Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!coplex!disk!corpane!herman
From: herman@corpane.uucp (Harry Herman)
Newsgroups: comp.sys.handhelds
Subject: Re: A Bug or not ?
Message-ID: <1991Apr13.110917.13554@corpane.uucp>
Date: 13 Apr 91 11:09:17 GMT
References: <00946F29.17B42000@MAPLE.CIRCA.UFL.EDU>
Organization: Corpane Industries Inc.
Lines: 55

In <00946F29.17B42000@MAPLE.CIRCA.UFL.EDU> bobdobbs@oak.circa.ufl.edu (Amiga Dude) writes:

>Well I haven't had my HP48SX long now, but I was messing around with the units,
>and did this simple operation:

>           45_F (45 Degrees F)
>           45_F (     "      )
> Well I put them on the stack, levels 2: and 1: then pushed the nifty + (plus)
>key, and I expected to see 90_F, but instead I saw 549.67_F, well needless to
>say, I looked in the manual, and it said something about Convert performs an
>absolute temp. scale conversion, including the additive constants.  Well I did
>not Convert, I merely added the 2.  I suppose I am forgetting something from my
>old basic Science days, so what gives ??? Anyone know a simple soln' to the
>problem (not that it is of earth-shaking importance, but just curious) ?

>Thanks,
>Scott D. Alden

What is happening is that the HP48 converts all temperature units to absolute
scales (Degrees R or Degrees K), does the operation on the converted values
and then converts the result back to the units of the number on level 1.
I assume the calculator does its work in Degrees K, but don't know for sure.

Also, remember that you are asking the calculator to add the absolute
temperature 45 Degrees F to the absolute temperature 45 Degrees F.  What
you really want to be able to do is to add 45 F Degrees (temperature
difference) to the absolute temperature 45 Degrees F.  Unfortunetly, the
HP48 does not know anything about temperature differences, only about
absolute temperatures.  Fortunately, HP put two functions in the Equation
Library Card that DO know about temperature differences.  TINC expects
the base temperature in level 2, and the delta temperature (positive or
negative) in level 1, and adds the delta to the base temperature and
the delta temperature together, and returns the result in the units
specified for the base temperature in level 2 (not the units specified
in level 1 like most unit math operations do).  TDELTA subtracts two
absolute temperatures and returns the temperature difference, again using
the units specified in level 2.

There was a discussion about this about a year ago on the HP Bulliten
Board Service (I think it was in feedback.to.HP), prior to the release
of the Equation Library Card.  The reason that HP chose to only use
absolute units is explained in that discussion, but it's been so long
that I don't remember the reason anymore.  

p.s.  Also note that:
	59_\^oF		@Base temperature
	59_\^oR		@Temperature difference in degrees R.
	SWAP		@Swap so that result will be in degrees F.
	+		@Add base+difference.
does give 118 degrees F, since the temperature difference was expressed
as a temperature relative to absolute 0, instead of a temperature
relative to 0 degrees F.

					Harry Herman
					herman@ukma!corpane