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Thermal Conductivity and Diffusivity of Soil Using a Cylindrical Heat Source¹

ABSTRACT

The interference of moisture migration upon measurements of the thermal constants in soil has been reduced by using a large-diameter, cylindrical thermal probe which is heated at a constant rate. Using the initial portion of the experimental record (temperature versus time) and by taking account of the thermal contact resistance between the heat source and sample material, it is possible to obtain values of thermal conductivity accurate to within 4%. Real thermal conductivity is obtained by subtracting from the measured value the small effect of distillation which is estimated with the help of the theory of vapor diffusion in porous media. Results obtained for a sandy loam soil show that real thermal conductivity increases rapidly with water content from 6.05 x 10^-4 cal/cm per °C per sec., when air dry, to 26.0 x 10^-4 cal/°C per cm per sec. at 30% of saturation. Thereafter it increases less rapidly (in proportion to the increase of volumetric heat capacity) to a value of 59.5 x 10^-4 cal/°C per cm per sec. at complete saturation. Real thermal diffusivity remains essentially constant at water contents above 30% of saturation. The method can also be used for rapid and accurate measurement of the thermal conductivity of liquids provided their volumetric heat capacities are known.

¹ Technical paper 1544, Arizona Agr. Exp. Sta., Tucson. Contribution from the Department of Agricultural Chemistry & Soils under Regional Research Project W-68 with additional financial support from the Office of Water Resources Research, Department of Interior. Paper presented before Div. S-1, American Society of Agronomy, New Orleans, Nov. 1968, for which the senior author received travel support from the Arizona Chapter of Sigma Xi.

² Research Associate and Professor, University of Arizona. Senior author now with Illinois Water Survey, Urbana, Illinois.

Received for publication September 15, 1969. Accepted for publication February 19, 1970.