DUNIWAY Stockroom Terranova 908A User manual

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Instruction Manual
Terranova
Model 908A Dual Capacitance Diaphragm
Gauge Controller
Copyright © 2000 by Duniway Stockroom Corp.
rev100410sr
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Table of Contents
I Safety Information page 4
II Overview page 5
A. Front View, Back View and Dimensions
B. General Description
C. Specifications
D. Controls and Indicators
C. Capacitance Diaphragm Gauge Suppliers
III Installation page 9
A. Unpack the Controller
B. Mount the Controller
C. Select the CDG
D. Connect the CDG
E. Attach the CDG Cable
F. Make Accessory Connections
G. Check Supply Voltage
H. Attach the Power Cord
IV Operation page 13
A. Turn Power On
B. Front Panel Controls
C. Set Pt 1 High
D. Set Pt 1 low
E. Set Pt 2 High
F. Set Pt 2 Low
G. Units mTorr/Torr/kTorr -- Bar/mBar/microBar -- Pascal/kPascal -- Arb
H. Calibrate
I. Full Scale
J. Zero
K. Reset of Stored (Default) Values
L. Set Point Operation
M. Reading Pressure
N. Analog Output
O. Serial Interface
P. Pascal/kPascal Pressure Units Only Set-Up (-J Mode)
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V Maintenance page 25
A. Changing Fuses
B. Schematic Diagrams
VI Trouble Shooting page 26
VII Return Procedure page 27
VIII Warranty page 28
List of Figures
Figure 1: Model 908A front view page 5
Figure 2: Model 908A rear view page 5
Figure 3: Model 908A dimensions page 5
Figure 4: Sensor Connector -- 9 Pin Female D-Sub page 10
Figure 5: MKS Baratron Pin Connections (15 pin) page 10
Figure 6: MKS Baratron Pin Connections (5 pin) page 11
Figure 7. MKS Baratron Pin Connections (9 pin ‘D’) page 11
Figure 8: Accessory Connector -- 15 Pin Female D-Sub page 12
Figure 9: Accessory Connector Signals and Pins page 12
Figure 10: Protective Circuits/Set Point Relays page 17
Figure 11: Choosing Components/Set-Point Circuits page 18
Figure 12 908A Ranges: Sensor, High, Low, Set Points page 19
Figure 13: Analog Output and Displayed Pressure page 20
Figure 14: Serial Interface Cables frim PC to 908A page 21
Figure 15: Serial Output and Displayed Pressure page 22
Figure 16: Serial Output and Full Scale Range Setting page 22
Figure 17: Torr-Pascal Relationship - Terranova 908J page 24
Figure 18: Error Codes page 26
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I Safety Information
Explosive Gases
WARNING!
Do not use the Model 908A Dual Capacitance Diaphragm Gauge Con-
troller to measure the pressure of combustible gas mixtures. The gauge
normally operates at low temperatures, but it is possible that momen-
tary transients or controller malfunction may cause ignition of combus-
tible mixtures, which then might explode and cause damage to
equipment and injury to personnel.
Limitation on use of Compression Mounts
WARNING!
Do not use a compression mount (quick-connect) for attaching the
gauge tube to the vacuum system in applications that may develop pos-
itive pressures. Positive pressures may cause the tube to be blown out
of a compression fitting and damage equipment and injure personnel.
Chemicals
WARNING!
Many organic cleaning solvents, such as acetone, produce fumes that
are toxic or flammable. Use such solvents only in areas that are well
ventilated to the outdoors and away from electronic equipment, open
flames, or other potential ignition sources.
Please let us Know...
Terranova products are the most advance instruments of their type available
from any manufacturer. We have made this Instruction Manual as complete
and clear as possible. Let us know if you have any comments that can make
this manual or our products more useful.
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II Overview
A. Front View, Back View and Dimensions
Figure 1: Model 908A front view
Figure 2: Model 908A rear view
Figure 3: Model 908A dimensions
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B. General Description
The Terranova Scientific Model 908A Dual Capacitance Diaphragm Gauge (CDG) Controller
displays vacuum pressure as measured from capacitance diaphragm gauges. The 908A supplies
±15 volts at up to 0.75 amp; this is sufficient to operate most heated capacitance diaphragm gaug-
es. The 908A precisely measures the 0 to 10 volt signal from the CDG to determine pressure. The
Model 908A controller covers full scale ranges from 20 mTorr to 10,000 Torr. The 908A is
housed in a 1/8 DIN enclosure and is simple to operate.
C. Specifications
1. Useful Measuring Range
4 decades; for CDG tubes with full scale of 20 mTorr to 10.00 ktorr; full scale range selection is
entered on the front panel by the user
2. Display Range
-9.9 torr to 10.00 ktorr; pressures higher than 130% of Sensor Full Scale displays HI;
3. Display Resolution
varies according to full scale range, from 0.01 mTorr to 1 Torr
4. Gauge Interface
The 908A incorporates a high-resolution input circuit which allows the use of the capacitance di-
aphragm gauge over its entire 4 decades.
5. Input to the 908A Controller
0 to 10 volts for full scale of the gauge
6. Units of Display
torr (mtorr), mBar (Bar), pascal (kpascal), or arb (no units); user selectable
7. Full Scale
user selectable range to match CDG FULL SCALE : 20, 50, 100 mTorr; 1, 2, 10, 100, 1000, 5,000
and 10,000 torr
8. Calibration Adjust
for calibration of display; allows user to multiply CDG response by 0.50 to 2.00
9. Vacuum Gauge
one or two capacitance diaphragm gauges which have 1-10 VDC signal output and require up to
0.75 amp total from ± 15 volt supplies; this is sufficient to operate most heated gauges
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10. Operating Temperature Range
+2 to +50 deg. Celsius
11. Process Control Set Points
two, with independent High and Low set points for each relay, for flexible control of hysteresis
12. Process Control Relays
two relays; contacts rated for resistive loads:
AC: 120VA: 2.0 Amps @ 60VAC, 1.0 Amps @120 VAC, 0.5 Amps@/240 VAC.
DC: 60 Watts: 2.0 Amps @ 30 VDC, 1.0 Amps @ 60 VDC, 0.4 Amps @ 150 VDC.
13. Nonvolatile Memory
for all user specified parameters
14. Analog Output
logarithmic, 0.5 volts/decade; 0.10 mTorr=0.5 volts, displays pressure for selected gauge
15. Output Power
+15 at 0.75 amp and -15 volts at 0.75 amp; sufficient to operate temperature-controlled gauges
16. Mounting
The 908A may be used as a bench-top instrument or it may be mounted in an instrument panel.
Clips are provided for panel mounting.
17. RS-232 Input/Output
allows user to read pressure and set points; 9600 baud, 8-N-1; available through the accessory
connector
18. Operating Voltage
The Model 908A has a universal power supply, which operates on input voltages from 90 VAC to
240 VAC 47 to 65 Hz; input is through a standard IEC 320 instrument power input receptacle on
the rear panel; input power is protected by fuses in both lines of the input power.
19. Weight, Model 908A only; does not include cable or CDG
0.9 lb. /0.4 kg
D. Controls and Indicators
1. GAUGE SELECT button
allows the user to select which of the two gauges are shown on the digital display
2. SELECT button
Allows selection of parameters to be adjusted, e.g. Set Points
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3. RAISE and LOWER buttons
used for adjustment of gauge and instrument parameters
4. Digital Display
4-digit 7-segment bright red LED, 10 mm high
5. Display Indicators
bright red individual LED for miscellaneous indicators
E. Capacitance Diaphragm Gauge Suppliers
CDGs may be ordered from several sources, including the following:
Duniway Stockroom Corp.
1305 Space Park Way
Mountain View, California 94043
USA
telephone [phone]
toll-free [phone]
facsimile [phone]
email [email protected]
internet www.duniway.com
MKS Instruments, Inc.
Six Shattuck Rd.
Andover, Massachusetts 01810
USA
telephone [phone]
toll-free [phone])
facsimile [phone]
internet www.mksinst.com
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III Installation
A. Unpack the Controller
Carefully unpack the Model 908A Dual Capacitance Diaphragm Gauge Controller. The shipment
includes these components:
• controller unit
• power cord
• mounting clips, fuses
• D-sub 15 accessory connector
• this instruction manual
If your controller does not have all of these items, call Duniway Stockroom. If anything appears
to have been damaged in shipment, contact the shipper.
If you need a gauge cable for connection to the Model 908A, please contact Duniway Stockroom.
Do not plug the power cord in yet.
B. Mount the Controller
You can rest the controller unit on a bench, table top, or shelf, or you can mount it in a rack or
cabinet. The controller unit is housed in a standard 1/8 DIN box. If you are mounting the unit in
a panel, the cutout dimensions are 1.78 inch by 3.60 inch (45.2 mm by 91.4 mm), see fig. 3, page
9. One mounting clip attaches to each of the sides of the controller unit. To attach the clip, slide
the beveled surfaces of the clip under the cutout on the side of the box and push the clip toward
the back of the unit.
Be sure to leave enough clearance at the back of the controller unit for easy access to cable con-
nections.
C. Select the CDG
The Model 908A controller is designed to work with standard capacitance diaphragm gauges
which operate on ±15 volts. If you have difficulty obtaining a CDG, please contact us at Duniway
Stockroom.
CAUTION
Use of a CDG other than those which operate from ±15 volts may cause
damage to the CDG.
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D. Connect the CDG
Make sure that the CDG is securely connected to the vacuum system, using good vacuum
practice.
E. Attach the CDG Cable
The CDG cable has a 9-pin D-sub connector on one end, which plugs into the 908A, see Figure
4, below. There are a variety of conventions for connection to the CDG; you may assemble or
modify the cable to adapt to the CDG as needed. Connect the 9-pin D-sub plug of the gauge cable
to the 9-pin connector on the back of the 908A controller unit. Push the plug onto the connector
until it is firmly in place. Tighten the retaining screws to make certain the connector remains in
place. Loose connections can cause faulty readings.
Figure 4: Model 908A Sensor Connector -- 9 Pin Female D-Sub Connector
NOTE
The following information will allow you to make modifications to the ca-
ble as needed to interface with your CDG. Please refer to Figure 4 above;
this shows the 9-pin connector which is on the rear panel of the 908A. This
connector has female sockets; the mating connector on the CDG cable must
have male pins
908A 9-pin ‘D’ to MKS Baratron 15-pin ‘D’
908A Pin Terranova Name Color Tube Pin/Connection Name
1 Tube #1 Signal white to: Tube#1, Pin 2/Signal Output
2 Tube #2 Signal white to: Tube#2, Pin 2/Signal Output
3 Tube #2 Signal Return brown to: Tube#2, Pin 12/Signal Common
4 +15 volts red to: Tube#1, Pin 7/+15 VDC
5 +15 volts red to: Tube#2, Pin 7/+15 VDC
6 -15 volts green to: Tube#1, Pin 6/-15 VDC
7 -15 volts green to: Tube#2, Pin 6/-15 VDC
8 Tube #1 Signal Return brown to: Tube#1, Pin 12/Signal Common
9 Electronics Common black/shield to: Tube#1,Pins5/Pwr.Comm & 15/& Chassis Gnd
9 Electronics Common black/shield to: Tube#2,Pins5/Pwr.Comm & 15/& Chassis Gnd
Figure 5: 908A -- MKS Baratron Pin Connections (15 pin)
-----------------------------------------------------------------------------------------------
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Note: Many other manufacturer’s gauge pin connections use this same pin configuration.
908A 9-pin ‘D’ to MKS Baratron 5-pin Molex
908A Pin Terranova Name Color Tube Pin/Connection Name
1 Tube #1 Signal white to: Tube#1, Pin 3/Pressure Out
2 Tube #2 Signal white to: Tube#2, Pin 3/Pressure Out
3 Tube #2 Signal Return brown to: Tube#2, Pin 2/Output Return
4 +15 volts red to: Tube#1, Pin 5/+15 VDC
5 +15 volts red to: Tube#2, Pin 5/+15 VDC
6 -15 volts green to: Tube#1, Pin 4/-15 VDC
7 -15 volts green to: Tube#2, Pin 4/-15 VDC
8 Tube #1 Signal Return brown to: Tube#1, Pin 2/Output Return
9 Electronics Common black/shield to: Tube#1, Pin 1/Pwr.Comm & none
9 Electronics Common black/shield to: Tube#2, Pin 1/Pwr.Comm & none
Figure 6: 908A -- MKS Baratron Pin Connections (5 pin)
Current available for +15 volts from pins 4 and 5, and for -15 volts from pins 6 and 7 is internally
protected and limited to approximately 0.75 amp total for each supply. For example, this allows
CDG #1 and CDG #2 to draw 0.375 amp each; or CDG #1 may draw 0.72 amp and CDG #2 may
draw 0.03 amp. If excessive current is taken through any of the power output pins, the internal
protection will shut off the power for the affected voltage. It will be necessary to disconnect pow-
er for the 908A for a few minutes to allow the internal protection device to cool down and reset
itself.
908A 9-pin ‘D’ to MKS Baratron 9-pin ‘D’ (i.e. 750B)
908A Pin Terranova NameColorTube Pin/Connection Name
1 Tube #1 Signal white to: Tube#1, Pin 1/Pressure Out
2 Tube #2 Signal white to: Tube#2, Pin 1/Pressure Out
3 Tube #2 Signal Return brown to: Tube#2, Pin 8/Output Return
4 +15 volts red to: Tube#1, Pin 4/+15 VDC
5 +15 volts red to: Tube#2, Pin 4/+15 VDC
6 -15 volts green to: n/a
7 -15 volts green to: n/a
8 Tube #1 Signal Return brown to: Tube#1, Pin 8/Output Return
9 Electronics Common black/shield to: Tube#1, Pin 9/Pwr.Comm & none
9 Electronics Common black/shield to: Tube#2, Pin 9/Pwr.Comm & none
F i g u r e 7 : 9 0 8 A - - M K S B a r a t r o n P i n C o n n e c t i o n s ( 9 p i n ‘ D ’ )
(This applies to CDG sensor tubes such as the Baratron 750B)
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F. Make Accessory Connections
The 15-pin D-sub Accessory Connector is on the rear panel of the 908A, see Figure 8, below. The
connector has female pins; the mating connector must have male pins. Mating D-sub 15 connec-
tors are available from many of the normal electronic sources. If you need help identifying a
source, please contact us.
Figure 8: Model 908A Accessory Connector -- 15 Pin Female D-Sub Connector
Following are pin assignments for the Accessory Connector:
15-pin Accessory Connector
pin 1 set point #1 relay, normally closed
pin 2 set point #1 relay, common
pin 3 set point #1 relay, normally open
pin 4 set point #2 relay, normally closed
pin 5 set point #2 relay, common
pin 6 set point #2 relay, normally open
pin 7 Tx, RS-232 signal out of the 908A; 9600-N-8-1
pin 8 Rx, RS-232 signal into the 908A
pin 9 ground, RS-232 and analog common
pin 10 no function
pin 11 CDG#2 buffered analog signal; 1Kohm output
pin 12 no function
pin 13 log analog output, 1Kohm output, 0.5 volts/decade
pin 14 no function
pin 15 CDG#1 buffered analog signal; 1Kohm output
Figure 9: 908A -- Accessory Connector Signals and Pins
G. Check Supply Voltage
The Model 908A incorporates a universal power supply. This allows the 908A to operate on any
input voltage from 90 VAC to 240 VAC, 47 to 65 Hz.
H. Attach the Power Cord
Plug the power cord into the receptacle in the power module on the rear of the 908A.
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IV Operation
A. Turn Power On
Plug the AC power end of the power cord into an electrical outlet. The loudspeaker will “beep”
and test all indicators while the controller executes its self test. After being turned on, the instru-
ment will go through the following sequence:
• “beeper”
• indicators for TORR, MTORR, MBAR, ΜBAR, PASCAL, KPASCAL, GAGE 1, GAGE 2
• 10 LED indicators for set points and other functions
• all four digits will light, including decimal points
• display shows the model number of the instrument, 908A
• display shows software version, e.g. 1.10
The 908A will go into normal operation and begin measuring pressure. If the CDG is not connect-
ed, the display will show OFF. If the system pressure is between 100% and 105% of the CDG
Full Scale Range the display will flash, if the pressure is greater than 105% of the CDG Full Scale
Range the display will show OFF.
B. Front Panel Controls
The Model 908A allows flexible configuration of operation using simple entry from the front pan-
el buttons labeled GAUGE SELECT, SELECT, RAISE and LOWER. Parameters which you may adjust
are selected by scrolling through list which begins with SET PT 1 HIGH. Each time the SELECT but-
ton is pushed, the led indicator advances to the next parameter. The LED indicators will be lit to
indicate which parameter is being adjusted, and the digital display will flash to indicate the value
of the parameter being adjusted.
Each push of a button will give a short “beep” from the loudspeaker to confirm the button was
pushed. If you have reached the limit of adjustment or if the button push is not allowed, the loud-
speaker will give a long “beep”.
Following is a detailed description of parameter selection and adjustment:
C. Set Pt 1 High
Default Value: OFF
This sets the high limit of the set point. Above this pressure, the set point relay will be de-ener-
gized. Press the RAISE or LOWER buttons to enter the value desired. The minimum value is OFF;
this shuts the set point off. The next increment is 0.2% of the full scale range; for example, if the
full scale range is 1 torr, the increment sequence is: OFF, 2.0 mTorr, 3.0 mTorr, etc.
When the RAISE or LOWER buttons are pressed, the display will change slowly at first. If you hold
the button down for a few seconds, the rate of change will increase to allow you to make large
changes more quickly.
SET PT 1 HIGH operates in conjunction with SET PT 1 LOW. While the 908A is in this mode, the set
point may be assigned to either GAGE 1 or GAGE 2 by pressing the GAUGE SELECT button.
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D. Set Pt 1 low
Default Value: OFF
This sets the low limit of the set point. This is the pressure at which the set point relay will be
energized. Operation is similar to that of SET PT1 HIGH above. The minimum value is OFF; this
shuts the set point off. The next increment is 0.1% of the full scale range; for example, if the full
scale range is 1 torr, the increment sequence is: OFF, 1.0 mTorr, 2.0 mTorr, etc.
SET PT 1 LOW operates in conjunction with SET PT 1 HIGH.
NOTE: The High and Low set point allow the user to set the hysteresis of the
set point operation. As the system is pumped down, the set point relay will
be energized (set point turns on) as the pressure drops below SET PT 1 LOW.
The relay will remain energized until the pressure rises above SET PT 1 HIGH.
It is not possible to adjust the High set point to be lower than the Low set point. If you adjust the
High set point below the pressure previously selected for the Low set point, the 908A will auto-
matically reduce the value for the Low set point so that it is the next increment lower than that of
the High set point.
E. Set Pt 2 High
Default Value: OFF
This operates in the same manner as SET PT 1 HIGH, described above.
F. Set Pt 2 Low
Default Value: OFF
This operates in the same manner as SET PT 1 LOW, described above.
G. Units mTorr/Torr/kTorr -- Bar/mBar/microBar -- Pascal/kPascal -- Arb
Default Value: Torr
This allows selection of the units to be used in display of the pressure. Press either the RAISE or
LOWER buttons to alternate between Torr, mBar and Pascal. You will notice that both the GAGE 1
and GAGE 2 indicators will be lit also. This is to let you know that the units of measure apply to
both gauges; it is not possible to select Torr for one gauge and mBar for the other gauge. The
“Arb” value allows arbitrary units to be used; the value displayed is the same as for Torr units,
and can be adjusted using the “Full Scale” and “Calibrate” functions.
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H. Calibrate
Default Value: 1.00 (Internal value)
This allows the user to modify the reading for either gauge by multiplying by a value between
0.500 and 2.000. This is convenient for calibration of the CDG. CDGs are calibrated by the man-
ufacturer before shipment; we suggest you use this adjustment only if you have reliable calibra-
tion data. The multiplier is internal, and is not seen by the user. The digital display shows the
result of the pressure multiplied by the internal multiplier.
This adjustment may also be used to set the display to some value which is unique to your appli-
cation or experiment.
NOTE
This adjustment is applicable to either gauge; please select the appropriate
gauge by pressing GAUGE SELECT until the desired gauge is indicated. Use
RAISE and LOWER as described above to set to the desired value.
The CALIBRATE function may be adjusted only when the CDG pressure is at 50% of full scale or
higher.
NOTE — ATMOSPHERIC PRESSURE
You can use this function to set the atmospheric pressure reading for 1000
Torr CDGs. You should know the local barometric pressure before proceed-
ing; your local airport or on the internet at www.weather.com may have this
information. Normal barometric pressure is approximately 760 Torr (1000
mBar) at sea level and decreases by approximately 1 torr of every 47 feet
above sea level.
I. Full Scale
Default Value: 1.00 Torr
This adjustment is applicable to either gauge; please select the gauge by pressing GAUGE SELECT
until the desired gauge is indicated. Use RAISE and LOWER as described above to set to the desired
value.
This allows the user to select the full scale range for each CDG. Press RAISE or LOWER to set the
desired full scale range. The full scale ranges available are 20 mTorr, 50 mTorr, 100 mTorr, 1
Torr, 2 Torr, 10 Torr, 100 Torr, 1000 Torr. 5000 Torr and 10,000 Torr.
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J. Zero
Default Value: 0.0 (internal value)
This allows the user to adjust the zero for each CDG. Before making this adjustment, the CDG
should be connected to a vacuum system at a pressure lower than 0.01% of the full scale for the
CDG. This adjustment may also be used to set the display to a specific value if you know the pres-
sure through other means. For example, if the CDG to be adjusted is attached to a system that has
another CDG which has been independently calibrated, the CDG may be made to read the same
as the calibrated gauge.
The ZERO function may be adjusted only when the CDG pressure is at 10% of full scale or lower.
NOTE: INITIAL ZERO
When using installing a CDG for the first time, it is good practice to reset
the 908A internal settings. This will prevent errors in set up, e.g. if the CDG
has not had its internal zero properly adjusted. To reset the 908A, see Reset
of Stored Values, just below. To adjust the zero, the vacuum system to
which the CDG is connected should be at a pressure lower than 0.001 times
the full scale range of the CDG. For example for a 10 Torr CDG, the system
should be at a pressure lower than 0.01 torr, or 10-2 torr.
Adjust the zero adjustment on the CDG (usually a trimpot) until the 908A display shows a value
close to zero. After this initial adjustment, the display may be conveniently be set to zero using
front panel controls on the Model 908A.
NOTE: HEATED CDGs
When using heated or temperature-controlled CDGs, you should wait for an
hour or two before making any adjustments to the 908A or the CDG. This
will allow the CDG to come to its regulated temperature. When you are con-
fident the CDG is at a stable temperature, adjust the zero adjustment on the
CDG until the 908A display shows a value close to zero.
K. Reset of Stored (Default) Values
This allows you to recover the factory (default) settings for all stored values and resets the SET
POINTS to off. For a system that is far out of calibration, the factory settings provide a good start-
ing point for re-calibrating or adjusting the gauge controller. To recover the factory settings, un-
plug the 908A from its power source. Press and hold RAISE and LOWER buttons at the same time;
while holding the RAISE and LOWER buttons depressed, plug the power cord in. You will hear a
few short ‘chirps’ from the loudspeaker confirming the factory settings have been entered. The
digital display will show RST to confirm the reset has been entered.
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L. Set Point Operation
When pressure values have been entered for a set point (1 or 2) and assigned to a gauge (1 or 2),
the set point relay operates as follows:
As the pressure on the assigned gauge falls through the chosen “Set Point N Low” pressure, the
relay actuates, either opening (if the Normally Closed pin has been used on the Auxiliary I/O con-
nector) or closing (if the Normally Open pin has been used on the Auxiliary I/O connector).
Likewise, as the pressure on the assigned gauge rises through the chosen “Set Point N High” pres-
sure, the relay de-actuates, either closing (if the Normally Closed pin has been used on the Aux-
iliary I/O connector) or opening (if the Normally Open pin has been used on the Auxiliary I/O
connector).
Figure 10: Protective Circuits for Connecting Loads to Set Point Relays
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Figure 11: Choosing Component Values for Set-Point Protective Circuits
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M. Reading Pressure
Pressure display and ranging are automatic in the 908A. Most readings will take place between
zero pressure and the full scale of the 908A. The following table will help explain operation:
Sensor
Range
Full-
Scale
908A
Lowest
Scale
908A
Highest
Scale
Highest
Display
Resolution
Lowest
Recommended
Reliable Value
Lowest
Set Point
Highest
Set
Point
10 ktorr X torr XXE3 torr 1 torr 5 torr 50 torr 9990 torr
1000 torr X.X torr XXXX torr 0.1 torr 0.5 torr 5 torr 999.0 torr
100 torr X.XX torr XXX.X torr 0.01 torr 50 mtorr 0.50 torr 99.90 torr
20 torr X.XXX torr XX.XX torr 1 mtorr 10 mtorr 100 mtorr 19.98 torr
10 torr X.XXX torr XX.XX torr 1 mtorr 5 mtorr 50 mtorr 9.990 torr
2 torr X.X mtorr X.XXX torr 0.1 mtorr 1 mtorr 10 mtorr 1.998 torr
1 torr X.X mtorr X.XXX torr 0.1 mtorr 0.5 mtorr 5 mtorr 999.0 mtorr
100 mtorr X.XX mtorr XX.XX mtorr 0.01 mtorr 0.05 mtorr 0.5 mtorr 99.90 mtorr
50 mtorr X.XX mtorr XX.XX mtorr 0.01 mtorr 0.025 mtorr 0.25 mtorr 49.95 mtorr
20 mtorr X.XX mtorr XX.XX mtorr 0.01 mtorr 0.01mtorr 0.1 mtorr 19.90 mtorr
Notes:
1. From 100% of full scale to approximately 130% of full scale, the display will flash.
2. Above approximately 130% of full scale, the display will indicate “HI”.
3. Prior to proper setting of zero, the display may show a negative value, as low as –1% of full scale.
Of course negative readings are meaningless, but only provide span for adjustment.
Below approximately –1% of full scale, the display will indicate “LO”.
Proper adjustment of the 908A zero and sensor zero will move readings out of the negative range.
Figure 12: 908A Ranges: Sensor, High, Low, Set Points
N. Analog Output
The analog output is calculated from the value of the digital display and for the gauge selected.
The output is logarithmic, 0.5 volt/decade; the source impedance for the output is 1 K ohm. The
output voltage is calculated from:
V=0.50*(log10(100*Pressure))
where V is the Analog Output in volts; P is the pressure in mTorr or µbar. Some examples fol-
low; because of normal tolerances in the electronics, there may be minor differences in the values
you observe compared to those shown:
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Displayed pressure Analog Output - volts
LO 0.00
0 mTorr (µBar) or less 0.00
0.016 mTorr (µBar) 0.10
0.10 mTorr (µBar) 0.50
0.20 mTorr (µBar) 0.65
1.0 mTorr (µBar) 1.00
1.58 mTorr (µBar) 1.10
10.0 mTorr (µBar) 1.50
100 mTorr (µBar) 2.00
1.00 torr (mBar) 2.50
10.0 Torr (µBar) 3.00
100 Torr (mBar) 3.50
999 Torr (mBar) 4.00
10.00 kTorr (mBar) 4.50
OFF or HI 5.00
Figure 13: Analog Output and Displayed Pressure
The pressure as a function of the Analog Output voltage is:
P=0.01*log-1(2V) or
P=0.01*10(2V)
NOTE:
The analog output is valid for the gauge which is selected on the display.
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