What is a Thermocouple? Definition, Working Principle, Diagram & Applications

In the year 1821, a physicist namely “Thomas Seebeck” revealed that when two different metal wires were linked at both ends of one junction in a circuit when the temperature applied to the junction, there will be a flow of current through the circuit which is known as electromagnetic field (EMF). The energy which is produced by the circuit is named as the Seebeck Effect. Using Thomas Seebeck’s effect as his guideline, both Italian physicists namely Leopoldo Nobili and Macedonio Melloni were collaborated to design a thermoelectrical- battery in the year 1826, that is called as a thermal multiplier, it drew from the discovery of Seebeck’s thermoelectricity by merging a galvanometer as well as a thermopile to calculate radiation. For his effort, some people identified Nobili as the discoverer of the thermocouple.

What is a Thermocouple?

The thermocouple can be defined as a kind of temperature sensor that is used to measure the temperature at one specific point in the form of the EMF or an electric current. This sensor comprises two dissimilar metal wires that are connected together at one junction. The temperature can be measured at this junction, and the change in temperature of the metal wire stimulates the voltages.

Thermocouple
Thermocouple

The amount of EMF generated in the thermocouple is very minute (millivolts), so very sensitive devices must be utilized for calculating the e.m.f produced in the circuit. The common devices used to calculate the e.m.f are voltage balancing potentiometer and the ordinary galvanometer. From these two, a balancing potentiometer is utilized physically or mechanically.

Thermocouple Working Principle

The thermocouple principle mainly depends on the three effects namely Seebeck, Peltier and Thompson.

See beck-effect

This type of effect occurs among two dissimilar metals. When the heat offers to any one of the metal wire, then the flow of electrons supplies from hot metal wire to cold metal wire. Therefore, direct current stimulates in the circuit.

Peltier-effect

This Peltier effect is opposite to the Seebeck effect. This effect states that the difference of the temperature can be formed among any two dissimilar conductors by applying the potential variation among them.

Thompson-effect

This effect states that as two disparate metals fix together & if they form two joints then the voltage induces the total conductor’s length due to the gradient of temperature. This is a physical word which demonstrates the change in rate and direction of temperature at an exact position.

Construction of Thermocouple

The construction of thermocouple is shown below. It comprises of two different metal wires and that are connected together at the junction end. The junction thinks as the measuring end. The end of the junction is classified into three type’s namely ungrounded, grounded and exposed junction.

Ungrounded-Junction

In this type of junction, the conductors are totally separated from the protecting cover. The applications of this junction mainly include high-pressure application works. The main benefit of using this junction is to decrease the stray magnetic field effect.

Grounded-Junction

In this type of junction, the metal wires as well as protecting cover are connected together. This function is used to measure the temperature in the acidic atmosphere, and it supplies resistance to the noise.

Exposed-Junction

The exposed junction is applicable in the areas where quick response is required. This type of junction is used to measure the gas temperature. The metal used to make the thermocouple basically depends on the calculating range of temperature.

Construction of Thermocouple
Construction of Thermocouple

Generally, a thermocouple is designed with two different metal wires namely iron and constantan that makes in detecting element by connecting at one junction that is named as a hot junction. This consist of two junctions, one junction is connected by voltmeter or transmitter where the cold junction and second junction is associated in a process that is called as a hot junction.

Working of Thermocouple

The thermocouple schematic diagram is shown in the below figure. This circuit can be built with two different metals, and that are coupled together by generating two junctions. The two metals are surrounded to the connection through welding.

In the above diagram, the junctions are denoted by P & Q, and the temperatures are denoted by T1, & T2. When the temperature of the junction is dissimilar from each other, then the electromagnetic force generates in the circuit.

Working of Thermocouple
Working of Thermocouple

If the temperate at the junction end turn into equivalent, then the equivalent, as well as reverse electromagnetic force, produces in the circuit, and there is no flow of current through it. Similarly, the temperature at the junction end become imbalanced, then the potential variation induces in this circuit.

The magnitude of the electromagnetic force induces in the circuit rely on the sorts of material utilized for thermocouple making. The entire flow of current throughout the circuit is calculated by the measuring tools.

The electromagnetic force induced in the circuit is calculated by the following equation

E = a (∆Ө) + b (∆Ө)2

Where ∆Ө is the temperature difference among the hot thermocouple junction end as well as the reference thermocouple junction end, a & b are constants

Advantages & Disadvantages of Thermocouple

The advantages include the following.

  • Accuracy is high
  • It is Robust and can be used in environments like harsh as well as high vibration.
  • Thermal reaction is fast
  • The operating range of temperature is wide.
  • Wide operating temperature range
  • Cost is low and extremely consistent

The disadvantages include the following.

  • It has low-accuracy.
  • The thermocouple recalibration is hard

Thermocouple Applications

Some of the applications of thermocouple include the following.

  • These are used as the temperature sensors in thermostats in offices, homes, offices & businesses.
  • These are used in industries for monitoring temperatures of metals in iron, aluminum, and metal.
  • These are used in the food industry for cryogenic and Low-temperature applications. Thermocouples are used as a heat pump for performing thermoelectric cooling.
  • These are used to test temperature in the chemical plants, petroleum plants.
  • These are used in gas machines for detecting the pilot flame.

Thus, this is all about an overview of the thermocouple. From the above information finally, we can conclude that the measurement of thermocouple output can be calculated by using the methods like a multimeter, potentiometer, and amplifier by output devices. The main purpose of the thermocouple is to build consistent & direct temperature measurements in several different applications.

Image Source: Nptel

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