This invention relates generally to grass mowing machines, and more specifically to material collection system sensors. A material collection system comprises:an ultrasonic sensor (16) mounted in a collection container (14) above a discharge chute (15) and directed to a portion of the collection container (14) where a pile of grass clippings accumulates, and providing data to an electronic control unit (19) regarding the distance between the pile and the ultrasonic sensor (16)the electronic control unit (19) filtering the data based on certain pre-specified criteria and correlating the distance data to a fill level of the collection container (14) anda fill level indicator (20) connected to the electronic control (19) unit anddisplaying information about any fill level of the collection container (14).
