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.
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 build-up of pressure.
Use the blow off tube for primary fermentation of beer and mead, as they are especially violent. If you can, place the pitcher or glass from the blow off tube in a sink. A violent fermentation can cause the glass to overflow. An overflow in your sink is easier to clean up. If you can find tubing large enough to seal your fermenter, you can skip the rubber stopper. Use the ballon method for secondary fermentation of beer and mead and all stages of wine fermentation. When using the balloon method, the airlock only works as long as fermentation is active and the balloon is inflated. Once fermentation stops, drink or bottle your brew.
Keep all fermentation equipment sterile.