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Old 03-08-2012, 08:53 PM   #1
HappyWino
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Default Calculating alcohol after the fact

This is a re-post of a question I asked in the Brew Science forum before I realized that it was a sub-forum of "Beer". Hopefully posting here will get some more wine\cider related feedback as that is what I am making!

I have long wished (as have many others I am sure) that there were an easy way to calculate actual ABV of a finished product but always believed it required more complex lab equipment than I am willing to get involved in.

Recently a friend came across a couple of different sites offering a calculation that should yield ABV based off of Hydrometer and Refractometer readings of the finished brew. VinoCalc has one here

It seems too good to be true, does anybody understand the science behind this enough to endorse it? Are there any caveats such as only working for liquids with little to no residual sugars?

Ultimately I would like to use on off-dry ciders as well as dry wines.

Thanks for any insight.

Cheers

HW

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Old 03-08-2012, 08:56 PM   #2
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This is very frequently used. The basic gist is that a hydrometer measures dissolved sugars. So if you measure the change in dissolved sugars (initial reading vs final reading) during fermentation, you know the amount of sugars the yeast converted to alcohol, and can estimate ABV.

EDIT: Perhaps I misread your question - are you asking if there's a way to tell solely from the finished brew...i.e., without an initial reading, by comparing the hydro to refract readings? If so, 1) please don't take my first answer as condescending, and 2) yes, this method seems reliable to me, though I've never used it. I'd be concerned that the margin of error would be too wide for my liking without precise measurements.

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Old 03-08-2012, 09:02 PM   #3
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With a Hydrometer you don't even need to calculate anything. Take an Original Gravity, take a Final Gravity subtract the FG from the OG, and then look at a chart to see what the ABV is. It is a piece of cake and the way most of us do it. With what you are wanting, it is even easier because you want dry wines and cider. So all you will need is the OG no FG (taking an FG is still a good idea to insure the brew is truly dry) the OG will give you the Potential Alcohol, and as long as it is dry the PA will become ABV. nothing to it. I love using my Refractometer for taking OGs due to the small sample size and the fact that solids in the liquid do not effect it, but it doesn't work as well once alcohol has been produced because the alcohol throws off the reading.

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Old 03-08-2012, 09:56 PM   #4
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The short answer is (OG-FG) x 131= ABV%

For example, if your OG is 1.100, and your FG is .990, the ABV is roughly 14.4%

(1.100-.990) x 131
(.11) x 131= 14.4

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Old 03-08-2012, 11:16 PM   #5
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OP - can you clarify what you're asking?

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Old 03-08-2012, 11:35 PM   #6
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I'm not sure about the OP, but last time I made a wine (plum wine) - I wondered something similar. I crushed many pounds of plums and added sugar on top of that, but I have no idea how much sugar I was getting from the plums because a hydrometer reading would only show how much sugar was in solution - not the sugars that were still within the plum pulp and juice contained therein.

For the OP, you can take a refractometer reading and a hydrometer reading at the end and you should be able to calculate the original gravity/abv. I'm no expert, but my understanding is that the reason for this is that the hydrometer reading isn't affected by alcohol in solution, but the refractometer's reading is. Having both numbers allows us to calculate how much alcohol is in solution.

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Old 03-09-2012, 12:10 PM   #7
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A hydrometer doesn't only measure "dissolved sugars", it measures the specific gravity of a solution. That can be affected by sugars, pulps, yeast, shoe dirt, cat hair...anything that makes the liquid thicker. We (the brewing community) use the generic ("sugar") version to give us an estimate of potential alcohol.

If it ONLY measured sugar, the SG would never go below 1.000. It is also measuring how much of a thinner liquid is mixed in to the water (in our case, alcohol). Pure alcohol measures .787, by the way.

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Old 03-09-2012, 02:15 PM   #8
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Quote:
Originally Posted by DoctorCAD View Post
A hydrometer doesn't only measure "dissolved sugars", it measures the specific gravity of a solution. That can be affected by sugars, pulps, yeast, shoe dirt, cat hair...anything that makes the liquid thicker. We (the brewing community) use the generic ("sugar") version to give us an estimate of potential alcohol.

If it ONLY measured sugar, the SG would never go below 1.000. It is also measuring how much of a thinner liquid is mixed in to the water (in our case, alcohol). Pure alcohol measures .787, by the way.

Fantastic point!!!!!!!! That is why a refractometer is also a nice tool to have. Before any alcohol has been created a refractometer is still slightly vulnerable to things throwing off a hydrometer but not by much, so it will give you a much closer OG. After the alcohol has been produced a refractometer's readings are no longer accurate due to the alcohol so a hydrometer is better for FG.
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Old 03-09-2012, 05:17 PM   #9
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Quote:
Originally Posted by Daze View Post
After the alcohol has been produced a refractometer's readings are no longer accurate due to the alcohol so a hydrometer is better for FG.
Totally disagree here. The refractometer can't be read directly (can't simply multiple brix*4 as you can pre-ferment) and you need to know the OG, but I almost exclusively use refractometer for post fermentation readings and FG. I periodically double check my readings against a hydrometer and it has yet to be anything but spot on accurate.
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Old 03-09-2012, 06:29 PM   #10
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Quote:
Originally Posted by bruin_ale View Post
The refractometer can't be read directly (can't simply multiple brix*4 as you can pre-ferment) and you need to know the OG, but I almost exclusively use refractometer for post fermentation readings and FG.
Agreed. Which is exactly why, as you said, you can infer the ABV using the difference between the refract & hydro readings.
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