ALL ABOUT AIR
WHITE PAPER #23 – BAROMETRIC PRESSURE © Tom Kreher
Please indulge a brief rant about the common terminology for Atmospheric Pressure, Absolute Pressure
and Gauge Pressure. We specify that atmospheric pressure added to gauge pressure equals absolute
pressure: PSIG + PSIA = PSIA. ???????
Atmospheric pressure is barometric pressure and might be designated PSIB then the equation for
absolute pressure would make sense. Gauge pressure plus barometric pressure equals absolute
pressure, PSIG + PSIB = PSIA.
Many of you know that when we use the standard atmospheric (barometric) pressure at sea level for
calculations about compressed air it is an approximation from the average of the fluctuating atmospheric
pressure. Also this close approximation based on the most common barometric pressure is incorrect for
every application except right at sea level.
If your application is not floating in a raft in the Ocean the local atmospheric (barometric) pressure is most
often lower than the standard atmospheric (barometric) pressure which is given as 29.92 inches of
Mercury (Hg) and 14.7 (14.696) PSIA (Should be PSIB) and 1.013 Bar. (A bar is .986 atmospheres). One
bar is 14.5 PSI.
It is uncommon for a gauge or pressure switch to show absolute pressure. When absolute pressure is
required we read the gauge and add atmospheric pressure.
Most of us have been convinced that 14.7 is the atmospheric pressure. When you change the word from
atmospheric to barometric it changes he perspective.
By the way what is atmospheric pressure? If we had a cubic inch of air so that any one side was one
square inch and laid that on a scale in a prefect vacuum it would weigh .0000434 lbs (.075 lb/cu ft / 1728
cu in/cu ft). How the heck can it have almost 15 pounds of pressure per square inch? The air around us is
stacked up approximately 200,000 feet. Let us imagine that you got on the scale and weighed 200 Lbs. If
a person who weighed 150 pounds sat on your shoulders and a 100 pounder on theirs and a 50 pounder
on top. You would be the onl