Abstract: The present invention provides a monitoring apparatus for an outlet of a vessel storing gas under pressure. The monitoring apparatus comprises 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, a valve position detector connected to the flow control valve to detect the position of the flow control valve, an internal pressure sensor to sense an internal pressure Pint(t) of the gas in the vessel at different times, a processor, a memory and an alarm. The processor calculates an actual rate of change in pressure dPint/dt of the gas in the vessel over time, and compares dPint/dt with an expected rate of change.