Method for calibrating a peristaltic pump in a medical apparatus, wherein the medical apparatus comprises a medical machine and a manifold assembly, wherein the medical machine comprises a peristaltic pump and a pressure transducer, wherein the manifold assembly comprises: a casing delimiting internally a first compartment and a second compartment; a yielding pump tube having a first end connected to the first compartment and a second end connected to the second compartment, wherein the yielding pump tube extends outside the casing and is coupled to the peristaltic pump of the medical machine; a plurality of line tubes (58, 59, 61, 63, 65) each having a first end connected to the first compartment or to the second compartment and a second end connected or connectable to a fluid source (60, 62, 64) or to a drain or to a patient; wherein, when the manifold assembly is properly mounted on the medical machine, an upper part of the second compartment delimits an air buffer volume, the air buffer volume being in communication with the pressure transducer; the method comprising: i. rotating the peristaltic pump of a predetermined rotation (Rotor_rev) to pump a liquid from the fluid source (60, 62, 64; 64) into the second compartment and raise a level of the liquid in the second compartment to compress air in the air buffer volume; ii. measuring pressure of air in the air buffer volume; iii. calculating, from the measured pressure of air in the air buffer volume, a variation of liquid volume (Vol _Moved) in the second compartment due to the rotation of the peristaltic pump; iv. calculating, from the variation of liquid volume (Vol _Moved) and the predetermined rotation (Rotor_rev), a stroke liquid volume (Stroke_Vol_Press) of the peristaltic pump. wherein the medical machine comprises a peristaltic pump and a pressure transducer, wherein the manifold assembly comprises: a casing delimiting internally a first compartment and a second compartment; a yielding pump tube having a first end connected to the first compartment and a second end connected to the second compartment, wherein the yielding pump tube extends outside the casing and is coupled to the peristaltic pump of the medical machine; a plurality of line tubes (58, 59, 61, 63, 65) each having a first end connected to the first compartment or to the second compartment and a second end connected or connectable to a fluid source (60, 62, 64) or to a drain or to a patient; wherein, when the manifold assembly is properly mounted on the medical machine, an upper part of the second compartment delimits an air buffer volume, the air buffer volume being in communication with the pressure transducer; a casing delimiting internally a first compartment and a second compartment; a yielding pump tube having a first end connected to the first compartment and a second end connected to the second compartment, wherein the yielding pump tube extends outside the casing and is coupled to the peristaltic pump of the medical machine; a plurality of line tubes (58, 59, 61, 63, 65) each having a first end connected to the first compartment or to the second compartment and a second end connected or connectable to a fluid source (60, 62, 64) or to a drain or to a patient; wherein, when the manifold assembly is properly mounted on the medical machine, an upper part of the second compartment delimits an air buffer volume, the air buffer volume being in communication with the pressure transducer; the method comprising: i. rotating the peristaltic pump of a predetermined rotation (Rotor_rev) to pump a liquid from the fluid source (60, 62, 64; 64) into the second compartment and raise a level of the liquid in the second compartment to compress air in the air buffer volume; ii. measuring pressure of air in the air buffer volume; iii. calculating, from the measured pressure of air in the air buffer volume, a variation of liquid volume (Vol _Moved) in the second compartment due to the rotation of the peristaltic pump; iv. calculating, from the variation of liquid volume (Vol _Moved) and the predetermined rotation (Rotor_rev), a stroke liquid volume (Stroke_Vol_Press) of the peristaltic pump. i. rotating the peristaltic pump of a predetermined rotation (Rotor_rev) to pump a liquid from the fluid source (60, 62, 64; 64) into the second compartment and raise a level of the liquid in the second compartment to compress air in the air buffer volume; ii. measuring pressure of air in the air buffer volume; iii. calculating, from the measured pressure of air in the air buffer volume, a variation of liquid volume (Vol _Moved) in the second compartment due to the rotation of the peristaltic pump; iv. calculating, from the variation of liquid volume (Vol _Moved) and the predetermined rotation (Rotor_rev), a stroke liquid volume (Stroke_Vol_Press) of the peristaltic pump.