A combine harvester comprising: a storage tank defining an interior region for holding grain; a conduit extending into the interior region of the storage tank, said conduit defining an interior space that is sized to receive a vertically oriented grain conveyance device that is configured to remove the grain from the storage tank, said conduit defining an opening (409, 509) for receiving grain from the interior region of the storage tank; a panel (410, 510) that is positioned adjacent the opening (409, 509) in the conduit, said panel (410, 510) being movable between a first position in which the panel (410, 510) conceals the opening (409, 509) to prevent the entrance of grain through the opening (409, 509) and into the conduit, and a second position in which the panel (410, 510) does not conceal the opening (409, 509) to freely permit the entrance of grain through the opening (409, 509) and into the conduit; a controller that is configured to actively adjust the panel (410, 510) to positions between the first and second positions during a grain unloading operation in order to meter the entrance of grain through the opening (409, 509); the combine harvester being characterised in that it further comprises a sensor that is connected to the controller and is configured to sense a level of grain within the storage tank, and, when the sensed level of grain exceeds a pre-determined value, the system is configured to (i) deactivate an auger (326, 332) Z in the storage tank, and (ii) move the panel (410, 510) between the first position and the second position to meter the volume of grain expelled from the conduit. a storage tank defining an interior region for holding grain; a conduit extending into the interior region of the storage tank, said conduit defining an interior space that is sized to receive a vertically oriented grain conveyance device that is configured to remove the grain from the storage tank, said conduit defining an opening (409, 509) for receiving grain from the interior region of the storage tank; a panel (410, 510) that is positioned adjacent the opening (409, 509) in the conduit, said panel (410, 510) being movable between a first position in which the panel (410, 510) conceals the opening (409, 509) to prevent the entrance of grain through the opening (409, 509) and into the conduit, and a second position in which the panel (410, 510) does not conceal the opening (409, 509) to freely permit the entrance of grain through the opening (409, 509) and into the conduit; a controller that is configured to actively adjust the panel (410, 510) to positions between the first and second positions during a grain unloading operation in order to meter the entrance of grain through the opening (409, 509); the combine harvester being characterised in that it further comprises a sensor that is connected to the controller and is configured to sense a level of grain within the storage tank, and, when the sensed level of grain exceeds a pre-determined value, the system is configured to (i) deactivate an auger (326, 332) Z in the storage tank, and (ii) move the panel (410, 510) between the first position and the second position to meter the volume of grain expelled from the conduit.