Half a croissant, on a plate, with a sign in front of it saying '50c'
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Expensive, difficult, slightly dangerous, not particularly effective... I'm on a roll.

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Dry-ice soda machine

Use dry ice rather than CO2 cartridges to make fizzy water
  (+5, -1)
(+5, -1)
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Carbonated water costs about $0.75 or more per 2-liter bottle. Although machines are available to make soda water at home, the inexpensive machines use 12g CO2 cartridges which do about two quarts each and cost as much as simply buying fizzy water.

Buying CO2 in dry-ice form is a lot cheaper than buying it in 12g cartridges. Although one would have to make soda water in batches (dry ice won't keep, so one would have to use it all at once) I should think the kit would be simple enough that someone could buy enough units to get good use out of a 1lb block of dry ice. The simplest form of the kit would consist of a screw-top for a 2-liter bottle that would contain a small dry ice reservoir, a safety valve, and a safety failure point. To use, fill the bottle with the proper amount of water, don safety goggles, measure a suitable quantity of dry ice into the reservoir, screw on the cap, and then invert the bottle briefly to allow the dry ice to fall into the water. The dry ice measure should provide just the right amount of CO2 to provide suitable carbonation; in case of excess, the safety valve would prevent explosion. In the event the valve failed, the safety failure point would break before the bottle itself expoded; such failure would destroy the device but avoid injury risk.

It would also be possible to construct a system which used a master CO2 reservoir and pressure regulator that could then be used to pressurize individual bottles. That might cost somewhat less per bottle (the equipment for each bottle would simply be a cap with an attachment point and a check valve) but would add the cost of the separate unit.

BTW, another entirely different approach for home soda making would be to have interconnected bottles with check valves; most of the bottles would contain straight water, but one would contain a mixture of water, sugar, and yeast. Since it would take more than pounds of sugar to yield one pound of CO2, I would guess that CO2 generated via fermentation would be a little cheaper than dry ice, but not as cool.

supercat, Jan 03 2009

How to carbonate a beverage http://www.wikihow....arbonate-a-Beverage
Yep, thought so. [nineteenthly, Jan 03 2009]

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       I'm almost sure this is actually done. I make cola at home and i looked into carbonation, and though i chickened out and do it using mineral water, this is another way of doing it. See link.
If you do it by fermentation, it ends up mildly alcoholic and really not very fizzy. I've tried it that way as well but gave up doing it because it's not suitable for Muslims or other teetotalers, like me.
nineteenthly, Jan 03 2009

       Cheaper overall to use commercial beverage CO2 cylinders. A 50 litre cylinder costs about £40 ($80) to refill - this is nothing compared to the cost of those little canisters.
MaxwellBuchanan, Jan 03 2009

       (1) For soda to remain fizzy for any period of time, there must be excess CO2 pressure. If one added the right amount of dry ice to the water and then capped it immediately, rather than waiting for the fizzing to subside, one could achieve this. Unless the cap included a pressure-relief valve, however, there would be a significant risk of explosion. A cap which included the dry-ice holder and an integrated safety valve would provide a more effective solution.   

       (2) My fermentation-based CO2 idea would produce the CO2 in one bottle and pipe it into the others. Although alcohol would be produced in the fermentation bottle, the liquid in the other bottles would remain alcohol free. If one did not wish to drink the alcohol, one could simply dispose of it.   

       (3) What is the cost of the equipment required to use commercial CO2? I was thinking of the dry-ice equipment as being a lower-capital-cost alternative, though obviously businesses that produce a lot of fizzy beverages would go the commercial route.
supercat, Jan 04 2009

       //What is the cost of the equipment required to use commercial CO2?//   

       Well, you're looking at a few tens of pounds (or dollars) for the initial cylinder, and probably a few more pounds for a pressure regulator and a few connectors.   

       I'm not sure of the size of the little CO2 bulbs you buy for domestic use, but I'd imagine that a £40 commercial cylinder is equivalent to a few thousand of the little bulbs.   

       So, you're looking at a total outlay of less than £100 for maybe 1000 bulbs-worth of fizzy drinks, or 10p a shot. After that, the cost becomes lower because you already have the equipment.
MaxwellBuchanan, Jan 04 2009

       There ought to be a way, and there possibly is, of extracting carbon dioxide by cooling the air on a small scale and using it as dry ice to carbonate. I just had a look but i can only find info on making it with canisters.
My problem is always cashflow. I could spend a hundred quid on a canister but i'd then need to be able to shift loads of cola to pay for that, and the number of clients i have is tiny. There are more crucial bills for doing what i do. A hundred quid is three months income.
nineteenthly, Jan 04 2009

       aren't we missing the really great applications for this idea? the mobility bit where we can have an all terrain soda machine, self cooling, without the weight or bulk of compressed gas tanks. I think it's brilliant. Set up a bicycle trailer mounted vending business and work the summer traffic jams. Now all we need is a light weight ice shaver and blender and we're all set. (+)
WcW, Jan 04 2009

       I think it'd be better to aim at actually getting the carbon dioxide out of the air itself. You could dehumidify it, pump it into a piston, seal it, then yank it up _really_ hard so the temperature of the air falls below the freezing point of carbon dioxide, then remove it and keep accumulating it until you've got enough to dunk in the liquid. Or maybe use Peltier devices.
nineteenthly, Jan 04 2009

       "...and really not very fizzy." Check your yeast!   

       I used to make ginger ale in my home fermenter and I'll tell you, no factory-made carbonated beverage ever wanted out of its container like my homebrew did. It took a few messy tries but I finally learned how to finesse the cap off in itty-tiny increments to let the pressure off gradually. And believe me, it might be a hassle you wouldn't bother with for a Coke, but for my kitchen sink ginger ale, it was worth it.
elhigh, Jan 05 2009

       I've tried making a basic ginger ale by adding powdered ginger to soda water (and vice versa) with amusing-to-the-audience but less than stellar results.
FlyingToaster, Jan 05 2009

       [Elhigh], it may have been my reluctance to leave it for too long due to being teetotal, so i wanted to minimise the ethanol. Since that was presumably part of the molecule which later generated the gas, there´s presumably a link, so there must be a relationship between the two, based around moles of the sugar, associated moles of carbon dioxide and solubility.
nineteenthly, Jan 05 2009

       If not allowed to vent CO2, a bottle of homebrew beer would explode before it had produced enough alcohol to be worthy of the term "beer". Thus, naturally-carbonated beer is fermented in two steps--in the first step, the CO2 is vented (with the goal of producing alcohol) and the second it is not (with the goal of providing carbonation).
supercat, Jan 07 2009

       I seem to remember that carbon dioxide in most carbonated beverages is at about eight bars, but that could be complete rubbish. I used an airlock with the ginger beer, so i was losing a fair amount owing to a lack of desire to have the demijohn explode.
nineteenthly, Jan 07 2009

       If you use an airlock that will limit pressure buildup to a few inches, the results aren't going to be very fizzy no matter how much yeast you use. To get fizzy results, you must either (1) use a pressure relief valve that's calibrated for a suitably high pressure, (2) use a safety cap that will vent if the pressure reaches dangerous levels, and adjust the amount of yeast and fermentation time to ensure that it does not, or (3) use an ordinary cap and hope for the best.
supercat, Jan 07 2009

       Thanks. I may try that. The smell around here has given me a hankering for ginger beer, which makes me think i´ve knocked something over.
nineteenthly, Jan 07 2009

       err... it's not an infinite process; eventually the CO2 content will kill the yeast.
FlyingToaster, Jan 07 2009

       //eventually the CO2 content will kill the yeast.//   

       If done in a sufficiently-strong container, the process will limit itself. Yeast can, however, produce sufficient pressure to explode soda bottles.
supercat, Jan 08 2009


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