This book examines the process of accumulation of gases in a known volume, initially at high vacuum, which is referred to as a constant volume (CV) technique. This technique allows for the measurement of very low gas flows, otherwise not detectable by any direct flow measuring device. The CV technique has been implemented into a time lag method used commonly for the determination of diffusivity, solubility and permeability coefficients in homogeneous films. Although not explicitly stated, the CV technique and the time method assume no resistance to gas transport within the volume where the gas accumulates. Starting from principles, this book shows that gas molecules encounter resistance while accumulating in vacuum systems, whenever the flow is in the Knudsen or in the slip flow regimes. Ignoring the resistance incorporates errors in the evaluation of gas flow rates and in membrane characterization process. This book has provided the foundation of further research in more complex CV systems, the results of which have been published by the Journal of Membrane Science.