"frederick ploegman" > wrote in message >...
>
> The only thing I find are "pre-pitch" (original gravity) tables. These
> cannot be used in the manner stated above because it fails to consider
> the effect that alcohol has on the "post ferment" reading. So far, I have
> not seen a single answer in this thread that considers this, so the answers
> thus obtained are wrong.
>
> I am hoping that Jack or someone else here has a "post ferment/end
> alcohol" table that they could share with us. Tables, after all, are
> nothing more than a simple listing of all of the answers that a formula
> can provide. They have the advantage that folks that can't even
> _spell_the word algebra can look up the answers they need without
> having to understand the math. So - if someone has a suitable formula,
> it should be no problem converting it to a table.
>
> Frederick

Frederick:

I certainly agree with you that alcohol will affect the final sg
reading, but I disagree with your conclusions in this thread. As you
say, a table just reflects an underlying formula. There are formulas
out there that use final sg reading to calculate the PA, so tables
based on these formulas then reflect both the starting and final sg.

To take one example, Duncan and Acton's Progressive Winemaking
mentions a simple formula PA = (Gs - Gf)/7.4, where G is the gravity,
defined as SG*1000. The constant 7.4 is actually an approximation -
they also give a more complicated formula, where the constant depends
on the starting gravity, but for simplicity, we can stick with the one
value 7.4 here.

The point is that this does not falsely increase the amount of sugar
in the must. Instead, the formula describes PA in terms of the total
drop in gravity during fermentation (which directly depends on the
sugar content of the must). As such, it's not better or worse by
itself than formulas that describe the PA only in terms of the initial
sg (sugar content). Which formula is better is an empirical question -
any formula gives only a rough estimate of the final alcohol, so the
quality of any formula would have to be measured against decent amount
of real data.

To continue with the above formula - since it's linear in terms of the
total gravity drop, we can use the method that Jack described and
shift the final gravity to 1000 (sg to 1.000) if we shift the initial
gravity by the same amount. And then a table that is based on this
formula will show the PA reading for the adjusted Gs - the adjustment
is needed because it allows us to start tabulating from sg 1.000.

One advantage of this approach is that it works equally well for all
wines - dry or sweet. Only the Gf reading is needed for all cases.
Formulas that work only with initial sg assume fermentation to
dryness. If there is significant residual sugar left, the PA has to be
adjusted, in which case one has to find out the RS amount and
manipulate the table accordingly. So in this case, the final sg value
becomes significant again.

The simple formula above also automatically corrects for presence of
solids because of the subtraction. In terms of precision, the book
claims the estimated PA is within +-0.5% of the actual value measured
by an ebullioscope, for the alcohol range 10-14%. That's about as good
as one could expect from any PA formula. The sample size is not
mentioned.

I personally don't use this formula, but I can see the value of this
approach. Mainly, I am hoping to make it clear here that approaches
like this don't really compare apples with oranges, as there is
nothing methodologically wrong in calculating PA in terms of total
gravity drop vs in terms of initial gravity only. What matters is
which formula makes a better prediction. In the absence of a decent
comparative study, the choice is up to individual winemakers'
preferences.

Pp