A Spherical Steady-State Method to Measure Soil Thermal Conductivity

Abstract

The reliable prediction of coupled heat and moisture transfer in three-phase soils involves authentic estimation of soil thermal conductivity. In the current work, a steady-state method was developed to measure soil thermal conductivity. Soil samples for two soils were packed in a plastic of 6.6 cm inner diameter and 3 mm thickness. A 130 Ohm resistor as a heat generator being connected to 5 V electricity source was located in the center of the ball. Three thermocouples were installed at the distances of 0.5, 1.5, and 2.5 cm from the center, respectively. After unpacking the ball, thermal conductivity was measured using the single probe method. The thermal conductivity values measured by both methods were almost the same in air dry soils; overall, the observed differences between the two methods were less than 3%. However, in the intermediate moisture content ranges, the values measured by the steady-state method were slightly lower than those obtained by the single probe method. Considering the effect of convective vapor flux due to thermal expansion of soil air in the single probe method, values measured with the new steady-state method may be more reliable.