```On Thursday, August 24, 2006, at 1:15 AM, Paul Lutus wrote:
>Peter Vanbroekhoven wrote:
>
>/ ...
>
>>> 3. Facet Units doesn't seem to do temperature conversions.  Maybe it
>>> does, but I can't find any documentation on it.
>>
>> It doesn't as temperature conversions are ill-defined.
>
>No, actually, temperature conversions are perfectly well-defined.
>Unfortunately, absolute temperature conversions require two mathematical
>operations instead of one (a multiplier and an offset), which sets them
>apart from nearly all other conversions. It even sets them apart from
>temperature differential conversions.
>
>> As you know,
>> both absolute temperatures and temperature differences have the same
>> unit. Any operation on two temperatures expressed in different units
>> cannot be correct for absolute temperatures and temperature
>> differences.
>
>That is because converting a temperature differential is not the same as
>converting a temperature. But each of the operations is clearly defined.
>
>> This is an inherent problem, so we had three choices: do
>> no conversions, use the conversion for absolute temperatures, or use
>> the conversion for temperature differences.
>
>There is a third choice -- convert temperatures and temperature
>differentials separately. Name one absolute temperatures, and the other
>delta-t or something similar.
>
>It is basically a Calculus issue. You may know that taking a differential
>(or a derivative) involves the loss of information, and when the converse
>operation (integration, a term only loosely applicable here) is performed,
>a constant (usually unknown) is included to remind the practitioner about
>the lost information. In the same way, moving from a temperature to a
>differential of temperature loses information, so the reciprocal operation
>(difference to absolute) must be carefully handled (when it can be
>performed at all).
>
>Someone versed in Calculus wouldn't think of mixing functions and their
>derivatives without being vigilant that they represent different things.
>The temperature conversion problem should be looked on in the same way.
>
>> We chose the first of
>> these three. We're open to additional ideas on this.
>
>Well, this is just something to think about. I've written a lot of unit
>conversion routines over the years and I haven't considered handling
>temperatures and temperature differentials separately, but it's an obvious
>solution.
>
>I just noticed something. The Linux utility "units" handles both
>temperature
>and temperature differences, and they are distinguished by syntax:
>
>\$ units "tempC(0)" tempF # absolute temperature
>   32
>
>\$ units "32 degF" degC   # temperature difference
>        * 17.777778
>        / 0.05625
>
>So it seems it has been done.
>
>--
>Paul Lutus
>http://www.arachnoid.com
>

Yeah, I'm considering a variation on this approach.  It's a little
tricky, but not insurmountable.

_Kevin
www.sciwerks.com

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