O2 saturation question

When using 100% O2 in our 70f wort, why worry about over oxygenation when you can only get 9ppm in 70f water at saturation? Higher SG would raise the saturation level right? So as I can see it, it would be very hard to over oxygenate wort.

Am I off here?

Yes. That 9 ppm number is for air (which is 20% O2). With pure O2 you can easily go much higher. I regularly wind up over 20 mg/L (the upper limit of what my meter will read).

In the recent BYO, Mr. Wizard did a simple calculation of how much oxygen you need to add and the time length for a specific flow rate. Can you comment on how accurate of an estimate you can make, assuming the flow rate is slow enough that most of the O2 is dissolving (judged by absence of bubbles forming on the surface)?

Here's the math:
20 L batch (5 gal)
10 mg/L oxygen (for 10 ppm O2 "optimal" concentration)
20*10 = 200 mg = 0.2 g O2

31.99880 g/mol O2 molar mass
0.2/31.99880 = 0.00625023438 moles O2

1 mole ideal gas = 24.06 L @ 20°C
0.00625023438*24.06 = 0.15038063918 L (7.5 mL O2 per L)

Flow ~1 min 12 sec @ 1/8 LPM (This is the highest flow I can set on my O2 regulator without seeing bubbles forming)

A great experiment would be to compare the calculations to observed DO levels in low (S.G. 1.040), mid (1.070), and high gravity wort (1.100) at 20°C.

The only thing here I might argue about is the idea that 10 mg/L is optimum. What's the optimality criterion? IMO the 'best' amount of oxygen is strain and pitch rate dependent. The latter is easy to see as, presumably, each cell will consume a certain amount and, thus, the more cells in a liter of wort the more O2 you need to supply to that liter. The other (strain dependence) is just common sense.

As for the accuracy of the calculation - that will depend on the accuracy of the flow meter. The typical gauge type flow meter measures pressure drop across an orifice and is accurate only against a given outflow pressure (typical medical regulator). If your oxygenation stone is under a foot or 2 or wort obviously the head adds back pressure.The pith ball type meters depend on the density of the fluid and thus, for accurate readings, you must have one calibrated for oxygen. The other factor is insuring that no bubbles are indeed breaking the surface. There are bubbles that are too small to see (hence the flashlight trick for early detection of the start of fermentation). Putting all this together I'd say the attainable accuracy is 'close enough for government work'.

ajdelange said:

Yes. That 9 ppm number is for air (which is 20% O2). With pure O2 you can easily go much higher. I regularly wind up over 20 mg/L (the upper limit of what my meter will read).

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I thought saturation was saturation, anything over 9ppm is over-saturation.

The heck with all of that. Are you sure your dissolving all O2? No way. Your speaking in a ideal situation. Unless you have like 5 plus meters of turbulent flow to allow the O2 to be absorbed all this theory is a waste of time.

trimixdiver1 said:

I thought saturation was saturation, anything over 9ppm is over-saturation.

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No, saturation is WRT a given set of conditions. If you expose pure water to air where the partial pressure of oxygen is about 0.2 bar oxygen will dissolve in the water until the partial pressure of oxygen in the water is 0.2 bar and that, at room temperature, corresponds to about 9 mg/L. There is no way, by shaking, stirring, or injecting air at atmospheric pressure that you can induce more to dissolve. But if you increase the air pressure to 2 bar, whence the partial pressure of O2 is 0.4 bar, oxygen will continue to dissolve until the partial pressure in the water is 0.4 bar corresponding to about 18 mg/L. Or, if you increase the relative percentage of oxygen in the air so that it's partial pressure is 0.4 bar (40% O2, 60% N2), the same will happen. With pure O2 at 1 bar the partial pressure of O2 is clearly 1 bar and if water is exposed to that O2 will dissolve until the partial pressure of O2 is 1 bar corresponding to a dissolved gas level of 5*9 = 45 mg/L. That is the saturation level of O2. Now if you take that water out from under the pure O2 blanket and remove it back to normal air it will be supersaturated WRT oxygen and oxygen will leave it until equilibrium is reached with the air at 0.2 bar (9 mg/L). Same as opening a beer bottle.

Forgive my asking but as a 'Tri Mix Diver' haven't you been schooled in Henry's law?

trimixdiver1 said:

The heck with all of that. Are you sure your dissolving all O2? No way. Your speaking in a ideal situation. Unless you have like 5 plus meters of turbulent flow to allow the O2 to be absorbed all this theory is a waste of time.

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Clearly, if it doesn't exit at the surface it is all dissolved. Where else could it go?

Of course you're not going to accurately calculate a value out to 11 decimal places (I was lazy), but you don't need that kind of precision.

I am curious to find out just how accurate it is, though.

In the end it comes down to learning your equipment and process for repeatability and meaningful (strictly for you) adjustments.

I think the "optimal" concentration was chosen for ease of calculation although ~10 ppm is also a commonly cited general target.