Good day. I installed an ABB TTH300-EX temperature transmitter and after a few days the transmitter readings started fluctuating and giving the wrong temperature. Replaced it with a new transmitter (same model) and the same thing occured but this time within a couple hours. The process line is natural gas. The line has cathodic protection on it also. I also tried another new transmitter, but this time i didn't install it onto the line. I left it inside the RTU panel connected for a few days and no issues was found.

 

Check on the PLC side, the ABB TTH 300 is good product.
I use in my project in almost year and not find any issue.

I think the temperature transmitter is the easiest instrument to install.
Check you cable continuity, loop check and AI card in the PLC.

Br,

Golan