Schmidt-Boelter Heat Flux Gage Sensitivity Coefficients in Radiative and Convective Environments

Commercial Schmidt-Boelter heat flux gages are always calibrated by using a radiative heat flux source where convection is minimized. This is because one can establish a reliable link to a National Institute of Standards and Technology (NIST) calibration standard. To the authors’ knowledge, no NIST traceable link exists for convective heat flux calibration. When heat flux gages are used in typical applications, convection is often not negligible. It has been common practice to assume that the sensitivity coefficient supplied by the manufacturer also applies for convective environments. This assumption is believed to be incorrect. If incorrect, this would result in uncertainties larger than typically reported (e.g., ±3%). This paper analyzes the heat transfer from an idealized Schmidt-Boelter heat flux gage. The analysis shows that the theoretical sensitivity coefficients in purely radiative and convective environments are not the same and, in fact, differ by the emissivity of the gage surface. The implication of this difference is that the accuracy specification supplied by the manufacturer (typically ± 3%) is not correct for measurement applications where convection is not negligible.