Atmospheric Pressure is the pressure exerted by weight of atmosphere on different fluids. At sea level, its standard value is considered as 101.325 Pa.
In fluid mechanics, certain other phenomena are also considered such as gauge pressure, vacuum pressure and absolute pressure as shown in Fig. 2.4
P = Total Pressure
Pabs = Absolute Pressure
Pg = Gauge Pressure
Pat = Atmospheric Pressure
Pvac= Vacuum Pressure
The pressure which is measured from absolute zero level and below vacuum pressure is called absolute pressure.
The pressure above atmospheric pressure is the gauge pressure.
The below atmospheric pressure is called vacuum pressure.
Mathematically, these pressures can be formulated as,