Thermocouple - Definition (What Is It?), Principle of Operation, Construction, Types, Advantages, Disadvantages and Its Applications

 What is Thermocouples?

The thermocouple is defined as an active temperature sensor or transducer which induces an emf that is a function of the temperature between the two junctions.

 Principle of Operation of Thermocouple

The principle of operation of thermocouples is based on the see back effect. It states that a current flow in a closed circuit is made of two different dissimilar metals if the junctions of the two metals are kept at different temperatures. Due to that current flow in a closed circuit hence an emf is induced proportional to the temperature difference.

 Construction of Thermocouple

The basic thermocouple circuit is shown in the figure below. In which two different/dissimilar metals are joined together to form two junctions J₁ and J₂.

If both junctions are kept at temperature T₁ and T₂ then a thermal emf is induced which is proportional to the temperature difference T₁-T_2.

                                                                e = K (T₁-T₂)

In the measurement system, one junction can be maintained at a known temperature that junction is called a “reference” junction or “cold” junction while the other junction is called the “sensing” junction or “hot” junction. The reference temperature generally used is 0⁰ C or 273⁰ C.

When a temperature difference exists between the sensing(hot) junction and the reference(cold) junction, an emf is induced that causes current to flow in the circuit.

A millivoltmeter is used to measure the thermally generated emf. The magnitude of emf depends on the temperature difference between the hot junction and cold junction. The circuit diagram for thermocouple measurement is shown in the figure below.

The thermocouple elements are enclosed in a housing assembly for their protection. The assembly is called a thermocouple probe.   

 Types of Thermocouple

The thermocouples are classified into various types i.e., type E, type K, type J, type R, type S, and type T, etc.

 Type E

        The type E thermocouple comprises a Chromel or Copper alloy as the positive electrode and Constantan alloy as the negative electrode.

         The temperature range for type E thermocouple is 0-2000⁰ F.

 Type S, Type R, Type B

       The type S, type R, and type B thermocouple comprises of a Platinum-Platinum/Rhodium alloy. 

    The type S thermocouple produces the least output voltage but it can be used over the greatest temperature range.

        The temperature range for S, R, and B type thermocouple is 0-3000⁰ F. 

 Type T

        The type T thermocouple comprises of a Copper-Constantan alloy.  

 Type J

        The type J thermocouple comprises of an Iron-Constantan alloy.
        The temperature range for type E thermocouple is 0-1200⁰ F.

 Advantages of Thermocouple

 Some of the advantages of the thermocouple are listed below.

1.  External dc supply is not required.
2. Wide temperature range between -270⁰ to 2700⁰.
3.  Good sensitivity i.e., small temperature changes can be sensed.
4. Fast dynamic response i.e., thermocouple responds quickly to any temperature changes.
5. Less expensive and small in size.
6. No need for a bridge circuit.
7. Rugged construction.
8. Simpler to use than a resistance thermometer.

Disadvantages of Thermocouple

 Some of the disadvantages of the thermocouple are listed below.

1. The temperature characteristics of thermocouples are slightly nonlinear.
2. Thermally generated emf is small hence amplification is required.
3. Cold junction compensation is required for accurate measurement of temperature.
4. Stray voltages may get induced into the thermocouple conductors.
5. For control applications, they need expensive accessories.
6. They have limited use in temperature spans of less than about 33⁰ because of the relatively small change in the junction voltages with temperature. 

Applications of Thermocouple

 Some of the applications of the thermocouple are listed below.

1. Thermocouples have a small heat capacity. Hence, they are suitable for an indication of rapidly changing temperatures.
2. Thermocouples are used in pyrometers i.e., in resistance pyrometers for measurements at several points.
3. It is used to measure surface temperature.