Non-SI units mentioned in the SI
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Unit accepted for use in the International System of Units
This is a list of units that are not defined as part of the International System of Units (SI) but are otherwise mentioned in the SI Brochure,[1] listed as being accepted for use alongside SI-units, or for explanatory purposes.
Units officially accepted for use with the SI[edit]
The SI prefixes can be used with several of these units, but not, for example, with the non-SI units of time.
Other units defined but not officially sanctioned[edit]
The following table lists units that are effectively defined in side- and footnotes in the 9th SI brochure. Units that are mentioned without a definition or that occur in historical material recorded in the appendices are not included.
Changes to units mentioned in the SI[edit]
With the publication of each edition of the SI brochure, the list of non-SI units listed in tables changed compared to the preceding SI brochures.[3] The table below compares the status of each unit for which the status has changed between editions of the SI Brochure.
| Name | Symbol | 1st–3rd SI Brochures | 4–6th SI Brochures | 7th SI Brochure | 8th SI Brochure | 9th SI Brochure |
|---|---|---|---|---|---|---|
| astronomical unit | au[note 4] | accepted | not mentioned | accepted | accepted | accepted |
| parsec | pc | accepted | not mentioned | not mentioned | not mentioned | not mentioned |
| neper | Np | not mentioned | not mentioned | accepted | listed | accepted |
| bel | b | not mentioned | not mentioned | accepted | listed | accepted |
| decibel | db | not mentioned | not mentioned | not mentioned | listed | accepted |
| unified atomic mass unit[note 3] | u | accepted | accepted | accepted | accepted | footnote only |
| dalton[note 3] | Da | not mentioned | not mentioned | footnote only | accepted | accepted |
| natural unit (n.u.) of speed | c0 | not mentioned | not mentioned | not mentioned | listed | not mentioned |
| n.u. of action | ħ | not mentioned | not mentioned | not mentioned | listed | not mentioned |
| n.u. of mass | me | not mentioned | not mentioned | not mentioned | listed | not mentioned |
| n.u. of time | ħ/(mec02) | not mentioned | not mentioned | not mentioned | listed | not mentioned |
| atomic unit (a.u.) of charge | e | not mentioned | not mentioned | not mentioned | listed | not mentioned |
| a.u. of mass | me | not mentioned | not mentioned | not mentioned | listed | not mentioned |
| a.u. of action | ħ | not mentioned | not mentioned | not mentioned | listed | not mentioned |
| a.u. of length | a0 | not mentioned | not mentioned | not mentioned | listed | not mentioned |
| a.u. of energy | Eh | not mentioned | not mentioned | not mentioned | listed | not mentioned |
| a.u. of time | ħ/Eh | not mentioned | not mentioned | not mentioned | listed | not mentioned |
| nautical mile | M | temporarily accepted | temporarily accepted | temporarily accepted | listed | not mentioned |
| knot | kn | temporarily accepted | temporarily accepted | temporarily accepted | listed | not mentioned |
| ångström | Å | temporarily accepted | temporarily accepted | temporarily accepted | listed | not mentioned |
| are | a | temporarily accepted | temporarily accepted | temporarily accepted | not mentioned | not mentioned |
| hectare | ha | temporarily accepted | temporarily accepted | temporarily accepted | accepted | accepted |
| barn | b | temporarily accepted | temporarily accepted | temporarily accepted | listed | not mentioned |
| bar | bar | temporarily accepted | temporarily accepted | temporarily accepted | listed | not mentioned |
| standard atmosphere | atm | temporarily accepted | listed | listed | not mentioned | not mentioned |
| gal | Gal | temporarily accepted | temporarily accepted | listed | listed | footnote only |
| curie | Ci | temporarily accepted | temporarily accepted | listed | not mentioned | not mentioned |
| roentgen | R | temporarily accepted | temporarily accepted | listed | not mentioned | not mentioned |
| rad | rad | temporarily accepted | temporarily accepted | listed | not mentioned | not mentioned |
| rem | rem | not mentioned | temporarily accepted | listed | not mentioned | not mentioned |
| erg | erg | listed | listed | listed | listed | not mentioned |
| dyne | dyn | listed | listed | listed | listed | not mentioned |
| poise | P | listed | listed | listed | listed | not mentioned |
| stokes | st | listed | listed | listed | listed | not mentioned |
| maxwell | Mx | listed | listed | listed | listed | not mentioned |
| gauss | G | listed | listed | listed | listed | not mentioned |
| ørsted | Oe | listed | listed | listed | listed | not mentioned |
| phot | ph | listed | listed | listed | listed | not mentioned |
| fermi | listed | listed | listed | not mentioned | not mentioned | |
| metric carat | listed | listed | listed | not mentioned | not mentioned | |
| torr | Torr | listed | listed | listed | not mentioned | not mentioned |
| kilogram-force | kgf | listed | listed | not mentioned | not mentioned | not mentioned |
| calorie | cal | listed | listed | listed | not mentioned | not mentioned |
| micron | μ | listed | listed | listed | not mentioned | not mentioned |
| x-unit | xu | listed | listed | not mentioned | not mentioned | not mentioned |
| stilb | sb | listed | listed | not mentioned | not mentioned | not mentioned |
| gamma (Mass) | γ | listed | listed | listed | not mentioned | not mentioned |
| γ (magnetic flux density) | γ | listed | listed | not mentioned | not mentioned | not mentioned |
| lambda | λ | listed | listed | not mentioned | not mentioned | not mentioned |
| jansky | Jy | not mentioned | not mentioned | listed | not mentioned | not mentioned |
| millimetre of mercury | mmHg | not mentioned | not mentioned | not mentioned | listed | not mentioned |
In this table, the status descriptions have the following meanings:
- “accepted”: The unit is accepted for use with the SI.
- “temporarily accepted”: The unit is accepted for use with the SI, but is planned to be phased out.
- “listed”: The unit is defined in a table of units in the brochure but not accepted for use with the SI.
- “footnote only”: The unit is defined in a footnote or side note, without any mention in the main text.
- “not mentioned”: There is no mention of the unit in the brochure, excluding historical appendices.
See also[edit]
Notes and references[edit]
Notes[edit]
- ^ A footnote in the 9th SI Brochure gives an exact definition of the dalton.
- ^ This is a unit employed in geodesy and geophysics to express acceleration due to gravity.
- ^ a b c The unified mass unit (u) is a synonym of the dalton (Da). In edition 9 (2019) of the SI Brochure, the unified mass unit is no longer listed as being accepted for use with SI units, though it notes its equivalence with the dalton in a footnote. In edition 8 (2006), both names were mentioned in parallel. In edition 7 (1998), position was the reverse of that in edition 9. Earlier editions mentioned only u.
- ^ The symbol given for the astronomical unit changed from ua in the 8th to au in the 9th SI Brochure.
References[edit]
- ^ Bureau international des poids et mesures, “Non-SI units that are accepted for use with the SI”, in: Le Système international d’unités (SI) / The International System of Units (SI), 9th ed. (Sèvres: 2019), ISBN 9789282222720, c. 4, pp. 145–146.
- ^ a b “2018 CODATA Value: atomic mass constant”. The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-05-20.
- ^ Bureau international des poids et mesures, Le Système international d’unités (SI) / The International System of Units (SI), 8th ed. (Sèvres: Organisation Intergouvernementale de la Convention du Mètre, 2006‑05), ISBN 9282222136.
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