Design of a User-Friendly Automated Multistep Outflow Apparatus

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The multistep outflow (MSO) test is a widely used laboratory method for direct measurement of the soil moisture retention curve, θ(h), or indirect estimation of θ(h) and the unsaturated hydraulic conductivity curve, K(h). Manual MSO testing is time consuming and while automated systems have been described in the literature, many require significant effort to fabricate and operate, or have not been described in sufficient detail to allow rapid setup and implementation by others. In this work, a user-friendly, easily assembled, automated multistep outflow (AMSO) test apparatus was designed and built using National Instruments (Austin, TX) hardware and LabVIEW software. Control codes were written to support four testing regimes. The first option allows the pressure steps to be set manually while continuously measuring the water level in the burette. Three fully automated options are also available in which pressure steps are changed automatically after (i) a user-specified time period has elapsed, (ii) water level changes in the burette are negligible, or (iii) tensiometer measurements of the soil-water pressure head in the sample become constant for a specified period of time. To verify performance, five soil samples were subjected to a manual hanging column test and then retested using the automated system. Good correspondence between soil moisture values at the applied pressure steps with manual and AMSO tests was observed (RMSE values < 0.0125 m3 m−3). Two samples were also drained and rewetted with the new system to show its utility for studying hysteresis.