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The principle and type of thermocouple
Thermocouples are one of the 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 measured continuously from -50 to +1600 °C . Some special thermocouples can measure -269 °C (such as gold-iron nickel-chromium) and zui can reach +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 . Basic principle of thermocouple temperature measurement
Solder the conductors or semiconductors A and B of two different materials 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 , and thus a current of a magnitude is formed in the loop . This phenomenon is called a thermoelectric effect. Thermocouples use this effect to work.
2 . Thermocouple type and structure formation
(1) Type of thermocouple
Commonly used thermocouples can be divided into two categories: 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 not as good as standardized thermocouples in terms of use or order of magnitude. Generally, there is no uniform indexing table, which is mainly used for measurement in some special occasions. Standardized thermocouple China from January 1, 1988, all of the thermocouple and thermal resistance according to IEC international standards, and designated S, B, E, K, R, J, T for the seven kinds of unified design standard thermocouple type thermocouple.
(2) Structural form 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 constituting 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 . Temperature compensation of the cold junction of the thermocouple
Because thermocouple materials are generally more expensive (especially expensive Metal time), and the temperature measurement point is far away from the meter, in order to save heat Galvanic materials, reducing costs, usually using compensation wires to cool the thermocouple The end (free end) extends into a relatively stable temperature control room, connected to On the meter terminal. 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 instrument terminal of the control room, which itself does not eliminate the influence of the temperature change of the cold end on the temperature measurement, and does not 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 should not exceed 100 °C.
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