Dear people:
Just look at your northern neighbors, your northern neighbors. They know how
to do it.
Regards
John F-L
----- Original Message -----
From: Bill Hooper
To: U.S. Metric Association
Sent: Friday, December 04, 2009 9:51 PM
Subject: [USMA:46250] Re: Simple Metric web site.
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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