carbing, "serving pressure", etc.

i've been thinking about this "serving pressure" thing, and I may have gotten myself confused....

the driving force in diffusion is concentration levels; that is, molecules in areas of high concentration will move to areas of low concentration. So, if we had a beer that's carbed to 3 volumes, and put 5psi of CO2 gas in the headspace (at 40F, this is all assuming constant temps), why will the CO2 in the beer diffuse out? We've already filled the headspace with a sufficient volume of CO2 to get to our headspace pressure of 5psi. In fact, if any CO2 does effuse, the pressure in the headspace has to increase, right? Pv = nRT and all that. So this isn't happening, at least my gauge says so, and it's certainly not going back into the tank for various (pressure, check valve) reasons, so the only way i can think of for that to happen is for gas to escape the system entirely.

CO2 escapes from a bottle because it's left open to air, which is about 0.039% CO2, so not much. in the example above, there's certainly a higher concentration of CO2 in the liquid than there is in the headspace (concentration being mols / volume), but i think there are two seperate subsystem inside the keg - the headspace and the beer. If the headspace is at equilibrium, how can CO2 in the beer diffuse out? my original belief was that the "whole system" (beer and headspace) has to maintain some equilibrium based on pressure. i don't think this is the case, simply because we have a constant supply of CO2 to the headspace. I understand we are also removing pressure from the beer - but my question is, how can CO2 inside the beer get out if the headspace is saturated with CO2? and the headspace pressure is not increasing?

it's not an isolated soda can - if we somehow take some CO2 out of the headspace of a soda can and seal it back up, the soda inside the can will lose CO2. venting a keg will still certainly remove CO2 from the beer, because the headspace is no longer saturated.

motobrewer said:

We've already filled the headspace with a sufficient volume of CO2 to get to our headspace pressure of 5psi. In fact, if any CO2 does effuse, the pressure in the headspace has to increase, right? Pv = nRT and all that. So this isn't happening, at least my gauge says so

Click to expand...

I don't think the gauge on the regulator is reading the actual pressure of the inside of the keg. I believe it might be reading the pressure of a "chamber" within the gauge itself.

I've had kegs that were pressurized to way more than 10 psi and have connected them to a regulator that was set at 10psi. Nothing happened. No gas moved around (could go backward through the regulator) and the gauge still read 10psi. I vented a little gas. Still read 10psi. I vented a lot of gas, and then it dropped down to below 10psi while the regulator opened up and tried to get back to 10psi.

I'm not sure what you are asking, so you have confused me as well!

I DO see my gauge increase if I have an over-carbed beer and its hooked to a regulator that is set at serving pressure (5psi or whatever). Over the next 12 hours of so after hooking up, the over-carbed beer in the keg is cause the regulator to drift up as the pressure in the head space equalizes.

I imagine the same thing is happening in a properly-carbonated beer and a reg. set at 5-10psi. However, the pressure differential isn't as severe, so you don't see the difference as much?

Not sure I answered your question....

edit to add: I don't have check valves on my lines, so that is why I observe an increase in pressure in the above situation.

Walker said:

I don't think the gauge on the regulator is reading the actual pressure of the inside of the keg. I believe it might be reading the pressure of a "chamber" within the gauge itself.

I've had kegs that were pressurized to way more than 10 psi and have connected them to a regulator that was set at 10psi. Nothing happened. No gas moved around (could go backward through the regulator) and the gauge still read 10psi. I vented a little gas. Still read 10psi. I vented a lot of gas, and then it dropped down to below 10psi while the regulator opened up and tried to get back to 10psi.

Click to expand...

Do you have check-valves on your system?

Walker said:

I don't think the gauge on the regulator is reading the actual pressure of the inside of the keg. I believe it might be reading the pressure of a "chamber" within the gauge itself.

I've had kegs that were pressurized to way more than 10 psi and have connected them to a regulator that was set at 10psi. Nothing happened. No gas moved around (could go backward through the regulator) and the gauge still read 10psi. I vented a little gas. Still read 10psi. I vented a lot of gas, and then it dropped down to below 10psi while the regulator opened up and tried to get back to 10psi.

Click to expand...

ahhhh, this makes sense. my gauges are behind check valves, so that certainly changes my logic.

i haven't really tried anything, for some reason this thought came into my head this morning.

More pondering....

It doesn't matter that the headspace is filled with only CO2. It's the pressure that matters.

it takes a certain amount of pressure to keep a certain amount of gas dissolved in the beer. If that pressure is backed off, gas will come out of solution until equilibrium is reached and the pressure on top of the liquid is enough to keep further gas from coming out.

In your soda can example, it's not that the gas on top of the soda is air and only partially CO2. It's that the gas on top of the soda is at 1 atmosphere of pressure. If you opened a can of soda and placed it in a 1 atmospheric room filled with just CO2, the soda would still go flat because there isn't anything to stop the CO2 from bubbling out.

broadbill said:

Do you have check-valves on your system?

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I don't think so.

I bought my kegging set-up used off craiglist in 2006 or something like that. There is a little ball-valve with a lever at the output of the regulator, but I don't think it has a check-valve in it.

I really don't know, though. I just bought it and started using it and remained blissfully ignorant.

Walker said:

More pondering....

It doesn't matter that the headspace is filled with only CO2. It's the pressure that matters.

it takes a certain amount of pressure to keep a certain amount of gas dissolved in the beer. If that pressure is backed off, gas will come out of solution until equilibrium is reached and the pressure on top of the liquid is enough to keep further gas from coming out.

In your soda can example, it's not that the gas on top of the soda is air and only partially CO2. It's that the gas on top of the soda is at 1 atmosphere of pressure. If you opened a can of soda and placed it in a 1 atmospheric room filled with just CO2, the soda would still go flat because there isn't anything to stop the CO2 from bubbling out.

Click to expand...

henry's law states that partial pressure is the driving force of gas diffusion, not total pressure.

the only way for CO2 to come out of solution is if it goes into the headspace - for that to happen, the headspace pressure has to increase. i thought this wasn't happening based on my system, but it probably is.

the ball valve is probably a combo ball/check valve.

motobrewer said:

the ball valve is probably a combo ball/check valve.

Click to expand...

Yeah... it might very well be. I have never really thought much about it.