These vaporisers consist of a vaporising chamber, usually a glass bowl or bottle, containing liquid anaesthetic and a tap controlling the proportion of gas flowing through it. Examples include the Ohio and Boyle's bottles, and the Stephens and Goldman vaporisers.
The vaporising chamber may contain a wick to improve vaporisation. Nonetheless, the gas in the chamber is unlikely to be fully saturated with anaesthetic. The concentration leaving the vaporising chamber is, therefore, less than the saturated vapour pressure and is usually unknown. The concentration of anaesthetic emerging from the vaporiser, therefore, is also not known. Also, higher gas flow rates will tend to lower the concentration further, since the carrier gas spends less time in the vaporising chamber and less anaesthetic is vaporised.
Changes in temperature
No compensation is made for the effect of changes in temperature on saturated vapour pressure, so the output of such vaporisers is highly dependent upon the ambient temperature:
Effect of changes in temperature on the output concentration of a simple halothane vaporiser calibrated at 20 degrees C.
The delivered concentration will also be reduced over time, since loss of the latent heat of vaporisation causes the liquid to cool and lowers the saturated vapour pressure. This is most noticeable in the case of ether vaporisers, where ice may form on the outside of the vaporisation chamber due to the extreme cold which develops owing to evaporation.
Because of these causes of variability in their output, simple vaporisers cannot generally be provided with a scale indicating the concentration of anaesthetic being delivered. Most merely have some qualitative scale such as Off to On, or 0 to 10. Adjustments to the vaporiser can only be made on the basis of the response of the patient, as judged by changes in the plane of anaesthesia.
The major advantage of simple vaporisers is their relatively low cost. Many are also designed to have a low internal resistance which allows them to be used as draw-over vaporisers.
Although simple vaporisers may in principle be used to vaporise any agent, the large difference in the saturated vapour pressure of different anaesthetics presents some practical difficulties. For example, vaporisers intended for use with halothane or isoflurane are designed to be relatively inefficient in order to produce a maximum output of around 3%. The disadvantage of simple vaporisers is their unknown and variable output. The rapid changes in depth of anaesthesia which may be achieved with halothane and isoflurane may cause major difficulties to occur if their concentration is not known.
Precision vaporisers incorporate mechanisms to compensate for these effects. Their output is reasonably constant over a wide range of conditions.