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
> >
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> >
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----- End forwarded message -----
Received on Thu Jan 08 2015 - 06:34:40 GMT
