Determining the quality of thermocouples.
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I used a multimeter set to 200 ohms to measure the resistance between the two ends of the thermocouple; regardless of whether I measured it in the positive or negative direction, the resistance was always 0.8 ohms. What could be the reason for this?
The mV signal generated by the thermocouple can be tested for quality using the mV function of a multimeter.
The original poster doesn’t understand the principle of thermocouples. In simple terms, a thermocouple is made by welding two types of metal wires together; its resistance value is very low. It’s not a semiconductor with unidirectional conductivity, so the resistance value measured with a multimeter in either direction remains the same. During inspection, heat the hot end and measure the thermoelectromotive force at the cold end; the millivolt signal can be used to make the judgment.
The resistance values of the other thermocouples were all around 2 ohms, while the resistance value of this one was 0.8 ohms
Swapping the test leads between the positive and negative terminals will result in a different resistance value
I tested this by swapping the test leads for the positive and negative terminals, and the resistance value remained the same in both cases
Then you must have measured it at room temperature
The resistance value you measured is correct; the other readings are higher. Perhaps there is an oxide layer on the surface of the terminals, or it might be a thermocouple with a different calibration. In any case, the resistance value is very low
Thermocouples measure voltage or on/off status, while thermal resistors measure resistance
As the temperature at the hot side increases, the forward resistance rises, but the reverse resistance remains close to zero. It’s better to measure the potential
Testing at 200Ω is the method used for identifying thermal resistors; for thermocouples, use the mV setting for testing. First, use a multimeter to check whether the two wires of the thermocouple are conductive, and then proceed with the mV testing