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
Thermistor temperature measurement is based on the characteristic that the resistance value of a metal conductor increases with increasing temperature to measure temperature.
Thermal resistors are mostly made of pure metal materials. Currently, platinum and copper are the most widely used. In addition, materials such as copper, nickel, manganese and rhodium have been used to make thermal resistors.
2. Structure of thermal resistance
(1) Proficient thermal resistance The structure and characteristics of industrial thermal resistance temperature sensing elements (resistors) 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 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 .
3 (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 face 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
Thermistor temperature measurement system generally consists of thermal resistance, connecting wires and display instruments. You must pay attention to the following two points:
①The division 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, 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 (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 face 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 expose the explosive mixed gas inside its casing due to the effects of sparks or arcs.
Repair of the resistor body 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 must be verified after welding before it can be used.