In our literature, you’ll often see the abbreviation “PSI”, which stands for “Pounds per Square Inch” or, more precisely, “Pound force per Square Inch”. What it expresses is the number of pounds of force applied to an area of one square inch.
We use “PSI” in reference to tank pressure, i.e. the reading on a pressure gauge. The technical term for this reading is “PSIG”, where the “G” stands for “gauge”. When the subject is atmospheric pressure or pressure readings that are affected by atmospheric pressure, such as a balloon or an inflatable bicycle tire, this is known as “PSIA”, where the “A” stands for “absolute”.
The difference between PSIA and PSIG is atmospheric pressure itself. Imagine that you have a tank of air with a pressure gauge that reads 0 PSI. If you were to take the pressure gauge off of the tank at that reading, you would create an opening from which air would neither escape nor enter. Does that mean that the tank is empty?
Not quite. Let’s assume that our tank is at sea level. With the pressure gauge removed, the hole allows the tank to equalize with atmospheric pressure. Atmospheric pressure is essentially the weight of the big blanket of air that extends up to the outer atmosphere, which, at sea level, exerts a force of 14.7 PSIA. Put the regulator back on the tank and the pressure gauge will still read 0 PSIG, even though there is 14.7 PSIA of pressure from the atmosphere.
Thus, PSIG includes atmospheric pressure. In fact, to convert PSIA to PSIG, just add the current atmospheric pressure.