The Sodastream machine's Achilles heel is its tiny, proprietary CO2 tank. The company (formerly known as Soda Club) sells seven different home carbonation machines ranging from $80 to $200 (enhancements along the line are mostly cosmetic), but only offers CO2 canisters in 14.5 and 33-ounce sizes. (The measurements refer to the weight of the gas.) Sodastream claims somewhat optimistically that these are good to make a total of 60 and 130 liters of seltzer respectively. What's worse, these tanks have a proprietary cap which can only be filled by the company. My local kitchen supply store charges a hefty $15 to swap an empty 14.5-ounce tank for a new one. Compare that to the $15 charged by my friendly neighborhood welding supply shop to fill my new five pound tank, and the economics start to come into focus. With luck, my five-pounder won't need a refill until early 2013, and if you've got the room, you can go even bigger on the tank. Web carbonation guru Richard Kinch yields over 1,000 liters from his 20-pound tank—and he likes his seltzer extremely fizzy (more on that in a minute).