1, the principle of temperature measurement of thermal resistance
Different from the temperature measurement principle of the thermocouple, the thermal resistance is based on the thermal effect of the resistance, that is, the resistance of the resistor changes with temperature. Therefore, as long as the resistance change of the 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 the metal thermal resistance can generally be expressed by the following approximate relationship, that is,
Rt=Rt0[1+α(t-t0)]
Where Rt is the resistance at temperature t; Rt0 is the corresponding resistance value at temperature t0 (usually t0 = 0 °C); α is the temperature coefficient.
The relationship between the resistance and temperature of the semiconductor thermistor is
Rt=AeB/t
Where Rt is the resistance at a temperature of t; A, B depends on the constant of the structure of the semiconductor material.
In comparison, the thermistor has a higher temperature coefficient and a higher resistance value at normal temperature (usually above several thousand ohms), but the interchangeability is poor, the nonlinearity is severe, and the temperature range is only -50 to 300 °C. Left and right, a large number of temperature detection and control for home appliances and automobiles. Metal thermal resistance is generally suitable for temperature measurement in the range of -200~500 °C. It is characterized by accurate measurement, good stability and reliable performance. It is widely used in process control.
2, commonly used metal thermal resistance in industry
From the point of view of the change of resistance with temperature, most metal conductors have this property, but they can not be used as temperature measuring resistors. Metal materials as thermal resistors generally require: as large and stable temperature coefficient and resistivity as possible. Larger (reducing the size of the sensor under the same sensitivity), stable chemical and physical properties in the temperature range used, good material replication, and resistance value change with temperature (best linear relationship) ).
At present, the most widely used thermal resistance materials are platinum and copper: platinum has high precision and is suitable for neutral and oxidizing media. It has good stability and has certain nonlinearity. The higher the temperature, the smaller the rate of change of resistance; the copper resistance is measured. The resistance value and the temperature are linear in the temperature range, the temperature line is large, and it is suitable for non-corrosive media, and more than 150 is easily oxidized. The most commonly used in China are R0=10Ω, R0=100Ω and R0=1000Ω. Their index numbers are Pt10, Pt100 and Pt1000 respectively; copper resistors are R0=50Ω and R0=100Ω, their indexing The numbers are Cu50 and Cu100. Among them, Pt100 and Cu50 are the most widely used.
3. Signal connection method of thermal resistance
A thermal resistor is a primary component that converts a change in temperature into a change in resistance. It is usually necessary to pass a resistive signal through a lead to a computer control device or other instrument. Industrial thermal resistors are installed at the production site, and there is a certain distance from the control room. Therefore, the leads of the thermal resistors have a great influence on the measurement results.
There are three main ways to wire the thermal resistance.
â—‹1 two-wire system: The method of connecting a wire at each end of the thermal resistor to extract the resistance signal is called two-wire system: the lead method is simple, but the lead wire must have the lead resistance r, the size of the r and the material of the wire. The length factor is related, so this lead method is only suitable for occasions with low measurement accuracy.
â—‹ 2 three-wire system: one lead is connected to one end of the root of the thermal resistor, and the other end is connected to two leads. This method is usually used together with the bridge, which can better eliminate the influence of lead resistance. It is the most commonly used lead resistance in industrial process control.
â—‹3 four-wire system: The method of connecting two wires at the two ends of the thermal resistance is called a four-wire system. Two of the leads provide a constant current I for the thermal resistance, convert R into a voltage signal U, and pass the other two. The root lead leads U to the secondary meter. It can be seen that this type of lead wire can completely eliminate the influence of the resistance of the lead wire, and is mainly used for high-precision temperature detection.
4, the structural form of the thermal resistance
Similar to the thermocouple temperature sensor, the thermal resistors commonly used in the industry mainly include two types of common assembly type thermal resistance and armored thermal resistance.
Ordinary universal assembled thermal resistance is made up of temperature sensing body, stainless steel outer protective tube, junction box and various fixing devices. The mounting fixture has fixed external thread, movable flange, fixed flange and belt fixing. Bolt tapered protection tube and other forms. The armored thermal resistance outer protective sleeve is made of stainless steel and filled with high-density oxide insulator. It has strong anti-pollution performance and excellent mechanical strength. Compared with the former, the armored thermal resistance has the advantages of small diameter, easy bending, good shock resistance, fast thermal response time and long service life.
For some special temperature measurement occasions, some professional type thermal resistances can also be selected. For example, the end surface thermal resistance can be selected for measuring the solid surface temperature. Explosion-proof type thermal resistance can be selected for the flammable and explosive occasions. The temperature on the vibration equipment can be selected. Thermal resistance with shockproof structure, etc.
Toner Cartridge For Brother,New Brother Compatible Toner,Tn2225 Black Toner Cartridge,Brand Brother Compatible Toner
jiangmen jinheng office equipment Co. Ltd. , https://www.jm-jinheng.com