Extrapolating hydrometer readings

[Zakrabbit]

Im wondering if anyone has any info on how best to extrapolate a hydrometer reading taken at a particular volume of must, in order to easily account for the extra water that will be added later?

What im thinking is this - im currently putting on a summer fruits wine, which I know will be rather frothy as it starts to ferment, so plan on starting it with a good headspace in the demijohn, then when the fermentation dies down topping it up with water to 5L, this of course will dilute it a bit. It also means that my initial SG reading of the must will be taken at about 3L not the final 5L.

So I want to know how to adjust/extrapolate the initial reading to take into account the subsequent dilution


with any luck the above will make sense to someone!

 

[Oneiroi]

Afaik it’s a straightforward linear relationship between s.g and the amount of sugar per litre.

I.e If you double the volume of water you half the gravity.

So to work out the new gravity, multiply the old gravity points by Old volume/New volume.

If you had 10 litres of wort/must at 1.050 and added 2 litres water the new gravity would be 50*(10/12) = 41.667.

Edit: To go the other way for your example, your 3L gravity will need to be 67% higher than your final desired gravity at 5L, so multiply target grav by 1.67

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