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 Home Brew Forums > Estimating alcohol by total weight during fermentation?

08-30-2011, 03:22 AM   #1
weaselchew
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 Estimating alcohol by total weight during fermentation?

I tried doing a search but wasn't very fruitful given some of the common words. I'm curious if anyone has tried estimating alcohol content by weighing the entire fermenter?

As a bit of an experiment, I thought I'd sit the carboy on a fairly accurate (.05 lb resolution) digital scale during fermentation. The total weight of my 5.5 gallon batch (including the 6.5 gallon carboy) was 64.9 lbs right after I popped the airlock on. 24 hours later, the total weight is now 63.5 lbs.

I should have weighed my empty carboy to get a tare weight, but this idea didn't even come to me until after I started filling it. From what I've seen, a 6.5 gallon carboy weighs about 15 lbs. I'm using 8.33 lbs as the weight of water per gallon, which isn't exact, but just as a test I figured it was close enough. The temperature of the batch is within 2 degrees of when I first weighed it, but I haven't taken that into account either. I started playing with some calculations and this is what I came up with:

((64.9 - 15) / 5.5) / 8.33 = 1.08916294 (My OG was 1.084, so this is actually pretty close)

And now the batch is at 63.5 lbs, so:

((63.5 - 15) / 5.5) / 8.33 = 1.05860526

The difference of which should mean it's roughly about 4.0% right now. Sound correct? Or have I completely lost it?

I'm still obviously going to take some actual samples to measure with my hydrometer, but I thought I'd see how close the scale would be.

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08-30-2011, 04:20 AM   #2
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Well I did just find this thread, http://www.homebrewtalk.com/f13/monitor-sg-beer-fermenting-251137/, which mentions something I didn't really think of at the time... the added weight of the multiplying yeast cells. I suppose if anything, the alcohol content should actually be slightly higher than calculated based on weight then?

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08-30-2011, 05:33 AM   #3
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Extract is lost to the production of c02, yeast biomass, and ethanol (and some other stuff in small amounts). The proportions of each are estimated by Balling.

Your scale at .05 lb resolution is sorta crap compared to a hydrometer for this sort of thing. I am not sure I understand the point.

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08-30-2011, 05:50 AM   #4
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Quote:
 Originally Posted by remilard Extract is lost to the production of c02, yeast biomass, and ethanol (and some other stuff in small amounts). The proportions of each are estimated by Balling. Your scale at .05 lb resolution is sorta crap compared to a hydrometer for this sort of thing. I am not sure I understand the point.
If you have to ask, you'll never know.
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08-30-2011, 06:20 AM   #5
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Nope

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08-30-2011, 11:19 AM   #6
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Conceptually you could do this. As fermentation proceeds the carboy will become lighter as CO2 is lost. Balling found that each 2.0665 grams of extract that is fermented is converted to 1 gram of alcohol, 0.9565 grams of CO2 and 0.11 grams of yeast. Thus for each 0.9565 gram loss in carboy weight you could assume you had 1 gram of alcohol. Subtract 0.11 grams of yeast weight for each 0.9565 gram of weight loss from the total net weight of the carboy and you have the net weight of the beer. Divide into the weight of the alcohol and you have an estimate of ABW. This is simply converted to ABV by multiplying by the specific gravity of the beer and dividing by the specific gravity of alcohol (0.0791). Thus you also need to know the volume of beer in the carboy in order to calculate the specific gravity. The volume estimate will be off because of the volume of the yeast cells, of course, and the other problem is that all the weight loss isn't from CO2 but includes the water vapor the CO2 sweeps out with it.

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08-30-2011, 01:40 PM   #7
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Quote:
 Originally Posted by remilard Extract is lost to the production of c02, yeast biomass, and ethanol (and some other stuff in small amounts). The proportions of each are estimated by Balling. Your scale at .05 lb resolution is sorta crap compared to a hydrometer for this sort of thing. I am not sure I understand the point.
While the scale I used is accurate for most things (accurate to 8 tenths of an ounce), it's certainly not "perfect" for this. My scale that is accurate to 1 tenth of a gram is only good to a little over 1/4 lb, so clearly that won't work. It really was just an experiment... I should be able to "guesstimate" what % it is and also when it stops. Granted you can tell a bit just by looking at the activity. I already had the scale, so it's not like I paid anything to run the test. I'm still going to do hydrometer readings.

Quote:
 Originally Posted by ajdelange Conceptually you could do this. As fermentation proceeds the carboy will become lighter as CO2 is lost. Balling found that each 2.0665 grams of extract that is fermented is converted to 1 gram of alcohol, 0.9565 grams of CO2 and 0.11 grams of yeast. Thus for each 0.9565 gram loss in carboy weight you could assume you had 1 gram of alcohol. Subtract 0.11 grams of yeast weight for each 0.9565 gram of weight loss from the total net weight of the carboy and you have the net weight of the beer. Divide into the weight of the alcohol and you have an estimate of ABW. This is simply converted to ABV by multiplying by the specific gravity of the beer and dividing by the specific gravity of alcohol (0.0791). Thus you also need to know the volume of beer in the carboy in order to calculate the specific gravity. The volume estimate will be off because of the volume of the yeast cells, of course, and the other problem is that all the weight loss isn't from CO2 but includes the water vapor the CO2 sweeps out with it.
Thanks for the info, that gives me a bit more insight.
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08-31-2011, 12:19 PM   #8
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Quote:
 Originally Posted by weaselchew Well I did just find this thread, http://www.homebrewtalk.com/f13/monitor-sg-beer-fermenting-251137/, which mentions something I didn't really think of at the time... the added weight of the multiplying yeast cells. I suppose if anything, the alcohol content should actually be slightly higher than calculated based on weight then?
Uh, last time I checked, yeast cells can not break the second law of thermodynamics. The yeast can only multiply with stuff that's in the wort, and any added oxygen.
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08-31-2011, 02:08 PM   #9
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Chemical reactions take place within the yeast cells producing biomass, alcohol and CO2. These reactions all follow the second law - they continue until the entropy of the system is maximized i.e. until all the nutrients are consumed. There is heat flow from the yeast cells into the surrounding beer (and subsequently into the air surrounding the fermentor). Thus the entropy of the surroundings increases and, as the entropy of the system (chemicals in the cells) also increased the entropy of the universe increased. The second law is satisfied.

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09-01-2011, 02:04 PM   #10
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Quote:
 Originally Posted by ajdelange Chemical reactions take place within the yeast cells producing biomass, alcohol and CO2. These reactions all follow the second law - they continue until the entropy of the system is maximized i.e. until all the nutrients are consumed. There is heat flow from the yeast cells into the surrounding beer (and subsequently into the air surrounding the fermentor). Thus the entropy of the surroundings increases and, as the entropy of the system (chemicals in the cells) also increased the entropy of the universe increased. The second law is satisfied.
Yes, but they cannot "add" any weight to the wort.

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