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 (a) and hence volumetric water content () and bulk electrical conductivity (a). Difficulties arise for and a determination, however, when uncoated TDR probes (UP) are used in highly conductive media. This work shows that a 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 a 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, r, 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 a estimations using an UP. The method was subsequently used for determining a in four different soils with different values of and a (0–6 dS m–1) and again compared with the standard TDR procedure. The low error (RMSE = 1.5) for the comparison between the a measured with the CP and that calculated with the analytical solution for coaxial probes indicates that the CP is accurate enough for a estimations. For a values <4 dS m–1, the UP allows determinations of a and the most accurate estimations of a using the standard TDR method (R2 = 0.99). For higher a values, however, estimations of a were only possible using the CP, where a 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.