Application of infinite line source and cylindrical‐perfect‐conductors theories to heat pulse measurements with large sensors

Abstract

AbstractThe infinite line source (ILS) theory for soil thermal property determination with heat pulse (HP) sensors is simple and widely‐used, but ignores the finite probe radius (r) and heat capacity (Cp). The cylindrical‐perfect‐conductors (CPC) theory, which accounts for r and Cp by using the identical‐cylindrical‐perfect‐conductors (ICPC) or the dissimilar‐cylindrical‐perfect‐conductors (DCPC) approaches, can be applied to estimate soil thermal property values with improved accuracy. In this study, the ILS and CPC theories were evaluated, and the finite r and Cp effects were quantified using numerical simulations and laboratory measurements with a large HP sensor of dissimilar probes. The errors due to finite probe properties were saturation dependent: Dry soils had a 14% reduction in the maximum temperature rise of the HP signal, while only slight temperature differences occurred in wet sandy soils. The finite probe effects were minor on ICPC‐ and DCPC‐thermal property values with relative errors generally less than 5%, but the absolute values of relative errors for dry soils were greater than 6%. Errors caused by ignoring the finite probe effects changed linearly with the ratio of soil heat capacity (C) versus C of the heating and sensing probes. The dissimilar probe r had negligible effect on HP signals and thermal property estimates with the specific sensor used in this study. The effects of finite probe size and properties should be considered in HP sensor design. The CPC theory is recommended for estimating soil thermal properties with large HP sensors.