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 <[email protected]> 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 <[email protected]> ----- > > > Date: Wed, 7 Jan 2015 10:37:58 -0700 > > From: Steve Emmerson <[email protected]> > > To: Maarten Sneep <[email protected]> > > CC: CF Metadata Mail List <[email protected]> > > 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 <[email protected]> > > 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: [email protected] > > > R: A2.14 > > > > > > _______________________________________________ > > > CF-metadata mailing list > > > [email protected] > > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > > > > _______________________________________________ > > CF-metadata mailing list > > [email protected] > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > ----- End forwarded message ----- > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >
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