I would think it is a grounding issue with the thermocouples... There is a "Cold Junction" compensator in the Low side of the thermocouple and If it were seeing a ground or in your case the lack of one... A thermocouple's output voltage is measured in millivolts and any small potential difference there will shift the readings you observe the indicator is the difference in readings when you shift the ground from the cabinet (earth ground) to no ground with the PC. If the Arduino had no ground to earth and it's outputs were opticoupled out you might well change things as if the Arduino were truly floating then the ground leakage/galvanic issue you are having might go away. I am thinking that the issue is galvanic in nature... brass and steel between the ground and the thermocouples contact with the measured medium (Water. I believe and potable (drinkable) water at that). There was a guy that had an issue with a PH Probe in a large Reservoir. His answer was total isolation from contact between the probe and the mains, when he got the device PSU to look like an isolated battery all the issues went away... Very similar to the isolation methods used in medical instruments.IMO, then and now.DocHi, Would it be useful if I ground the arduino all the way to AC Main Ground?