Opened 13 months ago

Last modified 7 months ago

#5775 accepted enhancement

Avogadro's number changed (and AMU with that)

Reported by: Gunther Schadow Owned by: Brenée Mitchell
Priority: major Milestone: Version 2.2
Component: unit definitions Keywords:
Cc:

Description (last modified by Gunther Schadow)

Originally from Lynch, Paul (NIH/NLM/LHC) [E] <plynch@…> for Clem McDonald?:

A needed change to a couple of UCUM constants:

In 2019 the definition of “mole” was revised (https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units#Mole) to be exactly 6.02214076×1023 (instead of being experimentally determined), but UCUM has an older value of 6.0221367×1023.

Also, the conversion of an atomic mass unit to g (experimentally determined) needs an update. Per the 2018 CODATA recommendations (https://physics.nist.gov/cuu/pdf/wallet_2018.pdf) the recommended value is now 1.660 539 066 60(50) × 10−24 g. UCUM has 1.6605402 × 10-24. Note that the “(50)” is the uncertainty on the previous two digits and should not be included in the value.

The re-definition of a mole as a number affects how conversions between mass and moles are done, because now molecular weight is no longer exactly equal numerically (modulo units) to molar weight, though the difference is very slight (about the 7th significant digit). However, I don’t think that affects any part of the text in the specification, as far as I can tell.

So, there is the definition of the two constants to change (“Table 4: SI units”, and “Table 5: Other units…”), and also there are several units in “Table 26: Example Unit Terms by Term” which involve “mol” whose “canonical form value” will need to be revised.

I don’t know how updates to constants have been handled in the past, but perhaps it is worth considering incrementing the version number of the standard from 2.1 to 2.1.1.

Change History (8)

comment:1 Changed 13 months ago by Gunther Schadow

Description: modified (diff)

comment:2 Changed 13 months ago by Gunther Schadow

[443] - tentative update of Avogadro's number and definition value of AMU, needs some more checking of text and also needs more commentary to describe the update history and the new official status of the mol."

comment:4 Changed 9 months ago by Brenée Mitchell

Owner: set to cjmcdonald
Status: newassigned

comment:5 Changed 8 months ago by Brenée Mitchell

note from Clem: we should fix these. Believe it NLM may have changed it in their ucum converter

comment:6 Changed 8 months ago by Gunther Schadow

This I had already applied in a joint session with the Regenstrief people. [443]

comment:7 Changed 8 months ago by Simon Cox

I didn't see it in the ucum-essence.xml file. But now I understand that 'essence' is not the normative artefact. That was perhaps a misunderstanding on my part.

However, I think it also exposes a problem with UCUM - the normative definitions should be available in a machine-processable form, not just a table embedded in an HTML spec. At the least a machine-processable artefact should be automatically synced from the spec.

comment:8 Changed 7 months ago by Brenée Mitchell

Owner: changed from cjmcdonald to Brenée Mitchell
Status: assignedaccepted

Decision from 17 February 2021 Board Meeting was to update all constants.

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