Thermocouple types can be subdivided in 2 groups, base metal and rare (noble) metal:
-200°C up to 1200°C – These thermocouples use base metals
Type K – Chromel-Alumel
The best known and dominant thermocouple belonging to the group chromium nickel aluminium is type K. Its temperature range is extended (-200 up to 1100°C). Its e.m.f./ temperature curve is reasonably linear and its sensitivity is 41µV/°C
Type J – Iron-Constantan
Though in thermometry the conventional type J is still popular it has less importance in Mineral Insulated form because of its limited temperature range, -200°C to +750°C. Type J is mainly still in use based on the widespread applications of old instruments calibrated for this type. Their sensitivity rises to 55µV/°C.
Type E – Chromel-Constantan
Due to its high sensitivity (68µV/°C) Chromel-Constantan is mainly used in the cryogenic low temperature range (-200 up to +900°C). The fact that it is non magnetic could be a further advantage in some special applications.
Type N – Nicrosil-Nisil
This thermocouple has very good thermoelectric stability, which is superior to other base metal thermocouples and has excellent resistance to high temperature oxidation. The Nicrosil-Nisil thermocouple is ideally suited for accurate measurements in air up to 1200°C. In vacuum or controlled atmosphere, it can withstand temperatures in excess of 1200°C. Its sensitivity of 39µV/°C at 900°C is slightly lower than type K (41µV/°C). Interchangeability tolerances are the same as for type K.
Type T – Copper-Constantan
This thermocouple is used less frequently. Its temperature range is limited to -200°C up to +350°C. It is however very useful in food, environmental and refrigeration applications. Tolerance class is superior to other base metal types and close tolerance versions are readily obtainable. The e.m.f/temperature curve is quite nonlinear especially around 0°C and sensitivity is 42µV/°C.
0°C up to +1600°C – Platinum-Rhodium (Noble metal) Thermocouples
Type S – Platinum rhodium 10% Rh-Platinum
They are normally used in oxidising atmosphere up to 1600°C. Their sensitivity is between 6 and 12 µV/°C.
Type R – Platinum rhodium 13% Rh-Platinum
Similar version to type S with a sensitivity between 6 and 14µV/°C.
Type B – Platinum rhodium 30% Rh-Platinum rhodium 6% Rh
It allows measurements up to 1700°C. Very stable thermocouple but less sensitive in the lower range. (Output is negligible at room temperature).
Historically these thermocouples have been the basis of high temperature in spite of their high cost and their low thermoelectric power. Until the launching of the Nicrosil-Nisil thermocouples, type N, they remained the sole option for good thermoelectric stability.
Additionally, there are specialised thermocouple types which are not described here; these include Tungsten Rhenium types, Pallaplat, Nickel Molybdenum and other Platinum Rhodium alloys.