DigiVac 45 / v, 100 Tc, 100 LED, 100 Tc/Hast/Batt, 100 Tc/F, 100 / 760 Digital Vacuum Gauge Instruction manual

INSTRUCTION MANUAL
DIGIVAC Model 45 and 100 Series
Digital Vacuum Gauges
Digivac Models
Model 45 / v
Model 100 Tc
Model 100 LED
Model 100 Tc/Hast/Batt
Model 100 Tc/F
Model 100 / 760
Ranges
1 to 1999 microns
1.to 199.9 microns
1 to 19.99 Torr
1 to 760 Torr and 1 to 199.9 Torr
0 to 29.9 Inches of Mercury
0 to 1 Bar or 0 to 1013 Millibar
The DigiVac
DigiVac Company
105B Church Street
Matawan, NJ 07747
Ph: 732.765.0900
Fax: 732.765.1800
www.digivac.com
CONTENTS
1.0
Description and Principle of Operation
2.0
Construction
3.0
Unpacking and Inspection
4.0
Installation
5.0
Operation
6.0
Servicing and Calibration
6.1 Factory Repair and Calibration
6.2 Field Calibration
6.3 Field Calibration of Tranducer (760) Gauges
7.0
Notes on Calibration
8.0
Understanding Microns, Torr, and Absolute Pressure
9.0
Accessories and Modifications
10.0
Attachments and Illustrations
2
1.0
DESCRIPTION AND PRINCIPLE OF OPERATION
DIGIVACTM 100 series gauges are compact, digital vacuum sensing instruments. They
use either thermocouple or strain gauge transducers to sense vacuum.
If in doubt about what gauge you have, consult the Digivac packing list that came with
your instrument for positive identification.
Major models and variations are as follows:
Model 45 / v
Plastic slant front case with D-cell battery.
Model 100 Tc
Plastic Case with Liquid crystal display (LCD) with AC wall
adapter.
Model 100 LED
Extruded Aluminum Case, Red LED display and internal power
supply.
Model 100 Tc/Hast
Similar to Model 100 Tc except uses Hastings DV6 or DV4
sensors
Model 100 Tc/Hast/Batt
Model 100 Tc/Fred
Similar to 100 TC above; powered by 9 volt battery.
Uses Frederics 2A gauge tube; very popular for Neon Sign
processing.
Model 100 Tc/Fred/BattAs above except runs on C-cell batteries
Model 100/760
Pressure transducer Model; 0 to 760 Torr (independent of
gas).
Model 100/29.9
Pressure transducer Model; 0 to 29.9 Inches of Mercury
(independent of gas).
Consult the Digivac website www.digivac.com for information about other Digivac
gauges.
Units with a /V suffix use the Varian 531 and compatible tubes. As tabulated above,
some units use an absolute pressure transducer which reads from 0 to 760 torr or a
differential transducer that reads 0 to 29.9 inches of mercury. Other models are
available for gauge tubes from other manufacturers.
Other Digivac vacuum gauges (the 200 series and 801w ) use thermocouple tubes and
sense from atmosphere to 1 millitorr while auto ranging and providing improved
accuracy. These instruments are discussed in the Digivac 200 series instruction manual.
All Digivac products (except 0 to 760 Torr and 0 to 29.9 inches of mercury) operate by
measuring the temperature rise of an electrically heated thermocouple exposed to a
vacuum. As vacuum increases, absolute pressure decreases, fewer and fewer molecules
of gas are available to cool the thermocouple, and its temperature rises.
Digivac gauges with transducers use precision strain gauge pressure sensors. The
circuitry in the Digivac puts a precisely controlled exitation current on the transducer.
Op amp circuitry then amplifies the low lever electrical output from the transducer to a
signal large enough to be displayed on by the Digivac.
A precision reference inside the Digivac, in conjunction with an integrated circuit
amplifier, controls the electrical excitation of the filament. In gauges that use Hastings
tubes, a precision, temperature compensated AC square wave oscillator is included in
the electronics.
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A CMOS A to D converter then digitizes the signal and drives the LCD (or LED) display.
The Instrument then reads either 0 to 1999 microns, 0.0 to 199.9 microns, or 0.00 to
19.99 Torr, depending on the model.
2.0
CONSTRUCTION
The Digivac consists of the indicating and controlling instrument, the gauge tube, the
gauge tube cable, and an AC adapter (on applicable models).
The instrument is housed in a rugged free-standing plastic or extruded aluminum
enclosure. It is normally simply placed on a suitable surface, but can be flush mounted
in a panel if desired. (Digivac 100P/200P series instruments are designed to mount in a
panel.)
The gauge tube is a nickel-plated assembly which houses the various thermocouple
sensing, heating and compensating elements and terminates in an octal connector. On
most models, the connector wiring terminates at the instrument with a D-subminiature
connector. If longer leads are necessary, an extension can be made simply by adding
appropriate male and female connectors, and wiring pin to pin. Extension cables are
also available from Digivac. Regulating circuity in the Digivac provides constant current
for gauge tube excitation, and thus compensates for resistance in the probe leads.
Leads up to 100 feet in length will not affect the accuracy of the instrument. Hastingscompatible units use a constant voltage excitation, and are affected by cable length.
Adjustments for zero, span, and gauge tube excitation are accessible from inside the
instrument. Most instruments also have a hole in the top of the case for the zero adjust.
Internal adjustments are provided for setting of the heater excitation. Refer to
Calibration instructions.
In the case of the Model 45, zero and span adjustments are provided at the top of the
instrument.
3.0
UNPACKING AND INSPECTION
After the DIGIVAC is received, it should be carefully unpacked and inspected for damage
during shipment and for completeness. The package should contain, at a minimum, the
instrument, the thermocouple gage tube, the tube connecting cable, and an instruction
manual. In the event of a loss during shipment, a claim should immediately be made to
the common carrier or the postal service, as applicable. The Digivac warranty pertains
only to the instrument, and does not cover losses in shipping.
4.0
INSTALLATION
Instrument
The instrument should be located in a clean, dry environment for best results. Its AC
adapter or line cord should be plugged into a suitable AC outlet, and plugged into the
power jack at the rear of the instrument. The electrical connections to the apparatus
that is to be controlled should be made where applicable. The leads are normally
identified by wire tags or markings.
Gauge Tube
Thermocouple gauge tubes must be installed in a stem-down orientation. Transducer
tubes are not sensitive to orientation.
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If the gauge is used in a Neon sign processing facility, the following is recommended to
protect the gauge from damage from bombarding:
The gauge tube should be isolated from the system with a stopcock. The stopcock
should be closed when bombarding.
There should be at least 2 feet of tubing between the electrode and the Digivac. For best
results, the tubing should be metal.
In extreme cases, the gauge can be absolutely protected by installing a normally open
solenoid valve between the gauge tube and the system. The solenoid valve coil should
be in parallel with the bombarding transformer. In this way, the solenoid will be closed
and the gauge tube will be positively protected whenever bombarding is done.
If a gauge is damaged by bombarding, it can generally be brought back to operating
condition by replacing the Op amp which controls the gauge tube current. Consult
Digivac.
Gauges which are damaged by bombarding will normally be repaired by Digivac free of
charge.
The tube should be installed, threaded end down, in a clean dry vacuum system. While
threading the gauge tube in, the cable should be disconnected. In this way, twisting of
the cable and the octal socket on the tube is avoided.
Care should be exercised to install the tubes in a dry part of the system. Since the
instrument works on the principle of temperature rise, the probes will not work if they
become filled with a liquid such as vacuum or diffusion pump oil.
See section 6.0 for gauge tube cleaning instructions.
Some Digivac gauges, particularly gauges which read gauge (as opposed to absolute)
pressure, have an internal sensor. These gauges have either an O-Ring fitting or a hose
barb. Make the appropriate connection to your system.
5.0
OPERATION
After installation, the DIGIVAC is ready for immediate operation.
The unit will normally provide accurate readings immediately; however, occasionally a
gauge tube will have absorbed material during storage, and may require as much as 24
hours of operation before accurate readings are attained.
It is recommended that the DIGIVAC be energized continuously during vacuum system
operation. In this way, the hot filament will not allow contaminants to condense.
In cases where the system has contaminants, as is often the case with metalizing and
coating equipment, it is often effective to isolate the gauge tube with a solenoid or
manual valve during periods when contamination is most active.
6.0
SERVICING
Gauge Tube Cleaning
In many cases, a gauge tube may become fouled with oil or other foreign matter. It is
often possible to restore the functionality of contaminated probes with cleaning. If the
contaminant is known, the tube should be filled with a fluid that is known to be a
solvent to that contaminant. As an example, ether is often effective in removing
residues of some oils. Commercial carburetor cleaners are very powerful solvents and
are highly effective against some contaminents.
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After cleaning with solvents, the gauge tube should be completely dried or flushed with
a volatile solvent to assure that it is dry prior to re-installing it. If this is not done,
contamination of the system may result.
6.1
FACTORY REPAIR AND CALIBRATION
The DIGIVAC is designed to provide years of trouble-free service, and the liberal internal
use of plug-in components make it easily repairable.
No field servicing of the unit is recommended, other than replacement of the gauge
tube, but factory servicing and calibration are available at a nominal cost and turnaround times of 24 hours are typical.
Unless an instrument has been grossly damaged, it can normally be repaired and
restored to original specifications for a cost of less than 25% of the purchase price of a
replacement instrument.
6.2
FIELD CALIBRATION
Although it is preferable that all calibration be performed at Digivac, field calibration can
be accomplished.
Before re-calibrating the instrument, it should be ascertained that the instrument is in
fact incorrect. In many cases, the problem will be with a tube that is fouled, or a system
that is operating improperly. It is recommended that a spare tube be kept on hand and
stored in a clean, dry place.Then, in cases of suspect readings, the tube should be
changed before proceeding further.
If adjustments are to be made, proceed as follows:
Operate the vacuum system at the lowest attainable pressure, and allow the system
and the gauge tube to stabilize for several minutes. Factory zero setting is done at a
pressure of .1 millitorr (.1 micron) or less.
Adjust the zero setting potentiometer. This adjustment is accessible from a small hole
in the top of 100 tc series gauges. It is accessible by removing the front panel of LED
series gauges.
Check the operation of the gauge at other pressures. Normally, adjustment of the zero
will not be interactive with the readings of the instrument at higher pressures.
If necessary, adjust the span with the span potentiometer. This potentiometer is
located on the vertical voltmeter board of the Model 100 tc and on the main board of
the Model 45. In the case of the Model 45, the span potentiometer acces hole is on the
right side of the instrument.
6.3
FIELD CALIBRATION OF TRANSDUCER (760)
(760) GAUGES
These gauges utilize a strain gauge absolute pressure transducer that is incorporated
into its gauge tube. (On some gauges, the transducer is internal to the gauge.)
Overall Gain Potentiometer (in cable)
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The transducers have varying gain factors. An adjustment for this is provided in the
gauge tube cable. If a transducer tube is changed, it may be necessary to adjust this.
Determine the local ambient pressure
(If at or near sea level, this is the “Altimeter Setting” number at any local airport. It
decreases 1 inch of mercury per 1000 feet of altitude.)
Remove the hood from the gauge tube cable at the instrument end.
Adjust the potentiometer as required. This is a span adjustment only.
Instrument Calibration
The Model 100Led760/sw has three potentiometers.
They are marked “Z”, “S” and “.S”
Adjust the “Z” for zero pressure. The gauge tube should be exposed to a pressure less
than 0.1 torr to make this adjustment.
Adjust the “S” potentiometer for span at atmosphere.
After “S” is adjusted, expose the sensor to a pressure of aprox. 150 torr as seen on the
high scale, then switch to the low scale and adjust “.S”
Adjust the potentiometer as required. This is a span adjustment only.
7.0
NOTES ON CALIBRATION
The DIGIVAC is calibrated in nitrogen, which has thermal properties virtually identical to
air. Other gasses will affect the readings by an amount proportional to the thermal
conductivity of the gases. In most cases, the gases present in a vacuum system will be
air, nitrogen, or oxygen, and no appreciable errors will occur.
Certain other gases, however, have thermal conductivity significantly greater than air
and will cause the instrument to read higher than the actual amount of pressure.
Examples of such gasses are water vapor, fluorocarbon refrigerants, and acetone.
Conversely, other gasses have thermal conductivity significantly lower than air and will
cause the instrument to read lower than actual pressure. Examples of such gasses
include helium, oxygen and to a lesser extent, CO2.
When interpreting readings using gasses other than air, it should be kept in mind that
the DIGIVAC reads microns, which are a measure of absolute pressure, which is the
opposite of vacuum. Thus, a lower numerical reading actually is a higher level of
vacuum. For more information, refer to section 8.0. When in doubt, consult Digivac.
Expected accuracy:
The table below shows the expected uncertainty of gauges with a Varian 531 gauge
tube.
To achieve this accuracy, the tube must be calibrated to the gauge, with the zero and
span adjusted.
Actual
Tolerance +/+/-
1
10
25
50
100
2
3
8
15
25
7
200
500
750
1000
35
40
50
70
The table below shows the expected uncertainty of gauges with a Thermionics Tg 500
gauge tube.
To achieve this accuracy, the tube must be calibrated to the gauge, with the zero and
span adjusted.
Actual
Tolerance +/+/-
1
10
25
50
100
200
500
750
1000
2
2
5
10
15
25
40
50
70
*Above 1000 millitorr, all thermocouple gauge tubes have a very low incremental
output. The Digivac readings are, however, guaranteed to be monotonic (There are no
reversals; increasing pressure always results in an increasing reading) and continuous
(There are now "dead" spots, readings will always increase)
Digivac 200 series gauges read all the way to atmosphere.
Expected Accuracy of Transducer gauges
Digivac gauges with transducers have an accuracy of better than +/- 1/2% of full scale.
8.0
UNDERSTANDING MICRONS
The DIGIVAC and most similar instruments are calibrated in microns or "millitorr." It is
appropriate to discuss what microns are and how to relate microns to other measures of
pressure and vacuum.
Microns are not really a measure of vacuum at all, but rather of absolute pressure.
It will be recalled that the pressure of the atmosphere is 14.696 or approximately 14.7
pounds per square inch at sea level. This pressure is due to the weight of all of the air in
the earth's atmosphere above any particular square inch.
This 14.696 psi is equivalent to the pressure produced by a mercury column of
approximately 29.92 inches high or .76 meters (about 3/4 of a yard) or 760 millimeters
of mercury. Atmospheric pressure varies greatly with altitude. It decreases
approximately 1 inch of mercury per 1000 feet of altitude. It also varies widely with local
weather conditions. (Variations of one half inch in a single day are common.)
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The word vacuum means pressure lower than atmospheric or "suction," but, in
describing negative pressure, the atmosphere is only a
satisfactory reference if we are dealing with values of vacuum down to about 27 inches
of mercury. Below that, it is much more useful to talk in terms of absolute pressure,
starting from absolute zero. The DIGIVAC and all similar instruments do just this.
One TORR, a commonly used unit, is an absolute pressure of one millimeter of mercury.
A millitorr is equal to 1/1000 of a TORR. A MICRON is the same as a millitorr. The full
scale reading of a DIGIVAC is 1999 microns and is equivalent to 1.999 TORR of
approximately 2/760 of atmospheric pressure.
9.0
ACCESSORIES AND
AND MODIFICATIONS
The following are offered as accessory equipment or field-installed modifications.
MultiMulti-tube Switching
Multiple gauge tube switching can easily be accomplished with the DIGIVAC 100 Tc. This
can be accomplished either with an external multi-probe harness, which plugs into the
probe connection at the rear of the unit, or with internally installed multi-probe wiring.
The filaments are connected in series, and continuously energized. Gauge tube
switching is accomplished by switching the input amplifier between the sensing
thermocouples. Because the filaments are constantly heated, the readings after
switching probes are virtually instantaneous.
Padded shoulder strap Case with velcro closure
For instruments that will be used in the field, particularly in cryogenic applications, a
padded shoulder strap case is available. This case holds a Digivac 100tc battery
powered gauge in the optimal reading position. The operator can open the Velcro
cover, pull out the gauge tube cable, plug it into the tube on the equipment, and see the
reading.
It was developed to assist in field service of cryogenic tank farms and vacuum jacketed
piping.
230 VAC 50 and 60 HZ AC Power Adapter
A special adapter can be ordered that will enable the DIGIVAC to operate from 230 VAC
single phase 50 Hz power as is commonly used overseas. The Model 100LED has an
internal transformer which can be connected for 230 vac operation.
Recorder Output
The DIGIVAC has an internal analog signal of approximately 0 to 1.000 volt with a
source impedance of approximately 75 ohms. This signal can be brought out to the rear
of the unit and can be used for driving external recorders, data acquisition systems, etc.
The signal is monotonic and reasonably linear in most of its useful range. Typical
pressure/voltage curves are found at the end of this instruction book.
Other features, such as Linearized recorder outputs, 4 to 20 milliamp outputs, Rs 232
outputs and control setpoints are available on the microprocessor based 200 series
Digivac gauges.
Compatibility with other Gauge Tubes
On special order, Digivac Instruments can be provided to use with most other vacuum
gauge tubes. AC and DC excitation are available. Gauges have been provided for
Hastings, Varian, Thermionics, Veeco, VRC, and Frederics gauge tubes.
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Wide Range Gauges
Digivac 200 Series instruments, which are only slightly higher in cost than the 100
series, read the entire range of pressure from Atmosphere to below 1 Micron.
Consult Digivac or our representative for further information. The 200 series
instruments are based on a microprocessor and are programmed in a high level
language. Because of this design, sophisticated features such as Rs232 output, control
setpoints and linearized analog outputs can be economically provided.
Process
Process Computer / Controller / Printer Interface
Digivac can offer a combination gauge and single board computer which can monitor a
vacuum gauge tube, record readings, drive an external chart recorder, and control
external apparatus such as valves, diffusion pumps, vaporizers, turbo pumps etc. It can
be programmed to drive any printer in a manner to simulate a strip-chart recorder. The
hard copy output from the recorder will be labeled in pressure units, and can also
incorporate custom labels, such as the customer's name. Standard tractor-feed printer
paper is used. The module consists of a single-board computer, which digitizes the
signal and performs other housekeeping functions as necessary to drive the printer.
The module is programmed in BASIC and includes RS-232 connections which enable it
to be connected to any computer for software modification. Actual software is stored in
non-volatile memory. A real time clock-calendar is included.
Visual Basic interfaces are provided which draw graphical representations of a process
on a PC screen, and log operating data to a hard drive.
Consult Digivac or www.digivac.com for details.
SPECIAL REQUIREMENTS
It is the policy of the Digivac Company to customize instruments for specialized
requirements whenever it is economically feasible to do so. We encourage inquiries
about your special needs.
For repair or recalibration, return gauges to:
The DigiVac Company
105B Church Street
Matawan, NJ 07747
Ph:
Fax:
732.765.0900
732.765.1800
E-mail: Direct from our website www.digivac.com
www.digivac.com
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The Digivac Company manufactures a complete line of vacuum gauges and
computers. Contact us or your distributor if you wish for further information.
See www.digivac.com for our latest offerings
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