Condenser Vacuum Transmitters

A range of 0-760mmHg would encompass the absolute air pressure at that location with some margin.

Other options would tend towards a pseudo-gauge pressure reading, and are therefore not applicable in the context of an absolute pressure transmitter used on a condenser.

ditto what PW wrote. The pressure in a surface condenser is independent from atmospheric pressure.

Does the device analyzing the pressure signal expect to see absolute or gauge pressure?

Changing your settings based on altitude would be similar to changing them again every time a cyclone comes through, due to the inherent pressure change associated with that.

An absolute pressure transmitter is already reference to zero. This is the reason it can give you a vacuum reading that is in "absolute" terms and not in relation to the changing outside pressure of the condenser. ie: they are not effected by elevation or external pressure.

If the pressure is measured with low side to the condenser and high side to the atmosphere then you need to put the elevation compensation.

If both the high and low sides are connected to the process then you use absolute measurement where the external elevation difference does not affect the process.

Thanks everyone

we sorted it out turns out it was called to 0 to atmosphere and then had the output inverted.

so it was out by 30 all the time...

reset the instrument to factory calibration and it was all good.