Corrections to Graham's Law of Effusion for Predicting Leak Rates Through Hermetic Seals

Graham's law of effusion has been used for decades to facilitate the prediction of the flow rates of gases penetrating into hermetically sealed devices. Typically, a trace gas such as He is used as a measure of gas flow into or out of a device. He flow is then related via Graham's law to the flow of an unwanted gas, and this in turn is used to predict the service lifetime of the device based on the maximum allowable leak rate. As hermetically sealed devices become smaller and lower leak rates are required, there is evidence to suggest that Graham's law is inaccurate. We evaluate Graham's law by measuring the flow rates of noble gases (He, Kr, and Xe) and diatomic gases (D
₂
and N
₂
) through realistic leaks. Our findings indicate that Graham's law is not always accurate, and the incorporation of a relative tangential momentum accommodation coefficient into Graham's law leads to improved correlation between different gases and thus improved predictions of service lifetime.