Evaluation of factors affecting heat flux sensors

A program to evaluate heat-flux sensors employed in thermal measurements of passive solar structures has been performed. A guarded hot box was designed and built in accordance with ASTM Standard C236-80 to generate known heat fluxes through a variety of structural materials. These fluxes were compared with those measured by a heat flux transducer calibrated by both by the manufacturer and an independent laboratory. The guarded hot plate method, ASTM C117-76, was used in the latter case. The three sets of values were often in substantial disagreement, the extent of which varied with the substrate to which the transducer was attached. An analysis of the data indicates that the cause of disagreement lies in a local distortion of the heat flux through the substrate caused by the presence of the transducer. Disturbance of the air flow over the transducer and mismatch of surface thermal emissivities of the transducer and substrate are contributing factors. This work has demonstrated a need for a standardized procedure for heat flux sensor calibration, and the ASTM guarded hot plate method is recommended for this purpose. An approximate method for correcting the heat flux indicated by a transducer calibrated in this manner to bring it inmore » agreement with the measured by the guarded hot box is presented.« less