Difference and Function of Thermal Resistance and Thermocouple
Author:vacorda Source:vacorda.com Date: 2020-04-13 15:16:53
Thermal resistance and thermocouple are both temperature detection sensors. Due to the delay of temperature transmission, PID is needed to control the temperature in advance. As temperature detecting elements, they all seem to measure temperature on the surface, but they are quite different.
The first is their temperature measurement principle. Thermocouples work on the basis of thermoelectric effect. Thermoelectric resistance works on the property that the resistance of conductor changes with the temperature. According to the difference of their temperature measurement principle, the output signal of thermal resistance is thermal resistance signal, while the output signal of thermocouple is millivolt signal and polarity should be distinguished. The second is wiring connection. Thermocouple adopts two-wire system and thermal resistance adopts three wire system. In the practical application of the two wire thermocouple, a special compensation wire is needed, which is used to compensate the cold end temperature. In the application process of thermal resistance, the change of wire resistance may overlap with the change of thermal resistance value, resulting in the measurement error of thermal resistance. Therefore, three wire system connection method should be used to eliminate the influence of wire resistance.
The above only refers to their temperature measurement principle and wiring connection mode, as well as their actual use environment. Thermocouple is usually used in high temperature environment, while thermal resistance is generally used in low temperature environment, so it is obvious that the temperature measurement width of thermal resistance is smaller than that of thermocouple. According to the principle of thermocouple temperature measurement, as long as two kinds of conductors can be made into thermocouples in theory, but due to a series of requirements for measurement accuracy and use of actual temperature measurement, there are also requirements for thermoelectric materials of thermocouples. For the thermal resistance, it is almost the same. Although the resistance of most metal conductors will change with the change of temperature, it can't all be the material of thermal resistance.
According to the above mentioned, it is very difficult to find materials that fully meet the requirements of thermocouple and thermal resistance temperature measurement conditions. According to the known international standards of thermocouples, there are eight types of thermocouples (K-type E-type S-type R, etc.), and there are two kinds of thermal resistance materials, namely platinum thermal resistance and copper thermal resistance.
The temperature measurement range of thermal resistance and thermocouple sensor, and the thermoelectric potential and resistance value of 100 ℃ are listed.
Pt Rh 10 Pt (s type) (0-1300 ℃), t = 100 ℃, e (100,0) = 0.646mv.
Platinum rhodium 13 platinum (R type) (0-1300 ℃), t = 100 ℃, e (100,0) = 0.647mv.
Ni Cr Ni Si (type K) (0-1200 ℃) t = 100 ℃ e (100,0) = 4.096mv.
Nichrome constantan (type E) (- 200-760 ℃), t = 100 ℃, e (100,0) = 6.319mv
Platinum thermistor (PT100) (- 200-850 ℃), t = 100 ℃, r = 138.50 Ω)
Copper thermal resistance, (Cu 50) (- 50-150 ℃), t = 100 ℃, r = 71.4 Ω.