A Nutrient Delivery System (NDS) capable of working in microgravity is an essential component of growing plants in space. A substrate-based NDS is a conventional and mechanically simple way to grow plants in microgravity, especially when a nutrient impregnated substrate is used. A substrate moisture sensor is a key element of the control system needed to successfully grow plants in this type NDS. A heat pulse-type moisture sensor has been developed for use in microgravity that has the advantages of a simple, compact design and a low average power requirement.The heat pulse-type moisture sensor was developed to be used in the “Greenhouse 2” Experiment with the Russian plant growth unit, “Svet.” This experiment is part of the SpaceLab/Mir-1 mission which is being conducted on Mir in 1995 and 1996. The design of the sensor allows installation of the sensors into the Svet root module aboard Mir. These sensors provide the ability to measure the moisture distribution in the substrate of the root module. A moisture sensor calibration procedure was developed which allows sensor data from on- board Mir to be interpreted using calibration curves obtained on the ground. The effects of contact resistance and particle diameter on sensor performance were estimated. A method of accounting for uncertainty in contact resistance is presented.