PROPERTY OF PROPERTY OF PRESSUREPRESSUREz (m)

Pressure the same inall directions P=gz

Pressure always perpendicular to a surface.

A B

Pressure the same at A and B.

ABSOLUTE- ,GAGE PRESSURE

Vacuum: 0 N/m2 (Pa)Atmospheric pressure (Patm): 1.013 . 105 N/m2

(Pa)Gage pressure (Pgage): Pressure reading at gageAbsolute Pressure (Pabs): Pabs=Patm+Pgage

Patm=101 kPa

Ex Pabs=151 kPa Pabs=51 kPa

Pabs=151

Pgage=50 kPa

Pabs =51 kPa

Pgage= -50 kPa

Pabs=0

Pgage=0Patm=gage reference

Pabs=0

PIEZOMETRIC HEADPressure head h=P/ (m) h

Piezometric headThe sum of the elevation abovean arbitrary datum plus thepressure head.H= h + z = P/ + z

hP

z z

P

PRESSURE MEASUREMENTS

a

h (m)

a

Piezometer/Manometer

Gage pressure at surface = 0 Pa=0+ . h

Ex: Calculate the heigth (h) of the water column if thepressure at point a is 50 kPa.

PRESSURE MEASUREMENTS

U-TUBE/DIFFERENTIAL MANOMETER

a a

a

h1h2

Pa=0+h1 . mercury - l1 . H2O

l1

Pa=0-h2 . mercury - l2 . H2O

l2

Manometer liquid:mercury

EXAMPLE U-TUBEEXAMPLE U-TUBE

a a

h1h2

l1

Calculate the gage - and absolute pressure in the pipes. mercury=13600 kg/m3 . The atmospheric pressure is 100 kPa.a)h1=10 cm ; l1=0.3 m b)h2=1 cm ; l2=0.4 mc) Calculate h if the manometer liquid is water in a).

l2

a

l1

h

DIFFERENTIAL MANOMETER

h

a bl1 l2

Calculate the pressure difference between b and a.h=1.5 cml1=30 cmmercury=13600 kg/m3

Pa Pb

Manometer liquidmercury

EXAMPLE

water

0.5 m

1.5 m

2.5 m

airoil

1.4 m

0.4 m

Calculate the gage pressure of air.oil=900 kg/m3

mercury=13600 kg/m3

Manometer liquid:mercury