An instrument calibrating demand flow regulator for providing a demanded amount of calibration gas from a calibration gas source to an instrument to be calibrated to permit calibration of the instrument includes a housing having an interior, an inlet for connection to the calibration gas source and an outlet for connection to the instrument to be calibrated. A pressure regulating valve device positioned within the interior of the housing regulates the pressure of the calibrated gas entering the interior of the housing through the inlet and a demand flow valve device delivers calibration gas to the outlet of the housing upon demand by a pump operatively associated with the instrument.