Improved field probes for soil water content and electrical conductivity measurement using time domain reflectometry

Abstract

Volumetric water content 0 and soil electrical conductivity rr may be measured in situ using time domain reflectometry (TDR). The parallel-wire or two-wire transmission line TDR probes currently in field use suffer from unwanted noise and information loss due to impedance mismatch between the probe and the coaxial connecting cable. Here we describe symmetric, multiwire probes designed to minimize these problems and eliminate the need for a balancing transformer between probe and TDR device. Analysis of the electric field distributions around these new probes shows that they emulate a coaxial transmission line, and their measured characteristic probe impedances approach that of coaxial probes. Signals from the new probes permit more reliable and accurate 0 and rr measurement and are superior to those of two-wire probes with balancing transformer. The enhanced signal clarity of the new probes extends to sample diameters of at least 0.2 m. We show that electrical conductivity determined with the new probes is identical to that found with a coaxial cell and substantially different from that measured by a two-wire probe. Our results indicate that values of rr, determined using the Giese-Tiemann thin sample approach and measured characteristic probe impedances of coaxial or multiwire probes, agree with values of rr measured using an ac bridge for both electrolyte solutions and soil samples to within _+ 10%, provided rr exceeds 10 mS m -1 . Finally, we give an example of the use of multiplexed three-wire probes in following rainfall infiltration and redistribution during and after a simulated rainfall event in the field. Infiltrated quantities of water estimated from the TDR water content profiles agreed within _+ 10% with the amount applied. 1. Comparison of TDR signals L=150 , d =4.7, s=30 (mm) -,-J J•-2.7ns ß ß ß ß ß Balum S E S E ß ß ß ß S E S E

Soil profile method for soil thermal diffusivity, conductivity and heat flux: Comparison to soil heat flux plates

“…A three-wire TDR probe configuration is semi-coaxial in nature and eliminates the need for an impedance matching transformer (balun) used with a two-wire design [22]. Also, the range of sensitivity above and below the plane of the rods is narrower for the 3 wire configuration than for the 2 wire configuration most commonly used in the past [6,7] allowing for better discrimination of soil water content with depth.…”

Influence of water and terpenes on flammability in some dominant Mediterranean species

“…1b). Soil water content was assessed using a time domain reflectometry device (Zegellin et al 1989) every 2 weeks. Four probes were randomly placed on each plot at 25-cm soil depth.…”

A numerical analysis of the effects of coatings and gaps upon relative dielectric permittivity measurement with time domain reflectometry

“…If the boundaries of the rods and gaps coincide with equipotentials, the gradient of the potential is perpendicular to the material and domain boundaries for isotropic media. The resulting equipotentials for this heterogeneous medium also conform to Zegelin et al [1989] introduced the three-rod TDR probe and derived an approximate analytical expression for the voltage distribution in the transverse plane for probes with one central rod and a number of rods arranged in a circle around it. Knight [1992] showed the importance of the spatial energy distribution in the transverse plane and analyzed the response of some TDR probes to small lateral perturbations of the relative dielectric permittivity about a uniform value.…”