Dear Bill,
It seems to me that the history goes like this:
101.325 kPa is regarded as a 'standard' because it is a soft
conversion of 760 millimetres of mercury in a glass column.
760 mm Hg was regarded as a 'standard' because it was a soft
conversion of 30 inches of mercury in a glass column.
The numbers 30 and 760 were chosen from rounded (to the nearest inch
or to the nearest 10 millimetres) guesstimations based on many air
pressure observations and having a rough stab at where the mid-point
might usually be.
The second 1 and the decimal places in the number 101.325 imply an
accuracy and a precision that probably never existed. Just taking the
highest pressure ever recorded (109 kPa in Siberia) and the lowest (87
kPa in Cyclone 'Tip' in 1979) gives us an average of 98 kPa.
If you stick to the original – rather sloppy – methods used arriving
at rounded values for inches and millimetres, you would simply round
your average of air pressure observations to say that 'average
atmospheric pressure is about 100 kPa and, I think, that to say that
'101.325 is exact' is simply wrong.
As to the idea of a 'standard' average air pressure, this is clearly a
mess that is still in dispute. You could start at http://en.wikipedia.org/wiki/Atmospheric_pressure
to get an idea of all the different 'standards'.
By the way, as you read the Wikipedia page you could imagine what this
page will look like when the Wikipedia people get over their fixation
with conversions. As you know, I think that conversions are are
complete waste of time. See http://www.metricationmatters.com/metric_conversion.html
Cheers,
Pat Naughtin
Author of the ebook, Metrication Leaders Guide, that you can obtain
from http://metricationmatters.com/MetricationLeadersGuideInfo.html
PO Box 305 Belmont 3216,
Geelong, Australia
Phone: 61 3 5241 2008
Metric system consultant, writer, and speaker, Pat Naughtin, has
helped thousands of people and hundreds of companies upgrade to the
modern metric system smoothly, quickly, and so economically that they
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On 2009/12/05, at 08:51 , Bill Hooper wrote:
On Dec 4 , at 11:38 AM, Pierre Abbat wrote (in response to my
earlier note):
(I wrote) Normal or
average atmospheric pressure is about 101.3 kPa. (There may be
different
standards for identifying "normal" or "average" air pressure.)
(Pierre replied)
101.325 kPa, to be exact, is the standard.
The value of 101.325 kPa is the exact value (the "standard")
specified by CGPM for the standard atmospheric pressure, as Pierre
correctly points out. That value is as close to a universal value as
is possible.
In addition, however, there are other circumstances where different
standards are used. Here are two I've encountered (from Wikipedia):
... International Union of Pure and Applied Chemistry (IUPAC)
recommended that for the purposes of specifying the properties of
substances, “the standard pressure” should be defined as precisely
100 kPa ... rather than the 101.325 kPa value of “one standard
atmosphere”. ... For natural gas, the petroleum industry uses a
standard temperature of 15.6 °C (60.1 °F), pressure 101.56 kPa
(14.730 psi). (air pressure)
The CGPM standard is probably more precise than is reasonable for
many uses, such as reporting air pressure in weather reports. For
such measurements, a rounder value of 101.3 kPa or even 101 kPa
might be suitable as a state "normal" or "average".
Certainly, atmospheric pressure is affected by altitude and other
factors. For specific locations or specific special conditions, the
CGPM standard atmosphere may not be too close to the actual average
of the ambient pressure.
This is not meant to be argumentative. I recognize the value of
101.325 kPa as the best value to use for most technical purposes. I
just felt it was necessary to indicate in my note that there are
different possible averages or normal values that might be used.
Regards,
Bill Hooper
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