The invention relates to a method for evaluating a particle signal (P) by means of an evaluation unit (101) associated with a control device (102), wherein a) the particle signal (P) is generated by means of a sensor (205) within a flow element (202), and is at least dependent on the number of particles in a two-phase flow generated when cleaning a surface (7) by means of a suction device connected to the flow element (202), b) the evaluation unit (101) determines a control signal (103) from the particle signal (P) for further controlling an actuator influenced by the control device (102). In order to reliably consider further influence factors for the presence of dust, besides the quantity of dust, the invention proposes that the evaluation unit (101) considers, in addition to the particle signal, the velocity (G) with which the flow element (202) is moved over the surface (7) when determining the control signal (103).