Historical Review of the Metered Section Area for the Guarded-Hot-Plate Method.

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 generally used to determine the thermal conductivity of insulating and building materials. The review examined technical publications from 1885 to 1990 and identified 31 papers of interest. Historical versions of ASTM C177 were also researched as well as test methods from other standard development organizations. The investigation revealed that, over the past 100 years, researchers 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 with diameters from 250 to 1,000 mm, a guard-to-meter aspect ratio of 2, and guard gap widths of 1-4 mm. The gap effects are not negligible because large gaps (4 mm) on small plates (250 mm) can lead to errors of 10 % or more on the computation of the metered section area, ultimately affecting the uncertainty of the test results of the guarded-hot-plate method. The results of this study are applicable to other thermal conductivity test methods that employ a primary thermal guard to promote 1-D heat flow.