Vacuum Technology Book, Volume II

1.3.6 Venting

To avoid undesired contamination, vacuum chambers should be vented with dry nitrogen instead of air. This prevents water vapor and other condensable contents of the ambient air (such as solvent vapors) from depositing (adsorbing) on the vessel walls. Every condensate would extend the subsequent evacuation due to the slow process of desorption compared to a pump-down operation. If a vessel had been vented with inert gas, it should only be opened to allow the necessary work to be carried out to the inside the vessel. Opening the vessel for long periods will result in water vapor from the ambient air entering due to convection caused by movement of maintenance personnel or due to diffusion.