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National Advisory Committee for Aeronautics, Technical Notes - Experimental Analysis of a Pressure Sensitive System for Sensing Gas Temperature

various theoretical and practical aspects of a pressure-
sensitive system for the measurement of gas temperatures in gas—
turbine-engine combustion chambers are analyzed. An experimental
setup was operated under controlled conditions of temperature,
weight flow, and wall temperature. The gas temperature after com—
bustion was obtained by application of a relation equating thermo-
dynamic conditions before and after combustion. This method of gas-
temperature sensing appears practical for high-temperature appli-
cations. The accuracy with which gas temperature may be determined
by this method is within.i2 percent. Measurements made by the
thermodynandc method were used as a temperature standard for com-
parison of temperature data obtained from conventional thermocouple
probes.

With the advent of gas-turbine engines, the controls designer
has been confronted with the serious problem of operating engines
not only at high temperatures for maximum efficiency, but also at
temperatures low enough to be within the safe operating range for
the materials used.

It therefore becomes imperative to incorporated temperature-
limiting controls to prevent engine failure by overheating.
Temperature can be used both as a primary control parameter and as a
temperature-sensing parameter. Any means of sensing gas temperature
for application to controls must meet the following requirements:
have an output that is'a simple function of true gas temperature
under various conditions of weight flow, temperature of_surrounding
materials, density, composition of gas, and temperature level;
provide an output that is easily incorporated in a control;
have a rapid response to transient conditions; (4) have an
extended temperature range; (5) have a long life; and (6) be
independent of material characteristics subject to change with
time and use.

The most commonly used methods for measuring temperature
employ thermocouples or resistanceiwire thermometers, both of which
have the advantages of simplicity and durability. These devices,
however, do not provide sufficiently ideal_outputs when consider-
ation is given to the inherent errors resulting from radiation,
heat conduction, and velocity losses that occur at elevated gas
temperatures.