Hi.
I can't tell if this last post was intended to be humorous or not. I'm
in the mood to have fun and take it seriously, so here goes.
avogadros_number molecules = 1 mole molecules
A mole is like dozen. It is a count of whatever it is you are counting.
How many things are in a dozen? 12. How many things are in a mole?
avogadros_number. So, working with dozens I have:
12 eggs = 1 dozen eggs
1 egg = (1 dozen eggs) / 12
To make a quantity analogous to avogadro_constant, let's make
twelve_constant = 12 / dozen. I can then rewrite my formula as:
12 eggs = 1 * twelve_constant * dozen eggs
If I rearrange, I can write
1 egg = twelve_constant * dozen / 12 egg
1 egg = (twelve_constant / twelve_constant) egg
The inverse of twelve_constant is not 1. It is dozen / 12.
Twelve_constant and avogadro_constant are conversion factors, not units.
They convert scales without converting units (unlike conversion factors
between feet and meters, and like conversion factors between grams and
kilograms).
It is incorrect to write molecule = 1/avogadro_constant. In addition,
avogadro_constant should never be used as a unit, either straight up or
as a reciprocal.
Grace and peace,
Jim
On 1/8/15 1:39 PM, Steve Emmerson wrote:
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] <mailto:[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]
<mailto:[email protected]>> -----
> Date: Wed, 7 Jan 2015 10:37:58 -0700
> From: Steve Emmerson <[email protected] <mailto:[email protected]>>
> To: Maarten Sneep <[email protected]
<mailto:[email protected]>>
> CC: CF Metadata Mail List <[email protected]
<mailto:[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] <mailto:[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] <mailto:[email protected]>
> > R: A2.14
> >
> > _______________________________________________
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> > [email protected] <mailto:[email protected]>
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> >
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