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You are in Home >> Resources >> Physics and equipment >> Vapourisers

Precision vaporisers

Created: 13/7/2004

 

These vaporisers include the Ohmeda 'Tec series (Fluotec, Pentec and Isotec/Fortec for halothane, methoxyflurane and isoflurane, respectively), the Dräger Vapor 19 series, Foregger, Penlon and Ohio vaporisers.

Operation

The fundamental difference between the precision vaporiser and the simple vaporiser is that the vaporising chamber incorporates a network of internal channels and wicks which ensures that the gas emerging from the chamber is fully saturated with anaesthetic vapour. The concentration of the anaesthetic agent in this gas is therefore known (from its saturated vapour pressure [SVP]), so, when this gas is mixed with the anaesthetic-free bypass gas, the concentration of anaesthetic in the gas leaving the vaporiser is also known. The proportion of the total gas flow passing through the vaporising chamber is controlled by a dial which accurately indicates the concentration of the anaesthetic delivered by the vaporiser.

Changes in temperature

Compensation for the effects of changes in temperature on the SVP of the anaesthetic may be achieved in a number of ways:

 Minimise temperature fluctuations by a heavy copper construction, which acts as heat sink. 
 Compensate for temperature changes by a valve that varies the proportion of the gas that flows through the vaporising chamber. 

Tec 2 - a bimetallic strip decreases flow through the vaporising chamber when temperature increases. Tec 3- a bimetallic strip increases flow through the bypass chamber when temperature increases.
Ohio 100 - a bellows and thimble valve increases flow through the bypass chamber when temperature increases. Dräger 19 - an annular valve constructed of dissimilar metals increases flow through the bypass chamber when temperature increases.

Gas flow rate

Different designs of vapouriser vary in their ability to deliver accurate concentrations of anaesthetic at different fresh gas flow rates. The following graphs demonstrate the accuracy of various halothane vaporisers, but those designed for other agents are generally similar in performance.

Fluotec 2 - Very inaccurate at low flows, but good for flow rates greater than 5 L/min.

 

Fluotec 3 - Accuracy tends to fall off at high dial setting and flow rates.

 

Ohio 100 - Output increases at high flow rates and vice versa. Dräger 19 - Very accurate at all dial settings over a wide range of flow rates.

The Tec 6 desflurane vaporiser

Desflurane is extremely volatile: at 22.8 degrees C, its boiling point is only slightly above normal room temperature, which precludes the use of a normal variable-bypass type vapouriser. The Tec 6 vaporiser avoids this problem by heating the desflurane liquid to above its boiling point in a sealed chamber and mixing pure desflurane gas with the carrier gas. 

Tec 6 vaporizer
Tec 6 schematic

The desflurane liquid is heated to 39 degrees C by a heating element (H), at which temperature its SVP is approximately 1300 mmHg. The carrier gas flow is restricted by an orifice (O) so that the pressure of the carrier gas within the vapouriser is proportional to gas flow. This pressure is sensed by a differential pressure transducer (P), which adjusts a resistor (R1) so that the flow of desflurane out of the vaporising chamber is proportional to the carrier gas flow. This enables the output concentration to be made independent of carrier gas flow rate. The control dial adjusts a second resistor (R2), which controls the output of desflurane gas, and thus the output concentration.

The Tec 6 is electrically powered and electronically controlled and so requires a mains power supply. Since the vaporising chamber is sealed from the atmosphere, a special filler arrangement is required.

Uses

Precision vaporisers can provide a highly accurate output of anaesthetic which, over a wide range, is independent of time, gas flow rate and temperature. 

Precision vaporisers are built for use with a specific agent and can be lethal if the wrong agent is used (e.g. a Pentec, designed for use with methoxyflurane, filled with halothane would deliver over 30% halothane when full on). Their high internal resistance prevents them from being used in the breathing circuit.

Fillers

Vaporisers may be filled by a conventional funnel-fill mechanism, in which the liquid anaesthetic is simply poured into a funnel in the vapouriser. The problem with this method is that, if more than one anaesthetic is used in a facility, there is nothing to prevent the vaporiser being filled with the wrong agent. This may be prevented with the use of a key-filler system. 

Isotec 4 funnel fill
Isotec 4 key fill
Isotec 4 vaporiser with funnel-filler
Isotec 4 vaporiser with key-filler
Isotec 4 funnel fill
Isotec 4 key fill
Detail of Isotec 4 funnel filler
Detail of Isotec 4 key filler



In this system, an agent-specific filler tube is used, one end of which slots into a fitting on the vaporiser, and the other end slots into a collar on the bottle of anaesthetic. The fitting on the vaporiser and the collar on the bottle are specific to each agent, making it impossible (or, at least, extremely difficult) to fill the vaporiser with the wrong agent.

Key filler inlet
Key filler bottle
Key-filler vaporiser inlet
Key-filler vaporiser bottles


ArticleDate:20040713
SiteSection: Article
 
   
    
                                            
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