A fluid conduit (2) comprises a wall (4) defining a fluid flow path (6) and a confinement feature (24) within the wall (4) and being configured to confine energy within a cavity (26), wherein at least a portion of the fluid flow path (6) extends through the cavity (26). The confinement feature (24) may be configured to confine electromagnetic energy. The fluid conduit (2) may comprise an oscillator defined by the cavity (26) and a positive feedback arrangement (34). The fluid conduit (2) may be configured for sensing a property of a fluid present in or flowing through the fluid conduit (2) or for use in sensing a property of a fluid present in or flowing through the fluid conduit (2). More specifically, the present invention deals with a microwave cavity sensor wherein the cavity member (24) is embedded in the wall (4) of the fluid conduit (2), the wall (4) including a composite region (20).