Chapter 9 Notes - Chapter 9 Pressure and Velocity...

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1 Chapter 9 Pressure and Velocity Measurements Material from Theory and Design for Mechanical Measurements; Figliola, Third Edition Pressure • Pressure is force per unit area. • It acts inward or outward, normal to the surface of any physical boundary. Pressure •P g = P abs –P o – Common “P o ” is P abs at local atmospheric conditions. • Absolute pressure will be a positive number. • Gauge pressure can be positive or negative depending on the value of measured pressure relative to the reference pressure. • A differential pressure, such as p1- p2, is a relative measure and can’t be written as an absolute pressure. Figliola, 2000
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2 Pressure • Pressure can also be described in terms of the pressure exerted on a surface submerged in a column of fluid at a depth, h, as depicted. • The pressure at any depth within a fluid of specific weight γ can be written as: p abs (h) = p 0 (h 0 ) + γ h •p 0 is pre-determined and h is measured from an arbitrary datum line, h 0 . • The fluid specific weight is given by γ = ρ g / g c . • The equivalent head of fluid of depth, h = (p abs –p)/ γ Figliola, 2000 Pressure • Pressure transducers are usually calibrated using a reference pressure instrument such as: – McLeod Gauge – Barometer – Manometer McLeod Gauge • The McLeod gauge is a pressure-measuring instrument and laboratory reference standard used to establish gas pressures in the subatmospheric range of 1 mm Hg abs down to 0.1 µ m Hg abs . • The glass tubing is arranged so that a sample of the gas at the low pressure to be determined can be trapped by inverting the gauge from the sensing position to that of the measuring position.
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3 McLeod Gauge • The trapped gas within the capillary is isothermically compressed by a rising column of mercury. • Boyle’s law is then used to relate the two pressures on either side of the mercury to the distance of travel of the mercury within the capillary. Figliola, 2000 Barometer • A barometer consists of an initially evacuated tube that is closed on one end.
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This note was uploaded on 11/22/2011 for the course ABE 6031 taught by Professor Burks during the Summer '11 term at University of Florida.

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Chapter 9 Notes - Chapter 9 Pressure and Velocity...

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