Abstract: An assembly comprising an upstream pipe, a downstream pipe, and a system (1) for measuring variation in the gas content of a two-phase flow, the assembly comprising: an insulating sheath (6), an upstream ground (2), a measurement electrode (3), a guard electrode (7), and a downstream ground (4) arranged in succession in said insulating sheath (6) and each presenting an identical internal section defining an internal flow duct for a two-phase flow from the upstream ground (2) towards the downstream ground (4) in line with the upstream pipe and the downstream pipe, the guard electrode (7) being subjected to the same potential as the measurement electrode (3), the measurement electrode (3) measuring the capacitance of the two-phase flow relative to the upstream ground (2) and variation in that capacitance, the upstream ground (2) and the downstream ground (4) being electrically connected to the upstream pipe and to the downstream pipe, respectively.