Potential alcohol on hydrometer

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Professor Duck
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Potential alcohol on hydrometer

Post by Professor Duck » Sun Jan 30, 2005 10:41 pm

My dad gave me his old hydrometer which has a potential alcohol scale on it. 0% potential alcohol corresponds to 1.000 SG. There's something about is which confuses me though:

It says on the hydrometer that to work out the alcohol content, you take a potential alcohol reading (A) before it starts fermenting and then take another one (B) when it's finished, or whenever you want to know the %abv, then subtract B from A and you get the %abv. I don't really understand this, and here's an example to explain why:

Say you put in enough sugar to ferment to 20% and so the potential alcohol reading is 20%. If you subtract the end reading from the start reading, then when it's fermented to 20% you would subtract 0% off the scale to get the %abv of 20%. Since 0% potential alcohol on the scale corresponds to 1.000 SG, this implies that when all the sugar has fermented the SG is 1.000. But how can that be, when the SG of ethanol is less than 1.000?

I probably could have explained this more clearly, but if you know what I'm going on about I would appreciate some clarification on the issue.
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Uncle Jesse
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alcohol

Post by Uncle Jesse » Mon Jan 31, 2005 12:04 am

potential alcohol is just that, the potential. chances are you won't get that, and ending gravity can be hard to predict/keep consistent.

the way i figure proof is to take starting gravity, subtract ending gravity, multiply the result by 131.

so, 1.060 start, 1.005 end, that's .055*131 or 7.205% ABV.
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Grayson_Stewart
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Post by Grayson_Stewart » Mon Jan 31, 2005 6:18 am

PD,

You can do it like Uncle Jesse stated or since you are already working in % potentials, just use the absolute value of the DIFFERENCE in the starting and finishing percentages.

Ex. #1
If you have an initial alcohol potential of 20% and at the end of fermentation you have a alcohol potential of 2%, you can assume you have a wash that is 18 %.

Ex. #2
If you have an initial alcohol potential of 20% and at the end of fermentation you have a alcohol potential of - 1%, you have a wash with 21% alcohol.

It's definately not exact... there could be sugars not completly disolved in the wash when you take an initial reading, temperature variances, eyeballing the miniscus on a hydrometer, etc.
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The Chemist
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Post by The Chemist » Mon Jan 31, 2005 11:50 am

Trust your Uncle Jesse! You got lost by comparing apples to oranges. The hydrometers with SG and potential alcohol are good only in fermentation, you'll need another to measure the alcohol content of a distilled product. The reason for this is that the "potential alcohol" scale is actually a measure of how much sugar is present. When you take the difference, you are really calculating how much sugar was there at the beginning, and isn't now--and assuming it all turned into alcohol. If you start with no sugar (1.000SG, 0% potential alcohol), you obviously can't ferment. If you start with 15% (w/v) sucrose (~1.060SG, ~8%PA) and ferment to completion, the SG of the ~8% ethanol, 0% sugar would be so close to 1.000 as to not be noticable (0.9889 for pure ethanol/water) given that there are other things coming from the yeast and such that will affect the density. Take-home lesson? Trust your Uncle Jesse--the hydrometer doesn't measure alcohol!
As an aside, I will also mention that if you are adding sugar to 20% PA, you're likely wasting sugar. Anything above about 13-15% ethanol kills most yeast (although I have heard of some strains of "super-yeast"), and the residual sugar will simply be lost (or worse, burn) during distillation.
Hope this clears something up. Didn't mean to ramble!!!

Professor Duck
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Post by Professor Duck » Mon Jan 31, 2005 10:31 pm

Grayson_Stewart wrote:PD,

You can do it like Uncle Jesse stated or since you are already working in % potentials, just use the absolute value of the DIFFERENCE in the starting and finishing percentages.

Ex. #1
If you have an initial alcohol potential of 20% and at the end of fermentation you have a alcohol potential of 2%, you can assume you have a wash that is 18 %.

Ex. #2
If you have an initial alcohol potential of 20% and at the end of fermentation you have a alcohol potential of - 1%, you have a wash with 21% alcohol.

It's definately not exact... there could be sugars not completly disolved in the wash when you take an initial reading, temperature variances, eyeballing the miniscus on a hydrometer, etc.
Yeah, I understand how to do it, that wasn't exactly the problem that was bothering me. It's just that, as The Chemist said, it only measures how much sugar is in there and doesn't take the ethanol into account which would, I thought, effect the reading. But...
The Chemist wrote:If you start with 15% (w/v) sucrose (~1.060SG, ~8%PA) and ferment to completion, the SG of the ~8% ethanol, 0% sugar would be so close to 1.000 as to not be noticable (0.9889 for pure ethanol/water) given that there are other things coming from the yeast and such that will affect the density.
This cleared up the problem. I didn't realise the ethanol would have such an insignificant effect. I did the maths and saw that the "difference in potential alcohol" method perfectly matches the "difference of SG, times 131" rule, which convinced me that it works fine.

Thanks for the help, chaps.

PS, I'm not actually putting in 20%abv worth of sugar, that was just an example.
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