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.
Download ReportTranscript 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.
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 – 10.2 cmH 2 O = 1 kPa – 1 atm = 101.325 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 = kg.m
-1 .s
-1 / m 2
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 • “ T ry T o L ook 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
Benedict-Roth Spirometer
Volume
Dry Gas Meter
Volume
Vitalograph
Volume
Wright’s Respirometer
Volume
Flow
Wright’s Peak Flow Meter • Variable orifice, constant pressure
Flow
Flow
Pitot Tube
Flow
Electronic Flowmeter
Flow
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: 5 th 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/