An apparatus is described for extracorporeal blood treatment, comprising a treatment unit, an extracorporeal blood circuit and a fluid evacuation line. The apparatus comprises a control unit connected to a pressure sensor and configured to move a blood pump, 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, a pressure progression that is variable over time with a pressure component oscillating about a mean value. The control unit receives a plurality of values over a period of time comprising a plurality of oscillations of the pressure about the mean value, calculates a control value representative of the oscillating pressure component, and then determines the verification or not of a condition of variation of the blood level in the expansion chamber.
