Gases and Vapours Flashcards
Define a vapour?
A vapour is a gaseous substance below its critical temperature. It could therefore be liquefied by pressure alone.
What is the saturated vapour pressure?
SVP is the pressure exerted by the molecules in the liquid component of a vapour when it is at equilibrium, at a specified temperature.
SVP can be thought of as an indicator of volatility, the greater the SVP the more volatile a gas.
Different gases have different SVPs
What is the relationship between SVP and temperature?
As temperature increases the kinetic energy of the liquid molecules will increase and more will vaporise. Once equilibrated the overall SVP will be increased.
Think about a pan of water on the hob, as you heat the water more of the water vaporises.
Note the relationship is not linear.
What is the relationship between SVP and ambient pressure?
SVP is not changed by ambient pressure.
However the proportion of total pressure
occupied by the given vapour changes.
I.e. if you deliver 5kPa of a gas at atmospheric pressure, the proportion would be 5%
If you went to altitude and the atmospheric pressure was 50kPa and you still delivered 5kPa the proportion would be 10%
Define latent heat of vaporisation?
Latent heat of vaporisation: heat energy required to convert a given mass of liquid into vapour whilst
maintaining the same temperature.
I.e. the temperature of the liquid molecules which have changed state to vapour is constant. The temperature of the remaining liquid will be decreased.
Latent heat should be expressed as J/Kg
Define latent heat of fusion?
Latent heat of fusion is the heat energy required to convert a given mass of solid into a liquid whilst maintaining the same temperature.
What is the difference between heat and temperature?
Heat is a form of energy whilst temperature
is the measure of the hotness/coldness of a
substance.
How is boiling point affected by pressure and what occurs when SVP=atmospheric pressure?
Boiling point is proportional to pressure therefore as pressure decreases boiling point decreases.
Once SVP= atmospheric pressure the liquid will boil
What is an adiabatic change?
When the state of a gas is altered (compressed/expanded) without a change in heat energy to or from the gas into the surroundings. However the temperature of the gas will change.
E.g. when a compressed gas expands energy is required to overcome the van der vaals forces of the compressed gas. As no heat energy is exchanged the energy must from the molecule’s own kinetic energy. As a result, the gas cools.
What happens when a gas is rapidly compressed and what is the practical relevance in anaesthetic practice?
When a gas is rapidly compressed there is a temperature rise in the gas (joule-kelvin principle).
May be seen if a cylinder connected to an anaesthetic machine is turned on too quickly with a rapid rise in temperature…may cause
an explosion or fire in presence of combustibles
What is an isothermal change?
If the compression or expansion occurs sufficiently slowly, heat can be
conducted through the walls of the container, leading to no change in the temperature of the gas.
Explain how a vaporiser works?
A vaporiser is used to deliver a controlled and predictable partial pressure of anaesthetic agent in a carrier gas at the common gas outlet.
If you take Sevoflurane for example it has a SVP of 22.
Therefore has exerts a partial pressure of 22kPa this would be the equivalent of a MAC ~11.
In a vaporiser there is a fresh gas flow of O2/air. The gas flow is split so that only a proportion goes into the vaporising chamber and the remaining gas flow bypasses the vaporising chamber. Therefore the resultant gas has a lower partial pressure of volatile.
What are the 2 types of vaporiser?
- Plenum Vaporisers use positive pressure upstream to force gas through the vaporiser and is the standard used in the UK. Because of high internal resistance, only appropriate to use as
vaporiser out of circle (VOC)). - Draw-over Vaporisers use negative pressure downstream through bellows or via respiratory effort and are more common in field or developing world medicine. May be incorporated as
a vaporiser in circle (VIC) or VOC)
If we have higher flow rates through the vapouriser, there isn’t much time for the gas flowing to equilibrilate and ‘pick up’ the vapour. What adaptations are there to counteract this?
Having a number of channels with wicks helps increase the surface area which will help with equilibration.
Having baffle’s help slow the flow of the gas through the vaporising chamber allowing more time to equilibrate.
By bubbling the fresh gas flow through the volatile liquid helps it equilibrate.
Why is it important that the correct volatile is put in the correct vaporiser?
As each vaporiser is calibrated for each volatile agent.
If you put a volatile with a higher SVP in a vaporiser designed for a gas with a lower SVP you would risk overdosing your patient.
