Principle and Application of Temperature Sensor

The principle and application of temperature sensors: Firstly, the application principle of thermocouples. Thermocouples are one of the most commonly used temperature detection components in industry It has the advantage of high measurement accuracy. Due to direct contact with the measured object, thermocouples are not affected by the intermediate medium Wide measurement range. Typical thermocouples can measure continuously from -50~+1600 ℃, while specific special thermocouples can measure up to -269 ℃ (such as iron nickel chromium alloy) and+2800 ℃ (such as tungsten rhenium) The structure is simple and easy to use. Thermocouples are usually composed of two different types of wires, and are not limited by size or opening. They have protective covers on the outside, making them easy to use. 1 . The basic principle of thermocouple temperature measurement is to weld two different materials of conductors or semiconductors a and b, as shown in Figure 2-1-1, to form a closed circuit. When there is a temperature difference between the two attachment points 1 and 2 of conductors a and b, an electromotive force is generated between them, resulting in a large current in the circuit, which is called the thermoelectric effect. Thermocouples utilize this effect to function. 2. Types and structures of thermocouples (1) Types of thermocouples Commonly used thermocouples can be divided into two types: standard thermocouples and non-standard thermocouples. The standard thermocouple used refers to a thermocouple that is specified in national standards for the relationship between thermoelectric potential and temperature, allowable errors, and a unified standard scale, and has a matching display. Non standardized thermocouples are not as suitable for use as thermocouples with standardized ranges or orders of magnitude, and generally do not have a unified scale. They are mainly used for measurements in special situations. Standardized Thermocouples: Since January 1, 1988, thermocouples and thermal resistances in China have been produced in accordance with IEC international standards. Seven standardized thermocouples, namely s, b, e, k, r, j, and t, have been designated as the unified design type thermocouples in China. (2) In order to ensure the reliable and stable operation of the thermocouple, the structure of the thermocouple includes: ① The welding of the two thermoelectric poles that make up the thermocouple must be firm ② The two thermoelectric poles must be insulated from each other to prevent short circuits ③ The connection between the compensating wire and the free end of the thermocouple must be simple and reliable The temperature compensation of the cold end of a thermocouple, due to the fact that the material of the thermocouple is generally valuable (especially when using precious metals), the distance from the temperature measuring point to the measuring instrument is relatively long. In order to save the material of the thermocouple and reduce costs, a compensation wire is usually used to extend the cold end (free end) of the thermocouple to a relatively stable temperature control room and connect it to the measuring instrument terminal. It must be pointed out that the function of the thermocouple compensation wire is only to move the cold end of the thermocouple to the instrument terminal in the control room, and cannot eliminate the influence of temperature changes in the cold end on temperature measurement, and cannot be compensated. Therefore, in order to compensate for the cooling temperature t0&ne, other correction methods are needed to affect the temperature measurement at 0 ℃. When using thermocouple compensation wires, it is important to ensure consistency in form and avoid polarity errors. The temperature at the connection end between the compensation wire and the thermocouple should not exceed 100 ℃. 2, The application principle of thermal resistance is that thermal resistance is the most commonly used temperature detector in the medium and low temperature range. Its main characteristics are high measurement accuracy and stable performance. Among them, platinum has the highest resistance measurement and has been widely used not only in industrial temperature measurement, but also in standard instruments. 1 . The temperature measurement principle and material of a temperature measuring resistor are based on the characteristic that the resistance value of a metal conductor increases with temperature. Thermistors are almost entirely made of pure metal materials, with platinum and copper being the most commonly used nowadays. In addition, materials such as median, nickel, manganese, and rhodium are also being used to manufacture thermistors. 2 . The structure and characteristics of thermal resistance sensors (resistors) commonly used in the industry are shown in Table 2-1-11. According to the temperature measurement principle of the temperature measuring resistor, the change in the measured temperature is directly measured by the change in the resistance value of the temperature measuring resistor. Therefore, the changes in the resistance of various leads such as the leads of the temperature measuring resistor have an impact on temperature measurement. (2) Sheath thermal resistor is a sturdy body that combines temperature sensing elements (resistors), leads, insulation materials, and stainless steel sleeves, as shown in Figure 2-1-7. The outer diameter is generally phi; 2~φ 8mm, the smallest achievable size; Mm. Compared with ordinary thermal resistors, it has the advantages of small volume, no internal air gap, and small measurement lag in thermal inertia. ② It has good mechanical performance, vibration resistance, and impact resistance. ③ It can bend and is easy to install. ④ It has a long service life. (3) The end face thermal resistance and temperature sensing element is wound with specially treated resistance wire, which is in close contact with the end face of the thermometer. Its structure is shown in Figure 2-1-8. Compared with typical axial thermal resistors, this can more accurately and quickly reflect the actual temperature of the measured end face, making it suitable for measuring the end face temperature of metals and other workpieces. (4) The explosion-proof thermal resistor partition of the explosion-proof thermal resistor body seals the explosive mixture gas inside the shell in the terminal box caused by sparks or arcs through a special structure terminal box, and no explosion occurs on the production site. Explosion proof thermal resistors can be used for temperature measurement in areas with explosion hazards such as Bla~B3c level. 3 . The temperature measuring resistor system that constitutes the temperature measuring resistor system generally consists of a temperature measuring resistor, connecting wires, a display, etc The division codes of the temperature measuring resistor and the display must be consistent. In order to eliminate the influence of changes in the resistance of the connecting wires, it is necessary to use a three wire connection method. Please refer to Chapter 3 of this article for specific content. (2) Sheath temperature measuring resistor is a sturdy body that combines temperature sensing elements (resistors), leads, insulation materials, and stainless steel sleeves, as shown in Figure 2-1-7. Its outer diameter is generally phi; 2~φ 8mm, the smallest achievable size; Compared with ordinary thermal resistors, it has the advantages of small size, no internal air gap, and small measurement lag in thermal inertia. ② It has good mechanical performance, vibration resistance, impact resistance, ③ It is easy to bend and install, and ④ It has a long service life. (3) The end face thermal resistance and temperature sensing element is wound with specially treated resistance wire, which is in close contact with the end face of the thermometer. Its structure is shown in Figure 2-1-8. Compared with typical axial thermal resistors, this can more accurately and quickly reflect the actual temperature of the measured end face, making it suitable for measuring the end face temperature of metals and other workpieces. (4) The explosion-proof thermistor antibody is repaired by a specially constructed terminal box. In order to allow the explosive mixed gas inside the shell to be affected by sparks or arcs, changing the length of the resistance wire has an impact on the resistance value. Therefore, it is better to replace the resistance antibody with a new one. When using welding repair, it should be verified as qualified after welding before use.