Thermocouple Feedthroughs

Thermocouple FeedthroughsThermocouple Feedthroughs are widely used for temperature measurements in vacuum applications, whereas the temperature is not measured directly but can be calculated from a thermoelectric voltage. The thermoelectric voltage is generated due to charge separation (EMF) caused by a temperature difference between the two ends of an electrical conductor.

The thermoelectric voltage (depending on the conductor material) is nearly proportional to the temperature gradient. Thermocouples  consist of two different electrically conductive materials connected at one end. Using thermocouple feedthroughs the thermoelectric voltage (EMF) can be transmitted from the inside of a chamber to an external indicating instrument. A high thermoelectric voltage, corrosion resistance and linearity are  decisive factors for the selection of the material combination. Noble and base metals as well as alloys - adapted to different application - are combined in thermocouples. Typical alloys are Alumel® (nickel, aluminium and manganese), Chromel® (nickel and chrome) and  constantan (copper and nickel).

Some thermocouple feedthrough types use compensating material ("compensating wire") instead of the actual thermocouple material. Compensating materials are characterized by thermoelectric properties ideally adapted to the corresponding thermocouple materials. For these types, we explicitly point out the use of compensating wire by footnotes ("Uses compensating wire"). Furthermore, it should be noted, that the temperature at junctions between a compensating wire and a true thermocouple conductor material must not exceed 250 °C ("Max. junction temperature 250 °C").
Typical ANSI-standardized thermocouple material pairs are listed in the table below.

Thermocouple (ANSI code) Polarity Measurable temperature range [°C]
T Copper
Constantan
+
-
 -200...350
K
Chromel®
Alumel®
+
-
-200...1250
R/S Platinum 13/10 % Rhodium*
Platinum*
+
-
0...1450
J Iron
Constantan
+
-
 0...750
E
Chromel®
Constantan
+
-
-200...900
C** Wolfram 5 % Rhenium*
Wolfram 26 % Rhenium*
+
-
0...2315
*Compensating wire is used; Tmax at flange 250 °C
**no ANSI code

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