GASES AND FLOW FRCA PRIMARY PHYSICS 1 Dr E Farnham-Davis MEPS 2016

States of Matter  Solid  Liquid  Vapour  Gas

Phases

Definitions  Triple point – pressure and temperature at which all three states can coexist  Critical temperature – temperature, above which, a gas cannot be compressed into a liquid  Critical pressure – minimum pressure required at the critical temperature to liquefy a gas

Change of Phase

Gas Laws

Universal Gas Law

More Gas Laws  Dalton’s Law  Total pressure = sum (partial pressure all other gases)  Henry’s Law  Solubility of a gas in a liquid is proportional to partial pressure above liquid

Flow  Quantity of a fluid passing a point in a given time  e.g. L/min  (fluid = liquid, vapour or gas)  Laminar vs Turbulent

Laminar vs Turbulent

Laminar Flow

Reynolds number

Laminar vs Turbulent

Measuring flow  Several devices used in different areas of anaesthesia  Fixed orifice variable pressure  Fixed pressure variable orifice

Rotameter  Forces acting in equilibrium

Pneumotachograph  Change in pressure proportional to flow (laminar)

Wright’s Respirometer

PEFR  Variable pressure, variable orifice

Benedict-Roth spirometer  Measures volumes

Spirometry trace

Bernoulli Effect  At a narrowing in a system in which fluid is moving the pressure will decrease as the velocity increases (conservation of energy)

Venturi  Utilisation of low pressure created by Bernoulli effect in order to entrain another gas (e.g. O2 in Venturi Mask).

Coanda Effect  Fluid will “cling” to surface as it flows past it (e.g. water from a tap on the back of a spoon).  Used in “fluid logic” ventilators.