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Dear Jonathan,
The IPK is just a lump (cylinder) of platinum and Iridium that initially 17 nations agreed would be the international reference standard for the kg. By that definition there could be no uncertainty as there was no other international reference with which
to compare it by measurement.
Similarly, with acceptance of the new definition the value of Plank's constant has no uncertainty as there is no other international reference to compare it with. However it does now allow the uncertainty of the 'old' IPK to be estimated by measurement.
I hope this may help you get out more!
Best wishes,
Graham White
------ACB discussion List Information-------- This is an open discussion list for the academic and clinical community working in clinical biochemistry. Please note, archived messages are public and can be viewed via the internet. Views expressed are those of the individual who posts and they are solely responsible for all message content. The ACB does not monitor posts. ACB Web Site http://www.acb.org.uk Green Laboratories Work http://www.laboratorymedicine.nhs.uk List Archives http://www.jiscmail.ac.uk/lists/ACB-CLIN-CHEM-GEN.html List Instructions (How to leave etc.) http://www.jiscmail.ac.uk/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
Best wishes
John O’Connor