Thermocouple Documentation

Thermocouple Details – 

Any two dissimilar metals can be used for this purpose, however there are many standard combinations on metals, which are internationally used and designated for use as thermocouple elements.

Most commonly used thermocouple elements are broadly divided in two categories: A) Base metal thermocouples (J, K, T, and E type) and B) Precious metal thermocouples ( R, S and B type).

• Thermocouple Type-
Given below is general description of commonly used thermocouple.

• IEC 584-1, PART1: S:
This thermocouples can be used in oxidising or inert atmosphere at temperatures described above.Recrystallised Alumina beads and protective sheaths are generally used. However, for lower temperatures upto 1100 Deg C metal sheaths with porcelain beads can be used.Please note that continuous use at elevated temperature may cause fusing of Rhodium in pure Platinum causing reduction in output emf.

• IEC 584-1, PART2: R:
This thermocouples can be used in applications similar to S type thermocouples.The advantage being slightly higher output at same temperature and better stability. This type is preferred over S type thermocouples.

• IEC 584-1, PART3: J:
This thermocouples are commonly known as iron/constantant thermocouples and can be used in reducing or inert atmosphere at temperatures described above.Restrict use in oxidizing atmosphere to temperatures up to 400 deg C, otherwise degradation will be rapid.Avoid using for application where condensation temp is often reached otherwise Fe {+leg} will rust rapidly.These are designed primarily for use in Oxidising atmosphere at temperatures described above.Great care should be taken when using for any other atmosphere.These thermocouple will get oxidised when used continuously at temp above 1000 Deg C and cause drift.For temp range from 250 to 600 Deg C stability is suspect. Cyclic temperature changes can cause error of several degrees at these temperatures. Other base metal thermocouples are better bet at this temp range.

• IEC 584-1, PART4:K
This is most commonly used thermocouple ind is are commonly known as Chromel/Alumel thermocouple these are designed primarily for use in oxidising atmosphere at temperatures described above. Great care should be taken when using for any other atmosphere. These thermocouple will get oxidised when use continuously at temp above 1000 Deg C and cause drift. For temp range from 250 to 600 Deg C stabilty is suspect. Other base metal thermocouples are better bet at this temp range. Cyclic temperature changes can cause error of several degrees at these temperatures.

• IEC 584-1, PART5: T:
This is commonly used in laboratory applications where sub zero temperatures are often maintained.Repeatability at -200 + 200 Deg C is excellent. However at temp above this Copper arm rapidly oxidizes causing drift.Note that the negative arm of T and J type are not same and are not interchangeable.

• IEC 584-1, PART6: E and IEC 584-1, PART7: B
These thermocouples are not in much use presently.E type is more stable in -250 + 200 Deg Range than K type and can be preferred over K in this range.Type B thermocouple can be used at temperature upto 1600 Deg c. Interesting point is that in this type cold junction compensation is not normally required as emf change from 0 to 50 is negligible.