Thermal resistance is a kind of temperature detector commonly used in low temperature area. Its main features are high measurement accuracy and stable performance. It is widely used in industrial temperature measurement and is made into a standard reference instrument.

Working principle

The temperature measurement principle of thermal resistance is based on the characteristic that the resistance value of conductor or semiconductor changes with temperature to measure temperature and temperature-related parameters. Most of the thermal resistance is made of pure metal materials, the most widely used is platinum and copper, and now has begun to use nickel, manganese and rhodium and other materials to make thermal resistance. Thermal resistors usually need to pass the resistance signal through the lead to a computer control device or other secondary instrument.

Different from the temperature measurement principle of thermocouple, the thermal resistance is based on the thermal effect of the resistance to measure the temperature, that is, the resistance of the resistor changes with the change of temperature. Therefore, as long as the resistance change of the temperature sensitive thermal resistance is measured, the temperature can be measured. At present, there are mainly two types of metal thermal resistance and semiconductor thermistor.

The resistance value and temperature of metal thermal resistance can generally be expressed by the following approximate relationship, namely

Rt=Rt0[1 α(t-t0)]

In the formula, Rt is the resistance value at temperature t; Rt0 is the corresponding resistance value at temperature t0 (usually t0 = 0 ℃); α is the temperature coefficient.

The relationship between resistance and temperature of a semiconductor thermistor is

Rt=AeB/t

In the formula, Rt is the resistance value when the temperature is t; A and B depend on the constant of the structure of the semiconductor material.

In comparison, the temperature coefficient of thermistor is larger, and the resistance value at room temperature is higher (usually more than thousands of ohms), but the interchangeability is poor, the nonlinearity is serious, and the temperature measurement range is only about -50~300 ℃, which is widely used for temperature detection and control of home appliances and automobiles. Metal thermal resistance is generally suitable for temperature measurement in the range of -200~500 ℃. It is characterized by accurate measurement, good stability and reliable performance, and is widely used in process control.

thermal resistance material

The thermal resistance temperature measurement is based on the characteristic that the resistance value of the metal conductor increases with the increase of temperature. Most of the thermal resistance is made of pure metal materials, the most widely used is platinum and copper, in addition, has now begun to use nickel, manganese and rhodium and other materials to make thermal resistance.
Thermal resistance type (1) precision thermal resistance: industrial commonly used thermal resistance temperature sensing element (resistor body) structure and characteristics. From the temperature measurement principle of thermal resistance, it can be seen that the change of the measured temperature is directly measured by the change of the resistance value 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, three-wire system or four-wire system is generally used.

(2) armored thermal resistance: armored thermal resistance is a solid body composed of temperature sensing elements (resistance body), leads, insulating materials and stainless steel sleeves. Its outer diameter is generally φ2 ~ φ8mm, with a minimum of φmm. Compared with ordinary thermal resistance, it has the following advantages:

①Small size, no air gap inside, thermal inertia, measurement lag is small;

Good mechanical properties, vibration resistance, impact resistance;

③ can be bent, easy to install;

④ Long service life.

(3) end thermal resistance: end thermal resistance temperature sensing element by special treatment of resistance wire wound, close to the end of the thermometer. Compared with the general axial thermal resistance, it can reflect the actual temperature of the measured end face more correctly and quickly, and is suitable for measuring the end face temperature of the bearing bush and other parts.

(4) Flameproof thermal resistance: Flameproof thermal resistance through the special structure of the junction box, the internal shell of the explosive gas mixture due to the impact of sparks or arcs and other explosions confined in the junction box, the production site will not lead to super explosion. Flameproof thermal resistance can be used for temperature measurement in places with explosion hazard in Bla ~ B3c level area.

Industrial commonly used metal thermal resistance

From the point of view of the change of resistance with temperature, most metal conductors have this property, but not all can be used as temperature measuring thermal resistance. Metal materials as thermal resistance generally require: as large as possible and stable temperature coefficient, resistivity to be large (in the same sensitivity to reduce the size of the sensor), in the temperature range of use has a stable chemical and physical properties, the material replication is good, the resistance value changes with temperature to have a functional relationship (linear relationship).

At present, the most widely used thermal resistance materials are platinum and copper: platinum resistance has high precision, is suitable for neutral and oxidizing media, has good stability, has certain nonlinearity, and the higher the temperature, the smaller the resistance change rate is. The resistance value of copper resistance is linear with temperature in the temperature measurement range, the number of temperature lines is large, it is suitable for non-corrosive medium, and it is easy to be oxidized over 150. The most commonly used in China are R0 = 10Ω, R0 = 100Ω and R0 = 1000Ω, and their indexing numbers are Pt10, Pt100 and Pt1000 respectively. There are two kinds of copper resistors: R0 = 50Ω and R0 = 100Ω, and their indexing numbers are Cu50 and Cu100. Pt100 and Cu50 are the most widely used.

The signal connection mode of the thermal resistance The thermal resistance is a component that converts the temperature change into the resistance value change. It is usually necessary to transmit the resistance signal to the computer control device or other primary instruments through the lead. Industrial thermal resistance is installed in the production site, and there is a certain distance between the control room, so the lead wire of the thermal resistance will have a greater impact on the measurement results.