Definition : The absolute pressure, or total pressure, is measured relative to absolute zero on the pressure scale-which is a perfect vacuum-and is the sum of gauge pressure and atmospheric press-ure. It is the same as the thermodynamic pressure.
Absolute pressure accounts for the atmospheric pressure, which in effect adds to the pressure in any fluid which is not enclosed in a rigid container i.e., the fluid is open to the atmosphere.
p = p0 + Pg
where p0 and pg are respectively atmospheric pressure and the gauge pressure.
Absolute pressure can be never negative.
