Well, there’s no fancy formula – just some basic cyphering. Anyone wanting to verify these numbers, please point out any errors.
To start with the easiest example, if you mix 1 gallon of must with gravity 1.130 and ABV 15% with 1 gallon of must with gravity 1.110 and ABV 0%, everything just falls halfway in between.
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1 gallon 1 gallon 2 gallons
1.130 + 1.110 = 1.120
15% 0% 7.5%
[digression] How do you put something in a tabular format and keep it that way? Someone please tell me.[/digression]
Now if your new must ferments down to 1.070, the ABV is
(1.110-1.070)*131 = 5.24%. I used 5% to keep it easy.
So if you take 1 gallon of must that has gravity 1.070 and ABV 5% and add 1 quart of must with gravity 1.130 and 15% ABV, your going to get a total of 5 quarts of must. Start with the ABV since that is easier.
(4/5 * 5%) + (1/5 * 15%) = 4% + 3% = 7% ABV
Then calculate up the gravity. For this it is easier to do it by converting the specific gravity to gravity points. 1.130 is 130 gravity point. 1.070 is 70 gravity points.
(4/5 * 70) + (1/5 * 130) = 56 + 26 = 82 gravity point (gravity 1.082)
In my prior post I just used the rounded number of 1.080 just to give an idea of the number to expect. With all the error that can occur with readings, you just want to be in the ballpark.
So for practice purposes let's try to figure out what to expect mixing in the quart of old must into his 2 gallon batch of new must. So we are now going to have 8 quarts of new must in a 9 quart final volume. First the ABV.
(8/9 * 5%) + (1/9 * 15%) = 4.44% + 1.67% = 6.1% ABV
And for the gravity calculation,
(8/9 * 70) + (1/9 * 130) = 62.2 + 14.4 = 76.6 gravity points (gravity 1.076)
So that's how I do the calculations - clear as mud, huh!
Obviously adding to two gallons would cause less of a jolt to the yeast, and should work well. Dave, if you took a measurement after adding in the quart, what were your numbers?
Medsen