Carbon dioxide is integral for those people home brewing with a keg. Home brew kegs (also called "cornelius kegs") are smaller than most commercial kegs, so the carbon dioxide in most CO2 tanks last longer than it would for larger quarter-barrel to full-barrel kegs. CO2 cartridges, however, are a good alternative to tanks, as they take up less room and do not need the more elaborate tube set up required for tanks.
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CO2 cartridges stand in the place of a CO2 tank (or cylinder), which adds carbonation to a beer keg. Most home brewers use 5-gallon kegs, attached to which are anywhere from 3.5 to 20 lb CO2 tanks. CO2 cartridges hook into a portable CO2 charger, which is less cumbersome to use when dispensing beer from a keg outside of the house, and thus is often used for serving at picnics, parties or anywhere the keg must be carried.
Sizes of CO2 Cartridges
CO2 cartridges range in size from 12g to 68g, and the length of their life depends both on how much they hold and how much carbon dioxide needs to be pumped into the beer. In general, old versions of CO2 chargers take 12g cartridges, while newer chargers handle 16g cartridges. Be sure that you buy the correct size for your charger.
Temperature and CO2
The other important factor in the life of your cartridge is temperature. Particularly when using a portable cartridge, temperature is something that will affect the pressure of the CO2 cartridge and tank. The pressure increases when exposed to heat, and decreases when cool. At an average of 72 degrees F, the pressure should read 750 PSIG.
Rules of Thumb for CO2
In general, it takes about ½ lb of CO2 to dispense a quarter barrel of draft beer. Since most home brewers use 5-gallon kegs, however, these numbers are somewhat meaningless. For an average 5 lb CO2 tank and a 5-gallon keg, that means the tank will pump 31 to 44 kegs of beer. On a smaller scale, for portable cartridges, one 16g cartridge will empty about a third of a cornelius keg. A larger 68g cartridge will push through anywhere between one to four kegs.
CO2 Cartridge Warnings
Beware when attaching the charger to the keg. Always attach the ball and pin lock valve first to the keg, then to the charger. If you attach the ball and pin lock valve to the charger first, it will fail by becoming over-pressurized.