A control unit (30) is arranged to control a dialysis fluid distribution system (12) comprising first and second volumetric pumps (PI, P2) arranged upstream and downstream of a dialyzer (13). The control unit (30) is operable in a calibration mode, to establish a bypass flow path that bypasses the dialyzer (13) and extends from the first pump (PI) to the second pump (P2) in fluid communication with a calibration chamber (26) and to operate the pumps (PI, P2) at different combinations of speeds that cause first and second changes in fluid level in the chamber (26). The first and second changes correspond to first and second known volumes in the chamber (26). The control unit (30) measures, by a level detector (28) in the chamber (26), first and second time periods for the first and second changes, and computes the stroke volumes of the pumps (PI, P2) as a function of the speeds of the pumps (PI, P2), the first and second time periods, and the first and second known volumes. By computing the stroke volumes, the control unit (30) is operable to achieve an accurate ultrafiltration (UF) rate in the dialyzer (13) by controlling the relative speeds of the pumps (PI, P2).
