Martel Electronics 235 - Manual (Page 44)

The Pressure Module is capable of operating as an input or output
function, even though it is a measurement only device. This allows
simultaneous measurement of pressure and milliamps if desired.
To operate the Pressure Module as an input function, press the OPT key
while the calibrator is in the input mode. This will permit any allowable
output function to be active simultaneously with pressure
measurements.
To operate the Pressure Module as an output function, press the OPT
key while the calibrator is in the output mode. This will allow pressure
readings to be displayed on the output side, and milliamps to be
displayed on the input side. In this mode, the calibrator is actually
alternating readings between pressure and milliamps. This may cause a
barely perceptible slowing of response time of the readings. If the
simultaneously display of pressure and milliamps readings are not
required, the Pressure Module should be operated as an input function
for quickest response times.
To reactivate pressure readings simply press the OPT Key after first
selecting INPUT or OUTPUT mode as desired.
Zeroing a gauge pressure module
When a pressure module with a gauge pressure sensor is first activated,
it will typically display a non-zero pressure reading.
This is true even if the pressure is being displayed on the output side of
the calibrator.
There may be a slight drift of the zero reading during the first few
minutes of operation after power is first applied to the Pressure
Module. The user should recheck the zero by venting any pressure and
Input Functions
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If pressure measurements are not being made, the calibrator should
be turned off and the pressure module disconnected.
Note: To zero the reading, press the CE/Zero Key while the
calibrator is in input mode.