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This paper briefly describes studies made on an 8-in. square guarded hot plate thermal conductivity set designed as outlined in ASTM Method of Test C 177-45. The limits which must be met in order to operate this instrument successfully are discussed. The results of various experiments in which the operational limits are changed are included and show such things as: the effect of ambient temperature on guarded hot plate results, the effect of thermal unbalance between the heater and guard, and the correlation of errors in results with equipment faults. It is shown that hot plates, as presently constructed, probably do not possess a sufficiently accurate temperature control on the guard heater and that extremely small amounts of unbalance affect the thermal conductivity results more than had previously been supposed. It is also demonstrated that the ambient temperature, when allowed to rise above the hot plate temperature, markedly influences the results. Future plans are discussed covering the construction of a new holder for the guarded hot plate apparatus and the development of a thin hot plate.
Gilbo, C. F.
Physicist, Armstrong Cork Co., Lancaster, Pa.