GAS LAWS AND THEIR APPLICATION.ppt
- 1. DR UFOEGBUNAM M.P. DEPARTMENT OF ANAESTHESIA UNTH ENUGU.
- 2. INTRODUCTION All matter exists in one of three states or phases-solid, liquid or gas. When a gas co-exists in equilibrium with its corresponding liquid, the gas is termed a vapour The critical temperature is defined as the temperature above which a substance cannot exist in a liquid state
- 3. Laws governing the flow of gases Pressure, Volume and Temperature : when describing the behavior of gases the temperature, pressure and volume are related in a consistent manner, which makes it possible to formulate three gas laws Boyle’s law Charles’s law Gay-lussac’s law Others are Avogadro’s law Henry’s law Dalton’s law
- 4. Boyle’s law This is the first gas law and states that at constant temperature, the volume of a fixed mass of a perfect gas varies inversely with pressure. PV=K (at constant temperature)
- 5. Charles’s law This is the second gas law and states that at constant pressure, the volume of a fixed mass of a gas is proportional to its temperature. V/T =K (at constant pressure)
- 6. Gay-Lussac’s law This is the third gas law and states that at constant volume, pressure of a fixed mass of gas is directly proportional to its temperature P/T =K (at a constant volume)
- 7. The combined gas law Is derived from all three gas laws and is as follows PT= T OR alternatively expressed P1V1=P2V2 T T1T2 Provided two of the variables are known, the third can be assumed to remain constant, the gas laws can be used to calculate changes in one of the known variables when the other alters. For example, if the initial pressure and volume of an oxygen cylinder is 137bar and 1.2 litres respectively(and temperature is assumed to remain constant), then the volume of the oxygen at a pressure of 1 bar can be calculated.
- 8. Cont… Assuming room pressure is also roughly 1 bar then this approximates to the volume of oxygen available to the patient, before the cylinder empties. Furthermore, if the rate of oxygen use is known, then the time remaining for use of that oxygen cylinder can be calculated When making these calculations it does not matter which units are used for pressure or volume provided consistency is maintained
- 9. Units of pressure Units Equivalent value Atmospheres (atm) Bar (bar) Kilopascal (KPa) Millimeters of mercury(mmHg) Torr (mmHg O0C) Centimeters of water (cmH20) Pounds per square inch(PSI) 1 1 1O1.3 760 760 1033.2 14.7
- 10. Avogadro’s law Avogadro’s law states that at a constant temperature and pressure a given volume of any gas will contain the same number of molecules. The converse of this is that at a given temperature and pressure, 1 mole of any gas will occupy the same volume( 22.4 litres at 1 atmosphere and 0oC Both Avogadro’s law and combined gas law are used to derive the ideal gas law PV=nRT n=number of moles of gas, R=universal gas constant(8.314). The ideal gas law is of relevance when considering the behavior of nitrous oxide in cylinder
- 11. Henry’s law States that at a constant temperature, the amount of a given gas dissolved in a given liquid is directly proportional to the partial pressure of the gas in contact with the liquid. Temperature affects the solubility of gases such that at a higher temperatures gas will be less soluble in a liquid Henry’s law is of importance when considering the way inhaled anaesthetic vapours and gases behave physiologically
- 12. Dalton’s law States that the pressure of a gas in a mixture of gases is independent of the pressure of the other gases in the mixture. Dalton’s law can be explained by the fact that in a mixture of gases the molecules are so far apart from one another, that each gas behaves as though the others were not present Dalton’s law explains why vapour pressure is not affected by ambient pressure.
- 13. Imperfections of the gas laws In reality, the gas laws described are not always true and they refer to a theoretical “ideal” gas However, these concepts generally hold true and as long as correction factors are used, the gas laws have practical applications Note that the gas laws are only accurate when applied to gases above their critical temperature
- 14. APPLICATIONS: OXYGEN Boiling point -1830C, critical temperature of -1190C , it exists as gas at room temperature and therefore obeys gas laws Boyle’s law can be applied to oxygen, which means that the reading on the pressure gauge of an oxygen cylinder gives a true indication of the volume remaining. However, inaccuracies may arise in this respect if large alterations in ambient temperature occur.
- 15. Cont.. Oxygen can be stored under pressure in cylinders, it can also be combined to form cylinder banks attached to manifold to reduce cost, transportation and constant change of exhausted cylinders. Oxygen has to be cooled to below -1180C to change to a liquid and as the change occurs , it occupies a much smaller volume. When a small volume of liquid oxygen is warmed it will make a very large volume of oxygen gas In liquid form, a very large quantity of oxygen can be transported or stored in a low volume
- 16. Cont.. Vacuum insulated evaporator (VIE): Is a container designed to store liquid oxygen
- 17. Nitrous oxide Boiling point of -88.60C and critical temperature is +360C. Since in most countries it exists as a vapour in equilibrium with its liquid phase, the gas laws do not apply to nitrous oxide. Unlike oxygen, the pressure gauge tells you nothing about the amount of nitrous oxide remaining in the cylinder- it always reads around 52 bar at room temperature To determine the quantity left in a cylinder it must be weighed, the weight of the empty cylinder subtracted, and then the number of moles of nitrous oxide calculated using the Avogadro’s number
- 18. Cont… The ideal gas law can then be used to calculate the approximate volume of gas remaining It is now easy to understand why nitrous oxide cylinders are not filled to a given pressure. A value called the filling ratio is used Is the ratio of the weight of the cylinder filled with nitrous to the weight of the cylinder when filled with water Filling ratio in U.K is 0.75, however this is reduced to 0.67 in hotter climates
- 19. Entonox BOC Medical trade name for 50% oxygen and 50% nitrous oxide. For Entonox the critical temp. is termed the pseudo critical temperature. The term is used to describe the temperature at which Entonox starts to separate into its constituent parts. The pseudo critical temperature of Entonox is -60C (occurs mainly in temperate climate) If there is a possibility of separation occurring, the manufacturers recommended that prior to use, cylinders should be stored horizontally for 24hours at temp. above 100C or warmed to 100C for two hrs (or body temp. for 5 mins and then completely inverted 3 times
- 20. Heliox A mixture of oxygen and helium. Percentage of O2 may be as low as 21% but not higher than 50% Heliox is useful in patients with upper airway obstruction. Theoretically patients with airway obstruction have greater amount of turbulent compared to laminar flow within their airways Helium has a lower density than oxygen ( and nitrogen) and this may increase airway gas flow when flow is turbulent. The density of a gas has no effect on flow when the flow is laminar.
- 21. Application to volatile anaesthetic agents Volatile anaesthetic agents exert their effects according to the partial pressure of the agent in the blood According to Henry’s law, the partial pressure of anaesthetic agent dissolved in the blood is proportional to the partial pressure exerted by the anaesthetic vapour in the alveli Dalton’s law tells us that the partial pressure exerted by the anaesthetic vapour in the alveoli is independent of the other gases present in the mixture.
- 22. Application to vaporizers A vaporizer does not generally alter the vapour pressure of an anaesthetic agent, and this remains relatively constant. One variable that can alter the anaesthetic vapour pressure in a vaporizer is temperature During vaporization latent heat of vaporization cools the liquid agent and cause a fall in the vapour pressure and agent concentration Modern vaporizers incorporates various mechanisms to compensate for any drop in temp. to overcome this problem
- 23. Do you need to adjust your vaporizer at high altitude Two concepts answer this question First, anaesthetic dept is controlled by altering the partial pressure of anaesthetic agent in the alveolus (Henry’s law) Second, only two factors affect the partial pressure namely the vapour pressure of agent in the vaporizer and the conc. of vapour in the inhaled gas mixture. The answer is that no change needs to be made to vaporizer setting at altitude
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