Attempting to calculate %ABW & %ABV per fermentation CO2 bubble

[Silver_Is_Money]

How much alcohol does each fermentation bubble of CO2 represent? This is my initial attempt at guesstimating it:

Assumption: Airlock bubbles each contain on average 1.7 ml of CO2
Assumption: 1 mole of CO2 gas = 22.4 liters of CO2
Assumption: 1 mole of CO2 creates 1 mole of ethanol (MW = 46 grams)
Assumption: 5 gallon batch, FG = 1.010, ABW = 5%

1/22.4 = 0.044643 moles CO2 per liter of bubbles

1000/1.7 = 588.24 airlock bubbles per liter of CO2

588.24 x 22.4 = 13,177 airlock bubbles per mole of CO2, and therefore per mole of ethanol

5 gallons = 19 liters
19L x 1.010 x 1,000 = 19,190 grams (batch weight)

19,190 x 0.05 = 960 total grams of ethanol in batch for this example

960/46 = 20.87 gram/moles of ethanol for this example

20.87 x 13,177 ~= 275,000 total CO2 bubbles per batch (for 5 gal at 5% ABW)

(5% ABW)/275,000 bubbles = 0.000018182% ABW contributed by each individual CO2 bubble

0.000018182% ABW / 0.789 = 0.000023044% ABV contributed by each individual CO2 bubble

Does this seem as if I did any of it correctly? I'm aware that based upon batch temperature a certain (and fair) percentage of these 275,000 bubbles will likely remain dissolved in solution and not come out via the airlock, but if they don't come out, they are in solution, and that is telling and vital information also.

Bottom line: If you could come up with a device to count the bubbles, could you use it to determine ABV, ABW, batch completion, CO2 remaining dissolved in solution, required sugar for bottle carbonating to a given CO2 target, or any other useful measures?

Or is using bubble counting as a means of forecasting a wasted endeavor?

 

[schatzke]

I'm a little confused. Are you saying you would have to watch a fermentor until it stops bubbling? I love checking on my fermentors as much as any other hombrewer but that might be taking it a bit far. How about a hydrometer reading?

 

[Silver_Is_Money]

I've seen a couple of economical electric eye type bubble counters. They can be linked to a computer, and let the computer monitor it for you so you don't have to.

Assuming that I calculated correctly, I believe that the % alcohol contributions per each bubble should at least reasonably apply to any and all 5 gallons in the fermenter batches, not just the specific example batch.

 

[Silver_Is_Money]

Follow-up: For the case of my airlock at least it turns out that the CO2 bubbles must actually be very close to 3.4 ml in volume (or twice my original estimate), and the ABW and ABV per bubble thereby end up being twice what I originally estimated above.

 

[beermanpete]

How will you account for gas density change related to temperture change?

 

[Silver_Is_Money]

How significant is that concern? The batch I'm fermenting presently was mainly fermented at 52 degrees F.

The last time I played around with PV = nRT was a very long time ago for this old timer.

 

[dmtaylor]

The trouble is, there is not just one reaction going on. There are probably hundreds of reactions. Also you did not indicate which reaction you assume, i.e., what sugar are you starting with, what are the specific products, how does all the chemical equation math work out.

I don't think what you propose is possible in any meaningful way.

And I'm a chemical engineer.

 

[beermanpete]

How significant is that concern? The batch I'm fermenting presently was mainly fermented at 52 degrees F.

The last time I played around with PV = nRT was a very long time ago for this old timer.

For a single ferment with good temperature control it is likely trivial but the difference at a differnent fermenting temparature could be a 20 or 30 percent difference in gas density. For example, 52°F vs. 72°F would be a 28% change in gas density, assuming the CO2 behaves as an ideal gas.

 

[timdillon36]

Am I the only one that read this and thought "airlock activity is not a sign of fermentation"?

 

[jbunton]

Wow too much thought going into this one. I'd be spending my time learning, designing recipes, brewing, drinking before I care about co2 bubbles. Way too many variables for this too be of any accuracy or consistency

 

[ajdelange]

When we estimate ABW (and from it, ABV) we generally do so by observing the amount of extract consumed (via reduction in wort specific gravity) and then use Balling's observation that each 2.0665 grams of extract yields 1 gram of ethanol. But he also observed that those 2.0665 grams of extract produce 0.11 grams of yeast biomass and 0.9565 grams of CO2. Thus, if one can measure the weight of CO2 evolved and add to it the amount retained by the beer he can easily calculate the weight of the ethanol produced. It seems to me that measuring the specific gravity of the beer is much easier as measuring the gas flow accurately (correcting for partial pressure of water vapor, having to account for volume/temperature/pressure interdependencies...) is not trivial and won't get you a result better than the extract consumption derived number in any case.

 

[ajdelange]

For a single ferment with good temperature control it is likely trivial but the difference at a differnent fermenting temparature could be a 20 or 30 percent difference in gas density. For example, 52°F vs. 72°F would be a 28% change in gas density, assuming the CO2 behaves as an ideal gas.

It's more like 4%
(273.15 + (72 - 32)/1.8)/ (273.15 + (52 - 32)/1.8) = 1.03909

 

[dmtaylor]

It's more like 4%
(273.15 + (72 - 32)/1.8)/ (273.15 + (52 - 32)/1.8 = 1.03909

Yup. And since almost every other measurement in the original post is only good to 2 sig figs, and maybe not even that, a difference of 4% is almost negligible.

In reality, you'd be lucky to figure all this out to 1 sig fig, honestly. You could try it, it will give you a ballpark notice of when fermentation might be done, with ABW or ABV good to about 1 sig fig, or maybe 1.5 sig figs if there were such a thing. But it's still best to use the trusty old hydrometer and monitor over the course of 3-4 days.

Still, this is all an intriguing exercise. I became curious enough to do some more math on it to come up with what I have here. It's kind of "fun".

 

[Silver_Is_Money]

If nothing else, counting bubbles instead of taking hydrometer readings maintains the low oxygen environment. Gaining significance is the difficult part though. Really need to measure the volume of the CO2.

 

[dmtaylor]

I still have to wonder how much hydrogen sulfide, ethanol, water vapor, esters, aldehydes, and other volatiles would be present in the off-gas, besides just the CO2. If we assume CO2 comprises more than about 95% of the total off-gas, then this is a moot point. But if for some reason there is quite a lot of other gases present, then this could throw off your results by quite a bit. I'm just not sure. Data might be out there someplace. But, are the specific constituents of the off-gas highly dependent on yeast strain, yeast health, temperatures, ??? Lots of unknowns, which may or may not matter. They likely don't matter, but I don't know.

 

[ajdelange]

The vapor pressure of water at 25°C is about 24 mmHg so its mole fraction in the CO2 over fermenting beer at 1 atm would be about 24/760 i.e about 3%. It's 58 mm/Hg for ethanol at that same temperature but that's over pure ethanol and beer of nominal strength is far from being pure ethanol (whereas it is pretty close to being pure water) so ethanol vapor wouldn't be an appreciable contributor.

Yes, it is quite possible to have fractional digit accuracy. Increasing the accuracy of a device by 1 digit means errors smaller by a factor of 10. Increasing it by 2 digits means error smaller by a factor of 100. Increasing it by a factor of 1/2 digit means error smaller by a factor sqrt(10). For example, the specified accuracy of an Anton Paar digital density meter is 5 and a half digits to the right of the decimal point.

 

[crane]

I can attest that there are more problems to determining abv/abw from counting bubbles than all the theory being talked about here. When I first built my fermentation chamber, I built a bubble counter for the sake of tracking fermentation progress without having to open the chamber all the time. Counting bubbles works great for that.

One of the main problems with counting bubbles is during peak fermentation. if your yeast is very active you have a continuous stream of bubbles. The very thin layer of water in between each bubble becomes indistinguishable to the photo sensor. At all other times of fermentation you have a steady rate of bubbles per minute. During peak fermentation my BPM would jump all over the place due to this problem.

Additionally, the bubble size changes based on how aggressive the fermentation is. At peak fermentation the bubbles are larger due to the higher volume of CO2 being released. This can be observed by eye as well. During peak fermentation the bubbles are large enough to displace all of the water in the bottom U of the airlock. Towards the end of fermentation the bubbles are small enough that some water remains in the bottom of the U as the bubble passes through. The picture below is at the end of fermentation. You can see a bubble is about to pass through and it isn't going to displace the full volume of the U.

Ultimately, I was never able to correlate, bubbles to abv.

If you are going to try any of this, don't use an IR photocoupler as they can't tell the difference between water and air. You need a sensor in the visible spectrum.

View attachment 1484661118623.jpg

 

[dyqik]

The bubbles are also going to change size with the viscosity of the fluid in the airlock - starsan solution will be different to water and blowoff yeast will makes things very different. And this will change with time as water evaporates from the starsan solution, for example.

Even water left in a slightly dirty airlock for a while can get a bit scummy and more viscous.

 

[dyqik]

BTW, I'd love if something like one of these sensors could be developed for the homebrew community. It's IR absorption based, I think, probably using filters matched to alcohol and sugar spectral lines.

http://www.vitalsensorstech.com/PDF... Fermentation Monitor Data Sheet.pdf#zoom=100

 

[kbaggen]

Hi there,
ressurecting this tread a bit, as also I tried to venture down this way of CO2 bubble dection as a mean to calcualte SG!

Code/description on:
https://github.com/kbaggen/Cheapfathers-Bubble-Logger

I used a cheap adriuno microphone to measue the big "blop" comming from airlock, but even I tried to ensure tghe fermneter was complet airtight, had same amount water in airloack I never got that SG part stable!

Still useable for following the fermentaion! And it doubles as a temperature controller too!

I plan on re-starting the testing with olviven-oil in airlock and then same recipe, yeast, fermentation-temperature (change hos maybe) a then see if I can get the SG estrimation better over the winter! The idea of oil is it is heavy and somewhay might help!

https://github.com/kbaggen/Cheapfathers-Bubble-Logger