Thermocouple introduction, working and types

What is Thermocouple : It is a such type of sensor which is used for measuring the temperature of any specific environment or object . It is made with two different material wires. The one end of these wires is connected together for making the junction .This junction is placed in that specific environment or object whose temperature want to measure. When the temperature is changed then two different materials are started to deform, as a result the resistance is changed .Actually, its output is milli volt signal whose voltages are changed when resistance is changed. This change in voltages could be measured easily with the help of 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, easy to use and capable for providing accurate reading. These are manufactured in wide range of styles such as infrared thermocouple, thermocouple probes, thermocouple probes with connectors, transition joint  probes and just wire or bare wire thermocouple probes etc. Means these are available in market in different styles or models, so each model has different shape and technical specification. Therefore for using it, 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 figure 1.

Figure 1 Basic  Diagram

Working Principle of Thermocouple

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

Seebeck Effect: According to seebeck, when two different metals are connected together for making two junctions then electromotive force would be developed at two junctions. The amount of this force would be different with metal material combinations.

Peltier Effect: According to Peltier state, when two different metals are connected together for making two junctions then electromotive force (EMF) would be developed with in circuit. The reason of this emf is due to different temperatures of two junction with in circuit.

Thomson Effect: : According to Thomson state, when two different metals are connected together for making two junctions then voltages or potential is existed within the circuit due to temperature gradient along the whole length of thermocouple conductor.

In most cases the emf, which is generated by Thomson effects statement is very small it could be neglected during the selection of metal material. But Peltier effects statement plays an important role in its working principle.

According to figure 1 two different metals such as metal A and B are connected together for making two junctions. whose name are  measuring end and reference end. Remember, two junction are necessary for making any thermocouple. Temperature of reference end is known but measuring end is unknown. When this unknown temperature end is placed at that place where we want to measure the temperature. In this condition, if both ends would be at same temperature level then there would be no emf generated. So the net current in whole circuit would be also zero. Similarly if both ends are at different temperature level then emf would be generate and current will be also flow in whole circuit. The value of this emf or current also depends on thermocouple metal material as well as the temperature of both ends. By the 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 have been currently using in market with respect to its material. These are divided in alphabetical order  .

 K Type Thermocouple (Nickel-Chromium / Nickel-Alumel): These K type thermocouples are  less costly, more accurate, more reliable and most common type thermocouple. It has wide range of temperature such as  –454 to 2,300F and wire extension 32 to 392F, shown in figure 2.

Figure 2 K type Thermocouple

J Type Thermocouple (Iron/Constantan): These J type  are almost similar with K type in terms of expensive and relatability. But it has smaller temperature range and shorter lifespan at high temperature. Its temperature range is in between -346 to 1400F and accuracy level is in between +/- 2.2C or +/- .75% shown in figure 3

Figure 3 J Type Thermocouple

T Type Thermocouple (Copper/Constantan): These T type  are very stable. These are made up with copper material and are used for very low temperature applications such as ultra low freezer or cryogenics etc. Its temperature range is in between  -454 to 1600F. and accuracy level is +/- 1.7C or +/- 0.5%, shown in figure 4.

Figure 4 T Type

E Type Thermocouple (Nickel-Chromium/Constantan): These E type  are made up with nickel chromium material and are highly accurate then above one . It has moderate temperature range in between -454 to 1600F and accuracy +/- 1.7C or +/- 0.5% shown in figure 5.

Figure 5 E Type

N Type Thermocouple (Nicrosil / Nisil): N type  are made up with nicrosil material and have almost same accuracy level and temperature range of K type thermocouples.

S Type Thermocouple ( (Platinum Rhodium – 10% / Platinum): S type are made up with platinum rhodium material and are used for high temperature applications such as in pharmaceutical and bio Teck industry. Its temperature rang is in between -58 to 2700F and accuracy level is in between +/- 1.5C or +/- .25%, shown in figure 6

Figure 6 S Type

Thermocouple Applications

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

1. It are used in gas industry for measuring the temperature of kilns and gas turbine exhaust.
2. There are used in diesel engine industry for measuring the temperature of diesel engine and fog machine.
3. These are used in steel industry for measuring the screen temperature as well as the type such as, B,S,R, and K are highly used in chemical process industry and for measuring the boiler temperature.
4. These are used in gas industry for the safety of gas appliances.
5. These are used as thermopile radiation sensors.
6. These are also used as a device for small measurements application such as thermistor, silicon bandgap temperature sensor and resistor types etc.

Thermocouple Advantages:

1. It device has high response time as compared to another devices.
2. It has wide temperature range almost 270 to 2700 degree Celsius. This range is not easily possible in other devices.
3. It is less expensive ,more reliable and efficient as compared to RTDs.
4. It has rugged in construction and has good accuracy.
5. It does not require any bridge circuit and also have good reproducibility.

Thermocouple Disadvantages:

1. They are made up with two different metals therefore in corrosion situation something could be dangerous .So, in presence of light corrosion, miss reading would be gained by thermocouple therefore proper care and maintenance is essential for accurate working of thermocouple.
2. The exact calibration of it is not so much easy, therefore for gaining accurate reading there must be always required another calibrated thermocouple. But during calibration the output is not exact reproduced.
3. They are not so much accurate as compared to
4. Output voltage is not so much high therefor always need to amplification.
5. It output shows non linearity.