The four laws of thermocouples

Thermocouples are made using the thermoelectric effect of metals. They are closely related to the material and chemical composition of the metal. Different thermocouples use different materials and compositions for the positive and negative electrodes. However, thermocouple measurements have commonalities, namely the four basic laws of thermocouples.

Law of homogeneous conductors. Homogeneity means uniform material composition. A closed loop composed of the same homogeneous material will not generate thermoelectric potential regardless of the conductor cross-section or temperature distribution, that is, the temperature cannot be measured. Therefore, the thermocouple must be composed of two homogeneous conductors of different materials. In addition, if the material is not uniform, additional thermoelectric potential will be generated, resulting in inaccurate temperature measurement.

Law of intermediate conductors. If a third conductor (intermediate conductor) is connected to the original thermocouple loop, as long as the temperatures at both ends of the intermediate conductor are the same, the connection of the intermediate conductor will have no effect on the temperature measurement results. Note that the key is that the temperatures at both ends of the intermediate conductor must be consistent. When connecting the third conductor, we should pay attention to the temperature difference environment at both ends.

Law of intermediate temperature. The thermoelectric potential at the hot and cold ends of the thermocouple cannot directly reflect the real-time temperature of the hot spot, because the temperature of the cold end is generally not 0℃. The actual temperature is equal to the temperature value of the thermoelectric potential lookup table plus the cold end temperature. The cold end compensation we often talk about is based on this principle. When we actually measure, we must pay attention to whether the cold end temperature is stable. If it is unstable, we must correct it in time, otherwise it will lead to inaccurate temperature measurement.

Reference electrode law. The reference electrode is a special electrode material, which is different from the positive and negative electrode materials of the thermocouple. Pair the reference electrode with the positive and negative electrodes of the thermocouple to form two pairs of thermocouples, then the measured value of the original thermocouple is equal to the sum of the measured values ​​of the two pairs of thermocouples. This law is generally used for the research and development or inspection of thermocouple materials, and it can be used to find suitable thermocouple materials.