Answer / carl_ellis

In order to 'wet'calibrate a transmitter (by applying
reference pressure), the transmitter needs to be powered,
but if the transmitter's output signal is an input to a
control loop or alarm, then those functions need to be
disabled (loop put in manual, alarm disabled) because the
transmitter's output is not representative of the process
during calibration.

The transmitter has to be isolated from the process, so that
cal pressure can be applied. This done through isolation
valving; typically a 3 or 5 valve manifold for a DP transmitter.

So, is a transmitter 'in service' if it's powered, but its
signal is not representative of the process and is, in fact,
isolated from the process?

My answer is no, a transmitter being calibrated is not 'in
service'.

Carl Ellis
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