Fermentation lock

When the pressure of the gas inside the fermentation vessel exceeds the prevailing atmospheric pressure the gas will push its way through the water as individual bubbles into the outside air.

This device may take the form of a tube connected to the headspace of the fermenting vessel into a tub of sanitized liquid or a simpler device mounted directly on top of the fermentation vessel.

Other models contain three bulbous chambers allowing for a broader range of pressure equalization.

The use of perforated rubber balloons offers an easy and inexpensive alternative to conventional airlocks: as used primarily in homebrewing, the balloon is stretched over the orifice of the fermentation vessel and, if necessary, tightened with rubber bands.

These punctures, while not completely airtight, sufficiently protect the vessel's contents from contamination and allow the gases produced by fermentation to evacuate from the vessel as the pressure rises and the balloon inflates.

Fermentation lock on homebrewing fermentation vessel
A 25 L ( 6 + 1 ⁄ 2 US gal) glass carboy acting as a fermentation vessel for beer. It is fitted with a fermentation lock.