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Effects of Variable Gravity on Porous Media Matric Potential and Water Content Measurements

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Paper number  034067,  2003 ASAE Annual Meeting . (doi: 10.13031/2013.13874) @2003
Authors:   J. H. Norikane, S. B. Jones, S. L. Steinberg, H. G. Levine, D. Or
Keywords:   Heat-pulse soil moisture sensors, KC-135, microgravity, tensiometers, Turface

Control of water and air in the root zone of plants remains a challenge in microgravity. Due to limited flight opportunities research aimed at resolving fluid dynamics in microgravity porous media must often be conducted on earth. KC135 flight offers an opportunity for earth-based researchers to study physical processes in a variable gravity environment. The objectives of this study were to obtain measurements of water content and matric potential during the parabolic profile flown by the KC135 aircraft. The flight profile was designed to give 20-25 seconds of microgravity at the top of the parabola, while pulling 1.8-g at the bottom. Temperature and Moisture Acquisition Sensors (TMAS; Orbital Technologies, Madison, WI) use a heat-pulse method to measure water content. Tensiometers were constructed using a porous membrane with a pressure transducer and were used to measure matric potential. The two types of sensors were placed at different depths in a substrate compartment filled with 1-2 mm Turface (calcined clay). The TMAS sensors were unable to monitor bulk changes in water content in the substrate compartment, but were able to track local moisture changes in the soil profile. There were differences in water content data recorded at zero-, one-, and 1.8-g, but these were not significant. Tensiometer readings tracked pressure differences due to the hydrostatic force changes with variable gravity. The readings may have been affected by changes in cabin air pressure that occurred during each parabola. Tensiometer porous membrane conductivity (function of pore size) and fluid volume both influence response time. Porous media sample height and water content influence time-to-equilibrium, where shorter samples and higher water content achieve faster equilibrium. Further testing is needed to develop these sensors for space flight applications.

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