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 -- Posted with http://DevLists.com. Sign up and save your mailbox.