A robust and reliable sensor to measure gas (air) superficial velocity (J(g)) continuously in flotation systems is introduced. It is based on the sampling of bubbles by buoyancy with the accumulating air allowed to exit through an orifice. At steady state, pressure drop is measured and related to the J(g) by prior calibration. The continuous device is readily automated and extended to a multi-unit set-up. The sensor and data collections are described. The governing gas flow equation and models of the dynamic response to air flow rate set point change and fluctuations in froth depth are derived. Model predictions are confirmed against plant data. By taking a moving average, the J(g) remains valid in the face of typical plant disturbances. (c) 2006 Elsevier B.V. All rights reserved.