This paper describes an 8-in. guarded hot plate apparatus constructed according to ASTM Standard C 177–45 but with a number of changes that increase the versatility of the apparatus and permit the securing of thermal conductivity values to ±1 per cent by automatic means.
These improvements include the use of mica for the insulating spacers and heating core in the hot plate permitting use up to 400 F. A new thermopile consisting of a string of 32 junctions made from chromel-constantan wire was woven across the gap, thus making a thinner and a more sensitive hot plate. Automatic controls were added to keep the heater to guard balance to ±0.004 F. This unit consists of a servo system that continually regulates the power to the guard. The cold plates were constructed with a continuous labyrinth instead of the usual manifold type shown in the ASTM method, and commercial constant temperature water baths were used to hold the cold plate temperature within 0.05 F. A frame was designed to hold the hot plate assembly under a constant pressure of 50 lb per sq ft. The completed apparatus was placed in a temperature and humidity controlled cabinet, with the temperature controlled automatically. With the above changes it was found possible to construct a guarded hot plate apparatus using commercially available equipment that will determine thermal conductivity values to within 1 per cent of those determined at the National Bureau of Standards.