International System of Units

by | Jun 1, 2020 | NPU Concept model, Measurement units

International System of Units

Abbreviation SI units
Synonym Metric system of units
Definition A set of measurement units defined accordance to the corresponding International System of Quantities (ISQ)
Scope

The system is founded on 7 base SI units corresponding to the 7 base kinds-of-quantity. The base units can be combined in various ways forming derived units.

Note: In laboratory medicine, number of entities with the corresponding base unit, 1, is well recognized, although it does not belong to the SI units.
Rules

 

The rules for the expression of quantities in SI units are to ensure consistent and legible formatting, including:

  • Recommended prefixes for units (e.g. milli (m) 10-3, micro (µ) 10-6)
  • Avoid preceding zeroes after a decimal point (e.g. 1.2 umol/L is preferable to 0.0012 mmol/L)
  • Numbers with many digits should be grouped into threes, separated by spaces
  • Very large numbers should be expressed using exponents and the required number of significant digits (e.g. 6.022 x 1023 versus 602 214 129 000 000 000 000 000)

IUPAC publishes detailed recommendations for properties, units and formats for the reporting of clinical laboratory results in the “Silver Book” (Compendium of Terminology and Nomenclature of Properties in the Clinical Laboratory Sciences).

Note:

Volume should be expressed litre (L, mL, µL, etc) rather than the strict SI unit of volume, the cubic metre (m3).
Reference
  1. BIPM measurement units, https://www.bipm.org/en/measurement-units/
  2. International vocabulary of metrology, https://www.bipm.org/en/publications/guides/vim.html
  3. Ferard G, Dybkaer R, Fuentes-Arderiu X. Compendium of Terminology and Nomenclature of Properties in Clinical Laboratory Sciences : Recommendations 2016. 1 ed: Royal Society of Chemistry; 2016. 182 p. doi:10.1039/9781782622451.
  4. Hansen YBL. Recommendations on Measurement Units – Why and How. EJIFCC. 2019;30(3):250-75.
Base units
Base kind-of-quantity Base unit Dimension
Term Term Symbol Symbol
Length metre m L
Mass kilogram kg M
Time second s T
Electrical current ampere A I
Thermodynamic temperature kelvin K An external file that holds a picture, illustration, etc. Object name is ejifcc-30-250-g001.jpg
Amount-of-substance mole mol N
Luminous intensity candela cd J

Number of entities

 

 one 1 1

Note: Number of entities does not belong to the ISQ, but is an accepted and neccessary kind-of-quantity in laboratory medicine.
Derived units
Derived kind-of-quantity Base units Derived units
Term Term Term Symbol
Mass concentration kilogram metre g/L
Substance content mole kg mol/kg
Number ratio one one 10³
Substance fraction one one 10¹²
Catalytic-activity mol s

kat

Note: The table represents only a fraction of derived units

Reference
  1. BIPM measurement units, https://www.bipm.org/en/measurement-units/
  2. International vocabulary of metrology, https://www.bipm.org/en/publications/guides/vim.html
  3. Ferard G, Dybkaer R, Fuentes-Arderiu X. Compendium of Terminology and Nomenclature of Properties in Clinical Laboratory Sciences : Recommendations 2016. 1 ed: Royal Society of Chemistry; 2016. 182 p. doi:10.1039/9781782622451.
  4. Hansen YBL. Recommendations on Measurement Units – Why and How. EJIFCC. 2019;30(3):250-75.
Abbreviation SI units
Synonym SI Units, Système international (d’unités)
Description

The SI units system is the most widely used system of measurement in the world, administered by the Bureau International des Poids et Mesures (BIPM).  There are seven base units of measurement, and by 20th May 2019 each will be defined by physical constants in nature.  For example, the second will be defined as a fixed number of transition of the caesium-133 atom, the metre will be based on the speed of light in a vacuum per unit time, the mole will equal a fixed number of elements/molecules (“the Avogadro constant”).

There are rules for the expression of quantities in SI units to ensure consistent and legible formatting, including:

  • Recommended prefixes for units (e.g. milli 10-3, micro 10-6)
  • Avoid preceding zeroes after a decimal point (e.g. 1.2 umol/L is preferable to 0.0012 mmol/L)
  • Numbers with many digits should be grouped into threes, separated by spaces
  • Very large numbers should be expressed using exponents and the required number of significant digits (e.g. 6.022 x 1023 versus 602 214 129 000 000 000 000 000)

IUPAC publishes detailed recommendations for properties, units and formats for the reporting of clinical laboratory results in the “Silver Book” (Compendium of Terminology and Nomenclature of Properties in the Clinical Laboratory Sciences).

Substance concentrations should be expressed in moles per litre (mol/L, mmol/L, umol/L, etc) rather than the strict SI unit of volume, the cubic metre.
Reference
  1. BIPM measurement units, https://www.bipm.org/en/measurement-units/
  2. International vocabulary of metrology, https://www.bipm.org/en/publications/guides/vim.html
  3. Ferard G, Dybkaer R, Fuentes-Arderiu X. Compendium of Terminology and Nomenclature of Properties in Clinical Laboratory Sciences : Recommendations 2016. 1 ed: Royal Society of Chemistry; 2016. 182 p. doi:10.1039/9781782622451.
  4. Hansen YBL. Recommendations on Measurement Units – Why and How. EJIFCC. 2019;30(3):250-75.