Discussion on the abnormally high temperature readings obtained with K-type thermocouples
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Hello everyone. Recently, I encountered such a situation on one of the devices: the actual temperature was 40 degrees, but the host computer displayed around 120 degrees. An analog signal was sent to the host computer via Const316, and the channel was working properly; by using 316 to read the thermocouple signal, the temperature was also measured at around 40 degrees, indicating that everything was normal. However, after reconnecting the thermocouple wires, the host computer still showed 120 degrees. By replacing one thermocouple, the temperature signal returned to normal. Therefore, it is determined as a “thermocouple fault”. So the question is, what is the cause of the failure in this thermocouple? Under normal circumstances, if there is poor wiring or deterioration of the core, the potential should be lower than the actual value, or it may fluctuate up and down, or there may be an open circuit. But what happened with this thermocouple was indeed an increase in potential. If there is interference at the point being tested, then the signal should also fluctuate up and down; or this phenomenon will continue to occur even after replacing the thermocouple. Therefore, I suspect that it is the connection between the thermocouple element and its housing that causes a change in the electron diffusion capacity, resulting in an increase in the thermocouple contact electromotive force and thus an artificially high measured temperature. I wonder if anyone has encountered such a problem? Is it this reason I’m suspicious about? Are there other reasons? I hope you can offer your guidance; thank you!
If contact between the thermocouple’s sensing element and its protective tube leads to an increase in electromotive force, then what’s the use of a junction box-type thermocouple? So I think there’s a little devil inside that faulty thermocouple, deliberately messing with you:lol:lol
It’s strange; could the temperature outside the device have an impact? Otherwise, there’s no reason for it to show 120°C. If there is an open circuit, the potential value is completely zero.
You’re overthinking it. The higher temperature indicates that the electromotive force at both ends is greater than before; as for why it’s higher, it can only be attributed to a problem with the sensor
LZ, go and measure the insulation resistance between the insulated wire and the armored enclosure; you can also directly measure the insulation resistance between the positive or negative lead and the enclosure. It’s possible that the insulation resistance has decreased.
I’m also wondering why a normal value was detected. The thermocouple used for temperature measurement in the tempering furnace shows a temperature of 40 degrees everywhere around it; since it is located at the central point, it is reasonable to assume that this value represents the actual measured temperature.
I couldn’t figure out the reason either, that’s why I came to discuss it with everyone.
Yeah, the strange thing is that its temperature increased. If it’s an open circuit directly, then it’s simple. See through at a glance. The temperature outside the equipment is around 40 degrees, and there are many thermocouples around, which allow it to be determined what the ambient temperature of those thermocouples is.
So, here’s the question: 1. The thermocouple itself does not have a sensor, so the possibility that the sensor is located within the thermocouple itself can be ruled out； 2. If it is a controller channel fault, then using a signal generator to simulate temperature and replacing the thermocouple are both methods that confirm the integrity of this channel ； 3. Replace this faulty thermocouple with one from another channel; the temperature reading at that measurement point still shows around 120 degrees.
It was this idea, and I also made an effort to verify my guess. It’s a pity that the apprentice casually threw the thermocouple in the waste pile; it couldn’t be found when tried to retrieve it. This bizarre phenomenon thus remained an unsolved mystery.
Problem with the thermocouple material: excessive impurities in the wire