Industrial resistance thermometerThe principle is to utilize the characteristic of the resistance of conductors or semiconductors changing with temperature. The main advantages of a thermistor thermometer are: high measurement accuracy and good reproducibility; Has a large measurement range, especially in low temperatures; Easy to use in automatic measurement and also convenient for long-distance measurement. Similarly, thermal resistors also have defects, such as poor accuracy in high temperature (greater than 850 ℃) measurements; Easy to oxidize and not resistant to corrosion.
The materials used for thermal resistance mainly include platinum, copper, nickel, etc. The use of these materials is mainly due to their linear relationship between temperature and resistance in common temperature ranges. Here, we mainly introduce platinum resistance thermometers.
Platinum is a precious metal with stable physical and chemical properties, especially strong oxidation resistance. It is easy to purify, has good processability, and can be made into extremely fine platinum wires. Compared with metals such as copper and nickel, it has higher electrical resistivity and high reproducibility, making it an ideal material for thermal resistance. However, its disadvantage is that the temperature coefficient of resistance is small, and it is prone to brittleness when working in reducing media, making it expensive. The purity of platinum is usually expressed in terms of resistance ratio: W (100)=R100/R0
R100 represents the resistance value at 100 ℃; R0 represents the resistance value at 0 ℃
According to IEC standards, platinum resistors with an initial resistance value of R0=100 Ω (R0=10 Ω) and W (100)=1.3850 are used as industrial standard platinum resistors. Platinum resistance thermometers with R0=10 Ω have thicker resistance wires and are mainly used to measure temperatures above 600 ℃. The resistance temperature equation of platinum resistance is a piecewise equation:
Rt=R0 [1+At+Bt2+C (t-100 ℃) t3] t represents the temperature range from -200 ℃ to 0 ℃
Rt=R0 (1+At+Bt2) t represents the temperature range from 0 to 850 ℃
By solving this equation, the temperature value can be known based on the resistance value. However, in practical work, the temperature value can be determined based on the resistance value by referring to the thermal resistance scale.
According to the standard regulations, platinum thermistors are divided into Class A and Class B. Class A has an allowable temperature measurement error of ± (0.15 ℃+0.002 | t |), while Class B has an allowable temperature measurement error of ± (0.3 ℃+0.005 | t |).
The thermal resistors used on site are generally armored thermal resistors, which are composed of a thermal resistor body, insulation material, and protective tube. The thermal resistor body and protective tube are welded together and filled with insulation material in the middle, which can effectively protect the thermal resistor body, withstand impact, shock, and corrosion.