Probe Body and Thermal Contact Conductivity Affect Error of Heat Pulse Method Based on Infinite Line Source Approximation

The dual probe (DP) method is a widely used method for determining the thermal conductivity ( K ), the specific heat capacity ( c ), and the thermal diffusivity (α) of soil, but this method overestimates c , particularly for dry soil. The objective of this study is to examine if the probe body and the thermal contact conductivity ( H ) lead to biased estimation of the thermal properties by using the DP method. We used the analytical solution for cylindrical heat sources with the finite H value derived by Moench and Evans (1970). For an air‐dried sand sample with c = 756 J kg −1 K −1 , the experimental estimated c from the infinite line source (ILS) model for DP units, with the heater needles of 3.175 and 1.27 mm, is 953 J kg −1 K −1 and 904 J kg −1 K −1 , respectively. For 3.175‐ and 1.27‐mm heater needles, the c estimated by applying the ILS model to temperatures simulated using the Moench and Evans solution is 926 J kg −1 K −1 and 845 J kg −1 K −1 , respectively. Simulations suggest that both the existing of the finite probe body and H make contribution to the overestimation of c from the ILS model. The overestimation of c increases with the increase in the radius of the heater needle and decreases with the increase of H value. Using ILS could lead to substantial errors in the estimated c of dry soil.