The NIST Reference on Constants, Units and Uncertainty

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Detailed Contents
This site addresses three topics: fundamental physical constants, the International System of Units (SI), which is the modern metric system, and expressing the uncertainty of measurement results. Both essential information and background information are given for each topic, as described below.


Fundamental Physical Constants
Values Access to the latest set of self-consistent values of the basic constants of physics and chemistry recommended by CODATA (Committee on Data for Science and Technology) for international use.
Access to the latest conversion factors for energy equivalents recommended by CODATA for international use.
Searchable bibliography giving the citations of over 6400 published papers on the constants and related precision measurements.
International aspects of establishing recommended values.
Introduction to the constants for nonexperts.      
Links to selected scientific data.

International System of Units (SI)
  Introduction   Introduction to the essentials of the International System of Units (SI), the modern metric system, adapted from NIST Special Publications (SP) 330 and 811, which are based on internationally accepted documents on the SI.
Units SI base units and SI derived units. Includes a graphical illustration of how the 22 SI derived units with special names and symbols are related to the seven SI base units.
Prefixes The 20 SI prefixes used to form decimal multiples and submultiples of SI units. Includes a discussion of prefixes for binary multiples.
Outside Important and widely used units outside the SI.
Rules Checklist of rules and style conventions for proper SI usage. Includes rules for correct typefaces for symbols in scientific manuscripts.
Background Definitions of the SI base units.
Historical context of the base units: meter, kilogram, second, ampere, kelvin, mole, and candela, including definitions of the abbreviations CGPM, CIPM, and BIPM; and brief history of the SI.
International aspects of establishing and maintaining the SI, including a list of the various Consultative Committees (Comités Consultatifs) of the International Committee for Weights and Measures (CIPM, Comité International des Poids et Mesures).
Links to related sites that deal with units.
Bibliography for SI units, including online NIST publications on the SI.
  SP 330, the U.S. edition of the definitive reference on the SI published by the International Bureau of   Weights and Measures (BIPM, Bureau International des Poids et Mesures), which describes the SI and gives the various historical recommendations concerning it. Available online.
  SP 811, which not only describes the SI but gives in detail the rules and style conventions for printing and using units and for expressing values of quantities. Available online in a PDF version (413 kB), or in an HTML version, a printed copy may be ordered at no charge.
  The current Federal Register notice that gives the interpretation of the SI for the United States. Available online.
  SP 447, which provides a brief history of U.S. weights and measures from the founding of the country to about 1975. Available online.

Uncertainty of Measurement Results
Basic Introduction to the essentials of the approach to expressing measurement uncertainty that is rapidly becoming the single internationally accepted method. Includes a glossary of basic uncertainty terms.
Evaluating A type A evaluation of a standard uncertainty component is based on any valid statistical method for treating data. A type B evaluation of a standard uncertainty component is usually based on scientific judgment using all of the relevant information available.
Combining Calculation of combined standard uncertainty, including simplified equation forms.
Expanded uncertainty and coverage factor.
Examples Examples of uncertainty statements that would be used in publications or correspondence.
  Background   International and U.S. perspectives on the effort that has led to the near world-wide adoption of a single approach to the expression of uncertainty in measurement.
Sources. The information on evaluating and expressing measurement uncertainty within this reference is adapted from NIST Technical Note 1297. Available online in a PDF version (136 kB) or in an html version, or a printed copy may be ordered at no charge. This publication is based on the comprehensive International Organization for Standardization (ISO) Guide to the Expression of Uncertainty in Measurement, or GUM (available from the BIPM website), available online in a PDF version. See also NIST Technical Note 1900, available online in a PDF version.
Related information. The International Vocabulary of Basic and General Terms in Metrology, or VIM, gives the definitions of many important terms relevant to the field of measurement. Available for purchase.
Other: Frequently asked questions and Links to on-line unit conversions

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Online: February 1998       Last update: January 2018