Some non-metallic thermocouple materials are affected by the relative amount of components, impurities and preparation methods. When comparing material samples from different sources, large differences in thermoelectric potential rates are still common. However, the situation is generally not as serious as graphite, because a sufficiently stable microstructure can be obtained without a long-term high-temperature heat treatment.
Usually non-metal thermocouples are brittle and have poor tensile strength. Therefore, the actual non-metallic thermocouple is usually a thick, bulky combined rod or probe. The most common design is to make a rod of one material and insert it into a tube made of another material. Add a pin at the hot junction where the two materials are in contact. The space between the rod and the tube can be empty, or filled with insulating material, or filled with ring gaskets. The cold end of the probe can be air-cooled or water-cooled. In some cases, a matching extension wire can be used from the stomach to lead to the usual reference point. Depending on the material and the required length, it may be possible to design this type of probe with a smaller size. In the case of graphite, it is possible to design some special devices, such as filamentary graphite thermocouples. Some people have used non-metallic thin film thermocouples. The same general process used to prepare thin-film thermocouples of other materials is used.
