A time domain reflectometry coaxial cell for manipulation and monitoring of water content and electrical conductivity in variably saturated porous media

Time domain reflectometry (TDR) is an effective and accurate method for determination of porous media dielectric permittivity (i.e., for soil water content) and electrical conductivity (EC). Characterization of these properties using controlled wetting or drying of the porous media is of interest for studying and quantifying porous media physical and chemical properties. Our objectives were to develop a TDR measurement cell providing sample water content adjustment and solution exchange capability without repacking or disturbing the sample. A coaxial cell provides a well-defined sample packing volume in which radial distribution of the electromagnetic field averages the transmission line measurements over the entire sample. Using concentric porous stainless steel tubes as electrical conductors, complete or partial saturation of the porous medium (via matric potential control) was possible with the additional key advantage of soil solution exchange without changes in pore space or sample porosity. Solution exchange and suction adjustment were controlled through the center tube with influent solution (EC(w)) entering the sample through the outer wall and for unsaturated measurements an air inlet port facilitates air exchange. The cell was temperature controlled using an external PVC jacket connected to a circulating water bath. Both saturated and unsaturated measurements of electrical conductivity resulted in excellent agreement between measurements and model predictions. Permittivity measurements were dependent on cell orientation, likely due to differences in the water distribution within the cell. The utility of this new coaxial TDR pressure cell design for porous media research is the ability to simultaneously measure dielectric and EC(b) in a fixed pore space where water content and solution electrical conductivity are adjustable.