Originally Posted by Pivovar_Koucky
Yes indeed. So let's assume your yeast starter is completely lacking in dissolved gas prior to turning on the stir plate. When you turn on the plate the gas will dissolve until it becomes saturated, at which point no more gas will enter the solution. So unless you have some way of increasing the pressure in the flask, the total amount of gas in the flask becomes consant after the solution becomes saturated.
Next, the yeast start producing a lot of CO2 from fermentation, probably several hundred times the headspace volume of your flask. This will force out the lighter gases (O2, N2, H2O, etc.) and push CO2 out into the environment. At this stage you would certainly have to agree that no net gas is entering the flask. The only way that O2 could enter the solution under these circumstances is by diffusing against the flow of CO2 out of the flask and becoming a significant portion of the gas immediately above the solution.
I didn't do the calculation, but it seems unlikely to me that this would happen. If anyone wants to crush Navier-Stokes and prove me wrong I'm all ears.
I agree with most of what you are saying. However, I think you are forgetting or ignoring that the O2 is used in synthesis of sterols. Using the O2 in synthesis would reduce the amount of molecular O2 in solution, in turn creating an unsaturated (in respect to O2 ) wort thus allowing more to be absorbed. I don't claim any of this fact but I am just inferring it from the logic of what I know thus far about yeast growth and sterol synthesis. I agree that once CO2 is starting to be produced in an appreciable amount that no O2 is probably being taken up but at that point the yeast have for the most part past their growth phase which is what the O2 is needed for anyways. So once the beer starts fermenting hard (producing a lot of CO2) I don't know that the O2 is really all that necessary anymore.