Abstract: A method and apparatus for generating an adjustable concentration of a selected gas within a carrier gas flow including a gas generator which is responsive to a drive signal. A gas analyzer detects the selected gas concentration within the carrier gas flow from the gas generator and provides a signal proportional to the measured concentration to a comparator. A gas concentration selector provides a second signal to the comparator, the signal being proportional to a desired concentration of the selected gas. In response to these signals, the comparator adjusts the drive signal to correct the gas generator output for the difference between the signals and to thereby substantially adjust the concentration of the selected gas within the carrier gas flow to the desired concentration. The method and apparatus automatically corrects for drift or other inaccuracies associated with the generation of the selected gas.