Automated Systems for Oxygen Diffusion Measurements in Porous Media at 1g and 0g

Liquid and gas exchange within a porous medium is likely to be altered in a microgravity environment. A difference in gravitational force can result in significant offsets in the water distribution in porous substrates such as plant growth media, due to the shift in hydrostatic force. The automated gas diffusion measurement systems being developed will examine the effects of variable gravity on water distribution and gas diffusion on Earth and on the International Space Station (ISS). The automated system will interface with the LADA control module currently on the ISS. To allow comparison of μg and 1g conditions, gas diffusion cell designs were horizontally oriented to minimize gravitational effects using a `thin rectangular profile' cell for 1g and 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. Continuous measurements of oxygen in sealed dual-chamber diffusion cells provided concentration data for fitting diffusion coefficients where water content was controlled by imposing a low matric suction in the porous media (in the range of 0 to 30 cm). The system will be launched to the ISS on a Russian Progress vehicle in 2005.