That nomograph makes no sense at all for cider. I can only assume it is
intended for some 'typical' beer and makes big assumptions about the
fermentability of the residual native wort sugars? I don't understand
the temperature scale either. Perhaps someone can interpret that for us?
Claude and I are obviously missing something!
Taking a worked example, it recommends 4 oz sugar per 5 US gallons to
get a carbonation of "3 vol" at 48F. None of that stacks up. Working
from first principles, 4 oz per 5 US gallons equates to (4*28)/(5*3.8) =
5.9 g/L of sugar. This will yield about 3 g/L of CO2. That is about 1.5
vol carbonation, not the 3 that's quoted. So, I can only assume the
difference is intended to be made up by the residual wort sugars. They
are not present in cider.
A typical carbonation level for cider is "3 vol" or 6 g/L. That can be
achieved by adding around 10 - 12 g/L of priming sugar. Just about what
Claude uses! You will find that is pretty much the standard level - one
flat teaspoon per pint in old money!
BTW "Carbonation volume" is defined as that volume of CO2 which is
dissolved in a unit volume of liquid of measured at STP (760 mm Hg
pressure, 0 degree C). "1 vol" corresponds to 1.964 g/L of CO2.
Wittenham Hill Cider Page
Ah, good thinking. That could explain some of the 'missing' CO2. I will
sit down later with my Henry's Law spreadsheet and try to work out how much!
I've done a few more calculations now .....
According to Henry's Law, saturation CO2 at 45F (7C) is around 1.25 vol.
Going back to the previous posting for that condition, we had a
shortfall of 1.5 vol at an indicated 3 vol carbonation. So if the beer
or cider is just saturated with CO2, the sum of these now gives a
shortfall of only 0.25 vol.
Taking another example, at 75F (25C) 5 oz of sugar per 5 Gallons is
shown by the nomograph to give 3 vol carbonation. That amount of added
sugar is (5*28)/(5*3.8) = 7.4 g/L which will give 3.7 g/L of CO2. That
is around 1.85 vol of CO2. Henry's Law shows that at 75F the saturation
of CO2 is 0.75 vol. Putting those together we get 2.6 vol, a shortfall
of 0.4 vol.
In either case there is a calculated shortfall which is presumably made
up by the supersaturation of CO2 in a freshly-fermented beer with
residual gravity together with the long-term fermentable potential of
that gravity, and the nomograph appears to be valid for that situation;
the temperature correction allowing for the lower saturation of the
existing CO2 at higher temperature.
However, it is not necessarily valid for bottle conditioning of a fully
fermented dry cider where, at very best, the existing CO2 level will be
at saturation only and quite probably a good bit less, and where there
is no residual gravity. Under those conditions, following the nomograph
will lead to less carbonation than expected. QED.