Using a "Kleer Drain" CO2 cartridge, the carbonator provides pressure through a valve and gauge to a 2L soda bottle. The CO2 dissolves into whatever cold liquid is inside the soda bottle, making it fizzy.
Everything's been built, although I'm not sure if my mechanism for puncturing the seal on the CO2 cartridge will function properly. I have to wait 24 hours for the PVC cement to dry, and then I can test the CO2 side of the device. Right now, I have a bottle of Pepsi hooked up to the modified bottle cap and hose, to make sure the gauge side of the device is working. It doesn't seem to be leaking, and has been holding around 11 psi for an hour or so.
Success! Although the puncturing cap didn't work very well, I've carbonated my first bottle of water! Amazingly, the pipe didn't explode, although there was some leakage in the gas valve.
I bought 50$ worth of brass fittings to try to improve the CO2 sleeve. I also intended to use a needle valve to replace the ball valve that had leakage problems. Unfortunately, when I finally got the new puncturing mechanism working, I couldn't get gas flow through either the needle valve or the old ball valve! After three hours of trying to get it to work and 4 CO2 cartridges wasted, I have elected to stop trying with the brass. Next step: return most of these components and buy a paintball CO2 bottle. Only question is where to get it filled in the area...
So, some friendly folks on the interwebs found a place I could get everything pretty cheap. The ball valve still doesn't put up much of a fight against this kind of pressure, but it's a convenient place to hook up the tube anyway, so I kept it. The cartridge container is gone, replaced by the connector to the giant paintball tank. It works great, so I'm calling this one completed!