How Thermocouple works

A thermocouple is a temperature measuring device which is formed when two dissimilar metals are joined together. When both the ends are joined together continuous current flows through the wires when one end is heated. This is called thermoelectric effect or see back effect. When there is difference in temperature at both the ends a small voltage(in mV) is generated. This voltage increases as the difference between the temperature increases. This voltage can be checked using voltmeter. When you connect the voltmeter probe it will not create additional junctions as per the law of intermediate metals.
The law of intermediate metals states that ‘a third metal inserted between two dissimilar metals of the thermocouple junction will have no effect provided that the two junctions are at the same temperature’. The differential voltage is used to measure the temperature. Since it measures the difference in temperature between the two ends cold junction compensation has to be done. To do this the manufacturer normally puts another temperature measuring device such as a RTD or thermistor to check the temperature at the cold junction end and then applies the compensation to give correct results.
There are various kinds of thermocouple made of different metals and their alloys. The most common thermocouple is the K type thermocouple. It is also called the general purpose thermocouple since it is widely used.
Below are the different kinds of thermocouples
The Advantages of the thermocouple are
  • Wide operating temperature range
  • Can be used to measure high temperatures in furnaces
  • Very robust and can be used in high vibration and other harsh environments
  • Low cost
  • Quick response time for low mass thermocouples
  • Reasonable repeatability and accuracy for high temperature measurements
The Disadvantages of the thermocouple are 
  • Low sensitivity
  • Since it measures the difference in temperature the output is in mV and there is a possibility that it is prone to electrical noise.
  • Requires another temperature measuring device such as RTD or the thermistor to measure the temperature at cold junction in order to do cold junction compensation.
  • Linearity is not good
  • Not the best for low temperature measurements because system errors of less than 1 degree Celsius is difficult to achieve.
The applications of Thermocouple are
  • The Thermocouples can be used to measure high temperature in blast furnaces
  • exhaust gas temperature.
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