A Kennedy Space Center team is developing the Microgravity Plant Nutrient Experiment (MPNE) middeck payload to support plant growth in microgravity for the United States Space Program. The fluid system in this payload employs porous plant tubes that deliver nutrient solution actively to the plant roots and replaces the need for soil to grow plants. The component controlling nutrient solution delivery to the plant tubes is the Water Availability Sensor (WAS). This sensor measures the thin film of plant nutrient solution or potable water on plant tubes in the MPNE payload and provides the feedback logic for nutrient delivery to plants. The WAS provides a noninvasive measurement of nutrient delivery for this unique hydroponic system. Nutrient solution is added to the fluid system network as nutrients are consumed by the plants. The team's long term goal is to produce a highly controllable nutrient delivery system that will be a prime candidate for long term plant growth on the Space Station.