Thermocouples are transducers used to measure temperature. When two dissimilar metals are connected together to form a junction. The junction creates a voltage difference across the wire when there is temperature difference is experienced at the junction. The classification is done based on the metal used and temperature range.
Base metal thermocouple
Noble metal thermocouple
Refractory metal thermocouple
Base metal thermocouple:
Thermocouple types K, J, E, T and N. Metals such as Nickel, Copper and Iron.
Positive leg - Chromel, Negative leg - Alumel Temperature range: -300 to 2300 Fahrenheit Wire colour: yellow and red
Used in oxidizing and completely inert environment
Positive leg - Chromel, Negative leg - Constantan Temperature range: -300 to 1600 Fahrenheit Wire colour: Purple and Red
It has the highest EMF Vs temperature values of all the commonly used thermocouples and can be used at sub-zero temperatures.
Positive leg - Iron, Negative leg - Constantan Temperature range: 32 to 1400 Fahrenheit Wire colour: White and Red
Type J thermocouples can be used in a vacuum, oxidizing, reducing and inert atmospheres.
Positive leg -Nicrosil, Negative leg - Nishil Temperature range: 32 to 2300 Fahrenheit Wire colour: Orange and Red
Type N is very similar to type K but is less susceptible to selective oxidation effects. Type N should not be used in a vacuum or in reducing atmospheres in an unsheathed condition.
Positive leg - Copper, Negative leg - Constantan Temperature range: 300to 700 Fahrenheit Wire colour: Blue and Red
Type T thermocouples can be used in oxidizing, reducing or inert atmospheres.
Noble metal thermocouple:
Noble metal thermocouples uses expensive metals like Platinum and Rhodium . There are three types of noble metal thermocouples, Type B, R, S.
Type B (Platinum/Platinum-30% Rhodium) Temperature range: 32 - 3100 Fahrenheit Wire colour: Gery and Red
Type R (Platinum/Platinum-13% Rhodium) Temperature range: 1000 - 2700 Fahrenheit wire colour: Black and Red The Type R is used in very high temperature applications.
Type S (Platinum/Platinum-10% Rhodium) Temperature range: 1000 - 2700 Fahrenheit wire colour: Black and Red
Refractory Metal Thermocouples
These thermocouples are made of the exotic metals Tungsten and Rhenium, which are expensive, difficult to manufacture, brittle, and must be handled carefully. All of the refractory metal thermocouple has the same temperature range.
Type G (Tungsten/Tungsten 26% Rhenium) Temperature range: 32 - 4200 Fahrenheit Wire colour: White with blue tracer and red
Type D (Tungsten 3% Rhenium/Tungsten 26% Rhenium) Temperature range : 32 - 4200 Fahrenheit Wire colour: Type D: White with yellow tracer and red
Type C (Tungsten 5% Rhenium/Tungsten 26% Rhenium) Temperature range: : 32 - 4200 Fahrenheit Wire colour: White with red tracer and red