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Multiplexer-Induced Interference on TDR Measurements of Electrical Conductivity

^a Land Resources and Environmental Sciences Dep., Montana State University, P.O. Box 173120, Bozeman, MT 59717-3120
^b USDA-ARS, George E. Brown Jr. Salinity Lab, 450 W. big Springs Road, Riverside, CA 92507

View larger version (9K): [in a new window]   Fig. 1. Schematic representation of the switching board of a two-channel coaxial multiplexer.

View larger version (17K): [in a new window]   Fig. 2. (a) Low frequency noise imposed on the time-domain reflectometry waveform in deionized water when an interfering probe is connected to the multiplexer; (b) the noise does not affect the measurement of the travel time.

View larger version (18K): [in a new window]   Fig. 3. Schematic of the experimental setup for three probes in an acrylic cylinder sequentially filled with deionized water and electrolyte solutions.

View larger version (135K): [in a new window]   Fig. 4. Insulated soil container with 12 probes for evaluation of multiplexer-induced signal interference under variable probe spacings and soil water contents.

View larger version (10K): [in a new window]   Fig. 5. Spatial relationships of time-domain reflectometry probes in the insulated soil container. Distances are in centimeters.

View larger version (23K): [in a new window]   Fig. 6. Low frequency noise in deionized water caused by interfering probes at different separation (4 and 8 cm) distances.

View larger version (17K): [in a new window]   Fig. 7. Effect of inter-probe interference in 0.673 dS m–1 electrolytic solution. The noise results in a vertical shift of the waveform at long times.

View larger version (16K): [in a new window]   Fig. 8. Effect of inter-probe interference on electrical conductance measurements in electrolyte solutions. The interference leads to incorrect values for the probe constant (inverse of the slope of the interpolating line).

View larger version (12K): [in a new window]   Fig. 9. Relative error in a measurements for different inter-probe distances in a loam soil at soil water content of 0.26 m3 m–3. Probe 1 and 4 are chosen as measuring probes.

View larger version (15K): [in a new window]   Fig. 10. Relative error in a measurements for different inter-probe distances in a loam soil at different values of water content. Probe 1 is the measuring probe.