NPL Realisation of SI Unit Definitions

I though that I should make it clearer exactly which of the SI units will be changed.  There are 7 fundamental SI units that all others are derived from.  This will probably be the biggest ever single change to our fundamental units since they started.  Great effort has been made to remove historical artifacts from our definitions because any variation in these artifacts change our standard to which everything else is measured against.  Its also very difficult to scale up or down from an artifact without further uncertainties creeping in.  It also means that as we get better and more accurate at measuring everything, the fundamental constants keep changing as they get more and more decimal places at the end.  If we fix the fundamental constants they will e forever constant and the unit definition can change getting more accurate with our increasing technology.  However, there are growing suspicions that our fundamental constants may not be absolutely constant after all, but this is not the place to be distracted.


The kg will change, and the Plank constant will be fixed forever at h = 6.62607015 x 10-34 Js exactly.


No change to the SI unit of length (m)

When this was previously changed, the speed of light was fixed forever at 299,792,458 m/s exactly.


No Change to the SI unit of time (s).

MOLE (mol)

The amount of substance (mol) will change by making Avogadro constant NA is 6.02214076 x 1023 mol-1 exactly.  The previously defined mass of carbon 12 will no longer be exactly 0.012 kg/mol.


The SI definition of current (A) will change, the elementary charge of an electron will be fixed forever at e = 1.602176634 x 10-19 C exactly.


The SI unit of temperature (K) will change, the Boltzmann constant will be fixed forever at k = 1.380649 x 10-23 J/K exactly.


No Change to the SI unit of light (cd).

Of course, many derived units will be affected by the redefinition’s, but great care has been taken so that all changes will lie within the area of uncertainty of the previously defined units, so it shouldn’t make any difference to any previous work.  All these changes will be approved during November 2018 and implemented on world metrology day on 20th May 2019.  No further changes are anticipated in the near or medium future after this date as all artifacts will have been removed and all fundamental units will rely on fundamental constants or atomic data.


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