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The Best Choice for the Application
In the sealed, high-oxygen environment of a space shuttle or space station, a fire can have tragic and disastrous results. The flammable chemicals, experimental materials, and often dangerous machinery used in spaceflight only increase risks. Therefore, manned space missions must have the most advanced and effective fire prevention, detection, and control methods.
Choosing smoke detectors for space missions is not the same as choosing a smoke detector for the home, because fire does not behave the same way in zero-gravity as it does on the ground. NASA research suggests that in zero-gravity environments, smoke is more likely to form large particles.
Because they are better-suited to detecting this kind of large-particle smoke, photoelectric smoke detectors may be more effective for space applications than the cheaper and more common ionization detectors fournd in many homes. To understand what's useful about photoelectric smoke detectors in spaceflight, it's important to know how photoelectric smoke detectors work.
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The Basics of Smoke Detection
All smoke detectors are based on the way smoke particles can affect an electrical circuit. When smoke particles enter a detector, they either interrupt or create electrical current across a circuit. (This current-dependent property is one reason why low batteries make a fire detector malfunction. It's important to regularly check smoke detector batteries to improve reliability and reduce the number of false alarms.)
There are two main classes of smoke detectors: ionization detectors and photoelectric detectors. The electrical current in an ionization detector is produced by the effect of alpha particles released by a tiny chip of radioactive americium-241 inside the detector. The current in a photoelectric detector is produced by a photocell and a small light source. Which kind of smoke detector is preferable depends on what type of smoke is most likely to be present.
Not all smoke has the same properties. Depending on the composition of the material that's burning, the temperature of the fire, and atmospheric conditions, smoke particles may range widely in size and chemical makeup. Ionization type smoke detectors are best at picking up smoke consisting of fine particles. This type of small-particle smoke is usually produced by hot fires that produce large flames. Photoelectric detectors are better at picking up larger-particle smoke, such as that produced by smoldering fires. They are particularly good at detecting electrical fires.
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Photoelectric Smoke Detectors Explained
The photoelectric smoke detector was developed by Duane Pearsall and Stanley Peterson in 1965, but did not go into wide use until 1975. In a photoelectric detector, current is generated when the light from a small light source (often an LED) hits a photosensitive unit (photocell) in the detector. When light hits the photocathode, it generates electrons that are, in turn, picked up by an anode. This generates current. (See an electrical diagram of a photoelectric smoke detector here.)
There are two ways a photoelectric detector can be set up: a straight line, in which a decrease in current triggers the alarm, and a T shape, in which the creation of current triggers the alarm. In the first type of photoelectric detector, the alarm sounds when smoke gets between the light and the photocell. In the more common T-shaped detector, the photocell is at the base of the T, and light travels along the top of the T. When smoke enters the detector, the light reflecting off the smoke reaches the photocell, triggering an alarm.
In photo-electric detectors, first current is generated by the interaction of light with a photocell. When the current across any of these circuits changes, it triggers an audible and/or visible alarm.
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Benefits of Photoelectric Detectors in Manned Spaceflight
Smoke and flames behave differently in the zero-gravity environments of space vessels than they do on Earth. Flames in zero gravity do not take the orange-tipped teardrop shape they ordinarily would. Instead, they form small, bluish balls. Smoke also forms differently. It collects into particles much more slowly, but when the particles do form they are larger. Larger-particle fires make photoelectric detectors a good choice for space missions. However, research and development continues in order to make sure that astronauts are protected by the best fire detection and protection equipment available.
Since many of the materials used in spacecraft are experimental, NASA and other space programs conduct extensive tests to figure out what kind of smoke they will produce when they burn. After a 1996 study showed that smoke particles in space are larger than on Earth, NASA conducted two experiments to determine the properties of smoke and the needs for detectors in space. DAFT, (Dust and Aerosol Measurement Feasibility Test), and the follow-on SAME (Smoke and Aerosol Measurement Experiment).
The results of these projects will not only have repercussions for smoke detection and fire prevention techniques in spaceflight, but back on Earth as well. Just as ionization detectors first showed their usefulness in space projects, learning more about photoelectric smoke detectors in spaceflight may provide new perspectives on smoke detection in homes, offices, and public buildings.
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- Photoelectric Detector: http://en.wikipedia.org/wiki/File:OpticalSD.jpg
- Candle Flame Comparison: http://science.nasa.gov/science-news/science-at-nasa/2002/21aug_flameballs/
Jack Rella. "History of the Photoelectric Smoke Detector." eHow.com. Accessed November 24, 2010. http://www.ehow.com/about_5241935_history-photoelectric-smoke-detector.html
Jan Wittry. "Smoking Out Space Fires: NASA Study Helps Prevent Fires in Space." September 29, 2006. http://www.nasa.gov/mission_pages/station/research/space_smoke.html
"Fact sheet: Dust and Aerosol Measurement Feasibility Test (DAFT)." October 19, 2010. ISS Program Scientist's Office. http://www.nasa.gov/mission_pages/station/research/experiments/DAFT.html"Fact sheet: Smoke and Aerosol Measurement Experiment (SAME)." October 19, 2010. ISS Program Scientist's Office. http://www.nasa.gov/mission_pages/station/research/experiments/SAME.html