Apparatus for extracorporeal blood treatment comprising: at least one treatment unit comprising at least a first compartment; at least one blood removal line connected to the first compartment and configured to remove blood from a patient; at least one blood return line connected to the first compartment and configured to return treated blood to the patient; the blood removal line, the blood return line and the first compartment being part of an extracorporeal blood circuit; at least one chamber placed in the extracorporeal blood circuit, the chamber being arranged in use to contain a volume of gas in an upper portion and blood at a predetermined level in a lower portion; at least one blood pump operating at the extracorporeal blood circuit and configured to move blood in the circuit; at least a first pressure sensor associated to the upper portion of the chamber and configured to enable determining pressure values internally of the upper portion; a control unit connected to the first pressure sensor and to the pump and configured to: - receive from the first pressure sensor a first signal (P1(t)) relating to at least one pulse of pressure of the blood flow; characterized in that the control unit is configured to: - calculate a phase shift (θ) between the first signal (P1(t)) and a reference signal (P2(t)) correlated to said pulse of pressure detected at a location distinct from the upper portion of the chamber, said phase shift (θ) depending on the volume of gas in the upper portion of the chamber; and - monitor said volume through said phase shift (θ). at least one treatment unit comprising at least a first compartment; at least one blood removal line connected to the first compartment and configured to remove blood from a patient; at least one blood return line connected to the first compartment and configured to return treated blood to the patient; the blood removal line, the blood return line and the first compartment being part of an extracorporeal blood circuit; at least one chamber placed in the extracorporeal blood circuit, the chamber being arranged in use to contain a volume of gas in an upper portion and blood at a predetermined level in a lower portion; at least one blood pump operating at the extracorporeal blood circuit and configured to move blood in the circuit; at least a first pressure sensor associated to the upper portion of the chamber and configured to enable determining pressure values internally of the upper portion; a control unit connected to the first pressure sensor and to the pump and configured to: - receive from the first pressure sensor a first signal (P1(t)) relating to at least one pulse of pressure of the blood flow; characterized in that the control unit is configured to: - calculate a phase shift (θ) between the first signal (P1(t)) and a reference signal (P2(t)) correlated to said pulse of pressure detected at a location distinct from the upper portion of the chamber, said phase shift (θ) depending on the volume of gas in the upper portion of the chamber; and - monitor said volume through said phase shift (θ). - receive from the first pressure sensor a first signal (P1(t)) relating to at least one pulse of pressure of the blood flow; characterized in that the control unit is configured to: - calculate a phase shift (θ) between the first signal (P1(t)) and a reference signal (P2(t)) correlated to said pulse of pressure detected at a location distinct from the upper portion of the chamber, said phase shift (θ) depending on the volume of gas in the upper portion of the chamber; and - monitor said volume through said phase shift (θ). - calculate a phase shift (θ) between the first signal (P1(t)) and a reference signal (P2(t)) correlated to said pulse of pressure detected at a location distinct from the upper portion of the chamber, said phase shift (θ) depending on the volume of gas in the upper portion of the chamber; and - monitor said volume through said phase shift (θ).