The malfunction I encountered today.
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Today I am responsible for reinstalling conduit at a temperature sensor. After I connected the wires to the tubes, this situation arose. This temperature reading comes from a J-type thermocouple; since the system is at rest, its temperature should be around 20 degrees, which is the room temperature. The measurement is carried out by using compensation wires to connect to the thermocouple interface in the DCS system. The DCS system uses FOXBORO’s FBM 202 interface module. There were two junction boxes in between. After connecting the wires, the temperature read around 70 degrees (which is abnormal). After a while, about an hour later, the temperature kept rising until it exceeded the range of 0-150 degrees Celsius, reaching 154 degrees. And it rises slowly (the first point of confusion). Then I went back behind the panel to measure the input terminals of the card connector, and there was actually a voltage of over 150 millivolts. (Second doubt) Is the hot spot thermocouple faulty and powered? I’m a newbie posting this; please forgive any unclear expressions. I hope to discuss this issue with other sailors.
Connect a good thermocouple to the terminal of the card to check if it works properly; if not, replace the card. If it works fine, then go to the site to check the wiring box and the measurement points!
I agree with the view from the second floor: if it’s truly impossible to determine where the fault lies, then use the elimination method and rule out each possibility one by one.
If it’s already 70 degrees at the start, that indicates something is abnormal. If the circuit is working fine, try another channel! I’m really curious! I don’t understand how it can keep rising on its own?
Thermocouples cannot pass through the junction box; they must be connected to the connector directly using thermocouple compensation wires, or connected to the connector after going through a safety barrier.
Agree with what was said above: thermocouples require compensation wires; if the connection from the junction box to the connector is not a compensation wire, then temperature compensation will be affected. It is said that the temperature is low because some of the voltage signal is lost along the line.
Compensating wires are also used from the junction box to the card holder