Realization

In metrology, the realization (or embodiment) of a measurement unit or a measurement scale is the practical method by which the unit can be measured (or the scale put into practice). The realization of a unit or of a scale creates one or more measurement standards against which an unknown physical quantity of the same kind can be compared.

There are three general methods of realizing a unit or a scale. The first and most obvious is simply to follow the definition of the unit or scale, for example by measuring the distance travelled by light in vacuum in ¹⁄_299,792,458 of a second to realize the metre. The second method is to use a physical phenomenon which is accepted to be equivalent to the definition, for example realizing the metre by laser interferometry with a measured frequency of light. The third method is to have a physical standard, for example the now-obsolete International Prototype Metre.

As a realization is a measurement, it is associated with a measurement uncertainty, which is called the standard measurement uncertainty. The standard measurement uncertainty is a component of the uncertainty in any measurement result which relies on the measurement standard, although it is often (indeed usually) negligeable compared to other components of the uncertainty.

International System of Units

The realizations of the base units in the International System of Units (SI) are known as mises en pratique (literally, "putting into practice"), and are approved by the International Committee for Weights and Measures (CIPM).

Unit	Principle of realization	ur	Link
Metre	laser interferometry using standard wavelengths	10−12	[1]
Kilogram	mass of the International Prototype Kilogram immediately after cleaning and washing	<10−8	[2]
Second	caesium fountain atomic clock	<10−15	[3]
Ampere	SI watt, ohm and volt (two out of three suffice) conventional volt and ohm with SI values of KJ and RK	<10−6 10−7	[4]
Kelvin	International Temperature Scale (ITS-90)		[5]
Mole	molar mass	<10−6	[6]
Candela	radiometry with absolutely calibrated detectors		[7]

The relative standard measurement uncertainties (u_r) are given for the best realizations, such as those carried out in national physical laboratories. Routine realizations may have uncertainites which are several orders of magnitude larger.