Thermal resistance issue
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Please describe the problem: The insulation of the PT100 was tested using a megohmmeter, and no issues were found. A resistance box was used on-site to generate resistance value signals for the DCS, and the DCS displayed normal readings. The actual temperature measured by the thermoresistor on-site was around 65°C; however, once it was connected to the circuit, the DCS showed a reading of 60°C, which is 3-4°C lower. The wiring setup on-site uses ABB’s method. Do any experts know what could be the issue?
3-4 degrees? That’s not even a problem; in my opinion, there’s no issue at all. What are the connection methods for ABB? They use a four-wire system; nothing special seems to be there.
Check again with a dial indicator. Generally, we just need to check the DCS channels (resistance boxes), verify the on-site thermal resistors (multimeter), and inspect the wiring (multimeter, megohmmeter). Make sure not to make a mistake in the conversion. If ABB makes a mistake, they still have to bow their heads.
I’m not sure how long the cable is from the field unit to the control panel; if it’s too long, it’s recommended that you install a temperature transmitter on-site. For your issue, after connecting the temperature cable, you can check the resistance value in the wiring terminal of the control panel to see whether it matches the resistance value measured on-site. Of course, there is always a certain amount of resistance in the cable itself.
There are two possibilities: either the instrument used to measure the resistance value of the thermistor gives inaccurate readings, or there is a problem with the thermistor itself.
Check the contact of the terminal connections; be sure to strip the wires again, clean the lead terminals once more, and reconnect them!
The poster’s description has already identified the reason. To further confirm the cause. The poster can use a multimeter to measure the resistance value of the thermistor at its connection points behind the DCS panel, as well as the corresponding temperature value, in order to determine the discrepancy between this value and the 65 degrees measured on-site. Manual measurements, the contact resistance of the test leads, as well as the performance of the multimeter itself and the precision errors in your calculations, all contribute to the discrepancies between the values displayed by the DCS and those obtained through measurement.
I think it’s just that the resistance value of your universal performance stage isn’t very accurate. The difference isn’t that big.
Have the ranges been checked? Are the DCS transmitters consistent? Don’t overlook the small details
To receive signals from the DCS, is a safety barrier used, a dedicated card, or temperature changes at the site? Are there any issues with these settings? Try it with a different resistor core. Try it with another multimeter. If it’s normal, then it’s a problem with the temperature change settings.