Measurement of soil bulk electrical conductivity in saline soils and solutions using TDR probes partially coated with high-dielectric material

Abstract

Time Domain Reflectometry (TDR) is a valuable technique that allows simultaneous estimation of the apparent permittivity (a) and the bulk electrical conductivity (a). However, in highly conductive media the signal is completely attenuated which precludes permittivity measurements. This paper shows that a can be estimated in conductive media by applying the 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 (P0) and probes with 50% (P50), 75% (P75) and 95% (P95) of the rod-length, respectively, coated with an insulator with a relative permittivity (r) equal to 32.3. A good relationship (r2 = 0.99) was found between the a estimated, if possible, with P0 immersed in several conductive and nonconductive media and the corresponding values estimated with P50, P75 and P95. The cell constant value (m-1) for P0 (5.10), P50 (9.60), P75 (16.92) and P95 (51.31), which were experimentally determined in different NaCl-water solutions, were on average 10% greater than the corresponding values calculated using a numerical model. The results show that, for a values ranging between 0.4 and 0.75 S m-1, simultaneous measurements of a and a were only possible using the partially coated probes. A good correlation was found between the a estimated with P0 inserted in different soil columns wetted with highly saline solutions (i.e. a > 0.2 S m-1) and those values estimated with P50, P75 and P95 (R2 = 0.96, RMSE = 1.08 and SD = 0.38). For a values lower than 0.2 S m-1 the accuracy of the partially coated TDR probes for estimations of a decreases as increases the percentage of the rod-coating, with errors up to 292% when a determined by P95 was compared to that estimated by P0.