Detectors 198
Detectors 198 : Particle Detectors: (4.26) Smoke Detector: (2) Design: (2. 1) Optical: An optical detector is a light sensor. When used as a smoke detector, it includes a light source (incandescent bulb or infrared LED), a lens to collimate the light into a beam, and a photodiode or other photoelectric sensor at an angle to the beam as a light detector. In the absence of smoke, the light passes in front of the detector in a straight line. When smoke enters the optical chamber across the path of the light beam, some light is scattered by the smoke particles, directing it at the sensor and thus triggering the alarm. Also seen in large rooms, such as a gymnasium or an auditorium, are devices that detect a projected beam. A wall-mounted unit sends out a beam, which is either received by a separate monitoring device or reflected back via a mirror. When the beam becomes less visible to the "eye" of the sensor, it sends an alarm signal to the fire alarm control panel. According to the National Fire Protection Association (NFPA ), "photoelectric smoke detection is generally more responsive to fires that begin with a long period of smoldering (called smoldering fires)". Also, studies by Texas A&M and the NFPA cited by the City of Palo Alto California state, "Photoelectric alarms react slower to rapidly growing fires than ionization alarms, but laboratory and field tests have shown that photoelectric smoke alarms provide adequate warning for all types of fires and have been shown to be far less likely to be deactivated by occupants"
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