2003 September 7
There has been remarks about some units not being SI.
For example, centimetre, millimetre.
All SI prefix multiples of the coherent SI units are SI units.
Reference: IEEE/ASTM SI 10 -2002 2.2 SI prefixes
"The term SI units includes the SI base units, the
SI derived units, and all units formed from them
using the SI prefixes."
At times one sees references to "SI units" meaning only
the base units. From this there have been statements
that a prefixed unit is not an SI unit. Such statements are
in error. It is not reasonable to tell the public to use SI
units, (usually said as "Use metric units") and then say
that millimetre is not an SI unit. The 1997 edition of
SI 10 was not clear on this matter.
The property of coherence applies to the base units and to
the derived units. Coherence means that various units are
related "with no numerical factor other than 1."
An expression with prefixes can be made coherent by removing
the prefixes and inserting the numerical value of each prefix.
If there is a requirement to "use SI units" or "use metric
units", it can be satisfied by reference to SI 10 3.3.2
"Compliance with this standard includes the use, as
needed and convenient, of certain non-SI units listed
in Table 6 and Table 7, as well as the SI units,
including the multiples and submultiples."
Note that these non-SI units are not coherent with SI units.
Particular questions can be submitted to ASTM Committee E43 on SI.
Robert H Bushnell PhD PE
member of the joint SI10 committee
