Donovan Posted December 21, 2021 Report Share Posted December 21, 2021 Hello! I have come across an interesting issue and am hoping someone can clarify what's going on: I have an M91-2-RA22 with 2 PT100 probes connected to it. What was happening was the input values from the probes were not accurate (ie: the indicated value in degress C would go off by a lot). One input would fluctuate based on the input of the other (ie: if one probe got hotter, the other would respond as if it got slightly hotter as well). I suspected a ground loop problem but eliminated that possibility (by further testing as indicated). I suspected a bad sensor but eliminated that as well (by replacing them) I removed one probe from the input channel of the PLC (leaving only one probe online) just to find that it did not matter: the watched value shown in U90Ladder (and displayed by the HMI) was simply wrong. I also confirmed the resistance values from all the probes were accurate, and that an independant PT100 monitor was having no problems reading correct values from them. Realizing This really had nothing to do with the probes or wiring, I decided to take another approach: I switched the PLC HW Configuration. I changed the Analog Inputs from PT100 (in Degrees C) to Resistance (in Ohms). Immediately I had accurate and stable (resistance) readings every time, from both inputs. From there I did a linear conversion in the ladder logic and got my readings in degrees C with no more fluctuations. So can anyone explain why the PT100 HW configuration would not be stable? I was hoping to get the most accuracy by using this feature, since PT100 probes are not perfectly linear across their full temperature range, but it seems I am getting the exact opposite. Is there a solution? Is there a good linearization formula for PT100 probes that will get the highest accuracy? Thanks. Quote Link to comment Share on other sites More sharing options...
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