Design and Fabrication Specifications for a User-Friendly Automated Multistep Outflow (AMSO) Apparatus

The definitive source for information about multistep outflow testing is Dr. Jan Hopmans’ web site, where the user can download a comprehensive report on multistep outflow testing that includes detailed descriptions of sample preparation procedures, complete system design specifications, pressure transducer calibration methods, hardware assembly and multiplexing instructions, data acquisition and system control codes, as well as data preparation and parameter estimation codes for analysis of transient flow and pressure data. Other key papers and book chapters related to multistep outflow testing are also presented on his site.

Thesis Abstract:  Determining the functional relationships between volumetric soil moisture content (θ) and pressure head (h) and unsaturated hydraulic conductivity (K) and pressure head (h) is required to predict flow and transport of water and solutes in unsaturated soils. These relationships can be determined using in-situ or laboratory methods. The multistep outflow test (MSO) is a widely used laboratory method for direct measurement of the soil water retention curve, θ(h), or indirect estimation of θ(h) and K(h). However, performing this test manually is time consuming and subject to significant operator error that can limit the usefulness of the results. Furthermore, while many automated systems have been described in the literature, most are complicated and have not been described in detail. 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 National Instruments LabVIEW software (links below) and validated via comparisons with manual test results (Fig. 1). Nitrogen is separated out of in-house compressed gas to be used as the pressurizing agent. Positive pressure is applied on top of a soil sample and held constant using two solenoid valves. Two pre-calibrated digital transducers are used to monitor pressure and cumulative outflow/inflow, respectively, and a tensiometer is installed vertically to the center of the soil sample to monitor soil-water pressure head. The automated system is capable of saturating, draining, and re-wetting samples so that drainage and imbibition soil characteristic curves can be developed for up to three samples in its current configuration (Fig. 2). Several undisturbed or reconstituted samples can be tested simultaneously and independently. The hardware is controlled and automated using LabVIEW software. Four versions of the code were written to control the system. In Version 1,  pressure steps are selected manually and applied for an indefinite period of time until the user selects a different pressure step or the program is stopped. In Version 2, pressure steps switch automatically after a specified period of time has elapsed. In Version 3, pressure steps switch automatically when the cumulative outflow or inflow from the sample is deemed negligible. Finally, in Version 4, pressure steps switch automatically when differences in soil-water pressure head measurements are negligible over a specified period of time.

Figure 1: Jordi Figueras working with the automated multistep outflow test apparatus.

Five soil samples were subjected to a manual MSO test, and then tested using the automated system developed in this work to verify the new system’s performance. Good correspondence between moisture contents at given pressure head steps were observed, with the overall average difference between moisture content readings of 3%, suggesting that the AMSO system developed for this thesis can reproduce the results obtained with the manual multistep outflow method. The new system was also shown to successfully drain and re-wet a soil sample to allow study of the effects of hysteresis from a single test.