Metrology is about measurement. To measure is to make a comparison between an item or quantity and a standard unit of measurement.
A unit is a value used to describe a quantity, e.g. weight is measured in kilograms.
We must have confidence that the unit for measuring a given quantity is the same for everyone everywhere, e.g. the kilogram of rice we buy in Senegal should be the same weight as the kilogram of rice we buy in Samoa.
To achieve this we need:
- Internationally accepted units and measurement standards
- Systems to verify the accuracy of instruments that are used for measurement
SI System of Units
The SI system was established in 1960 by the 11th General Conference on Weights and Measures CGPM:
- “The International System of Units, SI, is the coherent system of units adopted and recommended by the CGPM”.
- At the 14th CGPM in 1971 the SI was again extended by the addition of the mole as base unit for amount of substance.
- The SI system is now comprised of seven base units, which together with derived units make up a coherent system of units. In addition, certain other units outside the SI system are accepted for use with SI units.
Quantity |
Base unit | Symbol |
length | metre | m |
mass | kilogram | kg |
time second s | second | s |
electric current | ampere | A |
thermodynamic temperature | kelvin | K |
amount of substance | mole | mol |
luminous intensity | candela | cd |