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NIST Offers U.S. Interpretations of Recent SI (Metric) Change

The National Institute of Standards and Technology (NIST) has issued a new American version of the English language SI Brochure, the eighth edition of the international standard reference guide to the modern metric system, the International System of Units (known as SI from the French "Le Systeme International d'Unites"). NIST is the U.S. technical representative to the International Bureau of Weights and Measures that defines the SI and coordinates the federal government policy on the conversion to SI by federal agencies.Together NIST SP 330 and NIST SP 811 provide the legal interpretation of and guidelines for the use of the SI in the United States, as described in the Federal Register Notice PDF (May 16, 2008).

NIST SP 330:2008

SI plays an essential role in international commerce and is the common language of scientific and technological research and development. The 2008 edition of NIST Special Publications (SP) 330, The International System of Units (SI), cover correct U.S. usage of metric units, such as the accepted spelling of "meter" and "liter". In addition, the revised guide includes a new chapter on units for quantities that describe biological effects, and symbols for expressing values for enzyme catalytic activity in Biology and Medicine.

Seven major changes have occurred from the 7th edition (1998) to the 8th edition (2006) of the SI Brochure and these changes have been incorporated into the 2008 edition of NIST SP 330:

  • Full incorporation of the 2000 Supplement which clarified the definition of the second and incorporated the new coherent SI unit with a special name and symbol for the name and symbol for the mol · s-1, the katal with symbol kat, for expressing values of the quality catalytic activity;
  • A discussion that the base and coherent derived units of the SI form a coherent set, designated "the set of coherent SI Units";
  • New section "Units for quantities that describe biological effects";
  • New section "Historical note" has been added that contains updated information from Section 1.1 of the 7th edition;
  • Appendix 1 has been updated to include all the recent decisions of the CIPM and CGPM which impact the SI;
  • Appendix 2, which discusses the experimental realization of units, is only available in electronic form on the BIPM website, so that it may be easily updated between SI brochure versions; and
  • New Appendix 3 "Units for photochemical and photobiological quantities."

NIST SP 330 is edited by Barry N. Taylor and Ambler Thompson, and can be requested in hard copy by contacting TheSI@nist.gov or downloading electronic copy here:

NIST SP 811:2008

In another update, NIST Special Publication 811, Guide for the Use of the International System of Units (SI), complements SP330, and is published to assist NIST authors and others in correct SI usage and unit conversion. An extensive conversion factor appendix offers help in measurement unit conversions and in appropriate rounding-off of data. SP 811 also provides an editorial checklist for reviewing manuscripts' conformity with the SI and the basic principles of physical quantities and units. A color diagram has been added that illustrates the utilization of the SI base units in defining the 22 derived units with special names and symbols.

The U.S. Omnibus Trade and Competitiveness Act of 1988 designated "the metric system of measurement as the preferred system of weights and measures for United States trade and commerce." U.S. Public Law 110-69 in 2007, also known as the "American Competes Act" replaced the old definition of the metric system with the definition of the SI.

NIST SP 330 and NIST SP 811, both edited by Barry N. Taylor and Ambler Thompson, can be requested in hard copy by contacting TheSI@nist.gov or downloading electronic copies here:

Additional SI information can be found on the NIST Physics Laboratory website.

Further discussion of the Meter Convention, the SI, and international metrology may be found at the International Bureau of Weights and Measures (BIPM) website.

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