Incompressible fluid If Pi is on free water surface, i.e. PI=Pat P=P 2-Pi=pg h P P-P atm p g P2.abs =pgh+P atm For gauge pressure measurement, Patm=0 gauge pg h + P ,ga auge Gauge pressure at a point h below free surface =pg hIncompressible Fluid If P1 is on free water surface, i.e. P1 = Patm : P2 - P1 =  g h P2 - Patm =  g h P2,abs =  g h + Patm For gauge pressure measurement, Patm = 0 P2,gauge =  g h P2,abs = P2,gauge + Patm Gauge pressure at a point h below free surface =  g h  h P2 P1=Patm