Gas volume measurement Benedict Roth Spirometer Vitalograph Dry gas meter Wright respirometer (calibration inaccurate if used for continuous flow)

Liquid volume measurement Measuring cylinder Dilution techniques red cell labelling conc. labelled RBC= dose of labelled RBC red cell volume Red cell volume= dose of labelled RBC conc labelled RBC plasma volume– labelled albumin

Gas flow measurement Variable orifice flowmeter rotameter Wright peak flow meter Pneumotachograph

Gas flow measurement 1)Variable orifice flowmeter ≡ Rotameter 2) Wright peak flowmeter also works on the variable orifice principle 3) Pneumotachograph Measures a very small pressure change across a gauze screen which is transduced to an electrical signal

Rotameter Tapered tube, variable orifice around bobbin, pressure across bobbin stays constant (  2.5%) Needle valves at bottom of flowmeter At normal gas supply pressures the linear velocity of gas approaches the speed of sound

Pneumotachograph Measures a very small pressure change across a gauze screen which is transduced to an electrical signal Featuresrapid response minimal resistance head warmed to maintain temp Problemscalibration for gas mixture laminar vs turbulent

Liquid flow measurement Drop counter  20% depending on temp and fluid Infusion pump Fick principle (at steady state) Blood flow to organ = rate of addition/removal of substance A-V difference in concentration

Fick principle for measurement of cardiac output Q = oxygen consumption A-V difference oxygen content = 250ml min ml l -1 – 150 ml l -1 = 250 ml min -1 = 0.25l min ml l l l -1 = 5 l min -1

Gas laws Boyle’s law PV = constant (k 1 ) or V α 1 P The volume of a given mass of gas varies inversely with the absolute pressure at a constant temperature

Boyle’s law can be used to calculate the content of a full O 2 cylinder (10 litres) Pressure in full cylinder is 138 bar (absolute) 138 bar ≡ kPa kPa x 10 l = 1380 l x 100 kPA

Charles’s law V α T or V = constant (k 2 ) T The volume of a given mass of gas varies directly with the absolute temperature at a constant pressure

Third gas law P α T or P = constant (k 3 ) T The absolute pressure of a given mass of gas varies directly with the absolute temperature at a constant volume

Third gas law enables the effects of heating an O 2 cylinder from 17 o C to 307 o C to be calculated Absolute temperature increased from 290 K to 580 K Absolute pressure increases from 138 bar to …….? Cylinders will withstand a pressure of ?

Definitions STP0 o C K 760 mm Hg kPa Adiabatic change occurs when the state of a gas is altered without allowing the gas to exchange heat energy with the surroundings

Dalton’s law of partial pressures states that in a mixture of gases the pressure exerted by each gas is the same as if it alone occupied the space partial pressure of any gas is: total pressure x fractional concentration of gas for humidified gases the presence of water vapour must be taken into account

Avogadro’s hypothesis states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules One mole of any gas occupies 22.4 litres at STP Mole of any substance is x 10 23

Avogadro’s hypothesis and full N 2 O cylinder Molecular weight N 2 O is 44 Full cylinder contains 3.4 kg N 2 O Cylinder contains 3400 x 22.4 litres 44 = 1730 litres N 2 O

Universal gas constant PV = k 1 V = k 2 P = k 3 T T Thus PV= constant T For one mole of any gas there is a unique constant known as the universal gas constant = R So PV = nRT where n = number of moles of the gas

Critical temperature is defined as the temperature above which a substance cannot be liquefied however much pressure is applied critical temperature of N 2 O is 36.5 o C gas is defined as a substance above its critical temp at room temp – O 2, N 2 vapour is defined as a substance below its critical temp at room temp – N 2 O, CO 2 halothane

Filling ratio is used to describe how much gas is needed to fill a cylinder fr = mass of gas in cylinder mass of water to fill cylinder fr for nitrous oxide is 0.65 contents of nitrous oxide cylinder determined by weighing tare wt (empty) stamped on top of cylinder