Automated High-Temperature Guarded-Hot-Plate Apparatus for Measuring Apparent Thermal Conductivity

摘要

An automated guarded-hot-plate apparatus was designed and built to meet the requirements of ASTM Standard Test Method C-177 for measuring the thermal conductance of thermal insulation. Apparent thermal conductivity has been measured with this apparatus in the range from 40 to 100 mW/(m.K) at mean temperatures from 300 to 750 K, in environments of air and helium, at pressures ranging from atmospheric pressure to roughing-pump vacuum. The apparatus is controlled by a desk-top computer. A thermocouple device of novel design more accurately senses the average temperature over the surface of each heater plate. An improved algorithm for the control sequence leads to more stable heater powers and specimen temperatures. Overall uncertainties of apparent thermal conductivities at atmospheric pressure are 2 percent at 300 K and 5 percent at 750 K when measuring conductivities in the range from 40 to 100 mW/(m.K). The apparatus will be valuable in development of new Standard Reference Materials of low conductivity and for higher temperature ranges, and is being used in comparative interlaboratory measurement programs.