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The difference between the working principle of selecting thermocouple and thermal resistance in temperature transmitter

Temperature transmitter is a common instrument. The application range of temperature transmitter is very wide. There are temperature transmitters in various fields. What is the working principle of temperature transmitter? Thermocouple is generally used for The measurement of medium and high temperature, and the thermal resistance is mainly the measurement of low temperature. What is the difference between the working principle of selecting a thermocouple and a thermal resistor in a temperature transmitter? The following is a brief introduction to everyone.

Thermocouple
Thermocouple is one of the most commonly used temperature sensing components in the industry. The working principle of thermocouple is based on the Seeb effect, that is, two different conductors are connected at both ends into a loop. If the temperature of the two terminals is different, the loop is in the loop. A physical phenomenon in which a thermal current is generated. 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

The conductors or semiconductors A and B of two different materials are welded together to form a closed loop. 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. Types and structure formation of thermocouples

(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 Thermocouples Since January 1, 1988, thermocouples and thermal resistors have all been produced in accordance with IEC international standards, and seven standardized thermocouples, 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 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 when using precious metals), and the temperature measurement point is far away from the meter, in order to save thermocouple materials and reduce costs, the compensation wire is usually used to cool the cold end of the thermocouple (free The end extends to 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 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, you must pay attention to the model matching, the polarity can not be connected incorrectly, and the temperature between the compensation wire and the thermocouple connection terminal should not exceed 100 °C.

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