substantially constant in practice the control units 9 10 11 may be integrated in one and the same control equipment and or they may form for example a part of a controller provided with software and used for managing the process as a whole 0073 fig1 shows typical three phase screening in a screen room in which the pulp is screened in two consecutive screening phases or screens and the obtained reject is then screened in a separate reject screening phase however the basic idea of the invention may also be applied in other kinds of screen rooms in which the properties of the accept and reject can be measured or determined following the described principle the different screening phases may comprise either separate screens or multi phase screens forming one entity or other kinds of screen combinations the control units may be connected to control the screens either directly or according to the principle of the aforementioned bus b a specific screen being controlled either by a single control unit or the impact of several control units being taken into account by way of example control unit 9 may thus provide 70 of the control of the first screening phase 3 control unit 10 providing 20 and control unit.