A method to determine a type of blood circuit attached to an extracorporeal blood treatment console including: pumping a liquid through a blood passage of the blood circuit, sensing a first pressure in: the blood passage while the passage is closed and pumping stopped, pumping an additional amount of the liquid into the blood passage while the blood passage is and remains closed and thereafter sensing a second pressure, and determine a dimensional characteristic of a fluid passage in the blood passage based on the additional amount of the liquid and the second pressure.