I initially thought it was a ground loop issue as well... I am familiar with ground loop issues, and have ensured proper grounding of shielding, ground wires and 0V wires. There are too many other factors pointing away from that as the problem:
- The power supply is properly grounded, and I even tried various grounding combinations just to see if the behavior would change: it did not.
- There is a remote (independant) temperature monitor that monitors a second set of locally installed PT100 probes, and it has no ground loop errors whatsoever, even when I connect the same sensors the M91 uses to it.
- I have tested two probes installed, one probe installed, and two different (single) probes 'hanging freely' from the M91: all give me temperature values that are not correct.
- When I switch the HW config to 'Ohms Resistance', the values are solid: No fluctuation, and 100% accurate readings. If I do testing as mentioned in the above points, everything remains stable and accurate using this configuration. If there was a ground loop issue, the resistance readings would still fluctuate or be incorrect.
Given the last point above in particular, it would seem that whatever algorithm the M91 is using to convert the inputted resistance values to temperature value is not functioning correctly.
(I knew 385 was the most common, but now I know why, thanks. From what I do know, the only difference between the two is a small variance in the resistance to temperature conversion chart, both are equally accurate.)