Measuring the flow rate of RO permeate in bench-scale research systems can be challenging. A system for measuring the flow rate of RO cross-flow cells is presented. The apparatus (called a Flow Tower) is described in terms of construction and operation to allow others to replicate the design. Bench-scale RO permeate flux data is also presented with a comparison between manual readings and the Flow Tower to demonstrate the reliability of the system. The simplicity of the system and low-cost are highlighted as well as the advantages found in continuous online measurement compared with periodic measurement. Permeability data from a bench-scale RO experiment are also presented to demonstrate these advantages and the importance of operating bench-scale fouling experiments for relatively long durations. (C) 2011 Elsevier B.V. All rights reserved.