# Thermocouples

In this lecture, we are going to learn about the Thermocouples, the advantages of Thermocouples, disadvantages of Thermocouples and then we will learn the comparison between Thermistor, RTD, and Thermocouples. So let’s discuss the basic definition.

## Thermocouples

The principle of Thermocouples is based on the Seeback effect.

According to this Seeback effect a voltage is induced between the junction of two different metals when they are held at different temperatures.

The voltage induced in thermocouple can be given by,

\boxed{E=a(\Delta\Theta)+b(\Delta\Theta)^2}

where \Delta\Theta is a temperature difference between the junction (oC) a and b are constants.

a >>> b usually so, the equation may be written as,

E \approx a(\Delta\Theta)

\Rightarrow \boxed{\Delta\Theta \approx \frac{E}{a}}

The output of thermocouples in the range of mV can be detected with a PMMC instrument or D.C. potentiometer after amplifying it. Thermocouples are used for temperatures up to 1400 oC.

These are cheaper than resistance thermometers. Thermocouples follow the temperature changes with a small time lag in such a way that is suitable for recording rapid changes in temperature. These are very convenient for measuring the temperature at one particular point in a piece of apparatus.

They have lower accuracy and hence can not be used for precision work. It requires an open or closed-end metal protecting tube or well. Thermocouples are placed remote from measuring devices so it is expensive due to the connection of long wires. It has a very complex circuit.

## Comparison between Thermistors, RTD, and Thermocouples

\Rightarrow Non-linearity: Thermistors > Thermocouples > RTD

\Rightarrow Sensitivity: Thermistors > RTD > Thermocouples

## Frequently Asked Questions on Thermocouples

1. ### What is a thermocouple used for?

Answer: Thermocouples are used in applications that range from home appliances to industrial processes, to electric power generation, to furnace monitoring and control, to food and beverage processing, to automotive sensors, to aircraft engines, to rockets, satellites, and spacecraft.

2. ### What is the principle of a thermocouple?

Answer: The thermocouple working principle is based on the Seeback Effect. This effect states that when a closed circuit is formed by joining two dissimilar metals at two junctions, and junctions are maintained at different temperatures then an electromotive force (e.m.f.) is induced in this closed circuit.

3. ### Is it a thermocouple AC or DC?

Answer: It is used for both the AC and DC measurement. The thermocouple is an electric device that uses two wires of different metals. Thermocouple meters are AC meters that respond to the RMS value of a signal.

4. ### How does a thermocouple generate electricity?

Answer: How does a thermocouple generate electricity?

By taking two strips of dissimilar metals which are the same size and joining them together at each end a loop is created. By making one of the joins in the metal very hot and the other join very cold, an electric current will flow through the loop creating an electric circuit.

Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

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