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 (Q(t)) with a constant component (Qb) and a variable component (Qvar(t)) having a nil average 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 (Pvar(t)) oscillating about an average value (Pavg). The control unit receives, from the sensor, a plurality of values (Pj) and calculates the average value of the pressure (Pavg), acquires an estimated value of volume variation (AP) in the expansion chamber (11, 12) connected to the variable flow component (Qvar(t)), calculates, as a function of the pressure values (Pj), an estimated value of pressure variation (AP) in the expansion chamber (11； 12) that is representative of the oscillating pressure component (Pvar(t)) and determines a representative magnitude of a blood level (L) in the expansion chamber (11, 12) as a function of the average value (Pavg) of the pressure (P(t)), of the estimated value of volume variation (AV) and of the estimated pressure variation (AP) in the expansion chamber.L'invention concerne un appareil pour le traitement extracorporel du sang (1), comprenant une unité de traitement (2), un circuit extracorporel du sang (8) et une ligne d'évacuation du fluide (10). L'appareil comprend une unité de contrôle (21) connectée à un capteur de pression (13, 14) et à une pompe sanguine (9) et configurée pour bouger la pompe sanguine (9), en générant un flux variable (Q(t)) avec un composant constant (Qb) et un composant variable (Qvar(t)) ayant une valeur moyenne nulle； le flux variable génère, dans la chambre d'expansion (11, 12), une progression de la p
