Coaxial multiplexer for time domain reflectrometry measurement of soil water content and bulk electrical conductivity

Time domain reflectometry (TDR) is increasingly used for measurement of soil water content and bulk electrical conductivity in agricultural, hydrological, environmental, and infrastructure research and development. The TDR technique uses reflections (wave forms), from a probe buried in the soil, of a fast rise time voltage step. The advent of automatic systems for collecting many TDR wave forms has brought the need for a multiplexer to connect the probes to a TDR instrument without introducing distortion of the wave form. A 16-channel (1P16T), 50 omega coaxial multiplexer was developed to switch the TDR signal. Control is by synchronous serial addressing using three lines capable of producing TTL level signals similar to those from the parallel port of a personal computer. The multiplexer has 16 jumper selectable addresses. Testing on five multiplexers showed that quiescent power consumption was 6 mA at 12 VDC, peak power consumption was 101 mA, and average power consumption during switching was 54 mA. Wave form placement on a cable tester screen was not affected by the multiplexer channel used. The standard deviation (SD) of horizontal placement was 0.012 ns which compares favorably to the 15.6 ns full screen width of the wave form (five multiplexers by 16 channels each = 80 measurements; 30 cm probe length). Multiplexer channel also had negligible effect on computed travel times of the TDR pulse in a water content probe. The SD of 0.011 ns for a mean travel time of 8.68 ns was only slightly larger than the SD of 0.007 ns obtained when 80 wave forms were captured using the first channel of one multiplexer. Corresponding SD values for water content were 0.0006 and 0.0004 m3 m-3, respectively. The multiplexer used and the multiplexer channel had no important effect on the wave form voltage levels needed for determination of bulk electrical conductivity (BEC). The ratio of final voltage to incident voltage, which is directly proportional to conductivity, had a SD of 0.0002 when measured 80 times on channel one of one multiplexer and a SD of 0.0006 when measured once on each of the 16 channels of five multiplexers. Compared with a TDR probe connected directly to the cable tester, adding one multiplexer in series caused no change in travel time or water content, while adding two multiplexers in series caused a change in water content of 0.003 m3 m-3. As each multiplexer was added to the system the voltage ratio increased by 2% in a reproducible and expected way that can be included in system calibration for BEC measurements. The multiplexer developed is reliable and accurate for measurements of soil volumetric water content and bulk electrical conductivity.