Thermocouple application and principle
2021-04-18 18:01:36
Thermocouples are one of the most commonly used temperature sensing components in the industry. The advantages are:
1 Measurement accuracy is high. Because the thermocouple is in direct contact with the object to be tested, it is not affected by the intermediate medium.
2 Wide measurement range. Commonly used thermocouples can be continuously measured from -50 to +1600 °C. Some special thermocouples can measure -269 °C (such as gold-iron nickel-chromium), up to +2800 °C (such as tungsten-bismuth).
3 simple structure, easy to use. Thermocouples are usually made up of two different wires and are not limited by size and opening. They have a protective sleeve and are very convenient to use.
1. The basic principle of thermocouple temperature measurement solders two 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 attachment points 1 and 2 of the conductors A and B, an electromotive force is generated between the two, thereby forming a current of a magnitude in the loop, which is called a thermoelectric effect. Thermocouples use this effect to work.
2. Thermocouple type and structure formation
(1) Types of thermocouples Thermocouples can be divided into two types: standard thermocouples and non-standard thermocouples. The standard thermocouple that is called refers to the thermocouple that the national standard specifies the relationship between the thermoelectric potential and the temperature, the allowable error, and the uniform standard index table. It has a display instrument that is compatible with it. Non-standardized thermocouples are inferior to standardized thermocouples in terms of application range or magnitude, and generally do not have a uniform indexing table, which is mainly used for measurement in some special occasions.
Standardized thermocouples Since January 1, 1988, thermocouples and thermal resistors have all been produced in accordance with IEC international standards, and seven standardized thermocouples of S, B, E, K, R, J and T have been designated as China's unified design. Thermocouple.
(2) Structure of thermocouple In order to ensure reliable and stable operation of the thermocouple, its structural requirements are as follows:
1 The welding of the two hot electrodes that make up 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 compensation wire and the free end of the thermocouple should be convenient and reliable;
4 The protective sleeve should ensure that the hot electrode is sufficiently isolated from harmful media.
3. The temperature compensation of the cold junction of the thermocouple is generally expensive due to the thermocouple material (especially when using precious metals), and the distance between the temperature measurement point and the meter is very long. In order to save the thermocouple material and reduce the cost, the compensation wire is usually used to heat the electricity. The even cold end (free end) extends into a relatively stable temperature control room and is connected to the meter terminals. It must be pointed out that the function of the thermocouple compensation wire only extends the hot electrode, so that the cold end of the thermocouple moves to the meter terminal of the control room, which itself cannot eliminate the influence of the temperature change of the cold end on the temperature measurement, and cannot compensate. Therefore, other correction methods are needed to compensate for the influence of the cold junction temperature t0 ≠0 ° C on the temperature measurement.
When using the thermocouple compensation wire, it must be noted that the model is matched, the polarity cannot be connected incorrectly, and the temperature between the compensation wire and the thermocouple connection terminal cannot exceed 100 °C.
1 Measurement accuracy is high. Because the thermocouple is in direct contact with the object to be tested, it is not affected by the intermediate medium.
2 Wide measurement range. Commonly used thermocouples can be continuously measured from -50 to +1600 °C. Some special thermocouples can measure -269 °C (such as gold-iron nickel-chromium), up to +2800 °C (such as tungsten-bismuth).
3 simple structure, easy to use. Thermocouples are usually made up of two different wires and are not limited by size and opening. They have a protective sleeve and are very convenient to use.
1. The basic principle of thermocouple temperature measurement solders two 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 attachment points 1 and 2 of the conductors A and B, an electromotive force is generated between the two, thereby forming a current of a magnitude in the loop, which is called a thermoelectric effect. Thermocouples use this effect to work.
2. Thermocouple type and structure formation
(1) Types of thermocouples Thermocouples can be divided into two types: standard thermocouples and non-standard thermocouples. The standard thermocouple that is called refers to the thermocouple that the national standard specifies the relationship between the thermoelectric potential and the temperature, the allowable error, and the uniform standard index table. It has a display instrument that is compatible with it. Non-standardized thermocouples are inferior to standardized thermocouples in terms of application range or magnitude, and generally do not have a uniform indexing table, which is mainly used for measurement in some special occasions.
Standardized thermocouples Since January 1, 1988, thermocouples and thermal resistors have all been produced in accordance with IEC international standards, and seven standardized thermocouples of S, B, E, K, R, J and T have been designated as China's unified design. Thermocouple.
(2) Structure of thermocouple In order to ensure reliable and stable operation of the thermocouple, its structural requirements are as follows:
1 The welding of the two hot electrodes that make up 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 compensation wire and the free end of the thermocouple should be convenient and reliable;
4 The protective sleeve should ensure that the hot electrode is sufficiently isolated from harmful media.
3. The temperature compensation of the cold junction of the thermocouple is generally expensive due to the thermocouple material (especially when using precious metals), and the distance between the temperature measurement point and the meter is very long. In order to save the thermocouple material and reduce the cost, the compensation wire is usually used to heat the electricity. The even cold end (free end) extends into a relatively stable temperature control room and is connected to the meter terminals. It must be pointed out that the function of the thermocouple compensation wire only extends the hot electrode, so that the cold end of the thermocouple moves to the meter terminal of the control room, which itself cannot eliminate the influence of the temperature change of the cold end on the temperature measurement, and cannot compensate. Therefore, other correction methods are needed to compensate for the influence of the cold junction temperature t0 ≠0 ° C on the temperature measurement.
When using the thermocouple compensation wire, it must be noted that the model is matched, the polarity cannot be connected incorrectly, and the temperature between the compensation wire and the thermocouple connection terminal cannot exceed 100 °C.
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