To help our customers better understand thermocouples, engineers from Chongqing Duchin Temperature Instrument have compiled 15 common questions about thermocouples. Today we will explain the first three.
1. What is the measuring principle of a thermocouple?
The working principle of a thermocouple is based on the Seebeck effect: a physical phenomenon where a thermal current is generated in a circuit formed by two conductors of different materials, if the two junction temperatures are different.
A thermocouple consists of two different wires (thermoelements). One end is welded together to form the measuring junction (hot junction) of the thermocouple, which is inserted into the medium whose temperature is to be measured. The other end of the thermocouple (reference junction or cold junction) is connected to an indicating instrument.If there is a temperature difference between the measuring junction and the reference junction, the instrument will indicate the thermoelectric electromotive force (EMF) generated by the thermocouple.
2. What is the measuring principle of an RTD?
A Resistance Temperature Detector (RTD) measures temperature by utilizing the characteristic that the electrical resistance of a metal conductor or semiconductor changes with temperature.The heat-sensitive part (sensing element) of an RTD is made of fine metal wire uniformly wound on a frame made of insulating material, or formed on a substrate by laser sputtering technology.
When there is a temperature gradient in the measured medium, the measured temperature represents the average temperature of the medium layer within the range covered by the sensing element.
3. How to choose between thermocouples and RTDs?
- According to the temperature range:Thermocouples are generally selected for temperatures above 500℃, while RTDs are preferred below 500℃.
- According to the measurement accuracy:RTDs are chosen for applications requiring high accuracy, while thermocouples are used where accuracy requirements are relatively low.
- According to the measurement scope:Thermocouples generally measure point temperature, whereas RTDs typically measure spatial average temperature.