Originally Posted by cowain
I don't think your example holds true to real life situations. Yes, coke goes flat when you leave it in the bottle for a week. It could be attributed to the fact that there is 0 PSI in the bottle, not 10 or 5 or whatever. Perhaps some amount of PSI in the keg could cause the CO2 to stay diluted in the beer.
Ok, there seems to still be some confusion.
All the doubters please go memorize this first:
If you don't like his law go argue with him, not me. Now please read this regarding the coke analogy
The amount of CO2 dissolved in a liquid is directly linked to the temperature of the liquid and the pressure of the gas. If you change one or the other, the amount of gas dissolved in the liquid also changes. There is no physical law that states that once a gas is dissolved in a liquid it can't or won't come back out. It does.
If you carbonate a beer at 20psi and at typical serving temperatures, a fixed and finite amount of CO2 will dissolve in the beer, given sufficient time. The exact amount is quoted in a myriad of tables that are available online and in brewing books.
If you reduce that pressure to 19psi, a small amount of gas will come back out of solution and stabilize. If you reduce it to 1psi, almost all of the gas will come back out and stabilize. Boyle says so.
The Coke example I gave is correct and accurate, but extreme because the keg (bottle) pressure was reduced to atmospheric, and a large proportion of the contents was poured out. The remaining Coke went almost flat after the bottle was recapped because most of the remaining CO2 in the Coke came back out of solution to balance the partial pressure inside the bottle. Blame Boyle.
The goal with kegging beer is:
1) choosing our favourite serving temperature
2) choosing our favourite carbonation level
3) finding a way to get the beer out of the keg and into our glass with the temperature and carbonation level intact.
You can do all the gyrations you want with shaking kegs and playing with pressures up and down but Mr. Boyle states that it will make no difference in the end.
What you DON'T want to do with pouring a kegged beer is have all the CO2 come out of solution in a great rush- this causes the dreaded foaming.
Aside from having every piece of your system kept at serving temperature (including the serving line and tap) you need to find a way to 'ease' the beer from 10-12 psig in the keg, to your glass which is at atmospheric or zero psig. The standard way of doing this is to use restrictor tubing which is quoted to have 1.5 to 2 psi 'resistance' per foot. The resistance number is only applicable for a certain density of liquid traveling at a certain speed, but the quoted number works for beer.
Think of it this way - if your beer is stabilized at 12 psi, and you have 5 feet of tubing @ 2 psi drop per foot, then the beer will exit the tubing at 2 psi. Beer exiting the tubing at 2 psi into your glass (sitting at 0 psi) will do very little foaming.
There's other factors that come in to play that require some fine tuning (vertical height of your tap above the keg is one) and individual characteristics of serving taps, etc. but the concept is sound and is used in pubs and breweries around the world.
Hope this helps.