Printed circuit board time domain reflectometry probe: measurements of soil water content

Time domain reflectometry (TDR) is a widely used, nondestructive measurement technique for determining soil-water content (θ) and bulk soil electrical conductivity. Until recently, small scale applications of TDR have been restricted because of the lack of small-scale, high-resolution TDR probes. As a result of the introduction of the TDR coil probe principle and, in this study, the printed circuit board TDR probe (PCBP), the lower limit of the measurement scale for TDR is changing. The travel time of the electromagnetic waves in the PCBP was prolonged by forcing the electromagnetic waves to travel in a "three-rod" serpentine waveguide produced in the copper cladding of a circuit laminate (50 mm long, 10 mm width, 0.64 to 1.00 mm thickness). The apparent relative dielectric permittivity (Ka) measured by the PCBP (Ka, PCBP) was calibrated against Ka measured by a standard two-rod TDR probe in air and six fluids of various Ka. A two-phase dielectric mixing model was used to describe the contributions of the circuit laminate and the surrounding media to Ka, PCBP. Eleven PCBPs were produced on four different types of circuit laminate with well known water absorption properties. Minor changes in Ka attributable to water absorption could be observed for some of the circuit laminates. However, all four circuit laminates showed equal measurement performance during water infiltration in an initially air-dried soil. None of the circuit laminates was damaged by the soil environment during the water transport experiments. The waveguide of the PCBP is in direct contact with the soil, which should enable the PCBP to also measure electrical conductivity (EC). A calibration experiment was carried out where the load resistance (RL) and the EC were measured in deionized water and six KCl solutions by the PCBPs and a conductivity meter, respectively. A simple linear relationship was found between RL and EC. Therefore, in contrast to the TDR coil probe, the PCBP seems promising for obtaining simultaneous, small-scale and high-resolution TDR measurements of water and solute transport.