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If one ends-up with, say, half as much beer as needed to float the hydrometer, is dilution a viable way to get a measurement?

For instance, if one carefully measured the volume of available beer for the sample, mixed that beer with the same volume of water, then floated the hydrometer and took a reading, could that result be multiplied by 2 to get a valid gravity of the undiluted beer?

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Yes. Specific gravity is calculated on relative densities (densitybeer / densitywater in the case of brewing). If you double the volume of the beer as you describe by diluting it with water, then the density will be half (roughly, not accounting for intermolecular interactions). (Dbeer / 2)/ Dwater = (Dbeer / Dwater) /2, so multiplying by 2 would restore the actual original sg of the brew. Keep in mind that this is not exact as there is an inherent error in using the hydrometer (very temperature sensitive).

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Good answer and you can certainly do that. Me, I'd just put more beer in the hydrometer flask. –  Denny Conn Sep 23 '11 at 0:41
1  
If I HAD more beer! It was quite stupid of me, but I only realized it after I sealed the last bottle. But glad to know the idea in the question is valid. BTW, I do chunk the hydrometer reading with temp into Dave's dreaded calculator to normalize for temperature, and have gently weighted my hydrometer and tested with distilled water. –  Dale Sep 23 '11 at 23:14
    
Dave's is the best, I refer it to folk quite often. The other tool that I use is tastybrew's priming sugar calculator. I get much more consistent carbonation from using these two tools. cheers. –  drj Sep 24 '11 at 7:00

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