Thermocouple types

Thermocouples are categorized based on their use,Sensitivity and temperature range.They can be easily identified by using the color chart which is given here.

Type K (Chromel / Alumel)

Sensitivity -41 µV/°C.

Temperature range –270 to 1372°C.

Use: General purpose use like process plants etc.It is oldest and therefore most commonly used and popular type.Its probes are of all varieties and have its temperature range is quite wide making it applicable relatively vast process.Its can readily be used in Oxidizing or inert environment. However in reducing or vacuum it can deteriorate. Nickel part of the probe  is magnetic so its show abrupt variation in emf above temperature around 220.

Type E (Chromel / Constantan)

Sensitivity (68 µV/°C) Highest sensitivity in terms of  EMF /°C or °F of any of the thermocouples

Overall range –270 to 1000°C useful range 95-900 degrees C

It is mainly used for cryogenics or very low temperature measurement.Another advantage is Its none of the wires are magnetic.Suitable in oxidizing environment which are clean and not partially oxidizing or frequent change in environment from oxidizing to reducing.

Type J (Iron / Constantan)
Usefulrange (-45 to +760 °C)

Use: Equipment that can not accept ‘modern’ thermocouples.it is magnetic so above curie temperature around  750 it can loose calibration..Reducing, Vacuum, Inert environment .It can deteriote at high temperature at high temperature in oxidizing or sulphurous environment.below zero temperature applicationis also not suitable because of iron conductor may rust or form embritllement.So it is used upto 450 °C.

Type N (Nicrosil / Nisil)

650 -1260 degrees C –270 to 1300°C

Use: It is stable as it does not show abrupt emf change like type k.Also at high temperature it does not oxidizes. was made as a better replacement of type K.Type B s or R are TC  are also stable type of thermocouple but their sensitivity is not so good at low temperature so the are mostly used for high temperature.Also N  type is much cheaper.

Type D (

Type R/S/B (Platinum type thermocouple)

These are noble metal thermocouple and one of the most stable types.But they are  best applicable at high temperature because of the flat or constant emf  response  at temperature below 40-50°C Doe snot oxidizes in oxidizing condition like hot air. It is not enclosed in metal tubes because of high susceptibility to contamination ie diffusion of unwanted material etc.

They are mainly used in glass,semiconductor or steel industries to monitor the temperature of arch of this melting chamber of furnace where glass or steel is melted.They are generally enclosed in ceramic tube.

R (Platinum / Rhodium)
Overall range –50 to 1768°C.Useful range 870-1450 degrees C

S (Platinum / Rhodium)

Overall range –50 to 1768°C Useful range 980-1450 degrees C

sensitivity (10 µV/°C)

They are particularly used as standard for calibrating in melting pont of gold.

 B (Platinum / Rhodium)

Overall range 0 to 1820°C.Useful range1370-1700 degrees C (approx 10 µV/°C)

 

Type T (copper-constanan)

Overall range –270 to 400°C.Useful Range 200-350 degrees C.

Sensitivity 43 µV/°C.

They are good at low temperature so cryogenics very low temperature) application is its  standard characteristics. They are Particularly useful where long presence of moisture in unavoidable such as food industries.Can be used in all types of environment inert,oxidizing or reducing provided they are mild and not extreme.

Type C tungsten 5% rhenium – tungsten 26% rhenium)

Overall range 0 to 2320°C.Useful range 1650-2315 degrees C

Well-suited for vacuum furnaces at extremely high temperatures.Not useful in presence of oxygen above 250 °C.they are used vehicle for space,neclear reactor core,heating in industrial process etc. Tungeston is combustible at very high temperature so cannot be used in oxidizing environment.They are used in inert or hydrogen environment,Also in rigorous process of heating and cooling may cause embrittlement which then may cause breakage of wire due to vibration.This  limits it to be used in all type of industrial heating.

Type M nickel alloy for each wire

0-1287 degrees C

Use: These thermocouples are used in vacuum furnaces for the same reasons as with type C.

This chart shows the various color codes for identifying the type of wire and its polarity based on the type of wire.These color codes are different in different countries and Also different for different types of thermocouples. You can identify the type of thermocouple by noticing the insulation color and the color used for different polarities wire.

 

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