Jay Windley wrote:

There are many reasons why a system would not be consistent, or would not be
used consistently, or would be confused with informal equivalents.
(Celsius, for example, is not SI -- Kelvin is -- but is commonly used in the
Metric System along with other elements of SI, leading to confusion.)

Celsius is however an "SI derived unit with special name" (see
http://physics.nist.gov/cuu/Units/units.html), which puts it on the
same footing as for instance hertz, volt, and newton, so I would say
that for all practical purposes it's an SI unit (but not an SI *base*
unit). In practice there is never ever any confusion between celsius
and kelvin. (Although I must admit that a couple of times I've had to
explicitly tell second-year undergraduate students in physics to use
kelvin instead of celsius in formulas when they didn't come up with
the correct answer and didn't understand why. But they quickly learned
their lesson.)

A historical sidenote: Anders Celsius, professor in astronomy at Uppsala,
Sweden, originally placed the fixed-points of his thermometer scale the
other way around, with 0 C at the boiling point of water, and 100 C at
the freezing point. Carl von Linné is generally credited for turning
it around to the "modern" way.

I've seen kilograms-force (kgf) gain popularity as a competing unit of
force. In my mind that's just revisiting the fiasco between pounds-mass and
pounds-force in English units. I will happily admit that the SI was meant
from the start to be a clean, consistent, and simple system. I'm sorry to
see it start to get "polluted" like the English system.

I agree that in everyday usage some "polluting" units are still used, but
as Henry noted they are used less and less: in Sweden atmospheric pressure
is now given in hPa instead of mm of Hg, nails and lumber is specified in
metric units instead of inches, as is also bicycle tyres (but I'm not
sure about car tyres), etc. On the other hand the "size" of an bicycle
is specified as the size of the rims -- in inches.

Tomas

--
Tomas Lundberg | It doesn't make any difference how smart you are,
Ericsson AB | you better not ever prostitute physics.
Luleå, SWEDEN | Don Arabian
E-mail: - remove the obvious