units of measurement on altimeters

"Jukka K. Korpela" wrote in message
. ..

Describing them as hPa makes it clear what the unit is for someone
familiar with the SI, without risking accidents through unit
confusion.

Would it be clearer to use a non-recommended prefix than a recommended
prefix? Besides, your argument indicates a fundamental confusion. There
is only one SI unit of pressure, the pascal (Pa). That's part of the
beauty and practicality of the system. All the rest that is used to
express pressures relates just the way of expressing the numerical
value. For convenience, we can use multiplier prefixes of _the_ unit if
we like, or a multiplier of the number, consisting of a power of ten.

I'm not sure where you believe the "confusion" lies. Describing the unit as
hPa rather than mbar makes it clear that the unit is Pa and the prefix,
which is a standard SI prefix, gives the multiplier.

The preference to use powers of 1000 is just a preference because
practicality and pragmatism is sometimes more important than an
arbitrary recommendation. This is a perfect example of where
pragmatism should (and does) win.

The reason for preferring powers of 1,000, explicitly expressed in
several recommendations and standards, is its practicality, based on
the use of the system as a whole. If you take arbitrary special
aspects, you can always find arguments in favor of using non-SI units
or non-recommended SI expressions - but then you lose all the benefits
of a unified system.

Do you really believe that you lose *all* the benefits of a unified system
by using a prefix described (without deprecation, BTW) in the SI Brochure?

Using hPA is a half-hearted "solution" that
combines the trouble of transition

One man's half-hearted solution is another's essential compromise. :-)

Julian Scarfe