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All intensity-modulated fiber optic sensors are analog in nature, but they have a broad usage potential in digital applications. Until recently, they were used as absence/presence sensors, i.e., is the object there or not? Digital applications have now expanded to pressure, temperature, and liquid-level switches. Photoelectric sensors have been used for many years in absence/presence applications. Since fiber optic sensors are almost always more expensive than photoelectric sensors, the driving force for using fiber optics is generally due to environmental considerations. Fiber optic sensors, due to their small mass, are less susceptible to vibration. Fiber optic devices are immune to electromagnetic interference and radio frequency interference and, therefore, are free from electrical noise problems. They also are explosion proof. Their small size makes them easy to install, and conduit and shielding are generally not necessary. Figure 8.l shows illustrations of photoelectric and fiber optic sensors for comparison.
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