2003 March 14
USMA 25137 is about metric ton and megagram.
SI10-2002 is changed.
SI10-1997 in 3.3.2.4 on page 8 says "None of these terms is SI. The
terms "metric ton"and "tonne" are restricted to commercial usage."
SI10-2002 in 3.3.2.5 on page 9 these two sentences do not appear.
3.3.2.5 Mass
The SI unit of mass is the kilogram (kg). This unit, or a multiple
or submultiple formed by attaching an SI prefix to gram (g), is
preferred in all applications.
The name "ton" has been given to several large mass units that are
widely used in commerce and technology: the long ton of 2240 lb,
the short ton of 2000 lb, and the metric ton of 1000 kg or 1 Mg,
which is almost 2205 lb. "Tonne" is the name for metric ton that
is used in many countries. Only the megagram is SI.
The committee could not defend the restriction on tonne.
In SI10 page 7 Table 6 - Units in use with SI we see
mass metric ton t t = 1 Mg = 10^3 kg
The sentence "Only the megagram is SI." brings up the question of what
is an SI unit for a contract that specifies "Use SI units.". The
answer to this is in
2.2 SI prefixes
The prefixes listed Table 5 are used to form decimal multiples and
submultiples of the SI base and derived units. The term units
includes the SI base units, the SI derived units, and all units
formed from them using the SI prefixes.
and
3.3.2 Units in use with SI
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 all the SI units, including the multiples and
submultiples.
Now you know more than you want to know. Have fun.
Robert Bushnell PhD PE
member of the committee
