I am working on my HERMS setup and have a few PIDS and temp sensors to work with. The problem is that the temp sensors show about 5 degrees celsius off when I first set it up. I Raised the water in my HLT to 70 deg celsius and measured with three different ordinary thermometers. Two digital and one glass. All three was quite near each other. At least enough for my needs. Within 0.5-1 degree celcius. I calibrated the PIDs (tested with a MyPin and a Sestos PID) and everything was fine. When I lowered the temp the difference increased and when I reached 20 deg celsius there was about 5 deg difference again.
Both PIDs show the same difference. I have tried 4 different temp sensors PT100. Same problem with all of them.
Does anyone have any idea why it is not linear?