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Home Brew Forums > Home Brewing Beer > Brew Science > Calculating og and abv with hydrometer and refractometer
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Old 09-14-2012, 06:47 AM   #1
Travis0055
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Default Calculating og and abv with hydrometer and refractometer

So I failed to measure my beer untill after fermentation. I figure it's time to break out the refractometer to test the formulas on brewheads.com

www.brewheads.com/refract-getog.php
www.brewheads.com/refract-getabw.php

Using my tools on a control, I accurately (within about .1 %) measured a canned beer, so far so good.

This beer in question was 75 # of light dry dme and 3 # of piloncillo in an undetermined volume of beer. To ferment in a 60 wine barrel, I attempted to eyeball the final volume of 45 gallons. Any way, something isn't adding up...

My measurements were 8.6 Plato and 1.008 Sg. The above calculators indicated a OG of 1.076 and abv at nearly 10%.

That would put my attenuation over 90%! I fermented with a powdery yeast at above 80 F, ambient temp. That explains high attenuation but 90% seems far too high when the software I'm using suggests I should have expected 76%.
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Old 09-14-2012, 10:39 PM   #2
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Refractometer readings on fermented beer are notoriously inaccurate. Refractometery can be used to measure the alcohol content of beer provided that a calibration curve has been built first using other means (GC, dilstillation and densitometry etc.). Obviously this is of little value to a home brewer but it can be of value to a commercial operation who wants to check that gyle # 147 of Old Ovary is up to the specs for that label.
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Old 09-14-2012, 11:54 PM   #3
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And if your eyeball saw 46 or 48 gallons the numbers would be different.
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Old 09-15-2012, 07:46 AM   #4
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The dme I used was 75% fermentable, I guess I can try to work backwards from that. As for the final volume, it was very rough. It could have been as little as 35 gallons I suppose. Is there a general trend with how alcohol affects a refractometer? I would like to make a reasonable guess at the abv of this beer.
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Old 09-15-2012, 08:05 AM   #5
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Not really, just hopefully enjoy the results. Take more accurate readings next time. Volumes are more important than a measuring device.
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Old 09-15-2012, 09:15 AM   #6
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I should have just taken a gravity reading at the get go but it was a marathon brew day. 4, 4 gallon batches and 2, 5 gallon batches topped up with water. 100% pilsner light dry.
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Old 09-15-2012, 12:57 PM   #7
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You could use the rule of thumb that 2.0665 grams of extract produces 1 gram of alcohol and assume that the yeast did what the are rated to do, eat 75% (?) of the sugar you gave them and estimate how many grams of alcohol were produced. Now if you can even wag the beer volume and determine its density (with a hydrometer) you can calculate the total mass of beer and, with your estimate of the alcohol produced, the ABW. The ABV is ABW/0.79 i.e. about 25% higher.
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Old 09-18-2012, 04:43 AM   #8
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Just send a sample to a lab, you'll get exact results and it isn't very expensive...I send my wine out all the time for various tests.
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Old 09-18-2012, 04:54 AM   #9
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Recommended labs?
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