Vessel cap

A method for exchanging gas in and pressuring a vessel headspace, the method comprising:

• providing a cap inlet;
• providing a seal arranged to form a gas-tight seal on a vessel opening;
• providing a pressure reducing valve;
• providing a gas inlet port arranged to allow incoming gas into the vessel headspace;
• and
• providing a gas outlet port arranged to allow outgoing gas to escape from the vessel headspace, wherein:
• the pressure reducing valve is arranged to allow gas at a first pressure at the cap inlet to exit the gas inlet into the vessel headspace at a second pressure reduced from the first pressure,
• the pressure reducing valve comprises a movable member, the movable member having a first surface in fluid communication with the cap inlet,
• the gas outlet port further comprises an apparatus to seal the outlet port when pressure in the vessel headspace reaches a third pressure, and
• the movable member further comprises a second surface, wherein gas within the headspace of the vessel acts on the second surface to cause the movable member to move to a closed position after the pressure in the headspace rises following sealing of the outlet port.