A measurement section (M) of a conduit (110) receives a fluid medium containing a first fluid, e.g. a gas (G), and a second fluid, e.g. a liquid (L). A first sensor device (S1) in the measurement section (M) registers a first measure (ml) to represent a first type of physical characteristic of the fluid medium flowing through the measurement section (M). A first look-up table (T1) contains a first collection of values, where each value represents a first value of the flow (FG) of the first fluid (G) or the flow (FL) of the second fluid (L) for a given first measure (ml) and a second look-up table (T2) contains a second collection of values, where each value represents a second value of the flow (FQ) of the first fluid (G) or the flow (FL) of the second fluid (L) for a given first measure (ml). A processor (120) receives a series of first measures (ml) registered during a measurement interval, and based thereon, estimates a flow pattern of the fluid medium during said measurement interval. The processor (120) further derives the flow (FG) of the first fluid (G) or the flow (FL) of the second fluid (L) by using the first or the second look-up table (T1 T2). The first look-up table (T1) is used if the flow pattern fulfills a first flow-pattern criterion, and the second look-up table (T2) is used if the flow pattern fulfills a second flow-pattern criterion.
