Pressure

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Topics: Pressure
Module EG1083 Fluid Mechanics

PRESSURE , FLUID SYSTEMS and FLUID PRESSURES

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Pressure
Definition of Pressure

Fluid pressure: Force per unit area exerted by a fluid in a solid wall. Force acts perpendicularly to the surface in contacts. Fluid is a co u d s common word for gas a d/o liquid. o od o and/or qu d Pressure is a scalar quantity. It has the units of: N/m2 or Pa (or kPa) in SI system of units psi in Imperial system of units Pressure can also be expressed in terms of height of a column of liquid List of units of pressure measurements & conversion of units Pascal s Pascal’s law

Scalar quantity

Units of Pressure

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Pressure
Pressure measurements Absolute pressure Gauge Pressure

...divided into three different categories: 1. Absolute pressure – which is defined as the absolute value o pressure (force-per-unit-area) ac g o of p essu e ( o ce pe u a ea) acting on a surface by a fluid. su ace ud Abs. pressure = pressure at a local point of the surface due to fluid – absolute zero of pressure (see page 63 of lecture notes) 2. Gauge pressure – difference between abs. pressure and atmospheric pressure – is always positive 101.325 kPa or 14.7 psi

Equations

Pressure term relationships

– a –ve gauge pressure is vacuum ve vacuum. Pressure term relationships • Abs pressure = gauge pressure + atm pressure Abs. • Abs. pressure = – gauge pressure + atm pressure (vacuum) > atm < atm
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Pressure
Pressure measurements

Relation between abs., gauge and vacuum
Absolute pressure Gauge Pressure

gauge

Equations gauge) Pressure term relationships

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Pressure
Pressure term relationships Hydrostatic pressure

3. Differential pressure – measurement of an unknown pressure minus the reference to a o e u e e e ce o another unknown p essu e o pressure. – it is used to measure differential pressure i.e. pressure drop (ΔP) in a fluid system

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Fluid systems and Fluid pressures
Fluid systems

Two



References: •Transport Phenomena by Bird, Stewart, Lightfoot •Fluid Mechanics and Hydraulic machines by R K Rajput R.K. •http://www.freescale.com/files/sensors/doc/app_note/AN1573.pdf (18 F 10) •http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Pressure/Hydro Static.html (18 F 10) SM(15)

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