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Calculating ABV

Discussion in 'Homebrewing' started by tngolfer, Mar 12, 2012.

  1. tngolfer

    tngolfer Aficionado (200) Tennessee Feb 16, 2012

    I'm still learning the basics through extract kits and wanted to experiment a little. I wanted to try eliminating the secondary this time and I ended up coming up short on volume. I racked from my brew pot to keep a lot of the trub out of the primary so I had a little loss of volume there but my OG was in my expected range (1.052). I didn't rack to secondary so no loss there. The batch I just bottled this weekend yielded 46 bottles instead of the 51-53 I was expecting. It works out to 4.65 gallons or something with a FG of 1.020. My OG was in the primary for 13 days. I assume the 131.25 in the (OG-FG)x131.25 equation is based on a volume of 5 gallons.

    Q1: Is there a way to adjust the multiplier to give me the correct ABV for my smaller batch?

    Q2: Did I not enough yeast to get me to my expected FG of 1.012? (Krausen was good but it was done off-gassing by day 6.) The kit came with dehydrated yeast so it could have easily been poor preparation.

    Q3: Does racking to the secondary agitate everything just enough to liven the yeast up again?
     
  2. dublthink

    dublthink Initiate (0) Feb 22, 2012

  3. billandsuz

    billandsuz Savant (390) New York Sep 1, 2004

    OG, FG and the ABV calulation are not dependent on the volume of beer being brewed. if you have a target, and you used more or less fermentables than planned, then your readings will differ from the estimated target.
    but it does not matter, as you would take a post boil OG, and then your post ferment FG. calculate ABV.

    the only way to be sure if you reached FG is to check your gravity. when it stops falling its done. relying on the amount of CO2 being produced will eventually give you bottle bombs. almost gauranteed. what if your airlock has a leak, for instance?

    yes, it is possible your yeast crapped out. depends on the yeast and the size of the initial colony. and initial O2. and nutrients available. most likely it is done. don't sweat it too much, as even the experienced brewers can get unusual yeast activity for no apparent reason.

    yes, racking is a good way to wake up the yeast. but don't introduce oxygen at this stage. gentle rocking, gentle, will get the yeast back into suspension. you can also add some additional dry yeast. but it is likely the yeast are done. no problem.
    Cheers.
     
    tngolfer likes this.
  4. Q1: Is there a way to adjust the multiplier to give me the correct ABV for my smaller batch?

    ABV is solely a function of OG and FG. There is no need to ‘correct’ for batch size.

    Q2: Did I not enough yeast to get me to my expected FG of 1.012? (Krausen was good but it was done off-gassing by day 6.) The kit came with dehydrated yeast so it could have easily been poor preparation.

    The FG you achieve is a function a number of factors:

    · How fermentable is the extract you are using?

    · The particular yeast strain you use

    · Did you have a healthy fermentation (did you pitch an adequate amount of healthy yeast cell?)?

    I would be willing to guess that the extract you used was not extremely fermentable so you obtained a ‘higher’ FG value. I am making this guess since it sounds to me like you had a healthy fermentation: good Krausen and fermentation complete (as indicated by bubbling) in 6 days.

    Q3: Does racking to the secondary agitate everything just enough to liven the yeast up again?

    You really shouldn’t need a transfer to a secondary to ‘liven the yeast up again’. The yeast should be capable of completing fermentation just using primary fermentation.

    Cheers!
     
  5.  
  6. As a rule of thumb, unless your attenuation strays far from 70 to 75% the ABV can be read from the OG. Simply put, if the OG is 1.055 the beer will ferment out to 5.5%
    Small errors in the ABV don't matter very much-even commercial breweries are allowed significant leeway when declaring ABV anyway.
     
  7. telejunkie

    telejunkie Savant (340) Vermont Sep 14, 2007

    I'll be the stickler that says your oversimplifying to the point of being wrong. Change the wording to Original Sugar content and Final Sugar content and you would be correct. Problem with gravity is that it reads a lot everything in solution such as ethanol, not just sugar levels.
    Stickler out...
     
  8. VikeMan

    VikeMan Advocate (740) Pennsylvania Jul 12, 2009

    And the standard ABV formulae (all of which are estimates) account for this (approximately).
     
  9. telejunkie

    telejunkie Savant (340) Vermont Sep 14, 2007

    this is true but i took issue with the use of the word solely. We homebrewers are just making estimates when calculating ABV utilizing gravity which is just an estimate of sugar levels. The higher the alcohol level though, the worse the estimate gets...
     
  10. VikeMan

    VikeMan Advocate (740) Pennsylvania Jul 12, 2009

    There are alternate formulae to use with higher ABV beers, but yeah, it's an estimate no matter how you do it.
     
  11. It's curious that the formula for calculating SG from Brix deliberately returns these known incorrect values. Since both starting and ending Brix are known (they're both factors in the formula) we should be able to calculate sugar content exactly and get a real ABV number without bothering with SG.

    ...as if it's that important to begin with :rolleyes:
     
  12. VikeMan

    VikeMan Advocate (740) Pennsylvania Jul 12, 2009

    Unfortunately, the presence of alcohol corrupts the brix reading, which is why the refractometer calculators have to estimate the final sugar content (or FG), and the Original Brix (or OG) reading must be 'known' in order to do this. Even worse, the SG 'equivalent scale' on the right side of many refractometer displays is not accurate for beer wort (even for OG readings), because the refractometers we use are calibrated for sucrose. (Other sugars refract differently.) So rather than potentially solving the problem, refractometers arguably make it worse. But I'm not giving mine up. I like this calculator the best...

    http://seanterrill.com/2011/04/07/refractometer-fg-results/

    I agree that there should be a simpler ABV calculation from Brix Readings (without OG/FG conversions) though. Someone should come up with that in their spare time.
     
    HerbMeowing likes this.
  13. But that's exactly my point. We do know these numbers (the sucrose issue notwithstanding), so we should be able to do a reasonably accurate and consistent calculation, regardless of the size of the beer. (OG - FG) * 131 SG the way we all do is wrong simply because the sugar content is a function of both FG and alcohol. IOW, FG, by itself, tells us nothing about residual sugar. The 131 in the formula accounts for this, but it's only accurate at some arbitrary point. The error grows with the gravity of the beer. I suspect there's a useful formula for calculating ABV from Brix. I'll see what I can find.
     
  14. VikeMan

    VikeMan Advocate (740) Pennsylvania Jul 12, 2009

    I agree with most of what you're saying, but any refractometer reading taken in the presence of alcohol is inaccurate, and so we don't really know the final sugar content...it's an estimate based on formulas that are probably no more inherently accurate than the OG/FG ABV calcs...in fact there are multiple formulas for this final sugar estimate, depending on the assumed basic fermentability of the wort. IOW it's just a different set of (type of) estimates. But again, I agree that once you have estimated those final sugars, it should be possible to calculate an estimated ABV without having to go through the conversion to FG (and applying the OG/FG calc) again.
     
  15. (Damn! I hate it when people force me to do research). Given the starting Brix, ending Brix, and maybe the measured FG, the residual sugar should be determinable with some degree of accuracy. Once we have that, it should be a piece of cake to accurately calculate ABV.

    ...as if it matters ;)
     

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