Magnetic proximity sensor for measuring gap between opposed refiner plates

A gap sensor comprising a C-type core (38, 62, 126) wrapped with a coil (44, 68, 130), and arranged to be responsive to the variable reluctance in a magnetic circuit passing through the core, two refiner plates (28a, 28b, 194, 198) and the gap (34, 90) between the plates. The two sensing ends (134, 136) of the transducer core legs (128, 132) are positioned in close proximity to the circumferential surfaces, or rims (82, 86, 212, 214) of the two spaced apart refiner plates, respectively. The magnetic field generated by the coil follows a magnetic path or circuit including the core (38), the radial gaps (46), between the sensor and the discs (16, 18), the refiner disc material, the plates (28a, 28b) mounted on the disc, and the clearance gap (34) between the two refiner plates.