Beer Forum

I force-carbonate all of my beer in kags. Once a beer reaches the optimal level of carbonation, if Ireduce the CO2 pressure down to, say, 8-10 PSI for serving, will the beer loose carbonation over time?

thedrake wrote:I force-carbonate all of my beer in kags. Once a beer reaches the optimal level of carbonation, if Ireduce the CO2 pressure down to, say, 8-10 PSI for serving, will the beer loose carbonation over time?

As long as there is no leak in a keg of beer in which the CO2 saturation has stabilized, there will be no loss of pressure over time. That said, there are a number of things in your question that suggest you may see a change in level of carbonation.

You are apparently force carbonating at high pressure (25-30 psi?). Are you consulting a pressure/temperature chart to figure the proper pressure at the temperature you have the beer in order to get the desired level of carbonation? Using these charts, you can carbonate properly at just about any reasonable temperature. I carbonate at around 65°. Done with the charts, the carbonation level will remain the same when you chill it down to serving temperature. You just need to remember to not hook up the gas at the lower pressure until after the beer temperature has stabilized at serving temperature. If you bleed off the high pressure gas before the temperature has dropped, you will lose some carbonation as the beer needs to absorb the excess pressure from the head space. If you don't have a backflow valve in place, hooking up the lower pressure gas to a keg at higher pressure will result in beer backing up into your gas line and possibly into your regulator.

Also, you need to make sure your kegging system is balanced. There is much more involved than just turning your gas pressure down to 8-10 psi for serving. You need to consider the volumes of gas in the beer (carbonation level), temperature, and length of your serving line. The pressure of the CO2 applied to the keg to maintain carbonation level is determined by the temperature and desired carbonation level. The serving pressure is then determined by the length of your beer line. The longer the beer line, the more resistence to flow you will have. If you have the line too short, all you will get is foam from the beer gushing out too fast. Once the foam settles the beer will be flat. If the line is too long, the carbonation will be perfect but the flow may be too slow to fill the glass in a reasonable time. In most cases 5'-6' of 3/16" I.D. beer line will be just about right. If you are using a larger diameter line, such as 1/4" I.D., you will need a much longer line, perhaps up to 20'.

To help you figure all this out check out this link:
http://hbd.org/clubs/franklin/public_ht ... lance.html

Wayne

thedrake wrote:I force-carbonate all of my beer in kags. Once a beer reaches the optimal level of carbonation, if Ireduce the CO2 pressure down to, say, 8-10 PSI for serving, will the beer loose carbonation over time?

Yes it will unless the "optimal level of carbonation" happens to be the equilibrium volume at the storage/serving temperature and 8 - 10 psig. Ten (10) psig at 38°F will give you 2.38 volumes. If you carbonate at some other temperature and pressure to that level (how would you know you had acheived it) and then bring the beer to 38 °F under 10 psig CO2 it is in equilibrium and the 2.38 volumes will be maintained. If, OTOH you carbonated to 3.0 volumes at some other temperature and pressure and then put the beer on at 38 °F under 10 psig then it is not in equilibrium with the gas and as the beer is drawn out so that more and more of the headspace is gas at close to 10 psi the carbonation level will approach 2.38 volumes because that is the equilibrium level for those conditions.

It takes time for equilibrium to be reached. For a full sized Sankey keg it's 2 weeks or more. If I want 2.38 volumes can I get there faster by putting on say 30 psig at 38 °F for the first week and then dropping back to 10? Yes, I can and that is a workable strategy as long as you don't overshoot in which case your recourse is to bleed off the headspace, wait for the gas in the beer to equilibrate with the headspace, bleed again and keep doing this until you get headpace pressure of 10 psig (as in the example given here) at 38° F. At that point the gas is again at 2.38 vols and you can put the bottle back on for serving. The problem with a newly filled keg is that the headspace is small so that many bleedings will be required. You can, of course, improve the situation in this regard by enlarging the headspace. This is accomplished by removing beer and I'm sure you'll think of something to do with the beer you remove.

Here's an approximate (rms error 0.003 volumes, peak error 0.15 vols for 32 - 60 °F and 5 - 30 psig) formula for the volumes as a function of temperature and pressure: V=3.2694 + .076221*P - 0.042274*T
Here's a more accurate (rmse 0.00006 peak error 0.024 vols.) one: V = 3.4821 + 0.14562*P - 0.07437*T - 6.6194e-05 *P*P - 0.0012952*P*T + 0.00053484*T*T

Temperatures in Fahrenheit, pressures in psig. These are fits to the table in the ASBC's MOA (Method of Analysis).