Name for various devices made of glass that are placed on the sealing hole of the fermentation container and seal it tightly on one side. Other names are fermentation bell, fermentation control attachment, fermentation tube, fermentation bung and fermentation closure. This prevents the entry of air (oxygen) and the resulting undesired oxidation. This is why fermentation carried out in this way is also called "closed fermentation". The top is half-filled with water and fulfils the function of a valve. Frequently, sulphur dioxide is also added to the water. The carbon dioxide that escapes during fermentation is passed through and this is indicated visually or additionally acoustically. There are various forms or also very simple devices in which a hose placed on the bung is guided into a container filled with water. See also a list of relevant keywords under barrel.