Temperature transmitter problem?

My temperature transmitter is showing the value of 4.6ma at field but when I checked at panel side it is showing 3.5 ma. I have replaced my cable but the problem is still same.

Did you separate the transmitter from the DCS. Hook up parallel at the transmitter drive signal if it is good. The problem is in transmitter then preform calibration.

Check the 24vdc power supply. If power drop then MA also drop.

A temperature mismatch value is usually due to a misconfigured receiver whose range does not match the transmitter.

But a current mismatch can be either a transmitter issue, an AI issue or a ground loop issue.

Checking the input with a mA calibrated source can ‘prove’ the input Checking the measured temperature and then checking mA output of the transmitter will ‘prove’ the transmitter functionality.

Both can check out OK, but when connected, the current doesn’t match due to a ground loop issue.

A ground loop in a loop powered field transmitter can add or subtract current from the transmitter signal.

Ground loops can be caused by

  • non-isolated temperature transmitter using a grounded thermocouple (fix with an ungrounded thermocouple)
  • junction boxes that fill dirty water which provide a connection between signal and earth ground terminals. Fix the water leak and dry out the JB.
  • water intrusion into a temperature transmitter through the electrical conduit due to temperature cycling which can condense humidity into moisture. A path between the signal and earth ground can introduce ground loop currents. Fix the conduit with a lower ‘loop’ that will collect the water and empty the conduit on a schedule. Dry out the transmitter.
  • non-isolated analog inputs, particularly non-differential AI’s with a common terminal for all multiple inputs. Those modules typically have low common mode rejection levels. This particular problem is solved by using a loop repeater/isolator.