PROPERTY OF PRESSURE z (m) Pressure the same in all directions P=  gz Pressure always perpendicular to a surface. A B Pressure the same at A and B.

ABSOLUTE-,GAGE PRESSURE Vacuum:0 N/m 2 (Pa) Atmospheric pressure (P atm ): N/m 2 (Pa) Gage pressure (P gage ): Pressure reading at gage Absolute Pressure (P abs ): P abs =P atm +P gage P atm =101 kPa Ex P abs =151 kPa P abs =51 kPa P abs =151 P gage =50 kPa P abs =51 kPa P gage = -50 kPa Pabs=0 P gage =0 P atm =gage reference P abs =0

PIEZOMETRIC HEAD Pressure head h=P/  (m) h Piezometric head The sum of the elevation above an arbitrary datum plus the pressure head. H= h + z = P/  + z h P z z P

PRESSURE MEASUREMENTS a h (m) a Piezometer/Manometer Gage pressure at surface = 0 P a =0+ . h Ex: Calculate the heigth (h) of the water column if the pressure at point a is 50 kPa.

PRESSURE MEASUREMENTS U-TUBE/DIFFERENTIAL MANOMETER aa a h1h1 h2h2 P a =0+h 1.  mercury - l 1.  H 2 O l1l1 P a =0-h 2.  mercury - l 2.  H 2 O l2l2 Manometer liquid: mercury

EXAMPLE U-TUBE aa h1h1 h2h2 l1l1 Calculate the gage - and absolute pressure in the pipes.  mercury =13600 kg/m 3. The atmospheric pressure is 100 kPa. a)h 1 =10 cm ; l 1 =0.3 m b)h 2 =1 cm ; l 2 =0.4 m c) Calculate h if the manometer liquid is water in a). l2l2 a l1l1 h

DIFFERENTIAL MANOMETER h a b l1l1 l2l2 Calculate the pressure difference between b and a. h=1.5 cm l 1 =30 cm  mercury =13600 kg/m 3 PaPa PbPb Manometer liquid mercury

EXAMPLE water 0.5 m 1.5 m 2.5 m air oil 1.4 m 0.4 m Calculate the gage pressure of air.  oil =900 kg/m 3  mercury =13600 kg/m 3 Manometer liquid: mercury