Quote:
Originally Posted by menschmaschine
I've seen it written or implied that the maximum O2 concentration possible with "air" aeration is around 8ppm, which is pretty good... especially with the right size starter. Injecting pure O2 can get the concentration higher than with plain air.
|
From the bit I know about dissolved gasses in solution, this doesn't make much sense to me.
A solution exposed to the atmosphere can only hold a certain amount of O2. It's all about partial pressures.
O2 is about 20% of typical air/atmosphere. Expose a solution to that atmosphere, and 20% of the pressure on top of that solution is O2, so 1 atmosphere of pressure on top of a solution is really 0.2 atmospheres of O2 "pressing down" on the top of the solution. Gasses in solution also exhibit a pressure, pushing back against the 1 atmosphere of pressure on top of that solution.
Expose a solution that is devoid of O2 to the atmosphere, and the atmospheric pressure on top of the solution will force O2 into that solution until equilibrium is reached, i.e. there is as much O2 off-gassing from the solution as there is being forced in by atmospheric pressure. Likewise, take a solution that has had O2 forced into it under pressure until it is super-saturated with O2 and expose the super-saturated solution to the atmosphere. O2 will off-gas from the solution until, once again, equilibrium is reached.
This is precisely how we can force-carbonate beer in our kegerrators. In a sealed keg, we create an environment of pure CO2 above the beer at around 12 PSI or so. After about a week, the solution is in equilibrium, i.e. there is as much CO2 off-gassing from the beer as there being forced into the beer. Increase the CO2 pressure above the beer and the amount of CO2 in the beer rises. Decrease the pressure of CO2 above the beer and you get less CO2 in the beer.
Now, take our boiled wort. At atmospheric pressure that wort can only hold a certain amount of O2, that being the amount that puts the solution into equilibrium. (Remember, equilibrium is where there is just as much O2 off-gassing from the solution as is being forced in by the atmospheric pressure above that solution.) Indeed, we could get to that equilibrium state simply by exposing the wort to the atmosphere for a long enough period of time, however the stuff would be rotten by then.
So, it's incorrect to state that an air pump cannot achieve the same amount of dissolved O2 in the wort as can an oxygen tank. The oxygen tank will get you there quicker, that's all.
Edit: I'm referring to aqueous solutions in the above example. Same probably wouldn't hold true for solvent-based stuff like paint thinner, etc.
hehehehehe
Linear Mode
