Definition of pressure head Pressure=P Datum Pressure head at point=Plpg(m) Piezometric head=p/ g +y (m) Piezometric pressure =P+pgy h In fluid statics, gauge pressure head at depth h below free surface Lpg=pgh/pg-h
Definition of Pressure Head Pressure head at point = P/ g (m) Piezometric head = P/ g + y (m) Piezometric pressure = P + g y In fluid statics, gauge pressure head at depth h below free surface = P/ g = g h / g = h Datum Pressure = P y h
Example I KN 0.2m 0.05m 2.51 dial di lam P1=F/A=1000/(x0.0252)=5093kPa 2=P1-pg(2.5)=48477kPa f=PA =48477x兀0.12=1523kN
Example P1 = F/A = 1000 / ( 0.0252 ) = 509.3 kPa P2 = P1 - g (2.5) = 484.77 kPa F = P2 A2 = 484.77 x 0.12 = 15.23 kN 2.5m 1 kN F ? 0.2 m diam 0.05 m diam P1 P2
Pressure at liquid interface Liquid 1 Liquid 2 p2 Liquid 3 p 1=Pgh 22=pgh+ p2gh2 P3-P1gh1 p2gh2 pgh
Pressure at liquid interface P1 = 1gh1 P2 = 1gh1 + 2gh2 P3 = 1gh1 + 2gh2 + 3gh3 Liquid 1 1 Liquid 2 2 Liquid 3 3 h1 h2 h3 P1 P2 P3
Pressure measurement Absolute Pressure: Measured relative to perfect vacuum Perfect vacuum: 0 absolute pressure Gauge Pressure: Measured relative to local atmospheric pressure If no specified, pressure reading is usually assumed to be gauge Gauge pressure is positive if higher than atmospheric pressure, and negative if lower than atmospheric pressure Negative gauge pressure is also known as suction or Ⅴ acuum pressure
Pressure Measurement • Absolute Pressure : Measured relative to perfect vacuum • Perfect vacuum : 0 absolute pressure • Gauge Pressure : Measured relative to local atmospheric pressure • If no specified, pressure reading is usually assumed to be gauge • Gauge pressure is positive if higher than atmospheric pressure, and negative if lower than atmospheric pressure • Negative gauge pressure is also known as suction or vacuum pressure
Atmospheric Pressure ↓↓↓4↓ Mercury Atmospheric pressure is usually measured using a mercury barometer Patm =phg g ha 101, 300 N/m2 abs 0N/m2 gauge ≈1.013 bar abs ≈760 mm hg abs ≈10.3 m water abs
Atmospheric Pressure Atmospheric pressure is usually measured using a mercury barometer Patm = ρHg g h 101,300 N/m2 abs = 0 N/m2 gauge 1.013 bar abs 760 mm Hg abs 10.3 m water abs