Joe Collins wrote:
> I agree with this except for the possible implication that "the VIM is
>primarily
> about making measurements of quantities" is an unnecessarily narrow
>perspective.
> The length of an object only has (physical) meaning when arrived at via a
> measurement process.
>
I agree with that. I don't see how it contradicts the statement. I
rather thought that "making measurements" is "measurement process". The
VIM sees 'quantity' primarily as the 'property instance', the "trope".
Those are the ones that have physical meaning. The abstractions are
about assigning values to them by means of measurements.
> ++++snip++++
>
>
>>>> "Quantity value" is most generally a number and a reference to a
>>>> measurement
>>>> procedure. In the usual case where the quantity value is a
>>>> (multiplicative)
>>>> product of a number and a measurement unit, the measurement unit
>>>> refers to a
>>>> part of the measurement apparatus (the essential part).
>>>>
>>>>
>> I have several problems with this.
>> First, quantity value is defined to be a number and a reference to a
>> measurement _unit_.
>>
>
> Not only that:
>
> "A reference can be a measurement unit, a measurement procedure, a reference
> material, or a combination of such." (ISO/DIS 80000-1)
>
So your issue is about the fact that quantity values necessarily have
associated uncertainties that are derived from their provenance? That
is true, but it is the next step down the path. We are still trying to
get past the question of which of "the height of the Eiffel Tower" or
"183 metres" is what is meant by "quantity". (01)
If in DIS 80000 terms a 'quantity value' can be expressed in terms of a
number and a measurement procedure, or a number and a reference
material, then I will be of no help in using DIS 80000 as a basis for
the ontology. But I will be happy to try to make sense of the draft you
develop.
> In the case of the kilogram, the standard, which defines the unit, is an
> essential part of the apparatus. You cannot weigh something without it (or
>one
> of its "replicas").
>
The case of the kilogram is unique. The definition of the mole or the
second or the metre has nothing to do with apparatus. Ergo, the
generalization you stated is false. They are all referenced to specific
physical phenomena, but the whole idea of the last 50 years is to
reference them to invariant reproduceable physical phenomena, instead of
specific artifacts or artifices. The kilogram is the last holdout of
the 1876 approach.
> All but one of the SI reference units are defined in terms
>
>> of an invariant physical phenomenon that can be measured in any
>> laboratory with appropriate equipment. Moreover, the "best known
>> procedures" (in terms of "smallest uncertainty") and the corresponding
>> equipment have changed several times over the last 40 years, but those
>> changes don't change the units. Changes in the apparatus produce changes
>> (hopefully improvements) in the "uncertainty" of the measurement of the
>> phenomenon.
>>
>
> It is true that names of the units have not changed, but their definitions
>have.
> The only reason this does not present problems is because the newer
>definitions
> support greater precision and are made to be backwardly compatible with old
> standards.
>
That is correct. The definitions of the units have become somewhat more
precise, and in one case a different physical phenomenon was chosen.
That is a consequence of improved measurement apparatus and the need for
more precision for some modern applications. But, except for the
artifact kilogram, none of the SI base units of measure is defined by
reference to any apparatus. Each unit is defined by reference to a
physical phenomenon that we can now measure better. (02)
The actual measurement mechanisms used for the local reference standards
in the participating countries vary widely, with some consequences for
international trade. There is a joint international activity (recently
expanded) whose objective is to reconcile and align the results of the
multiple technologies and practices. And this is not just the EU vs.
Outer Slobbovia; this is U.S. v. Germany and India v. Japan. (This is
also off-topic, but my point is that the units are not defined in terms
of the practices, and the practices don't always agree.)
> I do not think of the kilogram artifact as somehow a "less valid" standard,
> though I agree it is certainly less convenient to distribute and does not
> provide opportunity for improvement in precision.
>
>
No one said it was "less valid". What I said was that it is the ONLY
unit that is dependent on apparatus, which means that apparatus is NOT a
common characteristic of units of measure.
> <snip>
>
you can take the rest of this up with the metrology experts. NIST
employs several hundred of them, but I ain't one.
>>>> The measurement instrument, in this case a
>>>> weighing scale, is calibrated in terms of the reference. The
>>>> kilogram standard
>>>> is the essential part of the measurement apparatus. The numbers that
>>>> the
>>>> weighing scale gives for masses are the "numbers" referred to in the
>>>> definition
>>>> of "quantity".
>>>>
>>>>
>>>>
>> The U.S. reference kilogram was made from the International Reference
>> kilogram by polishing a slightly overweight copy of the artifact and
>> comparing the magnitude on a very precise analytical balance. There was
>> no 'scale' of the kind described here involved.
>>
>
> Very precise, yes, but is not an analytical balance still a weighing scale?
>
It doesn't give "numbers". And the critical point here is that the
magnitude of a quantity is independent of numbers. When you introduce a
measurement unit, you then have a basis for a "number". But if you
introduce a different measurement unit, you get a different number. The
magnitude _is_; the quantity value _expresses_.
> While not of identical design, many labs use scales that are design
>variations
> of the beam balance with mass standard.
>
Balances tell us what the relationship between two quantity magnitudes
is: greater, equal, less. They don't provide numbers, and they don't
depend on them. This constitutes a valuable insight in distinguishing
'magnitude' from 'quantity value'.
> If the reference kilogram is used to calibrate a mass measurement device, is
>it
> not then an essential part of the measurement apparatus? I do not think it
>needs
> to be permanently attached to be an essential part of the apparatus.
>
What the essential parts of a measurement apparatus may be is a topic
that I am not qualified to address. And IMO, it is irrelevant to the
work at hand.
> I agree that supporting SI is most important.
>
Good. But we need to support other systems of units, and whole
categories of derived units that we don't want to enumerate as well. (03)
The question is: Can we restrict our 'scales' and 'units' to the ones
in which the quantity values are ratios? (OK, plus temperature.) Or do
we define broad categories and then define the subcategories we really
want to model and do those right? The problem I have with things like
"hardness" is that I have no idea what subset of the useful axioms would
include it.
> (out of time for today)
>
> Joe C.
>
You and me both. (04)
Thanks, Joe. (05)
-Ed (06)
--
Edward J. Barkmeyer Email: edbark@xxxxxxxx
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8263 Tel: +1 301-975-3528
Gaithersburg, MD 20899-8263 FAX: +1 301-975-4694 (07)
"The opinions expressed above do not reflect consensus of NIST,
and have not been reviewed by any Government authority." (08)
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