Method for eliminating or reducing reading errors of a non-contact microwave solids flow meter measuring solid particulate flow. The method is particularly suitable for calibration procedures prior to installation into an operating system. The flow meter is very sensitive and may pick up backscatter from dust and/or particulate clouds of the particulate product that builds up in the space above the collected particulate of the delivered product in the collecting bin. Readings that include backscatter from the dust cloud are in excess of the actual flow rate. The invention reduces or eliminates backscatter readings that include the particulate dust cloud by attaching a sheath reflective to backscatter energy and made for example from wire mesh to the delivery end of the output duct. The sheath extends toward the build up of the particulate material. During the calibration procedure, the reflective sheath is continuously moved so that it does not contact the material as it builds up.