A monitoring device and a product distribution system (34) with such is shown. The system is shown as embodied in an agricultural air seeder (10) in which a mass flow sensor (200, 220, 230) is used to measure the product flow through only a portion of a product flow passage (38, 50, 58, 84, 124) wherein the total mass flow is calculated from the portion that is measured. Furthermore, the output signal from the mass flow sensor is used to determine if a device (40) used for section control is functioning properly.
