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Difference between thermocouple and RTD

Author:vacorda    Source:vacorda.com    Date: 2016-12-15 16:55:14

Thermocouple and thermal resistance(RTD) are contact temperature measurement, although its role is the same, are measuring the temperature of the object, but their principles and characteristics are not the same.  


First of all, thermocouple temperature measurement is the most widely used temperature devices, it’s main feature is a wide temperature range, performance, relatively stable, while simple structure, good dynamic response, remote 4-20mA signal, easy to automatic and centralized control. The principle of thermocouple temperature measurement is based on the thermoelectric effect. Two different conductors or semiconductors connected into a closed loop, when the two contacts at different temperatures, the circuit will produce thermoelectric power, this phenomenon known as the thermoelectric effect, also known as the Seebeck effect. 


Thermocouples generated in closed loops are composed of two potentials: thermoelectric potential and contact potential. Temperature difference potential is the potential generated by the same conductor at both ends of the different temperature , different conductors with different electron density, so the potential they produce is not the same. The contact potential, as the name implies, refers to two different conductors in contact, and because of their different electron density so that a certain degree of electron diffusion, when they reach a certain balance, then form a potential, the size of contact potential depends on the material of two different conductors and the temperature at their point of contact.


At present, the international application of thermocouple has a standard specification, it is divided into eight different divisions, namely B, R, S, K, N, E, J and T. The minimum temperature measurement can be measured below minus 270 , up to 1800 . B, R, S belong to platinum series of thermocouple, platinum is precious metals, so they are called precious metal thermocouple and the remaining few are known as the cheap metal thermoelectricity. There are two thermocouple structures, common type and armored type. Ordinary thermocouple generally combined with the hot electrode, insulation tube, protective casing and junction boxes and other components. The armored thermocouple is assembling the thermocouple wire, insulation materials and metal protection sleeve, and after drawing from a solid combination of processing.


But the electrical signal of the thermocouple needs a special wire to transmit, this wire is called the compensation wire. Different thermocouples require different compensation wire, its main role is to connect with the thermocouple, so that the reference end of the thermocouple away from the power, so that the reference junction temperature stability. 


(Some types of temperature instruments: Thermocouple, RTD)

Secondly, the thermal resistance, Although the thermal resistance is wide used in the industrial application, but because of its temperature range limit its application, the working principle of thermal resistance is based on the resistance value of conductor or semiconductor  changes with the temperature change.It also has the advantages of high sensitivity, stability, interchangeability and better accuracy, and can also remote transmit the electrical signal, But it needs the power excitation, can not make the instantaneous measurement of temperature changes. In industrial, thermal resistance commonly use Pt100, Pt10, Cu50, Cu100. Platinum thermal resistance of the temperature range is generally minus 200-800℃, copper heat resistance is minus 40 to 140℃.  and thermocouple the same type of distinction, but the thermal resistance does not need the compensate wire, and cheaper than the thermocouple.

Finally, above information is just a simple introduction of the temperature instruments: thermocouple and RTD, and for selecting them,you must think of other parameters to choose the suitable instruments for your working environment.


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