Series 945 OPERATION AND MAINTENANCE MANUAL

Series 945  OPERATION AND MAINTENANCE MANUAL
Series 945
Digital Pirani
Vacuum Sensor System
OPERATION AND
MAINTENANCE MANUAL
Series 945
Digital Pirani
Vacuum Sensor System
Part #100014430
Digital Pirani Vacuum Sensor System
Part # K__________________
Serial # __ __ __ __ __ __
Please fill in these numbers and have
them readily available when calling for
service or additional information.
(The part number can be found on your
packing slip, and the serial number is
located on the back panel of the
controller.)
For more information or literature, contact:
Kurt J. Lesker Company
1925 Route 51
Clairton, PA 15025-3681 USA
Phone:
412-387-9200
800-245-1656
FAX:
412-384-2745
© 1996 by the Kurt J. Lesker Company. All rights reserved.
Teflon is a registered trademark of DuPont Co.
VCR ®is a registered trademark of Swagelok® Company.
Digital Pirani Vacuum Sensor System
Table of contents
Table of contents
Package Contents ..................................................................... 1
Symbols Used in this Manual .................................................... 2
Safety Precautions .................................................................... 3
Specifications ............................................................................ 5
Controller ................................................................................................. 5
Sensor Tube ............................................................................................ 6
Feature and Control Locations .................................................. 7
Typical Applications for the Series 945 Pirani System .............. 8
About Series 945 Pirani Vacuum Sensor System ...................... 9
Installing the Series 945 Pirani System ................................... 10
Pirani Sensor Installation .......................................................................10
Location ........................................................................................... 10
Orientation ....................................................................................... 10
Contamination .................................................................................. 10
Vacuum Connection ......................................................................... 10
Pirani Controller Installation ....................................................................11
Controller Mounting ..........................................................................11
Connecting the Sensor to the Controller ........................................... 12
AC Power Cord ................................................................................12
Fuse Replacement ........................................................................... 13
Accessory Connector .......................................................................13
Relay Inductive Loads and Arc Suppression ...................................14
Operating the Series 945 Pirani System.................................. 15
Reading Pressure ...................................................................................15
Sensor Adjustment ................................................................................. 16
Calibrating for Zero and Atmosphere ...................................................... 17
Adjusting the Set Point ........................................................................... 17
Venting to Atmosphere ........................................................................... 17
Buffered Analog Output ..........................................................................18
Use with Computers ............................................................................... 18
Leak Detection ....................................................................................... 18
Digital Pirani Vacuum Sensor System
Using the Series 945 Pirani System with Other Gases............ 20
Nitrogen Equivalent Pressure ................................................................. 20
Calibrating for Other Gases .................................................................... 21
Maintaining the Series 945 Pirani System ............................... 22
Troubleshooting and Service ................................................................... 22
Cleaning the Pirani Controller Front Panel .............................................. 22
Cleaning the Pirani Sensor ..................................................................... 23
Testing the Pirani Sensor ....................................................................... 23
Accessories ............................................................................. 24
Notes ....................................................................................... 25
Notes ....................................................................................... 26
Appendix A: How the Series 945 Pirani System Works .......... A.1
Theory of a Thermal Conductivity Sensor ............................................. A.1
Measurement Limits ............................................................................. A.1
The Series 945 Pirani System Design Overview .................................. A.2
Amplifier ........................................................................................ A.2
Output ............................................................................................ A.2
Set Point Relays ............................................................................ A.2
Comparator .................................................................................... A.2
Notes ...................................................................................... A.3
Digital Pirani Vacuum Sensor System
Package Contents
Before unpacking your Series 945 Digital Pirani Vacuum Sensor System, check
all surfaces of the packing material for shipping damage.
Please be sure that your Series 945 Pirani System package contains
these items:
1 Series 945 Pirani Controller
1 female, 9-pin, subminiature D (D-sub) Accessory connector
1 power cord
1 Series 945 Digital Pirani Vacuum Sensor System User's Manual.
The Series 945 System's Pirani Sensor and its connecting cable are sold
separately. Please refer to page 24 for necessary ordering information.
If any items are missing from the package, call Kurt J. Lesker
Company Customer Service at 1-412-387-9200 or 1-800-245-1656.
Inspect the Series 945 Pirani System for visible evidence of damage. If it has
been damaged in shipping, notify the carrier immediately. Keep all shipping
materials and packaging for claim verification. Do not return the product to
Kurt J. Lesker Company.
Digital Pirani Vacuum Sensor System
1
Symbols Used in this Manual
The first two symbols below, located on your Series 945 Controller and/or
Sensor, identify critical safety concerns. They are used throughout this
manual to further define the safety concerns associated with the product.
The last two symbols identify other information in this manual that is
essential to highlight or useful in achieving optimal performance from your
Series 945 Pirani System.
CAUTION: Risk of electrical shock.
CAUTION: Refer to manual.
Failure to heed message could result in personal injury
or serious damage to the equipment or both.
Failure to heed message could result in damage to the
equipment.
Information calling attention to important procedures,
practices, or conditions.
2
Digital Pirani Vacuum Sensor System
Safety Precautions
Do not use the Series 945 Pirani System's Sensor with
explosive gas mixtures or gases that are combustible in air.
If the control circuit fails or an exothermic catalytic reaction occurs at the
sensor wire, the wire could ignite the gas mixture.
When using the Series 945 Pirani System with gases other
than air or nitrogen, avoid overpressure to prevent any
possibility of explosion.
Since the Sensor is calibrated for air/nitrogen, the measured pressure of
another gas will differ from its true pressure. Calibrations diverge widely at
higher pressures. Since the different gases are not linearly related, a gas
correction factor could lead to hazardous overpressure. Capacitance
manometers and mechanical dial gauges are best suited for higher pressures.
Refer to page 20 for use with other gases.
In a system which is filled from a pressurized source, install a safety relief
valve or burst disc to safely limit pressure.
Turn off power to the Controller before connecting or
disconnecting a cable to the Sensor.
Connecting or disconnecting cables while the Controller is on may damage
the Sensor.
Properly ground the Controller.
The Controller is grounded through the conductor of the power cord. If the
protective ground connection is lost, all accessible conductive parts may
pose a risk of electrical shock. Plug the cord into a properly grounded
outlet only.
When applying a voltage or current to the ports on the rear of the panel, do
not exceed manufacturer's specifications. Electrical shock or Controller
damage may result.
Allow only qualified technicians to service the Series 945
Pirani System.
Users should not remove covers, casings, or plug-in components. Injury may
result. Any part replacement or internal adjustments must be performed by a
qualified technician only.
Digital Pirani Vacuum Sensor System
3
The Series 945 Pirani Controller contains lethal voltages
within the casing.
Avoid danger of electrical shock. Unplug the power cord before
opening the casing.
To prevent damage to the Series 945 Pirani Controller,
use the correct line voltage.
Use the Controller with the specified line voltage only. Use a fuse of the
correct type, voltage, and current rating. Only qualified personnel should
change line voltage or fuse ratings if required.
Outside of the U.S., use only a detachable cord set that has conductors with
a cross-sectional area equal to or greater than 0.75 mm2. Power cables
should have agency approval, for example, VDE, Semko, or SEV.
The Series 945 Pirani System and all its accessories must be used as
instructed by Kurt J. Lesker Company, to ensure safe operation. Use or
modification of the equipment in a manner not consistent with
specifications may render the product's inherent protection useless.
4
Digital Pirani Vacuum Sensor System
Specifications
Controller
Measuring Range
1.0 x 10-4 to 1.0 x 10+2 Torr
1.0 x 10-1 to 1.0 x 10+5 microns
1.0 x 10-4 to 1.3 x 10+2 mbar
Useful Set Point Range
2.0 x 10-3 to 9.9 x 10+1 Torr
2.0 to 9.9 x 10+4 microns
2.7 x 10-3 to 1.3 x 10+2 mbar
Resolution
1% of decade, 10% of decade for
pressures less than 10-3 Torr
Operating Temperature Range
5° to 40°C (41° to 104°F)
Storage Temperature Range
-10° to 55°C (14° to 131°F)
Operating Humidity
80% maximum (noncondensing)
Insulation Coordination
Installation (Overvoltage) Category
II, Pollution Degree 2
Number of Channels
1
Front Panel Controls
2 nonvolatile, independent relay set
points; power on-off switch; zero
adjustment; atmospheric
adjustment
Relay Contact Rating
SPDT, 1 A @ 24 VAC/DC, resistive
Relay Response
150 msec maximum
Power Requirement
90 to 130 VAC, 50/60 Hz
or
210 to 250 VAC, 50/60 Hz
Power Consumption
9W
Output Voltage
0.20 to 3.25 VDC, buffered
analog, 1 k ohms maximum output
impedance
Fuse Rating, Size
T 0.16 A, Ø 5 mm x 20 mm
for 90 to 130 VAC;
T 0.063 A, Ø 5 mm x 20 mm
for 210 to 250 VAC
Digital Pirani Vacuum Sensor System
5
Display
LED with 2 significant digits
(1 leading) and 1 signed exponent;
red, 3/5"-high, 7-segment digits; LED
set point indicators; units shown in
either Torr, mbar, Pascal, or microns
Electronic Casing
Aluminum, anodized
Dimensions
(W x D x H)
3¾" x 7" x 3¾"
(96 mm x 178 mm x 96 mm)
Size
¼ DIN
Weight
2.2 lb (1.0 kg)
Sensor Tube
Calibration Gas
Air/nitrogen
Installation Orientation
Any
Reproducibility
5% of indicated pressure @ Tconst
Materials Exposed to Vacuum
SS 304, platinum, alumina ceramic,
silver brazing alloy, nickel 200
Internal Volume
0.5 in.3 (8.0 cm3) maximum
Operating Temperature Range
0° to 50°C (32° to 122°F)
Maximum Bakeout Temperature
50°C (122°F)
Diameter
1.3 in. (34 mm)
Length
4.4 in. (112 mm)
Typical Weight (with KF Flange)
0.5 lb (0.2 kg)
Vacuum Connection
KF 16
1
/8" NPT-M
with ½" compression seal option
8 VCR®-F (½")
11/3" CF (non-rotatable)
2¾" CF (non-rotatable)
Design and/or specifications subject to change without notice.
6
Digital Pirani Vacuum Sensor System
Feature and Control Locations
Controller
1
14
Front View
Rear View
Side View
Sensor
1
z
2
z
3
z
4
z
5
z
6
z
7
z
8
z
9
z
10
z
11
z
12
z
13
z
14
z
Digital LED Display
Power On-Off Rocker Switch
View Set Point Push-buttons
Set Point Adjustment Potentiometers
LED Set Point Indicators
Atmospheric Adjustment Potentiometer
Zero Adjustment Potentiometer
Power Cord Inlet w/ Fuse Holder
Panel Mounting Fastener Holes
Female, 9-pin Gauge Port
Male, 9-pin Accessory Port
Slots (for Optional Mounting Brackets)
Male D-sub
Sensor Vacuum Port
Digital Pirani Vacuum Sensor System
7
Typical Applications for the
Series 945 Pirani System
Measuring foreline and roughing pressures generated by
mechanical vacuum pumps
Controlling valves and pumps to automate system pump down
using the relay set points
Sensing abnormal pressure and taking appropriate security
measures using the relay set points
Controlling system pressure using the analog output as input to
an automatic pressure controller
Starting or stopping system processes using the relay set
points
Measuring pressures of backfilled gases in the range of 10-4 to
10+2 Torr
Activating high vacuum sensors at the high end of their
operating pressure range
8
Digital Pirani Vacuum Sensor System
About the SensaVac® Series 945
Pirani Vacuum Sensor System
The Series 945 Pirani System, together with its Sensor and
connecting cable, provides accurate and reliable data for processes
which need pressure measurement from 10 +2 Torr down to 10 -4 Torr. It
is low in cost and easy to use.
The Pirani Sensor is available
with one of several standard
fittings for vacuum connection.
The Sensor connects to the
Controller with a cable that has a
female octal socket on one end
and a male, 9-pin D-sub
connector on the other. The
Controller easily mounts into a ¼
DIN panel cutout or stands alone.
All controls, even the power
switch, are conveniently
accessed from the front panel,
and cable connections are hidden
to the rear of the Controller.
The Series 945 Pirani Controller
is useful either as a small system
controller or as a module in more
sophisticated pressure control
environments. The Controller
features two standard relay set
points for process applications.
The pressure readout is a large,
easy-to-read digital display. LEDs
indicate the status of relay set
points, which are independently adjusted using both a push-button and
a potentiometer on the front panel.
The analog output and relay contacts are accessible from the
Accessory port on the rear panel.
The Sensor can be located away from the Controller using a cable as
long as 500 feet in length.
Digital Pirani Vacuum Sensor System
9
Installing the
Series 945 Pirani System
Pirani Sensor Installation
Location
Locate the Pirani Sensor where it can measure chamber or manifold pressure.
Install it away from pumps and gas sources and where vibration is minimal to
give the most representative values.
Orientation
Since a Pirani Sensor is designed to minimize convection, operating position
does not affect accuracy. In a standard Pirani system, the atmospheric
pressure reading of a horizontal sensor versus a vertical one differs negligibly
due to residual convection.
The Sensor is factory calibrated vertically with the vacuum port facing down.
Contamination
Locate and orient the Pirani Sensor to avoid contamination which would affect
the tube's element. For example, if the Sensor is installed directly above a
roughing pump, oil vapor could contaminate the tube's filament wire and
cause the calibration to shift.
Install the Sensor with the vacuum port facing down whenever possible to
prevent particulates falling or liquids flowing into it. Using a screen or porous
filter at the port is helpful.
Vacuum Connection
Standard vacuum interfaces for the Pirani Sensor include:
KF 16 flange
8 VCR®-F (½") fitting
11/3" CF flange
2 ¾" CF flange
1
/8" NPT-M fitting with a ½" compression seal
To fit the Sensor with the 1/8" NPT, do not use the case for tightening. The
Sensor’s vacuum tubing has 9/16" hex flats for tightening. Wrap a layer of
Teflon® tape on the threads of the tubing, in the direction of installation, to
ensure a leak-free seal.
Instead of the NPT fitting, you may opt to use the ½" O-ring compression
seal against the vacuum tubing above the thread, but the O-ring seal cannot
be used for positive pressure applications.
10
Digital Pirani Vacuum Sensor System
Ensure a solid electrical connection between the Sensor
and the grounded vacuum system to shield the tube
element from external electromagnetic sources. In
applications which the System will be exposed to large
voltage fluctuations, install a centering ring with a screen,
and then ground the screen and tubing.
Pirani Controller Installation
Controller Mounting
The Series 945 Pirani Controller is designed for either panel mounting or for
stand-alone use. Regardless of the method you choose, assure adequate
ventilation to the Controller with at least 1 inch left open above and below the
perforated panels.
An optional hardware kit, shown below, is also available for mounting the
Controller into a standard ¼ DIN cutout in a panel up to 3/16-inch thick (see
Accessories, p.24). Leave at least 3 inches of clearance behind the
Controller to accommodate the connectors and cables.
Mount the Controller into the panel by slipping it through the cutout from the
front. Slide the panel mounting brackets into the slots on either side of the
Controller from the rear, and secure them using the thumbscrews provided.
Adhesive backed rubber feet for bench top use are provided with the
mounting kit. Remove the adhesive backing from each foot and apply one to
each corner of the aluminum bottom surface.
Digital Pirani Vacuum Sensor System
11
Connecting the Sensor to the Controller
A sensor cable with a standard D-sub (see Accessories, p. 24) is required for
operation but is purchased separately from the Pirani System.
Cables are available in standard lengths of 10, 25, 50, and 100 feet and in
custom lengths up to 500 feet.
Connect the cable to the rear of the Controller at the Gauge port. Tighten the
connector jackscrews into the mating screw locks to ensure proper electrical
connection and to prevent stress on the connectors.
Turn off the Controller before connecting or disconnecting the
cable from the Sensor or Controller.
However, it is not possible for this preventative feature to always work, so
avoid any possible damage by always turning off the Controller before any
connections are made.
In applications for which special cables are required, i.e., where the
connection must be routed through restrictive barriers or through a conduit,
customize a cable using the figure shown on the following page. Use a D-sub
connector with jackscrews for strain relief in all applications.
AC Power Cord
The Series 945 Pirani System includes a standard, 120 VAC, 50/60 Hz power
cord with a female IEC-320 connector.
If the available power source or connection is different, use only a detachable
cord set with conductors that have a cross-sectional area equal to or greater
than 0.75 mm2. The power cable should be approved by a qualified agency
such as VDE, Semko, or SEV.
12
Digital Pirani Vacuum Sensor System
4UBE
#ONTROLLER
#USTOMIZINGASENSORCABLE
Fuse Replacement
The Series 945 Pirani System has a combined fuse holder and power
outlet located on the rear panel of the Controller. Replace the fuse with the
following steps.
1. Unplug the power cord from its outlet.
2. Snap out the fuse holder drawer.
3. Replace the fuse(s) with one of the following time-lag fuses:
T 0.16 A (Ø 5 mm x 20 mm) – 90 to 130 VAC
T 0.063 A (Ø 5 mm x 20 mm) – 210 to 250 VAC.
4. Close and secure the fuse holder drawer.
Accessory Connector
Relay set point contacts and the analog output voltage can be accessed from
the Accessory port on the rear panel of the Controller. A connector kit to mate
to the port is provided. The figure and chart on the next page identify the pin
functions of the Accessory port.
Do not inadvertently short circuit the set point relay terminals to
the analog output voltage.
Digital Pirani Vacuum Sensor System
13
Pin
1
2
3
4
5
6
7
8
9
Description
Set point relay 1 - normally closed contact
Set point relay 1 - normally open contact
Set point relay 2 - common
Not used
Analog output voltage (+)
Set point relay 1 - common
Set point relay 2 - normally closed contact
Set point relay 2 - normally open contact
Analog output voltage (-)
1
5
6
9
9-pin Accessory port
Relay Inductive Loads and Arc Suppression
If the set point relay is used to switch inductive loads, e.g., solenoids, relays,
transformers, etc., the arcing of the relay contacts may interfere with
Controller operation or reduce relay contact life. Therefore an arc suppression
network, shown schematically below, is recommended. The values of the
capacitance C and the resistance R are calculated by the equations,
C = I 2/(1 x 107) and R = E/10Ia
where,
C is in farads
R is in ohms
I is DC or ACpeak load current in amperes
E is DC or ACpeak source voltage in volts
a = 1 + (50/E).
Note that,
Rmin = 0.5 Ω and Cmin = 1.0 x 10-9 F.
14
Digital Pirani Vacuum Sensor System
E
Operating the Series
945 Pirani System
Reading Pressure
The Sensor must be connected to the Controller before turning
on the unit.
Turn the power on to display pressure or to operate the set point relays. The
table below shows the Series 945 Pirani System’s voltage output as a
function of pressure for nitrogen, argon, and helium, and the graph on the next
page gives the same information.
Since Pirani systems are gas dependent and the Series 945 Pirani System is
calibrated for air/nitrogen, it reads in nitrogen equivalent pressure for argon,
helium, or any other gas. The table and graph do not give true pressure of
gases other than nitrogen. Refer to page 20, Using the Series 945 Pirani
System with Other Gases, to determine true pressure.
In the measuring range of 1.0 x 10-4 to 1.0 x 10-3 Torr, only one significant
digit will be displayed, e.g., 2 -4 . At pressures above 100 Torr, the Series
945 System's upper measuring limit, the display will show.
H1 At
pressures below 1x10-4 The display should show LO x 10-4 (See page 17).
Series 945 Pirani System
Output Voltage vs. Pressure
Torr
1.0E-04
5.0E-04
1.0E-03
2.5E-03
5.0E-03
7.5E-03
1.0E-02
2.0E-02
3.0E-02
4.0E-02
5.0E-02
6.0E-02
7.0E-02
8.0E-02
9.0E-02
1.0E-01
2.0E-01
3.0E-01
4.0E-01
Pascal
1.3E-02
6.7E-02
1.3E-01
3.3E-01
6.7E-01
1.0E+00
1.3E+00
2.7E+00
4.0E+00
5.3E+00
6.7E+00
8.0E+00
9.3E+00
1.1E+01
1.2E+01
1.3E+01
2.7E+01
4.0E+01
5.3E+01
Voltage
Torr
Nitrogen
Argon
Helium
0.211
0.217
0.225
0.246
0.278
0.307
0.333
0.420
0.491
0.552
0.606
0.655
0.700
0.742
0.781
0.818
1.103
1.306
1.464
0.211
0.215
0.220
0.234
0.255
0.275
0.294
0.358
0.411
0.458
0.500
0.539
0.574
0.607
0.638
0.667
0.896
1.060
1.189
0.211
0.217
0.224
0.243
0.273
0.300
0.324
0.407
0.476
0.536
0.589
0.638
0.684
0.726
0.766
0.804
1.112
1.347
1.543
Pascal
Voltage
Nitrogen
5.0E-01
6.0E-01
7.0E-01
8.0E-01
9.0E-01
1.0E+00
2.0E+00
3.0E+00
4.0E+00
5.0E+00
6.0E+00
7.0E+00
8.0E+00
9.0E+00
1.0E+01
2.5E+01
5.0E+01
5.0E+01
1.0E+02
6.7E+01
8.0E+01
9.3E+01
1.1E+02
1.2E+02
1.3E+02
2.7E+02
4.0E+02
5.3E+02
6.7E+02
8.0E+02
9.3E+02
1.1E+03
1.2E+03
1.3E+03
3.3E+03
6.7E+03
6.7E+03
1.3E+04
Digital Pirani Vacuum Sensor System
1.593
1.702
1.796
1.877
1.949
2.013
2.411
2.609
2.729
2.810
2.868
2.911
2.945
2.973
2.995
3.127
3.172
3.172
3.193
Argon
1.295
1.385
1.462
1.530
1.590
1.643
1.976
2.143
2.245
2.313
2.362
2.399
2.428
2.452
2.471
2.583
2.622
2.622
2.638
Helium
1.713
1.865
2.003
2.129
2.246
2.355
3.174
3.723
4.133
4.456
4.718
4.937
5.122
5.282
15
He
4
N2
3
Output Voltage (VDC)
Ar
2
1
0
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
Pressure (Torr)
When using the graph, remember that the pressure scale is
logarithmic, and the voltage scale is linear. Equal increments of
distance along the pressure scale do not correspond to equal
pressure changes.
Sensor Adjustment
Series 945 Pirani controllers and their sensors are calibrated at manufacturer.
However, instances in which user-calibration will be required are:
After installation on your system, the controller and sensor may need
to be calibrated as a unit. They are not calibrated as a unit at the
factory. From sensor to sensor, readings may differ.
If the sensor becomes contaminated with process chemicals or
pump oil, the calibration may shift.
16
Digital Pirani Vacuum Sensor System
1.00E+01
1.00E+02
Calibrating for Zero and Atmosphere
Vacuum and atmosphere adjustment potentiometers are located on the front
panel as shown below. The two adjustments can be made independently, but
because of some interaction between them, we recommend making the
adjustments in the order below.
For best results, let the Series 945 Pirani System remain at the
calibration pressure for at least 15 minutes before adjusting
the potentiometers.
1. To adjust for atmosphere, an accurate,
high pressure reference sensor is required.
With the reference sensor at a known
pressure (e.g., 50 to 90 Torr), adjust the Atm
potentiometer on the Series 945 Controller
until the display indicates the correct
pressure.
Atmosphere
Adjust
Zero
Adjust
2. To adjust for vacuum, the Pirani Sensor
needs to be on a vacuum system pumped
down to less than 1.0 x 10-5 Torr. At this
pressure, the Series 945 Controller display
should read L0 -4 Torr ( L0 -1 for
microns, L0 -4 for mbar, or L0 -2 for
Pa). If the display shows TL -4 Torr, turn
the Vac potentiometer clockwise until L0 -4
Torr is displayed. If the reading is 1 -4
Torr or higher, turn the Vac potentiometer
counterclockwise until L0 -4 Torr is
displayed.
Adjusting the Set Point
To adjust a set point to open or close the relay contact at a particular
pressure, press the view button for the desired set point on the front panel.
Keeping this button depressed, use a small screwdriver to adjust the
corresponding potentiometer until the reading coincides with the desired set
point pressure.
When an LED is on, the measured pressure is below the set point value, the
normally open relay contact is closed, and the normally closed contact is
open. When an LED is off, the measured pressure is above the set point
value, the normally open relay contact is open, and the normally closed
contact is closed.
To prevent the relays from turning on at any pressure, adjust the trim pots
counter clockwise until nc is displayed.
Venting to Atmosphere
The Pirani Sensor senses pressure by measuring heat loss from a heated
fine wire. Sudden venting at its port can greatly stress the sensor physically.
Digital Pirani Vacuum Sensor System
17
To avoid damage to the Sensor, vent the vacuum system to
atmosphere before removing it.
Buffered Analog Output
The Series 945 Pirani Controller's analog output voltage is buffered. The
temperature compensated signal from the Sensor is buffered by a unity gain
amplifier, then connected to the Accessory port. This signal has an amplitude
of approximately 0.2 V in vacuum to 3.25 V at atmosphere and follows the
voltage-versus-pressure curve shown on the previous page. Since this signal
is not digitally processed, the Series 945 Pirani System is suitable for use in
applications in which measurement speed is important.
Use with Computers
The Series 945 Pirani System is designed to operate in highly automated
systems (especially those controlled by digital computers), as well as in
manual systems. It is compatible with a wide variety of computers,
interfaces, and software.
In order to take full advantage of the System’s capabilities, an analog-todigital (A/D) converter should be used with an input voltage span of 0 to 5 V.
The A/D conversion rate should be at least 6 Hz (150 msec conversion
time). A resolution of 12 bits is needed, corresponding to 1.22 mV per bit. The
A/D converter should have a differential input to prevent ground currents from
affecting the measurements.
Leak Detection
Its inherent sensitivity to gas type makes the Series 945 Pirani System
useful for detecting leaks, at rates greater than 10-4 std cc/sec of helium, in
foreline and roughing systems. It is a useful complement to a mass
spectrometer leak detector, which locates smaller leaks.
1) Pump your vacuum system to a base pressure.
2) Slowly and methodically probe with a small amount of gas.
Flooding the leak or moving quickly past the leak can confuse
the search since system time lags may be significant.
3) Note the pressure reading.
The pressure will either rise or fall, depending upon the thermal conductivity
of the probe gas relative to the system gas. The largest change in value
indicates the probe gas is nearest the leak location.
4) Repeat the test to confirm.
As with any leak testing, many factors can influence the sensitivity of the
test. Explained in further detail below, these include chamber volume; system
pressure; probe gas; type of vacuum pump; location of the Sensor, leak, and
pump; and others such as pumping speed and size of tubing in the system.
18
Digital Pirani Vacuum Sensor System
Minimize the chamber volume to reduce the search area and
increase the efficiency of the test.
Sensitivity to gas leaks is also pressure dependent due to the
complex nature of heat transfer at pressures above 10-1 Torr and the
presence of significant radiation and end losses below 10-3 Torr. In
general, leak test sensitivity is greater for lower system pressures.
The Series 945 Pirani System is sensitive to any leak probe gas
lighter or heavier than the system gas, however, for optimal
sensitivity, select a probe gas with the largest difference between its
molecular weight and that of the system gas. Lighter gases increase
thermal energy transfer while heavier gases reduce it. Helium or
argon gas is suitable to probe a system pumping air or nitrogen.
The type of vacuum pump used can also affect the accuracy of the
leak test. For moderate size leaks, pump down the system with a
high vacuum pump such as a diffusion or turbo pump if possible (ion
and cryo pumps are not recommended). Leak testing can be done
with a mechanical pump, however, cyclical variations in pressure
may result due to rotation of the vanes. This shows up as a large
background noise signal, possibly masking the leak signal.
Place the pump away from the suspected leak source and place the
Sensor between the leak and the pump to reduce the Sensor
response time. Vacuum tubing between the suspected leak and the
Pirani Sensor should be as short and wide as possible to shorten the
time required for the probe gas to reach the tubing.
If the above detection method does not locate a leak, unexpected high
pressures may be caused by a virtual leak, i.e., outgassing of a system
component. You can locate outgassing parts, or "virtual leaks", as well as
true gas leaks, using the rate-of-pressure-rise method.
1) With the Controller on, pump down the system to a base pressure.
2) Isolate the pump by closing a valve.
3) Measure the rise in pressure over a time interval. A very fast rise
indicates a leak.
4) Repeat this procedure as often as necessary.
Digital Pirani Vacuum Sensor System
19
Using the Series 945 Pirani
System with Other Gases
Before using the Series 945 Pirani System to measure
pressure of gases other than air or nitrogen, you should
read and understand this section. To answer further
questions, contact Applications Engineering at Kurt J. Lesker at
1-412-387-9200 or 1-800-245-1656
The Series 945 Pirani System is designed to give voltage output for air/
nitrogen according to the graph on page 16 or the table on page 15. If the
Pirani System is used to read pressure of gases with poorer heat transfer
properties than nitrogen, the true system pressure may be much higher than
indicated. This reading error could lead to dangerous overpressure.
A system which is backfilled from a pressurized gas source
should contain a safety device, such as a burst disc.
Nitrogen Equivalent Pressure
The thermal loss from a heated sensor element is a function of the
transporting gas (see page A.1, Theory of a Thermal Conductivity Sensor.)
Since the Series 945 Pirani System's Sensor is such a gauge, the voltage
output depends upon the gas measured.
1. Locate the indicated pressure on the
pressure axis of the graph on page 16 (as
shown in the example to the right). Locate the
point at which a vertical line from this point
intercepts the curve for N2.
2. Locate the point at which a horizontal line
from the point on the N2 curve intercepts the He
or Ar curve.
3. Locate the point at which a vertical line from
the He or Ar curve intercepts the pressure axis.
This point corresponds to the true pressure for
He or Ar.
20
Digital Pirani Vacuum Sensor System
Output Voltage
(VDC)
When used with gases other than nitrogen (N2), the System reads in nitrogen
equivalent pressure. For example, when the Sensor is set up to read pressure
for N2 but is used with the gases helium (He) or
argon (Ar), the data for He and Ar in the graph
on page 16 are used to convert nitrogen
equivalent pressure to true pressure.
4
He
#2
3
N2
#3
Ar
2
1
#1
0
10-4
10-3
10-2 10-1
1
Pressure
(Torr)
10+1
10+2
The nitrogen equivalent pressures of some gases can exceed
the measuring capabilities of the Series 945 Pirani Controller.
For example, a true pressure of 6.0 Torr of helium is equivalent
to a nitrogen equivalent pressure of several 1000 Torr. The
Series 945 Pirani Controller is not capable of measuring this
pressure of helium.
Calibrating for Other Gases
To determine the voltage/pressure relationship for gases which are not shown
in the graph or table, you need to calibrate the Series 945 Pirani System with
this gas. This calibration requires a gas-independent sensor, such as a
capacitance manometer, to act as a standard. A curve like that of page 16
can then be generated and used as described above.
The Series 945 Pirani Controller is not intended to be calibrated
for direct pressure readings of gases other than air or nitrogen.
The calibration is intended only to match the air/nitrogen curve
of the Controller to the characteristic of a particular sensor.
Digital Pirani Vacuum Sensor System
21
Maintaining the
Series 945 Pirani System
Troubleshooting and Service
The Series 945 Pirani Controller is designed to be maintenance-free under
normal operation. If a problem should occur, the following chart lists
symptoms, possible causes, and their remedies. With this guide, you should
be able to diagnose some problems and correct them. Those which fall
outside the scope of this chart are generally not serviceable by the user, and
the unit should be returned for repair.
Cleaning the Pirani Controller Front Panel
The front panel of the Pirani Controller is designed to resist many laboratory
solvents. The casing can be cleaned with water or alcohol.
Do not use acetone on the front panel.
Troubleshooting Chart
Symptom
Pressure readings are too
high or too low.
No analog output voltage,
and no display.
No analog output voltage,
but set points functional.
Possible Cause
Remedy
2. Gas other than air is used.
3. Controller incorrectly calibrated.
1. Test and replace if necessary.
Refer to page 23.
2. Refer to page 20.
3. Refer to page 17.
1. Power supply turned off.
2. Fuse is blown.
3. Defective PC board.
1. Turn power on.
2. Replace fuse.
3. Return to KJL for repair.
1. Analog output shorted to ground.
1. Check cable connections at
Accessory port.
2. Connect cable to its port.
1. Contaminated Sensor.
2. Accessory or Gauge connector
disconnected.
3. Defective PC board.
3. Return to KJL for repair.
Set point relay will not
operate.
1. Set point voltage incorrectly set.
2. Broken or shorted Sensor filament.
3. Defective PC board.
1. Check set point.
2. Test and replace if necessary.
Refer to page 23.
3. Return to KJL for repair.
Display shows "nc"
1. Cable or sensor not connected
2. Sensor filament broken
1. Connect properly (page 12)
2. Replace sensor (page 23)
22
Digital Pirani Vacuum Sensor System
Cleaning the Pirani Sensor
Roughing pump oils and other fluids condensing or decomposing on the
heated filament can contaminate the Sensor. This changes the emissivity of
the filament, which in turn can cause the calibration to change, especially at
low pressure.
It is not advisable to clean the Sensor. Trying to clean it
may either deform or break the filament, and the deformed
filament would then cause additional error from a shift in
the Sensor's output.
Replace the Sensor if it becomes contaminated (see Accessories, p.24).
Testing the Pirani Sensor
This test is for function only. Slight Sensor damage by
contamination or rough handling can affect calibration, but the
tube may still be functional.
The most common cause of Sensor failure is a broken filament (checked
from pin 4 to pin 6). This could be due to physical abuse or sudden venting of
the Sensor to atmosphere.
Test the Sensor with an ohmmeter with less than 5 mA of current. The
resistance readings of a normal Series 315/345 Sensor measured at
atmospheric pressure and at room temperature (20°C) are shown here.
345 D-sub
Pin Numbers
4 to 7
4 to 8
6 to 7
6 to 8
5 to 6
3 to 5
Resistance (Ω)
39
114
31
114
62
345
Digital Pirani Vacuum Sensor System
23
Accessories
Part #
Cables for the Pirani Sensor
10 ft ( 3.0 m)
25 ft ( 7.6 m)
50 ft (15.2 m)
K31706S
K31707S
K31708S
Mounting Hardware Kit
K5021
Pirani Sensors
KF 16
1
/8" NPT-M with ½" Compression Seal Option
8 VCR®-F (½")
11/3" CF (Non-rotatable)
2 ¾" CF (Non-rotatable)
NW 25
K34510
K34511
K34512
K34513
K34514
K34515
Series 945 Digital Pirani Vacuum Sensor System
User's Manual
100014430
Be sure to have on hand the serial number and flange type when ordering
replacement parts.
Please call Kurt J. Lesker Customer Service Department at 1-412-387-9200
or 1-800-245-1656 to order any of these accessories or to receive catalogs for
other Kurt J. Lesker products.
24
Digital Pirani Vacuum Sensor System
Notes:
Digital Pirani Vacuum Sensor System
25
Notes
26
Digital Pirani Vacuum Sensor System
Appendix A: How the Series 945
Pirani System Works
The Series 945 Pirani System is a heat-loss manometer which infers the
pressure of a gas by measuring thermal loss from a heated wire.
Theory of a Thermal Conductivity Sensor
A hot wire suspended from supports in a partial vacuum loses thermal energy
in three ways:
gas transport, which is pressure dependent,
end loss to the supports, and
radiation to surrounding surfaces.
Pirani and thermocouple sensors use pressure-dependent gas transport from a
hot wire to measure pressure. Because end loss and radiation are constant for
a wire at constant temperature, they provide a masking signal which largely
determines the low pressure limit of the sensor. Optimizing parameters of the
wire length and diameter, thermal emissivity, thermal conductivity, and wire
temperature can control end loss and radiation but not eliminate these terms.
A Pirani sensor may be operated at constant current, voltage, or resistance
(equivalent to temperature) at the sensor wire. The Series 945 Pirani System's
Sensor is operated at constant temperature to increase high pressure
sensitivity. At constant current or voltage, the wire temperature at high
pressure is much less than the temperature value at vacuum, reducing the
high pressure sensitivity.
Measurement Limits
At 10-3 Torr, the sum of end and radiation losses is about 10 times the energy
lost to the gas through conduction. This determines the practical lower limit for
thermal conductivity sensors.
At pressures between 20 to 200 Torr, the gas acts as an insulation layer
around the filament, which reduces the sensitivity of a Pirani sensor.
At pressures above 200 Torr, only convection currents contribute to energy
transport from the filament. Convection currents are extremely sensitive to
orientation. However, since the Pirani Sensor for the Series 945 System
operates to 100 Torr, it can be installed in any direction.
Digital Pairani Vacuum Sensor System
A.1
The Series 945 Pirani System Design Overview
This section is intended to give the reader an overview of the internal workings
of the Series 945 Pirani System. This manual does not provide sufficient detail
for advanced levels of troubleshooting. Refer to the block diagram while
reading this section.
Amplifier
The Sensor uses a wire
as one arm of a balanced
Wheatstone bridge. The
bridge amplifier
maintains the wire at a
constant temperature.
Thus, the bridge driving
voltage from the amplifier
is related to the pressure
in the Sensor.
Atm Adjust
Zero Adjust
Buffered Analog Output
Bridge
Amp
A/D
Converter
This voltage is buffered
by an op amp and then
fed to the analog output,
the Controller's digital
display, and the
comparator circuit.
microcontroller
Relay
Vref
Comparators
Output
Relay
The analog output is
driven by an op amp.
Although it has a fairly low output impedance, it is best to measure the analog
output signal with an instrument having at least 1 MW of input impedance.
Set Point Relays
A reference voltage and op amp form a precision adjustable reference that is
fed to the comparator circuit. A potentiometer can be adjusted to the proper
voltage to trigger the set point relay at the desired pressure.
Comparator
An analog comparator circuit compares the voltage from the buffer
amplifier (pressure signal) to the set point. When the pressure signal falls
below the adjusted reference voltage, a transistor turns on, which
energizes the set point relay. A resistor provides a feedback path around
the op amp, which increases the set point voltage by approximately 10
mV. This hysteresis provides stable operation even though some noise
may be present on the buffered output signal.
A.2
Digital Pairani Vacuum Sensor System
Display
Notes
Digital Pairani Vacuum Sensor System
A.3
A.4
Digital Pairani Vacuum Sensor System
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