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The International System of Units and the SI Brochure

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  • The International System of Units and the SI Brochure

    The International System of Units (SI, or "modern metric system") has recently been redefined to define the kilogram in terms of physical constants instead of a lump of metal. The BIPM has released the 9th edition of the SI Brochure, which is the official description of the SI and its proper usage, to address these changes. It is available as a free pdf download from their webpage:

    For the US, NIST publishes a version, NIST SP 330, with American, rather than British, spelling, and other minor editorial changes. The corresponding edition is usually delayed by a year or more. You can use the International version above and assume the usual US changes will be continued and eventually published:
    Spelling: meter, liter, and deka- in lieu of metre, litre and deca-
    Phrasing: metric ton is preferred to tonne, but symbol is "t" in either case
    Symbols: Preference for "L" as the symbol for liter vs no expressed preference between l and L.

    Prior US editions have discouraged but allowed continued use of some obsolete radiation units that the International editions does not recognize. I have no guess as to the position that will appear in the eventual 9th US edition.

    NIST also publishes NIST SP 811, an excellent, highly detailed guide to proper usage of the SI. Sections 1-5 largely repeat the old edition of the SI Brochure and should be considered obsolete until republished, but sections 6-10, and the appendix of conversion factors are still valuable. (free pdf download from NIST).