The MDF Duct Flowprobes are ideal to measure air volumes in round spiral ducts from 80 up to 500mm diameter (maximum 710mm with support bracket). The MDF produce an average differential velocity pressure to provide an air volume measurement for fan proving, VAV or CAV volume control in ducts and general ventilation applications. They are made of Aluminium and can be mounted easily by simply drilling a 25.4 mm round hole into the duct.