Thermocouple basics and types of thermocouple.

What is thermocouple?

A thermocouple is an active transducer which directly converts thermal energy into electrical energy. It is a simple device made by joining to dissimilar metals or semiconductor forming a junction. It produces a voltage when the temperature at the junction changes.

Principle:

The Seebeck Effect:

A thermocouple works on the Seebeck Effect. This is where (as previously mentioned) two wires of dissimilar metals are electrically connected at one end. When the junction is heated or cooled, a voltage is produced which is proportional to the temperature.

The Peltier Effect:

The reverse of Seebeck effect, Releasing or absorption of heat at a junction of thermocouple due to the passage of electric current. That is if the junctions of thermocouple kept at the same temperature and if the current is allowed to pass through then one junction get heated and other get cooled.

Operation:

The two dissimilar metal wires are connected together to form a junction j1 shown in the picture below. A voltmeter is connected across the metal wire to measure the voltage, this a simple thermocouple setup.

The voltmeter shows the change in voltage reading according to the temperature change in the junction J1.

Here comes another situation, the junction j2 forms another junction of two dissimilar metals, One the metal wire(iron in the picture) and the conductor wire. So this dissimilar metal junction also generates a voltage which will oppose the junction voltage at J1. This junction is known as cold junction/reference junction.

So that at room temperature the voltmeter reads 0V because the voltage generated by J1 will be opposed by the voltage generated at the J2 as both junctions are exposed to the same temperature.

For standard measurements and industrial porpuses this can’t be allowed so that the Junction maintained at 0°C as cold junction:

It is impossible to use ice water at industries, but there are many other alternate cold junction compensation techniques. All compensation techniques perform under the same principle to generate an opposing voltage to the cold junction voltage:

Thermocouple types:

Thermocouples exist in many different types, each with its own colour codes for the dissimilar-metal wires, Each type of thermocouples differ in using the different metal combo:

All the negative wires are colour coded as red. Each type is selected based on the temperature range and especially the metals used, for example, metals like iron are easily corroding that can’t be used in corroding situations.

T-type can be used in either oxidising or reducing atmospheres. This type of thermocouple has a high resistance to corrosion due to moisture. They also provide a relatively linear output and perform well from a medium to very low-temperature range.

J-type can also be used in reducing atmospheres and provide a good near-linear output. They are also the least expensive of commercially available thermocouples.

K-type can be used in oxidising atmospheres, and are the most linear thermocouple for general use. These are the most widely used.

E-type is the most sensitive thermocouple available and has the highest change in emf per temperature change, but they tend to drift more. They can be used in oxidising atmospheres.

R-type and B-type thermocouples are suitable in oxidising atmospheres, they are easily contaminated in other atmospheres.

Applications:

  • Manufacturing
  • HAVC systems
  • Thermoelectric cooling
  • Gas appliances safety
  • Thermopile radiation sensor

Advantages:

  • Low cost
  • It is rugged in construction
  • Small size
  • Cover a wide temperature range
  • Calibration can be easily checked
  • Fast response
  • Accurate for large temperature change
  • Reasonably stable

Disadvantages:

  • Very weak output, millivolts
  • Stray voltage pickup is possible
  • In many applications, amplification of required
  • Sensitive to electrical noise
  • Nonlinear
  • Complicated conversion from emf to temperature

To know about  Thermocouple transmitter calibration

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