A look inside a fluid flow rate sensor.

Ebay is full of industrial control components that used to cost an arm and a leg to buy from local distributors. With the current era of Arduino control systems and online ordering from China, you can buy these control components at a fraction of the cost. So low, that it's worth just buying them to take to bits.
This is a look inside a fluid control monitor that uses a turbine arrangement to detect the flow and speed of liquid, and represent it as a series of pulses that will be faster at higher flow rates.