Hydrometer vs Software ABV discrepancy.

Discussion in 'Beginners Brewing Forum' started by Wobbly Pop, Apr 15, 2018.

  1. Wobbly Pop

    Wobbly Pop New Member

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    So I use the brewers software and plug in 4kg of grain.

    OG= 1.045 and FG= 1.011 which make 4.38% ABV

    But when I look at those numbers on my hydrometer sheet, they read

    OG= 1.045 (5.52 potential alcohol) and FG= 1.011 (.35 potential approx) making it 5.17% ABV?!

    So not sure why the software reads different from my hydrometer table.
    I tend to believe my hydrometer as my friend and I both agree after a couple of beers if there isnt more
    alcohol than what my hydrometer is even telling me.
    I do add .3% for my bottle conditioning just as a margin for error.

    Anything I should be considering to feel more confident in my readings?
     
  2. Nosybear

    Nosybear Well-Known Member

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    Same inputs, different results, means different algorithm. My guess is one is the "simple" algorithm (OG-FG)*131.25 and the other the "complex" algorithm ((76.08 * (og-fg) / (1.775-og)) * (fg / 0.794)). Here's what matters: To three decimal places, you got the same reading with both instruments. The error, then, isn't the instruments, but the conversion between the readings and ABV.
     
  3. Yooper

    Yooper Administrator
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    It looks like to get "your" ABV estimate, you are just subtracting potential alcohol readings? The thing is, with a FG of 1.011 for beer, .35% PA isn't realistic.

    SG and PA rely on the hydrometer, which measures density in a solution. Whether the sugar is fermentable or not doesn't matter- it just measures the density. Since alcohol is lighter than water, the PA reading is no longer valid once alcohol is in solution, so a simple potential alcohol subtraction like that doesn't work.
     
  4. Wobbly Pop

    Wobbly Pop New Member

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    Well, my table and instructions that came with my hydrometer say to do just that. There are even examples with the instructions that show it done that way. The chart spells out the PA and if I follow the chart, it says exactaly what I described. One example directly from my instruction states as follows:
    OG= 1.09 = 12.35 PA
    FG= 1.01= .1 PA
    Alcohol content is 12.25
     
  5. Yooper

    Yooper Administrator
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    That's totally wrong- it's not even possible for that to happen.
     
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  6. Hawkbox

    Hawkbox Well-Known Member

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    I've honestly never seen math like that.
     
  7. Wobbly Pop

    Wobbly Pop New Member

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    Here.. pic is worth a thousand words. Maybe my head is on backwards, but this is a common hydrometer sold in every beer and wine making store in my city.
     

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  8. Nosybear

    Nosybear Well-Known Member

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    Formula is (OG-FG)*131.25. The result for the numbers you gave is 10.25%. It's easier to think in Plato, where the number represents the percentage of the solution that is sucrose (sugar). A 1.09 solution is 21.6 Plato (or Brix, they're the same to three decimal places), or 21.6% sugar by weight. What happens when you start fermenting, you start making alcohol, which is lighter by volume than water. So if your final gravity is 1.01 or 2.6 Plato, that doesn't mean you've fermented 19% of the solution by weight of sugar, the alcohol is making your gravity appear to be less than it would be if there were no alcohol in the solution. There's a calculator on this site that does well at calculating the ABV using both the equation above and a more complicated equation that takes the alcohol in solution into account better at higher gravities. There should be a PA scale on your refractometer - that's corrected for gravity so if you started with a PA of x and ended up with a PA of y, the ABV is x-y. Perhaps that's your confusion - right formula, wrong scale?
     
  9. Wobbly Pop

    Wobbly Pop New Member

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    Hmmmm, did some googling and ya, based on that formula you gave my beers have actually been coming out 1%ish weaker than I had thought. But the strange part is that I aim for 5%-5.5%, and after I have 3-4 of em I wonder if its more like 6.5%. Which is why I started to look into my ABV results. I was thinking they were coming out stronger than my readings...not weaker. Haha. My friend who I always give some to was the first one to mention how they seemed stronger than what I was telling him, and I agreed with him. But based on this formula you gave, all those beers have been more in the ballpark of 4.5%ish. I just don't know what to think anymore.... But at least it tastes like beer and it does the job right? Also that Example 1 on the first pic simply shows subtraction.?!
     
  10. Nosybear

    Nosybear Well-Known Member

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    I can't explain the directions with your hydrometer - maybe a bad translation from the language of origin? Anyway, as you said, it tastes like beer. I'm a stickler for detail and for knowing why I get the results I get so I may answer a bit too technically at times - use the calculator on this site. It does all that conversion for you.
     
  11. Wobbly Pop

    Wobbly Pop New Member

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    I appreciate the technical side of the craft so by all means.. I take detailed notes and strive toward being able to replicate a beer I love as a professional baker would. If I could just stop experimenting with ingredients each time I could work on perfecting one recipe. haha. But yeah, the only thing I have not been confident in, is accurately knowing my ABV. And it drives me nuts to not know everything down to the finest detail.
     
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  12. Mase

    Mase Well-Known Member

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    your hydrometer and the ABV calculator on this site will be your best friend.
     
  13. jeffpn

    jeffpn Well-Known Member

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    That’s why they say RDWHAHB!
     
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  14. Hawkbox

    Hawkbox Well-Known Member

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    It helps a lot if you forget to take a reading a couple times because you were verifying your existing product while brewing.

    Makes you realize it's not the end of the world. ;)
     
  15. Wobbly Pop

    Wobbly Pop New Member

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    Thats definitely happened before. And ya, still tasty.
     
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