A product distribution device is shown as embodied in an agricultural air seeder in which a mass flow sensor is used to measure the product flow through only a portion of a product flow passage wherein the total mass flow is calculated from the portion that is measured. The structural element has an opening in a wall of the cavity through which moisture located in the cavity can escape into the surroundings, wherein a drainage element for sealing the opening in the wall of the cavity is provided in the opening. The heights of the bent points of the frames are generally aligned with the height of the seat surface of the seat and disposed in the canopy.