2002 October 29
Several e-mails, 22961, 22969, etc. discuss the question of what is
the modern form of the metric system. The word "form" seems to
take a beating. The question appears to be related to what to tell
clients in relation to specifications and contracts. I think
"form" is not a problem. Use it if you want or not.
The SI brochure has used the phrases "SI units" and "units of the SI"
to separate coherent from non-coherent units. These words have been
a problem. For example, millimetre was not an SI unit but was a unit
of the SI. So, the CCU has proposed new text, not yet adopted by the
CGPM, to say all units, base, derived, and prefixed, are SI units.
Note that the prefixed units are not coherent (except kilogram).
For contracts and specifications, a problem remains. If a contract
says "Use SI units" may the work use millimetre?
I proposed text be developed to answer this to the joint IEEE/ASTM
committee revising SI 10, the ANSI standard on SI for the USA. The
committee declined to treat this matter. So do not look for the
answer in the revised SI 10 soon to appear.
I suggest that the following be standard boiler plate text for
contracts and specifications:
"Use SI units. SI base units, SI base units with
prefixes, SI derived units, and SI derived units
with prefixes comply with this requirement. Further,
the non-SI units listed in Tables 6 and 7 of the SI
brochure and hectare comply with this requirement."
Notice that "form" and "modern" do not appear.
For existing specifications and contracts I suggest that this text be
negotiated to be added.
You may reference the SI brochure, which is a free download, and
SI 10. When I visited BIPM I was told that BIPM encourages copying
and reprinting all of their publications without copyright restriction.
So copy what you want into specifications and contracts.
Robert H. Bushnell, PhD PE
member, joint IEEE/ASTM committee for SI 10
chair, ASTM committee E43 on SI
