Anesthesia Pharmacology:  Physics and Anesthesiology

Variable Bypass Systems


Section Table of Contents

Site Table of Contents

Previous Page

Next Page

Fig. 1: Variable Bypass Vaporizer

30, 42Variable Bypass Vaporizer: Drager  Vaporizer19.1

  • For this system, if the concentration dial is at zero (position 3) with the on/off switch closed (position 2),  gas would enter the vaporizer at the fresh gas in the (position 1) and exit through the outlet (position 10) completely bypassing the internal vaporizer mechanism.  

    • However, if the concentration dial is set to a particular % above 0.2 vol%, then the on-off switch (position 2) will open (automatically) allowing fresh gas to enter the vaporizing mechanism.  

    • A split internal circuit sends some fresh gas through a thermostatically regulated bypass circuit (position 7) which provides temperature compensation and maintains proper volume% vapor output concentration which correlates to the concentration dial setting. 

    • The rest of the fresh gas is diverted through a pressure compensator (position 4) preventing pressure changes (upstream or downstream in the vaporizer) to the affect the vaporizing process itself and therefore influence volume percent vapor output.  

    • Output from the pressure compensator line is sent to the vaporizing chamber (position 5) which contains the liquid volatile agent which is absorbed and evaporated using a wick assembly.  

    • The saturated gas exits the vaporizing chamber through a control cone (position 6) which is position-adjustable in accord with the setting on the concentration knob.  

    • Saturated vapor and the fresh gas (which did not passed through the vaporizing chamber) are combined, leading through the fresh gas outlet (position 10).  

    • The volume percent vapor output will be determined by the bypass opening (position seven) and the control cone opening (position 6). 

    •  Temperature compensation in this design occurs by dimensional change of the expansion element (position 8) resulting in a decrease vaporizing chamber gas flow which compensates for increased vaporization with increased temperature. Citation: (Narkomed 3 anesthesia system technical service manual, Telford, PA, 1988, North American Drager and reference 30.)



Drager  Vaporizer19.1



30,43Gas flow splitting ratios (20oC)

Desired anesthetic percentage

Halothane (Fluothane)

Enflurane (Ethrane)

Isoflurane (Forane)

Sevoflurane (Sevorane, Ultane)







1.7: 1












maximum possible = 2.7% at 20oC.

Figure below adapted from reference 30 and from Eisenkraft, JB: Vaporizers and vaporization of volatile anesthetics. In Eisenkraft JB, editor, Progress in Anesthesiology, vol 2, San Antonio,1988, Dannemiller Memorial Educational Foundation) (reference 40)

30,40Hypobaric Example

30,40Hyperbaric Example



Section Table of Contents

Site Table of Contents

Previous Page

Next Page