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From: TNAN0@ISUVAX.BITNET (XENO)
Newsgroups: comp.sys.handhelds
Subject: (none)
Message-ID: <9010011635.AA02071@ucbvax.Berkeley.EDU>
Date: 30 Sep 90 07:18:00 GMT
Sender: usenet@ucbvax.BERKELEY.EDU
Distribution: inet
Organization: The Internet
Lines: 43


 
 
 
A while back, somebody mentioned a bug in the HP involving adding two
numbers with units of degrees C or degrees F.  A simple example would be
that '1_C + 1_C' yields 275.15_C when EVAL (C is DEGREES C) is pressed.
 
Many people were quick to defend HP by saying that it is meaningless to add
temperatures unless you add them using absolute zero as a reference point. HP
wrote in their manual (vol I, page 201) "Pure temperature units [as opposed to
compound units] are converted to absolute temperatures before adding or
subtracting..."  Thus, HP did this intentionally (or noticed it after the fact
and described it as a "feature").
 
Ok, basic chemistry:
C and F are measured in degrees and K is a unit unto itself.  By definition, a
degree is a step or relative unit.  What does 1_C mean?  It means "one degree
of C"...  When we speak of temperature in C, we always assume that 0_C occurs
at the freezing point of water.  Therefore, 1_C is 1_C above the freezing point
of water (273.15_K).  If it is 26_C outside and the temperature drops 5_C, then
it is 21_C outside (not -252.15_C as the HP48sx would have us believe).  In
chemistry, there are many equations that use C units in this linear fashion.
Any equations NEEDING absolute temperature USE it...  1 degree of C is exactly
equal to 1 degree of K.  Here's an example to illustrate my point:
 
'(1_J*C + 2_J*C) / 3_J' EVALuates correctly to 1_C BUT now try the same
equation but this time type EXPAN (expand equation) first...  You will get
274.15_C instead... Interesting, huh?
 
If you are still hard nosed that there is no bug, and believe that 1_C + 1_C
should be 275.15_C then I'm sure that you will agree that 2*1_C should be the
same thing, right???  Try it...
 
As an actual example, try doing any specific heat or latent heat chemistry
problems without using x_K and without adding the numbers in your head... (ie.,
do it the way chemistry books tell you to).
 
 
---Xeno (Gary Snethen)
tnan0@isuvax            (Bitnet)
tnan0@ccvax.iastate.edu (Internet)