An airlock is essential to any home brewer, as it keeps oxygen away from the brews, be they wine, beer or mead. Yeast turn the sugars to alcohol and carbon dioxide. The carbon dioxide must be allowed to escape, or dangerous pressures can build up and shatter brewing vessels. Oxygen must be kept away from the brew, however, as alcohol exposed to oxygen turns into vinegar. An airlock is a gas-tight one-way valve designed to let the carbon dioxide out while preventing oxygen from the air from entering. Depending on the violence of the fermentation, there are two simple ways for a home brewer to make an airlock.
Blow Off Tube Technique
Drill a hole in the rubber stopper, just smaller than the outside diameter of the tubing.
Insert the tubing through the rubber stopper, so an inch or so of the tubing protrudes into your fermentation vessel.
Insert the stopper and tubing assembly into the fermentation vessel. The tube should end in the air space, called a head space, above your brew. If the tube ends in your brew, pull it back through the stopper a little bit.
Fill a drinking glass or pitcher halfway with water.
Place the end of the clear tubing into the drinking glass or pitcher.
Poke a hole in the bottom of a balloon with a straight pin.
Stretch the ballon over the neck of fermentation vessel.
When fermentation begins, the CO2 inflates the balloon, but the hole prevents a buildup of pressure.