Producing Thermal Dryout Curves for Buried Cable Applications
The relationship between water content and soil thermal conductivity or resistivity is often termed a thermal dryout curve (resistivity is just the reciprocal of conductivity). The thermal conductivity of a soil depends strongly on water content, but also depends on temperature, bulk density and soil composition. To speak of the thermal conductivity of soil, without specifying the water content, density, temperature and composition, is meaningless. For a soil in place the composition and density are fixed, and the temperature typically varies over a small enough range to have only a small effect on thermal conductivity (unless the soil freezes). The main variable for a soil in place is therefore moisture content. The purpose of the thermal dryout curve is to represent the effect on thermal conductivity of this variability. This application note presents some of the methods which have been used to obtain thermal dryout curves, and recommends a simple method, combining two of them, which will give reliable results.