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I tested my hydrometer in distilled water at 68F and it reads 0.994.

If I have an original gravity reading of 1.050 do I simply add .006 to this reading or is there a more advanced formula to correctly calculate the offset?

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Might want to check your numbers: did you mean the distilled water reading was 0.94? Or did you mean the reading was 0.994 and the correction should be 0.006? –  Henry Jackson Aug 29 '12 at 13:48
    
oops, thanks. corrected. Meant .006 added. –  corymathews Aug 29 '12 at 14:44
    
OK, so the original reading was 0.994, right? –  Henry Jackson Aug 29 '12 at 14:49
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2 Answers

up vote 4 down vote accepted

Before doing any math whatsoever, be sure to check the packaging for the specific hydrometer you are using. When mine broke recently, I went back to my LHBS and purchased a replacement (same store, same shelf, same brand) not only does it have different directions (above vs below meniscus) but it also has a different calibration temperature.

Once you're 100% sure your reading is accurate, and you're sure the adjustment is for a hydrometer with the same original calibration temp as yours, it's just addition.

NB: The Brewer's Friend hydrometer adjustment calculator now supports different calibration temps.

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Good point, but I think this error is highly unlikely to be the result of an unusual calibration temp. The calibration temp would have to be over 100 ˚F to explain this difference. –  Henry Jackson Aug 29 '12 at 13:47
    
Honestly, I was thinking that reading above vs below the meniscus would produce a bigger change than the calibration temp anyway, but point taken. –  Peter R Aug 29 '12 at 20:33
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Just add the difference. The error is probably due to the paper scale shifting. To get a non-linear error, the diameter of the stem would have to have changed.

Also, you should add the error back before you make any temperature correction (although it likely won't make much difference).

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