Abstract: Apparatus for calibrating the gas detector of a gas analyzer includes a gas source circuit for providing a standard sample gas at a substantially constant, predetermined pressure. The gas source circuit includes a gas bottle for containing the standard sample gas and a pressure regulator circuit for maintaining the output pressure of the gas source circuit to the substantially constant, predetermined pressure. Ideally, the gas source circuit is semipermanently coupled to the gas detector so that calibration may be performed at any time, even while the gas detector is coupled in an analysis circuit for analyzing an unidentified sample gas. The gas analyzer further includes a data processor that responds to programming instructions and data for controlling the gas source circuit and gas detector to perform the zero measurement and span measurement needed for calibration.