Abstract: Systems for monitoring fluid flow in an extracorporeal blood circuit are described. The blood circuit of such systems can include plod pump having a pumping chamber of the blood pump separated from a control chamber of the blood pump by a flexible diaphragm. The control chamber can be configured to transmit positive or negative pressure to operate the diaphragm. The system can include a pressure sensor configured to measure pressure in the control chamber of the blood pump, and a controller configured to receive information from the pressure sensor and to control the delivery of pressure to the control chamber of the blood pump. The controller can also be configured to cause the application of a time-varying pressure waveform on the blood pump diaphragm during a fill-stroke of the blood pump, and to monitor a pressure variation in the control chamber measured by the pressure sensor.