What components does a thermocouple consist of?

Using one thing to achieve another goal: Thermocouples are one of the most commonly used temperature sensing elements in industry. Its advantages are: ① High measurement accuracy. Since the thermocouple is in direct contact with the object being measured, it is not affected by any intermediate media. ②Wide measurement range. Common thermocouples can perform continuous measurements in the range from -50 to +1600°C; some special thermocouples can measure as low as -269°C (such as those made of gold, iron, nickel, and chromium) and as high as +2800°C (such as those made of tungsten and rhenium). ③It has a simple structure and is easy to use. Thermocouples are typically composed of two different types of wires; they are not restricted in terms of size or shape, come with a protective sleeve, and are very convenient to use. 1. Basic principle of thermocouple temperature measurement: Two conductors or semiconductors of different materials, A and B, are welded together to form a closed circuit, as shown in Figure 2-1-1. When there is a temperature difference between the two contact points 1 and 2 of conductors A and B, an electromotive force is generated between them, resulting in the flow of an electric current in the circuit. This phenomenon is known as the thermoelectric effect. Thermocouples operate by utilizing this effect. 2. Types and structural formation of thermocouples (1) Types of thermocouples Commonly used thermocouples can be divided into two main categories: standard thermocouples and non-standard thermocouples. A standard thermocouple refers to a thermocouple for which standards specify the relationship between its thermoelectric potential and temperature, as well as the allowable errors; it also comes with a standard calibration table, and there are corresponding display instruments available for use with it. Unstandardized thermocouples are inferior to standardized thermocouples in terms of range of use and quantity, and generally do not have a unified calibration table; they are mainly used for measurements in certain special applications. Standardized thermocouples: Since January 1, 1988, in China, thermocouples and thermal resistors have been manufactured in accordance with IEC international standards. Seven standardized thermocouple types, namely S, B, E, K, R, J, and T, have been designated as the standard thermocouple types for use in China. (2) Structure of thermocouples To ensure the reliable and stable operation of thermocouples, the following structural requirements apply: ① The welding of the two thermal electrodes that make up the thermocouple must be secure ； ② The two thermoelectrodes should be well insulated from each other to prevent short circuits ； ③ The connection between the compensation wire and the free end of the thermocouple must be convenient and reliable ； ④ The protective sleeve should ensure adequate isolation of the thermoelectrode from harmful media. 3. Temperature compensation for the cold end of the thermocouple: Since the materials used in thermocouples are generally expensive (especially when noble metals are employed), and the distance between the measurement point and the instrument is often large, in order to save on thermocouple material and reduce costs, compensation wires are typically used to extend the cold end (the free end) of the thermocouple into a control room where the temperature is more stable, thereby connecting it to the instrument terminals. It must be noted that the function of a thermocouple compensation wire is merely to extend the thermal electrode and move the cold end of the thermocouple to the instrument terminals in the control room; it cannot eliminate the impact of changes in the cold end temperature on temperature measurement, and thus it does not serve a compensating role. Therefore, other correction methods must also be employed to compensate for the effect of the cold-end temperature t0≠0℃ on temperature measurement. When using thermocouple compensation wires, it is essential to ensure that the models match, that the polarity is correct, and that the temperature at the connection point between the compensation wire and the thermocouple does not exceed 100°C.