A surprisingly sophisticated method of “pickup” for the plastic paddlewheel shown in the
photograph uses fiber-optic cables to send and receive light. One cable sends a beam of light to
the edge of the paddlewheel, and the other cable receives light on the other side of the paddlewheel.
As the paddlewheel turns, the paddles alternately block and pass the light beam, resulting in a pulsed
light beam at the receiving cable. The frequency of this pulsing is, of course, directly proportional
to volumetric flow rate