Dear Matthew, (01)
Thank you very much for sending in your perspectives despite the fact
that you cannot join us on the panel. I am including this among the
panel inputs[1], and am sure that it will help in our discussion
during the session this Thursday. (02)
[1] ref.
http://ontolog.cim3.net/cgibin/wiki.pl?ConferenceCall_2013_10_10#nid3YFS (03)
Thanks & regards. =ppy
 (04)
 original message 
From: Matthew West <dr.matthew.west@xxxxxxxxx>
Date: Mon, Oct 7, 2013 at 1:24 AM
Subject: [ontologybasedstandards] Some issues to consider in
developing an ontology of quantities and units
To: ontologybasedstandards <ontologybasedstandards@xxxxxxxxxxxxxxxx> (05)
Dear Colleagues, (06)
Below are some notes from my experiences from working on physical
quantities and units of measure. (07)
Introduction (08)
When we developed ISO 159262 some 10+ years ago, quantities and units
were one of the most challenging areas we addressed, and we were not
entirely happy with the results of our work. With the benefit now of
some 10 years hindsight, here are a few of the issues I think need to
be addressed in such an ontology that are not immediately evident. (09)
Missing scales (010)
What is really going on with the use of quantities and units is that a
quantity (say a particular degree of hotness) is mapped to a number
(say 20) on a scale (say the Celsius scale). It is a desirable
property of a scale that the mapping from quantities to numbers is
isomorphic (1:1 relationship) so you can go back and forth reliably
from quantities to numbers. (011)
Interestingly you rarely see scales mentioned, and BIPM talk very much
about physical quantities and units of measure. However, scales for
different types of physical quantity can have the same unit of
measure, so you do not know you have two comparable values unless you
have both the type of physical quantity and the unit of measure. For
example, both stress and pressure can be expressed in the unit Pascal,
but a stress value is not the directly comparable to a pressure value.
Even more so with dimensionless numbers that do not have units, a
Reynolds Number is not comparable to a mass ratio because they have
the same unit. (012)
It is not clear to me what it means for two different scales to have
the same unit of measure, when values on the two scales are not
comparable. (013)
What are units really about? (014)
It is interesting to notice the practical use of units, or more
particularly the labels we give to units. When we do scientific or
engineering calculations based on units, we quite often end up with
composite units, such as DegC/s. However, when we find the same unit
as a divisor and multiplier, we conventionally cancel them out. A
dimensionless number is an extreme example where all the units cancel
out. (015)
The utility of this approach is that if you have a quantity expressed
in a unit, say DegC/s, and you want to change the units to say
DegF/hr, then you know that it is only the conversion factors for DegC
to DegF and s to hr that you need to concern yourself with, the fact
that there were also mass flowrates involved in the calculation is
unimportant if the units cancelled out. Of course with dimensionless
numbers, once calculated, they never have to be converted, but really
they are just a special case. (016)
I would argue that this conventional dimensionality, though useful for
conversions, is potentially misleading, since there is a natural
inclination to assume that two values expressed in the same units are
comparable. On the other hand, if you consider what call full
dimensionality, the difference and the reason for them becomes clear.
So considering a mass ratio as M/M instead of just dimensionless,
makes the distinction clear with a length ration which is L/L. (017)
These of course are simple dimensionless numbers. Reynolds Number
really is worth looking at as a more complex example. Reynolds Number
for a pipe may be calculated as: (018)
Q*D/v*A (019)
Where: (020)
Q = volumetric flowrate (m3/s) Dimensions (L3*T1) (021)
D = the pipe diameter (m) dimension (L) (022)
V = the kinematic viscosity (m2/s) dimensions (L2*T1) (023)
A = pipe crosssectional area (m2) dimensions (L2) (024)
So although the standard dimensionality is 0, the full dimensionality is: (025)
(L4*T1)/(L4*T1) (026)
Is a Maximum Allowable Working Temperature a Temperature? (027)
Practical engineering quantities also present challenges. One example
we used a lot was Maximum Allowable Working Temperature for a furnace
tube. (028)
Let’s consider an ordinary temperature first. A temperature is an
intrinsic property of some physical object that can be determined
solely by observing the physical object itself. (029)
On the other, a maximum allowable working temperature (MAWT) depends
on a number of different factors. As an example I will consider the
maximum allowable working temperature of the furnace tubes for a Crude
Distillation Unit. (030)
The factors involved in determining the maximum allowable working
temperature in this case are: (031)
 The creep rate/temperature function for the furnace tube material, (032)
 The actual wall thickness for the furnace tube, (033)
 The minimum safe thickness for the furnace tube, determined
from maximum operating temperature and pressure, (034)
 The cost of replacing a furnace tube (035)
 The life required for the furnace tube, determined by the
time between shutdowns, and the economics of replacing furnace tubes. (036)
By the way, nothing dramatic happens if the MAWT is exceeded by a
modest amount, the creep rate will increase and the life of the
furnace tube is reduced. (037)
What is clear is that Maximum Allowable Working Temperature is not an
intrinsic property/quantity, since it depends on external factors
beyond the furnace tube itself, so it is not an ordinary temperature,
or a subtype of ordinary temperature, despite having the same unit of
measure. However, it is also clear that a Maximum Allowable Working
Temperature is comparable to an ordinary temperature, in that it makes
sense to compare the actual temperature of a furnace tube to its
Maximum Allowable Working Temperature and ask questions like whether
the actual temperature is greater or smaller than the maximum. (038)
The key lesson is that quantities are weird, especially when you start
looking at practical engineering quantities, and that in particular
you should be cautious about drawing conclusions from quantities being
expressed in the same Unit of Measure. (039)
Regards (040)
Matthew West (041)
Information Junction
Tel: +44 1489 880185
Mobile: +44 750 3385279
Skype: dr.matthew.west
matthew.west@xxxxxxxxxxxxxxxxxxxxxxxxx
http://www.informationjunction.co.uk/
https://www.matthewwest.org.uk/ (042)
This email originates from Information Junction Ltd. Registered in
England and Wales No. 6632177.
Registered office: 8 Ennismore Close, Letchworth Garden City,
Hertfordshire, SG6 2SU. (043)
_________________________________________________________________
Message Archives: http://ontolog.cim3.net/forum/ontologybasedstandards/
Subscribe: mailto:ontologybasedstandardsjoin@xxxxxxxxxxxxxxxx
Config/Unsubscribe:
http://ontolog.cim3.net/mailman/listinfo/ontologybasedstandards/
Shared Files: http://ontolog.cim3.net/file/work/OntologyBasedStandards/
Wiki: http://ontolog.cim3.net/cgibin/wiki.pl?OntologyBasedStandards (044)
