Rosemount line pressure

Dear all,
I have point pressure transmitter PT. When i accessed on hart i found the value is 140 psi but on plc (controllogix) is 114 psi. The ranges on transmitter and plc is 0 to 800 psi.when i isolate the transmitter from service and inject pressure by hand pump all values on hart and at ma is right with no deviation. When i put it in service but isolated it reads at hart 0 psi but at plc reads -60 psi. I inject plc channel found it passes the test without any deviation. At the end, i replace the transmitter, everything is ok and all readings are the same.I want to know what is happened to the transmitter? I mean can anyone show me what is the root cause of this problem...

>I mean can anyone show me what is the root cause of this problem...

Probably not. It would require a forensic investigation of the bad transmitter, something that only manufacturers are equipped for nowadays.

I have seen either faulty terminal blocks or faulty surge/lightning protection modules create a faulty 4-20mA output condition where current leaks to/from earth ground or the generated loop current is the incorrect value. Replacing the faulty component fixed the transmitter because the pressure body was still functional.

Please note that i don't have surge/lightning protection modules and i wondered from how the transmitter reads at HART 140 PSi and at the same time at PLc is 114 PSI.

The problem is a pressure value at the AI that is different than the pressure value at the transmitter.

That could be misconfiguration of the AI, but

shkoko2000 said:

At the end, i replace the transmitter, everything is ok and all readings are the same.

Click to expand...

you replaced the transmitter and everything worked OK, so the transmitter and the AI must have been working OK previously and something failed.

If you replaced the transmitter and the loop resumes functioning OK, then it is not an AI configuration issue. It also proves the AI was reading correctly whatever 'faulty' current it saw from the transmitter.

So that leaves the problem: the transmitter's current does not represent the true pressure:

current value is 140 psi at the transmitter
current value is 114 psi at the AI

That can be a ground loop problem where current is added or subtracted from the loop current through a ground connection.

But if you replaced the transmitter and that corrected the fault, then the ground loop was not outside the transmitter, the fault was internal to the transmitter. So that rules out an external ground loop caused, say, by water in junction box.

If you used a HART communicator to read the pressure 'seen' by the transmitter and the transmitter's output current did not match the AI value (knowing the AI is functional adn there's no external ground loop) then something in the transmitter failed; I'd speculate the most likely cause if the analog output 'module' or the terminal block.

An AO failure could be a ground loop within the transmitter, it could be faulty DAC, it could be some other component in the transmitter's AO.

The transmitter sitting on my bench has a potted AO assembly. Who knows what's inside it? Nobody but the manufacturer and they keep it to themselves.

If you have a spare working transmitter and time on your hands, you can try substituting components to try to determine which component or module failed. If not, I suspect that only Mr. Rosemount can tell you what's went wrong (the root cause and the probable reason why) with that particular transmitter and they probably want more money to do that than what a new transmitter costs (speculation on my part, I've never asked).

Thanks for ur great valuable knowledge