A gas mixer for computer calibration of an anesthetic mass spectrometer

To calibrate an anesthetic mass spectrometer without the use of premixed gases and vapors in cylinders, we devised a gas mixer using fixed resistances of capillary needle tubings and adjustable needle valves to dilute test gases and vapors with oxygen. The dilution ratio was determined during each calibration by diluting air with oxygen and noting the reduction in the ratio of nitrogen to oxygen. Empiric correction was made by the computer for the effects of density and viscosity, relative to air, on the flow of nitrous oxide, carbon dioxide, and the saturated vapors of the three anesthetics through the capillary resistor. The computer was programmed to control solenoid valves both for calibration and for the multiplexed sampling of operating rooms. Oxygen, nitrous oxide, and carbon dioxide were used as pure gases, and halothane, enflurane, and isoflurane were vaporized at room temperature in 50-ml vaporizers. The resulting calibrations were found to be accurate to within +/- 2%.