Cell size for water content-dielectric constant calibrations for time domain reflectometry

It has been shown that it is possible to measure the apparent dielectric constant of soil using time domain reflectometry (TDR). A component of the method is the calibration equation relating the apparent dielectric constant to the volumetric water content. This paper describes a study conducted to size a calibration cell that can be used efficiently for determining the relationship between the apparent dielectric constant and the volumetric water content. The results show that a polyvinyl chloride (PVC) cylinder having the same dimensions as a standard compaction mold (diameter = 102 mm, height = 116 mm) is a suitable calibration cell for two-rod TDR probes having diameter = 4 mm, center-to-center spacing = 30 mm, and length = 80 mm. The cell can also be used for three-rod probes having the same dimensions as the two-rod probe, and a center-to-center rod spacing of 20 mm. Calibrations made with this small cell are essentially identical to calibrations made in a much larger cell where boundaries are unlikely to be important.