Smoke Detectors

From Chempedia

Ian Sinclair
Danielle Lindgren
Minzee Kim
Babak Oskoui
David Windseth

Smoke Detectors: A Savior Invention


A Brief History of Smoke Alarms. Smoke detectors have come a long way since George Darby first invented in 1902. Before scientists knew how to capture ionizing molecules in a small enclosed space, they actually used an open/close electrical system along with a wedge of butter to detect fires and heat.[1] This system’s setup included two plates or electrical circuits, not unlike today’s Ionization alarms, with a wedge of butter between them. When the heat of the room became overwhelming and dangerous, the butter would melt, causing the two circuits to collapse onto one another, initiating the alarm. Since then, technology has found a way to capture light and molecules in a cheaper, more efficient, and safer way to save millions of lives each year.


Two Common Types There are two types of smoke detectors common to today\’s normal household: ionization and photoelectric smoke detectors.[2] [3] [4] [5] These smoke detectors are both used to detect fire, but not the same type of fire. Photoelectric Smoke Detectors are faster in detecting smoldering fires, while Ionization Smoke Detectors are better at detecting flaming fires due to their ability to detect smaller particles.2 There is a slight defect in these methods of detecting fire; high humidity or steam can also cause an alarm to go off.


Ionization Smoke Detectors An Ionization Smoke Detector has two key parts: the ionization chamber, and a source of radiation. This source of radiation consists of a very minute concentration of Americium-241, which produce alpha particles. The Ionization Chamber contains two plates: one plate is negatively charged, and the other is positively charged. The alpha particles created by the Americium-241 move at very high speeds and bump into oxygen and nitrogen molecules within the ionization chamber. The force exerted by this collision causes electrons to fall off from each molecule, creating an ion. The now positively charged ions are attracted to the negatively charged plate while the electrons attracted to the positively charged plate. This attraction causes a consistent electrical current within the chamber itself. When smoke travels into the chamber, its particles attach to the ionized molecules to neutralize them and pull them away from the plate. This disrupts the electrical current and triggers the alarm.


Figure 1. An Ionization Smoke Detector[6]

Image:Smokedetectors.GIF

The main reaction that takes place within a Smoke detector is a chemical reaction between Americium and alpha particles. This is shown in equation form below:[7] Image:Smokedetectors3.GIF

Many questions about public safety have arisen due to the radiation content within these detectors; however, there is not enough alpha radiation within the chambers to cause any serious damage. In fact, the content within the chambers of this type of radiation is so weak that the surrounding air particles are able to smother any toxicity secreted. Still one is always cautioned to never directly inhale this substance.


Photoelectric Smoke Detectors 2345 The Photoelectric Smoke Detector is less common and more expensive than the Ionization Smoke Detector. It consists of a chamber in the shape of a capital letter "T." The horizontal portion of this chamber consists of a light source called a Light Emitting Code. This beam of light travels across this horizontal bar, but never sends light vertically. At the base of the "T," is a photocell, which senses light from darkness. When smoke enters this "T" chamber, light from the beam is broken up and is scattered away from its straight beam. When a certain level of light reaches the photocell, which is usually in darkness, the alarm is initiated.


"light emitting diode" - "no light reaches photoelectric cell" "deflected light rays activate photocell"

Figure 2. An Photoelectric Smoke Detector[8] Image:Smokedetectors2.GIF

Technology to a Detectors Advantage [9] [10] The search for new and better ways to prevent fires today is still well underway. New sensors, analog sensors, are being installed in today’s new detectors. These sensors send information to a control computer which can detect whether the detectors themselves are too dirty to function or if they are too sensitive for the area in which they are placed. These new sensors also create an environment in which false alarms are rarely happen.


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Footnotes

  1. ^ “Smoke Detector”, in Wilipedia, http://en.wikipedia.org/wiki/smoke_detector, Accessed in September 25, 2005.
  2. ^ chemistry.about.com/cs/howthingswork/a/aa071401a.htm, Accessed in September25, 2005.
  3. ^ “How Smoke Detectors Work?”, in Howstuffworks, http://home.howstuffworks.com/smoke.htm, Accessed in September 25, 2005.
  4. ^ “How Do You Write the Chemical Reaction That Takes Place in a Smoke Detector?, in Howstuffworks, http://home.howstuffworks.com/smoke.htm, Accessed in September 25, 2005.
  5. ^ “How Do Smoke Alarms Work?: Ionization Technology”, in U.S. Environmental Protection Agency, http://www.epa.gov/radiation/sources/smoke_ion.htm, Accessed in September 25, 2005.
  6. ^ “How Do Smoke Alarms Work?: Ionization Technology”, in U.S. Environmental Protection Agency, http://www.epa.gov/radiation/sources/smoke_alarm.htm, Accessed in September 25, 2005.
  7. ^ “How Do You Write the Chemical Reaction That Takes Place in a Smoke Detector?, in Howstuffworks, http://home.howstuffworks.com/smoke.htm, Accessed in September 25, 2005.
  8. ^ 
  9. ^ “How Do Smoke Alarms Work?: Photoelectric Technology”, in U.S. Environmental Protection Agency, http://www.epa.gov/radiation/sources/smoke_alarm.htm, Accessed in September 25, 2005.
  10. ^ “Smoke Detection”, in Firenet, http://www.me.utexas.edu/~ezekoye/rsch.dir/firesite/smoke_detector, Accessed in September 25, 2005.
  11. ^ “Risk Analysis of Residential Fire Detector Performance”, Journal of Applied Fire Science, Volume 6, Number 2, June 1997.
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