Dandeman321 writes –

This simple CO2 regulator is used to slowly release CO2 gases from the fermentation process of wine or beer using water and a few parts that can be found quite easily. It works by allowing CO2 to get out of the fermenter without permitting oxygen to enter. If oxygen was to get into the fermenter, it would stop the yeast from making that delicious ethanol alcohol.

instructables : Simple, Cheap, CO2 Regulator – Link.