Metal‐coated printed circuit board time domain reflectometry probes for measuring water and solute transport in soil

Abstract

Metal‐coated printed circuit board (PCB) time domain reflectometry (TDR) probes for simultaneous small‐scale measurements of soil water content (θ) and bulk soil electrical conductivity (ECb) was developed and tested. To resolve corrosion problems associated with previous PCB probe designs, a nickel and gold plated copper waveguide is positioned at the surface of the PCB circuit laminate. In a numerical analysis the effect of PCB probe design parameters on the sample area distribution transverse to wave propagation was examined. The conclusions of the analysis were (1) two‐rod PCB probes have a larger sample area compared to three‐rod probes, (2) the probe rod separation has a larger effect on the sample area compared to rod width, and (3) if the thickness of the circuit laminate is 2/3 of the rod separation, a two‐rod PCB probe is insensitive to the medium below the laminate. On the basis of the numerical analysis, 20 different metal‐coated PCB probes were produced: five probe types on four laminates. Correct determination of PCB probe measured θ and ECb was ensured by calibration. The performance of the PCB probes in soil was tested in a water and solute transport experiment. Twenty PCB probes and five conventional two‐rod TDR probes were positioned in five horizontal transects. Tap water was added to an air‐dry soil and exchanged with a KCl solution. All probes registered changes in θ and ECb, but in some cases the level measured by the PCB probes differed significantly from the conventional probes. An ECb(θ) data analysis revealed that the differences was caused by local variations in θ rather than erroneous calibrations of the PCB probes. The metal‐coated PCB probe is a useful new TDR probe design for simultaneous small‐scale measurements of water and solute transport.