Thermocouple Electric Feedthroughs - Introduction
MDC uses the Thermocouple classification for all ISI™ feedthroughs used in applications for temperature measurement.
A thermocouple is a device that measures temperature as a function of the electromotive force induced when heat is applied to two dissimilar metal wires, which are joined at both ends. A Thermocouple Feedthrough is not a temperature sensing device as such, but a component used to conduct the thermocouple’s voltage (EMF) through the bulkhead of a chamber to external instrumentation. A Thermocouple Feedthrough provides an environmental splice in a thermocouple’s conductor path.
ISI™ Thermocouple Feedthroughs are not thermocouples and are designed for use as accessories or extensions to standard thermocouple elements. Thermocouples are divided into three basic categories: Base metal, Refractory metal and Noble metal types. (Note that the following information, although pertinent to Thermocouple Feedthroughs, is only given as reference for the selection of Thermocouples.)
Base Metal Thermocouple - Environmental Limits:
T Type Thermocouples
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
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
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
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
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
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
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
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. ISI offers thermocouple lead wire and accessories detailed in Catalog Sections 9 & 10.
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.
Intended Operating Conditions
Electrical ratings are safe operating limits. These ratings are determined by various factors, including dielectric strength, geometry and system operating pressure. Please note that all ISI catalog products are electrically rated for operation with one side in dry atmospheric conditions and the other side in a vacuum environment with a maximum system pressure of 1 x 10-4 Torr. We advise that users make allowances for deviations from stated operating parameters and take adequate safety precautions when feedthroughs are operated at high voltages or high currents.
Insulator Seal (ISI) brand catalog and custom engineered ceramic to metal seal products are manufactured to ISO 9001:2015 standard utilizing our qualified hermetic brazing and joining processes.
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