There are a couple of problems IMHO with this line of thinking. First of all, you are assuming that a nominal 6 gal carboy is, in fact, 6 gallons. Mine are about 6.25 gallons.
Second, there is a legitimate loss of volume due to, for example, racking off lees.
Ergo, I would advocate for topping off with a similar wine.
Well each one to their own taste.
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However, , My carboy is 6 gallons as per the measuring tools I have, The full carboy of water when I pour it into one any of my primaries matches approximately the 6 gallon mark on them. And then I make a definite mark.
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Also I checked all my carboys measuring litre by litre with a one litre measuring cup.
Considering that I don't really have perfect measuring tools , it is approximate but the plus or minus is not off by 1/4 of a gallon. That is a huge difference, almost a litre. Very non-standard.
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Which brings a good point of measurement. I wonder what do you and other people use as standards of measurement. I've bought all kinds of measuring cups eg one litre.
Very seldom are any of them standardized. It is very annoying, I can't trust any of them really when I buy them. I measure and measure one against the other and take the best average. I wish I could find something that I know absolutely is one exact litre then I can standardize everything else.
Absolute! Absolute!
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Anyway I don't see the problem of making 6.25 gallons of wine from a six gallon kit. It doesn't seem significant. It is possible that I don't know what I'm missing if I have gone over six gallons by maybe 250 ml, 500 ml or whatever , but I've never been disappointed with any of my wines. I prefer this method than the possibilty of ruining my wines with oxygen or using foreign to ups. or If it is considered significant to some people then maybe the best thing is something like a nitrogen top up.
Yes, I am aware that lees cost volume which is why I make sure there is enough water in the primary to result in six
gallons + of resulting wine after the primary lees have been discarded. . I keep an overflow bottle for the extra - I make six gallons plus, (usually around 250 ml depending on the varietal) enough for a top up for the second racking.
I regard a six gallon kit as meant to produce 6 gallons of wine, not wine + lees.
The instructions that come with kits are guidelines, they don't really cover all the specifics and they don't address some of the obvious problems such as ullage. If someone follows them precisely they are left in the lurch. The manufacturers simplify the instructions because they don't wnat to get in the weeds and/0r take on some kind of legal liability. People have to work out their own solutions.
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Topping up with another wine (store bought or self made) means to make an accurate ABV prediction you have to know the ABV and volume of the added wine. And then make a calculation.
I do want to know the taste of the wine I'm making. Making a blend doesn't give me a standard to determine future batches. And changing the ABV with another wine also throws out another benchmark for the future.
And also as a wine-maker, I just never will buy wine, I feel then just what's the point? At least buying wine for home purposes. . There are always spectacular wines to try and there is traveling in wine country , going to wineries etc.