A blood processing apparatus , comprising: - a measurement device having a first chamber element for measuring a haematocrit value of a blood fluid entering the blood processing apparatus , the first chamber element comprising a first inlet port connectable to a first reservoir container for allowing a flow from the first reservoir container into the first chamber element and a first outlet port for allowing a flow out of the first chamber element , the measurement device further having a second chamber element for measuring a haematocrit value of a blood fluid exiting the blood processing apparatus , the second chamber element comprising a second inlet port for allowing a flow into the second chamber element and a second outlet port connectable to a second reservoir container for allowing a flow out of the second chamber element towards the second reservoir container , - a first pump mechanism for pumping a blood fluid in a flow direction (F1) from the first reservoir container towards the blood processing apparatus , and - a second pump mechanism for pumping a blood fluid in a flow direction (F2) from the blood processing apparatus towards the second reservoir container , wherein the first pump mechanism is located upstream of the first inlet port of the first chamber element and the second pump mechanism is located upstream of the second inlet port of the second chamber element , characterized by a holder device for holding the measurement device , the holder device comprising a base having a reception opening for receiving the measurement device and a closure element movably arranged on the base for locking the measurement device in an inserted position in the reception opening , wherein the holder device comprises a first ultrasonic sensor element which in the inserted position of the measurement device faces the first chamber element , a second ultrasonic sensor element which in the inserted position of the measurement device faces the second chamber element , a first infrared sensor element which in the inserted position of the measurement device faces the first chamber element , and a second infrared sensor element which in the inserted position of the measurement device faces the second chamber element , wherein the base comprises a first tilted face and a second tilted face extending transversely with respect to the first tilted face , wherein the first and the second ultrasonic sensor element are arranged on the first tilted face and the first and the second infrared sensor element are arranged on the second tilted face . - a measurement device having a first chamber element for measuring a haematocrit value of a blood fluid entering the blood processing apparatus , the first chamber element comprising a first inlet port connectable to a first reservoir container for allowing a flow from the first reservoir container into the first chamber element and a first outlet port for allowing a flow out of the first chamber element , the measurement device further having a second chamber element for measuring a haematocrit value of a blood fluid exiting the blood processing apparatus , the second chamber element comprising a second inlet port for allowing a flow into the second chamber element and a second outlet port connectable to a second reservoir container for allowing a flow out of the second chamber element towards the second reservoir container , - a first pump mechanism for pumping a blood fluid in a flow direction (F1) from the first reservoir container towards the blood processing apparatus , and - a second pump mechanism for pumping a blood fluid in a flow direction (F2) from the blood processing apparatus towards the second reservoir container , <EOS>