Thermal resistance is the most commonly used temperature detector in the middle and low temperature areas. Its main features are high measurement accuracy and stable performance. Among them, platinum thermal resistance has the highest measurement accuracy. 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 materials
The thermal resistance temperature measurement is based on the characteristic that the resistance value of the metal conductor increases with the increase in temperature for temperature measurement. Thermal resistors are mostly made of pure metal materials. At present, platinum and copper are the most widely used. In addition, materials such as dian, nickel, manganese, and rhodium have been used to make thermal resistors.
2. The structure of thermal resistance
(1) Proficient thermal resistance
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 of the thermal resistance. Therefore, the change of the resistance of various wires such as the lead wire of the thermal resistance will affect the temperature measurement. In order to eliminate the influence of lead resistance, a three-wire system or a four-wire system is generally adopted.
(2) Armored thermal resistance
Armored thermal resistance is a solid body composed of temperature sensing element (resistor body), lead wire, insulating material, and stainless steel casing. Its outer diameter is generally φ2~φ8mm, and the smallest can reach φmm. Compared with ordinary thermal resistance, it has the following advantages:
①Small size, no internal air gap, thermal inertia, low measurement hysteresis;
②Good mechanical performance, vibration resistance and impact resistance;
③It can be bent and is easy to install;
④Long service life.
(3) End surface thermal resistance
The end surface thermal resistance temperature sensing element is wound by a specially treated resistance wire, and it is closely attached to the end surface of the thermometer. Its structure is shown in Figure 2-1-8. Compared with general axial thermal resistance, it can reflect the actual temperature of the measured end surface more accurately and quickly, and is suitable for measuring the end surface temperature of bearing bushes and other mechanical parts.
(4) Flameproof thermal resistance
Explosion-proof thermal resistance uses a junction box with a special structure to confine the explosion of the explosive mixed gas inside the shell due to the influence of sparks or arcs in the junction box, and the production site will not cause excessive explosion. Explosion-proof thermal resistance can be used for temperature measurement in places with explosion hazard in Bla~B3c zone.
3. The composition of the thermal resistance temperature measurement system
The thermal resistance temperature measurement system is generally composed of thermal resistance, connecting wires and display instruments. The following two points must be paid attention to:
(1) The graduation number of the thermal resistance and the display instrument must be consistent;
(2) In order to eliminate the influence of the resistance change of the connecting wire, a three-wire connection method must be adopted.
