Thermal Conductivity Measurements with the Plate Apparatus: Influence of the Guard Ring Width on the Accuracy of Measurements

The temperature field in circular and square plate test specimens is calculated and thereby the error from the heat exchange at the outer edge of the guard ring derived for single and double plate devices under conditions of isothermal hot and cold plates and ideal thermal contact. The error is shown to be proportional to the reduced temperature difference (difference between the mean specimen and the ambient temperatures divided by the difference between the hot and cold plate temperatures) and the margin of error, therefore, can be optimally reduced by an appropriate adjustment in the ambient temperature. Experiments on an extruded foam confirm the results of the analytical calculations. Diagrams and tables which are developed on the basis of the above derived relationships, make possible estimation of margin of error for different measuring arrangements for plate devices of usual sizes.