 5.4.1), this will often be converted to the
helium standard leak rate (Sect. 5.2.1). This standard leak rate value is frequently needed when issuing acceptance certificates but can also be of service when comparing leak rate values determined by helium leak detector
devices.
In spite of careful inspection of the individual engineering components,
leaks may also be present in an apparatus following its assembly – be it
due to poorly seated seals or damaged sealing surfaces. The processes
used to examine an apparatus will depend on the size of the leaks and on
the degree of tightness being targeted and also on whether the apparatus is
made of metal or glass or other materials. Some leak detection techniques
are sketched out below. They will be selected for use in accordance with
the particular application situations; economic factors may play an important
part here.
5.4.1 Pressure rise test
This leak testing method capitalizes on the fact that a leak will allow a
quantity of gas – remaining uniform through a period of time – to enter a
sufficiently evacuated device (impeded gas flow, see Fig. 1.1). In contrast,
the quantity of gas liberated from container walls and from the materials
used for sealing (if these are not sufficiently free of outgassing) will decline
through time since these will practically always be condensable vapors for
which an equilibrium pressure is reached at some time (see Fig. 5.5). The
valve at the pump end of the evacuated vacuum vessel will be closed in
preparation for pressure rise measurements. Then the time is measured
during which the pressure rises by a certain amount (by one power of ten,
for example). The valve is opened again and the pump is run again for
some time, following which the process will be repeated. If the time noted
for this same amount of pressure rise remains constant, then a leak is present, assuming that the waiting period between the two pressure rise trials
was long enough. The length of an appropriate waiting period will depend
on the nature and size of the device. If the pressure rise is more moderate
during the second phase, then the rise may be assumed to result from
gases liberated from the inner surfaces of the vessel. One may also attempt
to differentiate between leaks and contamination by interpreting the curve
depicting the rise in pressure. Plotted on a graph with linear scales, the
113
Leak detection
pend =
Q
L
Seff
=
6 ⋅ 10 – 5 mbar ⋅ ⋅ s – 1
30 ⋅ s – 1
= 2 ⋅10 – 6 mbar
Pressure
Naturally it is possible to improve this ultimate pressure, should it be insufficient, by using a larger-capacity pump (e.g. the TURBOVAC 151) and at
the same time to reduce the pump-down time required to reach ultimate
pressure.
Time
1 Leak
2 Gas evolved from the container walls
3 Leak + gas evolution
curve for the rise in pressure must be a straight line where a leak is present, even at higher pressures. If the pressure rise is due to gas being liberated from the walls (owing ultimately to contamination), then the pressure
rise will gradually taper off and will approach a final and stable value. In
most cases both phenomena will occur simultaneously so that separating
the two causes is often difficult if not impossible. These relationships are
shown schematically in Figure 5.5. Once it has become clear that the rise
in pressure is due solely to a real leak, then the leak rate can be determined quantitatively from the pressure rise, plotted against time, in accordance with the following equation:
Δp ⋅ V
Δt
QL =
(5.3)
Example: Once the vacuum vessel with a volume of 20 l has been isolated
from the pump, the pressure in the apparatus rises from 1 · 10-4 mbar to
1 · 10-3 mbar in 300 s. Thus, in accordance with equation 5.2, the leak rate
will be
⎛
⎞
⎜1 ⋅10 – 3 − 1 ⋅ 10 – 4 ⎟ ⋅ 20
⎝
⎠
QL =
mbar ⋅ 9 ⋅10 – 4 ⋅ 20
= 6 ⋅ 10 – 5
300
s
The leak rate, expressed as mass flow Δm / Δt, is derived from equation
5.1 at QL = 6 · 10-5 mbar · l/s, T = 20 °C and the molar mass for air
(M = 29 g/mole) at
mbar ⋅ g
Δm
= 6 ⋅10–5⋅
⋅ 29
⋅
Δt
s
mol
⋅
mol ⋅ K
g
= 7 ⋅10 – 8
s
. mbar ⋅ ⋅ 293 ⋅102 K
8314
If the container is evacuated with a TURBOVAC 50 turbomolecular pump,
for example (S = 50 l/s), which is attached to the vacuum vessel by way of
a shut-off valve, then one may expect an effective pumping speed of about
Seff = 30 l/s. Thus the ultimate pressure will be
114
5.4.2 Pressure drop test
The thinking here is analogous to that for the pressure rise method (Section
5.4.1). The method is, however, used only rarely to check for leaks in vacuum systems. If this is nonetheless done, then gauge pressure should not
exceed 1 bar since the flange connectors used in vacuum technology will
as a rule not tolerate higher pressures. Positive pressure testing is, on the
other hand, a technique commonly employed in tank engineering. When
dealing with large containers and the long test periods they require for the
pressure drop there it may under certain circumstances be necessary to
consider the effects of temperature changes. As a consequence it may happen, for example, that the system cools to below the saturation pressure for
water vapor, causing water to condense; this will have to be taken into
account when assessing the pressure decline.
300
=
QL =
Today leak tests for vacuum systems are usually carried out with helium
leak detectors and the vacuum method (see Section 5.7.1). The apparatus
is evacuated and a test gas is sprayed around the outside. In this case it
must be possible to detect (on the basis of samplings inside the apparatus)
the test gas which has passed through leaks and into the apparatus.
Another option is to use the positive-pressure leak test. A test gas (helium)
is used to fill the apparatus being inspected and to build up a slight positive
pressure; the test gas will pass to the outside through the leaks and will be
detected outside the device. The leaks are located with leak sprays (or
soap suds, 5.4.5) or – when using He or H2 as the test gas – with a leak
detector and sniffer unit (5.7.2).
5.4.3 Leak test using vacuum gauges which are
sensitive to the type of gas
The fact that the pressure reading at vacuum gauges (see Section 3.3) is
sensitive to the type of gas involved can, to a certain extent, be utilized for
leak detection purposes. Thus it is possible to brush or spray suspected
leaks with alcohol. The alcohol vapors which flow into the device – the thermal conductivity and ionizablity of which will vary greatly from the same
properties for air – will affect and change pressure indication to a greater or
lesser extent. The availability of more precise, easy-to-use helium leak
detectors has, however, rendered this method almost completely obsolete.
Leak detection
5.4.4 Bubble immersion test
5.4.7 Krypton 85 test
The pressurized test specimen is submerged in a liquid bath. Rising gas
bubbles indicate the leak. Leak detection will depend greatly on the attentiveness of the person conducting the test and involves the temptation to
increase the “sensitivity” by using ever higher temperatures, wherein the
applicable safety regulations are sometimes disregarded. This method is
very time-consuming for smaller leaks, as Table 5.3 shows. It references
leak testing on a refrigeration system using type R12 refrigerant. Here the
leak rate is specified in grams of refrigerant loss per year (g/a). Water is
used as a test liquid (which may be heated or to which a surfactant may be
added) or petroleum-based oils. The surface tension should not exceed
75 dyn/cm (1 dyn = 10-5 N).
When dealing with small, hermetically sealed parts where the enclosure is
leaky, krypton 85, a gaseous, radioactive isotope, can first be forced into
the device by applying pressure from the outside. Once an exactly measured holding period has elapsed the pressure will be relieved, the component flushed and the activity of the “gas charge” will be measured. In the
same way it is also possible to use helium as the test gas (see Section
5.7.4, bombing test).
Freon F12 loss Time taken to form Equivalent Detection time using
per year
a gas bubble
leak rate
helium leak detector
(g/a)
(s)
(cm3[STP]/s)
(s)
280
84
13.3
40
1.8 · 10–3
a few seconds
5.4 ·
10–4
a few seconds
10–4
a few seconds
28
145
1.8 ·
14
290
9.0· 10–5
a few seconds
2.8
24 min
1.8 · 10–5
a few seconds
0.28 *
6h
1.8 · 10–6
a few seconds
*) This leak rate represents the detection limit for good halogen leak detectors (≈ 0,1 g/a).
Table 5.3 Comparison of bubble test method (immersion technique) wit helium leak detec-
5.4.5 Foam-spray test
In many cases pressurized containers or gas lines (including the gas supply
lines for vacuum systems) can be checked quite conveniently for leaks by
brushing or spraying a surfactant solution on them. Corresponding leak
detection sprays are also available commercially. Escaping gas forms “soap
bubbles” at the leak points. Here, again, the detection of smaller leaks is
time-consuming and will depend greatly on the attentiveness of the inspector. The hydrogen gas cooling systems used in power plant generators represent a special case. These are indeed sometimes tested in the fashion
described above but they can be examined much better and at much higher
sensitivity by “sniffing” the hydrogen escaping at leaks using a helium leak
detector which has been adjusted to respond to H2 (see Section 5.7.2).
5.4.6 Vacuum box check bubble
As a variation on the spray technique mentioned above, in which the escaping gas causes the bubbles, it is possible to place a so-called “vacuum box”
with a seal (something like a diver’s goggles) on the surface being examined once it has been sprayed with a soap solution. This box is then evacuated with a vacuum pump. Air entering from the outside through leaks will
cause bubbles inside the box, which can be observed through a glass window in the box. In this way it is also possible, for example, to examine flat
sheet metal plates for leaks. Vacuum boxes are available for a variety of
applications, made to suit a wide range of surface contours.
5.4.8 High-frequency vacuum test
The so-called high-frequency vacuum tester can be used not only to check
the pressure in glass equipment but also to locate porous areas in plastic or
paint coatings on metals. This comprises a hand-held unit with a brush-like
high-frequency electrode and a power pack. The shape and color of the
electrical gas discharge can serve as a rough indicator for the pressure prevailing inside glass equipment. In the case of the vacuum tester – which
comprises primarily a tesla transformer (which delivers a high-voltage, highfrequency AC current) – the corona electrode approaching the apparatus
will trigger an electrode-free discharge inside the apparatus. The intensity
and color of this discharge will depend on the pressure and the type of gas.
The luminous discharge phenomenon allows us to draw conclusions
regarding the approximate value for the pressure prevailing inside the apparatus. The discharge luminosity will disappear at high and low pressures.
When searching for leaks in glass equipment the suspect sections will be
scanned or traced with the high-frequency vacuum tester electrode. Where
there is a leak an arc will strike through to the pore in the glass wall, tracing
a brightly lit discharge trail. Small pores can be enlarged by these sparks!
The corona discharge of the vacuum tester can also penetrate thin areas in
the glass particularly at weld points and transitional areas between intermediate components. Equipment which was originally leak-free can become
leaky in this fashion! In contrast to the actual leak detector units, the highfrequency vacuum tester is highly limited in its functioning.
5.4.9 Test with chemical reactions and dye penetration
Occasionally leaks can also be located or detected by means of chemical
reactions which result in a discoloration or by penetration of a dye solution
into fine openings. The discoloration of a flame due to halogen gas escaping through leaks was used earlier to locate leaks in solder joints for refrigeration units.
A less frequently employed example of a chemical effect would be that of
escaping ammonia when it makes contact with ozalid paper (blueprint
paper) or with other materials suitably prepared and wrapped around the
outside of the specimen. Leaks are then detected based on the discoloration of the paper.
An example of a dye penetration test is the inspection of the tightness of
rubber plugs or plungers in glass tubes, used for example in testing materials suitability for disposable syringes or pharmaceutical packages. When
evaluating tiny leaks for liquids it will be necessary to consider the wetability
of the surface of the solid and the capillary action; see also Table 5.1. Some
115
Leak detection
widely used leak detection methods are shown – together with the test gas,
application range and their particular features – in Table 5.4.
Method
Test gas
Smallest detectable
leak rate
Foaming
liquids
Air and others
10
Ultrasonic
microphone
Air and others
10
Thermal conductivity leak detector
Gases other
than air
10 – 10
Halogen
leak detection
Substances
containing
halogens
10
–5
(10 )
Universal
sniffer
leak detector
Refrigerants,
helium and
other gases
10
Helium
leak detection
Helium
10
–7
10
Bubble test
Air and other
gases
10
Water pressure
test
Water
10
Pressure
drop test
Air and other
gases
10
Pressure
rise test
Air
10
mbar /s
Quantitative
measurement
7 · 10
Positive pressure
No
70
Positive pressure
No
10 – 7
–1
Positive pressure
and vacuum
No
–3
Positive pressure
(vacuum)
With
limitations
–3
Positive pressure
Yes
–9
g/a R 134 a
–4
–1
–2
–3
Pressure range
–5
–6
7 · 10
–1
(10 )
–5
7 · 10
–12
7 · 10
–4
7 · 10
Vacuum,
positive pressure
Yes
–3
7
Positive pressure
No
–2
70
Positive pressure
No
–1
Positive pressure
Yes
–1
Vacuum
Yes
–4
7 · 10
–4
7 · 10
Table 5.4 Comparison of leak detection methods
5.5
Leak detectors and how they work
Most leak testing today is carried out using special leak detection devices.
These can detect far smaller leak rates than techniques which do not use
special equipment. These methods are all based on using specific gases
for testing purposes. The differences in the physical properties of these test
gases and the gases used in real-life applications or those surrounding the
test configuration will be measured by the leak detectors. This could, for
example, be the differing thermal conductivity of the test gas and surrounding air. The most widely used method today, however, is the detection of
helium used as the test gas.
The function of most leak detectors is based on the fact that testing is conducted with a special test gas, i.e. with a medium other than the one used
in normal operation. The leak test may, for example, be carried out using
helium, which is detected using a mass spectrometer, even though the
component being tested might, for example, be a cardiac pacemaker
whose interior components are to be protected against the ingress of bodily
fluids during normal operation. This example alone makes it clear that the
varying flow properties of the test and the working media need to be taken
into consideration.
116
5.5.1 Halogen leak detectors
(HLD 4000, D-Tek)
Gaseous chemical compounds whose molecules contain chlorine and/or
fluorine – such as refrigerants R12, R22 and R134a – will influence the
emissions of alkali ions from a surface impregnated with a mixture of KOH
and Iron(III)hydroxide and maintained at 800 °C to 900 °C by an external
Pt heater. The released ions flow to a cathode where the ion current is
measured and then amplified (halogen diode principle). This effect is so
great that partial pressures for halogens can be measured down to
10-7 mbar.
Whereas such devices were used in the past for leak testing in accordance
with the vacuum method, today – because of the problems associated with
the CFCs – more sniffer units are being built. The attainable detection limit
is about 1 · 10-6 mbar · l/s for all the devisces. Equipment operating in
accordance with the halogen diode principle can also detect SF6.
Consequently these sniffer units are used to determine whether refrigerants
are escaping from a refrigeration unit or from an SF6 type switch box (filled
with arc suppression gas).
5.5.2 Leak detectors with mass spectrometers
(MSLD)
The detection of a test gas using mass spectrometers is far and away the
most sensitive leak detection method and the one most widely used in
industry. The MS leak detectors developed for this purpose make possible
quantitative measurement of leak rates in a range extending across many
powers of ten (see Section 5.2) whereby the lower limit ≈ 10-12 mbar · l/s,
thus making it possible to demonstrate the inherent gas permeability of
solids where helium is used as the test gas. It is actually possible in principle to detect all gases using mass spectrometry. Of all the available
options, the use of helium as a tracer gas has proved to be especially practical. The detection of helium using the mass spectrometer is absolutely (!)
unequivocal. Helium is chemically inert, non-explosive, non-toxic, is present
in normal air in a concentration of only 5 ppm and is quite economical. Two
types of mass spectrometer are used in commercially available MSLD’s:
a) The quadrupole mass spectrometer, although this is used less frequently due to the more elaborate and complex design (above all due to the
electrical supply for the sensor), or
b) the 180° magnetic sector field mass spectrometer, primarily due to the
relatively simple design.
Regardless of the functional principle employed, every mass spectrometer
comprises three physically important sub-systems: the ion source, separation system and ion trap. The ions must be able to travel along the path
from the ion source and through the separation system to the ion trap, to
the greatest possible extent without colliding with gas molecules. This path
amounts to about 15 cm for all types of spectrometers and thus requires a
medium free path length of at least 60 cm, corresponding to pressure of
about 1 · 10-4 mbar; in other words, a mass spectrometer will operate only
in a vacuum. Due to the minimum vacuum level of 1 · 10-4 mbar, a high
vacuum will be required. Turbomolecular pumps and suitable roughing
pumps are used in modern leak detectors. Associated with the individual
component groups are the required electrical- and electronic supply sys-
Leak detection
tems and software which, via a microprocessor, allow for the greatest possible degree of automation in the operating sequence, including all adjustment and calibration routines and measured value display.
numerical constant and to have the leak rate displayed direct.
5.5.2.2
5.5.2.1
The operating principle for a MSLD
The basic function of a leak detector and the difference between a leak
detector and mass spectrometer can be explained using Figure 5.6. This
sketch shows the most commonly found configuration for leak detection
using the helium spray method (see Section 5.7.1) at a vacuum component.
When the sprayed helium is drawn into the component through a leak it is
pumped thorough the interior of the leak detector to the exhaust, where it
again leaves the detector. Assuming that the detector itself is free of leaks,
the amount of gas flowing through each pipe section (at any desired point)
per unit of time will remain constant regardless of the cross section and the
routing of the piping. The following applies for the entry into the pumping
port at the vacuum pump:
Q=p·S
(5.4)
At all other points
Q = p · Seff
(5.4a)
applies, taking the line losses into account.
The equation applies to all gases which are pumped through the piping and
thus also for helium.
QHe = pHe · Seff, He
(5.4b)
In this case the gas quantity per unit of time is the leak rate being sought;
the total pressure may not be used, but only the share for helium or the
partial pressure for helium. This signal is delivered by the mass spectrometer when it is set for atomic number 4 (helium). The value for Seff is a constant for every series of leak detectors, making it possible to use a microprocessor to multiply the signal arriving from the mass spectrometer by a
Detection limit, background, gas storage in oil
(gas ballast), floating zero-point suppression
The smallest detectable leak rate is dictated by the natural background
level for the gas to be detected. Even with the test connector at the leak
detector closed, every gas will pass – counter to the pumping direction –
through the exhaust and through the pumps (but will be reduced accordingly by their compression) through to the spectrometer and will be detected
there if the electronic means are adequate to do so. The signal generated
represents the detection limit. The high vacuum system used to evacuate
the mass spectrometer will normally comprise a turbomolecular pump and
an oil-sealed rotary vane pump. (Diffusion pumps were used earlier instead
of the turbomolecular pumps.) Like every liquid, the sealing oil in the rotary
vane pump has the capability of dissolving gases until equilibrium is
reached between the gas dissolved in the oil and the gas outside the oil.
When the pump is warm (operating temperature) this equilibrium state represents the detection limit for the leak detector. The helium stored in the oil
thus influences the detection limit for the leak detector. It is possible for test
gas to enter not only through the test connection and into the leak detector;
improper installation or inept handling of the test gas can allow test gas to
enter through the exhaust and the airing or gas ballast valve and into the
interior of the detector, to increase the helium level in the oil and the elastomer seals there and thus to induce a background signal in the mass
spectrometer which is well above the normal detection limit. When the
device is properly installed (see Fig. 5.7) the gas ballast valve and the airing valve will be connected to fresh air and the discharge line (oil filter!)
should at least be routed to outside the room where the leak test takes
place.
An increased test gas (helium) background level can be lowered by opening the gas ballast valve and introducing gas which is free of the test gas
(helium-free gas, fresh air). The dissolved helium will be flushed out, so to
speak. Since the effect always affects only the part of the oil present in the
pump body at the particular moment, the flushing procedure will have to
Test gas
e.g. He
Test connection
Test specimen
Test connection
Mass spectrometer
Turbomolecular
pump
MS
Venting
valve
Leak detector
QHe = pHe · SeffHe
Roughing pump
Exhaust
Exhaust
Fig. 5.6
Gas ballast
valve
Basic operating principle for a leak detector
Fig. 5.7
Correct set-up for a MSLD
117
Leak detection
be continued until all the oil from the pump’s oil pan has been recirculated
several times. This period of time will usually be 20 to 30 minutes.
In order to spare the user the trouble of always having to keep an eye on
the background level, what has been dubbed floating zero-point suppression has been integrated into the automatic operating concepts of all INFICON leak detectors (Section 5.5.2.5). Here the background level measured
after the inlet valve has been closed is placed in storage; when the valve is
then opened again this value will automatically be deducted from subsequent measurements. Only at a relatively high threshold level will the display panel show a warning indicating that the background noise level is too
high. Figure 5.8 is provided to illustrate the process followed in zero point
suppression. Chart on the left. The signal is clearly larger than the background. Center chart: the background has risen considerably; the signal
can hardly be discerned. Chart on the right: the background is suppressed
electrically; the signal can again be clearly identified.
Independent of this floating zero-point suppression, all the leak detectors
offer the capability for manual zero point shifting. Here the display for the
leak detector at the particular moment will be “reset to zero” so that only
rises in the leak rate from that point on will be shown. This serves only to
facilitate the evaluation of a display but can, of course, not influence its
accuracy.
Modern leak detectors are being more frequently equipped with oil-free
vacuum systems, the so-called “dry leak detectors” (UL 200 dry, UL 500
dry). Here the problem of gas being dissolved in oil does not occur but similar purging techniques will nonetheless be employed.
5.5.2.3
Calibrating leak detectors; test leaks
Calibrating a leak detector is to be understood as matching the display at a
leak detector unit, to which a test leak is attached, with the value shown on
the “label” or calibration certificate. The prerequisite for this is correct
adjustment of the ion paths in the spectrometer, also known as tuning.
Often the distinction is not made quite so carefully and both procedures
together are referred to as calibration.
entiate between two types of calibration: with an internal or external test
leak. When using a test leak built into the leak detector the unit can itself be
calibrated but it can only calibrate itself. When using an external test leak
not just the device but also a complete configuration, such as a partial flow
arrangement, can be included. Internal test leaks are permanently installed
and cannot be misplaced. At present all the leak detectors being distributed
by INFICON are fitted with an automatic calibration routine.
Sniffer units or configurations will as a rule have to be calibrated with special, external test leaks in which there is a guarantee that on the one hand
all the test gas issuing from the test leak reaches the tip of the probe and
on the other hand that the gas flow in the sniffer unit is not hindered by calibration. When making measurements using the sniffer technique (see
Section 5.7.2) it is also necessary to take into account the distance from the
probe tip to the surface of the specimen and the scanning speed; these
must be included as a part of the calibration. In the special case where helium concentration is being measured, calibration can be made using the
helium content in the air, which is a uniform 5 ppm world-wide.
Test leaks (also known as standard leaks or reference leaks) normally
comprise a gas supply, a choke with a defined conductance value, and a
valve. The configuration will be in accordance with the test leak rate
required. Figure 5.9 shows various test leaks. Permeation leaks are usually
used for leak rates of 10-10 < QL < 10-7, capillaries, between 10-8 and 10-4
and, for very large leak rates in a range from 10 to 1000 mbar · l/s, pipe
sections or orifice plates with exactly defined conductance values (dimensions).
Test leaks used with a refrigerant charge represent a special situation since
the refrigerants are liquid at room temperature. Such test leaks have a supply space for liquid from which, through a shut-off valve, the space filled
only with the refrigerant vapor (saturation vapor pressure) can be reached,
ahead of the capillary leak. One technological problem which is difficult to
solve is posed by the fact that all refrigerants are also very good solvents
for oil and grease and thus are often seriously contaminated so that it is difficult to fill the test leaks with pure refrigerant. Decisive here is not only the
chemical composition but above all dissolved particles which can repeatedly clog the fine capillaries.
In the calibration process proper the straight-line curve representing the
numerically correct, linear correlation between the gas flow per unit of time
and the leak rate is defined by two points: the zero point (no display where
no emissions are detected) and the value shown with the test leak (correct
display for a known leak).
In vacuum operations (spray technique, see Section 5.7.1) one must differ-
10–6
cau-
10–7
10–8
fine
10–9
prec
10–10
10–11
Equipment background level:
Leak:
Display:
Fig. 5.8
118
a
< 2 · 10–10
2 · 10–8
2 · 10–8
Example of zero-point suppression
1 · 10–8
2 · 10–8
3 · 10–8
1 · 10–10 (suppressed)
2 · 10–8
2 · 10–8
b
c
a Reference leak without gas supply, TL4, TL6
b Reference leak for sniffer and vacuum
applications, TL4-6
c (Internal) capillary test leak TL7
Fig. 5.9
d
e
d Permeation (diffusion) reference leak, TL8
e Refrigerant calibrated leak
Examples for the construction of test leaks
Leak detection
5.5.2.4
Leak detectors with quadrupole mass spectrometer (ECOTEC II)
INFICON builds leak detectors with quadrupole mass spectrometers to register masses greater than helium. Apart from special cases, these will be
refrigerants. These devices thus serve to examine the tightness of refrigeration units, particularly those for refrigerators and air conditioning equipment.
Figure 4.2 shows a functional diagram for a quadrupole mass spectrometer.
Of the four rods in the separation system, the two pairs of opposing rods
will have identical potential and excite the ions passing through along the
center line so that they oscillate transversely. Only when the amplitude of
these oscillations remains smaller than the distance between the rods can
the appropriate ion pass through the system of rods and ultimately reach
the ion trap, where it will discharge and thus be counted. The flow of electrons thus created in the line forms the measurement signal proper. The
other ions come into contact with one of the rods and will be neutralized
there.
Figure 5.10 shows the vacuum schematic for an ECOTEC II. The mass
spectrometer (4) only operates under high vacuum conditions, i.e. the pressure here must always remain below 10-4 mbar. This vacuum is generated
by the turbomolecular pump (3) with the support of the diaphragm pump
(1). The pressure PV between the two pumps is measured with a piezo
resistive measuring system (2) and this pressure lies in the range between
1 to 4 mbar while in the measurement mode. This pressure must not
exceed a value of 10 mbar as otherwise the turbomolecular pump will not
be capable of maintaining the vacuum in the mass spectrometer. The unit
can easily be switched over at the control unit from helium to any of various
refrigerants, some of which may be selected as desired. Naturally the unit
must be calibrated separately for each of these masses. Once set, however, the values remain available in storage so that after calibration has been
effected for all the gases (and a separate reference leak is required for
each gas!) it is possible to switch directly from one gas to another.
5.5.2.5
Helium leak detectors with 180° sector mass
spectrometer (UL 200, UL 500)
5
internal
flow
limiter 1 particle
filter
external
particle
filter
6
flow divider 1
QMA 200
flow
limiter 2
4
1 Diaphragm pump
2 Piezo-resistive
pressure sensor
3 Turbomolecular
pump
4 Quadrupole mass
spectrometer
5 Sniffer line
6 Gas flow limiter
7 Gas flow limiter
8 Gas flow meter
These units are the most sensitive and also provide the greatest degree of
certainty. Here “certain” is intended to mean that there is no other method
with which one can, with greater reliability and better stability, locate leaks
and measure them quantitatively. For this reason helium leak detectors,
even though the purchase price is relatively high, are often far more economical in the long run since much less time is required for the leak detection procedure itself.
A helium leak detector comprises basically two sub-systems in portable
units and three in stationary units. These are:
1. the mass spectrometer
2. the high vacuum pump and
3. the auxiliary roughing pump system in stationary units.
The mass spectrometer (see Fig. 5.11) comprises the ion source (1–4) and
the deflection system (5–9). The ion beam is extracted through the orifice
plate (5) and enters the magnetic field (8) at a certain energy level. Inside
the magnetic field the ions move along circular paths whereby the radius for
a low mass is smaller than that for higher masses. With the correct setting
of the acceleration voltage during tuning one can achieve a situation in
which the ions describe a circular arc with a defined curvature radius.
Where mass 4 (helium) is involved, they pass thorough the aperture (9) to
the ion trap (13). In some devices the discharge current for the ions impinging upon the total pressure electrodes will be measured and evaluated as a
total pressure signal. Ions with masses which are too small or too great
should not be allowed to reach the ion trap (13) at all, but some of these
ions will do so in spite of this, either because they are deflected by colli-
14
13
7
2
3
4
5
12
11
6
flow divider 2
3
1
10
flow
limiter 3
7
flow meter
pv
9
8
2
1
1
Fig. 5.10 Vacuum schematic for the ECOTEC II
2
1 Ion source flange
2 Cathode
(2 cathodes, Ir + Y2O3)
3 Anode
4 Shielding of the ion
source with discharge
orifice
8
5
6
7
8
9
Extractor
Ion traces for M > 4
Total pressure electrode
Ion traces for M = 4
Intermediate orifice
plate
10 Magnetic field
11
12
13
14
Suppressor
Shielding of the ion trap
Ion trap
Flange for ion trap with
preamplifier
Fig. 5.11 Configuration of the 180° sector mass spectrometer
119
Leak detection
sions with neutral gas particles or because their initial energy deviates too
far from the required energy level. These ions are then sorted out by the
suppressor (11) so that only ions exhibiting a mass of 4 (helium) can reach
the ion detector (13). The electron energy at the ion source is 80 eV. It is
kept this low so that components with a specific mass of 4 and higher –
such as multi-ionized carbon or quadruply ionized oxygen – cannot be created. The ion sources for the mass spectrometer are simple, rugged and
easy to replace. They are heated continuously during operation and are
thus sensitive to contamination. The two selectable yttrium oxide coated
iridium cathodes have a long service life. These cathodes are largely
insensitive to air ingress, i.e. the quick-acting safety cut-out will keep them
from burning out even if air enters. However, prolonged use of the ion
source may eventually lead to cathode embrittlement and can cause the
cathode to splinter if exposed to vibrations or shock.
Depending on the way in which the inlet is connected to the mass spectrometer, one can differentiate between two types of MSLD.
5.5.2.6
Direct-flow and counter-flow leak detectors
Figure 5.12 shows the vacuum schematic for the two leak detector types. In
both cases the mass spectrometer is evacuated by the high vacuum pumping system comprising a turbomolecular pump and a rotary vane pump. The
diagram on the left shows a direct-flow leak detector. Gas from the inlet
port is admitted to the spectrometer via a cold trap. It is actually equivalent
to a cryopump in which all the vapors and other contaminants condense.
(The cold trap in the past also provided effective protection against the oil
vapors of the diffusion pumps used at that time). The auxiliary roughing
pump system serves to pre-evacuate the components to be tested or the
connector line between the leak detector and the system to be tested. Once
the relatively low inlet pressure (pumping time!) has been reached, the
valve between the auxiliary pumping system and the cold trap will be
opened for the measurement. The Seff used in equation 5.4b is the pumping speed of the turbomolecular pump at the ion source location:
QHe = pHe · Seff,turbomolecular pump ion source
Solution 1: Direct-flow leak detector
(5.5a)
Solution 2: Counter-flow leak detector
Test specimen
Test specimen
Test gas stream
Test gas stream
In the case of direct-flow leak detectors, an increase in the sensitivity can
be achieved by reducing the pumping speed, for example by installing a
throttle between the turbomolecular pump and the cold trap. This is also
employed to achieve maximum sensitivity. To take an example:
The smallest detectable partial pressure for helium is
pmin,He = 1 · 10-12 mbar. The pumping speed for helium would be
SHe = 10 l/s. Then the smallest detectable leak rate is
Qmin = 1 · 10-12 mbar · 10 l/s = 1 · 10-11 mbar · l/s. If the pumping speed is
now reduced to 1/s, then one will achieve the smallest detectable leak rate
of 1 · 10-12 mbar · l/s. One must keep in mind, however, that with the
increase in the sensitivity the time constant for achieving a stable test gas
pressure in the test specimen will be correspondingly larger (see Section
5.5.2.9).
In Figure 5.12 the right hand diagram shows the schematic for the counterflow leak detector. The mass spectrometer, the high vacuum system and
also the auxiliary roughing pump system correspond exactly to the configuration for the direct-flow arrangement. The feed of the gas to be examined
is however connected between the roughing pump and the turbomolecular
pump. Helium which reaches this branch point after the valve is opened will
cause an increase in the helium pressure in the turbomolecular pump and
in the mass spectrometer. The pumping speed Seff inserted in equation
5.4b is the pumping speed for the rotary vane pump at the branch point.
The partial helium pressure established there, reduced by the helium compression factor for the turbomolecular pump, is measured at the mass spectrometer. The speed of the turbomolecular pump in the counter-flow leak
detectors is regulated so that pump compression also remains constant.
Equation 5.5b is derived from equation 5.5a:
QHe = pHe · Seff · K
(5.5b)
Seff = effective pumping speed at the rotary vane pump at the
branching point
K = Helium compression factor at the turbomolecular pump
The counter-flow leak detector is a particular benefit for automatic vacuum
units since there is a clearly measurable pressure at which the valve can be
opened, namely the roughing vacuum pressure at the turbomolecular
pump. Since the turbomolecular pump has a very large compression capacity for high masses, heavy molecules in comparison to the light test gas,
helium (M = 4), can in practice not reach the mass spectrometer. The turbomolecular pump thus provides ideal protection for the mass spectrometer
and thus eliminates the need for an LN2 cold tap, which is certainly the
greatest advantage for the user. Historically, counter-flow leak detectors
were developed later. This was due in part to inadequate pumping speed
stability, which for a long time was not sufficient with the rotary vane pumps
used here. For both types of leak detector, stationary units use a built-in
auxiliary pump to assist in the evacuation of the test port. With portable leak
detectors, it may be necessary to provide a separate, external pump, this
being for weight reasons.
p TOT < 10–4 mbar
LN 2
MS
pHe
MS
pHe
–4
p TOT < 10 mbar
High vacuum pump
High vacuum pump
Auxiliary pump
Roughing pump
Auxiliary pump
Cold trap:
2
S = 6.1 /s · cm
2
Fl ≈ 1000 cm
S = 6,100 /s
Fig. 5.12 Full-flow and counter-flow leak detector
120
Roughing pump
5.5.2.7
Partial flow operation
Where the size of the vacuum vessel or the leak makes it impossible to
evacuate the test specimen to the necessary inlet pressure, or where this
would simply take too long, then supplementary pumps will have to be
used. In this case the helium leak detector is operated in accordance with
Leak detection
the so-called “partial flow” concept. This means that usually the larger part
of the gas extracted from the test object will be removed by an additional,
suitably dimensioned pump system, so that only a part of the gas stream
reaches the helium leak detector (see Fig. 5.13). The splitting of the gas
flow is effected in accordance with the pumping speed prevailing at the
branching point. The following then applies:
QVacuum vessel = γ · DisplayLeak detector
(5.6)
where g is characterized as the partial flow ratio, i.e. that fraction of the
overall leak current which is displayed at the detector. Where the partial
flow ratio is unknown, g can be determined with a reference leak attached
at the vacuum vessel:
Display at the leak detector
γ = ———————————————
QL for the reference leak
Connection to vacuum systems
The partial flow concept is usually used in making the connection of a helium leak detector to vacuum systems with multi-stage vacuum pump sets.
When considering where to best make the connection, it must be kept in
mind that these are usually small, portable units which have only a low
pumping speed at the connection flange (often less than 1 l/s). This makes
Partial flow principle (example)
5.5.2.9
· (Leak rate)
QHe = 3 · 10–5 mbar
s
V = 150 A second method for coupling to larger systems, for example, those used
for removing the air from the turbines in power generating stations, is to
couple at the discharge. A sniffer unit is inserted in the system where it discharges to atmosphere. One then sniffs the increase in the helium concentration in the exhaust. Without a tight coupling to the exhaust, however, the
detection limit for this application will be limited to 5 ppm, the natural helium
content in the air. In power plants it is sufficient to insert the tip of the probe
at an angle of about 45 ° from the top into the discharge line (usually pointing upward) of the (water ring) pump.
Time constants
The time constant for a vacuum system is set by
τ=
SLD = 8 Leak detector (LD)
s
Seff = SPFP + SLD →
m3
SPFP = 60 s = 16.66 s Partial flow pump (PFP)
A) Signal amplitude:
Splitting of the gas flow (also of the test
gas!) in accordance with the effective pumping
speed at the partial flow branch point
V
Seff
(5.8)
τ = Time constant
V = Volume of the container
Seff = Effective pumping speed, at the test object
Figure 5.14 shows the course of the signal after spraying a leak in a test
specimen attached to a leak detector, for three different configurations:
Overal pumping speed: Seff = SLD + SPFP = 8 + 16.66 = 24.66 s
�
γ ... Partial flow ratio
再
Signal to Leak detector: 3 · 10–5 mbars · ·
–5
Signal to partial flow pump: 3 · 10
8 s
(8 + 16.66) s
mbar · · 16,668s s
(8 + 16.66)
s
�
Q
V
= 9.73 · 10–6
mbar · s
–5
mbar · s
= 2.02 · 10
S
S’
Q
S
MS
LD
= 3.00 · 10–5 mbars · QHe = QLD + QPFP
Q = 2p
S/2
MS
LD
LD
normal
Faster, less sensitive
Check: Overall signal
LD
or
γ=
SLD
1
=
S
SLD + SPFP 1 + nnn
S
PFP
LD
再
PFP
Display
100%
2,0
eff
s
Estimate: Value for S, V and γ are uncertain → certain: calibrate with reference leak
Fig. 5.13 Partial flow principle
�
Signal rise
Q =p
S
1,0
Q = P/
2
S + S’
0,5
100%
95%
Leak rate
150 B) Response time: t95% = 3 · SV = 3 · 24.66
= 18.25 s
m
Throttle
Slower, more sensitive
95%
Partial flow ratio = Fraction of the overall flow to the leak detector
Q
QLD
1
γ = LD =
=
Q
QHe QLD + QPFP 1 + nnn
Q
QLD = γ · QHe
Q
V
S/2
S
MS
Signal
amplitude
�
Q
V
ⱱ
5.5.2.8
(5.7)
it all the more important to estimate – based on the partial flow ratio to be
expected vis à vis a diffusion pump with pumping speed of 12000 l/s, for
example – which leak rates can be detected at all. In systems with high
vacuum- and Roots pumps, the surest option is to connect the leak detector
between the rotary vane pump and the roots pump or between the roots
pump and the high vacuum pump. If the pressure there is greater than the
permissible inlet pressure for the leak detector, then the leak detector will
have to be connected by way of a metering (variable leak) valve. Naturally
one will have to have a suitable connector flange available. It is also advisable to install a valve at this point from the outset so that, when needed, the
leak detector can quickly be coupled (with the system running) and leak
detection can commence immediately after opening the valve. In order to
avoid this valve being opened inadvertently, it should be sealed off with a
blank flange during normal vacuum system operation.
100%
95%
0
�
�
Compensation period, e.g. t95% = 3 · τ = 3 V
·
S
(τ = V ... Time constant)
S
to
3 · V = 1 (3 · V)
Dead time S + S’ 2
S
3· V
S
Time
3 · SV = 2 (3 · V )
S
/2
Fig. 5.14 Signal responses and pumping speed
121
Leak detection
1. Center: The specimen with volume of V is joined directly with the leak
detector LD (effective pumping speed of S).
2. Left: In addition to 1, a partial flow pump with the same effective pumping speed, Sl = S, is attached to the test specimen.
3. Right: As at 1, but S is throttled down to 0.5◊S.
either due to one large leak or several smaller leaks – more gas flows into
the unit than the maximum permissible throughput rate for the leak detector.
5.7
Leak detection techniques using helium
leak detectors
The signals can be interpreted as follow:
1: Following a “dead period” (or “delay time”) up to a discernible signal
level, the signal, which is proportional to the partial pressure for helium, will
rise to its full value of pHe = Q/Seff in accordance with equation 5.9:
pHe =
–t
⎞
Q ⎛
⋅ ⎜1 − e τ ⎟
Seff ⎝
⎠
(5.9 )
The signal will attain a prortion of its ultimate value after
t = 1 τ . . 63.3 %
t = 3 τ . . 95.0 %
t = 5 τ . . 99.3 %
t = 2 τ . . 86.5 %
t = 4 τ . . 98.2 %
t = 6 τ . . 99.8 %
The period required to reach 95 % of the ultimate value is normally referred
to as the response time.
2: With the installation of the partial flow pump both the time constant and
the signal amplitude will be reduced by a factor of 2; that means a quicker
rise but a signal which is only half as great. A small time constant means
quick changes and thus quick display and, in turn, short leak detection
times.
3: The throttling of the pumping speed to 0.5 S, increases both the time
constant and the signal amplitude by a factor of 2. A large value for t thus
increases the time required appropriately. Great sensitivity, achieved by
reducing the pumping speed, is always associated with greater time
requirements and thus by no means is always of advantage.
5.7.1 Spray technique (local leak test)
The test specimen, connected to the helium leak detector, is slowly traced
with a very fine stream of helium from the spray pistol, aimed at likely leakage points (welding seams, flange connectors, fused joints), bearing in
mind the time constant of the system as per Equation 5.8 (see Fig. 5.14).
The volume sprayed must be adjusted to suit the leak rate to be detected
and the size and accessibility of the object being tested. Although helium is
lighter than air and therefore will collect beneath the ceiling of the room, it
will be so well distributed by drafts and turbulence induced by movements
within the room that one need not assume that helium will be found primarily (or only) at the top of the room during search for leaks. In spite of this, it
is advisable, particularly when dealing with larger components, to start the
search for leaks at the top.
In order to avoid a surge of helium when the spray valve is opened (as this
would “contaminate” the entire environment) it is advisable to install a
choke valve to adjust the helium quantity, directly before or after the spray
pistol (see Fig. 5.15). The correct quantity can be determined easiest by
submerging the outlet opening in a container of water and setting the valve
on the basis of the rising bubbles. Variable-area flowmeters are indeed
available for the required small flow quantities but are actually too expensive. In addition, it is easy to use the water-filled container at any time to
determine whether helium is still flowing.
The helium content of the air can also be detected with helium leak detectors where large leaks allow so much air to enter the vessel that the
An estimate of the overall time constants for several volumes connected
one behind to another and to the associated pumps can be made in an initial approximation by adding the individual time constants.
5.6
Limit values / Specifications for the leak
detector
1. The smallest detectable leak rate.
2. The effective pumping speed at the test connection.
3. The maximum permissible pressure inside the test specimen (also
the maximum permissible inlet pressure). This pressure pmax will be
about 10-1 for LDs with classical PFPs and about 2 to 10 mbar for LDs
with compound PFPs. The product of this maximum permissible operating pressure and the pumping speed S of the pump system at the
detector’s test connection is the maximum permissible throughput:
Qmax = pmax · Seff, connector
(5.10)
This equation shows that it is by no means advantageous to attain high
sensitivity by throttling down the pumping speed. The maximum permissible
throughput would otherwise be too small. The unit is not functional when –
Avoiding the “helium surge” when the pistol valve is opened
a) Throttle hose or
b) Adjustable throttle valve ahead of the spay pistol
Minimum helium quantity for correct display: Changing the setting for the throttle shall not
affect indication.
The minimum quantity is always much smaller than one would set without a flowmeter (e.g.
by listening for flow or letting the helium flow across moistened lips). The simplest check
without a flowmeter: Letting gas bubble through water.
Fig. 5.15 Helium spray equipment
122
Leak detection
5 ppm share of helium in the air is sufficient for detection purposes. The
leak rate is then:
Display (pure He) Display (atmosph. He)
————————– = ——————————–
1
5 · 10-6
QL = Display (pure He)
= 2 · 10+5 · Display (atmospheric He)
(5.11)
5.7.2 Sniffer technology (local leak test using
the positive pressure method)
Here the points suspected of leaking at the pressurized test specimen (see
Fig. 5.4, d) are carefully traced with a test gas probe which is connected
with the leak detector by way of a hose. Either helium or hydrogen can be
detected with the INFICON helium leak detectors. The sensitivity of the
method and the accuracy of locating leaky points will depend on the nature
of the sniffer used and the response time for the leak detector to which it is
connected. In addition, it will depend on the speed at which the probe is
passed by the leak points and the distance between the tip of the probe
and the surface of the test specimen. The many parameters which play a
part here make it more difficult to determine the leak rates quantitatively.
Using sniffer processes it is possible, virtually independent of the type of
gas, to detect leak rates of about 10-7 mbar · l/s. The limitation of sensitivity
in the detection of helium is due primarily to the helium in the atmosphere
(see Chapter 9, Table VIII). In regard to quantitative measurements, the
leak detector and sniffer unit will have to be calibrated together. Here the
distance from the specimen and the tracing speed will have to be included
in calibration, too.
5.7.3 Vacuum envelope test (integral leak test)
Vacuum envelope tests are integral leak tests using helium as the test gas,
in which the test specimen is enclosed either in a rigid (usually metal)
enclosure or in a light plastic envelope. The helium which enters or leaves
(depending on the nature of the test) the test specimen is passed to a helium leak detector, where it is measured. Envelope tests are made either
with the test specimen pressurized with helium (Fig. 5.4c) or with the test
specimen evacuated (Fig. 5.4a). In both cases it may be necessary to convert the helium enrichment figure (accumulation) to the helium standard
leak rate.
5.7.3.1
Envelope test – test specimen pressurized with
helium
a) Envelope test with concentration measurement and subsequent
leak rate calculation
To determine overall leakiness of a test object pressurized with helium the
object shall be enclosed in an envelope which is either rigid or deformable
(plastic). The test gas leaving the leaks accumulates so that the helium
concentration in the envelope rises. Following an enrichment period to be
determined (operating period) the change in concentration inside the
envelope will be measured with a sniffer connected to the helium detection
unit. The overall leak rate (integral leak rate) can be calculated following
calibration of the test configuration with a reference concentration, e.g.
atmospheric air. This method makes it possible to detect even the smallest
overall leakiness and is suitable in particular for automated industrial leak
testing. Due to gas accumulation, the limits for normal sniffer techniques
are shifted toward lower leak rates and the ambient conditions such as temperature, air flow and sniffer tracing speed lose influence. When using plastic envelopes it is necessary to take into account helium permeation
through the plastic envelope during long enrichment periods.
b) Direct measurement of the leak rate with the leak detector
(rigid envelope)
When the test specimen, pressurized with helium, is placed in a rigid vacuum chamber, connected to a helium leak detector, the integral leak rate can
be read directly at the leak detector.
5.7.3.2
Envelope test with test specimen evacuated
a) Envelope = “plastic tent”
The evacuated test specimen is surrounded by a light-weight (plastic)
enclosure and this is then filled with helium once the atmospheric air has
been removed. When using a plastic bag as the envelope, the bag should
be pressed against the test specimen before filling it with helium in order to
expel as much air as possible and to make the measurement with the
purest helium charge possible. The entire outside surface of the test object
is in contact with the test gas. If test gas passes through leaks and into the
test specimen, then the integral leak rate will be indicated, regardless of the
number of leaks. In addition, it is necessary to observe when repeating testing in enclosed areas that the helium content of the room will rise quite
rapidly when the envelope is removed. Using plastic bags is thus more
advisable for “one-off” testing of large plants. The plastic envelope used
here is often referred to as a “tent”.
b) Rigid envelope
The use of a solid vacuum vessel as the rigid envelope, on the other hand,
is better for repetitive testing where an integral test is to be made. When
solid envelopes are used it is also possible to recover the helium once the
test has been completed.
5.7.4 “Bombing” test,
“Storage under pressure”
The “bombing” test is use to check the tightness of components which are
already hermetically sealed and which exhibit a gas-filled, internal cavity.
The components to be examined (e.g. transistors, IC housings, dry-reed
relays, reed contact switches, quartz oscillators, laser diodes and the like)
are placed in a pressure vessel which is filled with helium. Operating with
the test gas at relatively high pressure (5 to 10 bar) and leaving the system
standing over several hours the test gas (helium) will collect inside the
123
Leak detection
leaking specimens. This procedure is the actual “bombing”. To make the
leak test, then, the specimens are placed in a vacuum chamber following
“bombing”, in the same way as described for the vacuum envelope test.
The overall leak rate is then determined. Specimens with large leaks will,
however, lose their test gas concentration even as the vacuum chamber is
being evacuated, so that they will not be recognized as leaky during the
actual leak test using the detector. It is for this reason that another test to
register very large leaks will have to be made prior to the leak test in the
vacuum chamber.
5.8
Industrial leak testing
Industrial leak testing using helium as the test gas is characterized above
all by the fact that the leak detection equipment is fully integrated into the
manufacturing line. The design and construction of such test units will naturally take into account the task to be carried out in each case (e.g. leak
testing vehicle rims made of aluminum or leak testing for metal drums).
Mass-produced, standardized component modules will be used wherever
possible. The parts to be examined are fed to the leak testing system
(envelope test with rigid envelope and positive pressure [5.7.3.1b] or vacuum [5.7.3.2b] inside the specimen) by way of a conveyor system. There
they will be examined individually using the integral methods and automatically moved on. Specimens found to be leaking will be shunted to the side.
The advantages of the helium test method, seen from the industrial point of
view, may be summarized as follows:
• The leak rates which can be detected with this process go far beyond all
practical requirements.
• The integral leak test, i.e. the total leak rate for all individual leaks, facilitates the detection of microscopic and sponge-like distributed leaks
which altogether result in leakage losses similar to those for a larger
individual leak.
• The testing procedure and sequence can be fully automated.
• The cyclical, automatic test system check (self-monitoring) of the device
ensures great testing reliability.
• Helium is non-toxic and non-hazardous (no maximum allowable concentrations need be observed).
• Testing can be easily documented, indicating the parameters and
results, on a printer.
Use of the helium test method will result in considerable increases in efficiency (cycling times being only a matter of seconds in length) and lead to
a considerable increase in testing reliability. As a result of this and due to
the EN/ISO 9000 requirements, traditional industrial test methods (water
bath, soap bubble test, etc.) will now largely be abandoned.
124
Thin film controllers/control units
6
Thin film controllers and control units
with quartz oscillators
6.1
Introduction
It took a long time to go from the coating of quartz crystals for frequency
fine tuning, which has long been in practice, to utilization of frequency
change to determine the mass per unit area as a microbalance with the
present-day degree of precision. In 1880 two brothers, J. and P. Curie, discovered the piezoelectric effect. Under mechanical loads on certain quartz
crystal surfaces, electrical charges occur that are caused by the asymmetrical crystalline structure of SiO2. Conversely, in a piezocrystal deformations
appear in an electrical field and mechanical oscillations occur in an alternating field. A distinction is made between bending oscillations, thickness
shear mode and thickness shear oscillations. Depending on the orientation
of the cut plane to the crystal lattice, a number of different cuts are distinguished, of which only the so-called AT cut with a cut angle of
35°10" is used in thin film controllers because the frequency has a very low
temperature dependence in the range between 0 and 50 °C with this cut.
Accordingly, an attempt must be made not to exceed this temperature
range during coating (water cooling of crystal holder).
Since there is still a problem with “quartz capacity” (i.e. the maximum possible coating thickness of the quartz at which it still oscillates reliably) despite
refined technology, a number of approaches have been developed to
expand this capacity:
1. The use of several crystals, one behind the other, in a multiple crystal
holder with automatic change and data updating in the event of imminent failure of a quartz: CrystalSix.
2. The RateWatcher function, in which the quartz is alternately exposed to
the coating beam for a short time until all measurements and regulation
have been carried out and then remains covered by a shutter for a
longer period of time.
The selection of the “right” crystal holder thus plays an important role in all
measurements with quartz oscillators. Various crystal holder designs are
recommended for the different applications: with or without shutter, bakeable for UHV, double crystal holder or crystal six as well as special versions
for sputter applications. In addition to these important and more “mechanical” aspects, the advances in measuring and control technology and equipment features will be discussed in the following.
6.2
Basic principles of coating thickness measurement with quartz oscillators
The quartz oscillator coating thickness gauge (thin film controller) utilizes
the piezoelectric sensitivity of a quartz oscillator (monitor crystal) to the supplied mass. This property is utilized to monitor the coating rate and final
thickness during vacuum coating.
A very sharp electromechanical resonance occurs at certain discrete frequencies of the voltage applied. If mass is added to the surface of the
quartz crystal oscillating in resonance, this resonance frequency is diminished. This frequency shift is very reproducible and is now understood precisely for various oscillation modes of quartz. Today this phenomenon,
which is easy to understand in heuristic terms, is an indispensable measuring and process control tool, with which a coating increase of less than one
atomic layer can be detected.
In the late 1950s Sauerbrey and Lostis discovered that the frequency shift
connected with the coating of the quartz crystal is a function of the change
in mass due to the coating material in the following way:
Mf ΔF
ΔF
=
or Mf = Mq ·
with
Mq Fq
Fq
Mf
mass of the coating
Mq
mass of the quartz prior to coating
Fq
frequency prior to coating
Fc
frequency after coating
ΔF = Ff – Fc ... frequency shift due to coating
rel. frequency change (ppm)
If the following are now applied: Mf = (Mc – Mq) = Df · ρf · A and
Mq = Dq · ρq · A, where T = the coating thickness, ρ = density and A stands
for area while the index q stands for the state of the “uncoated quartz” and
c for the state after “frequency shift due to coating”, the following results are
obtained for the coating thickness:
Df =
K=
Fq
Fq
· Dq · ρ q ·
Dq · Fq · ρq
2
Fq
Temperature (°C)
Fig. 6.1
(6.1)
Natural frequency as a function of temperature in an AT cut quartz crystal
=
ΔF
ΔF
=K·
ρf
·
Fq ρ f
with
NAT · ρq
2
Fq
where
N = Fq · Dq is the frequency constant (for the AT cut NAT = 166100 Hz · cm)
and ρq = 2.649 g/cm3 is the density of the quartz. The coating thickness is
thus proportional to the frequency shift ΔF and inversely proportional to the
125
Thin film controllers/control units
↔
Node
Fig. 6.2
→
E
Thickness shear oscillations
Fig. 6.3
density ρf of the coating material. The equation
Df = K ·
ΔF
ρf
for the coating thickness was used in the first coating thickness measuring
units with “frequency measurement” ever used. According to this equation,
a crystal with a starting frequency of 6.0 MHz displays a decline in frequency of 2.27 Hz after coating with 1Å of aluminum (de = 2.77 g/cm3). In this
way the growth of a fixed coating due to evaporation or sputtering can be
monitored through precise measurement of the frequency shift of the crystal. It was only when knowledge of the quantitative interrelationship of this
effect was acquired that it became possible to determine precisely the
quantity of material that is deposited on a substrate in a vacuum.
Previously this had been practically impossible.
6.3
The shape of quartz oscillator crystals
Regardless of how sophisticated the electronic environment is, the main
component for coating measurement remains the monitor quartz crystal.
Originally monitor quartzes had a square shape. Fig. 6.4 shows the resonance spectrum of a quartz resonator with the design used today (Fig. 6.3).
The lowest resonance frequency is initially given by a thickness shear oscillation, which is called the fundamental wave. The characteristic
log Relative intensy
126
motions of the thickness shear oscillation are parallel to the main crystal
boundary surfaces. In other words: the surfaces are shift antinodes, see
Fig. 6.2. The resonance frequencies slightly above the basic frequency are
called “anharmonic” and are a combination of thickness shear and thickness rotation oscillation forms. The resonance frequency at around three
times the value for the fundamental wave is called “quasi-harmonic”. Near
the quasi-harmonic there are also a number of anharmonics with a slightly
higher frequency.
The design of the monitor crystals used nowadays (see Fig. 6.3) displays a
number of significant improvements over the original square crystals. The
first improvement was the use of round crystals. The enlarged symmetry
greatly reduced the number of possible oscillation modes. A second group
of improvements involved providing one of the surfaces with a contour and
making the excitation electrode smaller. The two together ensure that the
acoustic energy is recorded. Reducing the electrode diameter limits the
excitation to the middle area. The surface contour consumes the energy of
the moving acoustic waves before they reach the crystal edge. It is not
reflected into the center where it could interfere with new incoming waves.
Such a small crystal behaves like an infinitely expanded crystal. However, if
the crystal vibrations remain restricted to the center, one can clamp the
outer edge to a crystal holder, without engendering undesired side effects.
Moreover, contouring reduces the resonance intensity of undesired anharmonics. This limits the capacity of the resonator to maintain these oscillations considerably.
Use of an adhesive coating has enhanced the adhesion of the quartz electrode. Even the rate spikes occurring with increasing film stress (strain) and
caused by micro-tears in the coating were reduced. Coating material
remains at these micro-tears without adhesion and therefore cannot oscillate. These open areas are not registered and thus an incorrect thickness is
indicated.
Fig. 6.4 shows the frequency behavior of a quartz crystal shaped as in Fig.
6.3. The ordinate represents the amplitude of the oscillation or also the current flowing through the crystal as a function of the frequency on the
abscissa.
Frequency (MHz)
Fig. 6.4
Shape of LEYBOLD-Inficon quartz crystals
Frequency resonance spectrum
Usually an AT cut is chosen for the coating thickness measurement
because through the selection of the cut angle the frequency has a very
small temperature coefficient at room temperature.
Thin film controllers/control units
Since one cannot distinguish between
• coating: frequency reduction = negative influence
• temperature change:
negative or positive influence
• temperature gradients on the crystal, positive or negative
• stresses caused by the coating
it is important to minimize the temperature influence. This is the only way to
measure small differences in mass.
6.4
rate regulation becomes impossible (rate regulation: regulation of the energy supply to the coating source so that a specified coating thickness growth
per time unit is maintained). The great measurement uncertainty then causes more noise in the closed loop, which can only be countered with longer
time constants. This in turn makes the corrections due to system deviation
slow so that relatively long deviations from the desired rate result. This may
not be important for simple coatings, but for critical coatings, as in the case
of optical filters or very thin, slowly growing single-crystal coatings, errors
may result. In many cases, the desired properties of such coatings are lost
if the rate deviations are more than one or two percent. Finally, frequency
and stability of the reference oscillator determine the precision of the measurement.
Period measurement
Although the instruments that functioned according to equation 6.2 were
very useful, it soon became obvious that for the desired accuracy their area
of application was typically limited to ΔF < 0.02 Fq. Even at a relative frequency change of (Fq – Fc) / Fq < 2 %, errors of around 2 % occurred in the
coating thickness measurement so that the "usable service life" of the coating in the case of a 6-MHz monitor crystal was about 120 kHz.
In 1961 Behrndt discovered that:
Mf (Tc − Tq) ΔF
=
=
Mq
Tq
Fc
6.5
Miller and Bolef (1968) treated the quartz oscillator and coating system as a
single-dimensional, coherent acoustic resonator. Lu and Lewis (1972)
developed the simplified Z match equation on that basis. Simultaneous
advances in electronics, particularly the microprocessor, made it possible to
solve the Z match equation in real time. Most coating process control units
sold today use this sophisticated equation, which takes into account the
acoustic properties of the quartz oscillator/coating system:
with (6.3)
⎛
⎡ π ⋅ (Fq − Fc)⎤⎞
⎛ N ⋅d ⎞
Tf = ⎜⎜ AT q ⎟⎟ ⋅ arctg ⎜Z ⋅ tg ⎢
⎥⎟
⎜
Fq
⎝π ⋅ df ⋅ Fc ⋅ Z⎠
⎢⎣
⎥⎦⎟⎠
⎝
Tc = 1 / Fc ... oscillation period, coated
Tq = 1 / Fq ... oscillation period, uncoated
The period measurement (measurement of the oscillation duration) was the
result of the introduction of digital time measurement and the discovery of
the proportionality of crystal thickness Dq and oscillation duration Tq. The
necessary precision of thickness measurement permits application of equation 6.3 up to about ΔF < 0.05 Fq.
In period measurement a second crystal oscillator is essentially used as a
reference oscillator that is not coated and usually oscillates at a much higher frequency than the monitor crystal. The reference oscillator generates
small precision time intervals, with which the oscillation duration of the monitor crystal is determined. This is done by means of two pulse counters: the
first counts a fixed number of monitor oscillations m. The second is started
simultaneously with the first and counts the oscillations of the reference
crystal during m oscillations of the monitor crystal. Because the reference
frequency Fr is known and stable, the time for m monitor oscillations can be
determined accurately to ± 2/Fr. The monitor oscillation period is then
n
Fr · m
where n is the reading of the reference counter. The accuracy of the measurement is determined by the frequency of the reference oscillator and the
length of the counting time that is specified through the size of m.
For low coating rates, small densities of the coating material and fast measurements (that require short counting times), it is important to have a reference oscillator with a high frequency. All of this requires great time precision
so that the small coating-related frequency shifts can be resolved. If the frequency shift of the monitor crystal decreases between two measurements
on the order of magnitude of the frequency measurement accuracy, good
The Z match technique
Z=
dq ⋅ Uq
df ⋅ U f
(6.4)
acoustic impedance ratio
Uq = shear module, quartz
Uf = shear module, film
This led to basic understanding of the conversion of frequency shift into
thickness which enabled correct results in a practical time frame for process
control. To achieve this high degree of accuracy, the user must only enter
an additional material parameter Zf for the coating material. The validity of
the equation was confirmed for many materials and it applies to frequency
shifts up to ΔF < 0.4 Fq! Note that equation 6.2 was only valid up to
ΔF < 0.02 Fq. And equation 6.3 only up to ΔF < 0.05 Fq.
6.6
The active oscillator
All units developed up to now are based on use of an active oscillator, as
shown schematically in Fig. 6.5. This circuit keeps the crystal actively in
resonance so that any type of oscillation duration or frequency measurement can be carried out. In this type of circuit the oscillation is maintained
as long as sufficient energy is provided by the amplifier to compensate for
losses in the crystal oscillation circuit and the crystal can effect the necessary phase shift. The basic stability of the crystal oscillator is created
through the sudden phase change that takes place near the series resonance point even with a small change in crystal frequency, see Fig. 6.6.
127
Thin film controllers/control units
amplifier
output
log .Z. (Ohm)
| impedance |
phase (degrees)
phase
series resonance
Frequency (MHz)
crystal
Fig. 6.5
Circuit of the active oscillator
Fig. 6.6
Normally an oscillator circuit is designed such that the crystal requires a
phase shift of 0 degrees to permit work at the series resonance point. Longand short-term frequency stability are properties of crystal oscillators
because very small frequency differences are needed to maintain the phase
shift necessary for the oscillation. The frequency stability is ensured through
the quartz crystal, even if there are long-term shifts in the electrical values
that are caused by “phase jitter” due to temperature, ageing or short-term
noise. If mass is added to the crystal, its electrical properties change.
Fig. 6.7 shows the same graph as Fig 6.6, but for a thickly coated crystal. It
has lost the steep slope displayed in Fig. 6.6. Because the phase rise is
less steep, any noise in the oscillator circuit leads to a larger frequency shift
than would be the case with a new crystal. In extreme cases, the original
phase/frequency curve shape is not retained; the crystal is not able to carry
out a full 90 ° phase shift.
phase
| impedance |
series resonance
Frequency (MHz)
Fig. 6.7
128
Oscillations of a thickly coated crystal
phase (degress)
log .Z. (Ohm)
The impedance |Z| can increase to very high values. If this happens, the
oscillator prefers to oscillate in resonance with an anharmonic frequency.
Sometimes this condition is met for only a short time and the oscillator
oscillation jumps back and forth between a basic and an anharmonic oscillation or it remains as an anharmonic oscillation. This phenomenon is well
known as "mode hopping". In addition to the noise of the rate signal created, this may also lead to incorrect termination of a coating because of the
phase jump. It is important here that, nevertheless, the controller frequently
continues to work under these conditions. Whether this has occurred can
only be ascertained by noting that the coating thickness is suddenly signifi-
Crystal frequencies near the series resonance point
cantly smaller, i.e. by the amount of the frequency difference between the
fundamental wave and the anharmonic adopted by the oscillation.
6.7
The mode-lock oscillator
INFICON has developed a new technology for overcoming these constraints on the active oscillator. The new system constantly analyzes the
response of the crystal to an applied frequency: not only to determine the
(series) resonance frequency, but also to ensure that the quartz oscillates in
the desired mode. The new system is insensitive to mode hopping and the
resultant inaccuracy. It is fast and precise. The crystal frequency is determined 10 times a second with an accuracy to less than 0.0005 Hz.
The ability of the system to initially identify and then measure a certain
mode opens up new opportunities thanks to the advantages of the additional information content of these modes. This new, "intelligent" measuring
device makes use of the phase/frequency properties of the quartz crystal to
determine the resonance frequency. It works by applying a synthesized
sinus wave of a certain frequency to the crystal and measuring the phase
difference between the applied signal voltage and the current flowing
through the crystal. In the case of series resonance, this difference is exactly zero degrees; then the crystal behaves like an ohmic resistance. By disconnecting the applied voltage and the current that returns from the crystal,
one can determine with a phase comparator whether the applied frequency
is higher or lower than the crystal resonance point.
The crystal impedance is capacitive at frequencies below the fundamental
wave and inductive at frequencies above the resonance. This information is
useful if the resonance frequency of a crystal is unknown. A brief frequency
sweep is carried out until the phase comparator changes over and thus
marks the resonance. For AT quartzes we know that the lowest usable frequency is the fundamental wave. The anharmonics are slightly above that.
This information is not only important for the beginning, but also in the rare
case that the instrument loses “track” of the fundamental wave. Once the
frequency spectrum of the crystal is determined, the instrument must track
the shift in resonance frequency, constantly carry out frequency measurements and then convert them into thickness.
Use of the “intelligent” measuring system has a number of obvious advantages over the earlier generation of active oscillators, primarily insensitivity
Thin film controllers/control units
to mode hopping as well as speed and accuracy of measurement. This
technique also enables the introduction of sophisticated
properties which were not even conceivable with an active oscillator setup.
The same device that permits the new technology to identify the fundamental wave with one sweep can also be used to identify other oscillation
modes, such as the anharmonics or quasi-harmonics. The unit not only has
a device for constantly tracking the fundamental wave, but can also be
employed to jump back and forth between two or more modes. This query
of different modes can take place for two modes with 10 Hz on the same
crystal.
nances is shifted to lower frequencies. It has been observed that the three
above mentioned modes have a somewhat differing mass sensitivity and
thus experience somewhat different frequency shifts. This difference is utilized to determine the Z value of the material. By using the equations for
the individual modes and observing the frequencies for the (100) and the
(102) mode, one can calculate the ratio of the two elastic constants C60 and
C55. These two elastic constants are based on the shear motion. The key
element in Wajid’s theory is the following equation:
6.8
with
Auto Z match technique
The only catch in the use of equation 6.4 is that the acoustic impedance
must be known. There are a number of cases where a compromise has to
be made with accuracy due to incomplete or restricted knowledge of the
material constants of the coating material:
1) The Z values of the solid material often deviate from those of a coating.
Thin coatings are very sensitive to process parameters, especially in a
sputter environment. As a result, the existing values for solid material
are not adequate.
(C55 / C66 )coated
1
≈
(C55 / C66 )uncoated (1 + M ⋅ Z )
M ... area mass/density ratio (ratio of coating mass to quartz mass per area
unit)
Z ... Z value
It is a fortunate coincidence that the product M Z also appears in the LuLewis equation (equation 6.4). It can be used to assess the effective Z
value from the following equations:
⎛
⎛ F ⎞
F⎞
tg ⎜⎜M ⋅ Z ⋅ π ⋅ c⎟⎟ + Z ⋅ tg ⎜⎜ π ⋅ c ⎟⎟ = 0
F
⎝
⎝ Fq ⎠
q⎠
2) For many exotic substances, including alloys, the Z value is not known
and not easy to determine.
3) It is repeatedly necessary to carry out a precise coating thickness measurement for multiple coating with the same crystal sensor. This applies
in particular to optical multiple and semi-conductor coatings with a high
temperature coefficient TC. However, the effective Z value of the mixture
of multiple coatings is unknown.
In such a case, therefore, the only effective method is to assume a Z value
of 1, i.e. to ignore reality with respect to wave propagation in multi-substance systems. This incorrect assumption causes errors in the prediction
of thickness and rate. The magnitude of the error depends on the coating
thickness and the amount of deviation from the actual Z value.
In 1989 A. Wajid invented the mode-lock oscillator. He presumed that a
connection existed between the fundamental wave and one of the anharmonics, similar to that ascertained by Benes between the fundamental
oscillation and the third quasi-harmonic oscillation. The frequencies of the
fundamental and the anharmonic oscillations are very similar and they solve
the problem of the capacity of long cables. He found the necessary considerations for establishing this connection in works by Wilson (1954) as well
as Tiersten and Smythe (1979).
The contour of the crystal, i.e. the spherical shape of one side, has the
effect of separating the individual modes further from each other and preventing energy transfer from one mode to another. The usual method of
identification is to designate the fundamental oscillation as (100), the lowest
anharmonic frequency as (102) and the next higher anharmonic as (120).
These three indices of the mode nomenclature are based on the number of
phase reversals in the wave motion along the three crystal axes. The above
mentioned works by Wilson, Tiersten and Smythe examine the properties of
the modes by studying the influence of the radius of the cut on the position
of the anharmonic in relation to the fundamental oscillation.
(6.5)
⎛
or
Z=−
tg ⎜⎜ M ⋅ Z ⋅ π ⋅
⎝
⎛
tg ⎜⎜ π ⋅
⎝
(6.6)
Fc ⎞
⎟
Fq ⎟⎠
Fc ⎞
⎟
Fq ⎟⎠
Here Fq and Fc are the frequencies of the non-coated or coated quartz in
the (100) mode of the fundamental wave. Because of the ambiguity of the
mathematical functions used, the Z value calculated in this way is not
always a positively defined variable. This has no consequences of any significance because M is determined in another way by assessing Z and
measuring the frequency shift. Therefore, the thickness and rate of the
coating are calculated one after the other from the known M.
One must be aware of the limits of this technique. Since the assessment of
Z depends on frequency shifts of two modes, any minimal shift leads to
errors due to substantial mechanical or thermal stresses. It is not necessary
to mention that under such circumstances the Z match technique, too,
leads to similar errors. Nevertheless, the automatic Z value determination of
the Z match technique is somewhat more reliable regarding occurrence of
errors because the amplitude distribution of the (102) mode is asymmetric
over the active crystal surface and that of the (100) mode is symmetric.
According to our experience, coating-related stresses have the most unfavorable effect on the crystal. This effect is particularly pronounced in the
presence of gas, e.g. in sputter processes or reactive vacuum coating or
sputter processes. If the Z value for solid material is known, it is better to
use it than to carry out automatic determination of the “auto Z ratio”. In
cases of parallel coating and coating sequences, however, automatic Z
determination is significantly better.
If one side of the quartz is coated with material, the spectrum of the reso-
129
Thin film controllers/control units
6.9
Coating thickness regulation
The last point to be treated here is the theory of the closed loop for coating
thickness measuring units to effect coating growth at a controlled (constant)
growth rate. The measuring advantages of the instruments, such as speed,
precision and reliability, would not be completely exploited if this information
were not inputted into an improved process monitoring system. For a coating process this means the coating rate should be kept as close and stable
as possible to a setpoint. The purpose of the closed loop is to make use of
the information flow of the measuring system in order to regulate the
capacity for a special evaporation source in an appropriately adapted way.
When the system functions correctly, the controller translates small deviations of the controlled parameter (the rate) from the setpoint into correction
values of the re-adjusted evaporation capacity parameter. The ability of the
controller to measure quickly and precisely keeps the process from deviating significantly from the setpoint.
The most widespread type of controller is the PID controller. Here P stands
for proportional, I for integral and D for differential control function. In the
following some of the properties of this controller are described in detail.
Information on the system behavior is gained through a step response to a
control fault in certain controller settings.
This response is recorded, and then improved control parameters for a new
test are estimated. This procedure is continued until a satisfactory result is
achieved. At the end the controller is optimized so that its parameters
exactly match the characteristics of the evaporator source.
It is a long and frustrating process to adjust a controller to an evaporation
source, requiring several minutes for stabilization and hours to obtain satisfactory results. Often the parameters selected for a certain rate are not suitable for an altered rate. Thus, a controller should ideally adjust itself, as the
new controllers in INFICON coating measuring units do. At the beginning of
installation and connection the user has the unit measure the characteristics of the evaporation source. Either a PID controller is used as the basis
for slow sources or another type of controller for fast sources without significant dead time.
In relevant literature a distinction is made between three different ways of
setting controllers. Depending on which data are used for the setting, a distinction is made between the closed loop, open loop and resonance
response method.
Due to the simplicity with which the experimental data can be obtained, we
preferred the open loop method. Moreover, application of this technique
permits extensive elimination of the trial and error method.
The Auto Control Tune function developed by INFICON characterizes a
process on the basis of its step responses. After a step-by-step change in
the power the resulting changes in the rate as a function of time are
smoothed and stored. The important step responses are determined, see
Fig. 6.8.
In general, it is not possible to characterize all processes exactly, so several
approximations have to be made. Normally one assumes that the dynamic
characteristic can be reproduced by a process of the first order plus dead
time. The Laplace transformation for this assumption (transfer to the s
plane) is approximated:
−L
K p ⋅ 10 s
Output
=
τ ⋅ s+1
Input
with(6.8)
Kp = amplification in stationary state
L = dead time
τ = time constant
These three parameters are determined through the response curve of the
process. An attempt has been made by means of several methods to calculate the required parameters of the system response from curves, as shown
in Fig. 6.8. This results in a 1-point accordance at 63.2 % of the transition
(a time constant), an exponential accordance at two points and an
exponential accordance weighted according to the method of the smallest
squares. A process is sufficiently characterized by this information so that
the controller algorithm can be applied. Equation 6.9 shows the Laplace
transformation for the very often used PID controller:
⎛
M(s) = Kc · ⎜1 +
⎝
⎞
S
+ T · S⎟ · E (s)
Ti d ⎠
with
M(s) =
Kc =
Ti =
Td =
E(s) =
controlled variable or power
Control amplification (the proportional term)
integration time
differentiation time
process deviation
Fig. 6.9 shows the control algorithm and a process with a phase shift of the
first order and a dead time. The dynamics of the measuring device and the
control elements (in our case the evaporator and the power supply) are
1.00 K p
0.0632 K p
point of
maximum
rise
setpoint deviation
R(s)
+ E(s)
0
L
t (0.632)
(Σ)
–
Time t
s
Kc (1 + + Td * s)
Ti
[process]
T1 = t(0.632) – L
Kp = (change in output signal)/(change in control signal)
Fig. 6.8
130
(6.9)
Process response to a step change with t = 0 (open loop, control signal amplified)
Fig. 6.9
Block diagram of the PID controller
–L
K p · eaaa
s
T1s + 1
[controller]
precipitation rate
C(s)
Thin film controllers/control units
implicitly contained in the process block. R(s) represents the rate setpoint.
The return mechanism is the deviation created between the measured precipitation rate C(s) and the rate setpoint R(s).
Kc = ⎜⎜
The key to use of any control system is to select the correct values for Kc,
Td and Ti. The “optimum control” is a somewhat subjective term that is
made clear by the presence of different mathematical definitions:
Ti = ⎜
Usually the smallest square error ISE (Integral Square Error) is used as a
measure of the quality of the control:
⎛ L⎞
Td = (0.381⋅ T1) ⋅ ⎜ ⎟
⎝ T1⎠
ISE = ∫ e2(t) ⋅ dt
The integral of the absolute value of the deviation IAE (Integral Absolute
Error) was also proposed as a measure for control quality:
(6.11)
This is more sensitive for small deviations, but less sensitive for large deviations than ISE.
Graham and Lanthrop introduced the integral over time, multiplied by the
absolute error ITAE (Integral Time Absolute Error), as a measure for control
quality:
ITAE = ∫ t ⋅ e(t) ⋅ dt
⎛ 119
. ⎞ ⎛ L⎞
⎟ ⋅⎜ ⎟
⎝ T1 ⎠ ⎝ T1⎠
(6.13)
0.738
(6.14)
.
0995
(6.15)
(6.10)
Here e is the error (the deviation): e = rate setpoint minus measured rate.
ISE is relatively insensitive to small deviations, but large deviations contribute substantially to the value of the integral. The result is small “overshoots”, but long ripple times because deviations occurring late contribute
little to the integral.
IAE = ∫ e(t) ⋅ dt
– 0947
.
⎛ 136
. ⎞ ⎛ L⎞
⎟⎟ ⋅ ⎜ ⎟
K
⎝ p ⎠ ⎝ T1⎠
(6.12)
The ITAE is sensitive to initial and, to a certain extent, unavoidable deviations. Optimum control responses defined through ITAE consequently have
short response times and larger “overshoots” than in the case of the other
two criteria. However, ITAE has proven to be very useful for evaluating the
regulation of coating processes.
INFICON’s Auto Control Tune is based on measurements of the system
response with an open loop. The characteristic of the system response is
calculated on the basis of a step change in the control signal. It is determined experimentally through two kinds of curve accordance at two points.
This can be done either quickly with a random rate or more precisely with a
rate close to the desired setpoint. Since the process response depends on
the position of the system (in our case the coating growth rate), it is best
measured near the desired work point. The process information measured
in this way (process amplification Kp, time constant T1 and dead time L) are
used to generate the most appropriate PID control parameters.
The best results in evaluating coating control units are achieved with ITAE.
There are overshoots, but the reaction is fast and the ripple time short.
Controller setting conditions have been worked up for all integral evaluation
criteria just mentioned so as to minimize the related deviations. With a manual input as well as with experimental determination of the process
response coefficients, the ideal PID coefficients for the ITAE evaluation can
easily be calculated from equations 6.13, 6.14 and 6.15:
For slow systems the time interval between the forced changes in control
voltage is extended to avoid “hanging” the controller (hanging = rapid
growth of the control signal without the system being able to respond to the
altered signal). This makes a response to the previous change in the controller setting and “powerful” controller settings possible. Another advantage
is the greater insensitivity to process noise because the data used for control do not come from merely one measurement, but from several, so that
the mass-integrating nature of the quartz crystal is utilized.
In processes with short response times (short time constants) and small to
unmeasurable dead times, the PID controller often has difficulties with the
noise of the coating process (beam deflection, rapid thermal short-circuits
between melt and evaporator, etc.). In these cases a control algorithm of
the integral reset type is used with success. This controller always integrates the deviation and presses the system towards zero deviation. This
technique works well with small or completely imperceptible dead times.
However, if it is used with a noticeable phase shift or dead time, the controller tends to generate oscillations because it overcompensates the controller signal before the system has a chance to respond. Auto Control
Tune recognizes the properties of these fast systems during the measurement of a step response and utilizes the information to calculate the control
amplification for a non-PID control algorithm.
6.10
INFICON instrument variants
The instrument models available differ both in hardware and software
equipment: the simplest unit, the XTM/2, is purely a measuring or display
device that cannot control vacuum coating.
The XTC/2 and XTC/C group can control vacuum coating sources and up
to three different coatings of a process (not to be confused with nine different coating programs). In the case of XTM/2, XTC/2 and XTC/C units, the
AutoZero and AutoTune functions are not available, and measurement with
several sensors simultaneously as well as simultaneous control of two vacuum coating sources are not possible.
However, the IC/5 offers all comfort functions available today: measurement
with up to eight sensors with AutoZero and AutoTune as well as capability
of simultaneous control of two evaporator sources. Moreover, it offers 24
material programs, with which 250 coatings in 50 processes can be programmed. To simplify operation and avoid errors, the unit also has a
diskette drive. All types of crystal holders can be connected here. The thickness resolution is around 1 Å, the rate resolution for rates between 0 and
99.9 Å/s around 0.1 Å/s and for rates between 100 and 999 Å/s around 1
Å/s. A particularly attractive option offered by the IC/5 is a microbalance
131
Thin film controllers/control units
board with a highly stable reference quartz. This oscillator is 50 times more
stable than the standard oscillator; long-term stability and accuracy are
then 2 ppm over the entire temperature range. This option is specially
designed for coatings of material with low density and at low coating rates.
This is important for space contamination and sorption studies, for example.
132
Applications of vacuum technology
7.
Application of vacuum technology
for coating techniques
7.1
Vacuum coating technique
chemical compounds or layers of different materials applied in sandwich
form. A significant advantage of vacuum coating over other methods is that
many special coating properties desired, such as structure, hardness, electrical conductivity or refractive index, are obtained merely by selecting a
specific coating method and the process paramaters for a certain coating
material.
Vacuum technology has been increasingly used in industrial production
processes during the last two decades. Some of these processes and their
typical working pressure ranges are shown in Fig. 7.1.
Since a discussion of all processes is beyond the scope of this brochure,
this section will be restricted to a discussion of several examples of applications in the important field of coating technology.
Deposition of thin films is used to change the surface properties of the base
material, the substrate. For example, optical properties such as transmission or reflection of lenses and other glass products, can be adjusted by
applying suitable coating layer systems. Metal coatings on plastic web produce conductive coatings for film capacitors. Polymer layers on metals
enhance the corrosion resistance of the substrate.
Through the use of vacuum it is possible to create coatings with a high
degree of uniform thickness ranging from several nanometers to more than
100 mm while still achieving very good reproducibility of the coating properties. Flat substrates, web and strip, as well as complex molded-plastic parts
can be coated with virtually no restrictions as to the substrate material. For
example, metals, alloys, glass, ceramics, plastics and paper can be coated.
The variety of coating materials is also very large. In addition to metal and
alloy coatings, layers may be produced from various
Ultrahigh vacuum
7.2
Coating sources
In all vacuum coating methods layers are formed by deposition of material
from the gas phase. The coating material may be formed by physical
processes such as evaporation and sputtering, or by chemical reaction.
Therefore, a distinction is made between physical and chemical vapor
deposition:
• physical vapor deposition = PVD
• chemical vapor deposition = CVD.
7.2.1 Thermal evaporators (boats, wires etc.)
In the evaporation process the material to be deposited is heated to a temperature high enough to reach a sufficiently high vapor pressure and the
desired evaporation or condensation rate is set. The simplest sources used
in evaporation consist of wire filaments, boats of sheet metal or electrically
conductive ceramics that are heated by passing an electrical current
through them (Fig. 7.2). However, there are restrictions regarding the type
of material to be heated. In some cases it is not possible to achieve the
High vacuum
Medium vacuum
Rough vacuum
Annealing of metals
Degassing of melts
Electron beam melting
Electron beam welding
Evaporation
Sputtering of metals
Casting of resins and lacquers
Drying of plastics
Drying of insulating papers
Freeze-drying of bulk goods
Freeze-drying of pharmaceutical products
–10
10
–7
10
–3
10
0
10
3
10
Pressure [mbar]
Fig. 7.1
Pressure ranges for various industrial processes
133
Applications of vacuum technology
6
Hairpin-sheped evaporator
made of twisted tungsten wire
5
7
4
Evaporator made of electrically conductive
ceramics
8
3
2
Spiral evaporator made of twisted tungsten wire
Boat-shaped evaporator
1
9
1 Substrates
2 Sputtered
atoms
Fig. 7.3
3 Anode
4 Electrons
5 Target
6 Cathode
7 Magnetic
field lines
8 Argon ions
9 Substrate
Schematic diagram of a high-performance cathode sputter arrangement
7.2.3 Cathode sputtering
Trough-shaped evaporator
Trough-shaped evaporator with ceramic
coating
Fig. 7.2
Evaporator with ceramic
insert
Basket-shaped evaporator
Various thermal evaporators
necessary evaporator temperatures without significantly evaporating the
source holder and thus contaminating the coating. Furthermore chemical
reactions between the holder and the material to be evaporated can occur
resulting in either a reduction of the lifetime of the evaporator or contamination of the coating.
7.2.2 Electron beam evaporators (electron guns)
To evaporate coating material using an electron beam gun, the material,
which is kept in a water-cooled crucible, is bombarded by a focused electron beam and thereby heated. Since the crucible remains cold, in principle,
contamination of the coating by crucible material is avoided and a high
degree of coating purity is achieved. With the focused electron beam, very
high temperatures of the material to be evaporated can be obtained and
thus very high evaporation rates. Consequently, high-melting point compounds such as oxides can be evaporated in addition to metals and alloys.
By changing the power of the electron beam the evaporation rate is easily
and rapidly controlled.
134
In the cathode sputtering process, the target, a solid, is bombarded with
high energy ions in a gas discharge (Fig. 7.3). The impinging ions transfer
their momentum to the atoms in the target material, knocking them off.
These displaced atoms – the sputtered particles – condense on the substrate facing the target. Compared to evaporated particles, sputtered particles have considerably higher kinetic energy. Therefore, the conditions for
condensation and layer growth are very different in the two processes.
Sputtered layers usually have higher adhesive strength and a denser coating structure than evaporated ones. Sputter cathodes are available in many
different geometric shapes and sizes as well as electrical circuits configurations. What all sputter cathodes have in common is a large particle source
area compared to evaporators and the capability to coat large substrates
with a high degree of uniformity. In this type of process metals, alloys of any
composition as well as oxides can be used as coating materials.
7.2.4 Chemical vapor deposition
In contrast to PVD methods, where the substance to be deposited is either
solid or liquid, in chemical vapor deposition the substance is already in the
vapor phase when admitted to the vacuum system. To deposit it, the substance must be thermally excited, i.e. by means of appropriate high temperatures or with a plasma. Generally, in this type of process, a large number
of chemical reactions take place, some of which are taken advantage of to
control the desired composition and properties of the coating. For example,
using silicon-hydrogen monomers, soft Si-H polymer coatings, hard silicon
coatings or – by addition of oxygen – quartz coatings can be created by
controlling process parameters.
Coating methods
7.3
Vacuum coating technology/
coating systems
7.3.1 Coating of parts
For molded-plastic parts, vacuum coating techniques are increasingly
replacing conventional coating methods, such as electroplating. For example, using vacuum coating methods, automobile reflectors obtain a mirrorlike surface, plastic articles in the furniture, decoration, clock and watch as
well as electronics industry are metal-coated and optical effects are created
on articles in the decoration industry.
Fig. 7.4 shows a type of vacuum system in which large batches of moldedplastic parts can be coated simultaneously. The substrates are placed on a
cage that rotates past the coating source, a sputter cathode in this example. In some applications, by using a glow discharge treatment, the substrates are cleaned and the surface is activated prior to the coating
process. This enhances the adhesive strength and reproducibility of the
coating properties. A corrosion protection coating can be applied after sputtering. In this case, a monomer vapor is admitted into the system and a
high-frequency plasma discharge ignited. The monomer is actived in the
plasma and deposits on the substrates as a polymer coating. In this type of
system there may be plastic substrates with a surface area of several
10 m2 on the cage, causing a correspondingly high desorption gas flow.
The vacuum system must be able to attain the required pressures reliably
despite these high gas loads. In the example shown, the system is evacuated with a combination of a backing and Roots pump. A diffusion pump
along with a cold surface forms the high vacuum pump system. The cold
surfaces pump a large portion of the vapor and volatile substances emitted
by the plastic parts while the diffusion pump basically removes the non-condensable gases as well as the noble gas required for the sputter process.
A completely different concept for the same process steps is shown in Fig.
7.5. The system consists of four separate stations made up of a drum rotat-
1 Vacuum chamber
2 High-performance cathode
3 Substrate holdFig. 7.4
er
4
5
6
7
Substrates
Diffusion pump
Roots pump
Rotary piston
pump
8 Cold trap
9 High vacuum
valve
10 Valve for
Diagramm of a batch system for coating parts
ing around the vertical axis with four substrate chambers and process stations mounted in the vacuum chamber. During rotation, a substrate chamber moves from the loading and unloading station to the pretreatment station, to the metallization station, to the protective coating station and then
back to the initial position. Since each station has its own pumping system,
all four processes can run simultaneously with entirely independent
adjustable process parameters. The vacuum system comprises of turbomolecular pumps and backing pump sets consisting of Roots and rotary vane
pumps.
7.3.2 Web coating
Metal-coated plastic webs and papers play an important role in food packaging. They preserve food longer according to storage and transport logistics requirements and give packaging an attractive appearence. Another
important area of application of metal-coated web is the production of film
capacitors for electrical and electronics applications. Metal-coating is carried out in vacuum web coating systems. Fig. 7.6 shows a typical scheme.
The unit consists of two chambers, the winding chamber with the roll of web
to be coated and the winding system, as well as the coating chamber,
where the evaporators are located. The two chambers are sealed from
each other, except for two slits through which the web runs. This makes it
possible to pump high gas loads from the web roll using a relatively small
pumping set. The pressure in the winding chamber may be more than a
factor of 100 higher than the pressure simultaneously established in the
coating chamber. The pump set for the winding chamber usually consists of
a combination of Roots and rotary vane pumps.
With strongly degassing rolls of paper, it may be necessary to install a cold
surface in the winding chamber to act as a water vapor pump. The rolls of
the plastic web or paper typically have diameters between 400 and 1000
mm and a width of 400 to 3000 mm. A precise, electronically controlled
winding system is required for winding and unwinding as well as web guid-
bypass line
11 Foreline valve
12 Venting valve
Fig. 7.5
Multi-chamber parts-coating unit (rotationally symmetric in-line system DynaMet 4V)
135
Applications of vacuum technology
High-performance
plasma
source
Electron
beam
evaporator
O2
Monomer
1 Unwinder
2 Coating source
3 Coating roller
Fig. 7.6
4 Drawing roller
5 Take-up reel
Schematic diagram of a vacuum web coating system
ance.
During the coating process the web, at a speed of more than 10 m/s, passes a group of evaporators consisting of ceramic boats, from which aluminium is evaporated. To achieve the necessary Al-coating thickness at these
high web speeds, very high evaporation rates are required. The evaporators must be run at temperatures in excess of 1400 °C. The thermal radiation of the evaporators, together with the heat of condensation of the growing layer, yields a considerable thermal load for the web. With the help of
cooled rollers, the foil is cooled during and after coating so that it is not
damaged during coating and has cooled down sufficiently prior to winding.
During the entire coating process the coating thickness is continuously
monitored with an optical measuring system or by means of electrical resistance measurement devices. The measured values are compared with the
coating thickness setpoints in the system and the evaporator power is thus
automatically controlled.
7.3.3 Optical coatings
Vacuum coatings have a broad range of applications in production of ophthalmic optics, lenses for cameras and other optical instruments as well as
a wide variety of optical filters and special mirrors. To obtain the desired
transmission or reflection properties, at least three, but sometimes up to 50
coatings are applied to the glass or plastic substrates. The coating properties, such as thickness and refractive index of the individual coatings, must
be controlled very precisely and matched to each other. Most of these coatings are produced using electron beam evaporators in single-chamber units
(Fig. 7.7). The evaporators are installed at the bottom of the chamber, usually with automatically operated crucibles, in which there are several different materials. The substrates are mounted on a rotating calotte above the
evaporators. Application of suitable shieldings combined with relative move-
136
Ar
Fig. 7.7
Coating unit for optical coating systems
ment between evaporators and substrates, results in a very high degree of
coating uniformity. With the help of quartz coating thickness monitors (see
Section 6) and direct measurement of the attained optical properties of the
coating system during coating, the coating process is fully controlled automatically.
One of the key requirements of coatings is that they retain their properties
under usual ambient conditions over long periods of time. This requires to
produce the densest coatings possible, into which neither oxygen nor water
can penetrate. Using glass lenses, this is achieved by keeping the substrates at temperatures up to 300 °C during coating by means of radiation
heaters. However, plastic lenses, as those used in eyeglass optics, are not
allowed to be heated above 80 °C. To obtain dense, stable coatings these
substrates are bombarded with Ar ions from an ion source during coating.
Through the ion bombardement the right amount of energy is applied to the
growing layer so that the coated particles are arranged on the energetically
most favorable lattice sites, without the substrate temperature reaching
unacceptably high values. At the same time oxygen can be added to the
argon. The resulting oxygen ions are very reactive and ensure that the oxygen is included in the growing layer as desired.
The vacuum system of such a coating unit usually consists of a backing
pump set comprising a rotary vane pump and Roots pump as well as a high
vacuum pumping system. Depending on the requirements, diffusion pumps,
cryo pumps or turbomolecular pumps are used here, in most cases in connection with large refrigerator-cooled cold surfaces. The pumps must be
installed and protected by shieldings in a way that no coating material can
enter the pumps and the heaters in the system do not thermally overload
them. Since shielding always reduces the effective pumping speed, the system manufacturer must find a suitable compromise between shielding effect
and reduction of pumping speed.
Coating methods
7.3.4 Glass coating
Coated glass plays a major role in a number of applications: window panes
in moderate and cold climate zones are provided with heat-reflecting coating systems to lower heating costs; in countries with high intensity solar
radiation, solar protection coatings are used that reduce air conditioning
costs; coated car windows reduce the heating-up of the interior and mirrors
are used both in the furniture and the automobile industry. Most of these
coatings are produced in large in-line vacuum systems. Fig. 7.8 shows a
typical system. The individual glass panes are transported into a entrance
chamber at atmospheric pressure. After the entrance valve is closed, the
chamber is evacuated with a forepump set. As soon as the pressure is low
enough, the valve to the evacuated transfer chamber can be opened. The
glass pane is moved into the transfer chamber and from there at constant
speed to the process chambers, where coating is carried out by means of
sputter cathodes. On the exit side there is, in analogy to the entrance side,
a transfer chamber in which the pane is parked until it can be transferred
out through the exit chamber.
Most of the coatings consist of a stack of alternative layers of metal and
oxide. Since the metal layers may not be contaminated with oxygen, the
individual process stations have to be vacuum-isolated from each other and
from the transfer stations. Utilization of valves for separating process chambers is unsatisfactory because it increases plant dimensions. To avoid frequent and undesirable starting and stopping of the glass panes, the
process chambers are vacuum-separated through so-called “slit locks”, i.e.
constantly open slits combined with an intermediate chamber with its own
vacuum pump (Fig. 7.9). The gaps in the slits are kept as small as technically possible to minimize clearance and therefore conductance as the
glass panes are transported through them. The pumping speed at the intermediate chamber is kept as high as possible in order to achieve a considerably lower pressure in the intermediate chamber than in the process chambers. This lower pressure greatly reduces the gas flow from a process
chamber via the intermediate chamber to the adjacent process chamber.
For very stringent separation requirements it may be necessary to place
several intermediate chambers between two process chambers.
All major functions of a plant, such as glass transport, control of the sputter
processes and pump control, are carried out fully automatically. This is the
only way to ensure high productivity along with high product quality.
7.3.5 Systems for producing data storage disks
Coatings for magnetic- or magneto-optic data storage media usually consist
of several functional coatings that are applied to mechanically finished
disks. If several plates are placed on one common carrier, the coating
processes can be carried out in a system using a similar principle to that
used for glass coating. However, most disks must be coated on both sides
and there are substantially greater low particle contamination requirements
as compared to glass coating. Therefore, in-line systems for data memories
use a vertical carrier that runs through the system (Fig. 7.10). The sputter
cathodes in the process stations are mounted on both sides of the carrier
so that the front and back side of the disk can be coated simultaneously.
An entirely different concept is applied for coating of single disks. In this
case the different process stations are arranged in a circle in a vacuum
chamber (Fig. 7.11). The disks are transferred individually from a magazine
to a star-shaped transport arm. The transport arm cycles one station further
after each process step and in this way transports the substrates from one
process station to the next. During cycling all processes are switched off
and the stations are vacuum-linked to each other. As soon as the arm has
reached the process position, the individual stations are seperated from
each other by closing seals. Each station is pumped by means of its own
turbomolecular pump and the individual processes are started. As many
process stations as there are in the system as many processes can be performed in parallel. By sealing off the process stations, excellent vacuum
separation of the individual processes can be achieved. However, since the
slowest process step determines the cycle interval, two process stations
may have to be dedicated for particularly timeconsuming processes.
The glass coating process requires high gas flows for the sputter processes
as well as low hydrocarbon concentration. The only vacuum pump which
satisfies these requirements as well as high pumping speed stability over
time are turbo-molecular pumps which are used almost exclusively.
While the transfer and process chambers are constantly evacuated, the
entrance and exit chambers must be periodically vented and then evacuated again. Due to the large volumes of these chambers and the short cycle
times, a high pumping speed is required. It is provided by combinations of
rotary vane pumps and Roots pumps. For particularly short cycle times gas
cooled Roots pumps are also used.
Process chamber
1
Intermediate chamber
L1Z
←
S1
Slits
Process chamber
2
LZ2
→
SZ
S2
to backing pumps
Entrance chamber
Transfer chamber 2
Transfer chamber 1
Exit chamber
Sputter chambers
Fig. 7.8
Plant for coating glass panes – 3-chamber in-line system, throughput up to
3,600,000 m2 / year
L1Z, LZ2 = conductance between intermediate chamber and process chamber 1 or 2
= pumping speed at intermediate chamber
SZ
S1, S2 = pumping speed at process chamber 1 or 2
Fig. 7.9
Principle of chamber separation through pressure stages
137
Applications of vacuum technology
Fig. 7.10 Plant for coating data storage disks with carrier transport system
Fig. 7.11 Plant for individual coating of data storage disks
138
Instructions for equipment operation
8.
8.1
Instructions for vacuum equipment operation
Causes of faults where the desired ultimate pressure is not achieved
or is achieved too slowly
If the desired ultimate pressure is not reached at all in vacuum equipment
or if it is attained only after an excessively long pumping period, then the
following problems may be the reason:
If the desired ultimate pressure is not reached, then
• the apparatus may be leaky or dirty,
gauge) with and without a cold trap inserted in the line: Filling the cold trap
with liquid nitrogen will cause the pressure to drop abruptly, by one power
of ten or more, if the container is contaminated since the vapors will freeze
out in the trap.
Eliminating contamination for glass equipment
If the contaminants exhibit a high vapor pressure, then they can be pumped
out relatively quickly. If this is not successful, then the apparatus will have
to be cleaned. Contaminated glass components will first be cleaned with
chromic-sulfuric acid mixture or – if this is necessary – with dilute hydrofluoric acid (1:30). They are then rinsed with distilled water. If this does not
bring about the desired results, then an organic solvent can be tried. Then
the glass components will again have to be rinsed with methanol and distilled water. (Do not use denatured alcohol!)
In order to locate the fault, one normally proceeds by separating the evacuated vessel from the pump system (where this is possible) and checking the
vessel alone for leaks and contamination using the pressure rise method,
for example. If it has been found that the vessel is free of defects in this
regard, then the measurement system will be checked for cleanliness (see
Section 8.38) and ultimately – if required – the pump or the pumping system itself will be examined.
Eliminating contamination at metallic equipment
Metal components will usually exhibit traces of contamination by oil and
greases. If these cannot be readily removed by pumping down the vessel,
then an appropriate organic solvent (denatured alcohol is unsuitable in all
cases) will have to be used for cleaning. Maximum cleanliness can be
achieved with vapor baths such as those commonly found in industry. If one
desires to get down to extremely low pressure ranges (< 10-7 mbar), then –
after cleaning – the metal components will have to be baked out at temperatures of up to 200 °C. Seriously contaminated metal components will first
have to be cleaned by cutting away or sandblasting the top surface. These
methods suffer the disadvantage that the surface area for the surface thus
treated will be increased through roughening and active centers may potentially be formed which would readily adsorb vapor molecules.
Supplementary cleaning in the vapor bath (see above) is advisable. In
some cases electrolytic pickling of the surface may be beneficial. In the
case of high vacuum components it is necessary to pay attention to ensuring that the pickling does not turn into etching, which would seriously
increase the surface area. Polishing surfaces which have been sandblasted
is not necessary when working in the rough and medium vacuum ranges
since the surface plays only a subordinate role in these pressure regimes.
8.2
8.3
• the pump may dirty or damaged,
• the vacuum gauge may be defective.
If the desired ultimate pressure is reached only after a very long running
time, then
• the apparatus may be dirty,
• the pumping line may be restricted,
• the pump may be dirty or of too small a capacity,
• the pumping speed may be restricted due to other causes.
Contamination of vacuum vessels and
eliminating contamination
In addition to the pressure rise method (Section 6.1) there is a further
method for detecting contamination, based on the fact that condensable
vapors will generally account for the major share of the gas mix in dirty vessels: here the pressure reading at a compression vacuum gauge (partial
pressure for the non-condensable gases) is compared with that at an electrical vacuum gauge, e.g. a thermal conductivity or ionization vacuum
gauge (measuring total pressure). These two vacuum gauges must, however, be clean themselves. Where vapors are present the compression vacuum gauge will indicate much better pressure than the electrical vacuum
gauge. This is a sure sign that the vessel walls are contaminated, usually
with oil. Another commonly used procedure is to compare the pressure indication of one and the same vacuum gauge (not a compression vacuum
General operating information for vacuum
pumps
If no defects are found in the vacuum vessels and at the measurement
tubes or if the apparatus still does not operate satisfactorily after the faults
have been rectified, then one should first check the flange seals at the
pump end of the system and possibly the shut-off valve. Flange seals are
known to be places at which leaks can appear the most easily, resulting
from slight scratches and mechanical damage which initially appears
insignificant. If no defect can be discerned here, either, then it is advisable
to check to see whether the pumps have been maintained in accordance
with the operating instructions.
Given initially in this section are general instructions on pump maintenance,
to be followed in order to avoid such defects from the very outset. In addition, potential errors and their causes are discussed.
139
Instructions for equipment operation
8.3.1 Oil-sealed rotary vacuum pumps (Rotary
vane pumps and rotary piston pumps)
8.3.1.1
Oil consumption, oil contamination, oil change
The oil serves to:
• lubricate moving parts,
• seal moving parts against atmospheric pressure,
• seal the valve,
• fill the dead space below the valve,
• to seal the various operational spaces one from another.
In all pumps it is possible to check the oil charge during operation using the
built-in oil level sight glass. During continuous operation in particular it is
necessary to ensure that the oil charge never falls below the minimum
level. During a pumping process oil-sealed rotary pumps will emit oil vapors
from the discharge port, this being due to the high operating temperature.
This leads to oil loss to an extent which will depend on the quantity of gas
or vapor which is drawn into the pump. Larger oil droplets can be retained
by installing a coarse separator in the discharge line. This will reduce oil
loss considerably. The fine oil mist filter installed in some pumps will also
retain the finest oil droplets (fine separation) so that no oil at all will leave
the outlet of the pump and oil loss is reduced practically to zero since – as
in coarse separation – the oil which is separated out is returned to the
pump. With pumps without an integral fine separator this device is offered
as an optional extra.
If an oil-filled rotary pump is operated without an oil separation and return
device, then it will be necessary to expect a certain amount of oil consumption, the extent of which will depend on the size of the pump and the nature
of the operations. In the worst case this can amount to about 2 cm3 for
every cubic meter of air pumped (at STP and including the gas ballast also
drawn in). Figure 8.1 makes it possible to predict the amount of oil loss to
be expected in practical situations. The example demonstrates that greater
oil losses must be expected when operating the pump with gas ballast. This
situation, which is generally valid, is always to be taken into account in
practice.
If the pump oil has become unusabledue to exposure to the vapors or
contaminants which are encountered in the process, then the oil will have
to be replaced. It is impossible to formulate any hard-and-fast rules as to
when an oil change will be required since the nature of the operations will
determine how long the oil will remain good. Under clean conditions (e.g.
backing pumps for diffusion pumps in electro-nuclear accelerators) rotary
vacuum pumps can run for years without an oil change. Under extremely
“dirty” conditions (e.g. during impregnation) it may be necessary to change
the oil daily. The oil will have to be replaced when its original light brown
color, has turned dark brown or black due to ageing or has become cloudy
because liquid (such as water) has entered the pump. An oil change is also
necessary when flakes form in corrosion protection oil, indicating that the
corrosion protection agent is exhausted.
Changing the oil
The oil change should always be carried out with the pump switched off but
at operating temperature. The oil drain (or fill) opening provided for each
pump is to be used for this purpose. Where the pump is more seriously
contaminated, then it should be cleaned. The applicable operating instructions are to be observed in this case.
Pumping speed [m3 · h–1]∏
8.3.1.2
Intake pressure [mbar]
Example: Oil losses for a TRIVAC S 16 A at pressure of 1 mbar:
a) Without gas ballast: in accordance with the pumping speed curve S = 15 m3 / h; according
to diagram: oil loss = 0.03 cm3 / h (line a)
b) With gas ballast: S = 9 m3/h at 1 mbar; oil loss = 0.018 cm3 / h (line b1), plus additional loss
due to gas ballast quantity (0.1 times the 1.6 m3 / h rated pumping speed); that is: Chart on the
horizontal line b2 up to atmospheric pressure: additional loss: 3 cm3 / h (diagonal line). Overall
loss during gas ballast operation 0.018 + 3 = 3.018 cm3 / h.
Fig. 8.1
140
Oil loss for oil-sealed pumps (referenced to an approximate maximum value of 2 cm3
oil loss per cubic meter of air drawn in [STP])
Selection of the pump oil when handling aggressive vapors
If corrosive vapors (e.g. the vapors formed by acids) are to be pumped,
then a PROTELEN® corrosion protection oil should be used in place of the
normal pump oil (N 62). These types of vapors will then react with the basic
(alkaline) corrosion protection agent in the oil. The continuous neutralization
reactions will exhaust the corrosion protection agent at a rate depending on
the quantity and acidity of the vapors. The oil will have to be changed more
frequently, in accordance with these factors. Corrosion protection oils are
either very hygroscopic or they easily form emulsions with water.
Consequently a pump which is filled with corrosion protection oil will absorb
moisture from the air if it is out of service for an extended period of time. In
no case should one ever use a pump filled with corrosion protection oil in
order to pump water vapor since the lubricating and corrosion inhibition
properties of the oil would be adversely affected. Once the oil has absorbed
water it will no longer be possible for such pumps to achieve the ultimate
pressures which would be the case with fresh corrosion protection oil or
standard pump oil (N 62). Oil-filled pumps should, under normal operating
con-ditions, not be filled with corrosion protection oil. N 62 oil is preferred
when pumping air, water vapor and non-corrosive organic vapors in so far
as there is positive protection against the vapors condensing inside the
pump.
Instructions for equipment operation
8.3.1.3
Measures when pumping various chemical substances
This discussion cannot provide exhaustive coverage of the many and varied
application fields for oil-filled vacuum pumps in the chemicals industry. Our
many years of experience with the most difficult of chemicals applications
can be used to solve your particular problems. Three aspects should, however, be mentioned briefly: pumping explosive gas mixes, condensable
vapors, and corrosive vapors and gases.
Explosion protection
Applicable safety and environmental protection regulations shall be
observed when planning and engineering vacuum systems. The operator
must be familiar with the substances which the system will be pumping and
take into account not only normal operating conditions but also abnormal
situations, operating outside normal parameters. The most important aids to
avoiding explosive mixtures are – in addition to inertization by adding protective gases – maintaining the explosion limit values with the aid of condensers, adsorption traps and gas scrubbers.
Protection against condensation
LEYBOLD pumps offer three options for keeping vapors from condensing in
the pumps:
• The gas ballast principle (See Fig. 2.14). This increases considerably
the amount of vapor which the pump can tolerate.
• Increased pump temperature. The rugged design of our pumps makes it
possible to run them at temperatures of up to 120 °C. Thus the tolerance for pure water vapor, for example, will rise by a factor of five when
compared with normal gas ballast operation.
• Using vacuum condensers (see Section 2.15). These act as selective
pumps and should be sized so that the downstream gas ballast pump
will not receive more vapor than the amount corresponding to the appropriate vapor tolerance.
Corrosion protection
Oil-sealed pumps are already quite satisfactorily protected against corrosion
due to the oil film which will be present on all the component surfaces.
Corrosion is defined here as the electrochemical dissolution of metals, i.e.
the release of electrons by the metal atom and their acceptance by the oxidation agent (corrosive gas). A metal atom which is susceptible to corrosion
must therefore be exposed to an active atom of the oxidation agent.
This makes clear how the oil-sealed pump is protected against corrosion;
the concentration of the oxidation agent in the oil is negligible and thus the
opportunity for the metal to release electrons is equally small. This also
makes it clear that the use of so-called “non-rusting” or “stainless” steels
does not make sense since oxidation is necessary for the passivation of
these steels, in order to reach the so-called passive region for these steel
compounds. The critical passivation current density will normally not appear
in oil-sealed pumps.
a) Acids
Our pumps are fundamentally suited to pumping acids. In special situations
problems with the oil and with auxiliary equipment attached at the intake
and/or discharge end may occur. Our engineers in Cologne are available to
assist in solving such problems.
b) Anhydrides
CO (carbon monoxide) is a strong reducing agent. When CO is being
pumped it is therefore important that air not be used as the gas ballast but
rather that inert gases be used at the very outside (e.g. Ar or N2). Inert gas
ballast should also be used when pumping SO2, SO3, and H2S. A corrosion
inhibiting oil is also to be used when handling these three anhydrides.
Carbon dioxide (CO2) can be pumped without making any special
arrangements.
c) Alkaline solutions
Normal N 62 pump oil is to be used to pump basic (alkaline) solutions.
Sodium hydroxide and caustic potash solutions should not be pumped in
their concentrated form. Ammonia is highly amenable to pumping with the
gas ballast valve closed. Alkaline organic media such as methylamine and
dimethylamine can also be pumped satisfactorily, but with the gas ballast
valve open.
d) Elementary gases
Pumping nitrogen and inert gases requires no special measures.
When handling hydrogen it is necessary to make note of the hazard of creating an explosive mixture. The gas ballast valve may in no case be opened
when dealing with hydrogen. The motors driving the pumps must be of
explosion-proof design.
Oxygen: Particular caution is required when pumping pure oxygen!
Specially formulated pump oils must be used for this purpose. We can supply these, accompanied by an approval certificate issued by the German
Federal Materials Testing Authority (BAM), following consultation.
e) Alkanes
The low molecular weight alkanes such as methane and butane can be
pumped with the gas ballast valve closed or using inert gas as the gas ballast and/or at increased temperature of the pump. But important –
Increased explosion hazard!
f) Alcohols
Once operating temperature has been reached, methanol and ethanol can
be extracted without using gas ballast (N 62 pump oil). To pump higher molecular weight alcohols (e.g. butanol) the gas ballast valve will have to be
opened or other protective measures will have to be implemented to prevent condensation.
g) Solvents
Acetone: Can be extracted without difficulty; wait until normal operating
temperature is reached.
Benzene: Caution – vapors are highly flammable. Ultimate pressure is
degraded by dilution of the pump oil. Mixtures of air and benzene are explosive and flammable. Work without a gas ballast! Inert gases may possibly
be used as ballast gas.
Carbon tetrachloride and trichlorethylene: Amenable to pumping; nonflammable and non-explosive but will be dissolved in oil and thus increase
the ultimate pressure; wait until normal operating temperature is achieved.
Keep the gas ballast open when pumping carbon tet and other non-flamma-
141
Instructions for equipment operation
ble solvents. Use N 62 pump oil.
Toluene: Pump through the low-temperature baffle and without gas ballast.
Use inert gas, not air, as the gas ballast.
8.3.1.4
Operating defects while pumping with gas ballast
– Potential sources of error where the required
ultimate pressure is not achieved
a) The pump oil is contaminated (particularly with dissolved vapors). Check
the color and properties. Remedy: Allow the pump to run for an extended period of time with the vacuum vessel isolated and the gas ballast
valve open. In case of heavy contamination an oil change is advisable.
The pump should never be allowed to stand for a longer period of time
when it contains contaminated oil.
b) The sliding components in the pump are worn or damaged. Under clean
conditions our pumps can run for many years without any particular
mainte-nance effort. Where the pump has been allowed to run for a
longer period of time with dirty oil, however, the bearings and the gate
valves may exhibit mechanical damage. This must always be assumed
when the pump no longer achieves the ultimate pressure specified in
the catalog even though the oil has been changed. In this case the
pump should be sent in for repair or our customer service department
should be contacted.
c) The measurement instrument is soiled (see Section 8.4.2).
Potential sources of error when the pump no longer turns
• Check the pump electrical supply.
• The pump has stood for a long time containing contaminated or
resinous oil.
• The pump is colder than 10°C and the oil is stiff. Heat the pump.
• There is a mechanical error. Please contact our customer service
department.
Oil exits at the shaft
If oil is discharged at the shaft, then the Seeger rotary shaft circlip in the
drive bearing will have to be inspected and possibly replaced. The design
of the pumps makes it possible to replace this ring easily, following the
operating instructions provided with the unit.
8.3.2 Roots pumps
8.3.2.1
General operating instructions, installation and
commissioning
Roots pumps must be exactly level. When attaching the pump it is necessary to ensure that the pump is not under any tension or strain whatsoever.
Any strains in the pump casing caused by the connection lines shall be
avoided. Any strain to which the pump is subjected will endanger the pump
since the clearances inside the roots pump are very small.
Roots pumps are connected to the line power supply via the motor terminal
142
strip; a motor protection switch (overload/ overheating) shall be provided as
required by local codes.
The direction of motor rotation shall be checked with the intake and outlet
ports open prior to installing the pump. The drive shaft, seen from the motor
end, must rotate counter-clockwise. Note the arrow on the motor indicating
the direction of rotation! If the roots pump runs in the wrong direction, then
it is reversed by interchanging two of the phases at the motor connection
cord.
The roots pump may be switched on only after the roughing pump has
evacuated the vacuum vessel down to the cut-in pressure.
Permissible cut-in pressure PE will depend on the reduction ratios of the
roots pump as against the roughing pump and is calculated by dividing the
permissible pressure differential Δpmax by the compression ratio, reduced
by the value of 1:
p =
E
k th =
Δpmax
where
k th – 1
Theoretical pumping speed for the roots pump
Nominal pumping speed for the roughing pump
If the pump is protected using a diaphragm-type pressure switch, then the
pump will be switched on automatically. If a combination of roots pump and
roughing pump is to convey highly volatile substances such as liquids with
a low boiling point, then it is advisable to use a roots pump which is
equipped with an integral bypass line and a valve which will respond to a
pre-set pressure. Example: Roots vacuum pumps RUVAC WAU / WSU.
Roots pumps from the RUVAC-WAU/WSU series, being equipped with
bypass lines, can generally be switched on together with the forepump. The
bypass protects these roots pumps against overloading.
8.3.2.2
Oil change, maintenance work
Under clean operating conditions the oil in the roots pump will be loaded
only as a result of the natural wear in the bearings and in the gearing. We
nevertheless recommend making the first oil change after about 500 hours
in service in order to remove any metal particles which might have been
created by abrasion during the run-in period. Otherwise it will be sufficient
to change the oil every 3000 hours in operation. When extracting gases
containing dust or where other contaminants are present, it will be necessary to change the oil more frequently. If the pumps have to run at high
ambient temperatures, then the oil in the sealing ring chamber shall also be
replaced at each oil change.
We recommend using our specially formulated N 62 oil.
Under “dirty” operating conditions it is possible for dust deposits to form a
“crust” in the pump chamber. These contaminants will deposit in part in the
pumping chamber and in part on the pump’s impellers. They may be
removed, once the two connection lines have been detached, either by
blowing out the system with dry compressed air or by rinsing with a suitable
cleaning agent, such as petroleum ether (naphtha).
The oil in the roots pump will then have to be changed. The rotor may be
turned only by hand during cleaning since, due to the high speed when the
Instructions for equipment operation
motor is running, the deposits could damage the pump as they dislodge
from the surfaces.
8.3.3 Turbomolecular pumps
Grime which cannot be eliminated by washing can be removed mechanically using a wire brush, metallic scrubber or scraper.
8.3.3.1
Important!
The dislodged residues may not remain in the pump chamber. After cleaning is completed check the pump for operability by slowly turning the
impellers by hand. There may be no resistance to rotation. It is generally
not necessary to dismantle the roots pump. If this should nevertheless be
required due to heavy soiling, then it is highly advisable to have this done
by the manufacturer.
8.3.2.3
Actions in case of operational disturbances
1. Pump becomes too warm: (maximum operating temperature
100 to 115 °C)
Possible causes:
• Overloading: Excessive heat of compression due to an excessively
high pressure ratio. Check the pressure value adjustments and the tightness of the vacuum chamber!
• Incorrect clearances: The distances between the rotors and the housings have been narrowed due to dirt or mechanical strain.
• Soiled bearings: Excessive friction in the bearings
• Improper oil level: If the oil level is too high, the gears will touch the oil,
causing friction resistance. Where the oil level is too low the system will
not be lubricated.
• Incorrect oil type: An SAE 30 grade oil must be used for the pump.
2. Excessive power consumption: All the factors which can lead to elevated temperatures can also cause excessisve amounts of power to be
drawn. The motor is defective if excessive power requirements are not
accompanied by a rise in the temperature at the pump.
3. Oiling at the pump chamber:
Possible causes:
• Oil level too high: Oil is subjected to excessive thermal loading. Oil
foam is swept along.
• Oil mixed with the product: Azeotropic degasification of the oil.
• Pump leaking: Air ingress through the oil drain or filler screw will cause
a large stream of air and conveyance of oil into the pumping chamber.
4. Abnormal running noises:
Possible causes:
• Grime at the impeller
• Bearing or gearing damage
• Impellers are touching the housing
In the case of bearing or gearing damage or where the impeller scrapes the
housing the pump should be repaired only by the manufacturer.
General operating instructions
In spite of the relatively large gap between the pump rotor and the stator,
the turbomolecular pumps should be protected against foreign objects
entering through the intake port. It is for this reason that the pump should
never be operated without the supplied wire-mesh splinter guard. In addition, sharp shock to the pump when running and sudden changes in attitude are to be avoided.
Over and above this, and particularly for the larger types with long rotor
blades, airing the pump to atmospheric pressure while the impellers are
rotating may be done only when observing exactly the rules given in the
operating instructions. Under certain circumstances it is possible to operate
turbomolecular pumps under exceptional conditions.
The turbomolecular pump may, for example, be used unprotected inside
magnetic fields if the magnetic induction at the surface of the pump casing
does not exceed B = 3 · 10-3 T when radially impinging and B = 15 · 10-3 T
at axial impingement.
In a radioactive environment standard turbomolecular pumps can be used
without hazard at dose rates of from 104 to 105 rad. If higher dose rates are
encountered, then certain materials in the pump can be modified in order to
withstand the greater loading. The electronic frequency converters in such
cases are to be set up outside the radioactive areas since the semiconductors used inside them can tolerate a dose rate of only about 103 rad.
The use of motor-driven frequency converters which can withstand up to
108 rad represents another option.
Roughing (backing) pumps are required for the operation of turbomolecular
pumps. Depending on the size of the vessel to be evacuated, the turbomolecular pumps and forepumps may be switched on simultaneously. If the
time required to pump the vessel down to about 1 mbar using the particular
fore-pump is longer than the run-up time for the pump (see operating
instructions), then it is advisable to delay switching on the turbomolecular
pump. Bypass lines are advisable when using turbomolecular pumps in systems set up for batch (cyclical) operations in order to save the run-up time
for the pump. Opening the high vacuum valve is not dangerous at pressures of about 10-1 mbar.
8.3.3.2
Maintenance
Turbomolecular pumps and frequency converters are nearly maintenancefree. In the case of oil-lubricated pumps it is necessary to replace the bearing lubricant at certain intervals (between 1500 and 2500 hours in operation, depending on the type). This is not required in the case of greaselubricated pumps (lifetime lubrication). If it should become necessary to
clean the pump’s turbine unit, then this can easily be done
by the customer, observing the procedures described in the operating
instructions.
143
Instructions for equipment operation
8.3.4 Diffusion and vapor-jet vacuum pumps
8.3.4.1
Changing the pump fluid and cleaning the pump
Changing the pump fluid is always necessary whenever the pump no longer
achieves the required ultimate vacuum or when its pumping speed falls off.
The service life of the pump fluid will as a rule come to several weeks or
months – or even years – and will depend largely on the operating conditions for the pump. It is reduced by frequent pumping at high pressures, by
exposure to aggressive vapors and by air ingress of longer duration (this
does not apply to silicone oil and mercury).
In the case of oil diffusion and vapor-jet pumps the danger presented to the
pump fluid where there is air ingress with the pump hot is often overestimated. Where air ingress (up to atmospheric pressure) is encountered only
occasionally and for short periods of time then silicone oil will not be
attacked at all and the DIFFELEN pump fluid will only barely be affected.
The products with considerably higher vapor pressures which can be created through oxidation are removed again in a short period of time by the
fractionation and degassing equipment in the pump (see Section 2.1.6.1).
Even though the pump fluid which was originally light in color has been
turned brown by air ingress, this need not necessarily mean that the medium has become unusable. If, on the other hand, the pump fluid has turned
cloudy and has become more viscous, as well (which may be the case following periods of air ingress lasting for several minutes) then the medium
will have to be replaced. It is possible that under certain circumstances
cracking products from the pump fluid may make the oil in the forepump
unserviceable so that here, too, an oil change will have to be made.
Mercury diffusion and vapor-jet pumps are less sensitive to air ingress than
oil diffusion pumps. The oxidation of the hot mercury caused by the air
ingress is negligible in regard to the operating characteristics of the pump
when compared with the mercury loss in the forepump line.
Changing the pump fluid: The interior section will be extracted from the
pump and the contaminated pump fluid poured out. The interior section and
the pump body are then cleaned with pure petroleum ether (naphtha). The
interior section and pump body of mercury pumps should have been
cleaned beforehand with a clean brush; use a bottle brush for the nozzle
bores. Ensure that all the nozzle orifices are properly cleaned. It is advantageous to evaporate all solvent residues in a drying kiln. Then the inside
section is inserted once again and the fresh pump fluid is installed through
the forevacuum port. It is necessary to ensure that the upper nozzle cover
is not moistened with pump fluid! Do not install too much pump fluid!
8.3.4.2
Operating errors with diffusion and vapor-jet
pumps
Potential sources of defects when the desired ultimate pressure is not
reached
• Coolant temperature is too high; inadequate water throughput. The
coolant flow should always be monitored by a water flowmeter in order
to protect the pump from damage. Remedy: Measure the exit
temperature of the coolant water (it should not exceed 30 °C). Increase
the coolant flow-through rate. The cooling coils at the pump may have to
144
be decalcified.
• Forevacuum pressure too high: This is possible particularly where
vapors which are either evacuated from the vessel or are created as
cracking products from the driving medium (e.g. following air ingress)
get into the roughing pump. Check the forevacuum pressure with the oil
diffusion pump disconnected. Remedy: Run the forevacuum pump for an
extended period of time with gas ballast. If this is not sufficient, then the
oil in the forepump will have to be changed.
• Pump fluid at the diffusion pump spent or unserviceable: Replace the
driving medium.
• Heating is incorrect: Check the heating output and check for good thermal contact between the heating plate and the bottom of the boiler section. Replace the heating unit if necessary.
• Leaks, contamination:
Remedy: if the pump has been contaminated by vapors it may help to
use a metering valve to pass air through the apparatus for some period
of time; here the pressure should not exceed 10-2 mbar when DIFFELEN is being used.
• Measurement system old or soiled (see Section 8.4.2).
Potential sources of error where there is insufficient pumping speed:
• Forevacuum pressure is too high: Check the forevacuum; allow the gas
ballast pump to run for a longer period of time with the gas ballast valve
open. It may be necessary to change the oil in the forepump.
• The pump fluid in the diffusion pump has become unserviceable: Driving
medium will have to be replaced.
• Nozzles at the diffusion pump are clogged: Clean the diffusion pump.
• Heating is too weak: Check heating output; examine heating plate for
good thermal contact with the bottom of the boiler chamber.
• Substances are present in the vacuum vessel which have a higher
vapor pressure than the driving medium being used: among these are,
for example, mercury, which is particularly hazardous because the mercury vapors will form amalgams with the nonferrous metals in the oil diffusion pump and thus make it impossible to achieve perfect vacuums.
8.3.5 Adsorption pumps
8.3.5.1
Reduction of adsorption capacity
A considerable reduction in pumping speed and failure to reach the ultimate
pressure which is normally attainable in spite of thermal regeneration having been carried out indicates that the zeolite being used has become contaminated by outside substances. It does not make good sense to attempt
to rejuvenate the contaminated zeolite with special thermal processes. The
zeolite should simply be replaced.
8.3.5.2
Changing the molecular sieve
It will be necessary to rinse the adsorption pump thoroughly with solvents
before installing the new zeolite charge. Before putting the adsorption pump
which has been charged with fresh zeolite into service it is also necessary
to bake out the new zeolite charge, under vacuum and using the heating
Instructions for equipment operation
element associated with the pump, for a period of several hours so that
contaminants which might have collected during the storage period can dissipate.
8.3.6 Titanium sublimation pumps
Each of the turns in the titanium sublimation pump contains approximately
1.2 g of useable titanium supply. At a heating current of 50 A the surface
temperature comes to about 1850 K, the sublimation rate approximately
0.12 g/h, i.e. a turn can be operated continuously for about 10 hours. Since
at pressures below 1 · 10-6 mbar sublimation is not continuous but rather
only at intervals which – at low pressures (below 5 · 10-8 mbar) and low gas
volumes – are already more than ten times the actual sublimation period,
one may assume a pumping period of almost one month at a working pressure of 10-10 mbar per turn.
The effective pumping speed of a titanium sublimation pump will depend on
the getter screen surface and the geometry of the suction opening. The
pumping speed, referenced to the surface area of the getter surface, will be
dependent on the type of gas and the getter screen temperature. Since
inert gases, for example, cannot be pumped at all, titanium sublimation
pumps should always be used only with an auxiliary pump (sputter-ion
pump, turbomolecular pump) used to pump these gas components. The
supplementary pump can be much smaller than the titanium sublimation
pump. Only in a few special cases can one do without the additional pump.
The selection of the coolant is dictated by the working conditions and the
requirements in terms of ultimate pressure. At high pressures, above
1 · 10-6 mbar, more thermal energy is applied to the getter screen by frequent sublimation cycles. It is for this reason that cooling with liquid nitrogen is more favorable. At low pressures water cooling may be sufficient.
The getter screen should if at all possible be heated to room temperature
before airing the pump as otherwise the humidity in the air would condense
out on the surface.
8.3.7 Sputter-ion pumps
Sputter-ion pumps use high-voltage current. Installation and connection
should be carried out only under the supervision of a qualified specialist.
The operating instructions shall be observed!
The service life of sputter-ion pumps depends linearly on the pump’s operating pressure. In the case of pumps manufactured by LEYBOLD, the following applies:
p · T = 45 · 10-3 mbar · h
(p = operating pressure, T = service life)
This means that for operating pressure of
10-3 mbar the service life is 45
hours
10-6 mbar the service life is 45,000
hours
10-9 mbar the service life is 45,000,000 hours
pumps manufactured by LEYBOLD are so good that no problems will be
encountered when returning the units to service, even after a longer period
in storage.
When the sputter-ion pumps are installed one should ensure that the magnetic fields will not interfere with the operation of other devices (ionization
vacuum gauges, partial pressure measurement units, etc.). Mounting
devices for the sputter-ion pumps may not short circuit the inductance flow
and thus weaken the air gap inductance and pumping speed.
If the ultimate pressure which can be attained is not satisfactory even
though the apparatus is properly sealed, then it will usually be sufficient to
bake out the attached equipment at about 200 to 250 °C. If the pressure
here rises to about 1 · 10-5 mbar when this is done, then the sputter ion
pump will become so hot after evacuating the gases for two hours that it will
not be necessary to heat it any further in addition. It is also possible to heat
the pump by allowing air to enter for 2 hours at 10-5 mbar before the other
apparatus is then subsequently baked out. If the ultimate pressure is still
not satisfactory, then the pump itself will have to be baked out for a few
hours at 250 to 300°C (but not higher than 350 °C!). The pump should without fail remain in operation throughout this period! If the pressure rises
above 5 · 10-5 mbar it will be necessary either to heat more slowly or to
connect an auxiliary pump. Before airing one should allow the hot sputterion pump enough time to cool down at least to 150 °C.
8.4
Information on working with vacuum
gauges
8.4.1 Information on installing vacuum sensors
Here both the external situation in the immediate vicinity of the vacuum
apparatus and the operating conditions within the apparatus (e.g. working
pressure, composition of the gas content) will be important. It is initially necessary to ascertain whether the measurement system being installed is
sensitive in regard to its physical attitude. Sensors should only be installed
vertically with the vacuum flange at the bottom to keep condensates, metal
flakes and filings from collecting in the sensor or even small components
such as tiny screws and the like from falling into the sensorand the measurement system. The hot incandescent filaments could also bend and
deform improperly and cause electrical shorts inside the measurement system. This is the reason behind the following general rule: If at all possible,
install sensors vertically and open to the bottom. It is also very important to install measurement systems if at all possible at those points in the
vacuum system which will remain free of vibration during operation.
The outside temperature must be taken into account and above all it is necessary to avoid hot kilns, furnaces or stoves or other sources of intense
radiation which generate an ambient temperature around the measurement
system which lies above the specific acceptable value. Excessive ambient
temperatures will result in false pressure indications in thermal conductivity
vacuum sensors.
If a triode pump is not needed over an extended period of time it can either
be operated continuously at low pressure – with practically no influence on
the service life – or it can be aired, removed from the pump and packed in
a dust-tight container. The starting properties of the sputter-ion (triode)
145
Instructions for equipment operation
8.4.2 Contamination at the measurement system
and its removal
The vacuum gauges used in vacuum technology for pressure measurement
will certainly work under “dirty” conditions. This is quite understandable
since a vacuum device or system does not serve simply to produce low
pressures but rather and primarily have to run processes in chemistry, metallurgy or nuclear physics at low pressures. Here, depending on the nature
of the process, considerable quantities of gases or vapors will be liberated
either continuously or intermittently; these can pass into the measurement
systems provided for pressure measurement and installed in the vacuum
system and – due to surface reactions or through simple deposits – can falsify the pressure measurements considerably. This is true for all types of
vacuum gauges whereby, of course, high-sensitivity, high-accuracy measurement systems are particularly susceptible to soiling resulting from the
causes named. One can attempt to protect the measurement systems
against contamination by providing suitable shielding. This, however, will
often lead to the pressure registered by the measurement system – which
is indeed clean – deviating considerably from the pressure actually prevailing in the system.
In vacuum measurement systems used in the high and ultrahigh regimes it
is necessary to ensure in particular that the required high insulation values
for the high-voltage electrodes and ion traps also be maintained during
operation and sometimes even during bake-out procedures. Insulation
defects may occur both in the external feed line and inside the measurement system itself. Insufficient insulation at the trap (detector) lead may
allow creep currents – at low pressures – to stimulate overly high pressure
value readings. The very low ion trap currents make it necessary for this
lead to be particularly well insulated. Inside the measurement sensors, too,
creep currents can occur if the trap is not effectively shielded against the
other electrodes.
An error frequently made when connecting measurement sensors to the
vacuum system is the use of connector piping which is unacceptably long
and narrow. The conductance value must in all cases be kept as large as
possible. The most favorable solution is to use integrated measurement
systems. Whenever connector lines of lower conductance values are used
the pressure indication, depending on the cleanliness of the measurement
sensors and the connector line, may be either too high or too low. Here
measurement errors by more than one complete order of magnitude are
possible! Where systems can be baked out it is necessary to ensure that
the connector line can also be heated.
It is not fundamentally possible to keep the measurement system in a vacuum gauge from becoming soiled. Thus it is necessary to ensure that
• the influence of the contamination on pressure measurement remains
as small as possible and that
8.4.4 Connectors, power pack, measurement
systems
• the measurement system can readily be cleaned.
The measurement cables (connector cables between the sensor and the
vacuum gauge control unit) are normally 2 m long. If longer measurement
cables must be used – for installation in control panels, for example – then
it will be necessary to examine the situation to determine whether the pressure reading might be falsified. Information on the options for using overlength cables can be obtained from our technical consulting department.
These two conditions are not easy to satisfy by most vacuum gauges in
practice.
Dirt in a compression vacuum gauge will cause an incorrect and uncontrollable pressure indication. Dirty THERMOVAC sensors will show a pressure
which is too high in the lower measurement range since the surface of the
hot wire has changed. In Penning vacuum gauges contamination will
induce pressure readings which are far too low since the discharge currents will become smaller. In the case of ionization vacuum gauges with hot
cathodes, electrodes and the tube walls can be soiled which, under certain
circumstances, will result in a reduction of dielectric strengths. Here, however, the measurement systems can usually be baked out and degassed by
passing a current through or by electron bombardment, quite aside from
the fact that ionization vacuum gauges are often used in the ultrahigh vacuum range where it is necessary to ensure clean operating conditions for
other reasons.
8.4.3 The influence of magnetic and electrical
fields
In all those measurement instruments which use the ionization of gas molecules as the measurement principle (cold-cathode and hot-cathode ionization vacuum gauges), strong magnetic leakage fields or electrical potentials
can have a major influence on the pressure indication. At low pressures it is
also possible for wall potentials which deviate from the cathode potential to
influence the ion trap current.
146
Tables, Formulas, Diagrams
9.
Tables, formulas, nomograms and symbols
Unit
N · m–2, Pa 2) mbar
1 N · m–2 (= 1 Pa)
1 mbar
1 bar
1 Torr 3)
1
100
1 · 105
133
1 · 10–2
1
1 · 103
1.33
bar
Torr
1 · 10–5
1 · 10–3
1
1.33 · 10–3
7.5 · 10–3
0.75
750
1
1) The torr is included in the table only to facilitate the transition from this familiar unit to the
statutory units N · m-2, mbar and bar. In future the pressure units torr, mm water column, mm
mercury column (mm Hg), % vacuum, technical atmosphere (at), physicalatmosphere (atm),
atmosphere absolute (ata), pressure above atmospheric and pressure below atmospheric
may no longer be used. Reference is made to DIN 1314 in this context.
2) The unit Newton divided by square meters (N · m-2) is also designated as
Pascal (Pa): 1 N · m-2= 1 Pa.
Newton divided by square meters or Pascal is the SI unit for the pressure of fluids.
3) 1 torr = 4/3 mbar; fl torr = 1 mbar.
Table I: Permissible pressure units including the torr 1) and its conversion
Abbrev.
Gas
C* = λ · p
[cm · mbar]
H2
He
Ne
Ar
Kr
Xe
Hg
O2
N2
HCl
CO2
H2O
NH3
C2H5OH
Cl2
Air
Hydrogen
Helium
Neon
Argon
Krypton
Xenon
Mercury
Oxygen
Nitrogen
Hydrochloric acid
Carbon dioxide
Water vapor
Ammonia
Ethanol
Chlorine
Air
12.00 · 10–3
18.00 · 10–3
12.30 · 10–3
6.40 · 10–3
4.80 · 10–3
3.60 · 10–3
3.05 · 10–3
6.50 · 10–3
6.10 · 10–3
4.35 · 10–3
3.95 · 10–3
3.95 · 10–3
4.60 · 10–3
2.10 · 10–3
3.05 · 10–3
6.67 · 10–3
Table III:
1 ↓ = ... →
Mean free path l
Values of the product c* of the mean free path λ ( and pressure p for various gases at
20 °C (see also Fig. 9.1)
Pa
dyn · cm–2
atm
Torr
inch
Micron
cm
kp · cm–2
lb · in–2
(N/m3)
(μbar)
(phys.)
(mm Hg)
Hg
(μ)
H2O
(at tech.)
(psi)
mbar
lb · ft–2
mbar
1
102
103
9.87 · 10–4
0.75
2.953 · 10–2
7.5 · 102
1.02
1.02 · 10–3
1.45 · 10–2
2.089
Pa
10–2
1
10
9.87 · 10–6
7.5 · 10–3
2.953 · 10–4
7.5
1.02 · 10–2
1.02 · 10–5
1.45 · 10–4
2.089 · 10–2
2.089 · 10–3
μbar
10–3
0.1
1
9.87 · 10–7
7.5 · 10–4
2.953 · 10–5
7.5 · 10–1
1.02 · 10–3
1.02 · 10–6
1.45 · 10–5
atm
1013
1.01 · 105
1.01 · 106
1
760
29.92
7.6 · 105
1.03 · 103
1.033
14.697
2116.4
Torr
1.33
1.33 · 102
1.33 · 103
1.316 · 10–3
1
3.937 · 10–2
103
1.3595
1.36 · 10–3
1.934 · 10–2
2.7847
in Hg
33.86
33.9 · 102
33.9 · 103
3.342 · 10–2
25.4
1
2.54 · 104
34.53
3.453 · 10–2
0.48115
70.731
μ
1.33 · 10–3
1.33 · 10–1
1.333
1.316 · 10–6
10–3
3.937 · 10–5
1
1.36 · 10–3
1.36 · 10–6
1.934 · 10–5
2.785 · 10–3
cm H2O
0.9807
98.07
980.7
9.678 · 10–4
0.7356
2.896 · 10–2
7.36 · 102
1
10–3
1.422 · 10–2
2.0483
at
9.81 · 102
9.81 · 104
9.81 · 105
0.968
7.36 · 102
28.96
7.36 · 105
103
1
14.22
2048.3
psi
68.95
68.95 · 102
68.95 · 103
6.804 · 10–2
51.71
2.036
51.71 · 103
70.31
7.03 · 10–2
1
1.44 · 102
lb · ft–2
0.4788
47.88
478.8
4.725 · 10–4
0. 3591
1.414 · 10–2
359.1
0.488
4.88 · 10–4
6.94 · 10–3
1
Normal conditions: 0 °C and sea level, i.e. p = 1013 mbar = 760 mm Hg = 760 torr = 1 atm
in Hg = inches of mercury; 1 mtorr (millitorr) = 10-3 torr = 1 µ (micron … µm Hg column)
Pounds per square inch = lb · in-2 = lb / sqin = psi (psig = psi gauge … pressure above atmospheric, pressure gauge reading; psia = psi absolute … absolute pressure)
Pounds per square foot = lb / sqft = lb / ft2; kgf/sqcm2 = kg force per square cm = kp / cm2 = at; analogously also: lbf / squin = psi
1 dyn · cm-2 (cgs) = 1 µbar (microbar) = 1 barye; 1 bar = 0.1 Mpa; 1 cm water column (cm water column = g / cm2 at 4 °C) = 1 Ger (Geryk)
atm … physical atmosphere – at … technical atmosphere; 100 - (x mbar / 10.13) = y % vacuum
Table II: Conversion of pressure units
147
Tables, Formulas, Diagrams
Variable
General formula
Most probable speed
of particles cw
2⋅R⋅T
cw =
M
–c = 8 ⋅ R ⋅ T
π⋅M
–2 3 ⋅ R ⋅ T
c =
M
Mean velocity
of particles c–
Mean square of velocity
–
of particles c2
Gas pressure p of particles
Number density of particles n
Area-related impingement ZA
For easy calculation
冑
冑
p = 13.80 ⋅
p = 4.04 ⋅
2
1
· n · –c
4
c* =λ · p in cm · mbar
(see Tab. III)
k Boltzmann constant in mbar · l · K–1
cw = 410 [m/s]
–c = 464 [m/s]
[s ]
2
8
2
10–20
⋅ n ⋅ T [mbar]
10–17
⋅ n [mbar] (applies to all gases)
p
n = 7.25 ⋅ 1018 ⋅
T
[cm–3]
p = 2.5 ⋅ 1016 ⋅ p [cm–3] (applies to all gases)
ZA = 2.63 · 1022 ·
p
ZA = 2.85 ⋅ 1020 ⋅ p [cm–2 s–1] (see Fig. 78.2)
· p [cm–2 s–1]
A
1 n ⋅ –c
ZV = ⋅
2 λ
2 ⋅ NA
1
ZA =
⋅p2
c*
π⋅M⋅k⋅T
p⋅V=ν⋅R⋅T
ZV = 5.27 · 1022 ·
冑
Equation of state of ideal gas
Area-related mass flow rate qm, A
cm
s
cm
s
冑
M 
·T
N
·p
冑
2⋅π⋅M⋅k⋅T
ZA =
Volume collision rate ZV
cm2
–2
c = 25.16 ⋅ 104
2
4
n = p/kT
ZA =
冑 [ ]
–c = 1.46 ⋅ 10 冑
[ ]
T cm
–
c = 2.49 ⋅ 10
M [s ]
cw = 1.29 ⋅
p=n⋅k⋅T
1
–
p = ⋅ n ⋅ mT ⋅ c2
3
1
–
p = ⋅ ⋅ c2
3
Value for air at 20°C
T
M
T
M
104
p ⋅ V = 83.14 ⋅ ν ⋅ T [mbar ⋅ ]
冑
qm, A = ZA ⋅ mT =
M
⋅p
2 ⋅ π ⋅ k ⋅ T ⋅ NA
λ mean free path in cm
M molar mass in g · mol–1
mT particle mass in g
p2
[cm–3 s–1]

c* · 冑
M
·T
冑MT ⋅ p [g cm
Qm, A = 4.377 ⋅ 10–2
NA Avogadro constant in mol–1
n number density of particles in cm–3
ν amount of substance in mol
ZV = 8.6 ⋅ 1022 ⋅ p2 [cm–3 s–1] (see Fig. 78.2)
p ⋅ V = 2.44 ⋅ 104 ν [mbar ⋅ ] (for all gases)
–2 s–1]
qm, A = 1.38 ⋅ 10–2 ⋅ p g [cm–2 s–1]
p gas pressure in mbar
R molar gas constant
in mbar · l · mol–1 K–1
T thermodynamic temperature in K
V volume in l
Table IV: Compilation of important formulas pertaining to the kinetic theory of gases
Designation,
alphabetically
Symbol
Value and
unit
Atomic mass unit
Avogadro constant
mu
NA
1.6605 · 10–27 kg
6.0225 · 1023 mol–1
Boltzmann constant
k
Electron rest mass
Elementary charge
Molar gas constant
me
e
R
n
1.3805 · 10–23 J · K–1
mbar · l
13.805 · 10–23
K
9.1091 · 10–31 kg
1.6021 · 10–19 A · s
8.314 J · mol–1 K–1
mbar · l
= 83.14
mol · K
22.414 m3 kmol–1
22.414 l · mol–1
9.8066 m · s–2
6.6256 · 10–34 J · s
W
5.669 · 10–8 2 4
m K
A·s
– 1.7588 · 1011
kg
2.9979 · 108 m · s–1
kg · m–3
pn
Tn
101.325 Pa = 1013 mbar
Tn = 273.15 K, ϑ = 0 °C
Molar volume of
the ideal gas
Standard acceleration of free fall
Planck constant
Vo
gn
h
Stefan-Boltzmann constant
σ
Specific electron charge
Speed of light in vacuum
Standard reference density
of a gas
Standard reference pressure
Standard reference temperature
Table V: Important values
148
–e
me
c
Remarks
Number of particles per mol,
formerly: Loschmidt number
R = NA · k
DIN 1343; formerly: molar volume
at 0 °C and 1013 mbar
also: unit conductance, radiation constant
Density at ϑ = 0 °C and pn = 1013 mbar
DIN 1343 (Nov. 75)
DIN 1343 (Nov. 75)
Tables, Formulas, Diagrams
Unit
l · s–1
m3 · h–1
cm3 · s–1
cuft · min–1
1 l · s–1
1 m3 · h–1
1 cm3 · s–1
1 cuft · min–1
1
0.2778
10–3
0.4719
3.6
1
3.6 · 10–3
1.699
1000
277.8
1
471.95
2.12
0.589
2.1 · 10–3
1
Table VI: Conversion of pumping speed (volume flow rate) units
1↓ = ... →
mbar · l/s
cm3/s (NTP)
Torr · l/s
μ · cfm
lusec
Pa · l/s
slpm
3554
0.987
0.75
1.56 · 105
1.54 · 105
1593
7.52 · 102
100
59.2 · 10–3
kg · h–1 (20°C) kg · h–1 (0°C) cm3/h (NTP)
4.28 ·
10–3
10–3
1
kg · h–1 (20 °C)
234
1
1.073
8.31 · 105
231
175
–
–
37.2 · 104
1.75 · 105
23.4 · 103
13.86
kg · h–1 (0 °C)
218
0.932
1
7.74 · 105
215
163
–
–
34.6 · 104
1.63 · 105
21.8 · 103
12.91
cm3/h (NTP)
2.81 · 10–4
1.20 · 10–6
1.29 · 10–6
1
2.78 · 10–4
2.11 · 10–4
44
–
44.7 · 10–2
2.11 · 10–1
2.81 · 10–2 1.66 · 10–5
cm3/s (NTP)
1.013
4.33 · 10–3
4.65 · 10–3
3600
1
0.760
1.58 · 105
1611
760
10–3
10–3
Torr · l/s
1.33
g/a (F12. 20 °C)
6.39 · 10–6
–
g/a (F12. 25 °C)
6.50 · 10–6
–
μ · cfm
lusec
Pa · l/s
slpm
1
cm3
(NTP) = 1
6.28 ·
10–4
1.33 ·
10–3
5.70 ·
4.59 ·
g/a (F12. 20 °C) g/a (F12. 25 °C)
mbar · l/s
6.12 ·
–
2.69 ·
5.70 ·
10–6
2.89 ·
10–6
6.12 ·
10–6
–
6 · 10–2
133
78.8 · 10–3
1.32
1
4.80 · 10–6
1
–
10.2 · 10–3
4.8 · 10–3
6.39 · 10–4 37.9 · 10–8
–
–
4.88 · 10–6
–
1
10.4 · 10–3
4.89 · 10–3
6.5 · 10–4
38.5 · 10–8
6.28 ·
10–2
37.2 · 10–6
13.3 ·
10–2
2.24
6.21 ·
10–3
4.72 ·
98.16
96.58
1
0.472
78.8 · 10–6
4.737
1.32 ·
208
205
2.12
1
1 · 10–2
4.28 · 10–5
4.59 · 10–5
35.54
9.87 · 10–3
7.5 · 10–3
1.56 · 103
1.54 · 103
15.93
7.50
1
59.2 · 10–5
16.88
72.15 · 10–3
77.45 · 10–3
60.08 · 103
16.67
12.69
2.64 · 106
2.60 · 106
26.9 · 103
12.7 · 103
16.9 · 102
1
cm3
1·
10–3
1·
101
6.31 · 10–6
10–4
2119
103
4727
10–4
2.05 ·
105
2.27 · 10–2
–
10–6
2.08 ·
105
NTP = normal temperature and pressure (1 atm; 0 °C) R = 83.14 mbar · l · mol–1 · K–1
under normal conditions (T = 273.15 K; p = 1013.25 mbar)
1 cm3 (NTP) · h–1 = 1 atm · cm3 · h–1 = 1 Ncm3 · h–1 = 1 std cch
SI coherent: 1 Pa · m3 · s–1 = 10 mbar · l · s–1; R = 8.314 Pa · m3 · mol–1 · K–1; M in kg / mol
1 cm3 (NTP) · s–1 = 1 sccs = 60 cm3 (NTP) · min–1 60 sccm = 60 std ccm = 60 Ncm3 · min–1
1 lusec = 1 l · μ · s–1 1 · μ = 1 micron = 10–3 Torr 1 lusec = 10–3 Torr · l · s–1
Freon F 12 (CCl2F2) M = 120.92 g · mol–1; air M = 28.96 g · mol–1
1 sccm = 10–3 slpm = 10–3 N · l · min–1 = 60 sccs
Note: Anglo-American units are not abbreviated nonuniformly! Example: Standard cubic centimeter per minute → sccm = sccpm = std ccm = std ccpm
Table VIIa: Conversion of throughput (QpV) units; (leak rate) units
1↓= ... →
mbar · l/s
cm3/s **)
Torr · l/s
Pa · m3/s
g/a *)
μ · ft3/min
0.987
0.75
1
0.76
1.01 · 10–1
1.58 · 105
5.6 · 103
3.44 · 102
2.12 · 10–3
760
96.6 · 103
Torr · l/s
1.33
1.32
1
1.33 · 10–1
2.08 · 105
7.3 · 103
4.52 · 102
2.79 · 10–3
103
1.27 · 105
2119
Pa · m3/s
10
9.9
7.5
1
1.56 · 106
5.51 · 104
3.4 · 103
2.09 · 10–2
7.5 · 103
9.54 · 105
15.9 · 103
7.52 ·
102
μ · ft3/h
1
2.10 ·
10–3
μ · l/s
1.013
3.4 ·
102
atm · ft3/min
cm3/s **)
5.5 ·
103
lb/yr *)
mbar · l/s
1.56 ·
105
oz/yr *)
10–1
9.56 ·
104
1593
1614
g/a *)
6.39 · 10–6
6.31 · 10–6
4.80 · 10–6
6.41 · 10–7
1
3.5 · 10–2
2.17 · 10–3
1.34 · 10–8
4.8 · 10–3
0.612
10.2 · 10–3
oz/yr *)
1.82 · 10–4
1.79 · 10–4
1.36 · 10–4
1.82 · 10–5
28.33
1
6.18 ·· 10–2
3.80 · 10–7
0.136
17.34
0.289
lb/yr *)
2.94 · 10–3
2.86 · 10–3
2.17 · 10–3
2.94 · 10–4
4.57 · 102
16
1
6.17 · 10–6
2.18
280
4.68
atm · ft3/min
4.77 · 102
4.72 · 102
3.58 · 102
47.7
7.46 · 107
2.63 · 106
1.62 · 105
1
3.58 · 105
4.55 · 107
7.60 · 105
μ · l/s
1.33 · 10–3
1.32 · 10–3
10–3
1.33 · 10–4
208
7.34
4.52 · 10–1
2.79 · 10–6
1
127
2.12
μ · ft3/h
1.05 · 10–5
1.04 · 10–5
7.87 · 10–6
1.05 · 10–6
1.63
5.77 · 10–2
3.57 · 10–3
2.20 · 10–8
7.86 · 10–3
1
1.67 · 10–2
μ · ft3/min
6.28 · 10–4
6.20 · 10–4
4.72 · 10–4
6.28 · 10–5
98
3.46
2.14 · 10–1
1.32 · 10–6
0.472
60
1
1 · μ · ft3 · h–1 = 1.04 · 10–5 stsd cc per second
1 micron cubic foot per hour = 0.0079 micron liter per second
1 kg = 2.2046 pounds (lb)
1 cubic foot (cfut. cf) = 28.3168 dm3
1 cm3 · s–1 (NTP) = 1 atm · cm3 · s–1 = 1 scc · s–1 = 1 sccss
1 micron liter per second = 0.0013 std cc per second = 1 lusec
1 atm · ft3 · min–1 = 1 cfm (NTP)
1 micron cubic foot per minute = 1 μ · ft3 · min–1 = 1 μ · cuft · min–1 = 1μ · cfm
1 lb = 16 ounces (oz)
1 Pa · m3/s = 1 Pa · m3/s (anglo-am.) = 103 Pa · l/s
1 standard cc per second = 96.600 micron cubic feet per hour
1 lusec = 1 μ · l · s–1
1 μ · l · s–1 = 127 μ · ft3 · h–1 = 0.0013 std cc per second = 1 lusec
1 std cc/sec = 760 μ · l · s–1
*) F12 (20 °C) C.Cl2F2 M = 120.92 h/mol
**) (NTP) normal temperature and pressure 1 atm und 0 °C
Table VII b: Conversion of throughput (QpV) units; (leak rate) units
149
Tables, Formulas, Diagrams
% by weight
% by volume
Partial pressure mbar
N2
O2
Ar
CO2
Ne
He
CH4
Kr
N2O
H2
Xe
O3
75.51
23.01
1.29
0.04
1.2 · 10–3
7 · 10–5
2 · 10–4
3 · 10–4
6 · 10–5
5 · 10–6
4 · 10–5
9 · 10–6
Σ 100 %
78.1
20.93
0.93
0.03
1.8 · 10–3
7 · 10–5
2 · 10–4
1.1 · 10–4
5 · 10–5
5 · 10–5
8.7 · 10–6
7 · 10–6
Σ 100 %
792
212
9.47
0.31
1.9 · 10–2
5.3 · 10–3
2 · 10–3
1.1 · 10–3
5 · 10–4
5 · 10–4
9 · 10–5
7 · 10–5
Σ 1013
50 % RH at 20 °C
1.6
1.15
11.7
Note: In the composition of atmospheric air the relative humidity (RH) is indicated separately along with the temperature.
At the given relative humidity, therefore, the air pressure read on the barometer is 1024 mbar.
Table VIII: Composition of atmospheric air
Pressure
Particle number density
Mean free path
Impingement rate
Vol.-related collision rate
Monolayer time
Type of gas flow
p [mbar]
n [cm–3]
λ [cm]
Za [cm–2 · s–1]
ZV [cm–3 · s–1]
τ [s]
Other special features
Rough vacuum
Medium vacuum
High vacuum
Ultrahigh vacuum
1013 – 1
1019 – 1016
< 10–2
1023 – 1020
1029 – 1023
< 10–5
Viscous flow
1 – 10–3
1016 – 1013
10–2 – 10
1020 – 1017
1023 – 1017
10–5 – 10–2
Knudsen flow
10–3 – 10–7
1013 – 109
10 – 105
1017 – 1013
1017 – 109
10–2 – 100
Molecular flow
< 10–7
< 109
> 105
< 1013
< 109
> 100
Molecular flow
Convection dependent
on pressure
Significant change in
thermal conductivity
of a gas
Significant reductionin volume
related collision rate
Particles on the
surfaces dominate
to a great extend in
relation to particles in
gaseous space
Table IX: Pressure ranges used in vacuum technology and their characteristics (numbers rounded off to whole power of ten)
At room temperature
Standard values1
(mbar · l · s–1 · cm–2)
Metals
10–9 ... · 10–7
Nonmetals
10–7 ... · 10–5
Outgassing rates (standard values) as a function of time
Examples:
1/2 hr.
1 hr.
Ag
1.5 · 10–8
1.1 · 10–8
–8
Al
2 · 10
6 · 10–9
–8
Cu
4 · 10
2 · 10–8
Stainless steel
9 · 10–8
1 All values depend largely on pretreatment!
Table X: Outgassing rate of materials in mbar · l · s–1 · cm–2
150
3 hr.
2 · 10–9
5 hr.
6 · 10–9
3.5 · 10–8
3.5 · 10–9
2.5 · 10–8
Examples:
Silicone
NBR
Acrylic glass
FPM, FKM
1/2 hr.
1.5 · 10–5
4 · 10–6
1.5 · 10–6
7 · 10–7
1 hr.
8 · 10–6
3 · 10–6
1.2 · 10–6
4 · 10–7
3 hr.
3.5 · 10–6
1.5 · 10–6
8 · 10–7
2 · 10–7
5 hr.
1.5 · 10–6
1 · 10–6
5 · 10–7
1.5 · 10–7
Tables, Formulas, Diagrams
Nominal width (DN)
Series
R5
Internal diam. (mm)
R10
10
16
20
25
32
40
50
63
80
100
125
160
200
250
320
400
500
630
800
1000
10
16
21
24
34
41
51
70
83
102
127
153
213
261
318
400
501
651
800
1000
1 The nominal internal diameters correspond approximately to the internal diameters of the pipeline components” (DIN 2402 - Feb. 1976). The
left-hand column of the nominal internal diameter series is preferred in
practice.
Table XI: Nominal internal diameters (DN) and internal diameters of tubes, pipes and apertures
with circular cross-section (according to PNEUROP).
Solvent
Relative
molecular
mass
Density
g / cm3
(20 °C)
Acetone
Benzene (solution)
Petrol (light)
Carbon tetrachloride
Chloroform
Diethyl ether
Ethyl alcohol
Hexane
Isopropanol
Methanol
Methylene chloride
Nitromethane
Petroleum ether
Trichlorethylene („Tri“)
Water
58
78
0.798
0.8788
0.68 ... 0.72
1.592
1.48
0.7967
0.713
0.66
0.785
0.795
1.328
1.138
0.64
1.47
0.998
153.8
119.4
46
74
86
60.1
32
85
61
mixture
131.4
18.02
Melting
point
°C
5.49
– 22.9
– 63.5
–114.5
– 116.4
– 93.5
– 89.5
– 97.9
– 29.2
–
0.00
Boiling
point
°C
56
80.2
> 100
76.7
61
78
34.6
71
82.4
64.7
41
101.75
40 ... 60
55
100.0
Maximum admissible
concentration (MAC)
cm3 / m3
25
25
50
1000
400
500
400
200 (toxic!)
100
–
Table XII: Important data (characteristic figures) for common solvents
151
Tables, Formulas, Diagrams
t
°C
– 100
– 99
– 98
– 97
– 96
ps
mbar
1.403 · 10–5
1.719
2.101
2.561
3.117
D
g/m3
1.756 · 10–5
2.139
2.599
3.150
3.812
–
–
–
–
–
t
°C
35
34
33
32
31
ps
mbar
0.2233
0.2488
0.2769
0.3079
0.3421
D
g/m3
0.2032
0.2254
0.2498
0.2767
0.3061
t
°C
30
31
32
33
34
ps
mbar
42.43
44.93
47.55
50.31
53.20
D
g/m3
30.38
32.07
33.83
35.68
37.61
–
–
–
–
–
95
94
93
92
91
3.784 · 10–5
4.584
5.542
6.685
8.049
4.602 · 10–5
5.544
6.665
7.996
9.574
–
–
–
–
–
30
29
28
27
26
0.3798
0.4213
0.4669
0.5170
0.5720
0.3385
0.3739
0.4127
0.4551
0.5015
35
36
37
38
39
56.24
59.42
62.76
66.26
69.93
39.63
41.75
43.96
46.26
48.67
100
101
102
103
104
1013.2
1050
1088
1127
1167
597.8
618.0
638.8
660.2
682.2
–
–
–
–
–
90
89
88
87
86
9.672 · 10–5
11.60
13.88
16.58
19.77
11.44 · 10–5
13.65
16.24
19.30
22.89
–
–
–
–
–
25
24
23
22
21
0.6323
0.6985
0.7709
0.8502
0.9370
0.5521
0.6075
0.6678
0.7336
0.8053
40
41
42
43
44
73.78
77.80
82.02
86.42
91.03
51.19
53.82
56.56
59.41
62.39
105
106
107
108
109
1208
1250
1294
1339
1385
704.7
727.8
751.6
776.0
801.0
–
–
–
–
–
85
84
83
82
81
23.53 · 10–5
27.96
33.16
39.25
46.38
27.10 · 10–5
32.03
37.78
44.49
52.30
–
–
–
–
–
20
19
18
17
16
1.032
1.135
1.248
1.371
1.506
0.8835
0.9678
1.060
1.160
1.269
45
46
47
48
49
95.86
100.9
106.2
111.7
117.4
65.50
68.73
72.10
75.61
79.26
110
111
112
113
114
1433
1481
1532
1583
1636
826.7
853.0
880.0
907.7
936.1
–
–
–
–
–
80
79
78
77
76
0.5473 · 10–3
0.6444
0.7577
0.8894
1.042
0.6138 · 10–3
0.7191
0.8413
0.9824
1.145
–
–
–
–
–
15
14
13
12
11
1.652
1.811
1.984
2.172
2.376
1.387
1.515
1.653
1.803
1.964
50
51
52
53
54
123.4
129.7
136.2
143.0
150.1
83.06
87.01
91.12
95.39
99.83
115
116
117
118
119
1691
1746
1804
1863
1923
965.2
995.0
1026
1057
1089
–
–
–
–
–
75
74
73
72
71
1.220 · 10–3
1.425
1.662
1.936
2.252
1.334 · 10–3
1.550
1.799
2.085
2.414
– 10
– 9
– 8
– 7
– 6
2.597
2.837
3.097
3.379
3.685
2.139
2.328
2.532
2.752
2.990
55
56
57
58
59
157.5
165.2
173.2
181.5
190.2
104.4
109.2
114.2
119.4
124.7
120
121
122
123
124
1985
2049
2114
2182
2250
1122
1156
1190
1225
1262
–
–
–
–
–
70
69
68
67
66
2.615 · 10–3
3.032
3.511
4.060
4.688
2.789 · 10–3
3.218
3.708
4.267
4.903
–
–
–
–
–
5
4
3
2
1
4.015
4.372
4.757
5.173
5.623
3.246
3.521
3.817
4.136
4.479
60
61
62
63
64
199.2
208.6
218.4
228.5
293.1
130.2
135.9
141.9
148.1
154.5
125
126
127
128
129
2321
2393
2467
2543
2621
1299
1337
1375
1415
1456
–
–
–
–
–
65
64
63
62
61
5.406 · 10–3
6.225
7.159
8.223
9.432
5.627 · 10–3
6.449
7.381
8.438
9.633
0
1
2
3
4
6.108
6.566
7.055
7.575
8.129
4.847
5.192
5.559
5.947
6.360
65
66
67
68
69
250.1
261.5
273.3
285.6
298.4
161.2
168.1
175.2
182.6
190.2
130
131
132
133
134
2701
2783
2867
2953
3041
1497
1540
1583
1627
1673
–
–
–
–
–
60
59
58
57
56
10.80 · 10–3
12.36
14.13
16.12
18.38
10.98 · 10–3
12.51
14.23
16.16
18.34
5
6
7
8
9
8.719
9.347
10.01
10.72
11.47
6.797
7.260
7.750
8.270
8.819
70
71
72
73
74
311.6
325.3
339.6
354.3
369.6
198.1
206.3
214.7
223.5
232.5
135
136
137
138
139
3131
3223
3317
3414
3512
1719
1767
1815
1865
1915
–
–
–
–
–
55
54
53
52
51
20.92 · 10–3
23.80
27.03
30.67
34.76
20.78 · 10–3
23.53
26.60
30.05
33.90
10
11
12
13
14
12.27
13.12
14.02
14.97
15.98
9.399
10.01
10.66
11.35
12.07
75
76
77
78
79
385.5
401.9
418.9
436.5
454.7
241.8
251.5
261.4
271.7
282.3
140 3614
1967
–
–
–
–
–
50
49
48
47
46
39.35 · 10–3
44.49
50.26
56.71
63.93
38.21 · 10–3
43.01
48.37
54.33
60.98
15
16
17
18
19
17.04
18.17
19.37
20.63
21.96
12.83
13.63
14.48
15.37
16.31
80
81
82
83
84
473.6
493.1
513.3
534.2
555.7
293.3
304.6
316.3
328.3
340.7
–
–
–
–
–
45
44
43
42
41
71.98 · 10–3
80.97
90.98
102.1
114.5 · 10–3
68.36 · 10–3
76.56
85.65
95.70
106.9 · 10–3
20
21
22
23
24
23.37
24.86
26.43
28.09
29.83
17.30
18.34
19.43
20.58
21.78
85
86
87
88
89
578.0
601.0
624.9
649.5
674.9
353.5
366.6
380.2
394.2
408.6
–
–
–
–
–
40
39
38
37
36
0.1283
0.1436
0.1606
0.1794
0.2002
0.1192
0.1329
0.1480
0.1646
0.1829
25
26
27
28
29
31.67
33.61
35.65
37.80
40.06
23.05
24.38
25.78
27.24
28.78
90
91
92
93
94
701.1
728.2
756.1
784.9
814.6
423.5
438.8
454.5
470.7
487.4
1 Sources: Smithsonian Meteorological Tables 6th. ed. (1971) and VDI vapor tables 6th ed (1963).
Table XIII: Saturation pressure ps and vapor density D of water in a temperature range from –100°C to +140°C1
152
t
°C
95
96
97
98
99
ps
mbar
845.3
876.9
909.4
943.0
977.6
D
g/m3
504.5
522.1
540.3
558.9
578.1
Tables, Formulas, Diagrams
Group A3)
Group B3)
Group C3)
Methane
c
Ethylene
c
Hydrogen
c
Ethane
c
Buta-1,3-diene
c
Acetylene (ethyne)
c
Propane
c
Acrylonitrile
c
Carbon bisulfide
c
Butane
c
hydrogen cyanide
a
Pentane
c
Dethyl ether (s)
c
Hexane
c
Ethylene oxide (oxiran)
c
Heptane
c
1.4 Dioxan
a
Octane
a
Tetrahydrofuran
a
Cyclohexane
c
Tetrafluoroethylene
a
Propylene (propene)
a
Styrene (s)
b
Benzene (s)
c
Toluene (s)
–
Xylene
a
Legend
Group A
Group B
Group C
Naphthalene
–
MESG1
> 0.9 mm
0.5 ... 0.9 mm
< 0.5 mm
Methanol (s)
c
MIC2 ratio
> 0.8 mm
0.45 ... 0.8 mm
< 0.45 mm
Ethanol (s)
c
Propyl alcohol (propanol)
c
2
Butyl alcohol (butanol)
a
3
Phenol
–
Acetaldehyde (ethanal)
a
Acetone (s) (propanone)
c
Methyl ethyl ketone (s) (propan-2-one)
c
Ethyl acetate (s)
a
Butyl acetate (s)
c
Amyl acetate (s)
–
Ethyl methacrylate
–
Acetic acid (ethanoic acid)
b
Methyl chloride (s)
a
Methylene chloride (s) (dichlormethane)
–
Ammonia
a
acetonitrile
a
Aniline
–
Pyridine
–
1
Minimum Electrical Spark Gap
Minimum Ignition Current
The ratio is based on the MIC value for laboratory methane
Group allocation:
a – according to MESG value
b – according to MIC ratio
c – according to both MESG value and MIC ratio
(s) – solvent
Table XIV: Hazard classification of fluids according to their MESG1 and/or MIC2 values.
(Extract from European Standard EN 50.014)
153
x
x
x
x
x
x
–
Acetic acid, 50 %
Acetic acid, 80 %
Acetic anhydride
Aceto-acetic ester
Acetone
o x
o o
– x
x
o
o
x
x
x
x
x
Acetophenone
Acetylene
Acrylnitrile
Air, clean
Air, oily
o
x –
–
x
x
o
x
x
x
x
x
x
x
Ammonia liquid
Ammonia gas
Amyl acetate
Amyl alcohol
Aniline
x
x
o
–
o
x
x
o
–
o
–
–
o
o
o
o
x
Anthracene oil
ASTM oil No. 1
ASTM oil No. 2
ASTM oil No. 3
Benzaldehyde 100 %
o
x
x
–
o
x
x
–
x
x
x
–
Benzene
Benzene bromide
Benzoic acid
Bitumen
Blast furnace gas
o o
o o
Boron trifluoride
Bromine
Butadiene
Butane
Butyl acetate
x x
o o
x –
o o
Butyl alcohol
Butyl glykol
Butyraldehyde
Carbolineum
Carbon bisulfide
–
x
o
o
o
–
o –
x x
x x
x x
x
x
o
o
o
x
x
o
x
x
x
x
o
–
o
x
–
x
x
x
x
–
x
x
x
x
o
o
o
o
o
–
x –
–
x
o o
x
o o
x
o o
x
x
x
x
x
o
o
o
x
Cyclohexylamine
Decahydronaphtalene
Desmodur T
Desmophene 2000
Dibutylphthalate
x
x
x
x
x
x
x
x
x
x
o
–
o
o
o
–
Dichlorethylene
Dichlorethane
Diethylamine
Diethylene glycol
Diethyl ether
– o
x x
o –
o
o
–
Diethyl ether
Diethyl sebazate
Dichlorbenzene
Dichlorbutylene
Diesel oil
o
o
o
x
o
Di-isopropyl ketone
Dimethyl ether
Dimethylaniline
Dimethyl formamide
Dioctylphthalate
o
o
o o
o o
o o
x
–
o
o
x
x
o
x
o
x
x
x
x
x
EPDM
Teflon (PTFE)
Fluoro rubber (FPM, FKM) Viton
Silicone rubber
Chloroprene rubber (CR) Neoprene
Chloroform
Chloromethyl
Citrus oils
Coke furnace gas
Copra oil acid
x
x
x
o
o
x
x
x
o
o
o
– x
o o
o
o o
Cottonseed oil
Cresol
Crude petroleum
Cyclohexane
Cyclohexanone
x
x
x
x
x
x
x
x
o
o
Clorine, dry
Chlorine water
Chlorine, wet
Chlorobromomethane
Chlorobenzene
x
x
x
x
x
Table XV: Chemical resistance of commonly used elastomer gaskets and sealing materials
154
x
Carbon dioxide, dry
Carbon dioxide, wet
Carbon tetrachloride
Chloracetic acid
Chlorinated solvents
Nitrile-butadiene rubber (NBR) Perbunan
o
o
o o
– o
x
x x
x
x = resistant
– = conditionally resistant
o = not resistant
EPDM
Fluoro rubber (FPM, FKM) Viton
o
–
Acetaldehyde
Acetic acid (crystalline), pure
Acetic acid, industrial
Acetic acid vapors
Acetic acid, 20 %
Medium
Teflon (PTFE)
Silicone rubber
x = resistant
– = conditionally resistant
o = not resistant
Chloroprene rubber (CR) Neoprene
Medium
Nitrile-butadiene rubber (NBR) Perbunan
Tables, Formulas, Diagrams
x
o
x
x
x
x
x
x
x
x
x
–
o
x
x
x
x
x
o
o
o
x
x
x
x
x
–
o
o
o
o
o
o
o
x
o
x
x
x
x
x
x
x
o
x
o
x
x
–
x
x
x
x
o
o
o
o
–
x
o
x
o
x
x
x
x
x
x
o
x
x
o
x
x
x
x
x
–
–
o
o
o
x
x
x
x
x
x
x
–
–
o
o
Ethyl alcohol, pure
Ethyl cloride
Ethylene bromide
Ethylene chloride
Ethylene dichloride
– –
x –
o o
Ethylene glycol
Ethyl ether
Ethyl silicate
Ethyl acrylate
Fatty acids
x x
o o
x x
– –
Fatty alcohol
Fir leaf oil
Fluorbenzene
Hydrofluoric acid, cold, 5 %
Hydrofluoric acid, cold, pure
x
x
x
x
–
x
o
o
x
x
x
Formaldehyde
Formalin, 55 %
Formic acid
Formic acid methyl ester
Freon 11
x
x
–
o
x
–
x
–
–
x
x
Freon 12
Freon 22
Freon 113
Furane
Furfurol
x
o
x
o
o
x
x
x
o
o
Gas oill
Generator gas
Glycerine
Glycol
Halowax oil
x
x
x
x
o
–
–
x
x
o
o
x
o
o
–
x
x
x
x
x
x
o
o
–
x
x
x
x
x
x
–
x
– –
o
o o
–
o
x
o
o
–
o
–
o
x
o
o
x
x
x
x
x
x
o
–
–
o
–
Hydraulic fluids
Hydraulic oils DIN 51524
Phosphoric ester HFD
Polyglycol water HFC
Hydrobromic acid
x
o
x
o
–
o
–
o
Hydrobromic crystalline acid
Hydrocyanic acid
Hydrogen bromide
Hydrogen gas 20
Hydrogen sulfide
x
– –
x x
x
x
x
x
x
x
–
Isobutyl alcohol
Isopropyl acetate
Isopropyl alcohol
Isopropyl chloride
Isopropyl ether
–
o
–
o
x
x
x
–
Kerosene
Kerosine
Lighting gas
Maleic anhydride
Mercury
– –
x –
– –
Methane
Methane (pit gas)
Methylene chloride
Methyl acrylate
Methyl alcohol (methanol)
x –
x x
o o
– –
Methyl ethyl ketone
Methyl isobutyl ketone
Methyl methacrylate
Methyl salicylate
Naphtalene
o
o
o
o
o
Natural gas
Nitrobenzene
Nitrous oxide
Oleic acid
Orange oil
– x
o o
x
x
o o
o
x
–
o
x
x
o
x
x
x
x
x
x
x
x
–
x
x
x
x
x
–
x
x
x
x
x
x
o
o
x
x
o
x
x
x
o
x
–
o
x
x
x
x
–
x
x
x
o
x
o
o
EPDM
o
x –
o o
–
o
x
Teflon (PTFE)
Ethyl acetate (acetic ether)
Ethane
Ethyl acetate
Ethyl acrylate
Ethyl alcohol, denatured
x
x
x
x
x
Fluoro rubber (FPM, FKM) Viton
–
Heating fuel oil (coal base)
Heating fuel oil (petroleum crude base)
Heptane
Hexaldehyde
Hexane
x = resistant
– = conditionally resistant
o = not resistant
Silicone rubber
o o
Chloroprene rubber (CR) Neoprene
x
Dioxan
Diphenyl
Diphenyloxyd
Edenol 888
Essential oils
Nitrile-butadiene rubber (NBR) Perbunan
–
x = resistant
– = conditionally resistant
o = not resistant
Medium
EPDM
Fluoro rubber (FPM, FKM) Viton
o o
o o
o
Medium
Teflon (PTFE)
Silicone rubber
Chloroprene rubber (CR) Neoprene
Nitrile-butadiene rubber (NBR) Perbunan
Tables, Formulas, Diagrams
x
x
x
x
x
o
o
x
o
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x x
x
o
o
o
o
–
x
x
x
x
x
o
x
x
x
x
–
x
o
o
o
x
x
x
x
x
x
–
–
o
o
x
o
o
o
x
x
x
x
x
–
–
o
–
o
x
o
x
x
x
x
x
–
o
x
o
x
o
o
o
x
x
Table XV: Chemical resistance of commonly used elastomer gaskets and sealing materials
155
–
x
o
o
o
o
o
o
o
o
o
o
o
o
Petrol benzene 3:7
Petrol benzene spirit 5:3:2
Phenol
Phenyl ethyl ether
Phenylic acid (phenol)
o
o
o
o
o
o
o
o
o
o
Phosphorous chloride
Phthalic anhydride
Piperidine
Polyglycol
Propane, gas
o
x
o
x
x
–
x
o
x
x
x
Propylene oxyde
Propyl alcohol
Pydraul F-9
Pydraul AC
Pydraul A 150
o
x
o o
o
o
x
x
x
Pydraul A 200
Pyridine
Salicylic acid
Skydrol 500
Skydrol 7000
o
x
Stearic acid
Styrene
Sulfur
Sulfur dioxide
Sulfur trioxide, dry
–
o
–
o
o
Transformer oil
Train oil
Triethanolamine
Tributoxyethyl phosphate
Tributyl phosphate
x
o
o
o
o
o
o
o
x
x
x
x
x
x
x
x
x
x
o
o
o
o
Trichloroethane
Trichloroethylene
Trichloroethyl phosphate 20
Trichloroethyl phosphate 80
Trichloracetic acid 60
o
o
–
o
o
o
x
x
o
o
x
x
–
–
x
x
x
x
x
o
o
o
–
Tricresyl phosphate
Turpentine
Turpentine oil, pure
Vinyl acetate
Vinylaceto-acetic acid 3:2
o
– –
x
o
o –
x
x
x
x
x
x
x
Vinyl chloride, liquid
Water 50
Water 100
Wood oil
Xylamon
x x
x –
–
o o
x
x
x
x
x
–
x
Xylene
o o
x
x
x
x
x
o
x x
x
x
o
x
o
–
x
x
o
x
x
x
x
x
x
x
x
–
x
x
x
x
x
x
x
x
x
o
x
x
–
Table XV: Chemical resistance of commonly used elastomer gaskets and sealing materials
156
x
o
o
x
x
x
x
o
x
x
x
x
x
x
x
o
o
o
x
x
x
x
x
x
x
o
x
EPDM
Petrol alcohol 3:1
Petrol benzene 4:1
Petrol benzene 7:3
Petrol benzene 3:2
Petrol benzene 1:1
o
o
o
o
o
x
x
o
Teflon (PTFE)
o
x
x
x
x
x
x
o
Fluoro rubber (FPM, FKM) Viton
x
o
–
o o
o o
o o
x
o
Silicone rubber
x
o
Tar oil
Tetrachlorethylene
Tetrahydrofurane
Tetraline
Toluene
x
x
x
x
x
x
x
Chloroprene rubber (CR) Neoprene
x
o
x
o
x
x
x
Nitrile-butadiene rubber (NBR) Perbunan
Paraffin oil
Pentachlordiphenyl
Pentane
Perchloroethylene
Petrol
x
x = resistant
– = conditionally resistant
o = not resistant
EPDM
x
–
x
o
x
Medium
Teflon (PTFE)
x
o
x
–
x
x = resistant
– = conditionally resistant
o = not resistant
Fluoro rubber (FPM, FKM) Viton
Chloroprene rubber (CR) Neoprene
Oxygen
Ozone
Palmitic acid
Palm oil acid
Paraffin
Medium
Silicone rubber
Nitrile-butadiene rubber (NBR) Perbunan
Tables, Formulas, Diagrams
x
x
x
x
x
o
o
o
o
o
x
x
x
x
x
o
–
–
o
o
x
x
x
x
x
x
x
x
x
x
o
–
–
o
o
x
x
x
x
x
o
x
o
Tables, Formulas, Diagrams
Vacuum symbols
All symbols wit the exception of those marked with**) do not depend on the
position.
**) These symbols may only be used in the position shown here (tip of the
angle pointing down)
The symbols for vacuum pumps should always be arranged such that the
side with the constriction is allocated to the higher pressure
Vacuum pumps
Vacuum pump, general
Ejector vacuum pump
**
Piston vacuum pump
Diffusion pump
**
Diaphragm vacuum pump
Adsorption pump
**
Rotary positive displacement pump
**
Getter pump
Rotary plunger vacuum pump
**
Sputter-ion pump
Sliding vane rotary vacuum pump
**
Cryopump
Rotary piston vacuum pump
**
Scroll pump
Liquid ring vacuum pump
**
Evaporation pump
Roots vacuum pump
**
Turbine vacuum pump, general
**
Accessories
Radial flow vacuum pump
Condensate trap, general
Axial flow vacuum pump
Condensate trap with heat exchanger (e.g. cooled)
Turbomolecular pump
Gas filter, general
Table XVI: Symbols used in vacuum technology (extract from DIN 28401)
157
Tables, Formulas, Diagrams
Filtering apparatus, general
Right-angle stop cock
Baffle, general
Gate valve
Cooled baffle
Butterfly valve
Cold trap, general
Nonreturn valve
Cold trap with coolant reservoir
Safety shut-off valve
Sorption trap
Throttling
Modes of operation
Manual operation
Vacuum chambers
Variable leak valve
Vacuum chamber
Electromagnetic operation
Vacuum bell jar
Hydraulic or pneumatic operation
Electric motor drive
Shut-off devices
Weight-operated
Shut-off device, general
Shut-off valve, straight-through valve
Right-angle valve
Flange connection, general
Stop cock
Bolted flange connection
Three-way stop cock
Small flange connection
Table XVI: Symbols used in vacuum technology (extract from DIN 28401) (continuation)
158
Connections and piping
Tables, Formulas, Diagrams
Clamped flange connection
Threaded tube connection
Ball-and-socket joint
Spigot-and-socket joint
Taper ground joint connection
Intersection of two lines with connection
Intersection of two lines without connection
Branch-off point
Measurement and gauges
General symbol for vacuum **)
Vacuum measurement, vacuum gauge head **)
Vacuum gauge, operating and display unit for vacuum
gauge head **)
Vacuum gauge, recording **)
Vacuum gauge with analog measured-value display **)
Vacuum gauge with digital measured-value display **)
Measurement of throughput
Combination of ducts
Flexible connection (e.g. bellows, flexible tubing)
Linear-motion leadthrough, flange-mounted
Linear-motion leadthrough, without flange
Leadthrough for transmission of rotary and linear motion
Rotary transmission leadthrough
Electric current leadthrough
Table XVI: Symbols used in vacuum technology (extract from DIN 28401) (continuation)
159
Tables, Formulas, Diagrams
Boiling point H2O
Body temperature 37°C
Room temperature
Freezing point H2O
NaCl/H2O 50:50
Freezing point Hg
CO2 (dry ice)
Boiling point LN2
Absolute zero point
Kelvin Celsius
373
100
310
37
293
20
273
0
255
–18
34
–39
195
–78
77
–196
0
–273
Réaumur Fahrenheit Rankine
80
212
672
30
99
559
16
68
527
0
32
492
–14
0
460
–31
–39
422
–63
–109
352
–157
–321
170
–219
–460
0
K
Kelvin
°C
Celsius
K
Kelvin
1
K – 273
°C
Celsius
°C + 273
1
°C
Réaumur
5
· °R + 273
4
°F
Fahrenheit
°R
Rankine
Conversion in
°R
Réaumur
4
(K – 273)
5
9
(K – 273) + 32
5
°R
Rankine
9
K = 1,8 K
5
4
· °C
5
9
· °C + 32
5
9
(°C + 273)
5
5
· °R
4
1
9
· °R + 32
4
5 (°F – 32) + 273
9
5 (°F – 32)
9
4 (°F – 32)
9
1
°F + 460
5 (°R)
9
5 (°R – 273)
9
°R – 460
1
4
5
°F
Fahrenheit
[ 59 (°R – 273) ]
5
9
[ 54 (°R + 273) ]
λ~ 1
p
Pressure p [mbar]
Pressure p [mbar]
λ
n
ZA
ZV
Fig. 9.1:
160
V
2
–p
ZA
Z
Mean free path λ [cm]
– p2
Table XVII: Temperature comparison and conversion table (rounded off to whole degrees)
Variation of mean free path λ (cm) with pressure for various gases
Fig. 9.2:
: mean free path in cm (λ ~ 1/p)
: particle number density in cm–3 (n ~ p)
: area-related impingement rate in cm–3 · s–1 (ZA ~ p2)
: volume-related collision rate in cm–3 · s–1 (ZV ~ p2)
Diagram of kinetics of gases for air at 20 °C
Tables, Formulas, Diagrams
106
Pressure [mbar]
105
conductance [l · cm–1]
Temperature (K)
104
103
102
1018
6
Altitude [km]
4
Fig. 9.3:
2
Decrease in air pressure (1) and change in temperature (2) as a function of altitude
100
101
2
4
6 8102
103
104
Pipe length l [cm]
Conductance values for piping of commonly used nominal width with circular crosssection for laminar flow (p = 1 mbar) according to equation 53a. (Thick lines refer to
preferred DN) Flow medium: air (d, l in cm!)
conductance [l · s–1]
Altitude (km)
Fig. 9.5:
Molecules/atoms [cm–3]
Fig. 9.4:
Change in gas composition of the atmosphere as a function of altitude
Pipe length l [cm]
Fig. 9.6:
Conductance values for piping of commonly used nominal width with circular cross-section
for molecular flow according to equation 53b. (Thick lines refer to preferred DN) Flow medium: air (d, l in cm!)
161
Tables, Formulas, Diagrams
pSTART – pend, P
R=
pEND – pend, P
pSTART < 1013 mbar
Column ¿ : Vessel volume V in liters
Column ¡ : Maximum effective pumping speed Seff,max at the
vessel in (left) liters per second or (right) cubic
meters per hour.
Column ¬ : Pump-down time tp in (top right) seconds or (center left) minutes or (bottom right) hours.
Column √ : Right:
Pressure pEND in millibar at the END of the pumpdown time if the atmospheric pressure
pSTART ( pn = 1013 prevailed at the START of the
pump-down time. The desired pressure pEND is to
be reduced by the ultimate pressure of the pump
pult,p and the differential value is to be used in the
columns. If there is inflow qpV,in, the value
pend – pult,p – qpV,in / Seff,max is to be used in the
columns.
Left:
Pressure reduction ratio R = (pSTART – pult,p –
qpV,in / Seff,max)/(pend – pult,p – qpV,in / Seff,max), if
the pressure pSTART prevails at the beginning of
the pumping operation and the pressure is to be
lowered to pEND by pumping down.
The pressure dependence of the pumping speed
is taken into account in the nomogram and is
expressed in column ƒ by pult,p. If the pump
pressure pult,p is small in relation to the pressure
pend which is desired at the end of the pumpdown operation, this corresponds to a constant
pumping speed S or Seff during the entire pumping process.
Fig. 9.7:
162
pEND – pend, p
mbar
pSTART = 1013 mbar
Example 1 with regard to nomogram 9.7:
Example 2 with regard to nomogram 9.7:
A vessel with the volume V = 2000 l is to be pumped down
from a pressure of pSTART = 1000 mbar (atmospheric pressure) to a pressure of pEND = 10-2 mbar by means of a rotary
plunger pump with an effective pumping speed at the vessel of
Seff,max = 60 m3/h = 16.7 l · s-1. The pump-down time can be
obtained from the nomogram in two steps:
A clean and dry vacuum system (qpV,in = 0) with V = 2000 l (as
in example 1) is to be pumped down to a pressure of
pEND = 10-2 mbar. Since this pressure is smaller than the ultimate pressure of the rotary piston pump (Seff,max = 60 m3/h =
16.7 l ( s-1 = 3 · 10-2 mbar), a Roots pump must be used in
connection with a rotary piston pump. The former has a starting pressure of p1 = 20 mbar, a pumping speed of
Seff,max = 200 m3/h – 55 l · s-1 as well as pult,p – 4 · 10-3 mbar.
From pstart = 1000 mbar to p = 20 mbar one works with the
rotary piston pump and then connects the Roots pump from
p1 = 20 mbar to pEND = 10-2 mbar, where the rotary piston
pump acts as a backing pump. For the first pumping step one
obtains the time constant τ = 120 s = 2 min from the nomogram as in example 1 (straight line through V = 2000 l, Seff =
16.7 l · s-1). If this point in column ¬ is connected with the
point p1 - pult,p = 20 mbar – 3 · 10-2 mbar = 20 mbar (pult,p is
ignored here, i.e. the rotary piston pump has a constant pumping speed over the entire range from 1000 mbar to 20 mbar) in
column 5, one obtains tp,1 = 7.7 min. The Roots pump must
reduce the pressure from p1 = 20 mbar to
pEND = 10-2 mbar, i.e. the pressure reduction ratio
R = (20 mbar – 4 · 10-3 mbar) / (10-2 mbar-4 · 10-3) =
20/6 · 10-3 mbar = 3300.
1) Determination of τ: A straight line is drawn through
V = 2000 l (column ¿ ) and Seff = 60 m3/h-1 = 16.7 l · s-1 (column ¡ ) and the value t = 120 s = 2 min is read off at the
intersection of these straight lines with column ¬ (note that
the uncertainty of this procedure is around Δτ = ± 10 s so
that the relative uncertainty is about 10 %).
2) Determination of tp: The ultimate pressure of the rotary
pump is pult,p = 3 · 10-2 mbar, the apparatus is clean and leakage negligible (set qpV,in = 0); this is pSTART – pult,p =
10-1 mbar – 3 · 10-2 mbar = 7 · 10-2 mbar. Now a straight line
is drawn through the point found under 1) τ = 120 s (column
¬ ) and the point pEND – pult,p = 7 · 10-2 mbar (column ƒ )
and the intersection of these straight lines with column √
tp = 1100 s = 18.5 min is read off. (Again the relative uncertainty of the procedure is around 10 % so that the relative
uncertainty of tp is about 15 %.) Taking into account an additional safety factor of 20 %, one can assume a pump-down
time of
tp = 18.5 min · (1 + 15 % + 20 %) = 18.5 min · 1.35 = 25 min.
Nomogram for determination of pump-down time tp of a vessel in the rough vacuum pressure range
The time constant is obtained (straight line V = 2000 l in column ¿ , Seff = 55 l · s–1 in column ¡ ) at = 37 s (in column
¬ ). If this point in column ¬ is connected to R = 3300 in column ƒ , then one obtains in column √ tp, 2 = 290 s =
4.8 min. If one takes into account tu = 1 minfor the changeover
time, this results in a pump-down time of tp = tp1 + tu + tp2 =
7.7 min + 1 min + 4.8 min = 13.5 min.
Tables, Formulas, Diagrams
Example: What diameter d must a 1.5-m-long pipe have so
that it has a conductance of about C = 1000 l / sec in the
region of molecular flow? The points l = 1.5 m and
C = 1000 l/sec are joined by a straight line which is extended
to intersect the scale for the diameter d. The value d = 24 cm
is obtained. The input conductance of the tube, which depends
on the ratio d / l and must not be neglected in the case of
short tubes, is taken into account by means of a correction
factor α. For d / l < 0.1, α can be set equal to 1. In our exam-
Fig. 9.8:
Tube diameter
Conductance for molecular flow
Tube length
Correction factor for short tubes
C
ple d/l = 0.16 and α = 0.83 (intersection point of the straight
line with the a scale). Hence, the effective conductance of the
pipeline is reduced to
C · α = 1000 · 0.83 = 830 l/sec. If d is increased to 25 cm,
one obtains a conductance of 1200 · 0.82 = 985 l / sec
(dashed straight line).
Nomogram for determination of the conductance of tubes with a circular cross-section for air at 20 °C in the region of molecular flow (according to J. DELAFOSSE and G. MONGODIN: Les calculs de la Technique du Vide, special issue “Le Vide”, 1961).
163
Tables, Formulas, Diagrams
Air 20 °C
Tube internal diameter [cm]
Uncorrected conductance for mol. flow [m3 · h–1]
conductance [ l · s–1]
Clausing factor
Laminar flow
Mol. flow
Knudsen flow
Correction factor for higher pressures
Pressure [mbar]
Pressure [mbar]
Tube length [meters]
C
Procedure: For a given length (l) and internal diameter (d),
the conductance Cm, which is independent of pressure, must
be determined in the molecular flow region. To find the conductance C* in the laminar flow or Knudsen flow region with a
given mean pressure of p in the tube, the conductance value
previously calculated for Cm has to be multiplied by the correction factor a determined in the nomogram: C* = Cm · α.
Fig. 9.9:
164
Example: A tube with a length of 1 m and an internal diameter
of 5 cm has an (uncorrected) conductance C of around 17 l/s
in the molecular flow region, as determined using the appropriate connecting lines between the “l” scale and the “d” scale.
The conductance C found in this manner must be multiplied by
the clausing factor γ = 0.963 (intersection of connecting line
with the γ scale) to obtain the true conductance Cm in the molecular flow region:
Cm · γ = 17 · 0.963 = 16.37 l/s.
Nomogram for determination of conductance of tubes (air, 20 °C) in the entire pressure range.
In a tube with a length of 1 m and an internal diameter of 5 cm
a molecular flow prevails if the mean pressure p in the tube is
< 2.7 · 10-3 mbar.
To determine the conductance C* at higher pressures than
2.7 · 10-3 mbar, at 8 · 10-2 mbar (= 6 · 10-2 torr), for example,
the corresponding point on the p scale is connected with the
point d = 5 cm on the “d” scale. This connecting line intersects
the “α“ scale at the point α = 5.5. The conductance C* at
p = 8 · 10-2 mbar is: C* = Cm · α = 16.37 · 5.5 = 90 l/s.
Pump-down time t [min]
Volume V [min3]
[m3 · h–1] Pumping speed
Tables, Formulas, Diagrams
Example 2
ple 1
Gas evolution [mbar · l · s–1 · m–2]
Exam
weak
normal
strong
Area F [m2]
The nomogram indicates the relationship between the nominal
pumping speed of the pump, the chamber volume, size and
nature of the inner surface as well as the time required to
reduce the pressure from 10 mbar to 10-3 mbar.
Example 1: A given chamber has a volume of 70 m3 and an
inner surface area of 100 m2; a substantial gas evolution of
2 · 10-3 mbar · l · s-1 · m-2 is assumed. The first question is to
decide whether a pump with a nominal pumping speed of
1300 m3/h is generally suitable in this case. The coordinates
for the surface area concerned of 100 m2 and a gas evolution
of 2 · 10-3 mbar · l · s-1 · m-2 result in an intersection point A,
which is joined to point B by an upward sloping line and then
connected via a vertical line to the curve that is based on the
pumping speed of the pump of 1300 m3/h (D). If the projection
to the curve is within the marked curve area (F), the pumping
speed of the pump is adequate for gas evolution. The relevant
pump-down time (reduction of pressure from 10 mbar to
10-3 mbar) is then given as 30 min on the basis of the line
connecting the point 1300 m3/h on the pumping speed scale
to the point 70 m3 (C) on the volume scale: the extension
results in the intersection point at 30 min (E) on the time scale.
8 · 10-5 mbar · l · s-1 · m-2 is to be evacuated from 10 mbar to
10-3 mbar within a time of 10 min. The nomogram shows that
in this case a pump with a nominal pumping speed of
150 m3/h is appropriate.
In example 2 one has to determine what pumping speed the
pump must have if the vessel (volume = approx. 3 m3) with a
surface area of 16 m2 and a low gas evolution of
Fig. 9.10: Determination of pump-down time in the medium vacuum range taking into account the outgasing from the walls
165
Tables, Formulas, Diagrams
1000
1,00E+03
Mercury
100
Vapor pressure [mbar]
1,00E+02
Halocarbon 11
Halocarbon 12
1,00E+01
Halocarbon 13
1,00E+00
10
Halocarbon 22
1
Santovac 5
(similar to Ultralen)
10-1
Vapor pressure (mbar)
1,00E-01
Trichorethylene
Acetone
Aziepon 201
10-2
1,00E-02
10-3
1,00E-03
DC 704
Diffelen ultr a
10-4
1,00E-04
10-5
1,00E-05
DC 705
1,00E-06 -6
10
1,00E-07-7
10
Temperature [°C]
1,00E-08-8
10
Diffelen
light
Diffelen
normal
1,00E-09-9
10
Fig. 9.11: Saturation vapor pressure of various substances
-10
1,00E-10
10
10-11
1,00E-11
10-12
1,00E-12
0
25
50
75
100
150
200
250
Temperature (°C)
Temperature [°C]
Temperature [K]
Fig. 9.12: Saturation vapor pressure of pump fluids for oil and mercury fluid entrainment pumps
Vapor pressure
Fig. 9.13: Saturation vapor pressure of major metals used in vacuum technology
166
Vapor pressure [mbar]
Tables, Formulas, Diagrams
쑗 critical point
쎲 melting point
Temperature [°C]
1 NBR (Perbunan)
2 Silicone rubber
3 Teflon
Fig. 9.14: Vapor pressure of nonmetallic sealing materials (the vapor pressure curve for fluoro
rubber lies between the curves for silicone rubber and Teflon).
Ultrahigh vacuum
<10–7 mbar
<10–5 Pa
Fig. 9.15: Saturation vapor pressure ps of various substances relevant for cryogenic technology in
a temperaturerange of T = 2 – 80 K.
High vacuum
10–7 to 10–3 mbar
10–5 to 10–1 Pa
Medium vacuum
10–3 to 1 mbar
10–1 to 102 Pa
Rough vacuum
1 to approx. 103 mbar
102 to approx. 105 Pa
Piston vacuum pump
Diaphragm vacuum pump
Liquid-ring vacuum pump
Sliding-vane rotary vacuum pump
Multiple-vane rotary vacuum pump
Trochoide vacuum pump
Rotary plunger vacuum pump
Roots vacuum pump
Turbine vacuum pump
Gaseous-ring vacuum pump
Turbomolecular pump
Liquid jet vacuum pump
Vapor jet vacuum pump
Diffusion pump
Diffusion ejector pump
Adsorption pump
submilation pump
Sputter-ion pump
Cryopump
10–14
10–13
10–12
10–11
10–10
10–9
10–8
Working range for special model or special operating data
10–7
10–6
10–5
10–4
10–3
p in mbar →
10–2
10–1
100
101
102
103
Normal working range
Fig. 9.16: Common working ranges of vacuum pumps
167
Tables, Formulas, Diagrams
High vacuum
10–7 to 10–3 mbar
10–5 to 10–1 Pa
Ultrahigh vacuum
<10–7 mbar
<10–5 Pa
Medium vacuum
10–3 to 1 mbar
10–1 to 102 Pa
Rough vacuum
1 to approx. 103 mbar
102 to approx. 105 Pa
Pressure balance
Spring element vacuum gauge
Bourdon vacuum gauge
Diaphragm vacuum gauge
Capacitance diaphragm vacuum gauge
Piezoelectric vacuum gauge
Liquid level vacuum gauge
U-tube vacuum gauge
Compression vacuum gauge
(McLeod vacuum gauge)
Decrement vacuum gauge
Thermal conductivity vacuum gauge
Pirani vacuum gauge
Thermocouple vacuum gauge
Bimetallic vacuum gauge
Thermistor vacuum gauge
Cold-cathode ionization vacuum gauge
Penning ionization vacuum gauge
Magnetron gauge
Hot-cathode ionization vacuum gauge
Triode ionization vacuum gauge for medium vacuum
Triode ionization vacuum gauge for high vacuum
Bayard-Alpert ionization vacuum gauge
Bayard-Alpert ionization vacuum gauge with modulator
Extractor vacuum gauge
Partial pressure vacuum gauge
10–14
10–13
10–12
10–11
10–10
10–9
10–8
The customary limits are indicated in the diagram.
Working range for special models or special operating data
Fig. 9.16a: Measurement ranges of common vacuum gauges
168
10–7
10–6
10–5
10–4
10–3
p in mbar →
10–2
10–1
100
101
102
103
Tables, Formulas, Diagrams
Saturated vapor
Fig. 9.17: Specific volume Vsp of saturated water vapor in m3/kg within a range of 0.013 to 133 mbar.
Fig. 9.18: Breakdown voltage U between parallel electrodes in a homogeneous electrical field as
a function of gas pressure p distance between electrodes d (in mm) (Paschen curve),
for air.
169
Tables, Formulas, Diagrams
SOLID
LIQUID
Evaporation
Melting
Water vapor pressure [mbar]
Triple point
(0.01°C, 6.09 mbar)
Sublimation
GASEOUS
Temperature [°C]
Fig. 9.19: Phase diagram of water
170
Statutory units
10. The statutory units used in vacuum technology
10.1
Introduction
Two federal German laws and the related implementing provisions stipulate
which units must be used for measurements today (generally since January
1, 1978) in business and official documents and communications. The provisions resulted in a number of fundamental changes that also have to be
taken into account in vacuum technology. Many of the units commonly used
in the past, such as torr, gauss, standard cubic meter, atmosphere, poise,
kilocalorie, kilogram-force, etc., are no longer permissible. Instead, other
units are to be used, some of which are new while others were previously
used in other fields. The alphabetical list in Section 10.2 contains the major
variables relevant for vacuum technology along with their symbols and the
units now to be used, including the SI units (see below) and legally permissible units derived from them. The list is followed by a number of remarks in
Section 10.3. The purpose of the remarks is, on the one hand, to establish
a connection with previous practice wherever this is necessary and, on the
other hand, to provide explanations on practical use of the content of the
alphabetical list.
Statutory units are:
a) the basic SI units (Table 10.4.1)
b) units derived from the basic SI units, in some cases with special names
and unit symbols (Tables 10.4.2 and 10.4.4)
c) units used in atomic physics (Table 10.4.3)
d) decimal multiples and decimal parts of units, some with special names
Examples:
105 N ( m-2 = 1 bar)
1 dm3 = 1 l (liter)
103 kg = 1 t (ton)
Detailed descriptions are provided in publications by W. Haeder and E.
Gärtner (DIN), by IUPAP 1987 and by S. German, P. Draht (PTB). These
should always be referred to if the present summary tailored to vacuum
technology leaves any questions open.
The statutory units for measurements are based on the seven basic SI units
of the Système International (SI).
10.2
1 The
Alphabetical list1 of variables, symbols and units frequently used in vacuum technology and its
applications (see also DIN 28 402)
list is based on work done by Prof. Dr. I. Lückert, for which we would like to express our gratitude.
No. Variable
Symbol
SIunit
Preferred statutory
units
No. of remark
in Section 10.3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
A
s–1 (Bq)
s–1
3/1
–
W
mu
NA
a
k
ϑ (theta)
pv
t
ρ (ro)
ε (epsilon)
D
L
M
n, f
p
J
kg
mol–1
m · s–2
J · K–1
–
N · m–2, Pa
s
kg · m–3
F · m–1
m2 · s–1
N·s·m
N·m
s–1
N · m–2, Pa
J, kJ, kWh, Ws
kg, μg
mol–1
m · s–2, cm · s–2
j · K–1, mbar · l · K–1
°C
mbar, bar
s, min, h
kg · m–3, g · cm–3
F · m–1, As · V–1 · m–1
m2 · s–1, cm2 · s–1
N·s·m
N · m, kN · m
s–1, min–1
bar, mbar
Activity (of a radioactive substance)
(General gas constant)
Work
Atomic mass
Avogadro constant
Acceleration
Boltzmann constant
Celsius temperature
Vapor pressure
Time
Density (gas density)
Dielectric constant
Diffusion coefficient
Moment of momentum
Torque
Rotational speed, rotational frequency
Pressure in fluids
Notes
see no. 73
see Table V in Sect. 9
see Table V in Sect. 9
3/2
3/3
Pa = Pascal
see Table 10.4.4
3/6
F = Farad
3/3
Pa = Pascal
171
Statutory units
No. Variable
Symbol
SIunit
Preferred statutory
units
No. of remark
in Section 10.3
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Pressure as mechanical stress
Diameter
Dynamic viscosity
Effective pressure
Electric field strength
Electrical capacitance
Electrical conductivity
Electrical conductance
Electrical voltage
Electric current density
Electric current intensity
Electrical resistance
Quantity of electricity (electric charge)
Electron rest mass
Elementary charge
Ultimate pressure
Energy
Energy dose
Acceleration of free fall
Area
Area-related impingement rate
Frequency
Gas permeability
p
d
η (eta)
pe
E
C
σ (sigma)
G
U
S
I
R
Q
me
e
pult
E
D
g
A
ZA
f
Qperm
N · m–2, Pa
m
Pa · s
N · m–2, Pa
V · m–1
F
S · m–1
S
V
A · m–2
A
Ω (ohm)
C
kg
C
N · m–2, Pa
J
J · k–1
m · s–2
m2
m–2 · s–1
Hz
m3 (NTP)
––––––––––
m2 · s · Pa
N · mm–2
cm, mm
mPa · s
mbar
V · m–1
F, μF, pF
S · m–1
S
V, mV, kV
a · m–2, A · cm–2
A, mA, μA
Ω, kΩ, MΩ
C, As
kg, g
C, As
mbar
J, kJ, kWh, eV
3/4
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
Gas constant
Velocity
Weight (mass)
Weight (force)
Height
Lift
Ion dose
Pulse
Inductance
Isentropic exponent
Isobaric molar heat capacity
Isobaric specific heat capacity
Isochore molar heat capacity
Isochore specific heat capacity
Kinematic viscosity
Kinetic energy
Force
Length
Linear expansion coefficient
R
v
m
G
h
s
J
p^ (b)
L
κ (kappa)
Cmp
cp
Cmv
cv
ν (nü)
EK
F
l
α (alpha)
m · s–1, mm · s–1, km · h–1
kg, g, mg
N, kN
m, cm, mm
cm
c · kg–1, C · g–1
N·s
H, mH
–
J · mol–1 · K–1
J · kg–1 · K–1
60 Leak rate
QL
m · s–1
kg
N
m
m
C · kg–1
N·s
H
–
J · mol–1 · K–1
J · kg–1 · K–1
J · mol–1 · K–1
J · kg–1 · K–1
m2 · s–1
J
N
m
m
–––––
m·K
N · m · s–1
61
62
63
64
65
66
67
P
H
B
Φ (phi)
B
m
qm
W
A · m–1
T
Wb, V · s
T
kg
kg · s–1
172
Power
Magnetic field strength
Magnetic flux density
Magnetic flux
Magnetic induction
Mass
Mass flow rate
3/5
3/3
Notes
see also no. 126
F = Farad
S = Siemens
C = Coulomb
see Table V in Sect. 9
J = Joule
3/5 a
m · s–2
m2, cm2
m–2 · s–1; cm–2 · s–1
Hz, kHz, MHz
cm3 (NTP)
––––––––––
m2 · d · bar
J · kg–1 · K–1
mm2 · s–1, cm2 · s–1
J
N, kN, mN
m, cm, mm
m
–––––––– ; K–1
m·K
mbar · l ; cm3
––––––– –––– (NTP)
s
s
W, kW, mW
A · m–1
T
V·s
T
kg, g, mg
kg · s–1, kg · h–1, g · s–1
see Table V in Sect. 9
3/19
d =day (see Tab. 10.4.4
see no. 73 and no. 103)
3/6
3/7
see also no. 139
3/8
H = Henry
κ = cp · cv–1
3/9
3/10
3/11
N = Newton
3/12
3/13
3/14
3/15
3/6
T = Tesla
Wb = Weber
see no. 63
Statutory units
No. Variable
Symbol
SIunit
Preferred statutory
units
68
69
70
71
72
73
wi
ρi (ro-i)
J
λ
bi
R
kg · kg–1
kg · m–3
kg · m2
m
mol · kg–1
J
––––––––––
mol · K
kg mol–1
m3 · mol–1
m3 · mol–1
kg
N · m–2
kg · m–3
N · m–2, Pa
m3
N · m–2, Pa
s
m3 · m
–––––––––
s · m2 · bar
J·s
N · m · s–1
N·m
m
C · m–3
sr
–
–
–
N · m–2, Pa
N · m–2, Pa
N · m–2, Pa
–
%, o/oo, ppm
kg · m–3, g · m–3, g · cm–3
kg · m2
m, cm
mol · kg–1
mbar · l
––––––––––
mol · K
kg · kmol–1, g · mol–1
m3 · mol–1, l · mol–1
m3 · mol–1 (NTP)
l · mol–1 (NTP)
g
N · mm–2
kg · m–3, g · cm–3
mbar
m3 (NTP), cm3 (NTP)
mbar
s, ms, μs
cm2
––––––––
s · mbar
J·s
mbar · l · s–1
mbar · l
cm, mm, μm
C · m–3, As · m–3
sr
–
–
–
mbar
mbar
mbar
–
N · m–2, Pa
N · m · s–1
m3 · s–1
N · m–2
mbar
mbar · l · s–1
m3 · h–1, l · s–1
N · m–2, N · mm–2
102 Specific electron charge
103 Specific gas constant
ps
qpV, Q
S
ρ, σ, τ
(ro,
sigma, tau)
–e · me–1
Ri
C · kg–1
J · kg–1 · K–1
104
105
106
107
108
Specific ion charge
Specific electrical resistance
Specific volume
Specific heat capacity
Stefan-Boltzmann constant
e · m–1
ρ (ro)
v
c
σ (sigma)
109
110
111
112
Quantity of substance
Throughput of substance
Concentration of substance
Collision rate
ν (nü)
qv
ci
Z
C · kg–1
Ω·m
m3 · kg–1
J · kg–1 · K–1
W
––––––
m2 · K4
mol
mol · s–1
mol · m–3
s–1
C · kg–1, As · kg–1
mbar · l
––––––––
kg · K
C · kg–1, As · kg–1
Ω · cm, Ω · mm2 · m–1
m3 · kg–1; cm3 · g–1
J · kg–1 · K–1, J · g–1 · K–1
W
––––––
m2 · K4
mol, kmol
mol · s–1
mol · m–3, mol · l–1
s–1
Mass content
Mass concentration
Moment of inertia
Mean free path
Molality
Molar gas constant
74 Molar mass (quantity-related mass)
75 Molar volume
76 Molar volume, standard
M
Vm
Vmn
77
78
79
80
81
82
83
84
Molecular mass
Normal stress (mech.)
Standard density of a gas
Standard pressure
Standard volume
Partial pressure
Period
Permeation coefficient
m
σ (sigma)
ρn (ro-en)
pn
Vn
pi
T
P
85
86
87
88
89
90
91
92
93
94
95
96
97
Planck constant
pV throughput
pV value
Radius (also molecular radius)
Space charge density
Solid angle
Relative atomic mass
Relative molecular mass
Relative particle mass
Residual vapor pressure
Residual gas pressure
Residual (total) pressure
Reynold number nondimensional
variable
Saturation vapor pressure
Throughput (of a pump)
Pumping speed
Stress (mech.)
h
qpV
pV
r
ρ (ro)
Ω (omega)
AT
Mr
Mr
prd
prg
pr
Re
98
99
100
101
No. of remark
in Section 10.3
Notes
ppm = parts per million
see Table V in Sect. 9
see Table V in Sect. 9
see Table V in Sect. 9
3/16
3/17
3/18
see Table V in Sect. 9
3/19
3/19
3/20
3/21
sr = steradian
nondimensional variab.
nondimensional variab.
nondimensional variab.
nondimensional variab.
3/4
see no. 132
see no. 18
see Table V in Sect. 9
3/22
3/23
see Table V in Sect. 9
for substance „i“
173
Statutory units
No. Variable
Symbol
SIunit
Preferred statutory
units
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
Conductance
Flow resistance
Number of particles
Particle number density (volume-related)
Particle number density (time-related)
Particle throughput density
Particle mass
Particle flux
Particle flux density
Thermodyn. temperature
Temperature difference
Temperature conductivity
Total pressure
Overpressure
Ambient pressure
Speed of light in vacuum
Evaporation heat
Viscosity, dynamic
Volume
Volume throughput (volumetric flow)
Volume concentration
m3 · s–1
s · m–3
–
m–3
s–1
m–2 · s–1
kg
s–1
m–2 · s–1
K
K
m2 · s–1
N · m–2, Pa
N · m–2, Pa
N · m–2, Pa
m · s–1
J
Pa · s
m3
m3 · s–1
m3 · m–3
m3 · s–1, l · s–1
s · m–3, s · l–1
–
cm–3
s–1
m–2 · s–1, cm–2 · s–1
kg, g
s–1
m–2 · s–1, cm–2 · s–1
K, mK
K, °C
134
135
136
137
Volume-related collision rate
Quantity of heat
Heat capacity
Thermal conductivity
C, German: L
R
N
n
qN
jN
m
qN
jN
T
ΔT, Δϑ
a
pt
pe
Pamb
c
Ld
η (eta)
V
qv
σi
(sigma-i)
Zv
Q
C
λ
(lambda)
α (alpha)
s–1 · m–3
J
J · K–1
W
–––––
K·m
W
––––––
K · m2
m
m
rad
s–1 · m–3, s–1 · cm–3
J, kJ, kWh, Ws
J · K–1, kJ · K–1
W
––––––
K·m
rad · s–2
rad · s–1
rad · s–2
rad · s–1
–
s
s
–
s, min, h, nn, mn
s, min, h
138 Heat transfer coefficient
139 Path length
140 Wave length
141 Angle (plane)
142 Angular acceleration
143 Angular velocity
144 Efficiency
145 Time
146 Period of time
174
s
λ (lambda)
α, β, γ rad
(alpha,
beta, gamma)
α (alpha)
ω
(omega)
η (eta)
t
t, Δt
No. of remark
in Section 10.3
Notes
nondimensional variab.
see no. 120
see no. 121
see no. 117
see no. 118
3/24
a = λ · ρ–1 · cp
mbar
mbar
mbar, bar
m · s–1, km · s–1
kJ
mPa · s
m3, l, cm3
m3 · h–1, l · s–1
l · l–1, %, o/oo, ppm
m, cm
nm
rad, °, ‘, ‘’
3/3
3/3
3/3
see Table V in Sect. 9
see no. 20
ppm = parts per million
3/25
3/11
3/26
rad = radian
nondimensional variab.
see Table 10.44
see Table 10.44
Statutory units
10.3
Remarks on alphabetical list in
Section 10.2
3/5: Dynamic viscosity
The unit previously used was poise (P).
3/1: Activity
The unit previously used was curie (Ci).
1 P = 0.1 Pa · s = 1 g · cm–1 · s–1
1 Ci = 3.7 · 1010 · s-1 = 37 ns-1
3/5a: Energy dose
Rad (rd) is no longer permissible.
3/2: (°C) Celsius temperature
The term degrees Celsius is a special name for the SI unit kelvin (K) [see
no. 122] for indicating Celsius temperatures. The term degrees Celsius is
legally approved.
1
1 rd = –––– J · kg–1
100
3/3: Pressure
The revised version of DIN 1314 must be complied with. The specifications
of this standard primarily apply to fluids (liquids, gases, vapors). In DIN
1314, bar (1 bar = 0.1 MPA = 105 Pa) is stated in addition to the (derived)
SI unit, 1 Pa = 1 N · m-2, as a special name for one tenth of a megapascal
(Mpa). This is in accordance with ISO/1000 (11/92), p. 7. Accordingly the
millibar (mbar), a very useful unit for vacuum technology, is also permissible: 1 mbar = 102 Pa = 0.75 torr. The unit “torr” is no longer permissible.
Special note
Exclusively absolute pressures are measured and used for calculations in
vacuum technology.
In applications involving high pressures, frequently pressures are used that
are based on the respective atmospheric pressure (ambient pressure) pamb.
According to DIN 1314, the difference between a pressure p and the
respective atmospheric pressure (ambient pressure) pamb is designated as
overpressure pe: pe = p – pamb. The overpressure can have positive or negative values.
3/6: Weight
DIN 1305 is to be complied with in this context. Because of its previous
ambivalence, the word weight should only be used to designate a variable
of the nature of a mass as a weighing result for indicating quantities of
goods.
The designations “specific weight” and “specific gravity” should no longer be
used. Instead, one should say density.
3/7: Weight force
See DIN 1305. The previous units pond (p) and kilopond, i.e. kilogramforce, (kp) as well as other decimal multiples of p are no longer used.
1 kp = 9.81 N
3/8: Ion dose
The previously used unit was the Röntgen (R). 1 R = 2,58 · 10–4 C · kg–1
3/9: Kinematic viscosity
The previously used unit was stokes (St).
1 St = 1 cm2 · s–1; 1 cSt = 1 mm2 · s–1
Conversions
1 kg · cm-2 = 980.665 mbar = 981 mbar
3/10: Force
The dyne, the CGS unit for force, is no longer used.
1 at (technical atmosphere) = 980.665 mbar = 981 mbar
1 dyne = 10-5 N
1 atm (physical atmosphere) = 1013.25 mbar = 1013 mbar
1 atmosphere above atmospheric pressure (atmospheric overpressure) =
2026.50 mbar = 2 bar
3/11: Length/wavelength
The unit Ångström (Å) (e.g. for wavelength) will no longer be used in future
(see Table 4.6).
1 atm
1 torr = 1 mm Hg = –––––– = 133.322 Pa= 1.333 mbar
760
1 meter head of water = 9806.65 Pa = 98 mbar
1 Å = 10-8 cm = 0.1 nm
1 mm Hg = 133.332 Pa = 1.333 mbar = 4/3 mbar
The pressure as mechanical stress (strength) is generally given in pascal
(Pa) and in N · nm–2.
Conversions:
1 Pa = 1 N · m–2 = 10–6 N · mm–2
1 kg ·
cm–2
= 98,100 Pa = 0.981 N ·
mm–2
= 0,1 N
mm–2
1 kg · mm–2 = 9,810,000 Pa = 9.81 N · mm–2 = 10 N · mm–2
3/12: Leak rate
In DIN 40.046 sheet 102 (draft of August 1973 issue), the unit
mbar · dm3 · s-1 (= mbar · l · s-1) is used for the leak rate. Note that the leak
rate corresponding to the unit 1 mbar · l · s-1 at 20 °C is practically the
same as the leak rate 1 cm3 · s-1 (NTP). (See also 3/17)
3/13: Magnetic field strength
The previously used unit was the oersted (Oe).
1 Oe = 79.577 A · m-1
175
Statutory units
3/14: Magnetic flux density
The previously used unit was the gauss (G).
1 G = 10-4 Vs · m-2 = 10-4 T (T = Tesla)
3/15: Magnetic flux
The previously used unit was the maxwell (M).
1 M = 10-8 Wb (Weber)
3/16: Standard volume
DIN 1343 must be complied with.
The designation m3 (NTP) or m3 (pn, Tn) is proposed, though the expression in parentheses does not belong to the unit symbol m3 but points out
that it refers to the volume of a gas in its normal state
(Tn = 273 K, pn = 1013 mbar).
3/17: Partial pressure
The index “i” indicates that it is the partial pressure of the “i-th” gas that is
contained in a gas mixture.
3/18: Gas permeability
The permeation coefficient is defined as the gas flow m3 · s-1 (volumetric
flow pV) that goes through a fixed test unit of a given area (m2) and thickness (m) at a given pressure difference (bar).
According to DIN 53.380 and DIN 7740, Sheet 1, supplement, the gas permeability (see no. 40) is defined as “the volume of a gas, converted to
0 °C and 760 torr, which goes through 1 m2 of the product to be tested at a
certain temperature and a certain pressure differential during a day
(= 24 hours)”.
3/19: pV throughput/pV value
DIN 28.400, Sheet 1 is to be taken into account here. No. 86 and no. 87
have a quantitative physical significance only if the temperature is indicated
in each case.
3/20: Relative atomic mass
Misleadingly called “atomic weight” in the past!
3/21: Relative molecular mass
Misleadingly called “molecular weight” in the past!
3/22: Specific gas constant
As mass-related gas constant of the substance “i”. Ri = Rm ( Mi-1; Mi molar
mass (no. 74) of the substance “i”. See also DIN 1345.
3/23: Specific heat capacity
Also called specific heat:
Specific heat (capacity) at constant pressure: cp.
Specific heat (capacity) at constant volume: cV.
176
3/24: Temperature difference
Temperature differences are given in K, but can also be expressed in °C.
The designation degrees (deg) is no longer permissible.
3/25: Quantity of heat
The units calorie (cal) and kilocalorie (kcal) are no longer be used.
1 kcal = 4.2 kJ
3/26: Angle
1 radian (rad) is equal to the plane angle which, as the central angle of a
circle, cuts out an arc having a length of 1 m from the circle. See also DIN
1315 (8/82).
π
1° = ––––– rad: 1’ = 1°/60; 1’’ = 1’/60.
180
360°
1 rad = ––––– · 60°
2π
10.4
Tables
10.4.1 Basic SI units
Basic unit
Symbol
Variable
————————————————————————
meter
m
length
kilogramm
kg
mass
second
s
time, period; duration
ampere
A
electric current
kelvin
K
thermodyn. temperature
mole
mol
quantity of substance
candela
cd
luminous intensity
Statutory units
10.4.2 Derived coherent1 SI units with special
names and symbols (alphabetical)
10.4.4 Derived noncoherent SI units with special
names and symbols
Name of unit Symbol Variable
Relationship
—————————————————————————————————
coulomb
C
quantity of electricity
1C=1A·s
or electric charge
Basic unit
Symbol
Definition
—————————————————————————————————
Day
d
1 d = 86.400 s
farad
F
electrical capacitance
1 F = 1 A · s · V–1
henry
H
inductance
1 H = 1 V · s · A–1
hertz
Hz
frequency
1 Hz = 1 · s–1
joule
J
energy, work, quantity
of heat
1 J = 1 N · m = Ws
lumen
lm
luminous flux
1 lm = cd · sr
lux
lx
illuminance
1 lx = 1 lm · m–2
newton
N
force
1 N = 1 kgm · s–2
ohm
Ω
electrical resistance
1 Ω = 1 V · A–1
pascal
Pa
pressure, mechanical
stress
1 Pa = 1 N · m–2
radian
rad2
angle, plane angle
1 rad = 1 m · m–1
siemens
S
electrical conductance
1 S = 1 · Ω–1
steradian
sr2
solid angle
1 sr = 1 m2 · m–2
tesla
T
magnetic flux density or
induction
1 T = 1 Wb · m–2
volt
V
electrical voltage
or potential difference
1 V = 1 W · A–1
watt
W
power, energy flux, heat
flux
1 W = 1 J · s–1
weber
Wb
magnetic flux
1 Wb = 1 V · s
1
Hour
h
1 h = 3.600 s
Minute
min
1 min = 60 s
Round angle
–
2 π rad
Degree
(°)
π
–––– rad
180
Minute
(‘)
π
1
–––––– rad (= –––grad)
10.800
60
Second
(‘’)
π
1
––––––– rad (= ––––minute)
648.000
60
Formed with numeric factor 1; e.g. 1 C = 1 As, 1 Pa = 1 N · m–2
SI unit
2 Additional
10.4.3 Atomic units
Basic unit
Symbol
Vartiable
————————————————————————
Atomic
Mass for indication of
mass
mu
particle mass;
unit
1 mu = 1/12 mass of 12C
also amu (atomic mass
unit).
Electron
volt
eV
energy
177
Vacuum technology standards
11. National and international standards and
recommendations particularly relevant
to vacuum technology
11.1
National and international standards and
recommendations of special relevance to
vacuum technology
A) National agreements, Part 1: DIN
DIN
Title
For around 20 years now, numerous standards and recommendations have
been drawn up at national and international level and revised, whenever
necessary, in accordance with the state of the art. These standards and
recommendations must be observed whenever use is made of vacuum
equipment (pumps, gauges, valves, etc.) and vacuum apparatus, systems
and plants are assembled. They not only contain specifications applying
specially to vacuum technology, but also go beyond this specific field and
involve, for example, physical units, formulas, noise protection regulations,
etc.
1301
1304
National standards are primarily DIN standards, particularly those relating
to the area of vacuum technology in the DIN Standards Committee on
Mechanical Engineering (NAM). International standards and recommendations are drawn up and issued
Units
Part 1 – Names of units, symbols
Part 2 – Parts and multiples generally used
Part 3 – Conversions for units no longer used
General symbols
Part 1 – General symbols
Part 2 – Symbols for meteorology and geophysics
Part 3 – Symbols for electrical energy supply
Part 5 – Symbols for flow mechanics
Part 6 – Symbols for electrical
communications technology
Part 7 – Symbols for meteorology and geophysics
Part 7 – Symbols for electric machines
Issue
1993
2/78
6/79
3/94
9/89
3/89
9/89
5/92
1/91
a) by the International Standardization Organization (ISO), in particular by
ISO Committee TC 112 (vacuum technology)
1305
Mass; weighed value, force, weight force, weight
load, definitions
1/88
b) by the European Committee of Manufacturers of Compressors, vacuum
pumps and compressed air tools (PNEUROP), in particular by PNEUROP Subcommittee C5 (vacuum technology)
1306
Density; definitions
6/84
1313
Physical variables and equations,
definitions, spelling
4/78
c) by the European Committee for Standardization (CEN), in particular by
Technical Committee TC 138 (nondestructive testing) and Technical
Committee TC 318.
1314
Pressure; basic definitions, units
2/77
1319
Basic definitions for measurement technology
Part 1 – Basic definitions
Part 2 – Definitions for the use of gauges
Part 3 – Definitions for measurement
uncertainty and evaluation of gauges
and measuring equipment
Part 4 – Treatment of uncertainty in the
use of measurements
The content of the documents drawn up by the international organizations
in a) to c), with German participation (also by LEYBOLD), has been extensively incorporated into DIN standards, as reflected in designations such as
DIN / ISO or DIN / EN.
The most important standards to be complied with are listed in Table 11.1
below.
Abbreviations used:
D = draft
CD = Committee Draft
8/83
12/85
A) National agreements, Part 1: DIN (cont.)
DIN
Title
Issue
1343
Normal state, reference state
1/90
1345
Thermodynamics; basic definitions
12/93
1952
Flow measurement with screens, nozzles, etc.
7/82
2402
Pipelines; nominal internal diameters, definitions,
classification
2/76
Seals for gas supply – Part 6
4/94
3535
178
1/95
1/80
Vacuum technology standards
DIN
Title
8964
Circuit parts for refrigeration systems with
hermetic and semi-hermetic compressors
Part 1: Tests
Part 2: Requirements
16005
16006
19226
25436
28090
28400
Issue
3/96
9/86
(E 12/95)
Overpressure gauges with elastic measuring
element for general use Requirements
and testing
2/87
Overpressure gauges with Bourdon tube
Safety-related requirements and testing
2/87
– 1 Control and instrumentation technology;
control and regulation technology;
definitions – general principles
– 4 Control and instrumentation technology;
control and regulation technology;
definitions for control and regulation
systems
– 5 Control and instrumentation technology;
control and regulation technology;
functional definitions
Integral leak rate test of safety vessel with
absolute pressure method
Static seals for flange connections
Part 1 - Characteristic values for seals and
testing methods
Part 2 - Seals made of sealing plate; special
testing methods for quality assurance
Vacuum technology; designations and definitions
Part 1 - Basic definitions, units, ranges,
characteristic variables and basic principles
Part 2 - Vacuum pumps
Part 3 - Vacuum plants; characteristic
variables and gauges
Part 4 - Vacuum coating technology
Part 5 - Vacuum drying and vacuum
freeze-drying
Part 6 - Analytical techniques for surface
technology
Part 7 - Vacuum metallurgy
Part 8 - Vacuum systems, components and
accessories
Issue
28410
Vacuum technology; mass spectrometer partial
pressure gauges, definitions, characteristic
variables, operating conditions
11/86
Vacuum technology; acceptance specifications
for mass spectrometer leak detection devices,
definitions
3/76
Vacuum technology; calibration of vacuum
gauges in a range from 10-3 to 10-7 mbar.
General methods; pressure reduction
through constant flow
3/76
Vacuum technology; measuring pV mass flow
according to volumetric method at constant
pressure
3/76
Vacuum technology; standard method for
calibrating vacuum gauges through direct
comparison with a reference device
Part 1 – General principles
Part 2 – Ionization vacuum gauges
Part 3 – Thermal conductivity vacuum gauges
5/76
9/78
8/80
28411
28416
2/94
28418
2/94
2/94
28426
Vacuum technology; acceptance specifications
for rotary piston vacuum pumps
Part 1 – Rotary piston and vane type rotary
vacuum pumps in rough and medium vacuum range
Part 2 – Roots vacuum pumps in medium
vacuum range
28427
Vacuum technology; acceptance specifications
for diffusion pumps and ejector vacuum pumps for
pump fluid vapor pressures of less than 1 mbar
2/83
Vacuum technology; acceptance specifications
for turbo-molecular pumps
11/78
28429 Vacuum technology; acceptance
specifications for getter-ion pumps
8/85
Vacuum technology; measuring specifications for
ejector vacuum pumps and ejector compressors.
Pump fluid: water vapor
11/84
Acceptance specifications for liquid
ring vacuum pumps
1/87
Acceptance specifications for diaphragm
vacuum pumps
E 5/95
7/80
9/95
9/95
5/90
10/80
6/92
3/76
28428
28430
3/81
10/80
7/78
10/80
(E 7/91)
Vacuum technology; symbols – overview
11/76
28402
Vacuum technology: variables, symbols,
units - overview
12/76
Vacuum technology; quick connections,
small flange connections
9/86
Vacuum technology: flanges, dimensions
10/86
28404
Title
28417
28401
28403
DIN
28431
28432
53380
45635
Testing of plastic foils, determination of gas
permeability
Noise measurement at machines: measurement
of airborne noise, enveloping surface methods.
Part 13 – Compressors, including vacuum pumps,
positive displacement, turbo and steam
ejectors
6/69
(E 10/83)
2/77
179
Vacuum technology standards
DIN
Title
55350
Definitions of quality assurance and statistics
Part 11 – Basic definitions of quality assurance
Part 18 – Definitions regarding certification of
results of quality tests/quality test certificates
7/87
66038
Torr – millibar; millibar – torr conversion tables
4/71
–
Thesaurus Vacui (definition of terms)
1969
A) European/national agreements, EN, DIN/EN, CEN
DIN/EN Title
EN 473
837-1
837-2
837-3
Training and certification of personnel for
nondestructive testing (including leak test)
ISO
8/95
1607 / 2 Positive displacement vacuum pumps.
Measurement of performance characteristics.
Part 2 - Measurement of ultimate pressure
Issue
7/93
Title
Issue
11/89
1608 / 1 Vapor vacuum pumps.
Part 1: Measurement ofvolume rate of flow
12/93
1608 / 2 Vapor vacuum pumps.
Part 2: Measurement of critical backing pressure
12/89
1609
3/86
Vacuum technology. Flange dimensions
DIN/ISO Standard atmosphere
2533
12/79
2861 / 1 Quick release couplings. Dimensions
Part 1 - Clamped Type
8/74
2861 / 2 Quick release couplings. Dimensions
Part 2 - Screwed type
8/80
Pressure gauges, Part 1: Pressure gauges with
Bourdon tubes, dimensions, measurement
technology, requirements and testing
2/97
12/81
Pressure gauges, Part 2: Selection and installation
recommendations for pressure gauges
3529 / 1 Vacuum Technology Vocabulary
Part 1 - General terms
1/95
3529 / 2 Vacuum Technology Vocabulary
Part 2 - Vacuum pumps and related terms
12/81
3529 / 3 Vacuum Technology Vocabulary
Part 3 - Vacuum gauges
12/81
3556 / 1 Measurement of performance characteristics.
Part 1 - Sputter ion pumps (E)
1992
Pressure gauges, Part 3: Pressure gauges with
plate and capsule elements, dimensions,
measurement technology, requirements
and testing
1330-8 E Nondestructive testing - definitions for leak test –
terminology
1779 E
Issue
Nondestructive testing – leak test. Instructions
for selection of a testing method
1338-8 E Nondestructive testing – leak test.
Terminology on leak test
2/97
6/94
3/95
1994
3567
3568
Vacuum gauges. Calibration by direct
comparison with a reference gauge (CD)
2/91
Ionisation vacuum gauge. Calibration by direct
comparison with a reference gauge (CD)
2/91
1518 E
Nondestructive testing – determination of
characteristic variables for mass spectrometer
leak detectors
3570 / 1 Vacuum gauges – standard methods for
calibration
2/91
Part 1 - Pressure reduction by continuous
flow in the pressure range 10–1 ... 10–5 Pa.
1593 E
Nondestructive testing – bubble type testing method 12/94
3669
Vacuum Technology. Bakable flanges, dimensions.
Part 1: Clamped Type
EN/ISO
4080
Rubber and plastic hoses and hose
lines – determination of gas permeability
4/95
5167
Measurement of fluid flow by means of orifice
plates, nozzles etc.
1980
NMP 826 Calibration of gaseous reference leaks, CD
Nr. 09–95
B) International agreements, ISO, EN/ISO
ISO
Title
1000
1607 / 1
180
SI units and recommendations for the use of
their multiples and of certain other units
Positive displacement vacuum pumps.
Measurement of performance characteristics.
Part 1 - Measurement of volume rate of flow
(pumping speed)
9/95
Issue
11/92
12/93
2/86
5300
Vacuum gauges of the thermal conductivity type.
Calibration by direct comparison with a
reference gauge (CD)
2/91
9803
Pipeline fittings-mounting, dimensions (E)
2/93
DIN/ISO Requirements placed on quality assurance for
10012
measuring equipment
Part 1 – Confirmation system for measuring
equipment
8/92
Vacuum technology standards
C) PNEUROP/C5 (6.93)
Number Title/remark
identical Issue
to DIN
5607
Vacuum pumps; acceptance specifications
Part II: (Fluid entrainment pumps)
28427
1972
5608
Vacuum pumps; acceptance specifications
Part III: (Turbomolecular pumps)
28428
1973
5615
Vacuum pumps; acceptance specifications
Part IV: (Getter-ion pumps)
28429
1976
6601
Measurement of performance of ejector
vacuum pumps and ejector compressors
28430
5/78
Vacuum pumps; acceptance specifications
Part I: (Oil-sealed rotary pumps and
Roots pumps)
28426
1979
28403
and
28404
1985
6602
6606
Vacuum flanges and connections;
dimensions
PN5ASR Vacuum pumps, acceptance
CC/5
specifications refrigerator cooled
cryopumps
7/89
181
References
12. References
J. M. Lafferty et. al.
Foundations of Vacuum Science and Technology
704 pages, 1998, Wiley 1998
1.
A. Schubert
Normen und Empfehlungen für die Vakuum-Technik
Vakuum in der Praxis, Vol. 3, 1991, 211-217
Overview, definitions and history
K. Diels, R. Jaekel
Leybold Vacuum Handbook
Pergamon Press
1st Ed. 1966
W. Haeder, E. Gärtner
Die gesetzlichen Einheiten in der Technik
Beuth-Vertrieb GmbH, 5. Aufl. 1980,
Berlin 30, Köln, Frankfurt (Main)
H. Ebert
Vakuum-Chronik, A documentation on works concerning vacuum that were
published before 1928
PTB-Bericht ATWD-11, September 1977
M. Dunkel
„Gedenken an Wolfgang Gaede“
Physikalische Blätter Nr. 34 (1978), Heft 5, Pages 228-232 as well as
Vakuum-Technik, 27. Jahrgang, No. 4,
Pages 99-101
IUPAP (SUNANCO Commission)
Symbols, Units etc.
Document 25, 1987
Leybold AG
Vademekum, 93 pages, 1988
M. Wutz, H. Adam, W. Walcher
Theory and Practice of Vacuum Technology
5. Aufl., 696 pages, 1992,
Friedrich Vieweg u. Sohn, Braunschweig/ Wiesbaden
A. Guthrie and R. K. Wakerling
Vakuum Equipment and Techniques
264 pages, 1949, McGraw-Hill,
New York/London/Toronto
D. J. Hucknall
Vacuum Technology and Applications
1st Ed., 319 pages, 1991
Butterworth-Heinemann, Oxford
C. M. van Atta
Vacuum Science and Engineering
459 pages, 1965, McGraw-Hill
New York/San Francisco/Toronto/
London/Sydney
182
H. Scharmann
Vakuum – Gestern und Heute
Vakuum in der Praxis, Vol. 2, 1990, 276-281
M. Auwärter
Das Vakuum und W. Gaede
Vakuum-Technik, Vol. 32, 1983, 234-247
J. F. O’Hanlon
A User’s Guide to Vacuum Technology
3nd Ed., 402 pages, Wiley 1989, New York
G. Reich
Wolfgang Gaede – Einige Gedanken zu seinem 50. Todestag aus heutiger
Sicht
Vakuum in der Praxis, 7th year, 1995, 136-140
S. German, P. Draht
Handbuch SI Einheiten
Vieweg Braunschweig/Wiesbaden, 1979, 460 pages
„Gesetz über Einheiten im Meßwesen“ vom 2. Juli 1969
„Gesetz zur Änderung des Gesetzes über Einheiten im Meßwesen“ vom 6.
Juli 1973
„Ausführungsverordnungen“ vom 26. Juni 1970
In Vakuum-Technik Vol. 35, 1986:
Th. Mulder
Otto von Guericke
Pages 101-110
P. Schneider
Zur Entwicklung der LuftpumpenInitiationen und erste Reife bis 1730
Pages 111-123
L. Fabel
Physik in der 2. Hälfte des 19. Jahrhunderts und die
vakuumtechnische Entwicklung bis Gaede
Pages 128-138
H.-B. Bürger:
G. Ch. Lichtenberg und die Vakuumtechnik
Pages 124-127
G. Reich:
Gaede und seine Zeit
Pages 139-145
References
H. Adam
Vakuum-Technik in der Zeit nach Gaede (1945 to the present);
Pages 146-147
G. Reich
Die Entwicklung der Gasreibungspumpen von Gaede, über Holweck,
Siegbahn bis zu Pfleiderer und Becker (mit zahlreichen Literaturangaben)
Vakuum-Technik in der Praxis, Vol. 4, 1992, 206-213
G. Reich
Carl Hoffman (1844-1910), der Erfinder der Drehschieberpumpe
Vakuum in der Praxis, 1994, 205-208
Th. Mulder
Blaise Pascal und der Puy de Dôme – Große Männer der Vakuum-Technik
Vakuum in der Praxis, 1994, 283-289
W. Pupp und H. K. Hartmann
Vakuum-Technik, Grundlagen und Anwendungen
C. Hanser, München, 1991, Wien,
2.
Vacuum pumps
2.1
Positive displacement pumps, condensers
W. Gaede
Demonstration einer rotierenden Quecksilberpumpe
Physikalische Zeitschrift, 6, 1905, 758-760
W. Gaede
Gasballastpumpen
Zeitschrift für Naturforschung, 2a, 1947, 233-238
W. Armbruster und A. Lorenz
Das maximale Kompressionsverhältnis und der volumetrische
Wirkungsgrad von Vakuumpumpen nach dem Rootsprinzip
Vakuum-Technik, 7, 1958, 81-85
F. Fauser
Charakteristik von Pumpsystemen für größere Wasserdampfmengen unter
Vakuum und unter Anwendung von Kondensation und Kompression des
Wasserdampfes
1965 Transactions of the Third International Vacuum Congress, Stuttgart,
Bd. 2/II, 393-395, Pergamon Press, Oxford 1966
M. Wutz
Das Abpumpen von Dämpfen mit gekühlten Kondensatoren
Vakuum-Technik, 16, 1967, 53-56
H. Hamacher
Kennfeldberechnung für Rootspumpen
DLR FB 69-88, 1969
H. Hamacher
Beitrag zur Berechnung des Saugvermögens von Rootspumpen
Vakuum-Technik, 19, 1970, 215-221
H. Hamacher
Experimentelle Untersuchungen an Nachkühlern von Rootspumpen
Vakuum-Technik, 23, 1974, 129-135
M. Rannow
Ölgedichtete Vakuumpumpen in der Chemie
Chemie-Technik, No. 7, 1978, 39-41
H. P. Berges et al.
TRIVAC-B, ein neues Vakuumpumpen-Konzept für universelle
Anwendungen
Vakuum-Technik, 31, 1982, 168-171
H. Lang
Vakuumpumpen in der chemischen Industrie – Wälzkolbenpumpen
Vakuum-Technik, 1980, 72-82
H. F. Weber
Vakuumpumpen in der chemischen
Industrie – ölgedichtete Rotationsvakuumpumpen
Vakuum-Technik, 1980, 98-104
W. Armbruster und A. Lorenz
Die Kombination Rootspumpe-Wasserringpumpe
Vakuum-Technik, 7, 1958, 85-88
D. Bartels
Vakuumpumpen in der chemischen Indu-strie
Flüssigkeitsring-Vakuumpumpen – A
Vakuum-Technik, 1980, 131-140
H. Reylander
Über die Wasserdampfverträglichkeit von Gasballastpumpen
Vakuum-Technik, 7, 1958, 78-81
R. W. Adam und C. Dahmlos
Flüssigkeitsring-Vakuumpumpen – B
Vakuum-Technik, 1980, 141-148
U. Seegebrecht
Förderung trockener Luft und von gesättigtem Luft-Wasserdampfgemisch
mit Flüssigkeitsring-Vakuumpumpen
Vakuum-Technik, 1980, 246-252
183
References
H.-D. Bürger
Fortschritte beim Betrieb von Wälzkolbenpumpen
Vakuum-Technik 1983, 140-147
U. Seegebrecht
Einfluß der Temperatur des Fördermittels auf das Saugvermögen von
Flüssigkeitsring-Vakuumpumpen bei der Förderung von trockener Luft
Vakuum-Technik, 1985, 10-14
P. Bachmann und H.-P. Berger
Sicherheitsaspekte beim Einsatz von ölgedichteten
Drehschiebervakuumpumpen in CVD-Anwendungen
Vakuum-Technik, 1987, 41-47
U. Fussel
Trockenlaufende Vakuumpumpen in der chemischen Industrie
Vakuum in der Praxis, 1994, 85-88
L. Ripper
Explosionsschutz-Maßnahmen an Vakuumpumpen (with numerous references to relevant literature)
Vakuum in der Praxis, 1994, 91-100
K. P. Müller
Trockenlaufende Drehschiebervakuumpumpen in einer VielzweckProduktionsanlage
Vakuum in der Praxis, 1994, 109-112
F. J. Eckle, W. Jorisch, R. Lachenmann
Vakuum-Technik im Chemielabor
Vakuum in der Praxis, 1991, 126-133
P. Bachmann und M. Kuhn
Einsatz von Vorpumpen im Al-Ätzprozeß. Erprobung trockenverdichtender
Klauenpumpen und ölgedichteter Drehschieber-Vakuumpumpen im
Vergleich
Vakuum in der Praxis, 1990, 15 – 21
U. Gottschlich
Vakuumpumpen im Chemielabor
Vakuum in der Praxis, 1990, 257-260
W. Jorisch
Neue Wege bei der Vakuumerzeugung in der chemischen
Verfahrenstechnik
Vakuum in der Praxis, 1995, 115-118
D. Lamprecht
Trockenlaufende Vakuumpumpen
Vakuum in der Praxis, 1993, 255-259
P. Deckert et al.
Die Membranvakuumpumpe – Entwicklung und technischer Stand
Vakuum in der Praxis, 1993, 165-171
W. Jorisch und U. Gottschlich
Frischölschmierung – Umlaufschmierung, Gegensätze oder Ergänzung?
Vakuum in der Praxis, 1992, 115-118
W. Jitschin et al.
Das Saugvermögen von Pumpen: Untersuchung verschiedener
Meßverfahren im Grobvakuumbereich
Vakuum in Forschung und Praxis, 7, (1995) 183 -193
H.P. Berges and M. Kuhn
Handling of Particles in Forevacuum pumps
Vacuum, Vol. 41, 1990, 1828-1832
M. H. Hablanian
The emerging technologies of oil-free vacuum pumps
J. Vac. Sci. Technol. A6 (3), 1988, 1177-1182
E. Zakrzewski, P. L. May and B. S. Emslie
Developments in vacuum Pumping systems based on mechanical pumps
with an oil free swept volume
Vacuum, 38, 968, 757-760
H. Wycliffe
Mechanical high-vacuum pumps with an oil-free swept volume
J. Vac. Sci. Technol. A5 (4) 1987, 2608-2611
A. P. Troup and D. Turell
Dry pumps operating under harsh condictions in the semiconductor industry
J. Vac. Sci. Technol. A7 (3), 1989, 2381-2386
M. H. Hablanian
Aufbau und Eigenschaften verschiedener ölfreier Vakuumpumpen für den
Grob- und Feinvakuumbereich (wichtige Literaturangaben)
Vakuum in der Praxis, 1990, 96-102
P. Bachmann and M. Kuhn
Evaluation of dry pumps vs rotary vane pumps in aluminium etching
Vacuum 41, 1990, 1825-1827
B. W. Wenkebach und J. A. Wickhold
Vakuumerzeugung mit Flüssigkeitsring-Vakuumpumpen
Vakuum in der Praxis, 1989, 303-310
H. P. Berges and D. Götz
Oil-free vacuum pumps of compact design
Vacuum, Vol. 38, 1988, 761-763
U. Gottschlich und W. Jorisch
Mechanische Vakuumpumpen im Chemieeinsatz
Vakuum in Forschung und Praxis, 1989, 113-116
184
References
2.2
Turbomolecular pumps
W. Gaede
Die Molekularluftpumpe
Annalen der Physik, 41, 1913, 337-380
W. Becker
Eine neue Molekularpumpe
Vakuum-Technik, 7, 1958, 149-152
W. Armbruster
Vakuumpumpenkombinationen für Labor, Technikum und Produktion
Chemiker-Zeitung / Chemische Apparatur, 88, 1964, 895-899
W. Becker
Die Turbo-Molekularpumpe
Vakuum-Technik, 15, 1966, 211-218 and 254-260
R. Frank et al.
Leistungsdaten von Turbo-Molekularpumpen des Typs TURBOVAC mit
senkrecht angeordnetem Axialkompressor
Vakuum-Technik, 24, 1975, 78 -85
W. Becker
Eine gegenüberstellende Betrachtung von Diffusionspumpen und
Molekularpumpen
Ergebnisse europäischer Ultrahochvakuumforschung
Leybold-Heraeus GmbH u. Co., in its own publishing house, Cologne 1968,
41-48
R. Frank, E. Usselmann
Kohlenwasserstoffreier Betrieb mit Turbo-Molekularpumpen des Typs TURBOVAC
Vakuum-Technik, 25, 1976, 48-51
R. Frank, E. Usselmann
Magnetgelagerte Turbo-Molekularpumpen des Typs TURBOVAC
Vakuum-Technik, 25, 1976, 141-145
H.-H. Henning und G. Knorr
Neue luftgekühlte, lageunabhängige Turbo-Molekularpumpen für Industrie
und Forschung
Vakuum-Technik, 30, 1981, 98-101
H.-H. Henning und H. P. Caspar
Wälzlagerungen in Turbo-Molekularpumpen
Vakuum-Technik, 1982, 109-113
E. Kellner et al.
Einsatz von Turbo-Molekularpumpen bei Auspumpvorgängen im Grob- und
Feinvakuumbereich
Vakuum-Technik, 1983, 136-139
D. E. Götz und H.-H. Henning
Neue Turbo-Molekularpumpe für überwiegend industrielle Anwendungen
Vakuum-Technik, 1988, 130-135
J. Henning
30 Jahre Turbo-Molekularpumpe
Vakuum-Technik, 1988, 134-141
P. Duval et. al.
Die Spiromolekularpumpe
Vakuum-Technik, 1988, 142-148
G. Reich
Berechnung und Messung der Abhängigkeit des Saugvermögens von
Turbo-Molekularpumpen von der Gasart
Vakuum-Technik, 1989, 3-8
J. Henning
Die Entwicklung der Turbo-Molekularpumpe
Vakuum in der Praxis, 1991, 28-30
D. Urban
Moderne Bildröhrenfertigung mit Turbo-Molekularpumpen
Vakuum in der Praxis, 1991, 196-198
O. Ganschow et al.
Zuverlässigkeit von Turbo-Molekularpumpen
Vakuum in der Praxis, 1993, 90-96
M. H. Hablanian
Konstruktion und Eigenschaften von turbinenartigen Hochvakuumpumpen
Vakuum in der Praxis, 1994, 20-26
J. H. Fremerey und H.-P. Kabelitz
Turbo-Molekularpumpe mit einer neuartigen Magnetlagerung
Vakuum-Technik, 1989, 18-22
H. P. Kabelitz and J.K. Fremerey
Turbomolecular vacuum pumps with a new magnetic bearing concept
Vacuum 38, 1988, 673-676
E. Tazioukow et al.
Theoretical and experimental investigation of rarefied gas flow in molecular
pumps
Vakuum in Forschung und Praxis, 7, 1995, 53-56
2.3
Fluid entrainment pumps
W. Gaede
Die Diffusion der Gase durch Quecksilberdampf bei niederen Drücken und
die Diffusionspumpe
Annalen der Physik, 46, 1915, 357-392
W. Gaede
Die Öldiffusionspumpe
Z. techn. Physik, 13, 1932, 210-212
185
References
R. Jaeckel, H. G. Nöller und H. Kutscher
Die physikalischen Vorgänge in Diffusions- und Dampfstrahlpumpen
Vakuum-Technik, 3, 1954, 1-15
F. Hinrichs
Aufbau, Betriebsverhalten und Regelbarkeit von DampfstrahlVakuumpumpen
Vakuum in der Praxis, 1991, 102-108
W. Bächler und H. G. Nöller
Fraktionierung und Entgasung in Öl-Diffusionspumpen
Z. angew. Physik einschl. Nukleonik, 9, 1957, 612-616
2.4
H. G. Nöller
Weshalb sind systematische Fehler bei Saugvermögensmessungen besonders groß für Hochvakuumpumpen großer Leistung ?
Vakuum-Technik, 12, 1963, 291-293
W. Bächler und H.-J. Forth
Die wichtigsten Einflußgrößen bei der Entwicklung von Diffusionspumpen
Vakuum-Technik, 13, 1964, 71-75
W. Reichelt
Bemerkungen zur Arbeitsweise moderner Diffusionpumpen
Vakuum-Technik, 13, 1964, 148-152
H. G. Nöller
Theory of Vacuum Diffusion Pumps
Handbook of Physics, Vol.1, Part 6, (pp. 323...419) Ed. A. H. Beck,
Pergamon Press Ltd., London, W.I. (1966)
G. Herklotz
Enddruckversuche mit Diffusionspumpen hohen Saugvermögens und
Restgasspektren
Vakuum-Technik, 20, 1971, 11 – 14
H. G. Nöller
Die Bedeutung von Knudsenzahlen und Ähnlichkeitsgesetzen in Diffusionsund Dampfstrahlpumpen
Vakuum-Technik, 26, 1977, 72-78
R. Gösling
Treibmittelpumpen
Vakuum-Technik, 1980, 163-168
M. Wutz
Grundlagen zur Bestimmung der charakteristischen Daten von
Dampfstrahl-Ejektorpumpen
Vakuum-Technik, 1982, 146-153
H. Bayer
Dampfstrahlpumpen
Vakuum-Technik, 1980, 169-178
H. Bayer
Vakuumerzeugung durch Dampfstrahl-Vakuumpumpen
Vakuum in der Praxis, 1989, 127-135
186
Sorption pumps
G. Kienel
Zur Desorption von Gasen in Getter-Ionenpumpen in „Physik und Technik
von Sorptions- und Desorptionsvorgängen bei niederen Drücken“
Rudolf A. Lange Verlag, 1963, Esch/Taunus, 266-270
W. Bächler
Ionen-Zerstäuberpumen, ihre Wirkungsweise und Anwendung
Leybold-Heraeus GmbH u. Co., in its own publishing house, Cologne 1966
W. Espe
Zur Adsorption von Gasen und Dämpfen an Molekularsieben
Feinwerktechnik, 70, 1966, 269-273
G. Kienel
Vakuumerzeugung durch Kondensation und durch Sorption
Chemikerzeitung / Chem. Apparatur 91, 1967, 83-89 und 155-161
H. Hoch
Erzeugung von kohlenwasserstoffreiem Ultrahochvakuum
Vakuum-Technik, 16, 1967, 156-158
W. Bächler und H. Henning
Neuere Untersuchungen über den Edelgas-Pumpmechnismus von
Ionenzerstäuberpumpen des Diodentyps
Proc. of the Forth Intern. Vacuum Congress 1968, I. 365-368,
Inst. of Physics, Conference Series No. 5, London
H. Henning
Der Erinnerungseffekt für Argon bei Trioden-Ionenzerstäuberpumpen
Vakuum-Technik, 24, 1975, 37-43
2.5
Cryopumps and cryoengineering
R. A. Haefer
Cryo-Pumping
456 pages, 1989 Oxford University Press, Oxford
H. Frey und R-A. Haefer
Tieftemperaturtechnologie, 560 pages, VDI-Verlag, Düsseldorf, 1981
G. Klipping und W. Mascher
Vakuumerzeugung durch Kondensation an tiefgekühlten Flächen, I.
Kryopumpen
Vakuum-Technik, 11, 1962, 81-85
References
W. Bächler, G. Klipping und W. Mascher
Cryopump System operating down to 2,5 K, 1962 Trans. Ninth National
Vacuum Symposium, American Vacuum Society, 216-219, The Macmillan
Company, New York
G. Klipping
Kryotechnik – Experimentieren bei tiefen Temperaturen
Chemie-Ingenieur-Technik, 36, 1964, 430-441
M. Schinkmann
Messsen und Regeln tiefer Temperaturen, Teil I: Thermodynamische
Verfahren
Meßtechnik, 81, 1973, 175-181
G. Schäfer, M. Schinkmann
Messsen und Regeln tiefer Temperaturen, Teil II: Elektrische Verfahren,
Meßtechnik, 82, 1974, 31-38
R. Frank et al.
Entwicklung von Refrigeratoren für den Einbau in Kryopumpen
Vakuum-Technik, 30, 1981, 134-137
J. J. Scheer und J. Visser
Anwendungen von Kryopumpen in der industriellen Vakuumtechnik
Vakuum-Technik, 31, 1982, 34-45
P. Duval
Diffusionspumpen, Turbo-Molekularpumpen oder Kryopumpen ? –
Auswahlkriterien für Hochvakuumpumpen
Vakuum-Technik, 31, 1982, 99-105
2.6
Oil backstreaming
G. Levin
A quantitativ appraisal of the backstreaming of forepump oil vapor
J. Vac. Sci. Technol. A 3 (6), 1985, 2212-2213
M. A. Baker and L. Laurenson
A quartz crystal microbalance holder for low Temperature use in vacuum
Vacuum Vol. 17, (12), 647-648, 1967 (Letters to the Editor)
M. A. Baker and W. Steckelmacher
The Measurement of Contamination in Vacuum Systems
Vuoto, scienza e technologia, Bd.3 , (1/2), 3-17, 1970
J. P. Deville, L. Holland and L. Laurenson
Measurement of the rate of evaporation of Pump oils using a crystal vibrator
3rd. Internat. Vac. Congr Stuttgart 153-160, Pergamon Press, Oxford, 1965
L. Laurenson, S. Hickman and R. G. Livesey
Rotary pump backstreaming: An analytical appriasal of practical results and
the factors affecting them
J. Vac. Sci. Technol. A 6 (2), 238-242, 1988
B. D. Power, A. M. I. Mech, E. Crawley and D. J. Crawley
Sources, Measurement and Control of Backstreaming in Oil Vapour
Vacuum Pumps
Vacuum, Vol. 4 (4), 415-437, 1957
H. Henning und H.-H. Klein
Pumpen von Helium mit Refrigerator-Kryopumpen
Vakuum-Technik, 34, 1985, 181-184
M. A. Baker
A cooled quartz crystal microbalance methode for measuring diffusion pump
backstreaming
Journal of Scientific Instruments (Journal of Physics E), Series 2, Volume 1,
774-776, 1968
H.-H. Klein et al.
Einsatz von Kryopumpen in Produktionsanlagen
Vakuum-Technik, 34, 1986, 203-211
N. S. Harris
Diffusion pump back-streaming
Vacuum, Vol. 27 (9), 519-530, 1977
D. Müller und M. Sydow
Kryopumpen im Vergleich mit anderen Hochvakuumpumpen
Vakuum in der Praxis, 2, 1990, 270-274
M. A. Baker
Vapour and Gas Measurements in Vacuum with the Quartz Crystal
Microbalance
in Vol.1, Proceedings of the ninth Conference on Vacuum Microbalance
Techniques, „Progress in Vacuum Microbalance Techniques“
G. Kiese und G. Voß
Kryopumpen mit neuartiger Regenerationstechnik
Vakuum in der Praxis, 4, 1992, 189-192
Th. Gast and E. Robens ed.,
Heyden & Son Ldt., London, New York, Rheine, 1970
M. A. Baker and L. Laurenson
The use of a quartz crystal microbalance for measuring vapour backstreaming from mechanical pumps
Vacuum, Volume 16 (11), 633-637, 1966
R. D. Oswald and D. J. Crawley
A method of measuring back migration of oil through a baffle
Vacuum, Vol. 16 (11), 623-624, 1966
187
References
M. H. Hablanian
Backstreaming Measurements above Liquid-Nitrogen Traps
Vac. Sci. Tech., Vol. 6, 265-268, 1969
F. Grotelüschen
Das UHV-System bei DESY. 1. Teil
Vakuum in der Praxis, 4, 1991, 266-273
Z. Hulek, Z. Cespiro, R. Salomonovic, M. Setvak and J. Voltr
Measurement of oil deposit resulting from backstreaming in a diffusion
pump system by proton elastic scattering
Vacuum, Vol. 41 (7-9), 1853-1855, 1990
D. Trines
Das Strahlrohrvakuumsystem des Hera-Protonenringes
Vakuum in der Praxis, 2, 1992, 91-99
M. H. Hablanian
Elimination of backstreaming from mechanical vacuum pumps
J. Vac. Sci. Technol. A5 (4), 1987, 2612-2615
G. Schröder et al.
COSV- eine neue Forschungsanlage mit UHV-Technologie
Vakuum in der Praxis, 5, 1993, 229-235
W. Jacobi
Das Vakuumsystem der GSI-Beschleunigeranlage
Vakuum in der Praxis, 6, 1994, 273-281
3.
Ultrahigh vacuum technology
G. Kienel
Probleme und neuere Entwicklungen auf dem Ultrahochvakuum-Gebiet
VDI-Zeitschrift, 106, 1964, 777-786
G. Kienel und E. Wanetzky
Eine mehrmals verwendbare Metalldichtung für ausheizbare
Uktrahochvakuum-Ventile und Flanschdichtungen
Vakuum-Technik, 15, 1966, 59-61
H. G. Nöller
Physikalische und technische Voraussetzungen für die Herstellung und
Anwendung von UHV-Geräten.
„Ergebnisse europäischer Ultrahochvakuum Forschung“
LEYBOLD-HERAEUS GmbH u. Co., in its own publishing house, Cologne
1968, 49-58
W. Bächler
Probleme bei der Erzeugung von Ultrahochvakuum mit modernen Vakuumpumpen. „Ergebnisse europäischer Ultrahochvakuum Forschung“
Leybold-Heraeus GmbH u. Co., in its own publishing house, Cologne 1968,
139-148
P. Readhead, J. P. Hobson und E. V. Kornelsen
The Physical Basis of Ultrahigh Vacuum
Chapman and Hall, London, 1968
E. Bergandt und H. Henning
Methoden zur Erzeugung von Ultrahochvakuum
Vakuum-Technik, 25,1970, 131-140
H. Wahl
Das Hochvakuumsystem der CERN am 450 GeV Supersynchrotron und
Speichering (SPS)
Vakuum in der Praxis, 1989, 43-51
188
4.
Conductances, flanges,
valves, etc.
M. Knudsen
Gesetze der Molekularströmung und der inneren Reibungsströmung der
Gase durch Röhren
Annalen der Physik, 4th issue, 28, 1909, 75-130
P. Clausing
Über die Strömung sehr verdünnter Gase durch Röhren von beliebiger
Länge
Annalen der Physik, 5th issue, 12, 1932, 961-989
W. Röllinger
Die Verwendung von Klammerflanschen in der Vakuumtechnik
Vakuum-Technik, 13, 1964, 42-45
H. Hoch
Ausheizbare Verbindungen an Hochvakuum-Apparaturen
Vakuum-Technik, 10, 1961, 235-238
W. Bächler und I. Wikberg
Dual Seal Bakable Section Valves of the CERN Intersection Storage Ring
Vacuum, 21, 1971, 457-459
K. Teutenberg
UHV-Ganzmetallventile großer Nennweite
Vakuum-Technik, 21, 1972, 169-174
H. Henning
The approximate calculation of transmission probabilities
Vacuum, 28, 1978, No. 3, Seite 151
G. Kühn
Gasströme durch Spalte im Grobvakuum
Vakuum-Technik, 33, 1984, 171-175
References
R. Haberland und B. Vogt
UHV-Ventil für extrem viele Schließzyklen
Vakuum-Technik, 34, 1985, 184-185
A. Sele
Vakuum-Ventile (VAT)
Vakuum in der Praxis, 1, 1989, 206-212
L. Fikes
Berechnung von Auspumpkurven mit Hilfe der Analogie von Gasstrom und
elektrischem Strom
Vakuum in der Praxis, 4, 1992, 265-268
W. Herz
Zuverlässige Flanschverbindung im Anwendungsgebiet der Tieftemperaturund Vakuumtechnik
Vakuum-Technik, 29, 1980, 67-68
5.
Measurement of low pressures
C. Meinke und G. Reich
Vermeidung von Fehlmessungen mit dem System McLeod-Kühlfalle
Vakuum-Technik, 12, 1963, 79-82
P. A. Readhead and J. P. Hobson
Total Pressure Measurem. below 10–10 Torr with Nonmagnetic Ionisation
Gauge
Brit. J. Appl. Phys., 16, 1965, 1555-1556
C. Meinke und G. Reich
Comparison of Static and Dymanic Calibration Methods for Ionisation
Gauges
J. Vac. Sci. Techn., 4, 1967, 356-359
G. Reich und W. Schulz
Probleme bei der Verwendung von Ionisations-Vakuummetern im
Druckbereich oberhalb 10–2 Torr
Proc. of the Fourth Intern. Vacuum Congress, 1968,
II. Inst. of Physics Conference Series No. 6, London, 661-665
G. Reich
Probleme bei der Messung sehr niedriger Total- und Partialdrücke
„Ergebnisse europäischer Ultrahochvakuum Forschung“
Leybold-Heraeus GmbH u. Co., in its own publishing house, Cologne 1968,
99-106
A. Barz and P. Kocian
Extractor Gauge as a Nude System
J. Vac. Sci Techn. 7, 1970, 1, 200-203
U. Beeck and G. Reich
Comparison of the Pressure Indication of a Bayard-Alpert and an Extractor
Gauge
J. Vac. Sci. and Techn. 9, 1972, 1,126-128
U. Beeck
Untersuchungen über die Druckmessungen mit Glühkathoden-InisationsVakuummetern im Bereich größer als 10–3 Torr
Vakuum-Technik, 22, 1973, 16-20
G. Reich
Über die Möglichkeiten der Messung sehr niedriger Drücke
Meßtechnik, 2, 1973, 46-52
G. Reich
Spinning rotor viscosity gauge; a transfer standard for the laboratory or an
accurate gauge for vacuum process control
J. Vac. Sci. Technol., 20 (4), 1982, 1148-1152
G. Reich
Das Gasreibungs-Vakuummeter VISCOVAC VM 210
Vakuum-Technik, 31, 1982, 172-178
G. Grosse and G. Messer
Calibration of Vacuum Gauges at Pressures below 10–9 mbar with a molecular beam method
Vakuum-Technik, 30, 1981, 226-231
Chr. Edelmann et al.: Möglichkeiten der Meßbereichserweiterung bei
Glühkathoden-Ionisationsmanometern (numerous references to relevant literature)
Vakuum-Technik, 31, 1982, 2-10
Chr. Edelmann
Stand und Entwicklungstendenzen der Totaldruckmessung in der VakuumTechnik
Vakuum-Technik, 33, 1984, 162-180
J. K. Fremerey
Das Gasreibungsvakuummeter
Vakuum-Technik, 36, 1987, 205-209
G. Messer
Kalibrierung von Vakuummetern
Vakuum-Technik, 36, 1987, 185-192
G. Messer und W. Grosse
Entwicklung der Vakuum-Metrologie in der PTB (numerous references to
relevant literature)
Vakuum-Technik, 36, 1987, 173-184
G. Reich
Industrielle Vakuummeßtechnik
Vakuum-Technik, 36, 1987, 193-197
L. Schmidt und E. Eichler
Die Praxis einer DKD-Kalibrierstelle
Vakuum-Technik, 36, 1987, 78-82
C. Kündig
Vakuummeßgeräte für Totaldruck
Vakuum in der Praxis, 2, 1990, 167-176
189
References
Chr. Edelmann
Glühkahtoden-Ionisationsmanometer für hohe Drücke im Vakuumbereich
Vakuum in der Praxis, 3, 1991, 290-296
R. Heinen und W. Schwarz
Druckregelung bei Vakuumprozessen durch umrichtergespeiste
Rootspumpen
Vakuum-Technik, 35, 1986, 231-236
M. Ruschitzka and W. Jitschin
Physikalische Grundlagen des Wärmeleitungsvakuummeters
Vakuum in der Praxis, 4, 1992, 37-43
7.
T. Koopmann
Neue Trends in der Vakuum-Meßtechnik
Vakuum in der Praxis, 5, 1993, 249-254
Chr. Edelmann
Die Entwicklung der Totaldruckmessung im UHV- und
Extremvakuumbereich
Vakuum in der Praxis, 6, 1994, 213-219
W. Jitschin
Kalibrierung, Abnahme und Zertifizierung (with numerous references to relevant literature)
Vakuum in der Praxis, 6, 1994, 193-204
W. Jitschin
Obere Meßbereichsgrenze von Glühkatoden-Ionisationsvakuummetern
Vakuum in Forschung und Praxis, 7, 1995, 47-48
F. Mertens et al.
Einfluß von Gasadsorbaten auf die Eigenschaften eines GlühkatodenIonisationsvakuummeters mit axialer Emission nach Chen und Suen
Vakuum in der Praxis, 7, 1995, 145-149
6.
Pressure monitoring, control
and regulation
Mass spectrometer gas analysis at low
pressures
H. Hoch
Total- und Partialdruckmessungen bei Drücken zwischen 2 · 10–10 und 2 ·
10–2 Torr
Vakuum-Technik, 16, 1967, 8-13
H. Junge
Partialdruckmessung und Partialdruckmeßgeräte
G-I-T May 1967, 389-394 and June 1967, 533-538
A. Kluge
Ein neues Quadrupolmassenspektrometer mit massenunabhängiger
Empfindlichkeit
Vakuum-Technik, 23, 1974, 168-171
S. Burzynski
Microprocessor controlled quadrupole mass spectrometer
Vacuum, 32, 1982, 163-168
W. Große Bley
Quantitative Gasanalyse mit dem Quadrupol Massenspektrometer
Vakuum-Technik, 38, 1989, 9-17
A. J. B. Robertson
Mass Spectrometry
Methuen & Co, Ltd., London, 1954
K. G. Müller
Betriebsüberwachung, Steuerung und Automatisierung von Vakuumanlagen
Chemie-Ingenieur-Technik, 35, 1963, 73-77
C. Brunee und H. Voshage
Massenspektrometrie
Karl Thiemig Verlag, München, 1964
G. Kienel
Elektrische Schaltgeräte der Vakuumtechnik
Elektro-Technik, 50, 1968, 5-6
A. Cornu and R. Massot
Compilation of Mass Spectral Data
Heyden and Son Ltd., London, 1966
A. Bolz, H. Dohmen und H.-J. Schubert
Prozeßdruckregelung in der Vakuumtechnik
Leybold Firmendruckschrift 179.54.01
P. Dawson
Quadrupole Mass Spectroscopy
Elsevier, Amsterdam, 1976
H. Dohmen
Vakuumdruckmessung und -Regelung in der chemischen
Verfahrenstechnik
Vakuum in der Praxis, 6,1994, 113-115
J. Backus
Chap. 11 in „Characteristics of Electrical Discharges in Magnetic Fields“
National Nuclear Energy Series, Div. I, Vol. 5, McGraw-Hill Book Company
Inc., New York, 1949
N. Pöchheim
Druckregelung in Vakuumsystemen
Vakuum in Forschung und Praxis, 7, 1995, 39-46
J. Backus
University of California Radiation Laboratory Report, RL 20.6.36, Mar. 1945.
190
References
8.
Leaks and leak detection
8.1
Mass spectrometer leak detection
G. Kienel
Lecksuche an Vakuumanlagen auf elektrischem Wege
Elektrotechnik, 49, 1967, 592-594
U. Beeck
Möglichkeiten und Grenzen der automatischen Lecksuche im Bereich unter
10–8 Torr. l/s
Vakuum-Technik, 23, 1974, 77-80
Lecksuche an Chemieanlagen
Dechema Monographien (Ed. H. E. Bühler and K. Steiger), Vol. 89, Verlag
Chemie, Weinheim / New York
W. Jansen
Grundlagen der Dichtheitsprüfung mit Hilfe von Testgasen
Vakuum-Technik, 29, 1980, 105-113
K. Paasche
Lecksuche an Chemieanlagen
Vakuum-Technik, 29, 1980, 227-231
W. Große Bley
Moderne He-Leckdetektoren unterschiedlicher Prinzipien im praktischen
Einsatz
Vakuum in der Praxis, 1, 1989, 201-205
H. D. Bürger
Lecksucher (with references to relevant literature)
Vakuum in der Praxis, 2, 1990, 56-58
W. Fuhrmann
Einführung in die industrielle Dichtheitsprüftechnik
Vakuum in der Praxis, 3, 1991, 188-195
W. Fuhrmann
Industrielle Dichtheitsprüfung – ohne Testgas nach dem
Massenspektrometrieverfahren
Vakuum in Forschung und Praxis, 7, 1995, 179 -182
8.2
Leak detection with halogen leak detectors
H. B. Bürger
Lecksuche an Chemieanlagen mit He-Massenspektrometer-Lecksuchern
Vakuum-Technik, 29, 1980, 232-245
H. Moesta und P. Schuff
Über den thermionischen Halogendetektor
Berichte der Bunsengesellschaft für physikaische Chemie,
Bd. 69, 895-900, 1965
Verlag Chemie, GmbH, Weinheim, Bergstraße
Chr. Falland
Ein neuer Universal-Lecksucher mit luftgekühlter Turbo-Molekularpumpe
Vakuum-Technik, 29, 1980, 205-208
J. C. Leh and Chih-shun Lu
US Patent Nr. 3,751,968
Solid State Sensor
W. Jansen
Grundlagen der Dichtheitsprüfung mit Hilfe von technischen Gasen
Vakuum-Technik, 29, 1980, 105-113
9.
H. Mennenga
Dichtheitsprüfung von Kleinteilen
Vakuum-Technik, 29, 1980, 195-200
Chr. Falland
Entwicklung von He-Lecksuchtechniken für UHV-Systeme großer
Beschleuniger- und Speicherringe
Vakuum-Technik, 30, 1981, 41-44
W. Engelhardt et al.
Lecksuchanlagen in der Industrie
Vakuum-Technik, 33, 1984, 238-241
G. Sänger et al.
Über die Lecksuche bei Raumfahrzeugen
Vakuum-Technik, 33, 1984, 42-47
Film thickness measurement and control
G. Z. Sauerbrey
Phys. Verhandl. 8, 113, 1957
G. Z. Sauerbrey
Verwendung von Schwingquarzen zur Wägung dünner Schichten und zur
Mikrowägung
Zeitschrift für Physik 155, 206-222, 1959
L. Holland, L. Laurenson and J. P. Deville
Use of a Quartz Crystal Vibrator in Vacuum Destillation Invstigations
Nature, 206 (4987), 883-885, 1965
R. Bechmann
Über die Temperaturabhängigkeit der Frequenz von AT- und BTQuarzresonatoren
Archiv für Elektronik und Übertragungstechnik, Bd. 9, 513-518, 1955
W. Jitschin et al.
He-Diffusionslecks als sekundäre Normale für den Gasdurchfluß
Vakuum-Technik, 36, 1987, 230-233
191
References
K. H. Behrndt and R. W. Love
Automatic control of Film Deposition Rate with the crystal oscillator for
preparation of alloy films.
Vacuum 12 ,1-9, 1962
H. F. Tiersten and R. C. Smythe
An analysis of contowced crystal resonators operating in overtones of coupled thickness shear and thickness twist.
J. Acoustic Soc. Am. 65, (6) 1455, 1979
P. Lostis
Automatic Control of Film Deposition Rate with the Crystal Oscillator for
Preparation fo Alloy Films.
Rev. Opt. 38, 1 (1959)
R. E. Bennett, C. Rutkoeski and L. A. Taylor
Proceedings of the Thirteenth Annual Symposium on Frequency Controll,
479, 1959
K. H. Behrndt
Longterm operation of crystal oscillators in thin film deposition
J. Vac. Sci. Technol. 8, 622 (1971)
L. Wimmer, S. Hertl, J. Hemetsberger and E. Benes
New method of measuring vibration amplitudes of quartz crystals.
Rev. Sci. Instruments 55 (4) , 608, 1984
P. J. Cumpson and M. P. Seah
Meas. Sci. Technol., 1, 548, 1990
J. G. Miller and D. I. Bolef
Sensitivity Enhancement by the use of Acoustic Resonators in cw
Ultrasonic Spectroscopy.
J. Appl. Phys. 39, 4589, (1968)
J. G. Miller and D. I. Bolef
Acoustic Wave Analysis of the Operation of Quartz Crystal Film Thickness
Monitors.
J. Appl. Phys. 39, 5815, (1968)
C. Lu and O. Lewis
Investigation of Film thickness determination by oscillating quartz resonators with large mass load.
J. Appl. Phys. 43, 4385 (1972)
C. Lu
Mas determination with piezoelectric quartz crystal resonators.
J. Vac. Sci. Technol. Vol. 12 (1), 581-582, 1975
A. Wajid
U.S. Patent No. 505,112,642 (May 12, 1992)
C. Hurd
U.S. Patent No. 5,117,192 (May 26, 1992)
E. Benes
Improved Qartz Crystal Microbalance Technique
J. Appl. Phys. 56, (3), 608-626 (1984)
C. J. Wilson
Vibration modes of AT-cut convex quartz resonators.
J. Phys. d 7, 2449, (1974)
192
Chih-shun Lu
Improving the accuracy of Quartz csystal monitors
Research/Development, Vol. 25, 45-50, 1974, Technical Publishing
Company
A. Wajid
Improving the accuracy of a quartz crystal microbalance with automatic
determination of acoustic impedance ratio.
Rev. Sci. Instruments, Vol. 62 (8), 2026-2033, 1991
D. Graham and R. C. Lanthrop
The Synthesis fo Optimum Transient Response: Criteria and Standard
Forms
Transactions IEEE, Vol. 72 pt. II, Nov. 1953
A. M. Lopez, J. A. Miller, C. L. Smith and P. W. Murrill
Tuning Controllers with Error-Integral Criteria
Instrumentation Technology, Nov. 1969
C. L. Smith and P. W. Murril
A More Precise Method for Tuning Controllers
ISA Journal, May 1966
G. H. Cohen and G. A. Coon
Theoretical considerations of Retarded Control
Taylor Technical Data Sheet Taylor Instrument Companies, Rochester, New
York
J. G. Ziegler and N. B. Nichols
Optimum Settings for Automatic Controllers
Taylor Technical Data Sheet No. TDS 10A100, Taylor Instrument
Companies, Rochester, New York
C. Lu and A. W. Czanderna
Application of Piezoelectric Quarz Crystal Microbalances (Vol.7 of:
Methodes and Phenomena, Their Applications in Sience and Technology)
Elesvier, Amsterdam, Oxford, New York, Tokio, 1984
G. Simmons and H. Wang
Single Crystal Elastic Constants and Calculated Aggregate Properties – A
Handbook
The MIT Press, Cambridge, Massachusetts, 1971
C. D. Stockbridge
in Vol. 5 „Vacuum Microbalance Techniques“ K. Behrndt, editor, Plenum
Press, Inc., New York, 1966
References
S. Sotier
Schwingquarz-Schichtdickenmessung
Vakuum in der Praxis 1992, 182-188
Vakuumgerechte Werkstoffe und Verbindungstechnik, Part 3
Vakuum in der Praxis, 2, 1990, 179-184
11.
10.
Dictionaries
Materials and material processing
W. Espe
Werkstoffkunde der Hochvakuumtechnik
Vol. 1 1959, Vol. 2 1960, Vol. 3 1961,
VEB Deutscher Verlag der Wissenschaften, Berlin
F. Weber
Elsevier’s Dictionary of High Vacuum
Science and Technology (German, English, French, Spanish, Italian,
Russian)
Elsevier Verlag 1968
W. Espe
Werkstoffe für trennbare metallische Verbindungen der
Ultrahochvakuumtechnik
Feinwerktechnik, 68, 1964, 131-140
Hurrle / Jablonski / Roth
Technical Dictionary of Vacuum Physics and Vacuum Technology (German,
English, French, Russian)
Pergamon Press Verlag, Oxford, 1972
W. Espe
Synthetische Zeolithe und ihre Verwendung in der Hochvakuumtechnik
Experimentelle Technik der Physik, XII, 1964, 293-308
H. Adam
Allgemeiner Überblick über die Werkstoffe der Vakuumtechnik und deren
Auswahl
Haus der Technik Vortragsveröffentlichungen „Werkstoffe und
Werkstoffverbindungen in der Vakuumtechnik“ H. 172, Vulkan-Verlag, Dr.
W. Classen, Essen, 1968, 4 – 13
K. Verfuß
Bessere Oberflächenvergütung durch Elektropolieren – am Beispiel der
Vakuum-Technik
VDI-Berichte, 183, 1972, 29-34
K. Verfuß
Schweißen und Hartlöten
Haus der Technik, Vortragsveröffentlichungen „Werkstoffe und Werkstoffverbindungen in der Vakuumtechnik, H. 172
Vulkan-Verlag Dr. W. Classen, Essen, 1968, Seiten 39 -49
Chr. Edelmann
Gasabgabe von Festkörpern im Vakuum
Vakuum-Technik, 38, 1989, 223-243
R. Fritsch
Besonderheiten vakuumdichrter Schweißverbindungen
Vakuum-Technik, 38, 1989, 94-102
H. Henning
Vakuumgerechte Werkstoffe und Verbindungstechnik, Part 1
Vakuum in der Praxis, 2, 1990, 30-34
R. Fritsch
Vakuumgerechte Werkstoffe und Verbindungstechnik, Part 2
Vakuum in der Praxis, 2, 1990, 104-112
M. Mühlloff
193
Index
13. Index
Absolute pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Absorption isotherms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
Absorption pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50, 144
Absorption traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
Accessories for rotary displacement pumps . . . . . . . . . . . . . . . . . . . . . . .38
Active oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127, 128
Adjustment and calibration of vacuum gauges . . . . . . . . . . . . . . . . . . . . .86
Adsorption pumps,Instructions for operation . . . . . . . . . . . . . . . . . .144, 145
Aggressive vapors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .140
AGM (aggressive gas monitor) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99
Air, atmospheric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
ALL·ex pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32, 35
Ambient pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Amonton's law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Anticreep barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45
Anti-suckback valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
APIEZON AP 201 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44, 166
Atmospheric air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Atmospheric air, composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .150
Atmospheric pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Atomic units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .177
Autoc ontrol tune . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .131
Automatic protection, monitoring and control of vacuum systems . . . . . .89
Auto-Z match technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .129
Avogadro's law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Avogadro's number (Loschmidt number) . . . . . . . . . . . . . . . . . . . . . .14, 148
Backing line vessel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69
Baffle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
Baffles (vapor barriers) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41, 42, 44
Baking (degassing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60, 73, 146
Barrier gas operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
Basis SI units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .176
Bath cryostats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54
Bayard-Alpert gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86
Boat (thermal evaporator) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133
Boltzmann constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14, 148
Bombing-Test (storing under pressure) . . . . . . . . . . . . . . . . . . . . . . . . . .123
Booster (oil-jet) pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43, 51
Bourdon vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
Boyle-Mariotte law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Break down voltage (Paschen curve for air) . . . . . . . . . . . . . . . . . . . . . .169
Bubble (immersion) test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115
Calibration curves of THERMOVAC gauges . . . . . . . . . . . . . . . . . . . . . . .82
Calibration inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86
Coating sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133
Capacitance diaphragm gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78
Capsule vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
Causes of faults if/when the desired ultimate pressure
is not achieved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .139
Ceramic ball bearings (hybrid ball bearings) . . . . . . . . . . . . . . . . . . . . . . .47
CF-flange (conflathflange) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73
Changing the molecular sieve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .144
Charles' law (Gay-Lussac's law) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Chemical resistance of elastomer gaskets . . . . . . . . . . . . . . .154, 155, 156
Chemical vapor deposition (CVD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .134
Choked flow, critical pressure difference . . . . . . . . . . . . . . . . . . . . . . . . . .15
CIS (closed ion source) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99
Clamp flange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73
Classification of vacuum pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Clausius-Clapeyron equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
194
Claw pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Closed ion source, (CIS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99
Coating of parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .135
Coating thickness regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .130
Cold cap baffle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Cold cathode ionization vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . .83
Cold head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55
Cold surfaces, bonding of gases to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56
Cold traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Collision frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Collision rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Common solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1451
Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47, 48, 49
Compression vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79
Condensate traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
Condensers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38, 182
Conductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11, 15
Conductance of openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17, 187, 188
Conductance of piping . . . . . . . . . . . . . . . . . . . . . . . . . . . .16, 161, 187, 188
Conductance, nomographic determination . . . . . . . . . . . . . . . . . . . . . . . .18
Conductances, calculation of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Connection of leak detectors to vacuum systems . . . . . . . . . . . . . . . . . .121
Contamination of vacuum sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .144
Contamination of vacuum vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .139
Continous flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Continous flow cryopumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54
Continuum theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Conversion of leak rate units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Conversion of leak rate units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .149
Conversion of pressure units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .147
Conversion of pV-throughput units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .149
Corrosion protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .141
Counter-flow leak detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120
Cracking pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103
Critical pressure difference (choked flow) . . . . . . . . . . . . . . . . . . . . . . . . .15
Crossover value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58
Cryocondensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57
Cryopumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54, 186
Cryosorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57
Cryotrapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57
Crystal Six . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .125
Cut in (start) pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49, 60
CVD (chemical vapor deposition) . . . . . . . . . . . . . . . . . . . . . . . . . .133, 134
Dalton's law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Danger classes of fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .153
Data storage coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .137
DC 704, DC 705 (Silicone oils) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Degassing of the pump oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42
Derived coherent and not coherent SI units with special
names and symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .177
Detection limit (leak detectors) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .117
Determination of a suitable backing pump . . . . . . . . . . . . . . . . . . . . . . . . .69
Determination of pump down time from Nomograms . . . . . . . . . . . . . . . .70
Determination of pump sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66
DI series diffusion pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42
Diaphragm contoller, examples of application . . . . . . . . . . . . . . . . . . .91, 92
Diaphragm vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
Diaphragm vacuum pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
DIAVAC diaphragm vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
DIFFELEN, light, normal, ultra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44, 166
Diffusion / vapor-jet pumps, Instructions for operation . . . . . . . . . . . . . .144
Diffusion pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
Diode-type sputter ion pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52
Index
Direct-flow leak detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120
Discharge filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
Displacement pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19, 20, 182
DIVAC vacuum pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
DKD (Deutscher Kalibrierdienst) German calibration service . . . . . . . . . .87
Dry compressing rotary displacement pumps . . . . . . . . . . . . . . . . . . . . . .27
Dry processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62
Drying of paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72
Drying processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64
Drying processes, selection of pumps for . . . . . . . . . . . . . . . . . . . . . . . . .71
DRYVAC-Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
D-Tek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116
Duo seal (sealing passage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20, 21, 22
Dust separator (dust filter) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
Dynamic expansion method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88
ECOTEC II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .119
Effective pumping speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38, 67
Elastomer gaskets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .74, 154, 155, 156
Electrical break down voltage(Paschen curve air) . . . . . . . . . . . . . . . . . .169
Electron beam evaporators (electron guns) . . . . . . . . . . . . . . . . . . . . . . .134
Envelope test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122, 123
Envelope test (concentration measurement) . . . . . . . . . . . . . . . . . . . . . .123
Evacuation in the rough / medium / high vacuum region . . . . . . .66, 67, 68
Evacuation of gases / vapors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71
Evaluating spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102
Expansion method static / dynamic . . . . . . . . . . . . . . . . . . . . . . . . . . .87, 88
Extractor ionization vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86
Fast regeneration (partial regeneration) . . . . . . . . . . . . . . . . . . . . . . . . . .58
Fingerprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103
Flanges and their seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73, 187
Floating zero-point suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .117
Fluid entrainment pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40, 185
Foam spray leak test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115
Fractionation of pump fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42
Fragment distribution pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103
Fundamental pressure measurement methods . . . . . . . . . . . . . . . . . . . . .87
Gas analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95, 106, 107, 190
Gas ballast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24, 25, 117
Gas composition as a function of altitude . . . . . . . . . . . . . . . . . . . . . . . .161
Gas constant, general (molar) . . . . . . . . . . . . . . . . . . . . . . . . . . . .9, 14, 149
Gas density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Gas dependent pressure reading, vacuum gauges with . . . . . . . . . . . . . .81
Gas discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51, 83
Gas independent pressure reading, vacuum gauges with . . . . . . . . . . . . .77
Gas laws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Gas locks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75
Gas sorption (pumping) of vacuum gauges . . . . . . . . . . . . . . . . . . . . .83, 84
Gas storage in the oil of rotary vane pumps . . . . . . . . . . . . . . . . . . . . . .117
Gaskets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73, 154, 155, 156
Gay-Lussac's law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
General gas constant (Molar gas constant) . . . . . . . . . . . . . . . . . .9, 14, 148
Getter pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
Glass coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .137
Halogen leak detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116, 191
Helium leak detectors with 180° sector mass spectrometer . . . . . .114, 115
Helium spray equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122
Helium standard leak rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
High frequency vacuum test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115
High pressure ionization vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . .86
High vacuum range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67, 68
HLD 4000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116
HO-factor (diffusion pumps) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42
Hot cathode ionization vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . .84
HY.CONE pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49
Hybrid ball bearings (Ceramic ball bearings) . . . . . . . . . . . . . . . . . . . . . .47
Hydrocarbon-free vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44, 65
IC 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .131
Ideal gas law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 13
Impingement rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Industrial leak testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .124
Influence of magnetic / electrical fields . . . . . . . . . . . . . . . . . . . . . . . . . .146
Internal compression (claw pumps) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
Inside-out leak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Integral leak rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113
Internal reflow (roots pumps) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
Ion desorption effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .85
Ion sputter pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50, 51
Ionization vacuum gauge for higher pressures up to 1 mbar . . . . . . . . . .85
Ionization vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83
Ionization, specific (gas analysis) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103
Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102
Kammerer compression vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . .79
Kinetic gas theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Kinetic of gases, diagram of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .160
Kinetic of gases, formulas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .148
Knudsen flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Krypton 85 test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115
Laminar flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Langmuir-Taylor-effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .117
Laval nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
Leak detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110, 190
Leak detection using Helium leak detectors . . . . . . . . . . . . . . . . . . . . . .122
Leak detection without leak detector . . . . . . . . . . . . . . . . . . . . . . . . . . . .113
Leak detection, leak test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110
Leak detectors with 180° sector mass spectrometer . . . . . . . . . . . . . . . .119
Leak detectors with mass spectrometer . . . . . . . . . . . . . . . . . . . . . .116, 190
Leak detectors with quadrupole mass spectrometer . . . . . . . . . . . . . . . .119
Leak detectors, how they work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116
Leak rate, hole size, conversion . . . . . . . . . . . . . . . . . . . . .12, 110, 111, 112
Leak test (chemical reactions, dye penetration) . . . . . . . . . . . . . . . . . . . .115
Leak test, using vacuum gauges sensitive to the type of gas . . . . . . . . .114
LEYBODIFF Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42
LEYBOLD-INFICON Quartz crystal controllers . . . . . . . . . . . . . . . . . . . .131
Line width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
Linearity range of quadrupole sensors . . . . . . . . . . . . . . . . . . . . . . . . . . .102
Liquid filled (mercury) vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . .79
Liquid ring pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Liquid sealed rotary displacement pumps . . . . . . . . . . . . . . . . . . . . . . . . .20
Liquid-filled vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79
Literature references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .182 – 193
LN2 cold traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Local leak rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Loschmidt's number (Avogadro constant) . . . . . . . . . . . . . . . . . . . . .14, 148
Magnetic suspension (bearings) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47, 48
Mass flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11, 108
Mass flow (leak detection) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .108
Mass range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
Mass spectrometer, general, historical . . . . . . . . . . . . . . . . . . . . . . .95, 190
Maximum backing pressure (critical forevacuum pressure) . . . . . . . . . . .41
McLeod vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79
Mean free path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12, 147, 160
Measuring range of vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . .168
Measuring range, favorable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76
Measuring ranges of vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . .168
Measuring vacuum, vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . .76, 189
Medium vacuum adsorption trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
195
Index
MEMBRANOVAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78
Mercury (pump fluid) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41, 44,166
Mode-lock oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .128
Molar gas constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9, 14, 148
Molar mass (molecular weight) . . . . . . . . . . . . . . . . . . . . . . . . . . . .9, 12, 13
Molecular flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Molecular sieve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50, 145
Monolayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Monolayer formation time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12, 16, 65
National standards, resetting to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86
NEG pumps (non evaporable getter pumps) . . . . . . . . . . . . . . . . . . .50, 53
Neoprene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73, 154, 155, 156
Nitrogen equivalent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76, 83
Nominal internal diameter and internal diameter of tubes . . . . . . . . . . . .151
Nomogram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70
Nomogram: conductance of tubes / entire pressure range . . . . . . . . . . .164
Nomogram: conductance of tubes / laminar flow range . . . . . . . . . . . . .161
Nomogram: conductance of tubes / molecular flow range . . . . . . .161, 163
Nomogram: pump down time / medium vacuum, taking in
account the outgasing from the walls . . . . . . . . . . . . . . . . . . . . . . . . . . .165
Nomogram: pump down time / rough vacuum . . . . . . . . . . . . . . . . . . . . .162
Non evaporable getter (NEG) pumps . . . . . . . . . . . . . . . . . . . . . . . . .50, 53
Non gas-tight area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110
Nude gauge (nude system) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
Oil backstreaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44, 186
Oil change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .139
Oil consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .139, 140, 141, 142
Oil contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .139
Oil diffusion pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
Oil sealed rotary displacement pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Oil vapor ejector vacuum pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
Oil-free (hydrocarbon-free) vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . .44, 65
Oils (pump fluids) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Open (normal) ion source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96
Optical coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .136
Oscillation displacement pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Oscillator, ( active, mode-lock) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127, 128
Outgasing of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .150
Outgasing rate (referred to surface area) . . . . . . . . . . . . . . . . . . . . . .12, 65
Outside-in leak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Overpressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Oxide-coated cathodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84, 96
Partial final pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79
Partial flow opeartion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120
Partial flow ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .121
Partial pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Partial pressure measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106
Partial pressure regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109
Particle number density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Paschen curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .169
Penning vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83
Perbunan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73, 156, 167
Period measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127
Permissible pressure units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .147
Phase diagram of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .170
Photons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .85
PIEZOVAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78
Pirani vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81, 82
Plastic tent (envelope) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .123
Plate baffle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
PNEUROP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .178 – 181
PNEUROP flanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73
Poiseuille flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
196
Poisson's law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Positive pressure methode (leak detection) . . . . . . . . . . . . . . . . . . . . . . .112
Pre-admission cooling (roots pumps) . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Precision diaphragm vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78
Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Pressure and temperature as function of altitude . . . . . . . . . . . . . . . . . .161
Pressure converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99
Pressure dependence of the mean free path . . . . . . . . . . . . . . . . .150, 160
Pressure difference oil supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Pressure lubrication by geared oil pump . . . . . . . . . . . . . . . . . . . . . . . . . .22
Pressure measurement direct / indirect . . . . . . . . . . . . . . . . . . . . . . .76, 189
Pressure measurement, depending on / independent
of the type of gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76
Pressure ranges in vacuum technology . . . . . . . . . . . . . . . .14, 60, 61, 150
Pressure regulation / control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88, 190
Pressure regulation / control rough and medium vacuum systems . . . . .90
Pressure regulation in high and ultra high vacuum systems . . . . . . . . . .92
Pressure regulation, continuous / discontinuous . . . . . . . . . . . . . . . .90, 91
Pressure rise / drop (leak) test . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113, 114
Pressure units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9, 147
PTB (Federal physical-technical institute) . . . . . . . . . . . . . . . . . . . . . . . . .86
Pumpdown time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66 - 71
Pump fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Pump fluid backstreaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Pump fluid change cleaning (diffusion pumps) . . . . . . . . . . . . . . . . . . . .144
Pump oil, selection when handling aggressive vapors . . . . . . . . . . . . . .140
Pump throughput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Pumping (gas sorption) of vacuum gauges . . . . . . . . . . . . . . . . . . . . .83, 84
Pumping chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Pumping of gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62
Pumping of gases and vapors . . . . . . . . . . . . . . .24, 25, 40, 57, 62, 63, 140
Pumping speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Pumping speed units, conversion of . . . . . . . . . . . . . . . . . . . . . . . . . . . .149
Pumping various chemical substances . . . . . . . . . . . . . . . . . . . . . . . . . .141
Purge gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
PVD (physical vapor deposition) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133
pV-flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
pV-value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Quadrupole mass spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96
Quadrupole, design of the sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96
Quadrupole, gas admission / pressure adaptation . . . . . . . . . . . . . . . . . .99
Quadrupole, measurement system (detector) . . . . . . . . . . . . . . . . . . . . . .98
Quadrupole, separating system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .97
Quadrupole, specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
Qualitative gas analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106
Quantitative gas analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .107
Quantity of gas (pV value) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Quartz crystals, shape of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .126
Rate watcher . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .125
Reduction of adsorption capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .144
Reduction ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27, 28, 142
Refrigerator cryopump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54, 56
Regeneration time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58
Relative ionization probability (RIP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103
Residual gas composition (spectrum) . . . . . . . . . . . . . . . . . . . . . . . . .49, 50
Response time of leak detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122
Reynold's number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Rigid envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .123
Roots pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
Roots pumps, Instructions for operation . . . . . . . . . . . . . . . . . . . . . . . . .142
Rotary displacement pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Rotary plunger pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
Rotary vane / piston pumps, Instructions for operation . . . . . . . . . . . . . .140
Index
Rotary vane pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Salt, drying of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71
Saturation vapor pressure (nonmetallic gaskets) . . . . . . . . . . . . . . . . . . .167
Saturation vapor pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9, 26
Saturation vapor pressure (cryogenic technology) . . . . . . . . . . . . . . . . .167
Saturation vapor pressure (metals) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .166
Saturation vapor pressure (pump fluids) . . . . . . . . . . . . . . . . . . . . . . . . .166
Saturation vapor pressure (solvents) . . . . . . . . . . . . . . . . . . . . . . . . . . . .166
Saturation vapor pressure and vapor density of water . . . . . . . . . .152, 170
Sealing passage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20, 21
Seal-off fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75
Selection of pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60
Selection of pumps for drying processes . . . . . . . . . . . . . . . . . . . . . . . . . .71
Sensitivity of quadrupole sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
Sensitivity of vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84
Separating system of mass spectrometers . . . . . . . . . . . . . . . . . . . . . . . .96
Shell baffle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Silicone oils, DC 704, DC 705 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44, 166
Small flange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73
Smallest detectable concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
Smallest detectable partial pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
Smallest detectable partial pressure ratio . . . . . . . . . . . . . . . . . . . . . . . .101
Sniffer technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .123
Software for TRANSPECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .108
SOGEVAC pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .151
Sorption pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50, 186
Specific volume of water vapor . . . . . . . . . . . . . . . . . . . . . . . . . . . .152, 169
Spinning rotor gauge (SRG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81
Spray technique (Helium) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122
Sputter ion pumps, Instructions for operation . . . . . . . . . . . . . . . . . . . . .145
Sputter pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .134
Sputtering (cathode sputtering) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .134
Sputter-ion pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50, 51
SRG (spinning rotor gauge), VISCOVAC . . . . . . . . . . . . . . . . . . . . . . . . . .81
Stability for noble gases (sputter ion pumps) . . . . . . . . . . . . . . . .51, 52, 53
Standard pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Standards in vacuum technology . . . . . . . . . . . . . . . . . . . . . . . . . .178 – 181
Static expansion method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87, 88
Steam ejector pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46
Storage under pressure (bombing test) . . . . . . . . . . . . . . . . . . . . . . . . . .123
Stray magnetic field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52
Stray magnetic field (sputter ion pumps) . . . . . . . . . . . . . . . . . . . . . . . . . .52
Sublimation pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50, 51
Symbols and units, alphabetical list . . . . . . . . . . . . . . . . . . . . . . . .171 – 174
Symbols used in vacuum technology . . . . . . . . . . . . . . . . . . .157, 158, 159
Temperature comparison and conversion table . . . . . . . . . . . . . . . . . . . .160
Temperature in the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .161
Terms and definitions (leak detection) . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Test gas accumulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .124
Test leaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .118
Thermal conductivity vacuum gauge, constant / variable resistance . . . .82
Thermal conductivity vacuum gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . .82
Thermal evaporator (boat) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133
THERMOVAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81
Thickness control with quartz oscillators . . . . . . . . . . . . . . . . . . . . . . . . .125
Thickness measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .125
Thin film controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .125, 131, 191
Throtteling of pumping speed when using condensers . . . . . . . . . . . .39, 40
Time constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67, 121
Titanium sublimation pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51
Titanium sublimation pumps, Instructions for operation . . . . . . . . . . . . . .145
Torr and its conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .147
Total pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Transfer standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87
Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81
TRANSPECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96
Triode sputter ion pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52
TRIVAC pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Trochoid pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
Tuning / adjustment and calibration of leak detectors . . . . . . . . . . . . . . .118
Turbomolecular pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46, 185
Turbomolecular pumps, Instructions for operation . . . . . . . . . . . . . . . . . .143
TURBOVAC pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
Turbulent flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Types of leak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110
Types pV flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
UL 200 dry, UL 500 dry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .118
UL 200, UL 500 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .119
Ultimate pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Ultra high vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14, 16, 65, 66, 188
ULTRALEN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45, 160
Units, symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .171 – 177
U-tube vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79
Vacumm meters, instructions on installing . . . . . . . . . . . . . . . . . . . . . . . .145
Vacuum coating techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .135
Vacuum control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76
Vacuum equipment, Instructions for operation . . . . . . . . . . . . . . . . . . . . .139
Vacuum gauge contant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84
Vacuum method (leak detection) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113
Vacuum physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Vacuum pumps, literature references . . . . . . . . . . . . . . . . . . . . . . . . . . .182
Vacuum pumps, survey, classification . . . . . . . . . . . . . . . . . . . . . . . . .19, 20
Vacuum ranges (Pressure ranges) . . . . . . . . . . . . . . . . . . . . . . . . . . .16, 59,
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .61, 150, 167, 168
Vacuum regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76
Vacuum symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .157, 158, 159
Vacuum coating technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133
Values of important physical constants . . . . . . . . . . . . . . . . . . . . . . . . . .149
Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73, 188
Van der Waals' equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Vapor density of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .152, 170
Vapor pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9, 43, 166, 167, 170
Vapor-jet pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41, 43, 46, 144
Venturi nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
Viscous (continuum) flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
VISCOVAC vacuum gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81
Vitilan, Viton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73, 156, 167
Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Volumetric efficiency (roots pumps) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
Volumetric flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Water jet pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45
Water ring pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Water vapor tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
Web coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .135
Wet processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62
Working pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Working ranges of vacuum pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . .167
X-ray effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .85
XTC, XTM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .131
Zeolith . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50
Z-Match technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127
197
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