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Smoke Detectors Basics
Smoke detectors are designed to provide early warning for a fire involving ordinary combustible materials that is expected to progress through distinct incipient and/or smoldering stages. The type, volume, and density of smoke produced during the fire development process will vary greatly depending on the fuels involved and the amount of oxygen available. Typically, the greatest volume of visible smoke is produced during the ignition (incipient) stage and the smoldering stage.
Even with ready access to low cost and, at times, free smoke detectors, exposure to fire and smoke is responsible for thousands of deaths and billions of dollars in property damage each year. One challenge for many property owners is determining what type (i.e., design) smoke detector to purchase and where to install the units. Some detectors are more effective (respond quicker) to flaming combustion, while others respond better to smoldering combustion. Likewise, some detectors are more prone to false activation from environmental factors than others.
This report provides an overview of the primary smoke detector types and discusses research into the effectiveness of each of those types.
Smoke Detector Basic Types
The two most commonly used smoke detectors are the ionization and photoelectric types.
Ionization Detectors Smoke Detectors
The ionization smoke detector reacts to both visible and invisible products of combustion. This spot-type detector contains a small radiation source that produces electrically charged air molecules called ions. The presence of these ions allows a small electric current to flow in a chamber. When smoke particles enter the chamber, they attach themselves to the ions, reducing the flow of electric current. The change in the current sets off the alarm.
Many ionization detectors are of the multiple-chamber type. Probably the most common is the dual chamber ionization detector, which employs two sources of radiation - one in an essentially sealed chamber and one open to the atmosphere. The open chamber serves for sensing smoke particles, while the closed ionization chamber monitors ambient conditions and compensates for the effect of barometric pressure, temperature, and relative humidity on the ionization rate. This construction accepts a much wider range of pressure, temperature, and humidity changes without giving false alarms.
Ionization detectors are not suitable for use in applications where high ambient radioactivity levels are to be expected. High ambient radiation reduces the detector's sensitivity. Ionization detectors have been known to react to non-fire-generated particles of combustion and the presence of ozone, ammonia, or insects. Single chamber ionization detectors installed in higher altitudes usually require a modification in sensitivity during installation.
Photoelectric Smoke Detectors
There are two types of photoelectric detectors - beam and light scattering, both of which consist of a light source, a collimating lens system, and a photosensitive cell. Aerosols generated during the combustion process affect the propagation of light as they pass through the air. The combination of the aerosol and air mixture results in two conditions that can be used to detect the presence of a fire. These are: (1) the attenuation of the light intensity integrated over a beam path length, and (2) scattering of the light (i.e., Tyndall effect) at various angles to the beam path.
Beam-type smoke detectors.The beam-type photoelectric detector is a photoelectric smoke detector that works on the obscuration principle. A light beam is directed at a photocell. When no smoke interferes with this beam, the receiver accepts the beam at a specified voltage level, but when smoke interferes with the beam, the infrared light reaching the receiver drops below the predetermined sensitivity level of the receiver, initiating a signal. Beam-type photoelectric detectors can be sensitive to voltage variations, dirt on the lens or mirrors, building vibration, and insects.
Light scattering smoke detectors (Tyndall Principle).The Tyndall-principle photoelectric detector is of the spot-type and detects smoke by sensing the light reflected by smoke particles. The smoke particles enter the detector and reflect or scatter light from a small lamp, or LED, in the device. Some of that reflected light strikes a photocell that produces an electrical current. As the number of particles increases and more light strikes the photocell, the intensity of the electrical current increases. When the smoke particles are dense enough to reflect a predetermined amount of light, the detector’s circuit actuates the alarm.
See Fire Protection Report FP-21-01, Fire Detectors, for information on of other types of smoke detectors with specific applications.
Smoke Detector Effectiveness Studies
A number of groups have published reports and data related to the effectiveness of smoke detectors, including the National Fire Protection Association (NFPA), National Institute of Standards and Technology (NIST), and Texas A&M. The opinions of these groups often differ greatly, and each group’s rationale should be carefully evaluated when selecting detectors for a specific hazard. Some of the findings include:
National Fire Protection Association (NFPA)
The NFPA Technical Committee on Single- and Multiple-Station Alarms and Household Fire Alarm Systems released a Task Group report on Smoke Detection in 2008. That report “Minimum Performance Requirements for Smoke Alarm Detection Technology” reviewed data from various sources, including the 2007 NIST report, “Performance of Home Smoke Alarms Analysis of the Response of Several Available Technologies in Residential Fire Settings” (NIST Report TN 1455-1), to evaluate the effectiveness of various detector types, their installation locations, and the probability of nuisance alarms.
The 2008 NFPA report provides a detailed review of the topic and in closing offers a number of recommendations and observations, including the following: “Based on the Task Group on Smoke Detection Technology’s review of NIST TN 1455-1, using the escape scenarios considered in the NIST report, the Task Group concludes that smoke alarms using either ionization or photoelectric smoke detection technologies, installed per NFPA 72-2007, are generally providing acceptable response to smoldering fires. Additional study is needed regarding photoelectric alarm response in flaming scenarios.”
There was some push back from various groups following the release of the above report; thus, the Technical Committee commissioned a second task group to re-review the information. That Task group released the “Task Group on Smoke Detection Follow-Up Report” in July of 2009. In short, that group also found that: “The rate at which a particular type of detector did not provide adequate warning was similar for ionization and photoelectric detectors regardless of whether the Direct Escape or Indirect Escape was used. As expected, ionization detectors provided earlier warning to flaming fires, while photoelectric detectors provided earlier warning to smoldering fires.”
National Institute of Standards and Technology
Much of the research into detector response times was developed by the NIST research and is included in the NIST report “Performance of Home Smoke Alarms Analysis of the Response of Several Available Technologies in Residential Fire Settings,” which was originally published in 2004, then recommissioned in 2008. Both reports can be downloaded for free from the NIST Smoke Detector Study site.
The NIST site provides a summary of the findingh3>s, including: “Smoke alarms of either the ionization type or the photoelectric type consistently provided time for occupants to escape from most residential fires, although in some cases the escape time provided can be short. Consistent with prior findings, ionization-type alarms provided somewhat better response to flaming fires than photoelectric alarms, and photoelectric alarms provide (often) considerably faster response to smoldering fires than ionization-type alarms.”
A 1995 study by Texas A&M, “Risk Analysis of Residential Fire Detectors Performance,” used data from live fire testing. The authors developed a fault tree analysis for detector reliably predicative modeling. The closing statement in that report seems to mimic the latter reports, as follows: “Certain types of fire detectors are more reliable for different fires, therefore, recommendations as to the type and location of the fire detector should be including the type of fire ignition that would likely occur and the most reliable detectors that can be installed in that location.”
Smoke Detector Use
The 2011 NFPA Report, “Smoke Alarms in US Home Fires,” indicates that 385 of the home fire deaths occurred where there were no detectors. Further, the report reveals that 50% of the detectors that failed to operate were missing batteries, 23% had dead batteries, and 7% had the power disconnected. In short, the single greatest cause of detector failure does not have to do with the type of detector, but with a lack of power to the detector.
The NFPA smoke detector webpage offers the following advice: “For each type of smoke alarm, the advantage it provides may be critical to life safety in some fire situations. Home fatal fires, day or night, include a large number of smoldering fires and a large number of flaming fires. You cannot predict the type of fire you may have in your home or when it will occur. Any smoke alarm technology, to be acceptable, must perform acceptably for both types of fires in order to provide early warning of fire at all times of the day or night and whether you are asleep or awake.”
In summary, the importance of selecting the appropriate smoke detector lies in ensuring that the smoke detector is designed for the expected fire scenario and that they are installed and maintained in accordance with the manufacturer’s requirements.
In general, photoelectric detectors respond better to a smoldering fire and ionization-type detectors to a flaming fire. To address the differences in detector types, nearly all of the major manufacturers offer dual (ion and photo) sensor detectors. Additionally, most risk control specialists recommend that carbon monoxide detectors also be installed to ensure that all possible combustion products are detected.
For more information on loss control and managing business risks, check out the American Family Insurance Loss Control Resource Center.
1. Engineering and Safety Service. Fire Detectors. FP-21-01. Jersey City, NJ: ISO Services, Inc., 2011.
2. International Codes Council (ICC). International Fire Code. 2012 ed. Falls Church, VA: ICC, 2012.
3. National Fire Protection Association (NFPA). Fire Protection Handbook. 20th ed. Quincy, MA: NFPA, 2008.
4. ---. Minimum Performance Requirements for Smoke Alarm Detection Technology, Task Group Report. Quincy, MA: NFPA, 2008.
5. ---. Smoke Alarms in US Home Fires. Quincy, MA: NFPA, 2011.
6. ---. Task Group on Smoke Detection Follow-Up Report. Quincy, MA: NFPA, 2009.
7. National Institute of Standards and Technology (NIST). Performance of Home Smoke Alarms Analysis of the Response of Several Available Technologies in Residential Fire Settings. NIST TN 1455-1. Washington, DC: NIST, 2007.
8. Texas A&M University. Risk Analysis of Residential Fire Detectors Performance. College Station, TX: Texas A&M, 1995.
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The information contained in this publication was obtained from sources believed to be reliable. ISO Services, Inc., its companies and employees make no guarantee of results and assume no liability in connection with either the information herein contained or the safety suggestions herein made. Moreover, it cannot be assumed that every acceptable safety procedure is contained herein or that abnormal or unusual circumstances may not warrant or require further or additional procedure.