a device that allows the gas to escape but doesn't allow fresh air into the fermentation container

A simple water and bubble-type device used during closed or secondary fermentation that prevents ambient air from coming in contact withj the fermenting brew. At the same time, the fermentation lock permits the escape of carbon dioxide (a by-product of fermentaion). It fits into a rubber cork atop a carboy being used as a secondary fermenter.

A one-way valve attached to a fermenter to allow carbon dioxide gas to escape while excluding ambient wild yeasts, bacteria and contaminants. It can also serves as a guide to the fermentation process.

The fermentation lock or airlock is a device used in beer brewing and wine making that allows carbon dioxide released by the beer to escape the fermenter, while not allowing air to enter the fermenter, thus avoiding possible bacterial contamination.