What is the measurement method for thermal resistance?

The principle of a thermistor thermometer is to utilize the characteristic of the resistance of a conductor or semiconductor 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.
At present, the materials used for thermal resistors mainly include platinum, copper, nickel, etc. These materials are mainly used because their temperature to resistance ratio in common temperature ranges is linearly related. 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 temperature measurement allows an error of ± (0.15 ℃+0.002 | t |), The allowable error for B-level temperature measurement is ± (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.
Three wire platinum resistance measurement method:
Platinum thermistors have several types, including two-wire, three wire, and four wire systems. The two-wire system has significant measurement errors and is no longer used. Currently, industrial use generally uses the three wire system, while laboratory use generally uses the four wire system. Here we mainly introduce the wiring of the three wire platinum thermistor. A three wire platinum thermistor is connected in parallel to the A end and the C end of the resistor, thereby enabling the resistor to have three connection terminals A, B, and C. In this way, the resistance of the measuring wire introduced by the B wire can be compensated by the C wire, reducing the impact of lead resistance errors caused by temperature changes on the lead resistance. Three wire platinum thermistors, in secondary instruments, all have variable resistance bridges. Depending on the range of the platinum thermistors used, the platinum thermistors in the bridge of the secondary instrument can be fine tuned for more accurate measurements.
A new method for dividing thermal resistance thermometers:
Industrial platinum resistance thermometer is a widely used temperature measuring instrument. For a long time, the calculation method of CVD equation has been widely used in relevant standards or technical specifications at home and abroad to calibrate it. However, industrial platinum resistance thermometers that use the CVD equation for calibration have low accuracy, low stability, and high uncertainty, and cannot be used as transmission standards.
For this reason, most industrial temperature measurement fields or laboratories with low requirements can only use high-precision standard platinum resistance thermometers as traceability standards. However, in the actual industrial temperature measurement field, due to various limitations, standard platinum resistance thermometers cannot be used, making it impossible to achieve temperature value transmission and traceability in these places, and unable to carry out actual metrological calibration work.
The feasibility of calibrating and dividing industrial platinum resistance thermometers, and comparing them with the temperature resistance relationship calculation results given by the commonly used CVD equation, to identify the differences between the two, and to explore the ways and methods of establishing precision industrial platinum resistance thermometers as transfer standards. By conducting research and analysis on multiple industrial platinum thermistors manufactured by different models and manufacturers in different temperature ranges, the experimental results, data curves, and measurement errors caused by using two different methods to calibrate each thermometer are presented.
Experimental results have shown that the interpolation method of ITS-1990 international temperature scale is feasible for industrial platinum resistance thermometers. Compared with the calculation method of CVD equation for industrial platinum resistance calibration, it has better accuracy and consistency. Previously, the national metrology institutions of Italy and Canada conducted research on the industrial platinum resistance division method using the interpolation formula of the international temperature scale.
The traditional methods to improve the accuracy and stability of industrial resistance temperature measurement are focused on component purity, packaging technology, and manufacturing processes; A new approach has been proposed in terms of calculation methods, laying the foundation for the improvement of temperature value transmission and traceability systems for precision platinum resistors and industrial platinum resistors, which can be widely applied in the temperature measurement field of industrial platinum resistors.