Hall Effect Flow Sensor
It is simply a magnet on a spinning wheel that passes by a Hall sensor - counting the number of pulses counts the rate of wheel spin - which is proportional to fluid flow.
This would be a great way to implement a return-side Arduino hydraulic flow meter! Printing with ABS (7000 PSI) can easily get us measurements on the return hydraulic side.
This would be a great way to create an Arduino-based logging flow meter.