This procedure is used for adjusting the lower trim point to compensate for positioning effects. Set up the calibration equipment per your transmitter instruction.
Zero trim has no effect on the LRV. Zero Trim requires the application of zero pressure (or equal pressures on both sides of a differential pressure transmitter).
The LRV does not have to be zero. Do not use Zero Trim on an absolute pressure transmitter unless the transmitter has full vacuum applied.