COMPARISON OF SINGLE AND DUAL PROBES FOR MEASURING SOIL THERMAL-PROPERTIES WITH TRANSIENT HEATING

Storage and transfer of heat in soils is governed by the soil thermal properties and these properties are therefore needed in many agricultural and engineering applications. In this paper we discuss solutions of the heat flow equation applicable to single and dual probe transient heating methods, and describe measurements made on air-dry sand to show how these methods can be used to obtain soil thermal properties. Measurements show that the two methods yield similar values of thermal conductivity. When determining thermal conductivity from the single probe data, it is best to use nonlinear curve fitting and to include a correction term in the model to account for the presence of the probe. Measurements of volumetric heat capacity made by using the dual probe heat-pulse method agreed well with independent estimates obtained using the de Vries method of summing the heat capacities of the soil constituents. The advantage of using the dual probe method together with the appropriate heat-pulse theory rather than the single probe is that all three soil thermal properties, the thermal diffusivity, volumetric heat capacity, and thermal conductivity, can be determined from a single heat-pulse measurement. Instantaneous heat-pulse theory can be used with the dual probe method to determine heat capacity from short duration heat-pulse data, but it should not be used to determine the thermal diffusivity and thermal conductivity.