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A tube that is inserted directly into the neck of a carboy, or attached to a carboy cap or rubber stopper, while the other end is immersed in water to allow for vigorous fermentation and the overflow of krausen as well as the escape of CO2. The end of the tube is placed in a container of water to stop contamination of the wort. After the vigorous fermentation is finished the blow off tube is normally replaced with an airlock
