The pressure drop in a venturi meter varies only with the fluid density, pipe approach velocity, and diameter ratio of the meter. A model venturi meter tested in water at 20°C shows a 5-kPa drop when the approach velocity is 4 m/s. A geometrically similar prototype meter is used to measure gasoline at 20°C and a flow rate of 9 m3/min, if the prototype pressure gage is most accurate at 15 kPa, what should the upstream pipe diameter be?