A maintenance crew discovers an orifice meter while working on an old water line. The engineer in charge would like to know the size of the orifice plate opening without dismantling the delicate meter. A specification plate on the side of the meter reads "Cv = 0.605." For a flow rate of 0.00578 m3/sec, the attached mercury-water manometer registers a pressure difference of 9.0 cm. If the horizontal pipeline has an inside diameter of 10 cm, what is the size of the orifice plate?