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/