Does anyone know where the "131.25" figure comes from for calculating alcohol by volume? What is the unit of measure for the figure? I can't find anything on Google. Thanks..
Hang on for a few hours, I'll see if I can find it again.... I recently found the source, AND determined that it's silly, with way too many sig figs, especially considering that it is not a constant but varies between about 129-134. Can safely ignore the extra .25, it's just silly. I'll see if I can find it..........
Ok but what I'm asking is what the actual number represents. I am not currently grasping what it means because it doesn't have a unit of measure along with it. It's a conversion factor but I can't figure out what it represents (whether it's 131.25 or 131, or whatever, doesn't matter to me).
It's just a conversion factor that incorporates the relative density of alcohol and water, with final results being unitless because whether you measure in milliliters or gallons or anything else, it's like gallons of alcohol per gallon of beer, or milliters of alcohol per milliliter or beer, or whatever, and the units just cancel out, leaving just a fraction or percentage that is alcohol in the whole beer. You could also think of it as being a magical conversion factor of 1.31 times 100%, so using 131% instead of 1.31, you end up with a percentage instead of a fraction, e.g., 5.5% instead of 0.055. EDIT: Here, this might help... or might not...: https://www.quora.com/Where-does-the-alcohol-content-equation-A-OG-FG-*131-in-fermentation-come-from And then the density actually is different at different temperatures (but there are probably more variables that I'm not even thinking of right now).......
If I remember correctly the units are a little goofy to understand. It comes from converting your density change to ABW and finally to ABV. It typically falls into the range mentioned above. The higher the alcohol of the beer, the greater the error tends to be. Better explanation here: http://www.brewunited.com/abv_calculator.php
Here's how you use it: ABV (%) = (OG - FG) * 131, where OG and FG are the specific gravities (which are unitless) of unfermented wort and completely fermented beer, respectively. Therefore the units of the quantity 131 (if you really want to call them units) are "percent (%)". Cheers!
Ok thanks! This discussion really does help. Up until now it's like, "oh I don't worry about the math. I just plug my gravities into my calculator app on my phone". I appreciate the replies.
An alternative method (which is also just an estimate) is: [(OG-FG) * 1000]/7.46} + 0.2 I don't recall where I got this formula, but it always comes out close to the number from the method given above. I keep a spreadsheet of the beers that I brew, and I stored both formulas in the cells after the OG and FG cells so that it automatically calculate the ABV for every brew after entering the OF & FG in their cells, and then I use a number that is between these two answers for my ABV that I print on the labels that I make. If you are a math whiz you might recognize that you can divide the 1,000 by the 7.46 and you get 134.29 which is similar to the 131.25 used above (especially after adjusting for adding the 0.2), so maybe the ratio (the OG-FG and the 7.46) used in this formula could be a hint for the source of logic behind both of these calculations.
A long time ago I was told that the yeast growth will reduce the conversion number, so that is why 131.25 is probably more accurate than 134.29. Pros with a lab run it through an Anton Parr Alcolizer to determine the ABV.
