well i went a little mad scientist this week and built a carbonator of sorts. It's a little pricey but a proof of concept. brewed up my first bottle last night. (i'll post up some pics tonight or tomorrow when i make a new batch if anyone is interested)
What i did was use 2 bottles. I used 2 liter bottles cause that's what I happened to have. i drilled a hole into the top of each of the caps and inserted a length of tubing into the holes. Sealed it up air tight with hot glue. so what i end up with is 2 bottles linked by a tube. ("Tubacapa 2000" (TM) (C) all rights reserved

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now i fill my first bottle about 3/4 full with whatever i want carbonated...need room to shake...i used water and zero calorie watermelon/strawberry bottled water flavoring pack. (6 to be specific..enough packs to make 1.5 litres) I placed this in the fridge to get nice and cold. once it was chilled I screwed on the first end of the Tubacapa.
the second bottle is going to be our "C series o2 SUPER reaction vessel" (TM) (C) all rights reserved

don't worry we aren't going to be consuming this portion.
i filled the bottle... i dunno...1/4? full of vinegar... to the bottom of the label anyways. i prepared a bomb package by laying out 4 squares worth of toilet tissue and placing a line of baking soda on the length. (about 4 or 5 Tbsp of soda) I folded the ends and rolled it into a tube. this gives us the ability to suspend the package inside the bottle and get the other end of our Tubacapa ready. this way we don't waste precious Co2 trying to funnel the baking soda in.
drop the bomb and screw on the bottle. you will probably notice it starting to fizz. SCIENCE! while the science is happening you are going to start shaking your drink bottle. you may have to swish the reaction vessel a few times to keep things sciencey in there. i shook the bottle a few times and noted the bottle was quite hard... the pressure was getting pretty high up there. the reaction vessel seemed to still have a bit of science left to go so i burped some of the gas out so i didn't end up with an explosion.
I shook the bottle maybe a half dozen times and the left it for 10 or 15 minutes I wanted to make sure the Co2 had plenty of opportunity to dissolve into the juice. I opened up the bottle and voila! I was quite impressed with the results.
not quite as foamy as pop (because of the lack of sugar) but still very good. a half bottle overnight in the fridge didn't loose much carbonation and didn't repressurize the bottle like pop does after you close it back up again. a glass poured after work today was just as fizzy as when I made it last night. The Co2 had indeed penetrated quite well and was quite stable.
all in all I consider it a success. A little pricey since a half gallon jug of vineager is $6 and i used maybe 75 or 80 cents worth. add the cost of the flavoring and my 2 litre bottle of "pop" cost maybe $2... hard to beat pop for price, but this is a hell of a lot healthier for you. it would be cheaper if i had used just plain water or maybe lemon flavoring.
I may just spring the cash and buy a Co2 bottle...more expensive to start but that would drop the cost SIGNIFICANTLY.