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Method of Measurement of the Real Thermal Diffusivity of Moist Soil¹

ABSTRACT

Thermal diffusivity measurements on moist soils made by conventional methods yield only apparent values of the diffusivity because the heat applied for the measurement causes moisture transfer and hence attendant heat transfer not accounted for in the theory. In the present method an alternating source of heat is used so that moisture transfer occurs in alternate directions, reducing moisture movement effects. Moisture movement effects are further reduced by measuring the apparent values of the thermal diffusivity at a number of decreasingly smaller periods (with correspondingly smaller temperature gradients) of the alternating heat source. A plot is then made of the apparent value of diffusivity vs. the period of the applied heat wave, and the resulting curve extrapolated to zero period to yield a value of diffusivity, called the real diffusivity, for zero temperature gradient. The method has been used to determine real diffusivities on samples of a clay loam and a sand at a number of different moisture contents and tensions. The real diffusivities are found to increase steadily as the moisture content increases to saturation. Thus, the present results do not agree with previously reported data, in that previous data show maxima in the curves of diffusivity vs. moisture content at moisture contents less than saturation. The present data do, however, show maxima in curves of apparent diffusivity vs. moisture content. It is concluded that diffusivities formerly reported have been apparent diffusivities. From the real thermal diffusivities and soil data, real thermal conductivities have been computed and are presented.

¹ Journal Paper No. J-3247 of the Iowa Agr. and Home Econ. Exp. Sta., Ames, Iowa, Project 1235. Contribution from the Department of Agronomy (Soils). This investigation was supported in part by Consolidated Hatch Act Regional Research Funds, as a contributing project to Regional Project NC-17. Presented before Div. I, Soil Science Society of America, Atlanta, Ga., Nov. 20, 1957.

² Graduate Assistant and Professor. The senior author is now Soil Scientist with the Agricultural Research Service, USDA, Fort Collins, Colo.

Received for publication September 9, 1957. Accepted for publication August 13, 1958.