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Pronunciations of SI prefixes: kilo and giga; Unit Plurals
The American Society for Testing and Materials, ASTM, has published a
standard on SI units and prefixes, ASTM E 380-93. This standard has
been adopted by the U.S. Department of Energy. And, by the way, IMHO,
it is quite well done. ASTM E 380-93 contains 15 pages of conversions
for units from various systems. A joint ASTM-IEEE effort is currently
under way to consolidate ANSI/IEEE 268 and ASTM E 380 into a single ANSI
standard.
E 380-93 is the only "high-level" source I'm aware of for pronunciations
of SI prefixes.
1. ASTM E 380-93 specifies a soft g for giga, like "jig." This
pronunciation has been used for a long time in parts of the physics
community. It naturally derives from the same root as "gigantic" rather
than the root for "giggle." It has not yet caught on with most
Americans.
2. ASTM E 380-93 also specifies that prefixes are invariant in
pronunciation. Thus, kilometer is correctly pronounced "KIL o mee ter,"
just like kilogram and kiloton. We don't say "kil AHG ram" or "kil AH
ton," so we shouldn't say "kil AH mih ter" either. The reason for
invariant pronunciations is to simplify and clarify communication,
especially with non-native English or American speakers.
Unit Plurals
Like Strom et al. (1982) and NIST Special Publication 811 (McCoubrey
1995), ASTM E 380-93 agrees that units have regular plurals, and that
unit symbols are invariant and that one always uses [invariant] unit
symbols with numbers. Strangely, for rem and rad, the unit symbol and
the spelled-out unit are identical, leading to confusion. Examples:
"the rem is a unit of dose equivalent"
"he received a few rems"
"the limit is 5 rem" (read aloud as "five rems")
"the limit is 0.5 rem" (read aloud as "zero point five rem")
"a 5-rem limit"
"the sievert is a unit of equivalent dose"
"equivalent dose is measured in sieverts (Sv)"
"a 5-mSv investigation level"
"the rad is a unit of absorbed dose"
"if the absorbed dose rate exceeds a few rads per hour, ..."
"the SI unit of absorbed dose is the gray"
"a few hundred grays should inhibit sprouting"
And even Herb Parker talked about rems (with an "s;" Kathren et al.
1986), as did ICRP "Publication 0" (1955) and ICRP Publication 1 (1959).
On another note, for those arguing that "rem" and "rad" are acronyms:
well, they _were_ acronyms, once upon a time. However, the ICRU has
*never* defined them that way (at least back to 1962 and ICRU
Publication 10), but merely used them as words like joule or meter. The
ICRP has *never* defined the rem as an acronym, either. I have found
rem variously defined as "roentgen/radiation equivalent
man/mammal/matter" in USA sources of varying authority, but never in
ICRU or ICRP (ICRP 1955, 1959). Words like laser and radar once were
acronyms, too, but they have long since taken on a life as regular
words, with regular plurals.
References:
American Society for Testing and Materials (ASTM). Standard Practice
for Use of the International System of Units. The Modernized Metric
System. ASTM E 380-93. Philadelphia: American Society for Testing and
Materials; 1993. http://www.astm.org/ ASTM, 100 Barr Harbor Drive,
West Conshohocken, PA 19428-2959 USA 610-832-9585 as of 6/2/96.
International Commission on Radiological Protection (ICRP).
Recommendations of the International Commission on Radiological
Protection (revised December 1, 1954). British Journal of Radiology
Supplement No. 6; 1955.
International Commission on Radiological Protection (ICRP).
Recommendations of the International Commission on Radiological
Protection. ICRP Publication 1. Oxford: Pergamon Press; 1959.
Kathren, R.L.; Baalman, R.W.; Bair, W.J. Herbert M. Parker.
Publications and Other Contributions to Radiological and Health Physics.
Columbus, Ohio: Battelle Press; 1986.
McCoubrey, A.O. Guide for the Use of the International System of Units.
The Modernized Metric System. NIST Special Publication 811.
Springfield, Virginia: National Technical Information Service; 1995.
National Institute for Standards and Technology (NIST). The
International System of Units (SI). NIST Special Publication 330.
Springfield, Virginia: National Technical Information Service; 1995.
http://physics.nist.gov/
Strom, D.J.; Stansbury, P.S.; Watson, J.E., Jr. Units Should Have
Regular Plurals. Health Physics 42(3):387-388; 1982.
The opinions expressed above are my own, and have not been reviewed or
approved by Battelle, the Pacific Northwest National Laboratory, or the
U.S. Department of Energy.
Daniel J. Strom, Ph.D., CHP
Staff Scientist
Health Protection Department K3-56
Pacific Northwest National Laboratory
P.O. Box 999
Richland, WA 99352-0999 USA
(509) 375-2626
(509) 375-2019 fax
dj_strom@pnl.gov