The FloWatcher system provides real-time downhole flowrate and density measurement. Based on the venturi nozzle and a gradio-manometer, the system can provide two-phase total flowrate and water holdup measurements. The FloWatcher is initially calibrated in single phase flow which yields a linear relationship between the pressure drop across the venturi and the pressure drop across the gradio-manometer. If at any time after the calibration has been made data points are seen to be falling away from the calibration line, then there has been a change in flowing density. This allows early detection of water breakthrough and hence early intervention.