Application principle of thermocouple and thermal resistance First, application principle of thermocouple Thermocouple is one of the most commonly used temperature detection components in industry. Its advantages are:
1 high measurement accuracy. Because the thermocouple is directly in contact with the measured object, it is not affected by the intermediate medium.
2 wide measurement range. Commonly used thermocouples can be measured from -50 to +1600°C. Some special thermocouples can measure at least -269°C (such as gold, iron, nickel-chromium) and up to +2,800°C (such as tungsten-germanium).
3 simple structure, easy to use. Thermocouples are usually composed of two different types of wires, and are not limited by size and the beginning. They have protective sleeves and are very convenient to use.
1. Thermocouple Temperature Measurement Fundamentals
Two different material conductors or semiconductors A and B are welded together to form a closed loop, as shown in Figure 2-1-1. When there is a temperature difference between the two sticking points 1 and 2 of the conductors A and B, an electromotive force is generated between the two sticking points 1 and 2 so that a current of a size is formed in the loop. This phenomenon is called a thermoelectric effect. Thermocouples use this effect to work.
2. Types and Structures of Thermocouples (1) Types of Thermocouples
Commonly used thermocouples can be divided into standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to a thermocouple whose national standard stipulates the relationship between thermoelectric potential and temperature, tolerance, and a unified standard indexing table. It has its own display instrument for selection. Non-standardized thermocouples are less than standard thermocouples in terms of range of use or magnitude, and there is generally no uniform index table, which is mainly used for measurement in certain special occasions. Standardized Thermocouples From January 1, 1988, thermocouples and thermal resistances were all produced according to IEC international standards, and seven standardized thermocouples designated as S, B, E, K, R, J, and T were designed for unified design in China. Thermocouples.
(2) The structure of the thermocouple To ensure reliable and stable operation of the thermocouple, its structural requirements are as follows:
1 The welding of the two hot electrodes forming 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 ensure that the hot electrode is fully isolated from harmful media.
3. Thermocouple cold junction temperature compensation
Because thermocouple materials are generally more expensive (especially when precious metals are used), and the distance from the temperature measurement point to the instrument is very long. In order to save the thermocouple material and reduce the cost, the cold end of the thermocouple is usually used to compensate the wire (free The end) extends into a relatively stable temperature control room and is connected to the meter terminals. It must be pointed out that the role of thermocouple compensation wire only extends the thermode, so that the cold junction of the thermocouple moves to the instrument terminal of the control room. It does not eliminate the influence of the temperature change of the cold junction on the temperature measurement, and does not compensate. Therefore, other correction methods need to be used to compensate for the effect of temperature at the cold junction temperature t0≠0°C.
When using a thermocouple to compensate for the wire, it must be noted that the model matches, and the polarity cannot be connected wrongly. The temperature of the connection terminal of the compensation wire and the thermocouple cannot exceed 100°C.
Second, the principle of application of thermal resistance Thermal resistance is the most commonly used temperature detector in the low temperature region.
Its main features are high measurement accuracy and stable performance. Platinum thermal resistance is the highest measurement accuracy. It is widely used not only in industrial temperature measurement but also as 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 the metal conductor increases as the temperature increases. Most of the thermal resistance is made of pure metal materials. At present, platinum and copper are the most widely used materials. In addition, the use of dian, nickel, manganese, and tantalum has been used to manufacture thermal resistors.
2. Thermal resistance structure
(1) Proper type thermal resistance The structure and characteristics of the common thermal resistance temperature sensing element (resistor) are shown in Table 2-1-11. From the principle of temperature measurement of the thermal resistance, it can be seen that the change of the measured temperature is directly measured by the change of the resistance of the thermal resistor. Therefore, the change in the resistance of the lead wire, such as the thermal resistance body, will affect the temperature measurement. In order to eliminate the influence of the lead resistance, the three-wire or four-wire system is adopted in the same way. For details, refer to the first section of Chapter III of this paper.
(2) Armored Thermal Resistance An Armored Thermal Resistance is a solid body composed of a temperature sensing element (resistor), leads, insulating material, and a stainless steel sleeve, as shown in Figure 2-1-7. Generally φ2~φ8mm, the minimum can reach φmm.
Compared with common thermal resistance, it has the following advantages: 1 small size, no internal air gap, thermal inertia, small measurement lag; 2 good mechanical properties, vibration resistance, impact resistance; 3 can bend, easy to install 4 long service life .
(3) End face thermal resistance The end face thermal resistance temperature sensing element is wound by a specially treated resistance wire and is attached to the end face of the thermometer. The structure is shown in Figure 2-1-8. Compared with general axial thermal resistance, it can more accurately and quickly reflect the actual temperature of the tested end face, and is suitable for measuring the end face temperature of the bearing pad and other parts.
(4) Flameproof RTD flameproof RTDs are limited to the junction box by the junction box with a special structure, and the explosion of the explosive gas mixture inside the casing due to sparks or arcs is limited to the explosion at the production site. . Flameproof RTDs can be used for temperature measurement in explosion-hazardous areas in Bla-B3c zones.
3. The composition of the thermal resistance temperature measurement system The thermal resistance temperature measurement system is generally composed of a thermal resistance, connecting wires and display instruments. Must pay attention to the following two points:
1 The index number of the thermal resistance and the display instrument must be the same. 2 In order to eliminate the influence of the change in the resistance of the connecting wire, a three-wire connection must be used. For details, see Chapter 3 of this essay.
(2) Armored Thermal Resistance An Armored Thermal Resistance is a solid body composed of a temperature sensing element (resistor), leads, insulating material, and a stainless steel sleeve, as shown in Figure 2-1-7. Generally φ2~φ8mm, the minimum can reach φmm. Compared with common thermal resistance, it has the following advantages: 1 small size, no internal air gap, thermal inertia, small measurement lag; 2 good mechanical properties, vibration resistance, impact resistance, 3 can bend, easy to install 4 long service life .
(3) End face thermal resistance The end face thermal resistance temperature sensing element is wound by a specially treated resistance wire and is attached to the end face of the thermometer. The structure is shown in Figure 2-1-8. Compared with general axial thermal resistance, it can more accurately and quickly reflect the actual temperature of the tested end face, and is suitable for measuring the end face temperature of the bearing pad and other parts.
(4) Flameproof thermal resistance flameproof type thermal resistance through the special structure of the junction box, the explosion of the mixture of gas inside the shell due to sparks or arcs and other shadow resistor body repair must inevitably change the length of the resistance wire and affect the resistance value, It is better to replace the new resistor body for this purpose. If soldering is used for repair, it must be verified after passing the soldering.