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Title: MULTIPLEXER-INDUCED INTERFERENCE ON TDR MEASUREMENTS OF ELECTRICAL CONDUCTIVITY

Author
item CASTIGLIONE, PAOLO - MONTANA STATE UNIV.
item Shouse, Peter
item WRAITH, JON - MONTANA STATE UNIV.

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/2006
Publication Date: 8/3/2006
Citation: Castiglione, P., Shouse, P.J., Wraith, J.M. 2006. Multiplexer-induced interference on tdr measurements of electrical conductivity. Soil Science Society of America Journal. Vol. 70:1453-1458

Interpretive Summary: Coaxial multiplexers are used to automate multiple readings of soil water content and/or bulk soil electrical conductivity, and is one of the major benefits of using Time Domain Reflectometry (TDR) in soils research. We observed that different probes connected to a common multiplexer network interfere with the measurement of soil bulk electrical conductivity in all probes. This is due to the fact that the TDR probes share a common ground when connected to the multiplexer network. We also determined that the interference between probes was limited to the measurement of electrical conductivity and the measurement of the water content using TDR was unaffected. We further found that interference was independent of soil electrical properties (electrical conductivity or dielectric constant) but was strongly dependent on the probe geometry. Therefore we feel that newer probe designs will help to alleviate the problems.

Technical Abstract: 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 for the first time 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.