Pat Naughtin asked in USMA 26410:
Does anyone know the history of 'standard' air pressure of 29.92 inches of
mercury (inHg) or 760 millimeters of mercury (mmHg).
When I looked at the values that have been used previously I found:
One atmosphere of pressure is equivalent to:
760 millimeters of mercury
29.92 inches of mercury
14.7 pounds per square inch (psi)
1013.25 millibar (mb)
34 feet of water (approximately 10 meters of water)
Thinking that the 'standard' air pressure might be a mid-point between high
and low pressures, I investigated to find that:
The lowest air pressure recorded at sea level was 87 kilopascals, and
the highest air pressure recorded at sea level was 108.4 kilopascal, but
the mid-point between these values is 97.7 kilopascals, and this doesn't
seem to fit with any of the previously used 'standard' values.
I then thought that 10 metres of water might be a clue. Taking the standard
value of acceleration due to gravity (g) as 9.806 65 metres per second
squared, I found that the pressure below 10 metres of water is about
98 kilopascals, so I had no luck there either.
Can anyone help with the history of 'standard' air pressure?
Pat Naughtin LCAMS
Geelong, Australia
--
Air pressure varies widely. I think that 76 cm was chosen as a good
compromise for 'standard' pressure long before SI and the psascal
were invented. The acceleration due to gravity is a constant at any
given place (although it has a slight diurnal fluctuation due to the
moon). As nearly as I can discover, it is 9.806 65 m/s2 at sea level
and 45� latitude.
--
Joseph B. Reid
17 Glebe Road West
Toronto M5P 1C8 Telephone 416-486-6071
