Alcohol calculators - why the variance?

[jat147]

Howdy,
This has bugged me for a while now, alcohol content calculators seem to give different opinions on the
same input data. They don't vary by much, but one of them must be correct?

E.g:
Orig gravity = 1054
Final gravity= 1010

Put into 7 different calculators - gave these results:

6.2 - 6.4 - 6.2 - 5.8 - 5.9 - 5.8 - 5.9 ??

Usually I'd just go down the middle and call it 6% What do you think?

cheers,
James.

[boingy]

There was a decent discussion on this not so long ago. Thread here:

www.jimsbeerkit.co.uk/forum/viewtopic.php?f=6&t=31681

[coatesg]

I would say 5.7% using the HMRC formula (ie (54-10)*0.130 where 0.130 is the fudge factor in use when the gravity difference is in the range 36.1 – 46.5). This is the one that a commercial brewery should use given those two gravities (assuming no priming takes place, in which case you need to read the beer duty document further to add to 0.1/0.2% extra )

[jat147]

Thanks very much - looks like most of the calculators I've used have
been massaging my ego a little!

[Graham]

I'm not sure whether the HMRC calculation is going to give you the actual abv or just another estimation of the avb for tax purposes, the formula does cover a wide range of gravities

It will probably be weighted in their favour, but it can only be an approximation because they have to make a fixed assumption for the sugars used up in yeast growth that will not be converted into alcohol. This will be dependant pitching rate, wort aeration and, I assume, whether the yeast is dried or shoved in from a starter. If you believe the stuff from the dried yeast manufacturers, whereby they say that the energy reserves for yeast growth are already built into the cell walls, then less substrate will be used up during growth. Likewise, people whose use ruddy great starters will end up with a higher alcohol level because less sugar is used up for the growth phase. The downside of large starters and dried yeast is likely to be a lack of characteristic ale-type flavours. Many of the precursors of the flavour compounds are by-products of yeast growth, not fermentation.

Likewise a single divisor (or multiplier) number will not be accurate at all gravities. This is because if you add 100 grammes of sugar to a litre of water the volume will increase by (say) 64 millilitres. If you add another 100 grammes of sugar the volume will increase by only another (say) 60 millilitres. Likewise, if you add 500 millilitres of alcohol to 500 millilitres of water you will end up with less than a litre of total volume. I can't remember what the proper scientific term is for this effect, but it is something to do with the molecules nestling closer together in denser solutions. The end result is that specific gravity is not a linear function of sugar content. The non-linearity of the sugar divisor is almost certainly why there are steps in the HMRC table. A single number will be much more accurate if extract was done in Plato rather than S.G. As we use S.G., I suppose that the Excise table is most appropriate for us Brits, because it will match commercial practice closer than a fixed figure.