An important aspect of environmental control is the protection of equipment from the harmful effects of humidity. It is often desirable to desiccate a sealed volume in order that it satisfy a dryness requirement over the duration of its service life. Desiccant sizing, along with proper sealing, become important design elements in ensuring that such a requirement is achieved. To this end, Raytheon has developed a simulation tool to analyze and predict the humidity within sealed volumes. The pertinent physical principles that are modeled are permeation of water vapor by diffusion, pneumatic leakage, the hygroscopic properties of sealed materials, desiccant characteristics, and the kinetic model of ideal gas mixtures. The simulation has the capability of imposing any external environmental condition, consisting of diurnal cycles of temperature and humidity. The physical principles which underlie the simulation are described. An example of simulation results, design decisions and lessons-learned are presented.

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