Editing International System of Units (section)

== Realisation of units <span class="anchor" id="Realisation"></span> ==

{{Main|Realisation (metrology)}}
[[File:Silicon sphere for Avogadro project.jpg|thumb|upright|Silicon sphere for the [[Avogadro project]] used for measuring the Avogadro constant to a relative [[standard uncertainty]] of {{val|2|e=−8}} or less, held by [[Achim Leistner]]<ref>{{cite web |title=Avogadro Project |url=http://www.npl.co.uk/science-technology/mass-and-force/research/avogadro-project |publisher=National Physical Laboratory |access-date=19 August 2010}}</ref>]]
Metrologists carefully distinguish between the definition of a unit and its realisation. The SI units are defined by declaring that seven ''defining constants''<ref name="SIBrochure9thEd" />{{rp|pages=125–129}} have certain exact numerical values when expressed in terms of their SI units. The realisation of the definition of a unit is the procedure by which the definition may be used to establish the value and associated uncertainty of a quantity of the same kind as the unit.<ref name="SIBrochure9thEd"/>{{rp|page=135}}

For each base unit the BIPM publishes a {{lang|fr|mises en pratique}}, ([[French language|French]] for 'putting into practice; implementation',<ref name="NIST mise en pratique">{{cite journal |url=https://www.nist.gov/programs-projects/nist-mise-en-pratique-new-kilogram-definition |title=NIST Mise en Pratique of the New Kilogram Definition |journal=[[NIST]] | date=2013 |access-date=9 May 2020 |archive-url=https://web.archive.org/web/20170714202843/https://www.nist.gov/programs-projects/nist-mise-en-pratique-new-kilogram-definition |archive-date=14 July 2017 |url-status=live}}</ref>) describing the current best practical realisations of the unit.<ref name="MisesEnPratique">{{cite web |url=https://www.bipm.org/en/publications/mises-en-pratique/ |title=Practical realizations of the definitions of some important units |publisher=[[BIPM]] | date=2019 |access-date=11 April 2020 |archive-url=https://web.archive.org/web/20200409115245/http://www.bipm.org/en/publications/mises-en-pratique/ |archive-date=9 April 2020 |url-status=live}}</ref> The separation of the defining constants from the definitions of units means that improved measurements can be developed leading to changes in the {{lang|fr|mises en pratique}} as science and technology develop, without having to revise the definitions.

The published {{lang|fr|mise en pratique}} is not the only way in which a base unit can be determined: the SI Brochure states that "any method consistent with the laws of physics could be used to realise any SI unit".<ref name="SIBrochure"/>{{rp|page=111}} Various consultative committees of the [[CIPM]] decided in 2016 that more than one {{lang|fr|mise en pratique}} would be developed for determining the value of each unit.<ref>{{cite web|title=International Committee for Weights and Measures – Proceedings of the 106th meeting|url=https://www.bipm.org/utils/en/pdf/CIPM/CIPM2017-EN.pdf}}</ref> These methods include the following:
* At least three separate experiments be carried out yielding values having a relative [[standard uncertainty]] in the determination of the [[kilogram]] of no more than {{val|5|e=-8}} and at least one of these values should be better than {{val|2|e=-8}}. Both the [[Kibble balance]] and the [[Avogadro project]] should be included in the experiments and any differences between these be reconciled.<ref>{{cite web |url=http://www.bipm.org/utils/common/pdf/CCM12.pdf#page=23 |title=Recommendations of the Consultative Committee for Mass and Related Quantities to the International Committee for Weights and Measures |website=12th Meeting of the CCM |date=26 March 2010 |publisher=Bureau International des Poids et Mesures |location=Sèvres |access-date=27 June 2012 |archive-url=https://web.archive.org/web/20130514081750/http://www.bipm.org/utils/common/pdf/CCM12.pdf#page=23 |archive-date=14 May 2013 |url-status=dead}}</ref><ref>{{cite web |url=http://www.bipm.org/utils/common/pdf/CCQM16.pdf#page=40 |title=Recommendations of the Consultative Committee for Amount of Substance – Metrology in Chemistry to the International Committee for Weights and Measures |website=16th Meeting of the CCQM |date=15–16 April 2010 |publisher=Bureau International des Poids et Mesures |location=Sèvres |access-date=27 June 2012 |archive-url=https://web.archive.org/web/20130514072057/http://www.bipm.org/utils/common/pdf/CCQM16.pdf#page=40 |archive-date=14 May 2013 |url-status=dead}}</ref>
* The definition of the [[kelvin]] measured with a relative uncertainty of the [[Boltzmann constant]] derived from two fundamentally different methods such as acoustic gas [[thermometry]] and dielectric constant gas thermometry be better than one part in {{val|e=-6}} and that these values be corroborated by other measurements.<ref>{{cite web |url=http://www.bipm.org/utils/common/pdf/CCT25.pdf#page=53 |title=Recommendations of the Consultative Committee for Thermometry to the International Committee for Weights and Measures |website=25th Meeting of the CCT |date=6–7 May 2010 |publisher=Bureau International des Poids et Mesures |location=Sèvres |access-date=27 June 2012 |archive-url=https://web.archive.org/web/20130514064646/http://www.bipm.org/utils/common/pdf/CCT25.pdf#page=53 |archive-date=14 May 2013 |url-status=dead}}</ref>