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A recent work project involved a number of unit conversions between various systems of measurements. Due to the scale and critical nature of the project, we wanted to ascertain that our conversion factors were as accurate as possible.

While only tangentially related, what I couldn't determine was who held final authority in declaring what the "one true" conversion factors should be.

For example, NIST provides a set of conversions and does not make explicit reference to other standards organizations. This article on International Bureau of Weights and Measures (BIPM) implies that the BIPM is responsible for declaring those conversion factors.

Is there a single organization responsible for declaring and verifying the accuracy and precision of conversions?

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  • $\begingroup$ I would expect each system's defining body is responsible for also defining conversions to and from that particular system. $\endgroup$
    – Air
    Aug 7 '15 at 18:56
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    $\begingroup$ @Air That results in two bodies defining each conversion though. The conversions have to be two-way, so you can't have whoever defines the foot saying it's this many meters and whoever defines the meter saying it's this many feet unless they work together to ensure their answers are compatible. $\endgroup$ Aug 7 '15 at 21:02
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    $\begingroup$ I think each governing body defines it with respect to natural phenomenon so that the conversions are built in. The meter is defined by the speed light travels in a certain time for instance. The second is defined by a certain number of oscillations of a cesium atom. Etc. $\endgroup$ Aug 7 '15 at 22:56
  • $\begingroup$ Oops: The meter is defined by the distance light travels... $\endgroup$ Aug 8 '15 at 1:29
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In the US the National Institute of Standards and Technology (NIST) was established in 1901 by U.S. Code, Title 15, Ch. 7, sec. 272. Section (b) of that code states that, among other things, the functions of the Secretary and Institute is

(2) to develop, maintain, and retain custody of the national standards of measurement, and provide the means and methods for making measurements consistent with those standards;

(3) to compare standards used in scientific investigations, engineering, manufacturing, commerce, industry, and educational institutions with the standards adopted or recognized by the Federal Government and to coordinate the use by Federal agencies of private sector standards, emphasizing where possible the use of standards developed by private, consensus organizations;

(9) to assure the compatibility of United States national measurement standards with those of other nations;

so the US government has assigned NIST to maintain those standards.

Interestingly, the length of the inch/foot/yard was redefined (slightly) in 1959 to make it more consistent with the metric system. "The U.S., the United Kingdom and other Commonwealth countries agreed on this definition, and so it is often termed international measure." So, the current conversion between the imperial and metric units of length was agreed upon by the various governing bodies in the countries which use the imperial system.

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Internationally, the General Conference on Weights and Measures (French: Conférence générale des poids et mesures - CGPM) is the senior of the three Inter-governmental organizations established in 1875 to define standards for weights and measures. The other two organizations are:

  • the International Committee for Weights and Measures (French: Comité international des poids et mesures- CIPM) and
  • the International Bureau of Weights and Measures (French: Bureau international des poids et mesures - BIPM)

The conference meets in Sèvres (south-west of Paris) every four to six years. In 1960 the 11th CGPM approved the system of SI Units.

Each country has its own organization for establishing national standards, of which weights and measures is one, and their standards are legally binding for their country. Use the conversion factors set by the standards organization for the country in where the project will be realised/established.

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tl;dr National measurement institutes are responsible for ensuring the definition of SI units. They collaborate to ensure they get the same answer. Non-SI units are defined in terms of their SI equivalents.

The BIPM is responsible for determining and maintaining the standards for SI units in combination with national measurement institutes (NIST, NPL, PTB, etc.).

Traditionally, these were based on physical artifacts but nowadays they are mostly (except the kg) defined in terms of physical constants. Research on redefining the kg is currently ongoing.

Each country has its own measurement laboratory which provides standards and services to help industry ensure that units are correct. They also regularly do international comparisons to ensure they all give the same results.

Non-SI units are defined based on national and international laws and treaties. Historically, these were also physical artifacts. But now they are pretty much all defined in law in terms of their SI equivalent (either directly or indirectly). For example the inch is defined as exactly 25.4 mm and a US Gallon is 231 cubic inches.

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