Wed Jan 05, 2011 6:02 am
There is a bit of an art to filling a CO2 cylinder and if it is not done properly you will get 3 - 4# in a 5# container. Either the liquid must be pumped until the bottle, placed on a scale, reads 5 pounds plus the tare weight or, if filling from a syphon, the filling cycle must be repeated until the bottle, weighed, shows 5# plus the tare. Bar suppliers that fill from an on-site syphon are not willing to go through the trouble of insuring a good fill and there is the danger that if they use the multiple cycle approach (which requires cooling the bottle) that they will over fill and have to vent some CO2 or, failing that, wind up in a liability situation. An overfilled CO2 bottle can produce a spectacular event. I've had it happen to a 2.5 pounder - nothing bigger (fortunately). The best tips are to take the bottle to a large gas supplier (in my neck of the woods it's Roberts Oxygen) that fills at a site equipped with gear that will fill the bottle properly. The then truck the bottles to the distribution facilities where they swap your bottle for one of their full ones. The disadvantage is that your brand new shiny aluminum bottle will probably be replaced by a grungy, rusty, beat up (i.e. not shiny) old steel one. The good news is that you never have to worry about a hydro check. They take care of that for you.
If you must go to a bar supply store that fills on premises there is one tip that will help you. Don't take a hot bottle into the store for filling. This means don't leave it in the trunk of your car in the desert all morning and then take the bottle to be filled in the afternoon. This is because the tanks are filled with liquid and the pressure in the bottle depends only on the temperature of that liquid. When filling from a syphon the temperature in the syphon is at room temperature. If you connect a hot bottle to the syphon liquid will flow until the temperature of the liquid in the target bottle is the same as the liquid in the syphon. If the bottle is hot when you start obviously it will warm the liquid from the syphon more than if it is cold when you start. So the trick to getting a good fill is to take the target bottle out of the freezer just before connecting to the syphon. This you can do if you are filling yourself but it's kind of hard to arrange if you are going to a filling station across town.
The fact that the bottle is initially full of liquid means that the pressure gauge does not indicate how much CO2 is in the bottle but rather its temperature. It will read 302.86 + 5.6621*T + 0.025246*T*T where T is in °F whether the bottle contains one teaspoonfull of liquid or 5 pounds of liquid. Thus at 32 °F the gauge will read 510 psig whereas at 87°F it will read 986 psig. These fluctuations often cause brewers to think they have lost CO2 when in fact all that has happened is that the bottle has gotten colder. Now once enough liquid has evaporated such that there is nothing but CO2 gas in the bottle the gas laws do come into play and the pressure reading is proportional to the amount of gas remaining. So at 70 °F the gauge will hold steady at 823 psig until all the liquid is gone and the pressure will then start to drop. This drop in pressure is your signal that it is time to think about getting more gas though you can most likley draw another keg (15.5 gal) with what remains.
Also, at temperatures above 87°F (summertime picnic setups) CO2 cannot exist as a liquid and the gas laws come into play again (though there is deviation from the ideal gas law) and again pressure reading is proportional to the amount of gas remaining (but also its temperature).
