Thermocouple Introduction, Working and Types

In this tutorial, we will discuss the workings of a thermocouple. It’s a type of sensor that’s used for measuring the temperature of any specific environment or object. It is made with two wires of different materials. One end of these wires connects together to make the junction. This junction is placed in the specific environment or object whose temperature you want to measure. When the temperature changes, two different materials start to deform, and as a result, the resistance changes. Actually, its output is a millivolt signal whose voltage changes when resistance changes. This change in voltage is easily measurable with the help of a thermocouple.

Introduction to Thermocouple

So, it is a very reliable solution for measuring the temperature of any object or environment because its cost is extremely low, it is easy to use, and it is capable of providing an accurate reading. These are manufactured in a wide range of styles, such as infrared thermocouples, thermocouple probes, thermocouple probes with connectors, transition joint probes, just-wire or bare-wire thermocouple probes, etc. This means these are available on the market in different styles or models, so each model has a different shape and technical specification. Therefore, for any specific application, it is very important to understand the functionality, basic structure, and range of that one thermocouple. Its basic diagram is shown in the figure below.

Working Principle of Thermocouple

For understanding the working principle of this device, it is necessary to first understand the three effects that were developed by Seebeck, Peltier, and Thomson. The whole working principle of a thermocouple is based on these three effects.

Seebeck Effect

According to Seebeck, when two different metals are connected together to form two junctions, electromotive force will develop at both junctions. The amount of this force would be different with different metal material combinations.

Peltier Effect

According to Peltier, when two different metals connect together to form two junctions, electromotive force (EMF) will develop within the circuit. The reason for this emf is due to the different temperatures of two junctions in the circuit.

Thomson Effect

According to Thomson, when two different metals connect together to make two junctions, then voltages or potential exist within the circuit due to the temperature gradient along the whole length of the thermocouple conductor.

In most cases, the emf that the Thomson effect generates is very small and can be neglected during the selection of metal material. But Peltier’s effects play an important role in its working principles.

According to the figure, two different metals, such as metals A and B, connect together to form two junctions, whose names are measuring end and reference end. Remember, two junctions are necessary for making any thermocouple. The temperature of the reference end is known, but that of the measuring end is unknown. So, we place this unknown temperature end at the place to measure its temperature. If both ends are at the same temperature level, no emf is generated. So the net current in the whole circuit would also be zero.

Similarly, if both ends are at different temperature levels, then it generates emf, and current will also flow in the whole circuit. The value of this emf or current also depends on the thermocouple metal material as well as the temperature of both ends. By measuring the value of this current, or emf, the user can easily find out the temperature of that specific place.

Types of Thermocouple

There are eight different types of thermocouples on the market with respect to their material. These divisions, in alphabetical order, are as follows:

K Type Thermocouple (Nickel-Chromium / Nickel-Alumel)

These K-type thermocouples are less costly, more accurate, more reliable, and the most common type of thermocouple. It has a wide range of temperatures, such as -454 to 2,300 F, and wire extensions of 32 to 392 F. Refer to the figure below.

J Type Thermocouple (Iron/Constantan)

These J types are almost similar to K types in terms of cost and relatability. But it has a smaller temperature range and a shorter lifespan at high temperatures. Its temperature range is between -346 °F and 1400 °F, whereas its accuracy level is between +/- 2.2 °C or +/- .75%. As we can see in the figure below.

T Type Thermocouple (Copper/Constantan)

These T-types are very stable. These are made up of copper material and are useful for very low temperature applications such as ultra-low freezers, cryogenics, etc. Its temperature range is between -454 °F and 1600 °F, and its accuracy level is +/- 1.7 °C or +/- 0.5%. Refer to the following figure:

E Type Thermocouple (Nickel-Chromium/Constantan)

These E types have nickel-chromium material and are more accurate than the above ones. It has a moderate temperature range between -454 °F and 1600 °F and an accuracy of +/- 1.7 °C, or +/- 0.5%. As we can see in the figure below.

N Type Thermocouple (Nicrosil / Nisil)

N-type thermocouples have Nicrosil material and have almost the same accuracy level and temperature range as K-type thermocouples.

S Type Thermocouple (Platinum Rhodium – 10% / Platinum):

S types have platinum-rhodium materials, and these are useful for high-temperature applications such as in the pharmaceutical and biotech industries. Its temperature range is between -58 °F and 2700 °F, and its accuracy level is between +/- 1.5 °C and +/- .25%. Refer to the figure below.

Applications

Thermocouples are a very suitable solution for measuring the temperature of any object or specific place. It has a high temperature range of almost 2300 °C.

The gas industry uses them for measuring the temperature of kilns and gas turbine exhaust.
The diesel engine industry uses them for measuring the temperature of diesel engines and fog machines.
These are useful in the steel industry for measuring the screen temperature, such as B,S,R, and K, which are highly used in the chemical process industry and for measuring the boiler temperature.
Another application in the gas industry is for the safety of gas appliances.
These are used as thermopile radiation sensors.
These are also useful as devices for small measurement applications such as thermistors, silicon bandgap temperature sensors, resistor types, etc.

Advantages

It has a high response time as compared to other devices.
It has a wide temperature range, from almost 270 to 2700 degrees Celsius. This range is not easily possible on other devices.
It is less expensive, more reliable, and more efficient as compared to RTDs.
It is rugged in construction and has good accuracy.
It does not require any bridge circuits and also has good reproducibility.

Disadvantages

They are made up of two different metals; therefore, in a corrosion situation, something could be dangerous. So, in the presence of light corrosion, a misreading would be gained by the thermocouple; therefore, proper care and maintenance are essential for the accurate working of the thermocouple.
The exact calibration of it is not so easy; therefore, for an accurate reading, there must always be another calibrated thermocouple. But during calibration, the output is not exactly reproduced.
They are not so much accurate as compared to thermistors and temperature detectors.
Output voltage is not very high, so there is always a need for amplification.
Its output shows nonlinearity.

Conclusion

In conclusion, this tutorial provides an in-depth overview of thermocouples. It covers a basic introduction along with its working principles and types. At last, we discuss the application, advantages, and disadvantages of the thermocouple. Hopefully, this was helpful in expanding your knowledge of thermocouples.