Thermocouples are transducers that convert temperature directly into voltage. They are active transducers simply made by joining two different metals. It produces a voltage when the temperature in the joint changes.

Types of Thermocouples:

There are many different types of thermocouples, made of different types of cables and that have very different properties.
The five standard base-metal thermocouples are chromel–constantan (type E), iron–constantan (type J), chromel–alumel (type K), nicrosil–nisil (type N) and copper–constantan (type T).

Type J:

Type J thermocouples are most commonly used for industrial purposes. They are made of metal combo of Iron–constantan.
They have thermocouples have a sensitivity of 60 μV/°C and are the preferred type for general-purpose measurements in the temperature range 150°C to C1000°C, where the typical measurement inaccuracy is 0.75%.
Their performance is little affected by either oxidizing or reducing atmospheres.

Type E:

It has the highest emf of all standard thermocouples. Chromel–constantan devices give the highest measurement sensitivity of 80 μV/°C, with an inaccuracy of š0.5% and a useful measuring range of 200°C up to 900°C.
Unfortunately, whilst they can operate satisfactorily in oxidizing environments when unprotected, their performance and life are seriously affected by reducing atmospheres. These elements must be protected from sulfurous and marginally oxidizing atmospheres.

Type K:

Chromel–alumel thermocouples have a measurement sensitivity of only 45 μV/°C. They are used at high temperatures.
The operating temperature range is between 700°C and 1200°C. They are suitable for oxidizing atmospheres but not for reducing ones unless protected by a sheath. Their measurement inaccuracy is  0.75%.

Type T:

Copper–constantan devices have a measurement sensitivity of 60 μV/°C. Type T is recommended for use in moderately oxidizing and reducing atmospheres at temperatures from – 200°C to 350 ° C (32 ° to 660 ° F) and find their main application in measuring subzero temperatures down to with an inaccuracy of -/+0.75%.

Type N:

Nicrosil–nisil thermocouples are a recent development that resulted from attempts to improve the performance and stability of chromel–alumel thermocouples. Their characteristics are similar to type k. They have a measurement sensitivity of 40 μV/°C and measurement uncertainty of +/-0.75%.

Type B, R, S:

They are noble metal thermocouples, they are made of platinum and platinum alloys. They are designed for high
temperature applications, such as vacuum furnaces and a variety of process applications.
Type B is Platinum Rhodium – 30% / Platinum Rhodium – 6%, Temperature range, 32 to 3100F (0 to 1700C),Wire colour: Gery and Red.
Type S is Platinum/Platinum-10% Rhodium, Temperature range: 1000 – 2700 Fahrenheit and  wire colour: Black and Red.
Type R is Platinum/Platinum-13% Rhodium, Temperature range: 1000 – 2700 Fahrenheit and wire colour: Black and Red.
Because of the chance for contamination, they should be protected by a suitable protection tube, preferably of high-purity alumina. The use of mullite with types R and S is not recommended due to the possibility of contamination with silica, and the thermocouple should never be inserted directly into a metal tube.
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