On Tue, 28 Sep 2010 12:15:07 -0700, Keith Thompson <ks...@mib.org> wrote:
>George Neuner <gneun...@comcast.net> writes: >> On 28 Sep 2010 12:42:40 GMT, Albert van der Horst >> <alb...@spenarnc.xs4all.nl> wrote: >>>I would say the dimensional checking is underrated. It must be >>>complemented with a hard and fast rule about only using standard >>>(SI) units internally. >>> >>>Oil output internal : m^3/sec >>>Oil output printed: kbarrels/day >> >> "barrel" is not an SI unit. > >He didn't say it was. Internal calculations are done in SI units (in >this case, m^3/sec); on output, the internal units can be converted to >whatever is convenient. That's true. But it is a situation where the conversion to SI units loses precision and therefore probably shouldn't be done. > >> And when speaking about oil there isn't >> even a simple conversion. >> >> 42 US gallons ? 34.9723 imp gal ? 158.9873 L >> >> [In case those marks don't render, they are meant to be the >> double-tilda sign meaning "approximately equal".] > >There are multiple different kinds of "barrels", but "barrels of oil" >are (consistently, as far as I know) defined as 42 US liquid gallons. >A US liquid gallon is, by definition, 231 cubic inches; an inch >is, by definition, 0.0254 meter. So a barrel of oil is *exactly* >0.158987294928 m^3, and 1 m^3/sec is exactly 13.7365022817792 >kbarrels/day. (Please feel free to check my math.) That's >admittedly a lot of digits, but there's no need for approximations >(unless they're imposed by the numeric representation you're using). I don't care to check it ... the fact that the SI unit involves 12 decimal places whereas the imperial unit involves 3 tells me the conversion probably shouldn't be done in a program that wants accuracy. George -- http://mail.python.org/mailman/listinfo/python-list
