Thermal resistance is the most commonly used type of temperature detector in the low and medium temperature areas. Its main characteristics are high measurement accuracy, stable performance and good linearity. Among them, the measurement accuracy of platinum thermal resistance is the highest. It is not only widely used in industrial process temperature measurement, but also used to make a standard reference instrument.
1. Thermal resistance temperature measurement principle and material
Thermal resistance temperature measurement is based on the physical characteristic that the resistance value of a metal conductor increases with temperature. Most of the thermal resistances are made of pure metal materials. At present, the most commonly used are platinum and copper, and two metals. In addition, materials such as copper, nickel, manganese, and rhodium have been used to make thermal resistance.
2. Structure of thermal resistance
(1) Assembly-type thermal resistance From the temperature measurement principle of 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 Impact on temperature measurement. In order to eliminate the influence of the lead resistance, a three-wire system or a four-wire system is generally used.
(2) Armored thermal resistance Armored thermal resistance is a solid body composed of a combination of temperature sensing elements (resistors), leads, insulation materials, and stainless steel sleeves. Its outer diameter is generally φ2 ~ φ8mm, and the minimum can reach φ1mm. . 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. 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 the casing due to the impact of sparks or arcs, etc., within the junction box. explosion. 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 resistance change of the connecting wire, a three-wire connection method must be used.