COAXIAL MULTIPLEXER FOR TIME-DOMAIN REFLECTOMETRY MEASUREMENT OF SOIL-WATER CONTENT AND BULK ELECTRICAL-CONDUCTIVITY

Time domain reflectometry (TDR) is increasingly used for measurement o f soil water content and bulk electrical conductivity in agricultural, hydrological, environmental, and infrastructure research and developm ent The TDR technique uses reflections (wave forms), from a probe buri ed in the soil, of a fast rise time voltage step. The advent of automa tic systems for collecting many TDR wave forms has brought the need fo r a multiplexer to connect the probes to a TDR instrument without intr oducing distortion of the wave form. A 16-channel (1P16T), 50 Omega co axial multiplexer was developed to switch the TDR signal. Control is b y synchronous serial addressing using three lines capable of producing TTL level signals similar to those from the parallel port of a person al computer The multiplexer has 16 jumper selectable addresses. Testin g on five multiplexers showed that quiescent power consumption was 6 m A at 12 VDC, peak power consumption was 101 mA, and average power cons umption during switching was 54 mA. Wave form placement an a cable tes ter screen was not affected by the multiplexer channel used. The stand ard deviation (SD) of horizontal placement was 0.012 ns which compares favorably to the 15.6 ns full screen width of the waveform (five mult iplexers by Ia channels each = 80 measurements; 30 cm probe length). M ultiplexer channel also had negligible effect on computed travel times of the TDR pulse in a water content probe. The SD of 0.012 ns for a m ean travel time of 8.68 ns was only slightly larger than the SD of 0.0 07 ns obtained when 80 wave forms were captured using the first channe l of one multiplexer Corresponding SD values for water content were 0. 0006 and 0.0004 m(3) m(-3), respectively. The multiplexer used and the multiplexer channel had no important effect on the waveform voltage l evels needed for determination of bulk electrical conductivity (BEC). The ratio of final voltage to incident voltage, which is directly prop ortional 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 p robe 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 m(3) m(-3). As each multiplexer was added to the system the voltage ra tio increased by 2% in a reproducible and expected way that can be inc luded in system calibration for BEC measurements. The multiplexer deve loped is reliable and accurate for measurements of soil volumetric wat er content and bulk electrical conductivity. TDR, Time domain reflecto metry, Multiplexer, Soil water content, Bulk electrical conductivity, Soil salinity.