Its official – on the 16th November scientists from around the world have voted to revise the SI. This means that all the SI units will now be defined in terms of constants. The definition of the kilogram, ampere, kelvin and mole will formally change from the 20th May 2019.
The kilogram will no longer be defined as the cylinder of platinum alloy kept in vault at BIPM, but will be defined in terms of the Plank constant.
The definition of the Kelvin will be defined in terms of the Boltzmann constant rather than the water triple point. Whilst there is a fundamental change in the definition of the kelvin and there will be no noticeable change in the magnitude of the kelvin or °C.
The new definitions will allow for improved temperature metrology as technology improves whilst the value of the water triple point remains unchanged and the use of the International Temperature Scale (ITS-90) will continue with the same defined values and uncertainties.
More information can be found via the BIPM.