A process and apparatus are disclosed for the accurate measurement and control of liquid flow at low flow rates. The invention is particularly applicable to the measurement and control of continuous liquid blending operations, e.g. gasoline blending, in which a liquid additive, e.g. tetraethyl lead, is fed into a continuously flowing liquid stream, the additive being added in precisely controlled amounts requiring precise control of liquid flow at very low flow rates. According to the invention the additive is fed from a stock tank 1 alternately into one or the other of two reservoirs 6, 7 connected in parallel between the inlet line 3 from the stock tank and an outlet line 20. As the one reservoir is being charged, the other is being discharged through the four-way valve 5 to the outlet. Flow through the outlet line 20 is controlled by a valve 22. In accordance with the invention, the mass of liquid is continuously monitored by means in each reservoir sensitive to the weight of liquid in the reservoir which emits a signal proportional to the mass of liquid in the reservoir. This signal is passed to a differentiator for conversion into a flow signal proportional to the mass flow rate. The flow rate signal is fed to a ratio controller 36 for comparison with a demand signal fed via line 39 representing the desired flow rate. Depending on the difference, if any, between the flow signal and the demand signal, a control signal is sent via line 35 to automatically adjust the setting of the valve 22 and bring the flow rate to the desired value the control unit 33 also monitors the analogue signals via lines 30 and 31 to initiate the change – over from one reservoir to the other at the end of each filling and discharge phase of the operating cycle, in the preferred arrangement the rate of mass discharge of liquid from each reservoir is sensed by detecting the change in apparent weight of an elongated solid element suspended in each reservoir and extending into the liquid
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