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Measuring Alcohol Content

  1. Mar 24, 2011 #1
    Hello all,

    I'm a bit new to to the forum, but from the posts I've read, I can say that this is a fantastic site. I have a fish tank with some live plants in it, and I add CO2 to the water by putting some sugar, water and yeast in a two liter bottle and pumping the gas into the water. Worked great for the plants.

    However, I am curious how I would go about measuring the amount of alcohol that the yeast produces by measuring the density of the mixture before I start a batch and after. I initially had thought of something like this:

    1000(1 - x) + 789(x) = Density of mixture after some time

    where the 1000 represents the density of water in kg/m^3, and the 789 for the ethanol and x is the fraction of ethanol. You can noticeably observe the level of fluid in the two liter bottle rise slightly over the course of a few days, and so I would imagine that the density goes down after some fermentation has occured. Is this a solid approach to use? How, if necessary should I account for the CO2 that leaves the mixture?

    Any thoughts would be greatly appreciated.

    Last edited: Mar 24, 2011
  2. jcsd
  3. Mar 24, 2011 #2


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    Hi Drew. Welcome to the board.
    Have you looked into how alcohol concentration is measured in beer? It's basically a float (called a hydrometer) you drop in that measures fluid density. Sounds like it's just what you need to measure the alcohol in your batch. You can find them at your local beer and wine making store.
  4. Mar 28, 2011 #3
    I have, but I'm not sure I understand the math behind it. Are they taking the difference in densities divided by the density of ethanol to determine the volume created? Do you know where I could go to find some sort of explaination for the math; I've seen some formulas, but haven't found any explanation. Thanks!
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