Pressure, Flow & Measurement Dr James F Peerless November 2013
Objectives Pressure Flow Measurement of Volume and Flow
Pressure
“The force applied per unit area” P = f. a P = f. a
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
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
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
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
Base SI Units Temperature: K Time: s Length: m Current: A Amount of Substance: mol Luminous Intensity: cd Mass: kg “Try To Look CALM”
Measuring Pressure Manometers Aneroid gauges Barometers Electrical transducers: Wheatstone bridge
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!
Barometers Closed to atmosphere – Measure absolute pressure – Zeroed against a vacuum Not used in medicine
Aneroid Gauges Greek: “no water” E.g. Bourdon gauge High pressure measurements – e.g. cylinders Elliptical bourdon tube
Pressure Transducers
Flow
What is flow? “The amount of fluid passing a given point per unit time” F = Q/t = Q̇
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
Laminar Flow
Calculating Laminar Flow
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
Turbulent Flow
Predicting Flow Reynolds Number LAMINAR < 2000 < TURBULENT Can determine Critical Velocity
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
Venturi Principle
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
Measurement of Volume and Flow
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
Volume Benedict-Roth Spirometer
Dry Gas Meter Volume
Vitalograph Volume
Wright’s Respirometer Volume
Flow
Wright’s Peak Flow Meter Variable orifice, constant pressure
Flow
Pitot Tube
Flow Electronic Flowmeter
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.