Cell Size for Water Content-Dielectric Constant Calibrations for Time Domain Reflectometry

Time domain reflectometry (TDR) is a nondestructive electromagnetic technique used to measure the volumetric water content of soil. A key component of the method is the calibration equation relating the apparent dielectric constant (Ka) to the volumetric water content (ϑ). In this study, tests were conducted to evaluate dimensional requirements for a TDR calibration cell. The results show that a 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.