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A concise, easily evaluated analytical expression for the error in thermal conductivity measurement by the guarded hot plate, due to heat exchange with the ambient air, is derived assuming that the temperature distribution along the specimen edges may be represented by a mean temperature. This solution agrees closely with one presented by Somers and Cyphers (1) for the special case of the specimen edges held at the cold plate temperature.
The error is shown to depend upon three dimensionless parameters: (a) the ratio of guard ring width to specimen thickness; (b) the ratio of the length of side of the test area to specimen thickness; and (c) a number between zero and unity whose value is determined by the specimen-edge-temperature distribution. The ASTM specimen thickness requirements are based only upon the first parameter. The approximate effect of the size of the test area upon the error is described.
A procedure is proposed for testing, when necessary, very thick specimens. This involves the measurement of the specimen-edge-temperature distribution during test (with or without edge insulation), calculation of the error, and correction of the measured conductivity.
Research Officer, Division of Building Research, National Research Council of Canada, Ottawa, Canada