Designation for various devices made of glass, which are placed on the sealing hole of the fermentation tank and seal it on one side. Other names include bell, fermentation control, fermentation tube, fermentation tube, fermentation pint and fermentation seal. This will be the access of oxygen and unwanted oxidation prevented. Therefore, a so performed fermentation also referred to as "closed fermentation". The attachment is half filled with water and fulfills the function of a valve. Often the water is too sulfur dioxide added. The escaping during fermentation carbon dioxide is passed through and this visually or additionally also acoustically displayed. There are different forms or even very simple devices in which one on the bung attached hose is passed into a container filled with water.