Published: Jan 1967
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A guarded-hot-plate technique has been developed for making thermal-conductivity measurements at low temperature. The apparatus has been used to measure materials with conductivities in the range 0.01 < K < 10 Btu in./hr ft2 deg F. A thin electrical heater is placed between two panels of the test material. The heater system consists of two parts: a center heater surrounded by a guard heater. The heaters are fed by separate power supplies, making it possible to cancel out lateral heat flow. A cryogenic fluid provides a heat sink and fixes the outside panel wall temperature. Thermocouples measure the thermal balance between heaters and the gradient across the specimen. The electrical power establishes the inner wall temperature and determines the heat which flows through the guarded area. The advantages of this method are: (1) A fixed outside wall temperature is established by using the boiling and freezing points of various liquids. (2) The thin flexible heater helps reduce lateral heat flow and allows the center and guard heaters to conform to slight variations in the panels. Factors considered in the error analysis are: effective heater area, measurement of power to heater, thermocouples, panel thickness, contact resistance, panel bowing, and gas environment. Certain improvements in both apparatus and technique have been made by careful analysis of the data generated during several hundred measurements.
thermal conductivity, heat transfer, low temperature, plastics, contact resistance, guarded hot plate, cryogenics, thermal insulation
Haskins, J. F.
Chief of materials research, General Dynamics/Convair, San Diego, Calif.