The present invention relates to an especially medical apparatus (200) comprising at least one hard unit (201) having fluid lines (202) conducting an especially medical fluid, especially blood, through the hard unit (201)； a converter (205, 207, 209, 211), said converter (205, 207, 209, 211) being arranged for measuring a property (Q) of the fluid while the latter is present in one of the fluid lines (202)； and a device for determining an especially geometrical property (dx, dy) of the apparatus (200), especially of a fluid line (202).
