Smoke Detector
A smoke detector is a device that senses smoke, typically as an indicator of fire. Commercial and residential security devices issue a signal to a fire alarm control panel as part of a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible or visual alarm from the detector itself.
Smoke detectors are typically housed in a disk-shaped plastic enclosure about 150 millimetres (6 in) in diameter and 25 millimetres (1 in) thick, but the shape can vary by manufacturer or product line. Most smoke detectors work either by optical detection (photoelectric) or by physical process (ionization), while others use both detection methods to increase sensitivity to smoke. Sensitive alarms can be used to detect, and thus deter, smoking in areas where it is banned such as in public restrooms and schools. Smoke detectors in large commercial, industrial, and residential buildings are usually powered by a central fire alarm system, which is powered by the building power with a battery backup. However, in many single-family detached and smaller multiple family housings, a smoke alarm is often powered only by a single disposable battery.
As of November 2013 it is estimated that smoke detectors are installed in 93 percent of US homes and 85 percent of UK homes. 30 percent of these alarms are estimated to not work, due to aging, removal of batteries, or failure of owners to replace dead batteries. In the United States, the National Fire Protection Association estimates that nearly two-thirds of deaths from home fires occur in properties without working smoke alarms/detectors.
Design
Optical
An optical detector is a light sensor. The components of the light sensor are the light source (incandescent bulb or Light-emitting diode), a lens, and a photoelectric receiver (typically a photodiode). In spot-type detectors, all of these components are arranged inside a smoke chamber where smoke from a nearby fire will flow. In large open areas such as atria and auditoriums, optical beam smoke detectors are used. A wall-mounted unit emits a beam of infrared or ultraviolet light which is either received and processed by a separate device or reflected back to the transmitter/receiver by a reflector.
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."
Although optical alarms are highly effective at detecting smoldering fires and do provide adequate protection from flaming fires, fire safety experts and the National Fire Protection Agency recommend installing what are called combination alarms, which are alarms that either detect both heat and smoke, or use both the ionization and photoelectric / optical processes. Also some combination alarms may include a carbon monoxide detection capability.
Not all optical detection methods are the same. The type and sensitivity of light source and photoelectric sensor, and type of smoke chamber differ between manufacturers.
