Different ABV Values

[vrstud]

Hello Forum:

My hydrometer calculation is giving me a different value than online ABV calculators.
OG = 1.090
FG = 1.032.

ABV Calculators will get me around 7.57%.

When I follow the instructions from the hydrometer it has me convert the OG and FG values into potential ABV first and then subtract the two:
1.090 = 12.2%
1.032 = 3.2%
12.2 - 3.2 = 9.0% ABV.

Is the method my hydrometer is using correct?

Thanks

Chris

 

[foolsbrew]

If it makes you feel any better, even the commercial breweries will often have wrong ABV. There is just a lot of disagreement about it and it's hard to get it exactly right. https://www.theatlantic.com/business/archive/2013/02/budweiser-watered-down-beer-lawsuit/317949/ (just the first link I found, you can find a lot of cases people saying some beer has the wrong ABV)

There is a ~15% difference between your numbers. Or, equivalent to you misreading both OG and FG by 0.005 or one of them by 0.01 -- I doubt your measurements are exactly correct, so I wouldn't worry too much. For example, you may add 0.4% ABV(give or take 0.2%) just with the priming sugar and that's generally ignored.

Most calculators use the 7.5% value. Brewtarget and some others will use an equation that gives you the 9% value. I use the 9% value, mostly because I started with them and my result tasted similar when I made a 5% beer.

I doubt you can tell a 15% increase or decrease in ABV in a bottle of beer, so I wouldn't worry too much. 7.5% and 9% looks like a huge difference at a glance, but it isn't. The difference between 4% and 5% difference is more than that, for example. It's not a big deal.

 

[HB_ATL73]

If it makes you feel any better, even the commercial breweries will often have wrong ABV. There is just a lot of disagreement about it and it's hard to get it exactly right.

Not sure if I misunderstood the article.

That article isn't about gravity reading errors. Its about them using watering down the beer to stretch their supply/sales. I would imagine they know exactly what their ABV is? and using much more advanced technology than triple scale hydrometer to determine it.

 

[foolsbrew]

Not sure if I misunderstood the article.

That article isn't about gravity reading errors. Its about them using watering down the beer to stretch their supply/sales. I would imagine they know exactly what their ABV is? and using much more advanced technology than triple scale hydrometer to determine it.

The mass-brew stuff is already watered down, so it's not like they recently added watering-down to the pipeline. My guess is someone somewhere made an error and they had a faulty batch. They don't really have a lot of to gain by watering it down more than necessary.
I'd guess AB is usually pretty careful too, but all the craft beer stuff? A lot of it has to be misleading, even if it is unintentional and harmless.
I am just saying most people can't tell the difference between small changes in ABV, and even if we did, the stuff we buy isn't going to be exactly correct.

 

[vrstud]

There is a ~15% difference between your numbers. Or, equivalent to you misreading both OG and FG by 0.005 or one of them by 0.01 -- I doubt your measurements are exactly correct, so I wouldn't worry too much. For example, you may add 0.4% ABV(give or take 0.2%) just with the priming sugar and that's generally ignored.

Agreed - the priming sugar will affect the final value and that accuracy of reading will also have an impact.

Most calculators use the 7.5% value. Brewtarget and some others will use an equation that gives you the 9% value. I use the 9% value, mostly because I started with them and my result tasted similar when I made a 5% beer.

I guess my question is why are there two different equations and why is there such a difference between them, 20% difference (1.5%ABV/7.5%ABV ) in output value.

I doubt you can tell a 15% increase or decrease in ABV in a bottle of beer, so I wouldn't worry too much. 7.5% and 9% looks like a huge difference at a glance, but it isn't. The difference between 4% and 5% difference is more than that, for example. It's not a big deal.

Agreed - I likely can't tell/taste the difference. I was trying to make a barley wine and didn't achieve the ABV I was targeting. So being only 7.5% is much lower than I was hoping for. I just wished it was 1.5% the other way.

So what is the reason for the big difference between the two methods?

 

[balrog]

BrewersFriend has a little blurb that is a quick read, https://www.brewersfriend.com/2011/06/16/alcohol-by-volume-calculator-updated/

They give 7.6 or 8.4 for you numbers, using "simple" vs "more complex for higher gravity" equations.

 

[foolsbrew]

So what is the reason for the big difference between the two methods?

Mostly because they are correct enough. If we were brewing beers above 10% ABV, we'd probably be troubled by them and improve. But most people work around 5% ABV, and these equations produce good-enough values. Going by the equations @balrog posted, the error margin in your case is "only" 10%. There are a few more equations around by the way, these aren't the only ones. There is another common one that was (OG-FG)/(x-OG), but I forgot

In any case, this simplification results in larger errors as you start working with higher gravity.

Equation A. ((OG – FG) * 131.25)
Equation B. (76.08 * (OG - FG) / (1.775 - OG)) * (FG / 0.794)

Example A. 1.1 OG, 1.01 FG
Example B. 1.05 OG, 1.01 FG

Result A Eq A. ((1.1 – 1.01) * 131.25) = 11.81
Result A Eq B. (76.08 * (1.1 - 1.01) / (1.775 - 1.1)) * (1.01 / 0.794) = 12.9

Result B Eq A. ((1.05 – 1.01) * 131.25) = 5.25
Result B Eq B. (76.08 * (1.05 - 1.01) / (1.775 - 1.05)) * (1.01 / 0.794) = 5.33

Result A error margin = 8.82%
Result B error margin = 1.51%

You can(and in many cases I will agree with you) find a 10% or 20% margin too significant to be comfortable. If you ask a mathematician, they are most likely to be uncomfortable if they are off by 10% in their results. If you ask a computer engineer, a 10% difference in performance is usually considered random noise. albeit the serious ones will tune their algorithms. If you ask a cook, they won't even measure their ingredients most of the time and 10% is not even considered. So which view should prevail? I find the 10% margin to be comfortable for me in homebrewing.