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Gauging True Proof


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My suggestion would be to cool the spirit to bring it to within range of the tables in the Gauging Manual. AlchoDens may do the correction but if you can't give it a sanity check against the Gauging Manual I don't know how much stock I'd put in it.

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There are two factors that need to be corrected for when you use an hydrometer at a temperature other than its calibration temperature. The first is of course the expansion of the spirit. There is plenty of good data for the density of spirits as a function of proof and temperature, so this correction can be done accurately. The second factor that has to be corrected for is the expansion of the hydrometer bulb. In a hotter spirit the bulb will expand and float higher than expected, thus indicating a lower proof. The expansion coefficient of the glass varies from manufacturer to manufacturer, making this factor more difficult to correct for than the spirit expansion. Luckily the expansion of the hydrometer has a much smaller effect than the expansion of the spirit (for a given temperature variance) so it is generally not much of a problem. But of course, the further you are from the hydrometer's calibration temperature the bigger the error.

If you are trying to track the progress of a distilling run then the correction in AlcoDens should be fine at 100°F. In this instance rapid results are more important than fine accuracy. But if you are doing blending calculations or final proofing it would be better to cool the sample closer to the calibration temperature - in this situation you have the time to adjust the temperature.

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