A) Base Metal
Base metal thermocouples are the most common types of thermocouples. Over 90% of all thermocouples are in this group.
Type E, Chromel vs. Constantan
Suitable for use from -200 to 800°C. Can be applied in atmospheres ranging from vacuum to mildly oxidizing. Excellent choice for cryogenic applications. Has the highest EMF per degree of all the common elements.
Type J, Iron vs. Constantan
The standard selection for use from 0° to 750°C. Type J has good reliability at low temperatures. The positive leg will oxidize rapidly above 500°C. Very economical. Used extensively in the plastics industry but applicable to almost any process within its operating range.
Type K, Chromel vs. Alumel
Type K is the industry standard for use -200 to 1250°C. While stable in oxidizing atmospheres, it is prone to corrosion in reducing environments. Protection tubes are always recommended.
Type N, Nicrosil vs. Nisil
Similar to Type K but more resistant to oxidation and less subject to large drift in the EMF that is found in positive Type K thermocouples operating at approximately 500°C.
Type T, Copper vs. Constantan
Suitable for use from -200° to 400°C, Type T is widely used in the food processing industry. More stable than Type E or J for low temperature applications.
B) Noble Metal Thermocouples
Noble Metal Thermocouples are selected for their ability to withstand extremely high temperatures while maintaining their accuracy and lifespan. They are considerably more expensive than Base Metal Thermocouples.
Type R, Platinum / 13% Rhodium vs. Platinum
Type R has long been the industrial standard noble metal alloy used for high temperature applications to 1600°C. Platinum is prone to contamination if in contact with other metals. Ceramic protection tubes must be used. Very stable in an oxidizing atmosphere but will degrade rapidly in vacuum or a reducing atmosphere.
Type S, Platinum / 10% Rhodium vs. Platinum
Applications and conditions similar to Type R. Type S was applied as the “laboratory thermocouple” while Type R was considered the “industrial thermocouple”. This practice was based on tradition where Type S is not being used extensively as an industrial sensor.
Type B, Platinum/30% Rhodium vs. Platinum/6% Rhodium
Applications and considerations similar to Types R and S, but useful to 1800°C. Very low output at low temperature. Also very non-linear at low end. Generally not considered usable below 250°C. More stable than R or S at high temperature. Must be protected in ceramic tube.