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

Time domain reflectometry (TDR) is a widely used, nondestructive measuremen t technique for determining soil-water content (theta) and bulk soil electr ical conductivity. Until recently, small scale applications of TDR have bee n restricted because of the lack of small-scale, high-resolution TDR probes . As a result of the introduction of the TDR coil probe principle (Nissen e t al. 1998b) 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 produ ced in the copper cladding of a circuit laminate (50 mm long, 10 mm width, 0.64 to 1.00 nun thickness). The apparent relative dielectric permittivity (K-a) measured by the PCBP (K-a,K-PCBP) was calibrated against K-a measured by a standard two-rod TDR probe in air and six fluids of various K-a A two -phase dielectric mixing model was used to describe the contributions of th e circuit laminate and the surrounding media to K-a,K-PCBP. Eleven PCBPs we re produced on four different types of circuit laminate with well known wat er absorption properties. Minor changes in K-a attributable to water absorp tion could be observed for some of the circuit laminates. However, all four circuit laminates showed equal measurement performance during water infilt ration in an initially air-dried soil. None of the circuit laminates was da maged by the soil environment during the water transport experiments. The w aveguide of the PCBP is in direct contact with the soil, which should enabl e the PCBP to also measure electrical conductivity (EC). A calibration expe riment was carried out where the load resistance (R-L) and the EC were meas ured in deionized water and six KCI solutions by the PCBPs and a conductivi ty meter, respectively. A simple linear relationship vp as found between RL and EC. Therefore, in contrast to the TDR coil probe, the PCBP seems promi sing for obtaining simultaneous, small-scale and high-resolution TDR measur ements of water and solute transport.