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From: frank@grep.co.uk (Frank Wales)
Newsgroups: comp.sys.handhelds
Subject: Re: temperature conversion
Message-ID: <1991Jan24.233320.940@grep.co.uk>
Date: 24 Jan 91 23:33:20 GMT
References: <2D042619A000047E@gacvx1.gac.edu>
Reply-To: frank@grep.co.uk (Frank Wales)
Organization: Grep Limited, LEEDS, UK
Lines: 40

In article <2D042619A000047E@gacvx1.gac.edu> U5513172@NMSUVM1.BITNET 
 (Thomas L. Baca) writes:
>WhenI add 25_'C ( read degree Celcius)  and 1_'F
>on my HP 48SX, I get an answer of 537.67_'F.  What terribly simple thing
>am I overlooking? [...] Why is this happening? I've never seen a reference
>to this phenomenom, so I'm inclined to believe that the problem is not 
>really a problem.

[This has come up before, so probably deserves a mention in the FAQ posting.]

What you're encountering is a consequence of what it means to add
temperatures; in short, it means nothing.  You're attempting to add a
point on an absolute scale (degrees C) to another point on an absolute
scale (degrees F).  This has no physical meaning.  (Note: it would be
no more meaningful if both values were on the same scale.)  The units system
on the HP 48 tries to deliver an answer, and the answer comes about because
the 48 reduces both temperatures to the Kelvin scale, adds them, and
then converts the result to the units of the argument taken from level one.

See 'Adding and Subtracting Temperature Units' on page 201 in the
HP 48SX Owner's Manual [edition 4, July 1990], which basically gives the
same explanation, but without the statement: "P.S., this is meaningless."(*)

The current design of the units system doesn't accomodate the special needs
of things like temperature arithmetic; a full handling of this case would
need the existence of both degree_X units (where X is the temperature scale
of your choice) and X_degree units for representing absolute temperatures
and temperature differences respectively, as well as appropriate conversion
rules.  This may also impact other unit definitions (and could also be
used to implement weird things like units for C pointer arithmetic :-)).

If you need to perform temperature arithmetic, then the Equation Library
card functions TINC and TDELTA represent one way of handling this "problem".
-
(*) If you don't see why the operation is meaningless, imagine replacing
temperatures with dates (another absolute scale); now add January 12th, 1989
to August 28th, 1993.  What could the answer be?  And what could it mean?
--
Frank Wales, Grep Limited,             [frank@grep.co.uk<->uunet!grep!frank]
Kirkfields Business Centre, Kirk Lane, LEEDS, UK, LS19 7LX. (+44) 532 500303