Angular Sensitivity of Heat Flux Gages

The response of a heat flux gage depends on both the angular distribution of the source radiant flux and the angular sensitivity of the coating on a gage's heat-sensing element. The issue becomes important for the calibration of apparatuses designed to test the response of materials subjected to a known level of incident thermal radiation. In this study, the angular sensitivity of several different gage coatings are measured by rotating the gage sensing surface in front of a black body source. Ideally, gage output is proportional to the cosine of the angle of incidence with respect to the normal, known as Lambertian behavior. Some commercial black coatings become non-Lambertian for angles above 60° from the surface normal, but other coatings maintain a Lambertian response beyond 70°. The impact of these differences on the calibration of the Fire Propagation Apparatus and the Cone Calorimeter is evaluated.