COAXIAL MULTIPLEXER FOR TIME DOMAIN REFLECTOMETRY MEASUREMENT OF SOILWATER CONTENT AND BULK ELECTRICAL CONDUCTIVITY

Abstract

Time domain reflectometry (TDR) is increasingly used for measurement of soil water content and bulkelectrical conductivity in agricultural, hydrological, environmental, and infrastructure research and development. TheTDR technique uses reflections (wave forms), from a probe buried in the soil, of a fast rise time voltage step. The advent ofautomatic systems for collecting many TDR wave forms has brought the need for a multiplexer to connect the probes to aTDR instrument without introducing distortion of the wave form. A 16-channel (1P16T), 50 .coaxial multiplexer wasdeveloped to switch the TDR signal. Control is by synchronous serial addressing using three lines capable of producingTTL level signals similar to those from the parallel port of a personal computer. The multiplexer has 16 jumper selectableaddresses. Testing on five multiplexers showed that quiescent power consumption was 6 mA at 12 VDC, peak powerconsumption was 101 mA, and average power consumption during switching was 54 mA. Wave form placement on a cabletester screen was not affected by the multiplexer channel used. The standard deviation (SD) of horizontal placement was0.012 ns which compares favorably to the 15.6 ns full screen width of the wave form (five multiplexers by 16 channelseach = 80 measurements; 30 cm probe length). Multiplexer channel also had negligible effect on computed travel times ofthe 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 thanthe SD of 0.007 ns obtained when 80 wave forms were captured using the first channel of one multiplexer. CorrespondingSD values for water content were 0.0006 and 0.0004 m3 m3, respectively. The multiplexer used and the multiplexerchannel 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 0.0002when measured 80 times on channel one of one multiplexer and a SD of 0.0006 when measured once on each of the 16channels of five multiplexers. Compared with a TDR probe connected directly to the cable tester, adding one multiplexerin series caused no change in travel time or water content, while adding two multiplexers in series caused a change inwater content of 0.003 m3 m3. As each multiplexer was added to the system the voltage ratio increased by 2% in areproducible and expected way that can be included in system calibration for BEC measurements. The multiplexerdeveloped is reliable and accurate for measurements of soil volumetric water content and bulk electrical conductivity.