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Pressure, Flow & Measurement Dr James F Peerless November 2013.

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Presentation on theme: "Pressure, Flow & Measurement Dr James F Peerless November 2013."— Presentation transcript:

1 Pressure, Flow & Measurement Dr James F Peerless November 2013

2 Objectives Pressure Flow Measurement of Volume and Flow

3 Pressure

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

5 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

6 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

7 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

8 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

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

10 Measuring Pressure Manometers Aneroid gauges Barometers Electrical transducers: Wheatstone bridge

11 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!

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

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

14 Pressure Transducers

15 Flow

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

17 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

18 Laminar Flow

19 Calculating Laminar Flow

20 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

21 Turbulent Flow

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

23 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

24 Venturi Principle

25 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

26 Measurement of Volume and Flow

27 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

28 Volume Benedict-Roth Spirometer

29 Dry Gas Meter Volume

30 Vitalograph Volume

31 Wright’s Respirometer Volume

32 Flow

33 Wright’s Peak Flow Meter Variable orifice, constant pressure

34 Flow

35 Pitot Tube

36 Flow Electronic Flowmeter

37 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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