Most modern pressure gauges take into account the use of atmospheric pressure on the systems being read. Appliances and machines such as well pumps, air compressors and tire gauges will read by gauge pressure. This of course does not take into account the total system or absolute pressure. By using a basic formula, you can convert absolute pressure to gauge pressure or visa versa.
Understand that absolute pressure or total system pressure is generally defined as the pressure measured from an absolute vacuum or zero pounds per square inch (PSI). Gauge pressure is that which is generally read on a gauge and takes into consideration the atmospheric pressure we are placed under every day. Atmospheric pressure changes with temperature and altitude above sea level. On average this would be 14.6 pounds per square inch (PSI) above an absolute vacuum reading of 0 PSI. In other words, the reading of 0 on a gauge would really have 14.6 PSI already into account.
Realize that the basic formula for finding gauge pressure is Pg = Ps -- Pa. Where Pg is equal the gauge pressure reading, Ps is the total system pressure reading or absolute pressure, and Pa is the atmospheric pressure that on average will be 14.6 PSI.
Covert the absolute pressure or system pressure of 100 PSI into a gauge pressure reading. Plug the numbers into the formula and the equation will read Pg = 100 - 14.6.; the gauge pressure is equal to 85.4 PSI.
Find the absolute pressure or total system pressure on a tire gauge reading when testing a tire that reads 36 PSI. Translating the formula will read as follows, Ps = Pg + Pa. The answer will be 36 + 14.6 = 50.6 PSI.