A head gasket, head rubber, or colloquially foam ring is a large, flat rubber seal inside the cream whipper head. Its function is to prevent the escape of nitrous oxide gas at the junction of the siphon neck and head.
The two sides of the gasket are the same, but after a few uses, the neck of the aluminium flask will mark the rubber surface with a black ring.
As the gasket wears out or its material ages, the flask does not hold pressure, so it will decrease over time making some of the cream stay in the whipper. Replacement of the gasket is recommended during maintenance.
