Historical Review of the Metered Section Area for Guarded Hot Plates
Robert R. Zarr, Stacy M. Bruss, David McElroy
Results of an extensive literature review and investigation of the metered section area for the guarded-hot-plate method, standardized as ASTM C177, Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus, are presented. The guarded-hot-plate apparatus is a primary, linear-heat-flow method to determine the thermal conductivity of insulating and building materials. The review examined technical publications from 1885 to 1990, and identified 31 papers addressing the subject. Historical versions of C177 were also researched, as well as test methods from other standards organizations. The investigation revealed that researchers over the past 100 years have independently developed two main approaches for the computation of the metered section area. An assessment of the calculation techniques is presented for round plates having diameters from 250 mm to 1000 mm, a guard-to-meter aspect ratio of two, and guard gap widths of 1 mm to 4 mm. The gap effects are not negligible because large gaps (4 mm) on small plates (250 mm) can have an effect of 10 %, or more, on the computation of the metered section area. The results of this study are applicable to other thermal conductivity test methods that employ a primary thermal guard to promote one-dimensional heat flow.