Explain how a lower trim is different from a zero trim
Zero trim use for pressure reset in open to atmosphere condition in the transmitter
Lower and upper trim is used in conditions where an error in reading in a calibration range is different from reading.
If the range of a pressure transmitter is 0 to 10 bar and -25mmH2O to +25mmH20.
While you calibrate the transmitter which is 0 to 10 bar, to carry out the zero calibration you need to zero trim in Hart communicator with zero pressure g to the transmitter (open to atmosphere)
For the range -25mmH2O to +25mmH2O you need to give negative pressure of -25mmH2o to the transmitter (HP side if it is DP transmitter) and do a lower-trim from Hart communicator for the zero calibration.
Hence the zero trim is required for the TX range starts from exactly zero ( 0-10bar) and lower trim is required for the TX range starts below or above zero eg. -25mmH2O to +25mmH2o or 25 mmH2O to125 mmH2O.