Summary of information:
Thermocouple as a kind of commonly used temperature measurement instrument in industrial production, the utilization rate in all walks of life is very high, but many people use it, but they do not understand the working principle of this instrument.
In the actual use, to be able to understand the working principle of thermocouple is also a lot of benefits for the usual use. The following by Jiangsu Huanya electric heating instrument Co., Ltd. experts to explain.
Widespread use of thermocouples:
As one of the most widely used temperature sensors in industrial temperature measurement, thermocouple, together with platinum thermistor, accounts for about 60% of the total temperature sensor. Thermocouple is usually used together with display instrument to directly measure the surface temperature of liquid, vapor, gas medium and solid in the range of - 40 ~ 1800 ℃.
Advantages of thermocouple:
It has the advantages of high measurement accuracy, wide measurement range, simple structure and convenient use of thermocouple.
Basic principle of thermocouple temperature measurement
Thermocouple is a kind of temperature sensing element, which is a primary instrument. It directly measures the temperature, converts the temperature signal into thermoelectric EMF signal, and then converts it into the temperature of the measured medium through the electrical instrument (secondary instrument).
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 current in the circuit. At this time, there is an electromotive force thermoelectric EMF between the two ends, which is the so-called Seebeck effect.
Two kinds of homogeneous conductors with different compositions are thermoelectric electrodes. The higher temperature end is the working end, and the lower temperature end is the free end. The free end is usually at a constant temperature. According to the function relationship between thermoelectric EMF and temperature, a thermocouple index table is made, which is obtained when the free end temperature is 0 ℃. Different thermocouples have different index tables.
When the third metal material is connected into the thermocouple circuit, as long as the temperature of the two contacts 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 connection of the third metal into the circuit. Therefore, in the thermocouple temperature measurement, can be connected to the measuring instrument, after measuring the thermoelectric EMF, you can know the temperature of the measured medium.
Thermocouples weld two different materials of conductors or semiconductors A and B to form a closed loop.
When there is a temperature difference between the two attachment points 1 and 2 of conductor a and B, electromotive force will be generated between them, thus forming a large current in the circuit. This phenomenon is called thermoelectric effect. Thermocouples use this effect to work.
Two different components of conductors (called thermocouple wires or thermoelectric poles) are connected to a synthetic circuit at both ends. When the temperature of the junction is different, electromotive force will be generated in the circuit. This phenomenon is called thermoelectric effect, and this electromotive force is called thermoelectric potential. Thermocouple is based on this principle for temperature measurement, in which the end directly used for measuring medium temperature is called the working end (also known as the measuring end), and the other end is called the cold end (also known as the compensation end); the cold end is connected with the display instrument or supporting instrument, and the display instrument will point out the thermoelectric potential generated by the thermocouple.
Thermocouple is actually an energy converter, which converts heat energy into electric energy, and uses the generated thermoelectric force to measure temperature. For the thermoelectric potential of thermocouple, the following problems should be paid attention to:
1: the thermoelectric potential of thermocouple is the difference of temperature function at both ends of thermocouple, not the function of temperature difference between two ends of thermocouple;
2. When the material of the thermocouple is uniform, the thermoelectric potential is independent of the length and diameter of the thermocouple, but only related to the composition of the thermocouple material and the temperature difference between the two ends;
3: when the material composition of the two thermocouple wires of the thermocouple is determined, the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; if the temperature of the cold end of the thermocouple remains constant, the thermoelectric force entering the thermocouple is only a single value function of the temperature at the working end.
Difference between thermal resistance and thermocouple:
Thermocouple and thermal resistance belong to contact temperature measurement in temperature measurement. Although their functions are the same, they are used to measure the temperature of objects, but their principles and characteristics are different. .
Thermocouple is the most widely used temperature device in temperature measurement. Its main characteristics are wide measurement range, stable performance, simple structure and good dynamic response. It can transmit 4-20mA electrical signal remotely, which is convenient for automatic control and centralized control. Although the thermal resistance is widely used in industry, its application is limited because of its temperature measurement range. The principle of thermal resistance temperature measurement is based on the characteristic that the resistance value of conductor or semiconductor changes with temperature. It also has many advantages, such as remote transmission of electrical signals, high sensitivity, strong stability, good interchangeability and accuracy, but it needs power supply excitation and can not measure the temperature change instantaneously. Generally, PT100, pt10, cu50 and cu100 are used as industrial thermistors. The temperature measurement range of platinum thermistor is generally - 200-800 ℃ and that of copper thermistor is - 40-140 ℃. Thermistors are of the same type as thermocouples, but they do not require compensating wires and are cheaper than hot spot couples.
After the above detailed introduction, we should have some understanding of the working principle of thermocouple. Jiangsu Huanya electric heating instrument Co., Ltd. has rich experience in this regard, interested parties call for advice 。