Ionization and Photoelectric: Two Methods of Smoke Detection
Smoke detectors provide much more rapid response to fires than heat detectors. Fires will typically produce detectable amounts of smoke long before they produce detectable amounts of heat. Since smoke detectors respond to smoke, they are more prone to false alarm sources than heat detectors. Many benign environmental factors can look like smoke to a smoke detector; heat is more difficult to imitate.
Ionization smoke detectors use a small amount of radioactive material (typically Americium-241) to ionize air in a sensing chamber. The ionized air allows current flow between two charged plates. The detector monitors this current flow. When smoke enters the chamber, it attaches to the ionized particles and reduces the current flow between the plates. If the current flow drops low enough the detector goes into alarm.
Photoelectric smoke detectors detect smoke based on how it affects a projected light beam. There are two methods of detection for this type of detector. Light scattering detects light scattered or reflected into a sensor by the presence of smoke. Light obscuration detects when light received by a sensor is reduced by the presence of smoke.
Light-scattering detection is most commonly used in spot-type smoke detectors. The light source and the sensor are arranged so that the sensor cannot normally “see” the light source. When smoke enters the detector, it reflects (scatters) light into the sensor. When sufficient light is reflected into the sensor it goes into alarm condition.
Light obscuration detection is common with projected beam some detectors. In projected beam smoke detectors, the light transmitter and sensing element (receiver) are physically separate units (although some units house both together and use a reflector to return the signal). Light is projected from the transmitter to the receiver, and as long as the receiver can “see” the transmitter the receiver is in nonalarm condition. If the receiver gradually loses “sight” of the transmitter, it goes into alarm. Sudden losses of signal (from objects obstructing the path) usually cause a trouble signal rather than an alarm.
Projected beam detectors are commonly used to cover large open spaces, such as auditoriums or gymnasiums. A single detector can be used to protect an area more than 300 feet long and 60 feet wide. The large coverage area of projected beam smoke detectors allows them to be used in place of multiple spot type smoke detectors. The equipment and labor savings realized with this type of detector make it popular for large, open area applications.
The primary purpose of duct smoke detectors is to prevent the spread of smoke via the HVAC system. The detector is designed to detect fires within the system, or to detect smoke drawn in by the systems from rooms connected to it. It is not designed to be a replacement for primary fire detection in the building.
Duct detectors are typically mounted outside the duct with sampling tubes penetrating the duct to sample the air as it passes. Detectors may also be mounted in the duct, but this application is less common. The detectors in either application must be listed for use as duct smoke detectors. They must sample air passing the sensor or the sampling tube at a high volume and be able to detect smoke in it.
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