This was copied from this
link which I posted earlier in the thread, but it looks like it would only affect the abv by a few tenths of a percent.
e.g. OG 1.080 - FG 1.016 = 0.064 (or 64)
64 * .130 = 8.32 <---- what some people use
64 * .131 = 8.38
64 * .132 = 8.45 <---- probably more accurate if you believe what's written below
The rest is someone else's post:
For homebrew purposes you can say OG-FG*0.130 where OG and FG are whole
numbers (ie no decimal point) so if OG is 1040 and FG is 1010 then ABV is
1040-1010*0.130 which gives 3.9%
In reality the 0.130 varies and HMC&E use a figure between 0.125 and 0.135
as follows:
CALCULATION OF ALCOHOLIC STRENGTH
1 If you have no, or minimal, laboratory facilities, you may calculate the
alcoholic strength of your beer by multiplying the number of degrees by
which the beer has attenuated by a factor. In order to ensure that your
calculations are accurate, it is imperative that the OG. is established as
soon as possible after collection and before fermentation commences, which
will normally be within one hour of completion of filling the fermenting
vessel.
2 The present gravity (PG - also known as the specific or final gravity)
should be measured once fermentation is completed. For cask conditioned
beer this will be after secondary fermentation in the casks.
3 The OG and PG of representative samples should be measured using a
suitably calibrated sacharrometer adjusted for the temperature of the
sample.
4 When you have taken your readings, the formula for the calculation is:
(OG - PG) x f = a% ABV
where OG is the original gravity of the beer;
PG is the present gravity of the beer;
a is the beer's alcoholic strength; and
f is the factor connecting the change in gravity to alcoholic strength.
The value of "f " is not constant because the yield of alcohol is not
constant for all fermentations. In lower strength beers, more of the
sugars available for fermentation are consumed in yeast reproduction than
in producing alcohol.
5 The table below produced by the Laboratory of the Government Chemist
shows the changing value "f" depending on the alcoholic strength of the
beer.
(OG - PG) % ABV Factor
Up to 6.9 Up to 0.8 0.125
7.0 - 10.4 0.8 - 1.3 0.126
10.5 - 17.2 1.3 - 2.1 0.127
17.3 - 26.1 2.2 - 3.3 0.128
26.2 - 36.0 3.3 - 4.6 0.129
36.1 - 46.5 4.6 - 6.0 0.130
46.6 - 57.1 6.0 - 7.5 0.131
57.2 - 67.9 7.5 - 9.0 0.132
68.0 - 78.8 9.0 - 10.5 0.133
78.9 - 89.7 10.5- 12.0 0.134
89.8 - 100.7 12.0- 13.6 0.135
The table indicates that for the majority of popular UK beers, the factor
for calculating the estimated strength would lie in the range 0.128 and
0.129. For strong beers (exceeding 6% ABV), the factor would be in the
range of 0.131 - 0.133. Whilst it is important that the correct value for
"f" is used, you should be aware that this table is primarily for your
guidance. Some brewing methods and materials can affect the factor.
--
Andy Davison
[email protected]