A device that senses smoke is called Smoke Detector. It typically works as an indicator of fire. Commercial smoke detector issues signal to a fire alarm control panel while domestic smoke detectors issue a locally audible sound or visual alarm from the detector. Smoke detectors are plastic enclosures of varied shape and sizes but typically they are shaped like a disc of about 150 millimeters in diameter and 25 millimeters thickness. There are two ways to detect smoke. Smoke can be detected either optically or by physical process call ionization. Detectors may use either of these or both methods. Smoke detectors also deter smoking as sensitive alarms can be used to detect smoking in areas where it is banned. A central fire alarm system usually powers the smoke detectors in large commercial, industrial and residential buildings. They are powered by the building power with battery backup. Individual battery powered units powers the domestic smoke detectors and they are interlinked with battery backup so that if any unit detects smoke, the trigger will go off even in absence of electricity.
There are three types of smoke alarms:
1. Ionization smoke alarm: This type of alarm contains radioactive material in small quantity. The radiation permits a constant current between the electrodes when passed through and ionization chamber. The ionization chamber is an air-filled space between two electrodes. The alpha particles are absorbed from any smoke that enters the chamber. This process reduces the ionization and interrupts the current which sets off the alarm. These alarms are the best for raging fire.
2. Photoelectric smoke alarms: These type of alarm use a photoelectric sensor, a light beam collimating system, and a light source. Some light is scattered by the smoke particles when smoke enters the optical chamber and crosses the path of the light beam. This scattered light is directed at the sensor which activates the alarm. These alarms are best for slow fires.
3. Combination smoke alarm is a combination of both, ionization and photoelectric technologies as the name suggest. Combination smoke alarms are recommended by the NFPA for the best protection.