Smoke Detectors

1
Smoke Detectors
Jr. Lab II, sp01

• Brief history
• 1930s, Swiss scientist Walter Jaeger tried to
  invent poison gas detector thinking the particles
  would interact with an ion gas causing a
  detectable change in current. The idea failed.
  Frustrated, he lit a cigarette and voila, smoke
  detector. 1

2
Two Types

• Photoelectric
• Smoke particles encounter light beam
• Beam scatters to a photocell creating a current
• Better for larger particles (smoldering)
• Ionization
• Particles attach to ionized particles creating a
  detectable change in current
• Better for smaller particles (flaming)

3
Ionization Smoke Meters
4
Ionization Source

• Americium-241,243?
• Emits ? particle which ionizes the air molecules

Isotope Mass Half-life (years) Mode of decay
241Am 241.05682 432.2 ? to 237Np
243Am 243.061375 7370 ? to 239Np
2
5
References

• 1 Bunker, Merton Jr., Scientific American,
  Smoke Detectors, Apr. 1997. www.scientificameric
  an.com/0497issue/0497working.html
• 2 www.webelements.com
• chemistry.about.com/science/chemistry/library/week
  ly/aa020700a.htm
• Geoffrey, www.madsci.org/posts/archives/feb98/8880
  79238.Ph.r.html www.britannica.com/search?querya
  mericium