Time-domain reflectometry probe for water content and electrical conductivity measurements in saline porous media

A new coated time-domain reflectometry (TDR) probe design is described and evaluated. In contrast to previous coated TDR probes, our probe may be used to measure both the dielectric constant (K-a) and bulk electrical conductivity (sigma(a)) in saline porous media. This was made possible by attaching two coaxial cables to a 0.27-m three-rod with a coated central rod. The shield of the first cable was to one of the outer rods and the conductor was connected to the coated central rod. The conductor and shield of the other coaxial cable were connected to each of the two outer rods, respectively. Thus, our probe consists of two unbalanced, two-rod probes. The probe is called coated-uncoated probe (CUP). Four prototypes with two different coating materials (i.e., polyolefin and kynar heat-shrink tubes) were evaluated. The probes were calibrated in several fluids having different K-a and sigma(a). The K-a measurement of the coated part the probe was successfully fitted to target K-a using a two-phase dielectric mixing model. Due to signal attenuation, measurements of K-a were not possible for sigma(a) higher than 9 dS m(-1) for the polyolefin-coated probes whereas the upper limits for the kynar-coated probes and the uncoated probe were 5 and 2.5 dS m(-1), respectively. Measurements of sigma(a) are only possible with the uncoated part. Measurements of K-a and sigma(a) were also taken during three upward infiltration experiments in sand using soil solution electrical conductivities of 0.01, 6.31, and 12.03 dS m(-1). For the uncoated part, K-a could not be measured when sigma(a) was higher than about 2 dS m(-1), whereas K-a measurements were possible using the coated part even when sigma(a) was 3 dS m(-1).