An apparatus is described for extracorporeal blood treatment (1), comprising a treatment unit (2), an extracorporeal blood circuit (8) and a fluid evacuation line (10). The apparatus comprises a control unit (21) connected with a pressure sensor (13, 14) and with a blood pump (9) and configured to move the blood pump (9), generating a variable flow with a constant component equal to a desired blood flow value and a variable component having a nil mean value the variable flow generates, in the expansion chamber (11, 12), a progression of the pressure that is variable over time (P(t)) with a pressure component (ΔP) oscillating about a mean value (Pavg). The control unit receives, from the sensor, a plurality of values (Pj) over a period of time (T) comprising a plurality of oscillations of the pressure about the mean value and calculates, according to the pressure values (Pj), a control value that is representative of the oscillating pressure component (ΔP) and then compares the control value with a reference threshold to determine the verification or not of a condition of variation of the blood level in the expansion chamber (11 12).
