Types and working principles of thermocouples

Types of Thermocouples

Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The so-called standard thermocouple refers to a thermocouple whose national standard stipulates the relationship between its thermoelectric potential and temperature, the allowable error, and has a unified standard scale. It has a matching display instrument for selection. Non-standardized thermocouples are not as good as standardized thermocouples in terms of use range or order of magnitude, and generally do not have a unified scale. They are mainly used for measurements in certain special occasions. Standardized Thermocouples Since January 1, 1988, China has produced all thermocouples and thermal resistors in accordance with IEC international standards, and designated seven standardized thermocouples, S, B, E, K, R, J, and T, as China's unified design thermocouples.

Working principle of thermocouple Types and working principle of thermocouple

When two different conductors and semiconductors A and B form a loop and their two ends are connected to each other, as long as the temperature at the two junctions is different, one end has a temperature of T, called the working end or hot end, and the other end has a temperature of TO, called the free end (also called the reference end) or cold end, then there will be current in the loop, that is, the electromotive force in the loop is called thermoelectric electromotive force. This phenomenon of electromotive force generated due to different temperatures is called the Seebeck effect. There are two effects related to Seebeck: first, when a current flows through the connection between two different conductors, heat is absorbed or released here (depending on the direction of the current), which is called the Peltier effect; second, when a current flows through a conductor with a temperature gradient, the conductor absorbs or releases heat (depending on the direction of the current relative to the temperature gradient), which is called the Thomson effect. The combination of two different conductors or semiconductors is called a thermocouple. The thermoelectric potential EAB (T, T0) of a thermocouple is synthesized by the contact potential and the temperature difference potential. Contact potential refers to the potential generated by two different conductors or semiconductors at the contact point. This potential is related to the properties of the two conductors or semiconductors and the temperature at the contact point. Thermoelectric potential refers to the potential generated by the same conductor or semiconductor at two different temperatures. This potential is only related to the properties of the conductor or semiconductor and the temperature at the two ends, and has nothing to do with the length of the conductor, the cross-sectional size, and the temperature distribution along its length. Both contact potential and thermoelectric potential are potentials generated by the different number of electrons concentrated at the end points of the contact. The thermoelectric potential measured by thermocouples is the synthesis of the two. When the circuit is disconnected, there is an electromotive force difference △V between the disconnection points a and b, and its polarity and magnitude are consistent with the thermoelectric potential in the circuit. It is also stipulated that at the cold end, when the current flows from A to B, A is called the positive pole and B is called the negative pole. Experiments show that when △V is very small, △V is proportional to △T. The differential thermoelectric potential of △V to △T is defined as the thermoelectric potential rate, also known as the Seebeck coefficient. The sign and magnitude of the Seebeck coefficient depend on the thermoelectric properties of the two conductors that make up the thermocouple and the temperature difference at the junction.