A monitoring apparatus (1, 2) for an outlet of a vessel storing oxygen under pressure and for supplying oxygen gas to a patient, comprising: a flow control valve movable to a position between a fully open position and a fully closed position to adjust a flow of gas from the outlet of the vessel and to the patient; a valve position detector connected to the flow control valve configured to detect the position of the flow control valve; an internal pressure sensor configured to sense an internal pressure (P int (t)) of the gas in the vessel at different times; a processor connected to the internal pressure sensor configured to receive from the internal pressure sensor the pressure (P int (t)) sensed thereby at different times and configured to calculate an actual rate of change in pressure (dP int /dt) of the gas in the vessel over time from the pressure (P int (t)) of the gas in the vessel sensed at different times; a memory configured to store a volume of the vessel and for that volume, an expected rate of change in pressure ((dP int /dt) exp) of the gas in the vessel for each of a plurality of different positions of the flow control valve; the processor being connected to the valve position detector configured to receive from the valve position detector the position of the valve detected thereby, configured to retrieve from the memory the volume of the vessel and for that volume, the expected rate of change in pressure ((dP int /dt) exp) of the gas in the vessel for the position of the valve detected by the valve position detector, and configured to compare the actual rate of change in pressure (dP int /dt) with the expected rate of change in pressure ((dP int /dt) exp) for the same position of the valve as detected by the valve position detector and the same volume of the vessel as retrieved from the memory; and an alarm connected to the processor configured to receive from the processor an alarm signal (s) to activate the alarm if the actual rate of change in pressure (dP int /dt) is less than a first threshold ((dP/dt) min) defined in relation to the expected rate of change in pressure ((dP int /dt) exp) which is compared with the actual rate of change in pressure (dP int /dt) and is more than a second threshold ((dP/dt) max) defined in relation to the expected rate of change in pressure ((dP int /dt) exp) which is compared with the actual rate of change in pressure (dP int /dt). a flow control valve movable to a position between a fully open position and a fully closed position to adjust a flow of gas from the outlet of the vessel and to the patient; a valve position detector connected to the flow control valve configured to detect the position of the flow control valve; an internal pressure sensor configured to sense an internal pressure (P int (t)) of the gas in the vessel at different times; a processor connected to the internal pressure sensor configured to receive from the internal pressure sensor the pressure (P int (t)) sensed thereby at different times and configured to calculate an actual rate of change in pressure (dP int /dt) of the gas in the vessel over time from the pressure (P int (t)) of the gas in the vessel sensed at different times; a memory configured to store a volume of the vessel and for that volume, an expected rate of change in pressure ((dP int /dt) exp) of the gas in the vessel for each of a plurality of different positions of the flow control valve; the processor being connected to the valve position detector configured to receive from the valve position detector the position of the valve detected thereby, configured to retrieve from the memory the volume of the vessel and for that volume, the expected rate of change in pressure ((dP int /dt) exp) of the gas in the vessel for the position of the valve detected by the valve position detector, and configured to compare the actual rate of change in pressure (dP int /dt) with the expected rate of change in pressure ((dP int /dt) exp) for the same position of the valve as detected by the valve position detector and the same volume of the vessel as retrieved from the memory; and an alarm connected to the processor configured to receive from the processor an alarm signal (s) to activate the alarm if the actual rate of change in pressure (dP int /dt) is less than a first threshold ((dP/dt) min) defined in relation to the expected rate of change in pressure ((dP int /dt) exp) which is compared with the actual rate of change in pressure (dP int /dt) and is more than a second threshold ((dP/dt) max) defined in relation to the expected rate of change in pressure ((dP int /dt) exp) which is compared with the actual rate of change in pressure (dP int /dt).