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UFO Forum
other then ufo subjects
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<table border="0" cellpadding="3" cellspacing="1" width="100%" class="forumline"> <tr> <th class="thCornerL" width="22%" height="26">Author</th> <th class="thCornerR">Message</th> </tr> <tr> <td width="22%" align="left" valign="top" class="row1"><span class="name"><a name=""></a><b>op86oppa</b></span></td> <td class="row1" height="28" valign="top"><table width="100%" border="0" cellspacing="0" cellpadding="0"> <tr> <td width="100%"><img src="http://picsrv.fora.pl/Boyz/images/icon_minipost.gif" width="12" height="9" alt="Post" title="Post" border="0" /><span class="postdetails">Posted: Fri 6:58, 11 Mar 2011<span class="gen"> </span> Post subject: HowStuffWorks How Smoke Detectors Work</span></td> </tr> <tr> <td colspan="2"><hr /></td> </tr> <tr> <td colspan="2"><span class="postbody">by Marshall Brain <br /> Please copy/paste the following text to properly cite this HowStuffWorks article: <br /> Photoelectric Detectors <br />Did You Know? <br />Fire sensors were developed from technology NASA uses to enhance photos of distant planets. Learn more about cool NASA innovations in this interactive animation from Discovery Channel. <br />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 photodetector that can "see" the light. <br />When you cross the beam of light, you block it. The photodetector 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: <br />It's a pretty big smoke detector. <br />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. <br />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 this: <br /> <br /> <br /> <br /> <br />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,<a href="http://www.sail-trade.com/" target="_blank" class="postlink">Pump</a>, however, the smoke particles scatter the light and some amount of light hits the sensor: <br /> <br /> <br /> <br /> <br />The sensor then sets off the horn in the smoke detector. <br />Photoelectric detectors are better at sensing smoky fires, such as a smoldering mattress.</span></td> </tr> </table></td> </tr> <tr> <td colspan="2" height="1" class="spaceRow"><img src="http://picsrv.fora.pl/Boyz/images/spacer.gif" alt="" width="1" height="1" /></td> </tr> </table>
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