When you think of pressure, you most likely imagine something you can feel, either literally (as in the push of a pen tip against the palm of your hand) or in some symbolic but tangible way (as when preparing to take a final chemistry exam). However, you are not consciously aware of the one type of pressure with the greatest single impact in your very existence: atmospheric pressure.
Even if you cannot actively perceive it, you are probably at least aware of the phenomenon of atmospheric pressure, often called barometric pressure, because it is mentioned on practically every weather report or forecast these days. And you're also aware that certain closed objects on Earth, such as car tires and basketballs, have their own (artificially generated) air pressure.
Is there a way to relate the pressure of some gizmo in your hands to that of the "silent" atmospheric pressure? That is, are these in any way additive or canceling? The answer lies in the concept of pounds per square inch absolute versus pounds per square inch gauge, or PSIA vs. PSIG.
What Is Pressure in Physical Science?
Pressure, in your mind, probably translates to something resembling force. That's close, actually; pressure is just force per unit area, measured in pascals (Pa) or newtons per square meter (N/m2) in SI units. Force, in turn, is the product of mass and acceleration. Since Earth's gravity, g, (about 9.8 m/s2 at the planet's surface) has units of acceleration, anything with mass on Earth has a force resulting from gravity called weight.
If you distribute your own weight over the surface of a thinly frozen-over pond, you stand a better chance of not falling through than if you rise to your feet and stand. This is because your exert greater pressure on the ice you touch in the second case, because your feet touch a much smaller area of ice than when you lay sprawled as widely as possible on the surface.
Common Units of Pressure
You have already been introduced to the pascal, equal to 1 N/m2. Like a lot of fundamental units in physical science, pascals are not convenient to most everyday pressure measurements. Atmospheric pressure, for example, measures approximately 101,325 Pa, or 101.325 kilopascal (kPa). This is equal to 1 atmosphere, or 1 atm.
A unit called the bar approximates atmospheric pressure, with 1 bar = 1,000 millibar (mbar) = 100 kPa. On weather forecasts, you may see the pressure of hurricanes expressed in millibars; the lower the value, the stronger the storm, with values of around 900 mbar signifying extremely powerful weather events.
When you fill the tires of a car in the United States, the pressure is usually given in pounds per square inch (psi), with 1 atm = 14.7 psi. You're typically invited to inflate the tires to at least 32 psi. But what pressure does this actually measure?
PSIA vs. PSIG
At this point, you're probably eager to dispense with abbreviations and get to some meaningful pressure calculations.
In the above example, the tire pressure gauge is measuring, as the name befits, tire pressure relative to the gauge, or PSIG. The device has, much like your own body, been constructed to ignore atmospheric pressure and assign 0 to this value. This makes sense, since most everyday people don't operate in a vacuum, in which air and its pressure have been removed.
Absolute pressure (PSIA) is therefore just atmospheric pressure plus the gauge pressure:
PSIA = PSIG + atmospheric pressure
Thus a tire inflated to a gauge pressure of 32 psi, or a PSIG of 32, would have an absolute pressure, relative to a vacuum, of about 32 + 14.7 = 46.7 psi, or a PSIA of 46.7.
PSIA vs. PSIG Converter
See the Resources for a way to handle these calculations automatically and gain a little more insight into how and why these units are used.