Measurement of Soil Bulk Electrical Conductivity Using Partially Coated TDR Probes

The time domain reflectometry (TDR) technique allows simultaneous estimation of apparent permittivity (ε a ) and bulk electrical conductivity (σ a ). In highly conductive media, however, the signal is completely attenuated, which precludes permittivity measurements. This study showed that σ a can be estimated in conductive media by applying long‐time TDR waveform analysis to a TDR probe partially insulated with a high‐dielectric coating. Four 10‐cm‐long three‐rod TDR probes with identical geometry but different percentages of rod coating were tested: an uncoated sensor (P 0 ) and probes with 50% (P 50 ), 75% (P 75 ), and 95% (P 95 ) of the rod length coated with an insulator with a relative permittivity (ε r ) of 32.3. A good relationship ( r 2 = 0.99) was found between the ε a estimated, if possible, with P 0 immersed in several conductive and nonconductive media and the corresponding values estimated with P 50 , P 75 , and P 95 The results also show that, for σ a values ranging between 0.4 and 0.75 S m −1 , simultaneous measurements of ε a and σ a were only possible using the partially coated probes. A good correlation was found between the σ a estimated with P 0 inserted in different soil columns wetted with highly saline solutions (i.e., σ a > 0.2 S m −1 ) and those values estimated with P 50 , P 75 , and P 95 ( r 2 = 0.96, RMSE = 1.08, and SD = 0.38). For σ a < 0.2 S m −1 , the accuracy of the partially coated TDR probes for estimations of σ a decreased as the percentage of the rod coating increased, with errors up to 292% when σ a determined by P 95 was compared with that estimated by P 0