Introduction to the working principle of thermocouples

Thermocouple is a commonly used temperature measuring element in temperature

measuring instruments. It directly measures the temperature, converts the

temperature signal into a thermoelectromotive force signal, and converts it into the

temperature of the measured medium through an electrical instrument (secondary instrument).

The appearance of various thermocouples is often very different due to needs,

but their basic structure is roughly the same. They are usually composed of main parts

such as hot electrodes, insulating sleeve protection tubes and junction boxes,

and are usually combined with display instruments, recording instruments and

electronic adjustments. Used in conjunction with the device.

When there are two different conductors or semiconductors A and B forming a loop,

and their two ends are connected to each other, as long as the temperature at the

two junctions is different, the temperature at one end is T, which is called the working

end or the hot end, and the temperature at the other end is T0 , Called the free end

(also known as the reference end) or cold end, an electromotive force will be

generated in the circuit, and the direction and magnitude of the electromotive force

are related to the conductor material and the temperature of the two junctions.

This phenomenon is called the "thermoelectric effect", the loop composed of two

conductors is called a "thermocouple", these two conductors are called "thermoelectrodes",

and the electromotive force generated is called "thermoelectromotive force" 

The thermoelectromotive force consists of two parts of electromotive force,

one part is the contact electromotive force of two conductors, and the other part is

the thermoelectromotive force of a single conductor. The size of the thermoelectromotive

force in the thermocouple loop is only related to the temperature of the conductor material

and the two junctions that make up the thermocouple, and has nothing to do with the shape

and size of the thermocouple. When the two electrode materials of the thermocouple are

fixed, the thermoelectromotive force is the two junction temperature t and t0.

The function is poor. That is, this relationship has been widely used in actual temperature

measurement. Because the cold junction t0 is constant, the thermoelectromotive force

generated by the thermocouple only changes with the temperature of the hot junction

(measurement end), that is, a certain thermoelectromotive force corresponds to a certain

temperature. We can achieve the purpose of temperature measurement as long as we

use the method of measuring thermoelectromotive force. 

The basic principle of thermocouple temperature measurement is that two

different components of material conductors form a closed loop. When there

is a temperature gradient at both ends, there will be a current passing through

the loop. At this time, there is an electromotive force-thermoelectromotive force

between the two ends. It is the so-called Seebeck effect. Two homogeneous

conductors with different components are thermoelectrodes, the end with a

higher temperature is the working end, the end with a lower temperature is t

he free end, and the free end is usually at a certain constant temperature.

According to the functional relationship between thermoelectromotive force and

temperature, a thermocouple indexing table is made; the indexing table is obtained

when the free end temperature is 0℃, and different thermocouples have different indexing tables.