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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