Why does a 4-20 mA pressure transmitter read correctly on the bench but drift only after loop installation in the field?
Why does a 4 - 20 mA pressure transmitter read correctly on the bench but drift only after loop installation in the field?
Most of the time a 4-20mA pressure transmitter that reads accurately on the bench, but wanders in the field, has an installation problem, not an electronics fault. The transmitter on the bench sees clean test pressure, short tubing, constant temperature, and no mechanical stress. The same equipment in the plant has to cope with impulse lines, manifolds, vibration, temperature swings, trapped gas or liquid, and possibly pipe strain. Each may modify the apparent zero or the pressure delivered to the sensing diaphragm.
The first is the mechanical installation. Be sure to fully equalize the manifold, free the vents and drains, and slope the impulse lines properly during zero checks. Check for blockage by sludge, wax, condensate or frozen fluid. And make sure that the transmitter isn’t unduly tightened or twisted by hard pipes, as that can distort the body and produce misalignment. Also confirm that the process connection is correct for the service notably for gas, steam and liquid applications.
Then check the field loop. At the transmitter and at the DCS input, measure the loop current. If the signal is sensitive to temperature or vibration, look for improper grounding, damaged cables, or unstable power supply. If verify capillary route is remote installed and temperature effect. In many plants the drift only becomes evident after start up due to process pressure, heat and vibration exposing an issue which the bench test cannot simulate. The best remedy is to correct the installation and then retest zero, span and damping under operating conditions.