A system useable in a jet aircraft having installed therein a pressurized oxygen supply which feeds oxygen into the interior of the plane when it flies at high cabin altitudes, the system indicating the changing status of the supply as oxygen is drained therefrom. The system includes a pressure transducer coupled to the supply and means associated with the transducer to determine the lapse rate at which the pressure of the supply is reduced as oxygen is drained therefrom to yield a first signal representing this pressure lapse rate, and to concurrently determine the lapse rate at which the number of liters of oxygen in the supply is reduced as oxygen is drained therefrom to yield a second signal representing the liter lapse rate. These signals are applied to a microprocessor in whose data base is entered the total oxygen inventory of the supply and the oxygen demand of the plane in which the supply is installed. When oxygen is being drained from the supply, the microprocessor calculates and reads out the prevailing supply pressure, the number of liters remaining in the supply, and the time in hours and minutes remaining before the supply is exhausted based on the current rate of oxygen consumption.The invention provides both a method for calculating the oxygen required, as well as a real time monitoring and calculating system for emergency conditions. The invention is applicable to any pressurized gas system, such as for divers, compressed gas cooking or vehicles, therapeutic oxygen, and the like.