Direct Measurements of Dielectric Constant of Porous and Non-Porous Media.

Measurements of dielectric constants have been used for several decades to determine soil water content. These measurements rely on calibration procedures to relate measured electrical quantities to water content. Direct measurement of a samples’ water content provides not only water content but also a measure of the “boundness” of the water. To determine the dielectric constant of a sample using capacitance measurement requires knowledge of the geometrical factor of the sensor.  In sensors with electrically isolated probes, the measured capacitance consists of two capacitors in series: the insulator capacitor and bulk capacitor. Our analysis, using finite element models and experimental sensors, indicates that the geometrical factor depends on the dielectric constant of the media. An algorithm was developed to account for the non-linear behavior of the electric fields. Finite element modeling also provides insight into the probes’ volume of sensitivity. Our measurements call for measuring both the real and imaginary capacitances. Using the turn-on of a polarization mechanism due to the salinity effect, we determine both capacitances at the peak of the imaginary capacitance spectra. Using this approach allows us to determine both high- and low-frequency dielectric constants. This simple, single-frequency analysis, is very useful when analyzing samples with multiple Debye polarization mechanisms. In this paper we will describe (a) immersion experiments to obtain the capacitance per unit length in non-porous media that account for meniscus and end effects, (b) an algorithm for converting capacitance measurements into dielectric constants, (c) results from finite element modeling showing the extent of the potential lines (d) calibration procedures using a number of media with different dielectric constants, and (e) application to the water content soil samples.