Smoke Detectors

Jimmy H. Beard, M.S.E.E., P.E., CFEI

Smoke Detectors –
Critical elements for fire safety

Smoke detectors have been found to reduce the risk of dying in a home structure fire by 54% (Ahrens, 2019).  Clearly, smoke alarms are a critical element for fire safety.

Common Types of Smoke Detectors

There are two common types of residential smoke detectors: photoelectric detectors and ionization detectors (New World Encyclopedia, 2015).

Photoelectric Smoke Detectors

Photoelectric smoke detectors utilize an LED transmitter and an optical detector to detect the presence of smoke.  This technique works because smoke particles in the air tend to scatter any light that contacts them.  In a photoelectric smoke detector, an LED light source is used to illuminate a detection chamber while an optical detector senses the amount of light that reaches the detector.  If smoke enters the chamber, the amount of light reaching the optical detector changes.  This change in light indicates that smoke has entered the detection chamber, which results in the detector unit sounding an alarm.

Ionization Smoke Detectors

Smoke detectors that utilize an ionization process work differently than photoelectric detectors.  Ionization detectors utilize small amounts of a radioactive element as a radiation source.  The radiation enters the detection (ionization) chamber, which helps to ionize the air and causes an electrical current to flow between two electrically charged plates.  When smoke enters the detection chamber, it interrupts the ionization process, which reduces the amount of current flowing between the plates (National Fire Protection Association, 2014).  The change in current flow indicates that smoke has entered the chamber, and the smoke detector sounds the alarm.

Photoelectric smoke detectors generally work best with smoldering fires, while ionization smoke detectors work best with flaming fires (National Fire Protection Association, 2014).  The National Fire Protection Association recommends having both types of smoke detectors in each home because it is impossible to predict which type of fire may develop.

Testing and Utilization

The National Fire Protection Association has developed standards for the placement and testing of smoke detectors.  Smoke detectors are generally required in all bedrooms and on every level of a home, including the basement (National Fire Protection Association, 2010).  Testing should be performed in accordance with the manufacturer’s instructions and recommended schedule.  In addition, most smoke detectors have a specific lifespan which will also be listed in the manufacturer's instructions and on the smoke detector as well.

_________________________________________

References

Ahrens, M. (2019). Smoke Alarms in U.S. Home Fires. National Fire Protection Association.
National Fire Protection Association. (2010). National Fire Alarm and Signaling Code. NFPA.
National Fire Protection Association. (2014, February 26). Ionization vs photoelectric. Retrieved from www.nfpa.org: https://www.nfpa.org/Public-Education/By-topic/Smoke-alarms/Ionization-vs-photoelectric
New World Encyclopedia. (2015). Smoke Detector. Retrieved from www.newworldencyclopedia.org: http://www.newworldencyclopedia.org/entry/Smoke_detector