A description is given of a simple and cheap air admittance valve for automatic air inlet into turbomolecular pumped UHV system upon (controlled or uncontrolled) power cutoff. The valve is operated by a mechanical timer switch which can be adjusted such that dry air admittance starts just before the turbo pump has finished its pumping action completely. Thus, contamination of the UHV compartment by oil vapor from the turbo pump and other active gases from the force vacuum is prevented and quick reobtainment of vacuum in the 10−9Torr region after system restart becomes an easy matter.