Measuring Alcohol Content: How Do I Calculate It?

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The discussion revolves around measuring alcohol production in a homemade CO2 fermentation setup for a fish tank. The user, Drew, shares their method of generating CO2 using sugar, water, and yeast in a two-liter bottle and expresses interest in quantifying the alcohol produced by measuring the density of the mixture before and after fermentation. Drew proposes a mathematical approach to calculate the density changes, considering the densities of water and ethanol, but seeks clarification on the accuracy of this method and how to account for CO2 loss. Another forum member suggests using a hydrometer, a tool commonly used in brewing, to measure fluid density and determine alcohol concentration. Drew expresses uncertainty about the mathematical principles behind hydrometer readings and seeks further resources for understanding the calculations involved in measuring alcohol content.
Drew Sandlin
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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.

Drew
 
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Hi Drew. Welcome to the board.
Drew Sandlin said:
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.
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.
 
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 explanation for the math; I've seen some formulas, but haven't found any explanation. Thanks!
 
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