Rosemount Diff. Pressure transmitter problem


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In our site one of gas turbine liquid fuel nozzle differential pressure transmitter reading of pressure on HMI is not correct its reading (-21 bar) at 0 bar and its not changing when changing the pressure.

I checked the mA output its 4 mA at 0 bar and the reading of the transmitter on LCD screen is correct and changing with the pressure and also the mA output is changing with pressure.

the problem is the reading on HMI still -21 bar and didn't changed with pressure
I checked the wiring and I found it ok. for the doubt I replaced the transmitter by another one and I found that the reading on HMI is correct.

Please advise me

I had a similar problem today. a Rosemount transmitter connected to an AB 1794 IE8xt remote IO on a gas turbine. milliampes looked good on the HART meter, but I measured the current at the channel with a Fluke meter and it read Low. Swapped channels and problem followed, ran temporary wiring and problem followed, checked the scaling in PLC and it's good. will be changing out the transmitter.

I believed that the Hart meter always gave the true output reading, but I guess that's not the case.
>I believed that the Hart meter always gave the true output reading, but I guess that's not the case.

Depends on what you mean by HART meter.

Which HART meter were you using that displays milliamp values? What is the source of the milliamp value you read?

I know for a fact that the Meriam 5150 HART handheld communicator does not measure loop current (the ohm resistor is used to read the HART signal, not to read a precision mA signal). I suspect that a 475 does not read the mA current value, either.

The calibration of a transmitter's 4-20mA output done with a HART handheld communicator needs a precision milliampmeter (like a conventional Fluke multimeter) in the loop circuit to measure the current. The HART handheld does not make the loop current measurement.

There are HART enabled calibrator units, by vendors like Fluke or Beamex, that very likely would be able to display a measured loop current value because they are HART calibrators, not HART configuration tools. I say "very likely" because I've never used one, but would expect a calibrator to make that measurement.

If you mean the the HART meter was a transmitter's local display configured to display mA, then it might or might display true loop current. The 1990;s era Honeywell 'smart meter' (the dumbest display I ever ran into) local indicator was literally a DVM scaled in % or mA that read the actual loop current. Other meters might well read what HART says the transmitter thinks the current should be.

Problem nowadays is getting an explanation of exactly what the source of a reading is. It's all obfuscated in firmware and never explained the manual. Trust me.

It would be interesting to know if transmitter replacement fixed the problem because by running temporary wiring you eliminated about 80% of the chance of a ground loop. But there's still about 20% chance of a ground loop.