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The Water Offset Nutrient Delivery ExpeRiment (WONDER) studies two versions of a gravity-independent nutrient delivery system (NDS) to determine the optimum wetness set points for plants grown in microgravity. Control of root zone moisture content treatments used feedback from heat pulse sensors in the growing media. These sensors must be characterized for their precision and accuracy in the porous media substrate used in the NDS root modules. The objectives were to use a calibration vessel and technique to carefully set multi-step media wetness levels to test the sensors after several improvements in their hardware and operation were made. In order to test sensors with a larger surface area, several enlarged sensors were included in the experiment. The calibration vessel and methodology allowed no more than 4% total error in bulk relative water content (RWC) levels. A longer heating time and temperature measurement at the peak of heat pulse were used to correlate with RWC in the media. The results showed that in contrast to previous experiments, there was a clear linear relationship between the change in temperature of the probe and the change in bulk relative water content of the media. This was due both to the improved test configuration and changes in the operation of the sensors. Repeatability testing was only partially successful. For all sensors, there were significant problems repeating the same response across drying trials. There appeared to be more repeatability with the larger surface area sensors, indicated by lower root mean square errors of calibrated trial 1 and 3 measurements, using a calibration equation drawn from trial 2. Packing variations are likely to impact the response of the sensors. Further testing is required to characterize the accuracy of the sensors.