Thermocouples can be manufactured from a diversity of metals covering a temperature range of 200 oC to 2,600 oC. Various combinations of metals i.e. calibrations exist for thermocouples. There are approximately thirteen types of standard thermocouples available in the industry and out of them eight have been designated with an internationally known letters. The eight designated thermocouple types are listed below:
- Type K – This type of thermocouple is made up of Chromel-Alumel. It belongs to the chromium-nickel aluminum group and has a fairly linear e.m.f./temperature curve.
- Type J – This type of thermocouple having limited temperature range is made up of Iron-Constantan.
- Type E – It is made up of Chromel-Constantan and offers great sensitivity. It is primarily used in cryogenic i.e. low temperature range. Being a non magnetic type, it proves to be advantageous in variety of applications.
- Type N – It is made up of Nicros-Nisil and offers excellent thermoelectric stability. It is extremely resistant to high temperature oxidation.
- Type T – This type of thermocouple is made up of Copper-Constantan. It is found extremely useful in food, environmental and refrigeration applications.
- Type S – It is made up of Platinum rhodium
- Type R – It also contains Platinum rhodium and somewhat identical to type S
- Type B – It is an extremely stable thermocouple made up of Platinum rhodium only. Its output is found to be very small at room temperature and offers less sensitivity in the lower range.
The above thermocouple types can be categorized into two groups i.e. Base metal and Rate or Noble metal. Type K, J, E, N and T fall into the Base metal category whereas Type R, S and B come into the Noble metal category.
In general, there are four classes of thermocouples which are mentioned below:
- Home body class which is also referred to as Base metal Type. The temperature range for these types of thermocouples varies from -200°C to 1200°C.
- Upper crust class, also known as Rare metal or Precious metal or Noble metal Type. The temperature range for these types of thermocouples varies from 0°C to 1600°C.
- Rarified class, also known as Refractory metal Type
- Exotic class which consists of various tungsten alloy thermocouples and many other standard and developmental devices. It is typically designated by Type W.
Each thermocouple type or calibration has a specific range of temperature and environment. However, the maximum temperature possible depends upon the thermocouple wire diameter. The most preferred types of thermocouples are found to be K and N types since they perform well at high temperatures, whereas another industry preferred thermocouple is T type as it offers high sensitivity, low cost and simple operation.