Think of it this way: The potential alcohol scale is how much potential alcohol is *left* in the brew. A reading of 2% (which is equivalent to 1.015) means that there is enough sugar *left* to up your alcohol 2% (*if* you were to somehow manage to ferment it all).
[Notice the alcohol percentages go *down* not up. When you startee the wort the alcohol scale probably read something like 8% (equivalent to 1.062) potential alcohol (when you *know* there is *no* alcohol in it) and finished at 2% (when you know there is more alcohol than when you started) and if it kept going it could go to 0% if you ever got a s.g. of 1.000.]
The way to use the alcohol scale is to measure the original potential alcohol on brew day, let's say it read 8% (which means that if you converted *all* the sugar to alcohol you'd get something that was 8% alcohol), and to measure the final potential alcohol weeks later, let's say it read 2% (which means that if you converted all the *remaining* sugars, the remaining sugars would contribute alcohol that is 2% of the volume) and subtract the two (so, you had the potential to get an 8% beverage but you but didn't use it all and you are left with enough sugar to make a 2% ABV; that means the sugars that were converted were enough for a 6% ABV).
Basically this is the same thing as subtracting the two gravities and multiplying by 131. The potential alcohol scale is simply the gravity scale offset by one multiplied by 131. (2% => 1.015; .015*131 ~= 2... 8% => 1.061; .061*131 ~= 8. ABV = O.potential alcohol - F.potential alcohol = 8% - 2% = 6% *OR* ABV = (og - fg)* 131 = (1.061 - 1.015)*131 = .046*131 = 6%. Neat, huh?)
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To do that, you look at the "specific gravity" part of the scale, not the potential alcohol scale. The PA scale is for winemakers, and is not used by beer brewers.
So, say your OG was 1.050. And your FG was 1.010. Then you'd plug your numbers into the formula (1.050-1.010)x 131 = 5.24%.
Hmm. Well, you *can* use it. My LHBS guy didn't tell my not to use it. You just subtract the original reading from the final reading. Presto!
However, the scale and margin of error for beer use is so large on the PA that the Gravity scale is much more precise. But I'd use the Gravity scale for wine as well. It's simply that much more precise.
In theory they're the exact[*] same thing only on a different scale.
The important thing to understand is that the PA measures *potential* alcohol (from the sugars remaining) and *not* actual alcohol that currently exists. (Those are actually almost exact opposites).
[*]Oops, I guess I should be careful when I use the term "exact same". Potential Alcohol measures potential alcohol and Gravity scale measures weight of the liquid in relation to the weight of the same volume of water (1.062 means a gallon of wort weighs 1.062 times as much as a gallon of water). These are of course completely different things. When I say they are the "exact same" I mean they are related is such a way that the measurements are proportional and measuring one directly measures the other (by multiplying or dividing by 131).
The third measurement is the Balling or Brix, BTW, and it measures the percentage of sugar by weight and what I find interesting is that it *doesn't* seem to be proportional. 20 on the Brix is *over* 1.080 on the gravity, while 10 on the Brix is *under* 1.040 on the gravity.