From the HBD Archive
From: hplabs!gatech!raven!rcd (Dick Dunn)
Subject: doubts about SG temp corrections
Date: 1989-09-09 07:04:58 GMT

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,

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@raven.uucp

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