| Smoke Detector Information Presented By The Barre City Fire Department |
| I n t e r n a t i o n a l A s s o c i a t i o n O f F i r e F i g h t e r s L o c a l # 8 8 1 T h e B a r r e C i t y F i r e D e p a r t m e n t 15 Fourth St. Barre Vt. 05641 (802) 476-0254 Copyright © 2006-2010 All Rights Reserved. |
| Smoke Detectors ****How They Work**** |
| Smoke detectors are one of those amazing inventions that, because of mass production, cost practically nothing. You can get a smoke detector for as little as $7. And while they cost very little, smoke detectors save thousands of lives each year. In fact, it is recommended that every home have one smoke detector per floor. All smoke detectors consist of two basic parts: a sensor to sense the smoke and a very loud electronic horn to wake people up. Smoke detectors can run off of a 9-volt battery or 120-volt house current. On this website, we will examine the two most common types of smoke detectors used today: photoelectric detectors and ionization detectors. |
| Photoelectric Smoke Detectors Occasionally, you will walk into a store and a bell will go off as you cross the threshold. If you look, you will often notice that a photo beam detector is being used. Near the door on one side of the store is a light (either a white light and a lens or a low-power laser), and on the other side is a photo detector that can "see" the light. When you cross the beam of light, you block it. The photo detector senses the lack of light and triggers a bell. You can imagine how this same type of sensor could act as a smoke detector. If it ever got smoky enough in the store to block the light beam sufficiently, the bell would go off. But there are two problems here: It's a pretty big smoke detector. It is not very sensitive. There would have to be a LOT of smoke before the alarm would go off -- the smoke would have to be thick enough to completely block out the light. It takes quite a bit of smoke to do that. Photoelectric smoke detectors therefore use light in a different way. Inside the smoke detector there is a light and a sensor, but they are positioned at 90-degree angles to one another, like what you see in the side picture. In the normal case, the light from the light source on the left shoots straight across and misses the sensor. When smoke enters the chamber, however, the smoke particles scatter the light and some amount of light hits the sensor The sensor then sets off the horn in the smoke detector. Photoelectric detectors are better at sensing smoky fires, such as a smoldering mattress. Photoelectric Smoke Detectors are usually identified by: The letter P - or the word 'Photoelectric" |
A - Light source
B - Photo detector
B - Photo detector
| Ionization Smoke Detectors Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. This type of smoke detector is more common because it is inexpensive and better at detecting the smaller amounts of smoke produced by flaming fires. Inside an ionization detector is a small amount (perhaps 1/5000th of a gram) of americium - 241. The radioactive element americium has a half-life of 432 years, and is a good source of alpha particles. An ionization chamber is very simple. It consists of two plates with a voltage across them, along with a radioactive source of ionizing radiation, like seen in the side picture. The alpha particles generated by the americium have the following property: They ionize the oxygen and nitrogen atoms of the air in the chamber. To "ionize" means to "knock an electron off of." When you knock an electron off of an atom, you end up with a free electron (with a negative charge) and an atom missing one electron (with a positive charge). The negative electron is attracted to the plate with a positive voltage, and the positive atom is attracted to the plate with a negative voltage (opposites attract, just like with magnets). The electronics in the smoke detector sense the small amount of electrical current that these electrons and ions moving toward the plates represent. When smoke enters the ionization chamber, it disrupts this current -- the smoke particles attach to the ions and neutralize them. The smoke detector senses the drop in current between the plates and sets off the horn. Ionization Smoke Detectors are usually identified by: The letter " I " - This Symbol - or the word " Ionization" you may also find "americium - 241" |
| Follow the Manufactures Directions |
| Smoke Detectors How They Work |
| Smoke Detector Reports/Case Studies |
| Smoke Detector Documents |
| Smoke Detector Videos |
| Smoke Detector Letters |
| Smoke Detector Question Form |
| Firefighter Matthew J. Cetin & Deputy Chief Russell Ashe Mcetin@BarreCityFire.org Rashe@BarreCityFire.org |