Magnetic flowmeter, also can used in water meter and gas meter/energy meter/remote metering system

bargain

Generates output pulses when alternating magnetic fields (actuators) pass near sensor
Requires no external power
Operates from zero speed to 20kHz
Polarity of pulse determined by orientation of actuator
Operates in rugged environments
Applications:
- Electronic indexing for water, gas, electric meters and remote metering systems
- Tachometers, speedometers and other rotational counting devices
- Directional sensing
- Conveyor applications for process control
- Measuring shaft speed in engines and other machinery
- Automotive applications