| Flow through the vaporising chamber
Varying the proportion of gas passing through the vaporising chamber and bypass is the method by which vaporiser output is controlled.
Efficiency of vaporisation
Vaporisers may incorporate a system of wicks and channels in the vaporising chamber to improve efficiency of vaporisation and increase the output concentration of anaesthetic.
As temperature increases, the output of the vaporiser will increase, unless some compensatory mechanism is used.
Vaporisation causes the liquid anaesthetic to cool, since heat is lost because of the latent heat of vaporisation of the anaesthetic. Therefore, the output concentration will tend to fall over time.
Gas flow rate
Changes in carrier gas flow rate may affect vaporiser output by:
- Altering the proportion of the total gas flow that passes through the vaporising chamber.
- Altering the efficiency of vaporisation. For example, at high flow rates, the gas leaving the vaporising chamber will tend to be less saturated (since the gas spends less time in the chamber), so the output of the vaporiser will tend to fall.
Carrier gas composition
The composition of the carrier gas may affect vaporiser output by:
- Changes in the viscosity and density of the gas mixture affecting the proportion of the total flow that passes through the vaporising chamber
- Nitrous oxide dissolving in the anaesthetic, thus altering the effective volume that passes through the vaporising chamber.
Saturated vapour pressure (SVP) is solely a function of temperature. Therefore, if ambient pressure is reduced, the (constant) SVP becomes a greater proportion of the total (reduced) pressure, and the output concentration (in volumes %) rises.
For example, a halothane vaporiser calibrated at sea level and set to deliver 2% will produce about 2.7% halothane if used in Denver, Colorado.