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A thin-foil heating element is employed in a thermal conductivity apparatus that operates in accordance with the basic concept of the guarded hot plate. The metal foil is sufficiently thin that lateral heat flow along the plane of the heater is insignificant, at least in the central region, so there is no need for isolation and separate temperature control of a guard region. This makes the apparatus simple and inexpensive to construct and operate. The resultant low-mass configuration minimizes drift error and usually reaches steady state in less than 30 min with 0.5-cm-thick specimens of nonmetallic solids. Thermal conductivity values are calculated on an absolute basis with an estimated accuracy of ±3%.
Speed and simplicity make the apparatus well suited for product-development and quality-control applications. Because temperature differences within the sample do not exceed a few degrees, uncertainty in mean sample temperature is sharply limited, water-vapor-migration problems are minimized, and thermal conductivity can be measured close to melting points or other critical temperatures. New data obtained with dry calcium silicate give preliminary experimental verification of the validity of the apparatus in the temperature range extending between ambient and 550 K.
thermal conductivity, equipment, measurement, nonmetallic solids, calcium silicate
Senior research associate, Research and Development, Armstrong World Industries, Inc., Lancaster, PA