An Automated Oxygen Diffusion Measurement System for Porous Media in Microgravity

Liquid and gas exchange within a particulate plant-rooting medium is likely to be altered in a microgravity environment. A difference in gravitational force can result in significant offsets in control parameters developed on earth for optimum plant growth, due to the shift in hydrostatic water distribution. The experiment being developed will examine the effects of variable gravity on water distribution and gas diffusion. We are developing and testing an automated gas diffusion measurement system for use on the International Space Station (ISS). To allow comparison of μg and 1g conditions, gas diffusion cell designs were horizontally oriented to minimize gravitational effects using 1) a ‘thin rectangular profile’ cell and 2) a cylindrical cell design for flight. Electronic solenoid valves provide air and water flow control while pressure transducers measure water and substrate potential. Porous media water content is controlled using a porous membrane coupled with a metered pumping system. Diffusion measurements are made after purging two gas chambers separated by the porous medium with N₂ and with atmospheric air (∼20% O₂). The system allows continuous measurement of oxygen concentration for fitting the diffusion coefficient to measured data at a given water content set point. The entire measurement range includes 10 set points, which can be completed in a period of about 2 weeks. The LADA control system aboard ISS will provide control and monitoring capabilities for the ORZS system that will be launched to the ISS on a Russian Progress vehicle in 2005.