Abstract: 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.