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 (θ) 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 (θ).
