碳酸水的問題

Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonation in beer and sparkling wine comes about through natural fermentation, but some manufacturers carbonate these drinks artificially by dissolving carbon dioxide under pressure into the liquid.

Carbonating tap water to make seltzer is easy, fast, and absurdly inexpensive with my improvised apparatus. All that is required is to place CO2 (carbon dioxide) gas in agitated contact with chilled water for a few seconds.

The Gas Supply: Tank and Regulator
The essential ingredient is a supply of CO2 under pressure. CO2 is sold commercially in steel tanks of various sizes. It is actually a very inexpensive material, considering the special handling it requires. In my area you can refill a 20-pound tank for about $140, which will make many hundreds of gallons of carbonated beverages. Larger tanks are even cheaper per pound. Many soft-drink or beer distributors will swap your empty CO2 tank for a full tank at even lower cost, since they expect to make their profit on the other products that just use CO2 for propulsion. You shouldn't have to invest more than $600 to own such a swappable tank with a current inspection, and many dealers even prefer to just charge you a smaller refundable deposit to borrow one.

A "20-pound tank" is so called not because it weighs 20 lbs, but because it contains 20 lbs of CO2 in gas-over-liquid form (aka liquid-under-own-vapor). The empty steel tank and valve weight about 30 lbs (the "tare weight"), so the total filled weight is a hefty 50 lbs or so. I prefer this size tank because it is the largest that is still possible for me to carry by hand, and its 30-inch height also just fits under the counter in the kitchen. Tanks smaller than the 20-pound size do cost a bit less, and they're easier to lug around, but you will have to refill more often at a slightly greater cost per unit of CO2. On the other hand, the giant 50-pound tank weighs about 160 lbs when full, making it more of a piece of heavy equipment than a gadget.

The actual weight of my 20 lb tank on one occasion was 30.4 lbs (13.8 kg) empty and 46.3 lbs (21.0 kg) full. Apparently the "20 lbs" of payload is either a nominal figure, or I wasn't getting a complete fill from the supplier. The regulator adds about 3 lbs, if you're trying to weigh a connected tank. Tanks also vary slightly in size and weight depending on their construction.

The tank alone is not enough to supply the gas. One must attach a regulator to reduce the gas pressure inside the tank to a controlled, usable, low pressure.

Twenty pounds of CO2 carbonates a lot of water. We consume several liters per day in our household, and one tank lasts us for several years! The 20 pounds of CO2 in my full tank should theoretically yield 700 gallons of carbonated water at 100 percent saturation. Since my carbonator super-saturates the water with CO2 (the water is both chilled and pressurized while the gas is dissolved), and the gas in the head space of the bottle at the end of the process is vented, the yield must be somewhat less. But the process is so cheap that I haven't bothered to measure it! I estimate CO2 expense to be no more than a few cents per 2- or 3-liter bottle.

The Carbonation-in-a-Bottle Process and Its Economics
To carbonate a bottle of water, you must pre-chill the water first, because the solubility of CO2 in water is greatly increased by both lowering the temperature (Le Chatelier's principle), and by raising the pressure of the CO2 gas (Henry's law). Lukewarm water or low pressure CO2 gas will saturate with much less carbonation, so that the result is flat. In short, CO2 is easy to dissolve into water, but only up to an amount determined by pressure and temperature.

I hope this can help your understanding. :)