Forced carbonation pressure vs. serving pressure

Let’s assume that I want to force carbonate my beer at 20 psi and serve it at 8-10 psi. I carb up my beer at 20 psi for 7 days, then reduce the pressure to 8-10 psi and bleed out the excess CO2. If the beer line is balanced, all should be good, and the first pint should be just right.

But the CO2 in the beer and the pressure in the headspace is no longer in equilibrium, the CO2 in the beer will start to come out of solution until equilibrium is reached, right? Or am I missing something here?

If I leave it for 7 days, and come back for another pint, the pressure will be too high, because some of the CO2 has come out of solution, resulting in too much foam, and my beer will keep losing carbonation (as I consume it) until C02 level is equal to carbonation at 8-10 psi.

This concerns me. What is your take on this? Am I being overly concerned? Are there other factors that I have overlooked?

I want to use my kegerator for both conditioning, carbonation and serving. Therefore, I have set it to 10 C / 50 F, this is the reason for the 20 psi.
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Hi!
All I can say is that I set my regulator at serving pressure and it doesn't change. If there was a leeching of CO2 out of the beer into the headspace, surely the gauge would show an increased pressure.

If time is on your side, couldn't you just carb your beer at 8 - 10 psi in the first place? If you can leave the gas bottle connected, it should carb to whatever you set pressure to....eventually.....shouldn't it?

No, at 10 psi I will not get to my target CO2 vol. at 2.6. My question is more like: Do I lose carb over time when I reduce to serving perssure.

No, once the co2 is in your beer you only need around 10psi to keep it in there

Pawlo7671 said:

No, once the co2 is in your beer you only need around 10psi to keep it in there

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Sorry that's incorrect.
When the gas space pressure is reduced by venting, CO2 will slowly migrate out of solution and the pressure rises until an equilibrium is re-established (assuming constant temperature). Its basically the same mechanism that drives the force carbonation process, only in reverse.
By example if you open a bottle of sparkling water release the gas space pressure to ambient pressure and then reseal, if you open a few hours later the gas space will have repressurised from dissolved CO2 that's come out of solution. However the pressure will be slightly less than the first time, more so if you took out some liquid as well.

I'll preface this with saying that I've absolutely no experience of cornies, but I do know a bit of physical chemistry. Yes, once the pressure is reduced to serving pressure, the system will revert back to equilibrium. The question is, how long will this take? At room temperature this seems to happen quite fast, as this bloke found out. Indeed, if you over-carbonate your beer then the advice I've read is to take it out the kegerator and warm it up to room temp before depressurising in steps. If you plan on using your kegerator to condition, carbonate and serve, then this isn't so much of a problem, for two reasons. Firstly, the difference between carbonation pressure and serving pressure is smaller due to the higher solubility of CO2 at low temp, so the system isn't knocked as far out of equilibrium when you drop to serving pressure, and thus the rate is equilibration is lowered. Secondly, the rate of equilibration is lowered even further because the temperature is lower and chemistry happens slower at lower temps. The rate of gas exchange between the liquid and the head space is also proportional to the surface area between the two. When carbonating you bubble the CO2 through the beer to maximise this (and some people rock the keg too). However, once carbonated then the surface area available for exchange is only the area of the circular interface between the two phases. I guess this is the reason why the corny design is tall and narrow. For these reasons I don't think it will be a problem for you unless you plan on keeping it at serving pressure for a long time. So, I guess if you don't plan on having a beer for a while you can just crank it back up to 20psi. Regardless, a bit of googling seems to show that there are more people complaining about not being able to get the CO2 out of over-carbonated beer than people complaining about too much CO2 coming out of perfectly carbonated beer. That said, I've never used a corny so I'm happy to be corrected by someone experienced with them.

I find that carbonating to 20psi for about three days and then reduce to around 3-4 psi works for me. To me it's a matter of adjusting to suit, I don't think you can stick to hard and fast rules.

Brian

terrym said:

Sorry that's incorrect.
When the gas space pressure is reduced by venting, CO2 will slowly migrate out of solution and the pressure rises until an equilibrium is re-established (assuming constant temperature). Its basically the same mechanism that drives the force carbonation process, only in reverse.
By example if you open a bottle of sparkling water release the gas space pressure to ambient pressure and then reseal, if you open a few hours later the gas space will have repressurised from dissolved CO2 that's come out of solution. However the pressure will be slightly less than the first time, more so if you took out some liquid as well.

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Erm no its not !
I'v been using cornie kegs for quite some time and no matter how i force carb a serving pressure of around 10psi keeps the beer at the same level of carbonation until the keg kicks. See, if you force carb at 20psi for 4-5 days, not all that 20psi makes it in to the beer so you can easily maintain the carbonation with a serving pressure of 10psi. If you open a bottle of coke or sparkling water then yes the next time you open it you'll find its lost some sparkle, but a bottle of coke isn't connected up to a cylinder full of co2 now is it ?
Remember that at 20psi you are FORCE carbing, you could hook it up to 10psi and leave it longer and would end up with a decent level of carbonation because force carbing is a time and pressure thing.

Your experience with Cornie kegs clearly appears to be at odds with my knowledge of scientific fact.
I suggest therefore we agree to differ. :thumb:

terrym said:

Your experience with Cornie kegs clearly appears to be at odds with my knowledge of scientific fact.
I suggest therefore we agree to differ. :thumb:

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I guess so, I like a higher than normal level of fizz in my beer and 30psi for 3 days then 10psi serving pressure is just fine, maybe i drink my beer too fast :lol:

err, what was the question :)