Blow off tubes are used when fermentation is so vigorous that your Beer looks like it will enter the airlock.
I originally made a blow off tube because the 30 bottle wine kits i make ferment so vigorously the small airlocks i use run dry after a couple of days.
The how to bit -
Cut the bottom off an airlock then get a short piece of syphon tube and soften one end in boiling water, insert the airlock tube leaving enough sticking out to get a good seal in the FV grommet.
Pour enough water into a container (cup, glass or bottle) so the other end of the tube is covered and that's it, you can now empty the container if it gets messy which is a lot easier than cleaning airlocks.
A bigger version of the one i use (not mine) -
I originally made a blow off tube because the 30 bottle wine kits i make ferment so vigorously the small airlocks i use run dry after a couple of days.
The how to bit -
Cut the bottom off an airlock then get a short piece of syphon tube and soften one end in boiling water, insert the airlock tube leaving enough sticking out to get a good seal in the FV grommet.
Pour enough water into a container (cup, glass or bottle) so the other end of the tube is covered and that's it, you can now empty the container if it gets messy which is a lot easier than cleaning airlocks.
A bigger version of the one i use (not mine) -
