Evaluation of a Capacitance Probe Frequency Response Model Accounting for Bulk Electrical Conductivity: Comparison with TDR and Network Analyzer Measurements

Soils ranging in texture from sand to clay were used to compare permittivity measurements made using a Surface Capacitance Insertion Probe (SCIP) and time domain reflectometer (TDR). Measurements were made using the same electrodes embedded in each soil, making the measurements directly comparable. The objective of the work was to test a model describing the frequency response of the SCIP to both permittivity and electrode conductance, and to compare results with TDR and network analyzer measurements. The model was tested using liquids of known permittivity and in saline, dielectric solutions. Surface Capacitance Insertion Probe and TDR determined permittivity values are similar for sandy soils but diverge for loam and clay soils. Using Topp's values as a reference, the SCIP-determined permittivities for loams and clays lay close to the curve at water contents <0.25 m3 m−3, then often rose above the curve with increasing water content. Surface Capacitance Insertion Probe permittivity correction, using electrical conductivity (EC) measured at 1 kHz, corrected the results in sands reasonably well but not enough in loams and clays for reliable calibration. We propose three possible reasons for the higher than expected permittivity values observed using the SCIP: (i) higher than expected real permittivity created by dielectric dispersion, (ii) a large contribution of the imaginary permittivity due to relaxation processes assumed to be negligible, and (iii) poor model prediction of permittivity due to excessive damping of the oscillator circuit with high EC and dielectric losses. Results from network analyzer measurements for one of the clay soils were used to aid data interpretation. The TDR measurements were much more consistent, producing apparent relative permittivity values below those of the Topp curve for the finer textured soils.