A fermentation lock or airlock is a device used in beer brewing and wine making that allows carbon dioxide released during fermentation to escape the fermenter, while not allowing air to enter the fermenter, thus avoiding oxidation.
There are two main designs for the fermentation lock, or airlock. These designs work when half filled with water. 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.
Currently, a popular fermentation lock that mounts on top of the fermentation vessel is the three-piece fermentation lock. Other models contain three bulbous chambers allowing for a broader range of pressure equalization. These bulbous fermentation locks were generally made of hand blown glass and are nowadays often made of clear plastic or glass.