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Specific Gravity from Table 4

Jim at RVS

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I've been investigating the best way to assure proper bottle fill and have been convinced from what I have read here in the forum to fill by weight. That means, of course, knowing what 750 ml of spirit should weigh in the bottle, leading me to research a good density or specific gravity by %ABV chart. There is one I have seen from several sources that shows SG at every %ABV at 60 degrees F, with some sources including a 3 step temperature adjustment table. I then had the bright idea to use TTB Table 4 a a base, converting the "Wine gallons per pound" on the table to grams per milliliter using the conversions of gallons to milliliters and pounds to grams. I assumed the outcomes would be the same but they are not. For 40% ABV, the handy chart gives an SG (which by definition is grams per ml) of 0.9518. Converting Table 4 for 80 proof gave: 1/ (0.12616 *3785.41 ml/gal / 453.592 gm/lb)=0.9498. OK, the difference is 0.2% but I would have though density is density. 


Any wisdom on a) Where I went wrong on the assumptions or calculations, b) Does it matter given the pretty loose tolerances for fill, and c) What source is commonly used?

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Jim, you have opened a can of worms.  But they are all very small worms, so mostly they can be ignored.

Strictly speaking, SG is not the same thing as grams per ml. For most purposes they can be taken to be the same, but SG is actually the mass of 1 ml of the fluid divided by the mass of 1 ml of water - both at 60°F.  The mass of 1 ml water at 60°F is 0.9990 gram so 1 ml of 80 Pf has a mass of 0.9518 x 0.9990 = 0.95085 g at 60°F.  This makes its density 0.95085 g/ml.

The value of 0.12616 gallons per pound in Table 4 is measured in air.  This tells you what volume is occupied by alcohol that weighs 1 lb.  But normal density tables (not TTB tables) are based on mass, not weight, so we have to convert the 0.12616 gall per lb weight to 0.12602 gallon per lb mass.  Your conversion procedure was correct, so we can convert this directly to a density using your formula : 1/ (0.12602 * 3785.41 ml/gal / 453.592 g/lb)=0.95085.  Same as above.

Do your scales give "in air" values, or are they corrected to absolute mass?  As you pointed out, does it really matter?

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