Common thermocouples can be divided into two main categories: standard thermocouples and non-standard thermocouples. Standard thermocouples are those whose thermoelectric potential-temperature relationship and allowable error are defined by national standards, and for which a unified standard calibration table exists. They also have compatible display instruments available.
Non-standard thermocouples are not as widely used or available as standard thermocouples, and generally do not have a unified calibration table. They are mainly used for measurements in certain special applications. Since January 1, 1988, China has manufactured all thermocouples and resistance thermometers according to IEC international standards, and has designated seven types of standard thermocouples (S, B, E, K, R, J, T) as the unified design types for China.
Theoretically, any two different conductors (or semiconductors) can be combined to form a thermocouple, but as a practical temperature measuring element, it has many requirements. To ensure reliability and sufficient measurement accuracy in engineering technology, not all materials can be used to form a thermocouple. Generally, the basic requirements for thermocouple electrode materials are:
1. Stable thermoelectric properties within the temperature range, not changing over time, with sufficient physical and chemical stability, and resistance to oxidation or corrosion;
2. Low resistance temperature coefficient, high conductivity, and low specific heat;
3. A large thermoelectric potential generated during temperature measurement, and a linear or near-linear single-valued functional relationship between the thermoelectric potential and temperature;
4. Good material reproducibility, high mechanical strength, simple manufacturing process, and low cost.
