HOMEBREW Digest #0249

Published in

· 7 months ago

 

HOMEBREW Digest #249		             Sun 10 September 1989 

 
	FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES 
		Rob Gardner, Digest Coordinator 

 
Contents: 
  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@raven.uucp 

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End of HOMEBREW Digest #249, 09/10/89 
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