An apparatus for extracorporeal blood treatment, comprising: at least one treatment unit (2) comprising at least a first compartment (3)； at least one blood withdrawal line (6) connected to the first compartment (3) and configured to withdraw blood from a patient (M)； at least one blood return line (7) connected to the first compartment (3) and configured to return treated blood to the patient (M)； the blood withdrawal line (6), the blood return line (7) and the first compartment (3) being part of an extracorporeal blood circuit (8)； at least one chamber (12) placed in the extracorporeal blood circuit (8), the chamber (12) being used to contain a volume (V) of gas in an upper portion (120) and blood at a predetermined level in a lower portion； at least one blood pump (9) that works in the extracorporeal blood circuit (8) and configured to move blood in the circuit； at least a first pressure sensor (14) associated with the upper portion (120) of the chamber (12) and configured to allow the determination of pressure values inside the upper portion (120)； a control unit (21) connected to the first pressure sensor (14) and the pump (9), and configured to: - receive from the first pressure sensor (14) a first signal (P1 (t)) related to the minus a pulse of blood flow pressure； characterized in that the control unit (21) is configured to: - calculate a phase shift (θ the first signal (P1 (t)) and a reference signal (P2 (t)) correlated with said pressure pulse detected in a location other than the upper portion (120) of the chamber (12), said phase shift (θ volume (V) of gas depending on the upper portion (120) of the chamber (12)； and - monitoring said volume (V ) of gas through said phase shift (θ).Un aparato para tratamiento sanguíneo extracorporal, que comprende: al menos una unidad de tratamiento (2) que comprende al menos un primer compartimiento (3)； al menos una línea de retirada de la sangre (6) conectada al primer compartimiento (3) y configurada para retirar san
