Potential alcohol & non-sugar Brix

[acorn]

I have been wondering for quite some time why do triple-scale hydrometers have Brix and Potential Alcohol scales that fall below 0. Though I was suspecting it is because of non-fermentable solids found in wine, I wasn't sure how to estimate/calculate the final alcohol level based on this reading. Recently, I read a UC Davis publication that mentioned that the amount of non-sugar solids in wine approximately equals to about 3*Brix, which tends to explain why the Potential Alcohol and Brix scales fall about 3* Brix below 0 mark, when the wine is fermented bone dry.

However, my question is, how do I judge the final alcohol level if my wine doesn't ferment to 0.990 S.G. (or -3*B), nor, say, I want it to? In this case, do I still subtract 3*B from the initial reading and use the resulting number to calculate the final alcohol?

Hypothetical example:
I had a wine with initial S.G. of 1.095. It finished fermenting (or got stuck) at S.G. 1.005, and after tasting it I decided that I want to leave it at that point without restarting the fermentation. In this case, I probably would be wrong if I subtracted equivalent final reading from the initial reading on Potential Alcohol scale, as the use of this scale would only be valid, if the wine had finished at S.G. 0.990, right?

 

[grapeman]

To give you a ballpark estimate take 1.095 and subtract 1.005 giving 0.090. Looking at the PA scale you will see that a 1.090 reading has a PA of 11.5% ABV. Easy enough (there are many other ways of calculating it but this is close enough for your personal use).

 

[DoctorCAD]

Remember the word POTENTIAL. No device can tell you what the end result of fermentation will be, so thats why its called potential alcohol. They assume you will hit 1.000.

 

[GreginND]

0 is the reading for the density of pure water. Alcohol is less dense than water. So, the density in a fermentation actually ends up lower than 0 (or lower than 1.000 SG).

 

[acorn]

Thank you all for your feedback! So, what I gather from your responses is that for typical wine making purposes I should judge/calculate the final alcohol directly from the difference between the original and final readings on S.G. scale, sans other adjustments, even if wine doesn't ferment all its sugar (e.g. S.G. 0.990).

 

[sour_grapes]

Yes. Many of us just look at the specific gravity, rather than the brix. Then we estimate

ABV=(starting SG - final SG)*131.

 

[PAFruitWines]

the difference between starting and finished gravity is what matters. that is how much sugar was converted to alcohol. at that point the variable you are talking about would only come into play if back sweetening using a hydrometer.

 

[Deezil]

Then we estimate

ABV=(starting SG - final SG)*131.

Or

(Starting SG - Final SG)/.735 = ABV

Different equations.. Same answer, close enough