I'm not sure, but I think that the standard atmospheric pressure was derived from an average sea level pressure at mid latitude. The barometric pressure was developed from the mercury barometer measured in inches or centimeters depending on country. The name barometer comes from the metric unit bar which translates 1013.25 mbar into 101.325 kPa.
Stan Doore ----- Original Message ----- From: "Joseph B. Reid" <[EMAIL PROTECTED]> To: "U.S. Metric Association" <[EMAIL PROTECTED]> Sent: Friday, July 18, 2003 11:55 AM Subject: [USMA:26413] Re: Air pressure history 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
