TABLE OF CONTENTS Although almost all optical flow sensors are not strictly MEMS-based, they are, however, included in this chapter as they can be used in areas, which are important, but for which MEMS cannot yet cater for. Fiber optic sensors have a number of advantages over their electrical counterparts. They are safe around volatile chemicals, are free from electromagnetic interference, and provide electrical isolation. In some applications, fiber sensors show higher durability at elevated temperatures, and they are corrosion resistant. For example, Eckert et al. [108] developed a mechanooptical sensor to measure flow in metallic melts of about 350 C. Flow rates between 1 and 14 cm/s in eutectic InGaSn melt could be measured. Borosilicate glass can be used up to temperatures of 350 C and quartz glass up to 1,000 C [108]. The major disadvantage of optical measurement systems is their size. Lasers, optical power meters, lenses, couplers, and mirrors are needed, making the system setup rather expensive and not suitable for portable systems or for use in small, confined spaces. Optical devices are not suitable for operation in unclean conditions for long periods of time (e.g., on the engine block of a car) because dirt and condensation lead to problems.
A flow sensor using a silicon cantilever with a wave guide on its surface is described by Chun et al. [109]. It uses a similar principle to the sensors based on drag force, but here, the sensing is not detected by an implanted piezoresistor but...
