A fluid sensor comprises a base pipe 320 defining a fluid flow path and a cavity member 330. The cavity member 330 comprises an electrically-conductive composite material including a matrix and one or more reinforcing elements like carbon fibres 374 embedded within the matrix. In one embodiment the carbon fibres 374 in the main body portion 330a of the cavity member 330 are wound along a left-handed helical path at an angle of approximately 80 degrees relative to the longitudinal axis 376 and along a right-handed helical path at an angle of approximately 45 degrees relative to the longitudinal axis 376. Such an arrangement of carbon fibres may permit omni-directional current flow in the main body portion 330a, which thus supports the same modes that would be supported by a metallic cavity member of the same geometry.