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.
That's your 8 degrees. Wired as 2 wire the module measures the RTD plus both leads. 80 m of 0.75 is about 2 ohms a core, near 4 ohms for the pair, and at 0.385 ohm per degree that's 10 or so, close enough to your 8 with tolerances. The third wire lets the module measure one lead and subtract it for both, which works because the leads match on one cable.
Land it. The IR6 has three terminals per channel and the third is the compensation lead, it's on the terminal drawing in the module manual, shield to the ground bar. Set to 3 wire with that lead open, I reckon the module has been compensating against nothing, so you got the raw lead resistance.
Landed the third wire on the compensation terminal. The module had it all along and the contractor was just wrong. Reads within 0.5 of the portable now.
So the whole thing was a 3 wire RTD wired as 2 wire, and 80 m of lead resistance went straight into the reading. One wire.
That contractor commissioned five more tanks. Four checked and fixed the same way this morning. The fifth I can't get to until the outage. Thanks wanda, mike.
Five tanks, one guy. Check his 4-20 loops too.