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oldbagobones

Hydrometer reads

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Looking around some of the beer sites on the web I got a confusing issue. The equation that is used to determine th FG has different numbers and I wonder which one would be correct. One number I saw was (OG-FG)x131.25, and the other was (OG-FG)x132.715. Which one would be more accurate?

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So it really depends how accurate you want to be……… in measuring ABV.

I am not overly concerned except to be sure the yeast is done.

But for the concerned...………… here is more stuff to look at.🤓

 

What I read online is this based on water SG = 1 and ethyl alcohol SG = 0.8 

************

131.25 is to calc ABV (Alcohol by Volume)

105 is to calc calculate ABW (Alcohol by Weight)

((OG - FG) * 105) * 1.25 = ABV

So: (OG - FG) * 131.25 = ABV

These formulas are flawed in that they get progressively inaccurate with higher ABV, but are within 0.2% up to around 9%

************

But at what temperature - this chart from U.S. Bureau of Standards below  is at 60 deg. F.

I see Alcohol SG at 25 deg. C (77 deg. F) is 0.787 and at 60 deg. F. is 0.794 (100% on chart below)

So it looks like most standards are taken at 60 def F and (unless you work in metric lol)

 

This site https://learn.kegerator.com/specific-gravity/ 

uses the  132.715 number but tells you to use the temperature correction  given in your hydrometer instructions - but does not tell you what number to correct it for (duh)

I read a reference cited as from  Cooper's;(OG-FG)/7.64+.5=ABV%  Maybe this works for Australian beer?

There is also the issue of effect of additional dissolved compounds that will raise the SG, like unfermentable sugars. 

For pure water alcohol mix at 60 deg. F. you can use this U.S. Bureau of Standards  . 

fig5-3.jpg

Or for a glorious dissertation of the actual formulae and real numbers go here:

https://www.homebrewersassociation.org/attachments/0000/2497/Math_in_Mash_SummerZym95.pdf

This includes a temperature correction chart referenced to 60 deg. F.  At 70 deg F, the correction is 1.05, so relatively the 1% may really be small compared to other effects including reading inaccuracy 😮

Usually OG and FG will be made at different temperatures so unless corrected that too can cause errors.

Reminder - If you are calibrating your hydrometer use distilled water and check temperature correction.

Phew!

Now it is time for a homebrew and watching football.

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I made the assumption that he was either correcting for temperature or using a calculator that input temperature and adjusted in the ABV calc.  

 

However, people need to make sure they understand how small the adjustment is.  If I take an OG reading of 1.050 at 70 degrees, the adjustment for a 60 degree calibrated hydrometer is a new reading of 1.051.  If I adjust my final gravity of 1.012 at 65, the adjustment takes it to... 1.012.

 

So let's compare unadjusted vs. adjusted:

 

Unadjusted - (1.050 - 1.012) x 131.25 = 5% ABV

Adjusted - (1.051 - 1.012) x 131.25 = 5.1% ABV

 

In short, the ability of a homebrewer to accurate read a hydrometer has a higher degree of error than the adjustment for temperature...

 

 

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Nickfixit, thanks for making my head spin!  That is a lot of info to digest.  Seriously, that is a big help.  I am not overly concerned about an exact measurement for ABV but getting a close idea is what I strive to get.  As long as my beer has a good flavor and mouthfeel, I am happy.  The alcohol amount is just an added bonus.  Measuring the ABV is just out of curiosity more than of a critical observation.  Thanks for the info!!

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