Smoke Alarm Roundup: The Difference Between Ionization and Photoelectric Smoke Detectors
They’re the little disc-shaped devices on our walls and ceilings that let us rest easy, knowing that we’ll be alerted in time should fire or smoke put us in danger. But even though they tend to look similar and are all built with the same purpose in mind, smoke detectors can work in several different ways. Different smoke detection technologies are suited to different types of fires and smoke densities, and it’s best to be aware of the variations so you can make the smartest choices possible when protecting your home and family with smoke alarms.
The vast majority of residential smoke alarms fall into one of two different categories: ionization and photoelectric. Let’s take a look at the inner workings of each type to learn exactly how they’re triggered when smoke is in the air, and which types of smoke and fire they’re best at detecting.
Ionization: Equipped with ventilated ionization chambers that consist of two battery-charged metal plates (one positive, the other negative) and a trace amount of the element americium-241 (which acts as a source of radiation), ionization smoke alarms work by ionizing air’s nitrogen and oxygen atoms to detect harmful smoke in indoor environments.
Ionization takes place when the americium-241 releases radiation in the form of alpha particles, which separate singleelectrons from the oxygen and nitrogen atoms within the chamber. These separated electrons – known as “free electrons” – have a negative charge, which draws them toward the positively charged plate. In the meantime, the positively charged, electron-deficient nitrogen and oxygen ions are drawn to the chamber’s negatively charged plate.
As the electrons and ions move toward their respective plates, the smoke detector’s electronics are aware of the slight electrical current that their movement produces. When smoke particles enter an ionization chamber, they attach themselves to and absorb the ions, neutralizing them and disrupting the electrical current. The smoke detector senses the change in current, and an alarm is sounded.
Contrary to the images of danger that radioactive elements and radiation can bring to mind, ionization smoke alarms are very safe; the amount of americium-241 they contain is extremely minute (less than one milligram), and the element would have to be inhaled or ingested to do any harm. The radiation produced by americium-241 is made up almost exclusively of alpha rays, which are unable to penetrate a sheet of paper, much less the smoke detectors’ plastic casing.
A common fire safety measure in many homes, ionization smoke alarms are generally very cost effective, and are particularly good at detecting very small amounts of smoke, as would occur with a quick-flaring fire. We recommend Kidde’s Micro Ionization and Wireless Interconnectable Ionization smoke alarms.
Photoelectric: Sometimes referred to as optical smoke detectors, photoelectric smoke alarms use light-sensing technology to identify the presence ofsmoke. A photoelectric smoke detection system is typically made up of three main elements: a light source (either an incandescent bulb or an LED), a lens (which focuses light from the source we just mentioned into a concentrated beam), and a photo-sensor, which is usually placed at a 90° angle to the light beam.
As long as there’s no smoke present in the air, the beam shines straight ahead, parallel to the sensor’s surface. However, as soon as smoke particles drift into the detector, they disrupt the beam, scattering light in multiple directions – including toward the photo-sensor, which automatically triggers an alarm as soon as it detects the stray light waves.
Photoelectric smoke alarms are ideal for detecting slower-burning fires that produce a lot of smoke, and they’re less likely than ionization detectors to trigger a false alarm from kitchen and bathroom steam.
Combination Ionization/Photoelectric: Simply put, Ionization/Photoelectric smoke alarms combine the benefits of both detection technologies in a single unit to provide the most complete protection possible. Combination smoke detectors eliminate the guesswork involved in choosing the best type of smoke detector for each area of your home because they’ve got it all covered: ionization detection for sudden, low smoke flame-ups, and photoelectric sensors for smoky, slow-burning fires. Our top pick? Kidde’s Ionization/Photoelectric Smoke Alarm with 9V battery backup.
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