Multiplexer-Induced Interference on TDR Measurements of Electrical Conductivity

The possibility of automated multiple readings of water content and bulk soil electrical conductivity represents a major benefit in soil research, and is one of the most attractive characteristics of the time domain reflectometry (TDR) technique. Coaxial multiplexers are commonly employed to monitor up to hundreds of TDR probes through computer or datalogger interface.We observed that the different probes connected to a common multiplexer or multiplexer network interfere with one another. This is due to the electronics of most multiplexers, where the different channels share a common ground, while only the central electrode is switched. The effects of the multiplexer-induced interference were investigated through tests in electrolyte solutions and in a loam soil at variable water content. We determined that the interference did not affect the signal travel time, and therefore the water content measurement, but resulted in appreciable errors in measured electrical conductivity. We also found that the interference results in a variation of the cell constant Kp, and the errors in conductivity measurements could be easily corrected by determining Kp in presence of interference. The magnitude of the interference appears to be independent of the electrical conductivity and dielectric constant of the interposing medium, while it is strongly dependent on the inter-probe spacing and probe geometry. TIME DOMAIN REFLECTOMETRY has become an established method for measuring volumetric soil water content (u) and bulk soil electrical conductivity (sa) (Noborio, 2001; Robinson et al., 2003; Topp and Ferre, 2002). In TDR measurements, a fast rising step voltage pulse is transmitted through a coaxial cable to the sample, and reflected back to the generator. The incident and reflected voltages are then recorded in the time domain by a fast sampling oscilloscope. From the analysis of the reflected voltage (waveform) it is possible to determine important dielectric properties of the sampled medium. For instance, the apparent dielectric constant (ea, which can be considered to a first approximation the static value of the dielectric constant in non-dispersive dielectrics) can be measured from the signal travel time, Dt, as follows: