Bubble/Point relation

Hey kids, I've been reading for a while but this is the first post. I couldn't really find a answer through the search function to this silly question.

I've been brewing a ton of high gravity beers lately and got to wondering one day.

Let's say you've got your standard 3-piece airlock. Has anyone ever devised a number correlation to how many bubbles to point of gravity ratio? Example, for every 50 bubbles = one point of gravity having come down. How big would that number be roughly? In my brew books there are quite a few math equations made for IBU's and so forth. But what about bubbles to gravity points?

It don't work like that, matey. Bubbles are a mighty poor gauge of fermentation, and there are plenty of things that cause bubbles that aren't fermentation. Plus, it's not like a bubble is a standard size. How much air moves through your fermentor every time it "burps" depends on its shape, size, and fill-level.

It is, however, possible to figure out how much CO2 volume a gravity point would equate to. A starting point would be knowing that 1 mole of glucose ferments to 2 moles of alcohol and 2 moles of carbon dioxide...There would be a moderate bit of math involved, but you could then work it back and figure out volume using Boyle's law.

I figured an exact number wouldn't be possible, but surely a rough estimate like a thumb rule could be devised, no? Especially if you're using the same sized carboys over and over filling roughly to the same level.

Would like to know more though on your statement "plenty of things that cause bubbles that aren't fermentation", I figure some could be other suspended gases in solution, but once those are gone they're gone right? What else could cause regular bubbling? One reason this sparks my curiosity is because I had this big Belgian beer once that had an OG of 1.092 and seemingly finished out at 1.025 which was fairly low out of attenuation for the fresh yeast used, and I gave it the whole shebang (big starter, nutrients, oxygenation, etc). For a whole week I had the carboy heated up and saw many bubbles come out during the week it was heated and some yeasts rise back up to the surface for a bit, but after a whole week the gravity had only dropped one point.

simonbones said:

I figured an exact number wouldn't be possible, but surely a rough estimate like a thumb rule could be devised, no? Especially if you're using the same sized carboys over and over filling roughly to the same level.

Would like to know more though on your statement "plenty of things that cause bubbles that aren't fermentation", I figure some could be other suspended gases in solution, but once those are gone they're gone right? What else could cause regular bubbling? One reason this sparks my curiosity is because I had this big Belgian beer once that had an OG of 1.092 and seemingly finished out at 1.025 which was fairly low out of attenuation for the fresh yeast used, and I gave it the whole shebang (big starter, nutrients, oxygenation, etc). For a whole week I had the carboy heated up and saw many bubbles come out during the week it was heated and some yeasts rise back up to the surface for a bit, but after a whole week the gravity had only dropped one point.

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The problem with the bubbles is that so much of it has to do with shifting gas solubility saturation points of the water in your beer. A lot of CO2 gets dissolved into your beer early into fermentation. If you raise your liquid a few degrees, that will force a significant amount of gas to come out of suspension and leave your carboy. If you open up your fermentation cabinet and see bubbles going, there's no way to know that it is fermentation rather than just slight shifts in temperature or physical agitation. That's exactly what was happening with your belgian.

Furthermore, your carboy is not airtight, no matter how much it seems like it might be. Gas escapes from the seals around your carboy stopper and where the airlock plugs into the stopper. It might be a lot or it might be a little, but it is tough to know.

Any rule of thumb would be so incredibly vulnerable to shifting little things like this that I suspect that it would require a range of 10 fold to be accurate...i.e., each point is equal to between 10,000 and 100,000 bubbles. Not particularly useful. Plus, I'm assuming that, rather than counting each of the individual bubbles, you'd count the bubbles for a minute and then multiply it back out. That would throw things off by even more, as the rate of bubbling likely isn't all that steady.

Bubbles are a good indicator of when it may be time to take a gravity sample. I look at the fermenter and if it's bubbling I don't bother with it. That's about as technical as it gets though.