System for dosing at least two concentrates in water for online preparation of a medical fluid, said system comprising - a main duct (1) for feeding water, - a first conduit (2) with a first pump (3) to feed a first concentrate the main conduit in a first mixing point (4), - a second duct (5) with a second pump (6) to feed a second concentrate the main conduit (1) in a second mixing point ( 7), - a sensor main concentration (12) disposed in the main conduit downstream of both the first mixing point and the second mixing point (4, 7), characterized in that - the system is configured to initially control the dosage the first concentrate, while not supplied with any amount of the second concentrate, under the feedback control of the main concentration sensor (12) to the first pump (3) for feeding the first concentrate or, so that when the concentration measured by the sensor main concentration (12) has reached a first level of preselected concentration, a value for a parameter of power to the first pump (3) to feed the first concentrate it is determined - after determining the value for the parameter of power, the system is configured to control the dosage both of the first concentrate and second concentrate during the continuous preparation line of medical fluid, so that the feedback control of the sensor main concentration (12) is moved, the control of the first pump (3) for feeding the first concentrate, to control the second pump (6) for feeding the second concentrate, and so that the value for the parameter power is used to control the first pump (3) for feeding the first concentrate.Sistema para dosificar al menos dos concentrados en agua para la preparación en línea de un fluido médico, comprendiendo dicho sistema - un conducto principal (1) para alimentar con agua, - un primer conducto (2) con una primera bomba (3) para alimentar con un primer concentrado el conducto principal en un primer punto de mezcla (4), - un segundo conducto (5) con una segunda bomba (6)
