Suitable Steady-State Methods for Measurement of Effective Thermal Conductivity in Rigid Insulations

The ASTM Test for Thermal Conductivity of Materials by Means of the Guarded Hot Plate (C 177-63) is a recognized technique for measurement of thermal conductance of insulations. This method may not be the optimum choice due to temperature range, material availability, and other factors. This paper describes three other steady-state techniques that may be used. Two are modifications of the basic comparative rod apparatus, and the third is a modification of the radial inflow apparatus. The two modifications of the comparative rod apparatus are designed to minimize or monitor or both the axial heat bypass and radial heat interchange effects. The modification to the radial inflow apparatus converts a cylindrical specimen to obtain heat flows through a plane surface. These modifications represent an improvement in the state of the art.

The advantages of these procedures over the ASTM C 177-63 apparatus are smaller specimen size and wider temperature range. The disadvantages are that the precision and accuracy are not as good as that for the ASTM C 177-63 apparatus; however, an acceptable level for most engineering data may be obtained (< ±12 percent).

Data on various candidates for the reusable surface insulation of the shuttle were measured by these methods and are reported.