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Thermocouple Types
Iron-Constantan (ANSI Symbol J) The Iron Constantan "J" curve
thermocouple with a positive iron wire and a negative Constantan wire is recommended
for reducing atmospheres. The operating range for this alloy combination is 1600°F
for the largest wire sizes. Smaller size wires should operate in correspondingly
lower
Copper-Constantan (ANSI Symbol T) The Copper-Constantan "T" curve
thermocouple, with a positive copper wire and a negative Constantan wire is recommended
for use in mildly oxidizing and reducing atmospheres up to 750°F. They are suitable
for applications where moisture is present. This alloy is recommended for low temperature
work since the homogeneity of the component wires can be maintained better than
other base metal wires. Therefore, errors due to inhomogeneity of wires in zones
of temperature gradients is greatly reduced.
Chromel-Alumel' (ANSI Symbol K) The ChromelAlumel "K" curve thermocouple
with a positive Chromel wire and a negative Alumel wire is recommended for use in
clean oxidizing atmospheres. The operating range for this alloy is 2300°F for the
largest wire sizes. Smaller wires should operate in correspondingly lower temperatures.
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Chromel-Constantan (ANSI Symbol E) The Chromel-Constantan thermocouple
may be used for temperatures up to 1600°F in a vacuum or inert, mildly oxidizing
or reducing atmosphere. At sub-zero temperatures, the thermocouple is not subject
to corrosion. This thermo-couple has the highest emf output of any standard metallic
thermocouple.
Platinum-Rhodium Alloys (ANSI Symbol S, R and B) Three types of
"noble-metal" thermocouples are in common use; they are: 1) a positive wire of 90%
platinum and 10% rhodium used with a negative wire of pure platinum, 2) a positive
wire of 87% platinum and 13% rhodium used with a negative wire of pure platinum,
and 3) a positive wire of 70% platinum and 30% rhodium used with a negative wire
of 94% platinum and 6% rhodium. They have a high resistance to oxidation and corrosion.
However, hydrogen, carbon and many metal vapors can contaminate a platinum-rhodium
thermo-couple. The recommended operating range for the platinum-rhodium alloys is
2800°F although temperatures as high as 3270°F can be measured with the Pt-30% Rh
vs Pt-6% Rh alloy combination.
Tungsten-Rhenium Alloys Three types of tungsten-rhenium thermocouples
are in common use for measuring temperatures up to 5000°F These alloys have inherently
poor oxidation resistance and should be used in vacuum, hydrogen or inert atmospheres.