Setra Systems 761 Operating Instructions Manual - page 8
4.0 OPERATION
Mechanically connect the Model 761 to the vacuum chamber. As explained
in Section 2, then electrically connect as shown in Section 3. For the most
accurate pressure measurement, allow the Model 761 to warm up for at
least 15 minutes. After installation, periodically check the zero output read-
ing to verify correct output. Adjust the zero potentiometer if incorrect (See
Section 5 for zero adjustment instructions).
If the unit was just received or stored at an extreme temperature, allow 12
hours for it to acclimate. It is recommended that the Model 761 zero output
be nulled after installation.
The signal output of the Model 761 is linear with pressure; e.g., for a 10 VDC
FS Model 761, 10 VDC equals 100% FS output, 1 VDC equals 10% FS output.
Table 1 indicates the lowest pressures available for reading and pressure
control for each range of the Model 761. The lowest suggested pressure
available for reading is limited by the resolution and the accuracy of the
Model 761. This is directly related to the electrical noise on the signal
output and can be significantly effected by incorrect electrical ground con-
nection, or connection to an electronically noisy power supply or readout
instrument. Improved results may be obtained if the transducer is oper-
ated in an environment with stable temperature and air flow. The lowest
recommended pressure used for control applications, such as a closed loop
downstream pressure control system, is based on a signal output of 50 mV.
Table 1: Recommended Lowest Pressures
Available for Reading & Pressure Control
6
7
Full Scale Range
Recommended Lowest
Pressure Reading
Recommended Lowest
Pressure for Control
1 Torr
5 x 10
-4
Torr
5 x 10
-3
Torr
5.0 Calibration & Adjustment
5. 1 Checking & Adjustment of Zero Output
After installation on a system, the Model 761 may require initial zero adjust-
ment. Figure 9 shows the location of the zero adjustment potentiometer. The
access hole next to the “Z” on the label identifies the Zero potentiometer.
Use a digital voltmeter with 0.0005 VDC resolution to view the signal output
of the Model 761. Adjust the signal output of the Model 761 to be 0.001 to
–0.001 V. Make this adjustment at a pressure at below the Model 761’s resolu-
tion; e.g., for a 1 Torr FS unit the zero pressure should be less than 5E-5 Torr .
The Zero potentiometer is a multi–turn potentiometer providing very fine
adjustment of the zero over a +/- 200 mV range.
Figure 9 : Location of Calibration Adjustment Potentiometers
5. 2 Span (Full Scale) Adjustment and Calibration
The Zero adjustment is the only adjustment that should be made in the field.
Span (Full Scale) and linearity adjustments require a calibrated and certified
reference standard and should only be attempted by qualified personnel. Re-
turn the Model 761 to Setra Systems for periodic calibration, Span (Full Scale)
Adjustment and Calibration adjustments and servicing. Adjusting any output
other than zero voids the factory calibration.
Z
Use a small flat bladed
screwdriver to adjust
the zero (Z). Clockwise
to increase; Counter-
clockwise to decrease
signal.