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National Advisory Committee for Aeronautics, Technical Notes - Some Effects of Exposure to Exhaust Gas Streams on Emittance and Thermoelectric Power of Bare-Wire Platinum Rhodium - Platinum Thermocouples

Tests have been conducted to study the effect on unshielded platinum
rhodium - platinum thermoeouples of exposure to exhaust gases produced by
the combustion of propane, 72-octane gasoline, and MIL-F-SSZILA grade
J'P-4 fuel.

In all cases where an apparent error in temperature indication
occurred, the error was accounted for principally by an increase in rar
diation error caused by the increase in effective total hemispherical
emittance of the thermocouple wire. Representative values of effective
emittances obtained in the experiments were of the order of 0.2 for a
new thermocouple, 0.3 for a thermocouple exposed to exhaust gases which
left a dulled platinum surface, and 0.5 for a thermocouple exposed to a
luminous exhaust-gas stream.which contained large amounts of unburned
carbon and exhaust residue that coated the wires.

The exposure caused negligible change in the thermoelectric power
of the thermocouples, The value of the thermoelectric power after
exposure fell well within the standard Instrument Society of America
tolerances for such wires.

The use of thermocouple probes for measuring the jet-engine exhaust-
gas temperature poses problems of compromise among such factors as
ability to withstand high-velocity and high-temperature conditions, con-
duction and radiation errors, recovery characteristics, and time re-
sponse. Some of the factors affecting the comromise between ruggedness
and accuracy are discussed in reference 1.

Aside from the fact that the platinum rhodium - platinum thermo-
couples can be used above the temperature range of the common base metal
thermocouples, they have the added advantage of an initially low surface
emissivity which contributes a lower radiation error than that of a base
metal thermocouple under similar operating conditions. However, various
investigators have revealed the possibility of chemical contamination of
platinum rhodium —- platinum thermocouples in the combined presence of
silicon, sulfur, and a reducing atmosphere, which embrittle the wire and
also affect the thermoelectric power (ref. 2). There is also a small
change in thermoelectric power caused by oxidation or volatilization of
the wire and by diffusion of rhodium from the platinum rhodium alloy side
into the platinum side of the Junction (ref. 3).