Thermocouple problem
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We installed 24 T-type thermocouples in the heat exchanger, and used multi-core T-type compensation wires to connect them to the DCS. On the DCS screen, all temperatures are showing fluctuations, varying within a range of about 2-3 degrees. What could be causing this? How to solve it?
Do your 24 thermocouples share a common negative terminal? If so, could that be the reason?
Check the grounding of the cable’s shield; interference is suspected to be the cause.
Check whether the flow of the two media involved in the relative heat exchange is uniform as they enter and exit the heat exchanger
We also believe it is caused by interference; we checked the shielding grounding as well. We use single-end grounding in the control room, and no issues were found. So how do we solve this problem?
The cause of the fault is the unstable output of the thermocouple’s thermoelectric potential, and the reasons for this are as follows: 1. Poor contact between the thermocouple terminals and the thermal electrodes； 2. Insulation damage in the thermocouple measurement circuit causes continuous short circuits or grounding ； 3. The thermocouple is not securely installed or there is external vibration ； 4. Whether the thermoelectric electrode is broken or not ； 5. External interference (AC leakage, electromagnetic field induction, etc.)
The explanation on the eighth floor is quite comprehensive; I think it’s mainly due to poor foundations plus vibrations of the heat exchanger
If they are all vibrating, other factors can be ruled out, and electromagnetic interference should be considered. Please check to ensure that no large motors have been added near the heat exchanger, or that there is no strong electric current in the vicinity. Based on what you’ve provided, the interference should be of a fixed frequency.
I spent the whole day working on it today, and finally found the cause!
Possible causes: 1. Vibration 2. Electromagnetic interference. Looking into these areas should help identify the cause.
If this problem exists in all 24 units, factors such as the manufacturing process, thermocouples, compensation wires (unless sharing a common negative terminal), and poor wiring can be ruled out. This situation is mostly caused by external interference. You can look for the cause at the source of interference, and at the same time check whether the shielding is properly connected.
It’s because the grounding on the DCS card wasn’t properly connected; nobody thought of that, right? . . Hehe!