Hydra Probe and Twelve-Wire Probe Comparisons in Fluids and Soil Cores

Soil water content is often determined using various permittivity sensors. The Hydra Probe reports both the real (epsilon') and imaginary (epsilon'') components of complex permittivity (epsilon*) and estimates electrical conductivity (sigma'). Previously, a 12-wire probe was used with a vector network analyzer to determine epsilon' and epsilon'' and sigma' and sigma'' in undisturbed soil cores. The purpose of this study was to compare results from the Hydra Probe and the 12-wire probe for fluids as well as for soil samples taken from six locations, to better understand the interrelations of epsilon', epsilon'', and sigma' from the Hydra Probe. Undisturbed soil cores were collected from an Iowa prairie and adjacent soybean [Glycine max (L.) Merr.] field, an Iowa forest and adjacent soybean field, Idaho burned and unburned sites, Colorado grassland and dryland wheat (Triticum aestivum L.) sites, Ohio grassed areas, and Texas rangeland and an irrigated field. Because of their density, some of the Texas samples were sieved and repacked. The reported sigma', epsilon', and epsilon'' values of fluids were compared between the two measurement systems and with theoretical values. Because the theoretical values for soils are not known, the measured values were compared between systems. Both probes showed sigma', epsilon', and epsilon'' close to the theoretical values for fluids. In soils, we showed that an adjustment was needed in the calculated sigma' from the Hydra Probe to account for dielectric relaxation. The soil sigma' of the Hydra Probe matched sigma' from the 12-wire probe better after the adjustment was made.