A measurement section (M) of a conduit (110) receives a fluid medium containing a first fluid (G) and a second fluid (L). A first sensor device (S1 ) in the measurement section registers a first measure (m1 ) to represent a first type of physical characteristic of the fluid medium flowing through the measurement section (M). A second sensor device (S2) in the measurement section registers a second measure (m2) to represent a second type of physical characteristic of the fluid medium flowing through the measurement section (M). The second type of physical characteristic is different from the first type of physical characteristic, and thus the first and second measures (m1 m2) are independent from one another. However, both measures (m1, m2) are dependent on a flow (FG) of the first fluid (G) and a flow (FG) of the second fluid in the fluid medium. Based on the first and second measures (m1, m2), a processor (120) derives a flow (FG) of the first fluid (G) and/or a flow (F) of the second fluid (G) in the fluid medium.
