When you read about using compressed air, there are many references to the force generated by compressed air. It’s usually measured in PSI (Pounds of force per Square Inch of surface area).
The gauge on your compressor or regulator will show the air pressure of your air system. Though it’s not generally referred to, the readings on your compressed air gauge are almost always PSIG (Pounds of force per Square Inch of surface area, Gauge adjusted to zero).
If the pressure gauge wasn’t adjusted, at sea level, it would show 14.7 PSI, which is the pressure of the earth’s atmosphere at sea level. You would have to deduct 14.7 PSI from your gauge reading each time you wanted to know what the actual pressure of your compressed air system was.
The gauge manufacturer’s adjust the gauges to zero, even though the actual pressure they are reading at sea level is 14.7. Therefore, it’s easier for the gauge user to read the actual pressure of their system.
You can assume, unless you specify otherwise, that gauges you buy are PSIG and have already been adjusted to zero to account for the ambient air pressure of our atmosphere.