I do not know the basis of how all this is calibrated, but 1000 km - when
talking about standard atmospheres?? THINK about it - 1000 km is way above
what close proximity satellites orbit the earth at!!! Not much air up there.
Regards
JF-L
----- Original Message -----
From: John M. Steele
To: U.S. Metric Association
Sent: Friday, December 04, 2009 11:02 PM
Subject: [USMA:46251] Re: Simple Metric web site.
I believe the CGPM just copies it. It actually originates in the US,
ICAO, and ISO Standard atmospheres, which define that sea level pressure,
temperature (15 °C) and temperature lapse rate. (The lapse rate changes in
various segments above 11 km). This leads to the assumptions used in
calibration of aircraft altimeters, which really measure pressure but indicate
altitude.
The US defines a standard atmosphere to 1000 km, ICAO and ISO agree at
lower altitudes, but end at a lower altitude as no commercial aircraft flies at
1000 km.
--- On Fri, 12/4/09, Bill Hooper <billhoope...@gmail.com> wrote:
From: Bill Hooper <billhoope...@gmail.com>
Subject: [USMA:46250] Re: Simple Metric web site.
To: "U.S. Metric Association" <usma@colostate.edu>
Date: Friday, December 4, 2009, 4:51 PM
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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