Designation for various devices made of glass, which are placed on the closure hole of the fermentation container and seal it tightly on one side. Other designations are fermentation bell, fermentation control attachment, fermentation tube, fermentation tube, fermentation bung and fermentation closure. This prevents the ingress of air(oxygen) and the undesired oxidation caused by it. Therefore, a fermentation carried out in this way is also called "closed fermentation". The attachment is half filled with water and fulfils the function of a valve. Often sulphur dioxide is added to the water. The carbon dioxide escaping during fermentation is passed through it and this is indicated visually or additionally acoustically. There are various forms or even very simple devices in which a hose placed on the bung is led into a container filled with water. See also a list of relevant keywords under barrel.