gravity equation

jimmywit · 02-03-2010, 02:09 AM

Does anyone know where the 131 comes from in the abv equation. I have been told that og-fg X 131 is the best way to get your abv. But I just can't seem to find out where the 131 comes from. I know there is software and all but I just like to know how all this stuff is done and what it all means..If anyone knows of a good equation site that would be great as well.

phatuna · 02-03-2010, 02:30 AM

Sorry Jimmy - no idea where it comes from. Like many things in life, I think it just "is what it is".

schadelh · 02-03-2010, 03:08 AM

It is probably a derived constant factoring in 2 things

1) density reduction due to consumption of sugar (reducing the density of the water)
2) density reduction due to production of alcohol (less dense than water)

there is a stoicheometric (i.e. constant) relationship between the consumption of one and production of the other, but after that it gets ugly... There are probably a lot of nasty differential equations relating to 3 phase systems (sugar, water, alcohol) and density, with ugly non-ideal-fluid approximations and virial coefficient power series expansions.

Just thinking about it is giving my brain thermodynamics cramps... time for beer

Be very thankful someone figured it out for us!

ardentfrost · 02-03-2010, 03:16 AM

Well, the equation is actually [(OG - FG)(1.05)]/0.79 to get the percentage. 0.79 is something like the ratio of alcohol weight compared to water, but I don't know what the 1.05 is. It has something to do with attenuation or something like that (I think how much CO2 the yeast pulls from the wort), and it isn't linear. It changes as the amount of alcohol increases in the beer. So people use the 131 number because it usually applies to beer (in the range of ABV most beers are).

The 131 is just someone doing the simple math for you... [1.05/0.79]*100 = ~131 (after you use more precise numbers)

mordantly · 02-03-2010, 03:29 AM

131.25 is the widely accepted number, like stated, it converts from abw to abv in one step.

funkadelicturkey · 02-03-2010, 03:40 AM

few things in life is "is what it is", love of beer "is what it is." 131 is part of a mathematical equation. Schadelh is more on the righteous path. Good explanation here http://forum.northernbrewer.com/viewtopic.php?t=43158 wish i could give a better explanation but i've been peg legged by clipper city.

mparmer · 02-03-2010, 11:21 AM

No, no, you guy's got it all wrong, it's the volume of pst from poping the top off a homebrew divided by the sq root of the number of inches of lacing in your glass ;-]