- The temperature will eventually fall and modern vaporizers are therefore temperature compensated.
- Most use a bimetallic strip to automatically make fine adjustments to the splitting ratio.
- At lower temperatures therefore, the splitting ratio increases slightly and more gas is allowed into the vaporizing chamber to
ensure that the end concentration of anaesthetic stays constant (FGF will predominantly follow path of least resistance)
- Heavy, No Alarms
- Works within a small ambient temperature range WRT compensation
- High resistance = out of circle use
- Easy to use, no power supply needed.
- Accurate to within 15% of dialled conc when between 200ml-15L/min/FGF
Dr. David Lyness
Equipment in Anaesthesia and Critical Care: A complete guide for the FRCA - Aston et al (Vaporiser Schematic)
Modern plenum vaporisers have high resistance to gas flow and are available for use with sevoflurane, isoflurane, enflurane and halothane.
Each vaporizer is calibrated for a specific agent and cannot be interchanged because of the differing physical properties of the volatile agents.
<20% of the total FGF entering will enter the liquid chamber where wicks soak up the volatile agent creating a saturated high-content area.
As the FGF travels through, evaporation and vaporisation happens.
The gas leaving this chamber is fully saturated with the vapour and then be diluted by the other stream of FGF that bypassed the chamber.
The control of the FGF into the chamber is done by the % dial on the top of the unit - this reduces or increases % vapour content in total gas.
For example, sevoflurane has an SVP of 157mmHg at 20°C. Therefore, gas leaving the vaporizing chamber will contain sevoflurane at a vapour pressure of 157mmHg.
It must then be diluted 10–20 times by the bypass gas to give a useful concentration.
The baffles force the entering FGF closer to the surface of the volatile.
Due to the wicks and baffles, this is system
of high resistance.
As liquid evaporates, it loses energy as the latent heat of vaporization, causing cooling.
As the temperature falls, so does the SVP of the anaesthetic and therefore the output of the vaporiser.
Most modern vaporizers are made of a material such as copper that is a good conductor of heat and which, because of its high mass, acts as a heat sink and helps to keep the temperature inside stable.
Saturated vapour pressure (SVP) of a substance varies with its temperature but not with ambient pressure.- If the atmospheric pressure is altered (for example, at high altitude), the SVP of the anaesthetic does not change, and therefore the vapour pressure
of the anaesthetic in the gas leaving the vaporizing chamber also does not change.
- However, the output concentration will change because this is the vapour pressure divided by the atmospheric pressure.
e.g.: At sea level, dialling up 2% on a vaporizer would deliver approximately 2kPa of anaesthetic (2% of 100 kPa).
- If the barometric pressure is halved, setting the dial at ‘2%’ will still deliver 2kPa because SVP does not vary with ambient pressure.
- Therefore, a vaporizer can be used as normal at altitude because it is the vapour pressure that is clinically important.
- However, the delivered concentration will not be 2% (as read on the dial).
- If the delivery is 2 kPa at 50 kPa atmospheric pressure, the concentration delivered is actually 4% (2 kPa / 50 kPa = 4%).
1atm = 1kPa