Humidity Control for the Accelerated Aging of Rocket Propellants

Sensitive materials, such as rocket propellants, can be placed in harsh environments of increased temperature and relative humidity (RH) to study the effects of environmental conditions over time in accelerated aging studies. Programmable ovens are common to research laboratories and can be used to regulate the temperature of the aging environment for long durations. Some commercial units include a humidity control system but can be prohibitively expensive. In this study, a custom humidity control system was implemented in an existing oven. The target conditions were 30% RH at 70°C. A magnesium chloride salt solution was used to regulate the RH of the oven environment. The salt basin alone provides effective humidity control for periods up to 48 hours, after which more water has to be added to maintain the solution’s concentration. This is consistent with previously published results. A remote water filling system was implemented to control the salt solution concentration, so humidity levels could be maintained indefinitely without opening the chamber. A feedback control program was then developed to maintain the salt solution concentration, which automatically regulates the chamber environment RH for extended periods. After calibration and testing, the autonomous system can provide constant humidity levels for long durations with little system maintenance. The system can be operated at RH values between 27 and 75% ± 2.2% at 70°C ± 0.3°C. The custom system has been successfully used in a threeweek accelerated aging study, and it is now an established tool for rocket propellant development activities at Purdue University.