How thermocouples work

Thermocouple (thermocouple) is a temperature measuring element commonly used 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 shape of various thermocouples is often very different due to needs, but their basic structure is roughly the same, usually composed of main parts such as thermode, insulating sleeve protection tube and junction box, usually with display instruments, recording instruments and electronic adjustment. used in conjunction with the device.

Introduction In industrial production process, temperature is one of the important parameters that need to be measured and controlled. In temperature measurement, thermocouples are widely used. They have many advantages, such as simple structure, convenient manufacture, wide measurement range, high precision, small inertia, and easy remote transmission of output signals. In addition, because the thermocouple is an active sensor, it is very convenient to use without external power supply during measurement, so it is often used to measure the temperature of gas or liquid in furnaces and pipes and the surface temperature of solids [1].

Working principle When two different conductors or semiconductors A and B form a loop, and the two ends are connected to each other, as long as the temperatures at the two junctions are different, the temperature of one end is T, which is called the working end or the hot end, and the temperature of the other end is T. For T0, called the free end (also called the reference end) or the cold end, an electromotive force will be generated in the loop, and the direction and magnitude of the electromotive force are related to the material of the conductor and the temperature of the two junctions. This phenomenon is called "thermoelectric effect", the loop composed of two conductors is called "thermocouple", these two conductors are called "thermoelectrode", and the electromotive force generated is called "thermoelectric force. The thermoelectromotive force consists of two parts of the electromotive force. One part is the contact electromotive force of the two conductors, and the other part is the thermoelectric electromotive force of a single conductor. The size of the thermoelectromotive force in the thermocouple loop is only related to the conductor material and the temperature of the two junctions that make up the thermocouple, and is related to the shape of the thermocouple. The size is irrelevant. When the two electrode materials of the thermocouple are fixed, the thermoelectromotive force is the function difference between the two junction temperatures t and t0. That is, the formula (below) is 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 end (measuring end), that is, a certain thermoelectromotive force corresponds to a certain temperature. We only need to measure the thermoelectromotive force to achieve the purpose of temperature measurement

The basic principle of thermocouple temperature measurement is that two different material conductors form a closed loop. When there is a temperature gradient at both ends, a current will flow through the loop. At this time, there is an electromotive force between the two ends. This is the so-called Seebeck effect. Homogeneous conductors with two different compositions are hot electrodes, the higher temperature end is the working end, the lower temperature end is the free end, and the free end is usually at a constant temperature. According to the functional relationship between thermoelectromotive force and temperature, a thermocouple graduation table is made; the graduation table is obtained under the condition that the temperature of the free end is at 0°C, and different thermocouples have different graduation tables. When the third metal material is connected to the thermocouple loop, as long as the temperature of the two junctions of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, it will not be affected by the third metal being connected to the loop. Therefore, when measuring the temperature of the thermocouple, the measuring instrument can be connected, and the temperature of the measured medium can be known after measuring the thermoelectromotive force. When a thermocouple measures temperature, the temperature of its cold end (the measuring end is the hot end, and the end connected to the measuring circuit through the lead wire is called the cold end) is required to remain unchanged, and its thermoelectric potential is proportional to the measured temperature. If the (ambient) temperature of the cold end changes during measurement, it will seriously affect the accuracy of the measurement. Taking certain measures at the cold junction to compensate for the influence caused by the temperature change of the cold junction is called the cold junction compensation of the thermocouple is normal. Special compensation wire for connecting with measuring instrument. Calculation method of thermocouple cold junction compensation: From millivolt to temperature: measure the cold junction temperature, convert it to the corresponding millivolt value, add it to the millivolt value of the thermocouple, and convert the temperature; from temperature to millivolt: measure the actual temperature The temperature of the cold end is converted into millivolts respectively, and the millivolts are obtained after subtraction, that is, the temperature.