As part of a hydrocyclone Control system, different measured values have been tested, which can serve as a command variable for volume split regulation. Indicators for the operating state are the sediment mass stored in the conical part of the hydrocyclone, the formation of the air core and the shape of the underflow discharge. Principally, all tested variants including mechanical and optical detection of the discharge shape, hydrocyclone weighing and acoustic emission sensing are suitable for this purpose. Contact-free methods using laser signals for pattern analysis of rope/spray discharge or Fourier analysis in case of a laser vibrometer are given preference. (C) 2004 Elsevier Ltd. All rights reserved.