motobrewer
I'm no atheist scientist, but...
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.
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.