Author Topic: Carbonation (Henry's Law) question  (Read 1325 times)

Offline hubie

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Carbonation (Henry's Law) question
« on: March 20, 2013, 06:49:03 AM »
I have a question for those who are more chemistry-aware than I.  When bottle conditioning, I understand the reasoning behind calculating the priming sugar based upon the warmest temperature the beer had been at, namely that warmer liquids hold less CO2 than colder liquids.  I was wondering about the setup I have, which is a small chest freezer.  I have an ale that has been fermenting in the low 60's, and I let it warm up to the upper 60's for a few days.  If I cool it back down, for all intents and purposes my chest freezer is filled with CO2.  When the beer cools back down, does it take up the CO2 again in any significant amount, or do I still use the higher temperature in my carbonation calculation?  For the ale it is largely an academic question because the difference isn't that great, but I will be getting into lagering for the first time soon and there it can have a larger difference.

I can convince myself that the beer wouldn't take up additional CO2 because when it was colder it was in a supersaturated state and I wouldn't think the CO2 would diffuse back into solution such that it returns to a supersaturated state; however, at this point I'm arguing from the gut because I don't have much of a chemistry leg to stand on.

Offline euge

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Re: Carbonation (Henry's Law) question
« Reply #1 on: March 20, 2013, 07:46:49 AM »
My thoughts are that you need to be under a state of higher pressure for any diffusion into the beer. There's obviously a lowered risk of oxidation.

My second round of lagers were visibly carbonated after 2 months in the fermenter. But, it was only about half of what I wanted.
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Online mtnrockhopper

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Re: Carbonation (Henry's Law) question
« Reply #2 on: March 20, 2013, 08:37:15 AM »
It probably would start to dissolve more, but without a pressure differencial it will be very slow. I doubt it will be noticable.
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Offline Kaiser

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Re: Carbonation (Henry's Law) question
« Reply #3 on: March 20, 2013, 01:19:22 PM »
That’s a loaded question and I remember discussing this before.

I wouldn’t worry too much and base the calculations on the current temperature of the beer. If the beer has been warmed up before that and lost CO2 then it will also have reabsorbed the CO2 as it cooled down. One issue here is that as it absorbs CO2 from the headspace it would pull air into the airlock and thus lower the CO2 pressure in the head space. This will lead to less residual carbonation.

But I doubt that the effect is large enough that you have to worry about it for carbonation calculations.

There is also the issue of slow residual fermentation. How are you sure that only the added sugar will be metabolized during bottle conditioning?

Kai

Offline hubie

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Re: Carbonation (Henry's Law) question
« Reply #4 on: March 21, 2013, 06:00:28 AM »
I wouldn’t worry too much and base the calculations on the current temperature of the beer. If the beer has been warmed up before that and lost CO2 then it will also have reabsorbed the CO2 as it cooled down. One issue here is that as it absorbs CO2 from the headspace it would pull air into the airlock and thus lower the CO2 pressure in the head space. This will lead to less residual carbonation.

That is the part about Henry's Law that I'm not sure about.  Since my chest freezer is holding CO2 in, let's assume that when the beer cools back down and when it sucks gas back through the airlock that it is all CO2 coming back in.  The part I was wondering about is after the beer warms up and loses CO2, when it cools back to its original temperature does it pick up the same amount of CO2 that it lost, or is it a lower amount because the beer was originally sitting in a supersaturated state?  Since the CO2 partial pressure is the same before and after in this example, I suppose Henry's Law says that it would eventually take up the same amount of CO2.

I guess another way of looking at it is, say I have two beers that have just finished fermenting.  One I leave undisturbed and the other I use a wine whip and degas as much CO2 out of it that I can.  If I let them both sit under the same amount of CO2 headspace pressure, do they eventually end up with the same amount of dissolved CO2, or does the undisturbed one hold more because it started from a supersaturated state?

Offline Kaiser

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Re: Carbonation (Henry's Law) question
« Reply #5 on: March 21, 2013, 08:00:15 AM »

That is the part about Henry's Law that I'm not sure about.  Since my chest freezer is holding CO2 in, let's assume that when the beer cools back down and when it sucks gas back through the airlock that it is all CO2 coming back in.  The part I was wondering about is after the beer warms up and loses CO2, when it cools back to its original temperature does it pick up the same amount of CO2 that it lost, or is it a lower amount because the beer was originally sitting in a supersaturated state?  Since the CO2 partial pressure is the same before and after in this example, I suppose Henry's Law says that it would eventually take up the same amount of CO2.

I don’t think that your chest freezer is filled with 100% CO2. CO2 and air diffuse fairly quickly, otherwise we would not see a difference between yeast growth in an airlocked starter and one covered with foil. But there is much more CO2 in your chest freezer than there is in air.

If the headspace is 100% CO2 than a warmed up and cooled down beer will have the same CO2 level than a beer that remained at the same temp. This assumes that equilibrium was reached. How long it takes to reach that equilibrium, I don’t know. I think we are talking on the order of a few days here.

Quote
I guess another way of looking at it is, say I have two beers that have just finished fermenting.  One I leave undisturbed and the other I use a wine whip and degas as much CO2 out of it that I can.  If I let them both sit under the same amount of CO2 headspace pressure, do they eventually end up with the same amount of dissolved CO2, or does the undisturbed one hold more because it started from a supersaturated state?

A supersaturated liquid will move towards equilibrium once the production of CO2 stops. Both beers will eventually end up with the same amount of dissolved CO2

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Re: Carbonation (Henry's Law) question
« Reply #6 on: March 21, 2013, 08:06:10 AM »
I guess another way of looking at it is, say I have two beers that have just finished fermenting.  One I leave undisturbed and the other I use a wine whip and degas as much CO2 out of it that I can.  If I let them both sit under the same amount of CO2 headspace pressure, do they eventually end up with the same amount of dissolved CO2, or does the undisturbed one hold more because it started from a supersaturated state?
I don't think Henry's law would ever get you back to a supersaturated state. Also, eventually may be very long if the beer sits undisturbed. CO2 will dissolve into the surface layer, but without mixing it kind of stops. At least, this is what happens in lakes - only a few different system variables there. But the idea is the same as temperature gradients around an immersion chiller.
 
A few years back I read a thread on another forum. The person had calculated the CO2 contribution from an ounce of priming sugar and showed that the priming sugar and dissolved CO2 along were not enough to provide the volumes of CO2 predicted by those charts. The rest must come from supersaturation and/or small amounts of continued fermentation.
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Offline euge

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Re: Carbonation (Henry's Law) question
« Reply #7 on: March 21, 2013, 03:29:46 PM »
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In physics, Henry's law is one of the gas laws formulated by William Henry in 1803. It states:

    "At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid."

I think for any meaningful and immediate effects one needs to vary significantly from the standard atm in your brewery and will require a pressure vessel. Movement in an airlock IMO is not indicative of a large pressure change of any significant value. I would go from the highest temp if you let your beer warm up in a fermenter and then chilled it again, based on your scenario.
The first principle is that you must not fool yourself, and you are the easiest person to fool. -Richard P. Feynman