The pressure read in a pressure gauge is called gauge pressure, as 15 PSI, it is the pressure above atmospheric.
Absolute pressure is the total atmospheric pressure + gauge pressure. At sea level, standard atmospheric pressure.
There are three types of classification for instruments that measure atmospheric pressure:
PRESSURE GAUGES: measure pressures above atmospheric pressure.
Vacuum gauges: measure pressures below atmospheric pressure.
MANOVACUÔMETROS: measure the pressures both above and below atmospheric pressure.
