When you look at the pressure reading on the gauge, if it says 90 PSI, is that 90 PSI including or excluding the 14.7 PSI that exists all around us all the time?
Recognizing that we all exist in a sea of air at around 14.7 PSI, the industrial air gauges have been adjusted to read zero at one atmosphere, ensuring that the reading you see is the actual pressure of the air in the line.
An Air Gauge Usually Ignores Atmosphere. When compressed air users are discussing air pressure, the assumption is that the pressure referred to is PSIG, and that the atmosphere is not a factor in the pressure level being discussed.
Therefore, you will hear folks talk about 90 PSI, or 120 PSI, and that is the actual compressed air pressure level in the system. We all assume that the gauge has been factory adjusted to eliminate the force of the atmosphere. And when it comes to PSIG, unless specified otherwise, you can to.