Soil Bulk Electrical Conductivity Measurement using High‐Dielectric Coated Time Domain Reflectometry Probes

Abstract

Time domain reflectometry (TDR) is a technique that allows simultaneous estimates of apparent permittivity (εa) and hence volumetric water content (θ) and bulk electrical conductivity (σa). Difficulties arise for θ and σa determination, however, when uncoated TDR probes (UP) are used in highly conductive media. This work shows that σa can be estimated in highly conductive media using a TDR probe coated with a high‐dielectric insulator (CP). To this end, the Dalton method for σa estimations was applied to a 10‐cm‐long three‐rod TDR probe insulated with a 0.2‐mm‐thick epoxy‐ceramic composite coating with a relative permittivity, εr, of 32.3. This method was calibrated on different NaCl–water solutions (0–15 dS m−1) and compared with the standard long‐time TDR method for σa estimations using an UP. The method was subsequently used for determining σa in four different soils with different values of θ and σa (0–6 dS m−1) and again compared with the standard TDR procedure. The low error (RMSE = 1.5) for the comparison between the εa measured with the CP and that calculated with the analytical solution for coaxial probes indicates that the CP is accurate enough for εa estimations. For σa values <4 dS m−1, the UP allows determinations of εa and the most accurate estimations of σa using the standard TDR method (R2 = 0.99). For higher σa values, however, estimations of εa were only possible using the CP, where σa was satisfactorily determined (R2 = 0.99) using the Dalton method. To this end, a previous calibration between the real and the apparent bulk electrical conductivity estimated with the CP was required.