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Pressure, Flow & Measurement Dr James F Peerless November 2013

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Objectives Pressure Flow Measurement of Volume and Flow

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Pressure

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“The force applied per unit area” P = f. a P = f. a

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Pressure SI unit: pascal (Pa) (Nm -2 ) Other units – 1 bar = 100 kPa – 7.5 mmHg = 1 kPa – 10.2 cmH 2 O = 1 kPa – 1 atm = kPa

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SI Units of Pressure Pressure = force / area Force= mass x acceleration = kg.m.s -1 (where 1 N = force required to give a mass of 1 kg an acceleration of 1 second per second) Area = m 2 Therefore: Pressure= kg.m.s -1 / m 2 = kg.m -1.s -1

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Different Types of Pressure Partial pressure, Total pressure – Dalton’s Law The pressure exerted by an individual gas in a gas mixture is the same as if it was alone This is partial pressure of a gas – Total pressure is therefore the sum of partial pressures of a gas mixture

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Absolute, Gauge Pressure Gauge Pressure – Pressure measurements above/below atmospheric pressure – Empty cyclinder = 0 kPa Absolute Pressure – Is zeroed against a vacuum, so; – = Gauge Pressure + Atmospheric Pressure – Empty cylinder = 101 kPa

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Base SI Units Temperature: K Time: s Length: m Current: A Amount of Substance: mol Luminous Intensity: cd Mass: kg “Try To Look CALM”

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Measuring Pressure Manometers Aneroid gauges Barometers Electrical transducers: Wheatstone bridge

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Manometers Fluid-filled column Open to atmosphere – Read gauge pressure (not absolute) Measurers of low pressures Inaccuracies: – Surface tension H 2 0 – over-read Hg – under-read – No clinical significance but loved by MCQs!

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Barometers Closed to atmosphere – Measure absolute pressure – Zeroed against a vacuum Not used in medicine

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Aneroid Gauges Greek: “no water” E.g. Bourdon gauge High pressure measurements – e.g. cylinders Elliptical bourdon tube

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Pressure Transducers

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Flow

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What is flow? “The amount of fluid passing a given point per unit time” F = Q/t = Q̇

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Laminar Flow Fluid moves in a steady manner No eddies or turbulence Typically seen in smooth tubes at low rates Flow greatest at centre (x 2x̄) Pressure difference must exist for flow to occur Q̇ ∝ ΔP Viscosity is main component of laminar flow

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Laminar Flow

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Calculating Laminar Flow

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Turbulent Flow Characterised by swirls and eddies Can occur at constrictions Velocity varies across the tube Flow proportional to square root of p Density is important factor of determinant of turbulent flow

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Turbulent Flow

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Predicting Flow Reynolds Number LAMINAR < 2000 < TURBULENT Can determine Critical Velocity

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Bernoulli Effect Fall in pressure at a constriction in a tube There is a fall in potential energy (assoc. with pressure) Subsequent gain in kinetic energy (assoc. with flow) – no loss or gain of energy

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Venturi Principle

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Coanda Effect Tendency of a jet of fluid to attach itself to a curved surface due to areas of low pressure Fluid will preferentially flow down a limb of a Y-junction rather than being equally distributed. E.g. – ventilators – coronary vessels – bronchioles

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Measurement of Volume and Flow

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Measuring Volume & Flow Volume Benedict Roth Spirometer Dry Gas Meter Vitalograph Wright’s Respirometer Electronic Volume Monitor Flow Rotameters Wright’s Peak Flow Meter Fleisch Pneumotachograph Pitot Tubes Electronic Mass Flowmeter

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Volume Benedict-Roth Spirometer

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Dry Gas Meter Volume

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Vitalograph Volume

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Wright’s Respirometer Volume

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Flow

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Wright’s Peak Flow Meter Variable orifice, constant pressure

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Flow

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Pitot Tube

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Flow Electronic Flowmeter

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Reference Cross M, Plunkett E. Physics, Pharmacology and Physiology for Anaesthetists; Cambridge University Press, Cambridge. Davis P, Kenny G. Basic Physics and Measurement in Anaesthesia: 5 th Edition; Butterworth Heinemann, Edinburgh. Wijayasiri L, McCombe K, Patel A. The Primary FRCA Structured Oral Examination Study Guide 1; Radcliffe Publishing, Oxford.

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