The role of gas holdup in flotation has long been discussed but never demonstrated, arguably because a reliable measurement technique has not been available. Work was initiated to develop a gas holdup sensor for industrial operations based on the use of two so-called flow cells for measuring the conductivity of the pulp with and without air. These are the measurements required to estimate gas holdup using Maxwell's equation that relates conductivity to concentration of a dispersed non-conducting phase (i.e., bubbles) in a continuous liquid phase (pulp in this case). After a series of prototypes a unit robust enough for industrial use that continuously measures and delivers signals easily integrated into a plant PLC system has been developed. This communication describes the working principle along with some construction details. The experience of plant tests, ranging from paper to mineral pulps, and mechanical cells to columns, is reviewed. (C) 2003 Elsevier Science Ltd. All rights reserved.