I've been thinking about ways to measure the progress of the primary fermentation, looking for an alternative to taking samples and measuring the gravity.

I was wondering if anyone has tried to predict the amount of alcohol produced by capturing the CO2 and measuring its volume?

It seems like this should be simple to achieve, you'd just need to run a tube from the primary into a graduated cylinder/bottle submerged in water. As the CO2 enters the bottle, water is forced out, and you can measure the volume easily. It should be possible to calculate the amount of alcohol that has been produced given the volume of CO2 that's been released.

Anyone tried this?

4 Answers 4


I doubt anyone has tried this on a normal home brew scale because your "graduated cylinder/bottle" would need to hold 100 or 200 gallons of CO2 to capture all the CO2 for a batch. Even if you found a way to record the volume and reset, you still would need quite a large vessel during the most active stages of fermentation. You would also have the problem of leaks (standard homebrew equipment is not PSI tested, but it usually doesn't matter since there is postive pressure inside during active fermentation and you only care to keep air out). People have tried the "flow meter" approach (counting bubbles and such). But we have solved this problem in a much more efficient way. It's called taking an original gravity reading and a final gravity reading.

  • 2
    Plus, I don't see how this is easier than just taking a gravity reading!
    – Denny Conn
    Jun 10, 2012 at 16:27
  • I'm thinking about methods that are more easily automatable than a gravity reading from a sample. Refractometers look expensive. The flow meter sounds interesting though, do you have any links to writeups of that?
    – Symmetric
    Jun 11, 2012 at 17:09
  • What about using thermal readings? If you could get an accurate read on the internal fermenter temperature you could track fermentation by the fact that fermentation gives off heat. So as the fermentation subsides the temperature should return to ambient (or something closer to ambient). Would only require a thermometer that can be read on the outside of the fermenter, and an accurate measurement of ambient temperature. Jun 11, 2012 at 20:47
  • Sym, Is $22.49 expensive? ebay.com/itm/… Considering how much beer you'd waste floating a hydrometer (presuming, like most brewers, you don't risk contamination by returning the beer to the fermenter), $20 is darn cheap.
    – Dale
    Jun 12, 2012 at 1:36
  • Oh, here's something for your other question: sparkfun.com/tutorials/131
    – Dale
    Jun 12, 2012 at 1:37

For a standard 5gal(18.9 litre) carboy fermented to 12% alcohol content by weight (not volume, 14.5% by volume) approx 1100 litres (264 gallons) of CO2 at 1 atm, 68F. But if you knew that, you would also know that 2.268 kg of ethanol had been produced allowing the simple math to calculate % alcohol by either weight or volume.

The prediction, assuming 100% conversion, is that each molecule of sugar produces 2 molecules of ethanol and 2 molecules of CO2. Using atomic weights 0.51 x weight of the sugar tells you how much ethanol you can produce. That ethanol weight divided by the ethanol weight + the weight of the water used will give you % ethanol by weight, you would then use the density to calculate % alcohol by volume.

One could use CO2 production by unit of time (measure how long it takes to collect a liter of CO2) would be indication of how active fermentation was, but it would not indicate (assuming a constant temperature and pressure) whether it was because you were a) running out of sugar for the yeast to consume, b) yeast was dieing off from alcohol concentration, c) yeast was dieing because of infection, d) yeast was dieing because of pH. Though over several batches of recording these rates, taking SG readings, pH readings, do yeast counts you would be able to use the rate of CO2 generation as an indication of % alcohol as well as an indication of whether your pitch rates were consistent.


The simplest way I can think of is to use gas laws to calculate the quantity of CO2 produced based on pressure. It might get tricky since the greater the pressure gets, the more CO2 is dissolved in the beer, but you'd have to do that anyway.

Using a closed system and measuring pressure you'll learn a couple things:
1. How much sugar has been turned into CO2 and alcohol
2. How much pressure your fermenting equipment can handle

  • Care to explain the downvote?
    – MStodd
    Jun 25, 2012 at 16:20
  • It was me, by accident, either that or my kids on the laptop when my back was turned. I made a empty edit so I could remove the downvote. Sorry for the trouble.
    – mdma
    Jun 25, 2012 at 16:45
  • 1
    I just read the answer, I don't think this is feasible or useful. You'd need a huge vessel to capture all the co2 at a pressure that brewing equipment can tolerate. We don't need to know how much pressure the equipment can handle - for most fermentation equipment it's low, around 5 psi. An approach that doesn't need to retain all the co2 to maintain a tally would be more feasible and not require any pressure, e.g. a flowmeter.
    – mdma
    Jun 25, 2012 at 16:52

If you are annoy by the process of taking the sample of your beer before taking it gravity, why do you not just put your hydrometer directly into your fermenter? The reading will be a little more difficult to do and you have to be really carefull because you don't want to break it while it's in your beer. But you can keep it there and take your reading everyday.

  • 1
    I don't think this would work. The krausen would coat the hydrometer, making it difficult or impossible to read the markings. Jun 26, 2012 at 22:37
  • Yes it is hard to read the hydrometer when there is a lot of krausen. But usually, you don't need to read it in this situation, because you know that your beer is still fermenting. The most important readings are at the start and at the end, when there is no krausen.
    – NLemay
    Jun 27, 2012 at 4:54
  • But krausen crud would stick to the hydrometer, same as it sticks to the side of the fermenter, making it difficult-to-impossible to read without taking it out and cleaning it, thus defeating the purpose. Dec 13, 2012 at 7:10
  • I would also think that the krausen, though probably lightweight, would also throw off the accuracy of the hydrometer after it coats it.
    – jsmith
    Dec 13, 2012 at 15:55

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