What is a thermocouple and what is the function of a thermocouple

Thermocouples are one of the most commonly used temperature detection components in industry. Its advantages are:
① High measurement accuracy. Because the thermocouple is in direct contact with the measured object and is not affected by the intermediate medium.
② Wide measurement range. Commonly used thermocouples can measure continuously from -50~+1600 ℃, while some special thermocouples can measure temperatures as low as -269 ℃ (such as gold iron nickel chromium) and as high as+2800 ℃ (such as tungsten rhenium).
③ Simple construction and easy to use. Thermocouples are usually composed of two different types of metal wires, and are not limited by size or opening. They have protective sleeves on the outside, making them very convenient to use.
1. Basic principle of thermocouple temperature measurement
Weld two different materials of conductors or semiconductors A and B together to form a closed circuit, as shown in Figure 2-1-1. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between them, resulting in a current of a certain magnitude in the circuit. This phenomenon is called thermoelectric effect. Thermocouples work by utilizing this effect.
2. Types and Structure Formation of Thermocouples
(1) Types of thermocouples
Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The standard thermocouple referred to refers to a thermocouple that has a national standard that specifies the relationship between its thermoelectric potential and temperature, allows for errors, and has a unified standard scale. It has a matching display instrument available for selection. Non standardized thermocouples are not as widely used or of the same order of magnitude as standardized thermocouples, and generally do not have a unified calibration table. They are mainly used for measurements in certain special occasions. Since January 1, 1988, all thermocouples and thermistors in China have been produced according to IEC standards, and the seven standardized thermocouples S, B, E, K, R, J, and T have been designated as unified design thermocouples in China.
(2) In order to ensure the reliable and stable operation of thermocouples, the structural requirements for them are as follows:
① The welding of the two thermoelectric electrodes that make up the thermocouple must be firm;
② The two thermoelectric electrodes should be well insulated from each other to prevent short circuits;
③ The connection between the compensating wire and the free end of the thermocouple should be convenient and reliable;
④ The protective sleeve should ensure sufficient isolation between the thermoelectric electrode and harmful media.
3. Temperature compensation of thermocouple cold end
Due to the fact that the materials of thermocouples are generally expensive (especially when using precious metals), and the distance between the temperature measuring point and the instrument is far, in order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the thermocouple to a temperature stable control room and connect it to the instrument terminals. It must be pointed out that the function of the thermocouple compensation wire is only to extend the thermoelectric electrode and move the cold end of the thermocouple to the instrument terminal in the control room. It cannot eliminate the influence of temperature changes at the cold end on temperature measurement and does not have a compensation effect. Therefore, other correction methods need to be used to compensate for the impact of cold end temperature t0 ≠ 0 ℃ on temperature measurement.
When using thermocouple compensation wires, attention must be paid to model matching, polarity cannot be connected incorrectly, and the temperature at the connection end between the compensation wire and the thermocouple cannot exceed 100 ℃.
2, The application principle of thermal resistance
Thermistor is a commonly used temperature detector in the medium and low temperature range.
Its main features are high measurement accuracy and stable performance. Among them, platinum resistance has the highest measurement accuracy, and it is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.
1. Principle and Materials of Thermistor Temperature Measurement
Thermal resistance temperature measurement is based on the characteristic that the resistance value of metal conductors increases with temperature. Thermistors are mostly made of pure metal materials, with platinum and copper being the most commonly used. In addition, materials such as Dian, nickel, manganese, and rhodium have begun to be used to manufacture thermistors.
2. Structure of thermal resistance
(1) The structure and characteristics of commonly used thermistor temperature sensing elements (resistors) in the proficient thermistor industry are shown in Table 2-1-11. According to the temperature measurement principle of thermal resistance, the change in the measured temperature is directly measured by the change in the resistance value of the thermal resistance. Therefore, the change in resistance of various wires such as the lead wires of the thermal resistance body will have an impact on temperature measurement. To eliminate the influence of lead resistance, a three wire or four wire system is generally used. For specific details, please refer to Section * of Chapter 3 of this article
(2) Armored Thermistor Armored Thermistor is a solid body composed of temperature sensing elements (resistors), leads, insulation materials, and stainless steel sleeves, as shown in Figure 2-1-7. Its outer diameter is generally between 2 and 8mm, with the smallest being up to 8mm.
Compared with ordinary thermistors, it has the following advantages: ① Small size, no air gap inside, low measurement lag in terms of thermal inertia; ② Good mechanical performance, vibration resistance, and impact resistance; ③ Can bend, easy to install, and has a long service life.
(3) The end face thermistor temperature sensing element is made of specially processed resistance wire wound tightly against the end face of the thermometer, and its structure is shown in Figure 2-1-8. Compared with general axial resistance thermometers, it can more accurately and quickly reflect the actual temperature of the measured end face, and is suitable for measuring the end face temperature of bearing shells and other components.
(4) Explosion proof thermoelectric barrier type thermistor uses a special structure of junction box to confine the explosion of explosive mixed gas inside its shell caused by sparks or arcs within the junction box, so as not to cause excessive explosion in the production site. Explosion proof thermal resistors can be used for temperature measurement in areas with explosive hazards within Bla~B3c class zones.
3. Composition of Thermistor Temperature Measurement System
A thermistor temperature measurement system generally consists of a thermistor, connecting wires, and a display instrument. The following two points must be noted:
① The scale marks of the thermistor and display instrument must be consistent
② In order to eliminate the influence of changes in the resistance of the connecting wires, a three wire connection method must be adopted. Please refer to Chapter 3 of this article for specific content.
(2) Armored Thermistor Armored Thermistor is a solid body composed of temperature sensing elements (resistors), leads, insulation materials, and stainless steel sleeves, as shown in Figure 2-1-7. Its outer diameter is generally between 2 and 8mm, with the smallest being up to 8mm. Compared with ordinary thermistors, it has the following advantages: ① Small size, no air gap inside, low measurement lag in terms of thermal inertia; ② Good mechanical performance, vibration resistance, impact resistance, ③ bending ability, easy installation ④ long service life.
(3) The end face thermistor temperature sensing element is made of specially processed resistance wire wound tightly against the end face of the thermometer, and its structure is shown in Figure 2-1-8. Compared with general axial resistance thermometers, it can more accurately and quickly reflect the actual temperature of the measured end face, and is suitable for measuring the end face temperature of bearing shells and other components.
(4) Explosion proof thermoelectric barrier type thermistor uses a special structure of junction box to remove the explosive mixture gas inside its shell. If the resistor is damaged by sparks or arcs, the length of the resistance wire must be changed, which will affect the resistance value. Therefore, it is better to replace the resistor with a new one. If welding repair is used, it must be verified to be qualified after welding before use