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Pressure, Flow & Measurement

Pressure, Flow & Measurement. Dr James F Peerless November 2013. Objectives. Pressure Flow Measurement of Volume and Flow. Pressure. Pressure. “The force applied per unit area”. P = f . a. Pressure. SI unit: pascal (Pa) (Nm -2 ) Other units 1 bar = 100 kPa 7.5 mmHg = 1 kPa

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Pressure, Flow & Measurement

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

  2. Objectives • Pressure • Flow • Measurement of Volume and Flow

  3. Pressure

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

  5. Pressure • SI unit: pascal (Pa) (Nm-2) • Other units • 1 bar = 100 kPa • 7.5 mmHg = 1 kPa • 10.2 cmH2O = 1 kPa • 1 atm = 101.325 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 = m2 Therefore: Pressure = kg.m.s-1 / m2 = 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 • H20 – 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 Flow Rotameters Wright’s Peak Flow Meter Fleisch Pneumotachograph Pitot Tubes Electronic Mass Flowmeter • Benedict Roth Spirometer • Dry Gas Meter • Vitalograph • Wright’s Respirometer • Electronic Volume Monitor

  28. Benedict-Roth Spirometer Volume

  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 Flow

  34. Flow

  35. Pitot Tube Flow

  36. Electronic Flowmeter Flow

  37. Reference • Cross M, Plunkett E. Physics, Pharmacology and Physiology for Anaesthetists; 2008. Cambridge University Press, Cambridge. • Davis P, Kenny G. Basic Physics and Measurement in Anaesthesia: 5th Edition; 2003. Butterworth Heinemann, Edinburgh. • Wijayasiri L, McCombe K, Patel A. The Primary FRCA Structured Oral Examination Study Guide 1; 2010. Radcliffe Publishing, Oxford. • http://www.frca.co.uk/

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