fire detector

Smoke detector

A smoke detector is a device that detects smoke and issues an alarm. Smoke detectors alert people within hearing range; some also interface with a security system or notify emergency services.

A household smoke detector will typically be mounted in a disk-shaped plastic enclosure about in diameter and thick, but the shape can vary by manufacturer. Most smoke detectors work either by optical detection (photoelectric) or by physical process (ionization), but some of them use both detection methods to increase sensitivity to smoke. Smoke detectors are usually powered by battery; some are connected directly to power mains — often have a battery as a power supply backup in case the mains power fails. In either case, it is usually necessary to replace the batteries once a year to ensure appropriate protection if alkaline or carbon-zinc batteries are used.

The smoke detector is one of three items of fire safety apparatus that are recommended for homes and that can be installed by the consumer. The second is a fire extinguisher, and the third is a fire blanket, a section of fire retardant cloth, normally on each side, that can be applied to a small fire to smother it.


The first automatic electric fire alarm was invented in 1890 by Francis Robbins Upton and Fernando J. Dibble, (US patent no. 436,961). Upton was an associate of Thomas Edison, although there is no evidence that Edison contributed to this project.

It is a widely spread misinformation that the first electric fire alarm was patented in 1902 by George Andrew Darby of Birmingham, England, as this falls 10 years after the real first fire alarm was invented. There is a fun (and seemingly unconfirmable) anecdote regarding Darby's device. Apparently it indicated an increased temperature by closing an electrical circuit to sound an alarm if the temperature rose above the safe limit. The contact was made by bridging a gap with a conductor, or allowing one plate to fall on another. The connection of the two plates was caused simply by a block of butter which melted as the temperature rose.

In the late 1930s the Swiss physicist Walter Jaeger tried to invent a sensor for poison gas. He expected that gas entering the sensor would bind to ionized air molecules and thereby alter an electric current in a circuit in the instrument. His device failed: small concentrations of gas had no effect on the sensor's conductivity. Frustrated, Jaeger lit a cigarette--and was soon surprised to notice that a meter on the instrument had registered a drop in current. Smoke particles had apparently done what poison gas could not. Jaeger's experiment was one of the advances that paved the way for the modern smoke detector.

It was 30 years, however, before progress in nuclear chemistry and solid-state electronics made a cheap sensor possible. While home smoke detectors were available during most of the 1960s, the price of these devices was rather high. Before that, alarms were so expensive that only major businesses and theaters could afford them. The first truly affordable home smoke detectors were invented by Duane D. Pearsall in 1967, featuring individual battery powered units that could be easily installed and replaced. The first commercial smoke detectors came to market in 1969. Today they are installed in 93 percent of U.S. homes and 85% of UK homes. However it is estimated that any given time over 30% of these alarms don't work, as users forget to replace, or remove the batteries. Although commonly attributed to NASA, smoke detectors were not invented as a result of the space program, though a variant with adjustable sensitivity was developed for Skylab.


Virtually all modern smoke alarm units come equipped with a "test" button. Alternatively, artificial smoke can be purchased, which also tests the detection mechanism itself. One simple way to test a smoke alarm is to light and extinguish a match, then wave it beneath the detector. The smoke detector should be sensitive enough to trip its alarm if a small amount of smoke enters it.


An optical detector is a light sensor. When used as a smoke detector, it includes a light source (incandescent bulb or infrared LED), a lens to collimate the light into a beam, and a photodiode or other photoelectric sensor at an angle to the beam as a light detector. In the absence of smoke, the light passes in front of the detector in a straight line. When smoke enters the optical chamber across the path of the light beam, some light is scattered by the smoke particles, directing it at the sensor and thus triggering the alarm.

Also seen in large rooms, such as a gymnasium or an auditorium, are devices to detect a projected beam. A unit on the wall sends out a beam, which is either received by a receiver or reflected back via a mirror. When the beam is less visible to the "eye" of the sensor, it sends an alarm signal to the fire alarm control panel.

Optical smoke detectors are quick in detecting particulate (smoke) generated by smoldering (cool, smokey)fires. Many independent tests indicate that Optical smoke detectors typically detect particulate (smoke)from hot, flaming fires approximately 30 seconds later than ionization smoke alarms.

They are less sensitive to false alarms from steam or cooking fumes generated in kitchen or steam from the bathroom than are ionization smoke alarms. For the aforementioned reason, they are often referred to as 'toast proof' smoke alarms.


This type of detector is cheaper than the optical detector; however, it is sometimes rejected for environmental reasons and because it is more prone to false alarms than photoelectric smoke detectors. It can detect particles of smoke that are too small to be visible. It includes less than a milligram of radioactive americium 241 (241Am). The radiation passes through an ionization chamber, an air-filled space between two electrodes, and permits a small, constant current between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and interrupts this current, setting off the alarm. 241Am, an alpha emitter, has a half-life of 432.2 years. This means that it does not have to be replaced during the useful life of the detector, and also makes it safer for people at home, as it is less radioactive. Alpha radiation, as opposed to beta and gamma, is used for two additional reasons: Alpha particles have high ionization, so sufficient air particles will be ionized for the current to exist, and they have low penetrative power, meaning they will be stopped by the plastic of the smoke detector and/or the air, reducing the risk of harm to people.


An air-sampling smoke detector, sometimes called a VESDA® system, is capable of detecting microscopic particles of smoke. Most air-sampling detectors are aspirating smoke detectors, which work by actively drawing air through a network of small-bore pipes laid out above or below a ceiling in parallel runs covering a protected area. Small holes drilled into each pipe form a matrix of holes (sampling points), providing an even distribution across the pipe network. Air samples are drawn past a sensitive optical device, often a solid-state laser, tuned to detect the extremely small particles of combustion. Air-sampling detectors may be used to trigger an automatic fire response, such as a gaseous fire suppression system, in high-value or mission-critical areas, such as archives or computer server rooms.

Air-sampling smoke detection systems are classed as High Sensitivity Smoke Detectors (HSSDs) and provide multiple levels of alarm threshold, such as Alert, Action, Fire 1 and Fire 2. Thresholds may be set at levels across a very wide range of smoke levels. This allows early notification of a developing fire, allowing intervention before a fire has developed beyond the smoldering stage, thereby increasing the time available for evacuation and possibly enabling emergency firefighters to arrive earlier and minimize fire damage. Fire thresholds can be set to notify local or municipal emergency responders and ultimately to discharge fire suppression systems.

CO₂ detection

CO₂ sensors detect carbon dioxide, the most dangerous and quickly spreading gas in the smoke of a fire.

Performance differences

Optical or 'toast proof' smoke detectors are generally quicker in detecting particulate (smoke) generated by smoldering (cool, smokey) fires. Ionization smoke detectors are generally quicker in detecting particulate (smoke) generated by flaming (hot) fires. The Barre City Vermont Fire Department has performed testing and has video and data available on their website:

Many independent tests indicate that optical smoke detectors typically detect particulate (smoke) from hot, flaming fires approximately 30 seconds later than ionization smoke alarms, and that ionization smoke alarms detect particulate from smoldering (cool, smokey) fires 30 minutes later than photoelectric smoke detectors.

Tests have also been performed in which particulate generated by a smoldering fire has reduced the visibility within the test room to almost zero, and where the ionization smoke detector never sounded.

The NFPA (National Fire Protection Association) and most major smoke alarm manufacturers now recommend that both photoelectric and ionization smoke detectors be utilized.

A jury in the United States District Court for the Northern District of New York decided in 2006 that First Alert and its parent company, BRK Brands, was liable for millions of dollars in damages because the ionization technology in the smoke alarm in the Hackert's house was defective, failing to detect the slow-burning fire and choking smoke that filled the home as the family slept ( )


A smoke detector cannot detect carbon monoxide to warn of carbon monoxide poisoning, unless it has an integrated carbon monoxide detector. These are also available as a separate detector. Most companies that manufacture smoke detectors also manufacture carbon monoxide detectors.

Alarms and alerts

A second function of the detector is to alert persons at risk. Several methods are used and documented in industry specifications published by Underwriters Laboratories. Alerting methods include:

  • Audible tones
    • usually around 3200 Hz due to component constraints (Audio advancements for persons with hearing impairments have been made; see External links)
    • 85 dBA at 10 feet
  • Spoken voice alert
  • Visual strobe lights
  • Tactile stimulation, e.g., bed or pillow shaker (No standards exist as of 2008 for tactile stimulation alarm devices.)

While current technology is very effective at detecting smoke and fire conditions, the deaf and hard of hearing community has raised concerns about the effectiveness of the alerting function in awakening sleeping individuals in certain high risk groups such as the elderly, those with hearing loss and those who are intoxicated. Between 2005 and 2007, research sponsored by the NFPA has focused on understanding the cause of a higher number of deaths seen in such high risk groups. Initial research into the effectiveness of the various alerting methods is sparse. Research findings suggest that a low frequency (520 Hz) square wave output is significantly more effective at awakening high risk individuals.Wireless Wi-Safe smoke and carbon monoxide detectors linked to alert mechanisms such as vibrating pillow pads, strobes and remote warning handsets have been found to support the groups above.


In 2004, NIST issued a comprehensive report entitled Performance of Home Smoke Alarms — Analysis of the Response of Several Available Technologies in Residential Fire Settings The report concludes, among other things, that "smoke alarms of either the ionization type or the photoelectric type consistently provided time for occupants to escape from most residential fires", and "consistent with prior findings, ionization type alarms provided somewhat better response to flaming fires than photoelectric alarms, and photoelectric alarms provided (often) considerably faster response to smoldering fires than ionization type alarms".

The National Fire Protection Agency strongly recommand the replacement of home smoke alarms every 10 years.] Smoke alarms become less reliable with time, primarily due to aging of their electronic components, making them susceptible to nuisance false alarms. In ionization type alarms, decay of the 241Am radioactive source is a negligible factor, as its half-life is far greater than the expected useful life of the alarm unit.

Regular cleaning can prevent false alarms caused by the build up of dust or other objects such as flies, particularly on optical type alarms as they are more susceptible to these factors. A vacuum cleaner can be used to clean ionisation and optical detectors externally and internally. However, on commercial ionisation detectors it is not recommended for a lay person to clean internally. To reduce false alarms caused by cooking fumes, use an optical or 'toast proof' alarm near the kitchen.


Most residential smoke detectors run on 9-volt alkaline or carbon-zinc batteries. When these batteries run down, the smoke detector becomes inactive. Most smoke detectors will signal a low-battery condition. The alarm may chirp at intervals if the battery is low, though if there is more than one unit within earshot, it can be hard to locate. It is common, however, for houses to have smoke detectors with dead batteries. Is is estimated, in the UK, that over 30% of smoke alarms may have dead or removed batteries. As a result, public information campaigns have been created to remind people to change smoke detector batteries regularly. In Australia, for example, it is advertised that all smoke alarm batteries should be replaced on the first day of April every year. In regions using daylight saving time, these campaigns may suggest that people change their batteries when they change their clocks or on a birthday.

Some detectors are also being sold with a lithium battery that can run for about 7 to 10 years, though this might actually make it less likely for people to change batteries, since their replacement is needed so infrequently. By that time, the whole detector may need to be replaced. Though relatively expensive, user-replaceable 9-volt lithium batteries are also available.

Common NiMH and NiCd rechargeable batteries have a high self-discharge rate, making them unsuitable for use in smoke detectors. This is true even though they may provide much more power than alkaline batteries if used soon after charging, such as in a portable stereo. Also, a problem particularly prevalent in older technology rechargeable batteries is a rapid voltage drop at the end of their useful charge. This is of concern in devices such as smoke detectors, since the battery may transition from "charged" to "dead" so quickly that the low-battery warning period from the detector is either so brief as to go unnoticed, or may not occur at all.

The American National Fire Protection Association, through its fire protection program, urges homeowners to replace smoke detector batteries with a new alkaline battery every six months, for example when changing clocks for daylight saving time, and to replace the entire smoke detector after ten years of use. The used battery will probably still have the majority of its charge, and can be reused in less critical applications, such as a backup for a digital alarm clock.

Installation and placement

In the United States, most state and local laws regarding the required number and placement of smoke detectors are based upon standards established in Article 72 of NFPA fire code.

Laws governing the installation of smoke detectors vary depending on the locality. Homeowners with questions or concerns regarding smoke detector placement may contact their local fire marshal or building inspector for assistance. However, some rules and guidelines for existing homes are relatively consistent throughout the developed world. For example, Canada and Australia require a building to have a working smoke detector on every level. The United States requires smoke detectors on every habitable level and within the vicinity of all bedrooms. Habitable levels include attics that are tall enough to allow access.

In new construction, minimum requirements are typically more stringent. All smoke detectors must be hooked directly to the electrical wiring, be interconnected and have a battery backup. In addition, smoke detectors are required either inside or outside every bedroom, depending on local codes. Smoke detectors on the outside will detect fires more quickly, assuming the fire does not begin in the bedroom, but the sound of the alarm will be reduced and may not wake some people. Some areas also require smoke detectors in stairways, main hallways and garages.

Wired units with a third "interconnect" wire allow a dozen or more detectors to be connected, so that if one detects smoke, the alarms will sound on all the detectors in the network, improving the chances that occupants will be alerted, even if they are behind closed doors or if the alarm is triggered one or two floors removed from their location. Wired interconnection may only be practical for use in new construction, especially if the wire needs to be routed in areas that are inaccessible without cutting open walls and ceilings. As of the mid-2000s, development has begun on wirelessly networking smoke alarms, using technologies such as ZigBee, which will allow interconnected alarms to be easily retrofitted in a building without costly wire installations. Some wireless systems using Wi-Safe technology will also detect smoke or carbon monoxide through the detectors, which simultaneously alarm themselves with vibrating pads, strobes and remote warning handsets. As these systems are wireless they can easily be transferred from one property to another.

In the UK the placement of detectors are similar however the installation of smoke alarms in new builds need to comply to the British Standards BS5839 pt6. BS 5839: Pt.6: 2004 recommends that a new-build property consisting of no more than 3 floors (less than 200sqm per floor)) should be fitted with a Grade D, LD2 system. Building Regulations in England,Wales & Scotland recommend that BS 5839: Pt.6 should be followed, but as a minimum a Grade D, LD3 system should be installed. Building Regulations in Northern Ireland require a Grade D, LD2 system to be installed, with smoke alarms fitted in the escape routes and the main living room and a heat alarm in the kitchen, this standard also requires all detectors to have a main supply and a battery back up.



  • Journal of Applied Fire Science, Volume 6, Number 2, June 1997, article Risk Analysis of Residential Fire Detector Performance
  • National Fire Protection Code, Article 72
  • Domestic Alarm FAQ

External links

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