HOMEBREW Digest #249 Sun 10 September 1989

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		Rob Gardner, Digest Coordinator

  doubts about SG temp corrections (Dick Dunn)

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---------------------------------------------------------------------- Date: 9 Sep 89 01:04:58 MDT (Sat) From: hplabs!gatech!raven!rcd (Dick Dunn) Subject: doubts about SG temp corrections I've been watching the discussion of temperature corrections for hydrometer readings for a while. I guess it's time to toss in my twopence. A quick check in the Rubber Bible seems to indicate that the temperature coefficient of expansion of water over some interesting range (say 50-100 F), while not linear, is not warped enough to give you as much as a point of error relative to a 60 F reference. So that's cool...however: - How does the expansion change with sugar in the water? There are two questions here. First, how much does the coefficient change--that is, do you have to apply a significantly different temperature correction factor depending on the gravity? Second, does it get more or less linear than the expansion of water? - What about the expansion of the glass in the hydrometer itself? Is this really negligible? I've always gone by the (safe, but perhaps overly conservative) rule that you shouldn't try apply temperature corrections outside the range of per- haps 50-75 F. This seems like a good idea given the potential for non- linearity in expansion coefficients, difficulty in getting temperature measurements right, tendency to warm/cool while you take the measurement, etc. One other note-in-passing: Somewhere there was a discussion of the "poten- tial alcohol" scales on hydrometers. I have two hydrometers. Both show potential alcohol, and both clearly say "by volume", but they're not the same. However, the one that's different (:-) has the zero for potential alcohol at a gravity value other than 1.000. Is this just a simple screwup, or does someone know a reason this might make some sense? Seems wrong to me...I'd expect water to have SG of 1 and potential alcohol of 0. --- Dick Dunn {ncar;ico;stcvax}!raven!rcd (303)494-0965 or rcd at raven.uucp Return to table of contents
End of HOMEBREW Digest #249, 09/10/89 ************************************* -------
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