The pumping effect is due to positive pressure ventilation or use of the oxygen flush valve. It can increase vaporizer output. Modern vaporizers are relatively immune (older vaporizers are certainly not immune) due to check valves between the vaporizer outlet and the common gas outlet, smaller vaporizing chambers, or tortuous inlet chambers. Any of these design features prevent gas which has left the vaporizers from re-entering it. The check valves are why a negative pressure leak check is recommended by the FDA checklist .