Flow-meter using a marker responsive to a magnetic field

The meter comprises a rotatable member 12 coupled to a shaft member 14. A measuring circuit 22 comprising an AC power supply and a primary coil is mounted on the shaft member. The rotatable member or hub 12 comprises a passive non-permanent magnet marker 18 which is responsive to a magnetic field. When the rotatable member rotates in response to the motion of the surrounding fluid, the distance between the marker and the coil oscillates. The interaction of the marker with the magnetic field generated by the coil causes a measurable electrical response in the measuring circuit. Thus, the speed of rotation of the rotatable member can be determined. There is no physical contact between the rotatable member and the sensor, and the use of a permanent magnet is avoided. The marker could comprise an electrical circuit with a coil, capacitor and resistor, or could comprise an RFID tag, chip and aerial (26). Alternatively the marker could simply be made of a suitable substance such as steel. The rotatable member could have a number of rotatable blades 16, and there could be a plurality of such members.