On 17 Nov 2018, at 09:53, John <[log in to unmask]> wrote:

Just in case list members may have not noticed this. The redefinition of the SI units is now upon us. On 16th November 2018, measurement scientists from more than 60 countries have come together to witness the vote on the redefinition of the International System of Units (SI), changing the world's definition of the kilogram, the ampere, the kelvin and the mole.

The biggest issue was that, the base unit kilogram is based on an artifact, a platinum-iridium cylinder (the international prototype kilogram, IPK) manufactured in 1879 and stored at the BIPM. There is an intrinsic uncertainty in the long-term stability of the IPK. In principle, this instability also impacts other SI units that depend on the kilogram, among which is the mole. The redefinition links the unit of mass to fundamental or atomic constants.

In practice, nothing will really change, but in an ISO dominated scientific world, where the provenance/traceability of every measurement matters, this is of some relevance.

Nice  Utube video explaining the makeover from the BIPM

 https://www.bipm.org/en/about-us/

Introduction to  the new definition of the mol that does not rely  on the mass of a kilogram found here from IUPAC

https://iupac.org/new-definition-mole-arrived/

The paper describing the new definition in some detail here

https://www.degruyter.com/view/j/pac.2018.90.issue-1/pac-2017-0106/pac-2017-0106.xml

FAQ’s from the BIPM here

https://www.bipm.org/utils/en/pdf/SI_FAQs_EN.pdf

As far as temperature is concerned   The  new  definition  of  the  kelvin  has  no  immediate  impact  on  the  status  of  the  widely-used  ITS-90  and  PLTS-2000  temperature  scales. So traceability back to reference thermometers should not be a problem, this is covered in the FAQ link above