2002 March 28
What is an SI unit?
In 19104 Bill Hooper says "The litre is not an SI unit." Well, yes it is.
This long-time problem arose even more strongly with the millimetre. Could
we
really get away with telling people that the millimetre is not an SI unit?
We would
be laughed out of town. Or people would say to hell with SI.
The CCU has treated this issue. Any unit, with a prefix or not, formed from
the
base and derived units is an SI unit. Accordingly, Section 2.2 of draft 5 of
SI10
now says
"The term SI units includes the SI base units, the SI derived
units, and all units formed from them using the SI prefixes.
The coherent property applies to units without prefixes.
The second paragraph of Section C.1, called C.1.1 in draft 5, has the word
"coherent" added to the text to read:
"A great advantage of SI is that there is one and only one
coherent SI unit for each physical quantity."
Furthermore, what is the meaning of a requirement that SI units be used?
Section 3.3.2 Units in use with SI says:
"Compliance with this standard includes the use, as needed
and convenient, of certain non-SI units listed in Tables 6
and 7, as well as all the SI units, including the multiples
and submultiples."
This makes it now easier to promote SI units.
Robert Bushnell, PhD PE
meteorologist
chair ASTM E43 on SI
