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Home Brew Forums > Home Brewing Beer > Fermentation & Yeast > Stir plate efficiency
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Old 03-15-2011, 08:19 PM   #11
Pivovar_Koucky
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I'll tell you what, someone measure the internal diameter of the mouth of a 1L erlenmeyer flask and give me some ballpark numbers for their starting and ending gravities and the time it takes to ferment and I'll crunch the numbers and come back and post my results.


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Old 03-15-2011, 08:23 PM   #12
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My 2L flask (which I'm guessing is the size that most people use) is 1-5/8 (1.626") in diameter at the mouth. To be clear, this is not including the thickness of the glass.


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Old 03-15-2011, 08:26 PM   #13
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Originally Posted by kanzimonson View Post
I'm curious if there's a correlation between the way liquid moves in the starter and the way gas moves in and out of the flask.

What I mean is, the whirlpool of the starter creates a lower pressure region in the center of the flask. Does that mean that more CO2 is leaving the center of the starter, in general? Or do you think this is a negligible effect? If it's true, perhaps more O2 is able to enter the liquid at the edge of the surface?

Just throwin out some pseudo-science here...
I don't think that the whirlpool is due to lower pressure, it's due to the fluid flow being constrained at the walls. If anything I would expect that less CO2 is leaving at the center than at the edge because the height of the fluid column is less there, so less yeast is farting out CO2.
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Old 03-15-2011, 08:30 PM   #14
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Maybe I've got it wrong, but I thought I've heard it explained that the lower pressure is due to the whirlpool. Because of the constraint at the walls, the highest pressure is at the edge of the flask. The lowest pressure is in the dead center of the tornado. Fluid moves from the tornado, out to the wall, down to the bottom along the wall, towards the tornado along the bottom, and then up to the top of the flask through the tornado.
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Old 03-15-2011, 08:31 PM   #15
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Originally Posted by kanzimonson View Post
My 2L flask (which I'm guessing is the size that most people use) is 1-5/8 (1.626") in diameter at the mouth. To be clear, this is not including the thickness of the glass.
Would you say you usually fill the flask up all the way to the 2L mark or is it more like 1.5L?
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Old 03-15-2011, 09:30 PM   #16
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Interesting. That's a lot of gas!

It is constantly degassing co2, right? When it degasses, something needs to flush back in to take it's place. Otherwise it would create a vacuum.

If a bottled beer absorbs oxygen through a crown cap (which is does). Surely oxygen flushes back into a starter through foam or aluminimum foil. Right?

I feel like there is enough movement to cause gas movement in and out of the starter.

Are feelings part of science?
Nothing needs to come back in because the CO2 is being created by the yeast from sugar, this increase in the amount of gas causes a rather small increase in pressure which drives the CO2 above the liquid out.
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Old 03-16-2011, 01:51 AM   #17
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Would you say you usually fill the flask up all the way to the 2L mark or is it more like 1.5L?
Hmmm, it feels like I'm usually making a full 2L starter, usually because I'd rather use a small bit of my stored slurry and spend the money on the DME for the starter.

In reality, it's probably about 66% of the time, and the rest is a mix of 1-1.5L starters.
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Old 03-22-2011, 10:12 PM   #18
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So this problem was a hell of a lot more complicated than I had anticipated. Mostly because of boundary conditions. Its a 2nd order linear PDE so I need 2 boundary conditions and an initial condition to solve.

The initial condition was fine, C=C0 for all x (atmospheric concentration at all points).
The boundary conditions were harder. All of the conditions I could think of made the problem trivial (C=0 at x=0 for all t, i.e. what I was trying to prove), stupid (C=C0 at x=L for all t, i.e. atmospheric concentration at the mouth of the flask. This results in the concentraion not being a function of time.), or a beast (C=f(t) at x=0, for all t. Basically, specifying a function to describe the concentration at the liquid-gas interface).

What I ended up doing was to compare the diffusion distance versus the distance traveled by moving gas in a semi-infinite tube. The result of this is that after about 50 min the convection distance (v*t) exceeds the diffusion distance (2*sqrt(D*t)). I interpret this to mean that after 50 min there is no O2 left at the liquid-gas interface.

I realize that this is a vast oversiimplification so I certainly wont be offended if no one takes this as an absolute refutation of the stirplate increasing O2 in solution.
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Old 03-23-2011, 12:01 AM   #19
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When there is oxygen present the yeast multiple, not ferment... the reason the stir plat is so effective is that when it is stirring they just have sex... no eating the sugars at this point... hump hump hump all night long... and then when you stop there are crazy amounts of yeast who only then start fermenting... To my understanding, this is why the stir plate is so effective

Who has starters fermenting with in 24hrs anyways?
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Old 03-23-2011, 12:17 PM   #20
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Thanks Pivovar for the laborious calculations. Pretty interesting stuff, though I don't know what to think of the findings. In my head I was like, "So what, should I just turn off the stirplate after an hour?" But there's also got to be some benefit to keeping the yeast suspended and encouraging CO2 to blowout. Plus statistically, there's got to be some oxygen there. Science!


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