1. A method for supplying a patient having inspiratory and expiratory phases with respiratory oxygen, comprising the following steps: (a) providing an oxygen supply during only a part of an inspiratory phase; (b) initiating said inspiratory phase by detecting a reduction in pressure in a detecting chamber; (c) modifying current from a sensor means in response to an increase in optical flux due to a reduction in the distance between said sensor means and a reflector means which is due to an expansion of a windbag resulting from a pressure drop in said detecting chamber to insure a supply of respiratory gas; (d) storing, in a memory, during each inspiratory-expiratory phase, a reference output voltage of said sensor means representing the atmospheric pressure; and (e) causing the delivery of breathable gas to the patient at the start of the patient's inspiratory phase detected by comparison of said reference voltage with a voltage representing an inspiratory depression.