Simplified Permittivity Calibration of Custom-Designed TDR Probes for Measuring Soil Water Content

Time domain reflectometry (TDR), measuring the travel time of a microwave signal, has been recognized as a superior method of monitoring volumetric water content (θ) in soil. Although various custom-designed TDR probes having different advantages have been designed, the calibration requirement for any specific probe has been a major problem for their users. To propose a simpler calibration approach for custom-designed probes, we first examined their sensitivity to the change in permittivity for several fluid media (air, water, and ethanol-water mixtures), and then calibrated them for sand with a different θ. The custom-designed probes were less sensitive to the change in permittivity than a conventional probe because the detection part is surrounded by low-permittivity materials, and thus we could not adequately determine θ of sand from measured permittivity by applying widely used Topp equation. However, we confirmed that the equation is valid for estimating θ if the effect of the surrounding materials on the measured permittivity was excluded, which effect can be evaluated based on the data sets of permittivity measured for the fluid media. From this result, we concluded that the calibration of the custom-designed probes can be easily conducted by the testing for the fluid media.