A flow delivery system to deliver and monitor a liquid flow, using a pump, and a monitoring device (e.g. a flow meter) are described. A controller determines if the flow is a liquid flow or a gas flow by reducing a drive signal for the pump from a first drive signal to a second drive signal. This changes the characteristics of both the pump drive signal and the characteristics of the resulting flow. Based on the change in the characteristics, the flow is determined to be a liquid flow or a gas flow.