What is thermocouple

History of thermocouple:

Thomas Johann Seebeck accidentally discovered the thermocouple in the year of 1821. He was experimentally determined that a voltage exists between the two ends of a conductor when the conductor's end is at different temperatures

Meaning of thermocouple:

The thermocouple is basically a temperature measuring device.  The thermocouple is used for measuring the temperature at one particular point. It issued to measure the temperature at one specific point or in the form of the EMF or in an electric current. The temperature can be measured at this junction and the change in temperature of the metal wire stimulates the voltage.

So the thermocouple is comprised of at least two metals joined together to form two junctions. One is connected to the body whose temperature is to be measured this is the hot or measuring junction and the other junction is connected to a body known as temperature. This is the cold or reference junction.  So, therefore, the thermocouple measures the unknown temperature of the body with reference to the known temperature of the other body.

Working principle of thermocouple:
  • See back effect: This type of effect occurs among two different metals. When the heat offers to any one of the wire, the electrons start flowing from hot metal to cold metal wire. Therefore the direct current induces in the circuit. In short this phenomenon in which the temperature difference between the two different metal induces the potential difference between them. The see back effect produces small voltage for per kelvin of temperature.
  • Peltier effect: This effect totally opposite to see back effect. This effect state that the difference of the temperature can be created between any two dissimilar conductors by applying the potential difference or variation between them. 
  • Thompson effect: This effect states that when two disparate metal fixed and join together and if they create two joints then the voltage induces the entire length of the conductor because of the temperature gradient. The temperature gradient is a physical term which shows the direction and rate of change of temperature at a particular location.

Construction of thermocouple:

  • Ungrounded junction: In this type of junction, the conductor is entirely isolated from the protecting cover. So this one can be used in high-pressure application work. The major benefits of using such type of junction are that it decreases the stray magnetic field effect.
  • Grounded junction: In this type of junction the metal wires and protective cover are welded together. The grounded junction use for measuring the temperature in the acidic atmosphere and provides resistance to the noise.
  • Exposed junction: The exposed junction is applicable in the area where fast response requires. This type of junction is used for measuring the temperature of the gas. The metal used to make the thermocouple basically depends on the calculating range of temperature.
Thermocouple junction
Thermocouple junction

Working of thermocouple:

The circuit of the thermocouple is shown in the figure given below. The circuit consists of two dissimilar metals. These metals are joined together in such a manner that they are creating two junctions. The metal is bonded to the junction through welding

Let shown that in figure P and Q are two junctions of the thermocouple. The T1 and T2  are the temperatures at the junctions. As the temperature of the junction is different from each other, the EMF generated in the circuit 

If the temperature at the junction becomes equal, the equal and opposite EMF  generate in the circuit, and the zero current flows through it. If the temperature of the junction becomes unequal, the potential difference induces the circuit. The total current flowing through the circuit is measured through the measuring devices

Iron constant thermocouple
Iron constant thermocouple

The electromagnetic force induced in the circuit is calculated by the following equation
E = a (Δ𝛉) + b (Δ𝛉 ) 2
Where  Δ𝛉  is the temperature difference among the hot thermocouple junction end as well as the reference thermocouple junction end and here a and b are constant.  
Advantages  and disadvantages of thermocouple:

Advantages or benefits of thermocouple:
  • Fast response time
  • They are a simple construction
  • Low initial cost
  • Durable
  • Not required bridge circuit
  • Good accuracy
  • Does not required bridge circuit
  • Good reproducibility
  • High-speed response
  • They are rugged
  • They are a self-power active device
Disadvantages  or drawback of thermocouple:
  • Not as stable as RTD
  • Recalibration is difficult
  • More susceptible to RFI/EMI
  • Less sensitivity
  • They require a reference for operation
    The stray voltage pick up is possible
For detailed information :

You have also read:
1. RTD - Advantages and Disadvantages
2. Thermistor - Advantages and Disadvantages

Thermocouple applications:
  • These are used in gas machines for detecting the pilot flame
  • These are used to test temperature in the chemical plants, petroleum plants
  • These are used in the food industry for cryogenic and low-temperature application
  • These are used for a heat pump for performing thermoelectric cooling
  • These are used for medical equipment
  • It can be used for packing equipment
  • It can be used as Diesel engine
  • These are also used for plastic industry modeling machinery, steel industry
For detailed information
Read more >> Application of thermocouple

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