An apparatus (1) is described for extracorporeal blood treatment, comprising a treatment unit (2), an extracorporeal blood circuit (8) and a fluid evacuation line (10). A venous chamber (12) is placed in a blood return line (7) and is arranged in use to contain a gas in an upper portion (120) and blood at a predetermined level in a lower portion. The apparatus (1) comprises a control unit (21) connected to a first pressure sensor (14) and configured to: receive from the first pressure sensor (14) a first signal (P1(t)) relating to a time variable pressure (P(t)) of the blood flow； calculate a phase shift (¸) between the first signal (P1 (t)) and a reference signal (P2(t)) correlated to the time variable pressure (P(t)) detected at a location distinct from the upper portion (120) of the chamber (12)； monitor the volume (V) of gas in the upper portion (120) of the chamber (12) through the phase shift (¸).
