Abstract: A plant (1) for the treatment of a multiphasic fluid and method for line-characterization of the fluid wherein the plant (1) comprises a circuit (4) within which the fluid is intended to circulate and being characterized in that it comprises a characterization tool (3) comprising at least an analysis settler (6) adapted to separate the phases to obtain so-called separated phases and a means (9) for combining the separated phases. The plant (1) is designed so that a fraction of the fluid circulating within the circuit (4) circulates within the tool (3) to pass through the analysis settler (6) so that the phases of the fluid are separated then discharged into the circuit (4). The combining means (9) is designed so as to combine the separated phases.