Abstract: The level of such granular materials as fly ash and grain in a hopper presenting a differential internal hopper pressure above and below the level of materials in the hopper is sensed by sampling the pressure through a single flowless pressure conveying passageway extending by means of a port into the hopper interior. The pressure significantly changes above and below the level of materials. Thus, typically with a vacuum of about eight inches of water above the material level, the vacuum pressure is less than one-half an inch of water when the granular materials cover the port. Thus, a differential pressure gage connected between the atmosphere and the flowless pressure conveying passageway will indicate a significant difference in pressure when the level of materials covers the port. With a pressure gage operating relay contact an electric monitoring and control system may be used to automatically discharge the materials over long periods of time without maintenance.