Martel Electronics 135 - Manual (Page 40)

Enter the local barometric pressure in the engineering units that are
selected, and press ENT. The absolute pressure readings are then
compensated by using the entered value as the reference pressure.
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
observing the reading. If the reading does not match the local
barometric pressure, a new zero is obtained by pressing the CE/Zero
key while in input mode, and entering the barometric pressure. Note
that you must enter the barometric zero any time a new engineering
unit is selected. The barometric reference standard used should be
.02% FS accuracy.
Disconnected Pressure Module
If the Pressure Module is disconnected while pressure readings are
being made, the display will show invalid for the pressure reading.
After a few seconds, the calibrator will determine that the Pressure
Module has been disconnected or malfunctioned and the following
display will be seen:
Input Functions
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no external
module present