A semiquantitative analysis of the dynamics of a Goldman‐type vaporiser

Abstract

SummaryWhen a turbulent¯ow of a carrier gas is passed over a liquid anaesthetic agent contained in a vaporiser of Goldman design, evaporation from the cooled surface leads rapidly to a succession of uid instabilities which set in at characteristic critical conditions. An initially quiescent boundary layer at the surface is sequentially replaced by a thin layer of toroidal (Be Ânard±Marangoni) convection cells which are driven by surface tension gradients. These are then augmented by Rayleigh±Be Ânard convection driven by gravity, the whole process terminating in intermittent columnar plunging of cold¯uid from a chaotic surface layer of pulsating thickness to the base of the liquid pool. Residual striations from these plunging columns persist throughout the bulk of the liquid so long as evaporation continues. The ultimate state is then one in which turbulence occurs throughout both liquid and vapour phases. In this paper, a semiquantitative analysis of the system dynamics is given with supportive experimental evidence where possible. Modern anaesthetic vaporisers are large and complex devices having temperature compensation and designed for use in a steady¯ow of gas. They have a relatively high resistance to gas¯ow in order to reduce the effect of downstream pressure¯uctuations on vaporiser output. They are also calibrated, the concentration of agent emitted corresponding to the degree of rotation of the control knob being known. Both the temperature compensation and the calibration may be sources of error. In these vaporisers, saturation of gas by agent within the vaporising chamber is achieved by the use of wicks so that the gas is exposed to a large surface area of agent. The requirement of calibration and the use of wicks means that the vaporiser is agent speci®c. The nature of the gas¯ow through these vaporisers is not important except for the major proviso that the¯ow through either the bypass or the vaporising channels should not change from laminar to turbulent (or vice versa) at any point within the operating range of the vaporiser. If that did occur there would be a large and abrupt change in the calibration curve at that point.The availability of anaesthetic gas analysers for routine clinical use has changed the requirements governing the design of vaporisers. This is particularly true for systems using low fresh gas¯ows with which the concentration of agent inspired by the patient may not be the same as that delivered by the vaporiser. Three recent papers [1±3] have described the consistent and usable performance obtainable from simple vaporisers of the Goldman type mounted within circle systems (VIC). These uncalibrated vaporisers are of simple construction and have no temperature compensation. They have low resistance to gas¯ow and are not agent speci®c. The papers already referred to showed that vaporisers of this type had features useful in clinical practice. We have therefore undertaken an analysis of their physical performance believing that this would help improvement of the design of this type...

Delivery of sevoflurane to dogs using a Stephens anaesthetic machine

“…The delivered concentration of sevoflurane increased as the vaporizer temperature increased, again as expected for vaporizers that are not temperature compensated (Patel & Goa ; Smith ; Young et al. ). This may be of clinical importance in extremes of the ambient temperature.…”

“…As expected the vaporizer produced a higher delivered sevoflurane concentration with the sleeve down compared with the sleeve fully up under equivalent conditions, as the anaesthetic vapour concentration in a VIC system will be highest nearest to the surface of the liquid anaesthetic (Smith ; Young et al. ).…”