Experience obtained with a new hydrocyclone control system is reported. As command variable of the control serves an optical sensor detecting the underflow shape (rope or spray). These signals are transmitted to the computer and the control loop is completed with the pump power input, pressure in the feed and in the overflow. A volume split control of the hydrocyclone is performed by regulation of the overflow using a control valve. Groups of hydrocyclones can be controlled by installing only one control valve in a collecting pipe of all overflows. This control system stabilizes the separation at the transition point between spray and rope discharge at the cyclone underflow which in most cases is the operational optimum. One of the first applications was the regeneration of bentonite suspensions in tunnel driving. Since excavation of the material is discontinuous the solids content of the hydrocyclone feed varies over a wide range. By controlling the 150 mm hydrocyclone group it was possible to compensate for feed fluctuations and to stabilize the separation. (c) 2007 Elsevier Ltd. All rights reserved.