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Old 02-28-2012, 05:38 PM   #1
HappyWino
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Default Alcohol from Hydrometer & Refractometer?

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 02-28-2012, 06:31 PM   #2
ajdelange
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Assuming we are talking beer here: Refractometry does work and can be used in operations where the same beer is brewed time and time again. But the refractometer must be calibrated using one of the more absolute methods. Thus refractometry is great for telling you that this batch is 0.2% higher than that batch of Old Overshoe but not so great for telling you what the ABV of the first batch of Old Overshoe is. You would send a bottle of OO off to the lab to get the ABV measured and when you got the number build the ABV vs RI curve. ASBC has a Method of Analysis (MOA) based on this.

To measure beer accurately one must separate the alcohol from the beer by distillation then measure the density of the distillate. The former step requires some familiar lab gear (mantle, Kjeldahl adapter, condenser, receiver, volumetric flask) for the distillation part. Total investment of a couple of hundred $. It's in measuring the density of the distillate that you jeopardize your children's college educations. Tralle hydrometers don't work because the density of an alcohol/water solution at the strength of typical beer is so close to 1 that he marks on the instrument are too close together to read accurately. The way to go is a digital density meter. Put the sample in, push the button, read the ABV (or ABW, OIML or AOAC...) but these are out of the question even for fairly large breweries. An appreciably less expensive option is gas chromatography.

An affordable option is the simple pycnometer. Equipped with one of those and good analytical balance you can measure the density of the distillate and calculate the ABV. But this requires a good balance and while that is less expensive than a GC or digital density meter it is still a pretty expensive proposition.

For dry wines and meads ebuliometry and surface tension (Vinometer) are workable and for wines, meads and higher alcohol beers a Tralle hydrometer can be used.

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Old 02-29-2012, 05:26 PM   #3
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@ajdelange - so are you saying that the method VinoCalc (and others) offer just plain doesn't work at all?

Cheers

HW

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Old 02-29-2012, 05:35 PM   #4
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As I've never tested them I can't say but I do know what conditions ASBC recommends RI under (#2) and I do know that other algorithms I have tested work some or even a lot of the time but are sometimes way off. The problem is how do you know whether your beer is one of those 'sometimes'. What I really don't know is whether trying to use RI and apparent extract works better than apparent extract and original extract. There are certainly limitations to the latter too.

I've hoped for years to be able to gin up something for home brewers and pycnometry for the guy who has a good balance and is willing to invest a couple hundred more in the distillation gear and the pycnometer is the best I have been able to come up with.

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Old 02-29-2012, 06:41 PM   #5
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I've compared the refract calc to straight hydrometer readings on two batches of beer so far. And have found they differ by more than 1.5% Abv. I dont trust the refrac calc but will continue comparing the two

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