Thermocouples are one of the most commonly used temperature sensing elements in the industry. Its advantages are:
① High measurement accuracy. Because the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium.
② Wide measurement range. Common thermocouples can be continuously measured from -50 to + 1600 ° C. Some special thermocouples can measure as low as -269 ° C (such as gold-iron-nickel-chromium) and up to + 2800 ° C (such as tungsten-rhenium).
③ Simple structure and easy to use. Thermocouples are usually composed of two different metal wires, and they are not limited by size and opening. They have protective sleeves and are very convenient to use.
1. The basic principle of thermocouple temperature measurement
Solder two conductors or semiconductors A and B of 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 them, and a large current is formed in the circuit. This phenomenon is called a thermoelectric effect. Thermocouples use this effect to work.
2. Type and structure of thermocouple
(1) Type of thermocouple
Commonly used thermocouples can be divided into standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to the thermocouple whose national standard stipulates the relationship between thermoelectric potential and temperature, the allowable error, and has a unified standard index table. It has a matching display instrument to choose from. Non-standardized thermocouples are inferior to standardized thermocouples in the scope or magnitude of use. Generally, there is no unified indexing table, which is mainly used for measurement in some special occasions. Standardized thermocouples in China From January 1, 1988, all thermocouples and thermal resistors have 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 the unified design type Thermocouple.
(2) The structure of the thermocouple In order to ensure the 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 circuit;
③ The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;
④ The protective sleeve should be able to ensure that the hot electrode is fully isolated from the harmful medium.
3． Temperature compensation for the cold junction of a thermocouple
Because the materials of thermocouples are generally more expensive (especially when noble metals are used), and the distance between the temperature measurement point and the instrument is very long, in order to save the thermocouple materials and reduce costs, compensation wires are usually used to cool the thermocouples. The (free end) extends into the control room where the temperature is relatively stable, and is connected to the instrument 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 is moved to the instrument terminal in the control room. It cannot eliminate the influence of the cold junction temperature change on the temperature measurement, and it does not compensate. Therefore, other correction methods are needed to compensate for the effect on the temperature measurement when the cold junction temperature t0 ≠ 0 ° C. When using the thermocouple compensation wire, you must pay attention to the matching of the model, the polarity must not be wrong, and the temperature of the connection end of the compensation wire and the thermocouple cannot exceed 100 ° C.