An LED light beam strikes the interface between the column bottom and the liquid. Light striking the interface at less than the critical angle is refracted and the light striking at greater than the critical angle, is totally reflected and exits the opposite side of the column. A photodiode on that side of the column provides an output proportional to the amount of light energy received. This energy is related to the critical angle which is related to the fuel mixture index of refraction and the fuel alcohol concentration. In summary, optical techniques for FFV sensors present a challenge to opto-mechanical designers.
All optical surface are susceptible to contamination.
Reference Bill Siuru, Sensor, September 1992. It can measure flow over a large range with accuracy. It is small in size and can provide a remote signal. The flow meter consists of a member with an expandable volume having an inlet and outlet. The member is an elongated tube with outlet ports along its length.