NOVELTY - Fuel cell system comprises a fuel cell stack including a cathode side and an anode side; a compressor providing cathode air to the cathode side of the fuel cell stack; a water vapor transfer unit for humidifying the cathode air from the compressor before it is sent to the fuel cell stack; and a controller controlling the compressor and the water vapor transfer unit during a fuel cell stack purge.    USE - The system is useful for purging a fuel cell stack (claimed).    DETAILED DESCRIPTION - Fuel cell system comprises a fuel cell stack including a cathode side and an anode side; a compressor providing cathode air to the cathode side of the fuel cell stack; a water vapor transfer unit for humidifying the cathode air from the compressor before it is sent to the fuel cell stack; and a controller controlling the compressor and the water vapor transfer unit during a fuel cell stack purge, the fuel cell stack purge includes a first stage purge and a second stage purge, the compressor provides humidified cathode air through the water vapor transfer unit during the first stage purge to the fuel cell stack so that the stack hydration level drops to a known hydration value based on the relative humidity of the cathode inlet air during the first stage purge and then provides dry cathode air to the fuel cell stack during the second stage purge so that the stack hydration level drops from the known hydration value at the end of the first stage purge to a desired hydration level at the end of the second stage purge; or fuel cell system comprises a fuel cell stack including a cathode side and an anode side; a compressor providing cathode air to the cathode side of the fuel cell stack; a water vapor transfer unit for humidifying the cathode air from the compressor before it is sent to the fuel cell stack; and a controller controlling the compressor and the water vapor transfer unit during a fuel cell stack purge, the controller includes a stack hydration model that estimates the amount of water in the fuel cell stack at system shut-down, the fuel cell stack purge includes a first stage purge and a second stage purge, the compressor providing humidified cathode air through the water vapor transfer unit during the first stage purge to the fuel cell stack so that the stack hydration level drops in an asymptotic manner to a known hydration value based on the relative humidity of the cathode inlet air during the first stage purge and then provides dry cathode air to the fuel cell stack during the second stage purge so that the stack hydration level drops from the known hydration value at the end of the first stage purge to a desired hydration at the end of the second stage purge, where the stack hydration model allows the controller to more accurately determine the length of the first stage purge. An INDEPENDENT CLAIM is a method for purging a fuel cell stack in a fuel cell system, comprising purging the fuel cell stack with humidified air at a known relative humidity level; and purging the fuel cell stack with dry air to bring the fuel cell stack to a desired hydration level.