Thin-Heater Thermal Conductivity Apparatus

Abstract

Apparatus is described for measuring the thermal conductivity of insulating materials. A constant current is passed through a rectangular sheet of stainless steel foil in such a manner that the rate of generation of heat per unit area can be computed from electrical measurements. Identical sample slabs are placed against opposite sides of the heater, and the combination is enclosed in a plastic film and immersed in a bath having constant and uniform temperature. When thermal equilibrium is reached, the temperature drops across the central portions of the sample slabs are measured with differential thermocouples. The elementary one-dimensional form of the Fourier heat transfer equation is used to compute the thermal conductivity from the observed quantities. In pre liminary tests with representative insulating materials equilibrium is reached within 15 min. It is estimated that the apparatus is capable of giving measure ments accurate to within 2%, and experimental results are not inconsistent with this estimate. Thermal conductivity values are reported for polystyrene and a cork-rubber composition at mean temperatures ranging from liquid nitrogen temperature up to room temperature. Differential thermocouple techniques are discussed. The characteristics of the apparatus make it especially suitable for academic laboratory instruction and for low-temperature measurement.

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