Storage tank sits about 80 m from the panel. Pt100, 3 wire, in a thermowell on the tank side, into a 1769-IR6 on a CompactLogix rack. It reads 8 degrees high against the portable probe in the same well.
I pulled the RTD out and bench checked it, 100.0 ohm in the ice bath, 138.5 at boiling, so it's in spec. Moved it to a spare channel and got the same 8 high. Channel is set Pt100 385, 3 wire, degrees C. Cable is 3 core 0.75 mm2 with no junction box in between. Lost a day on it so far.
I don't want to drop an offset into the ladder and call that a fix. Is the IR6 just out of spec over 80 m of 0.75, or is the three wire bit not doing what I think it's doing here?
We had a Pt100 read high on a long run at our place and it turned out the third wire was never landed at the module. Photo of your module terminals for that channel?
Photo attached. Three cores come in from the tank, two landed on the channel, the third cut back and taped. Its been like that since the job was handed over. The commissioning contractor told the plant the module only had two terminals per channel. Not sure he ever looked at the drawing.