# UD EGTE 215 - PRESSURE & HYDROSTATICS (25 pages)

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## PRESSURE & HYDROSTATICS

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## PRESSURE & HYDROSTATICS

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Pages:
25
School:
University of Delaware
Course:
Egte 215 - Applied Fluid Mechanics
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PRESSURE HYDROSTATICS Pressure F A Pressure is typically measured with respect to a reference level Reference could be atmosphere or vaccum The reference level atmospheric pressure 95 105kPa Pressure measured with respect to atmosphere Gage pressure Pressure measured with respect to perfect vaccum Absolute pressure The three related by the equation Pabs Pgage Patm See Figure 3 1 1 Points Perfect vacuum lowest possible pressure Absolute pressure will always be positive Gage pressure above atmospheric is ve Gage pressure below atmospheric will be ve Units Pa abs or Pa gage always mention what the reference is Range of atmospheric pressure 95 kPa to 105 kPa 2 Assumed as 101 kPa in the text Or 14 7 psi a Problem 3 1 Given Pressure gage 155 kPa Atmospheric Pressure 98 kPa What is the Absolute Pressure Absolute pressure Pressure and Elevation Pressure is directly related to elevation and changes with elevation example swimming pool or diving deep into a water body The greater the depth elevation of fluid the greater the pressure Elevation in water measured with respect to a reference positive upwards negative downwards See Figure 3 2 3 It is advisable to choose the lowest point for a reference level so that you don t have to deal with negative elevation values Change in pressure p h Where is the specific weight of the fluid and h is the elevation 4 Key Points Equation is only valid for homogenous liquids at rest Points on the same horizontal level have the same pressure Pressure varies linearly with change in elevation depth Change in pressure is proportional to the specific weight of the fluid Above equation does not apply to gases because the specific weight of gas changes with elevation However the change in gas pressure with elevation is small An increase in elevation of 300 m changes gas pressure by only 3 4kPa Problem 3 5 Determine the pressure at a depth of 5m for water p h Specific wt of water 9 81 kN m3 Therefore p 9 81 x 5 49 05 kN m2 49 05 k Pa 5 Problem 3 7 SI

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