Base metal thermocouple - Environmental Limits.

T type Thermocouples with positive Copper wires and negative Constantan wires are recommended for use in mildly oxidizing and reducing atmospheres. The maximum operating temperature for these alloys is 400°C. These alloys are recommended for low temperature work since the homogeneity of its component wires is better than that in other base metal alloys.

J type Thermocouples with positive Iron wires and negative Constantan wires are recommended for use in reducing atmospheres. The maximum operating temperature for these alloys is 870°C for the largest wire sizes. Smaller wires should operate in correspondingly lower temperatures.

E type Thermocouples with positive Chromel wires and negative Constantan wires are recommended for use in vacuum or inert, mildly oxidizing or reducing atmospheres. The maximum operating temperature for these alloys is 870°C. At low temperatures the thermocouple is not subject to corrosion. This thermocouple has the highest EMF output of any standard metallic thermocouple.

K Type Thermocouples with positive Chromel wires and negative Alumel wires are recommended for use in clean oxidizing atmospheres. The maximum operating temperature for these alloys is 1260°C for the largest wire sizes. Smaller wires should operate in correspondingly lower temperatures.

N Type Thermocouples with positive Nicrosil wire and negative NiSil wire are used in place of K Type Thermocouples where high temperature stability is required. The alloys in this thermocouple offer excellent resistance to preferential oxidation in the range 1000°C to 1200°C. The EMF output differs from that of K Type Thermocouples, but the curves have similar slopes over the elevated temperature range.

Refractory Metal Thermocouple - Environmental Limits:

C Type Thermocouples have positive Tungsten 5%-Rhenium wires and negative Tungsten 26%-Rhenium wires. These alloys have inherently poor oxidation resistance and are therefore recommended for use in vacuum, hydrogen or inert atmospheres. The maximum operating temperature for this alloy is 2760°C.

Noble Metal Thermocouple - Environmental Limits:

R Type Thermocouples have positive Platinum 13%-Rhodium wires and negative Platinum wires. These alloys have a high resistance to oxidation and corrosion. However, hydrogen, carbon and many metal vapors can contaminate these types of thermocouples. The maximum operating temperature for
this alloy is 1540°C.

S Type Thermocouples have positive Platinum 10%-Rhodium wires and negative Platinum wires. These alloys have a high resistance to oxidation and corrosion. However, hydrogen, carbon and many metal vapors can contaminate these types of thermocouples. The maximum operating temperature for
this alloy is 1540°C.

The thermocouple application environments presented herein are those recommended by thermocouple manufacturers and suppliers and are provided for reference only. Thermocouple Feedthroughs have a maximum temperature rating of 450°C. Furthermore, Thermocouple Feedthroughs Type C, R and S are manufactured using extension grade thermocouple wire, which is rated for maximum junction temperatures of 250°C. 

A Thermocouple Feedthrough is limited thermally by ceramic to metal joints and vacuum mount style. Ceramic to metal seals should never be subjected to thermal gradients in excess of 25°C per minute.