Abstract: A diagnostic method for a gas supply system includes: determining a desired ramp rate for filling a vessel from a supply of compressed gas; monitoring the actual pressure of gas entering the vessel; and discontinuing the flow of gas into the vessel when the actual pressure deviates from the intended pressure at the desired ramp rate by an undesired amount. A system for carrying out the method includes a flow controller for controlling operation of the supply system to deliver compressed gas from a source to a vessel through a supply line at a desired ramp rate. The system employs a pressure monitor downstream of a control valve for measuring the pressure of gas directed into the vessel and transmitting pressure-related data to the flow controller, which closes the control valve to discontinue filling of the vessel if the actual pressure exceeds a permissible deviation from the intended pressure.