Today heat flux sensors are available with temperature ratings up to 800°C. In many situations these sensors do not have to be cooled at all, and therefore do not produce aerodynamic or thermal disturbances. However, the variation of their sensitivity with temperature can be an important source of error.
The High Temperature Heat Flux Calibration System described here is designed to perform calibrations of heat flux sensors at ambient temperatures up to 816°C and heat fluxes up to 341 watts/cm2. The response of the sensor being calibrated is compared with that of a reference sensor, with the two sensors located on opposite sides of a thin heated flat plate.
Calibrations with this system are performed in a vacuum of about 1 torr, effectively eliminating convective heat transfer and heated flat plate oxidation. A controlled temperature heated stage is provided for the sensor on test, and a constant temperature cooled stage for the reference sensor. The heated flat plate is powered by direct current, and shielded from the sensors during warm-up by a bifurcated water-cooled graphite shutter. After the temperature of the heated flat plate has stabilized, the shutter is opened. This exposes the sensor on test and the reference sensor simultaneously to steep wavefronts of heat flux. After a few seconds the shutter is closed again and power to the heated flat plate is cut off.
All functions of the calibration system are performed under control of Labview® software. After mounting the sensor to be tested, the operator enters a list of desired temperatures and heat fluxes. Tests are then performed automatically in the sequence which will utilize the resources of the system most efficiently. At the end of the test sequence, calculations are performed and a report of calibration results is generated and printed. During the test the operator may monitor system functions and intervene at any point if desired.
Problems encountered in the design of this system and typical results of calibrations performed during its development will be presented.