Realisation (metrology)

In metrology, the realisation of a unit of measure is the conversion of its definition into reality.

An example of the concept of realisation, is the unit of length, the meter. The metre was originally defined in 1791 as one ten-millionth of the distance from the equator to the North Pole along a great circle. To actually measure a length, this definition must be converted into a physical tool, which can be used to complete the measurement. The meter stick is the realisation of the meter.

The International Bureau of Weights and Measures maintains the techniques for realisation of the base units in the International System of Units (SI), all seven of which are defined in terms of natural physical constants, rather than human artefacts such as the standard kilogram. Following the 2019 revision of the SI all fundamental units of metrology are now defined in terms of natural physical constants, rather than human artefacts. The realization of these units is also defined by a published "Practice for the Realization of the Unit", for each unit. This is a detailed set of technical instructions for the construction of a device that will produce a practical realization of each unit. Any competent person, who follows these instructions can realize any unit.

For example, the metre is defined as 1/299792458 of the distance light travels in one second. The Practice for the Realization of the Metre describes how to build an apparatus to determine this distance. Using this apparatus it is possible to construct a metre stick which is the realization of the metre.

The International vocabulary of metrology identifies three distinct methods of realisation:

  1. Realisation of a measurement unit from its definition.
  2. Reproduction of measurement standards.
  3. Adopting a particular artefact as a standard.
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