BAKEOUT, a computer program for predicting diffusive outgassing from stainless steel

The outgassing load from a leak-tight and well-baked vacuum vessel is frequently dominated by the diffusive outgassing of hydrogen gas from the materials of which the vessel is constructed. Stainless steel, in particular, can be a major source of hydrogen. The diffusive transport of gas through materials is governed by the second order partial differential equation, D ∂2C/∂X2 = ∂C/∂t, known as Fick's Law. Two alternate solutions to this equation give exact numerical answers of the diffusive outgassing rate versus time. A simplified approximation of each solution allows prediction of both the short-term and the long-term behavior. BAKEOUT, a computer program based on Fick's Law, was developed to predict an elevated temperature step function of time (that is, a bakeout) that will achieve a desired hydrogen diffusive outgassing rate from a stainless steel vessel under vacuum. This paper presents the mathematical development of the computer program and an example case