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Application principle of thermocouple and thermal resistance I. Application principle of thermocouple

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 Since 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 China's 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 thermal 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 relatively 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 connect the cold end of the thermocouple (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.

Application principle of thermal resistance

Thermal resistance is the most commonly used type of temperature detector in low and medium temperature areas.

Its main features are high measurement accuracy and stable performance. Among them, the measurement accuracy of platinum thermal resistance is the highest. It is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.

1. Thermal resistance temperature measurement principle and material

Thermal resistance temperature measurement is based on the characteristic that the resistance value of a metal conductor increases with temperature. Most of the thermal resistances are made of pure metal materials, and platinum and copper are currently the most widely used. In addition, materials such as copper, nickel, manganese and rhodium have been used to make thermal resistances.

2. Structure of thermal resistance

(1) The structure and characteristics of the thermal resistance temperature sensing element (resistor) commonly used in the proficient thermal resistance industry are shown in Table 2-1-11. From the temperature measurement principle of the thermal resistance, it can be known that the change of the measured temperature is directly measured by the change of the resistance value of the thermal resistance. Therefore, changes in the resistance of various wires such as the lead wire of the thermal resistance body will affect the temperature measurement. In order to eliminate the influence of the lead resistance, a three-wire system or a four-wire system is generally used. For details, refer to the first section of Chapter Three of this chapter.

(2) Armored thermal resistance Armored thermal resistance is a solid body composed of a combination of a temperature sensing element (resistor), leads, insulation material, and stainless steel sleeve. As shown in Figure 2-1-7, its outer diameter Generally φ2 ~ φ8mm, the minimum can reach φmm.

Compared with ordinary thermal resistance, it has the following advantages: ① small size, no air gap inside, small measurement hysteresis in thermal inertia; ② good mechanical properties, vibration resistance, impact resistance; ③ can bend, easy to install ④ long life .

(3) End-face thermal resistance The end-face thermal resistance temperature sensing element is wound by a specially-treated resistance wire and is closely attached to the end surface of the thermometer. Its structure is shown in Figure 2-1-8. Compared with the general axial thermal resistance, it can more accurately and quickly reflect the actual temperature of the measured end face, and is suitable for measuring the end face temperature of the bearing pad and other parts.

(4) Explosion-proof thermal resistance Explosion-proof thermal resistance uses a special structure of the junction box to limit the explosion of the explosive mixed gas inside its casing to the junction box due to the effects of sparks or arcs. . Explosion-proof thermal resistance can be used for temperature measurement in explosion-hazardous areas in the Bla ~ B3c class area.

3． Composition of thermal resistance temperature measurement system

Thermal resistance temperature measurement system generally consists of thermal resistance, connecting wires and display instruments. You must pay attention to the following two points:

① The graduation number of the thermal resistance and the display instrument must be the same

② In order to eliminate the influence of the resistance change of the connecting wire, a three-wire connection method must be used. For details, see Chapter 3 of this article.

(2) Armored thermal resistance Armored thermal resistance is a solid body composed of a combination of a temperature sensing element (resistor), leads, insulation material, and stainless steel sleeve. As shown in Figure 2-1-7, its outer diameter Generally φ2 ~ φ8mm, the minimum can reach φmm. Compared with ordinary thermal resistance, it has the following advantages: ① small size, no air gap inside, small measurement hysteresis in thermal inertia; ② good mechanical properties, vibration resistance, impact resistance, ③ can bend, easy to install ④ long life .

(3) End-face thermal resistance The end-face thermal resistance temperature sensing element is wound by a specially-treated resistance wire and is closely attached to the end surface of the thermometer. Its structure is shown in Figure 2-1-8. Compared with the general axial thermal resistance, it can more accurately and quickly reflect the actual temperature of the measured end face, and is suitable for measuring the end face temperature of the bearing pad and other parts.

(4) Explosion-proof thermal resistance Explosion-proof thermal resistance uses a junction box with a special structure to repair the explosive mixed gas inside its casing due to the interruption of the spark resistor or the arc resistor, which must change the length of the resistance wire and affect the resistance value. For this reason, it is better to replace the new resistor body. If welding repair is used, it can only be used after it has been verified after welding.