How Do Smoke Detectors Work?

Smoke Detector

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Smoke detectors do their job above our heads efficiently and, for the most part, quietly. Sometimes, smoke detectors make themselves well-known when a frying pan gives off smoke or when they chirp to remind us that they need a new battery. Often called a smoke detector or a fire alarm—its official name is smoke alarm—this tiny device, packed with electronics and even a bit of radioactive material, has been responsible for cutting the number of fire deaths in half since its wide-spread adoption.

Smoke Detector Operations: Ionization and Photoelectric

Smoke detectors fall into either of two types: ionization and photoelectric. Ionization smoke detectors predate photoelectric alarms yet are considered effective at detecting flaming fires. Photoelectric smoke detectors are good at detecting fires in their early stages.

Ionization Smoke Detectors

Ionization smoke alarms have been used for years to detect advanced fires. An ionization smoke detector is roughly similar to another type of safety alarm: a window sensor burglar alarm. A sensor on the window and a sensor on the window frame touch, completing an electrical circuit. When that circuit is broken, the alarm sounds.

Smoke detectors work with the same circuit completion/break concept, though with a difference: There is no physical contact between the two sensors. With the ionization smoke detector:

  1. Wires extend from both the positive and negative ends of a battery in the detector.
  2. The wires attach to separate electrodes.
  3. The electrodes complete a circuit, but not physically. Instead, Americium-241 transforms the air molecules between the electrodes into positive and negative ions.
  4. The charged ions between the two plates complete the circuit.
  5. During a fire, smoke enters the smoke alarm through holes or slits in the housing.
  6. The positive and negative ions seek the smoke, not the plates.
  7. The circuit is broken and the alarm sounds.

Photoelectric Smoke Detectors

Photoelectric smoke alarms, sometimes called optical smoke alarms, sound when an LED light within the alarm chamber is broken. Sometimes referred to as smoldering alarms, photoelectric smoke detectors are capable of detecting a fire early in its stage before it breaks into a full-fledged fire.

Photoelectric smoke detectors' operation can be equated with window or door photoelectric alarms. With window and door alarms, an invisible beam of light passes from one sensor to another sensor. They are not physically connected. When the beam is broken, the alarm sounds. With the photoelectric smoke detector:

  1. An LED casts light in a straight line across the inner chamber.
  2. A photoelectric sensor at the other end detects the light, telling the system that the circuit is complete.
  3. Smoke enters the housing.
  4. The light is interrupted by the smoke.
  5. The smoke causes some of the light to be redirected to a different sensor.
  6. When this other sensor detects light, it sounds the alarm.

How to Buy and Install a Smoke Detector

Whether you are using a smoke detector for the first time or have done so many times already, learning the basics of smoke detector usage will keep you and your home safe in the event of a fire.

Best Type of Smoke Detector to Buy

The National Fire Protection Association recommends that you install both ionization and photoelectric smoke detectors in your home for maximum protection. Some alarms combine both ionization and photoelectric detection, and many even bundle in a carbon monoxide detector.

Many battery-powered smoke detectors come with 10-year lithium-ion batteries that parallel the life of the smoke detector; when the battery expires, it is time to purchase a new detector. Hardwired smoke detectors run off of household 120V current and have battery backups.

One benefit of hardwired smoke detectors is that, when they are interconnected, all alarms will sound at the same time. Newer homes may be required by code to have interconnected detectors.

Correct Way to Install a Smoke Detector

By code, at a minimum, you should install one smoke detector in every sleeping room. You should also install one smoke detector outside of every sleeping room and on each level of the house. Install the detector at least 10 feet away from the cooking area to minimize false alarms.

Install the smoke detector on the ceiling or high on the wall, no more than 12 inches away from the ceiling. Keep away from windows, doors, or ducts. On vaulted ceilings, do not install the detector closer than 3 feet away from the peak of the ceiling.

Never paint a smoke detector or add stickers, glue, or any type of material to it.