Jonathan,
avogadro_constant = 6.022...e23/mole
molecule = 1/avogadro_constant => 6.022...e23 molecules = 6.022...e23
(mole/6.022...e23) = 1 mole
Now if we could just get the people who want units of "photon", "enzyme",
"giraffe", etc. to use units of "avogadro_constant-1" (which is understood
by UDUNITS) then all would be well.
Regards,
Steve Emmerson
On Thu, Jan 8, 2015 at 6:34 AM, Jonathan Gregory <j.m.gregory at reading.ac.uk>
wrote:
> Dear Maarten and Steve
>
> > >> I think you meant to say that if a physical quantity has a different
> > >> dimensionality (not unit), then we have to give it a different name.
>
> Yes, that's right, I meant physical dimensionality. I said "unit" meaning
> "canonical unit". Sorry to be sloppy!
>
> If I'm not oversimplifying, the point is that mol of NO2 means the same as
> 6e23 (Avogadro number) of NO2 molecules. Maarten's argument implies that
> there
> is no need for mole as one of the basic SI units which can't be converted
> into
> other units, because instead mole could be defined as an alias for
> Avogadro's
> (dimensionless) number, like percent is defined as 0.01 by udunits. Then we
> would not need different standard_names referring to number of molecules or
> to moles.
>
> I think this makes physical sense but it would not be consistent with SI.
> In
> SI, Avogadro's number is not dimensionless; it is 6e23 mole-1. It would not
> be consistent with SI to allow mole to be interconvertible with
> dimensionless
> numbers. I suppose that udunits would like to be consistent with SI. If so,
> we cannot do this.
>
> I think I'm repeating myself so I must have missed the point still. Maarten
> suggests that molecule should be defined to mean 1/Avogadro. Why
> reciprocal?
> Defining molecule=Avogadro I think would be confusing. If a quantity is
> said
> to be 1 molecule m-2, I don't think that means 1 mole m-2.
>
> Best wishes
>
> Jonathan
>
>
> ----- Forwarded message from Steve Emmerson <emmerson at ucar.edu> -----
>
> > Date: Wed, 7 Jan 2015 10:37:58 -0700
> > From: Steve Emmerson <emmerson at ucar.edu>
> > To: Maarten Sneep <maarten.sneep at knmi.nl>
> > CC: CF Metadata Mail List <cf-metadata at cgd.ucar.edu>
> > Subject: Re: [CF-metadata] New UDUnits units for information: "byte" and
> > "octet"
> >
> > Maarten,
> >
> > I think I understand. You're fortunate in being "grandfathered-in", so to
> > speak.
> >
> > I just don't want to go down the road of adding support for all kinds of
> > different entities. I'm a bit sensitive in that regard. :-)
> >
> > Regards,
> > Steve Emmerson
> >
> > On Wed, Jan 7, 2015 at 10:31 AM, Maarten Sneep <maarten.sneep at knmi.nl>
> > wrote:
> >
> > > On 07-01-15 17:10, Steve Emmerson wrote:
> > >
> > >> Jonathan,
> > >>
> > >> I think you meant to say that if a physical quantity has a different
> > >> dimensionality (not unit), then we have to give it a different name.
> > >>
> > >> In my opinion, what's needed in this case is a package that
> understands
> > >> co-ordinate transformations -- in order to convert, for example,
> values
> > >> in units of "1e15/cm2" to values in units of "mol/m2". This is a
> rather
> > >> simple example and Maarten makes a good (though not yet convincing to
> > >> me) argument for simply modifying the UDUNITS database. One can
> imagine,
> > >> however, more complicated cases in which simple unit conversions are
> not
> > >> possible (e.g., converting between altitude and pressure). Such a
> > >> package would be easily capable of handling Maarten's conversion.
> > >>
> > >
> > > The only argument I'm making is that 'molecules' is available as a unit
> > > equivalent to 'avogadros_number-1' (which is the case with the current
> > > release of UDUnits). People who use 1/cm2 when they (implicitly) mean
> > > molecules/cm2 get what they deserve IMHO.
> > >
> > > As soon as you have molecules/cm2, then UDUnits can handle the
> conversion
> > > as is. For other densities, say an aerosol particle count, the
> conversion
> > > to mol is never needed, and number densities are fine (and a look at
> the
> > > standard_names will confirm this).
> > >
> > > So right now I'm not asking anything, as the most important scaled
> alias
> > > for 'mol' is available. This will ease the transition to mol/m2 quite
> > > significantly. It would be good to help the transition from photons to
> mol
> > > (photons) as well, which was a request that started this whole
> discussion
> > > in the first place.
> > >
> > > Does this make the argument clearer?
> > >
> > > Best,
> > >
> > > Maarten Sneep
> > > --
> > > KNMI
> > > T: 030 2206747
> > > E: maarten.sneep at knmi.nl
> > > R: A2.14
> > >
> > > _______________________________________________
> > > CF-metadata mailing list
> > > CF-metadata at cgd.ucar.edu
> > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> > >
>
> > _______________________________________________
> > CF-metadata mailing list
> > CF-metadata at cgd.ucar.edu
> > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
>
> ----- End forwarded message -----
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