Thermocouples are one of the most commonly used temperature detection devices in the industry. Its advantages are:
1 high measurement accuracy. Because the thermocouple is directly in contact with the measured object, it is not affected by the intermediate medium.
2 wide measurement range. Commonly used thermocouples can be measured from -50 to +1600°C. Some special thermocouples can measure at least -269°C (such as gold, iron, nickel-chromium) and up to +2,800°C (such as tungsten-germanium).
3 simple structure, easy to use. Thermocouples are usually composed of two different types of wires, and are not limited by size and the beginning. They have protective sleeves and are very convenient to use.
1. The basic principle of thermocouple temperature measurement is to weld two different material conductors or semiconductors A and B to form a closed loop, as shown in Figure 2-1-1. When there is a temperature difference between the two sticking points 1 and 2 of the conductors A and B, an electromotive force is generated between the two sticking points 1 and 2 so that a current of a size is formed in the loop. This phenomenon is called a thermoelectric effect. Thermocouples use this effect to work.
2. Types and Structures of Thermocouples (1) Types of Thermocouples Thermocouples can be classified into standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to a thermocouple whose national standard stipulates the relationship between thermoelectric potential and temperature, tolerance, and a unified standard indexing table. It has its own display instrument for selection. Non-standardized thermocouples are less than standard thermocouples in terms of range of use or magnitude, and there is generally no uniform index table, which is mainly used for measurement in certain special occasions.
Standardized Thermocouples From January 1, 1988, thermocouples and thermal resistances were all produced according to IEC international standards, and seven standardized thermocouples designated as S, B, E, K, R, J, and T were designed for unified design in China. Thermocouples.
(2) The structure of the thermocouple To ensure reliable and stable operation of the thermocouple, its structural requirements are as follows:
1 The welding of the two hot electrodes forming the thermocouple must be firm;
2 The two hot 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 that the hot electrode is fully isolated from harmful media.
3. Thermocouple cold junction temperature compensation
Because thermocouple materials are generally more expensive (especially when precious metals are used), and the distance from the temperature measurement point to the instrument is very long. In order to save the thermocouple material and reduce the cost, the cold end of the thermocouple is usually used to compensate the wire (free The end) extends into a relatively stable temperature control room and is connected to the meter terminals. It must be pointed out that the role of thermocouple compensation wire only extends the thermode, so that the cold junction of the thermocouple moves to the instrument terminal of the control room. It does not eliminate the influence of the temperature change of the cold junction on the temperature measurement, and does not compensate. Therefore, other correction methods need to be used to compensate for the effect of temperature at the cold junction temperature t0≠0°C.
When using a thermocouple to compensate for the wire, it must be noted that the model matches, and the polarity cannot be connected wrongly. The temperature of the connection terminal of the compensation wire and the thermocouple cannot exceed 100°C.
1 high measurement accuracy. Because the thermocouple is directly in contact with the measured object, it is not affected by the intermediate medium.
2 wide measurement range. Commonly used thermocouples can be measured from -50 to +1600°C. Some special thermocouples can measure at least -269°C (such as gold, iron, nickel-chromium) and up to +2,800°C (such as tungsten-germanium).
3 simple structure, easy to use. Thermocouples are usually composed of two different types of wires, and are not limited by size and the beginning. They have protective sleeves and are very convenient to use.
1. The basic principle of thermocouple temperature measurement is to weld two different material conductors or semiconductors A and B to form a closed loop, as shown in Figure 2-1-1. When there is a temperature difference between the two sticking points 1 and 2 of the conductors A and B, an electromotive force is generated between the two sticking points 1 and 2 so that a current of a size is formed in the loop. This phenomenon is called a thermoelectric effect. Thermocouples use this effect to work.
2. Types and Structures of Thermocouples (1) Types of Thermocouples Thermocouples can be classified into standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to a thermocouple whose national standard stipulates the relationship between thermoelectric potential and temperature, tolerance, and a unified standard indexing table. It has its own display instrument for selection. Non-standardized thermocouples are less than standard thermocouples in terms of range of use or magnitude, and there is generally no uniform index table, which is mainly used for measurement in certain special occasions.
Standardized Thermocouples From January 1, 1988, thermocouples and thermal resistances were all produced according to IEC international standards, and seven standardized thermocouples designated as S, B, E, K, R, J, and T were designed for unified design in China. Thermocouples.
(2) The structure of the thermocouple To ensure reliable and stable operation of the thermocouple, its structural requirements are as follows:
1 The welding of the two hot electrodes forming the thermocouple must be firm;
2 The two hot 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 that the hot electrode is fully isolated from harmful media.
3. Thermocouple cold junction temperature compensation
Because thermocouple materials are generally more expensive (especially when precious metals are used), and the distance from the temperature measurement point to the instrument is very long. In order to save the thermocouple material and reduce the cost, the cold end of the thermocouple is usually used to compensate the wire (free The end) extends into a relatively stable temperature control room and is connected to the meter terminals. It must be pointed out that the role of thermocouple compensation wire only extends the thermode, so that the cold junction of the thermocouple moves to the instrument terminal of the control room. It does not eliminate the influence of the temperature change of the cold junction on the temperature measurement, and does not compensate. Therefore, other correction methods need to be used to compensate for the effect of temperature at the cold junction temperature t0≠0°C.
When using a thermocouple to compensate for the wire, it must be noted that the model matches, and the polarity cannot be connected wrongly. The temperature of the connection terminal of the compensation wire and the thermocouple cannot exceed 100°C.