Smoke detector

A smoke detector or smoke alarm is an active fire protection device, subject to stringent bounding, that detects airborne smoke and issues an audible alarm, thereby alerting nearby people to the danger of fire. Most smoke detectors work either by optical detection or by ionization, but some of them use both detection methods to increase sensitivity to smoke. Smoke detectors may operate alone, be interconnected to cause all detectors in the premises to sound an alarm if one is triggered, or be integrated into a fire alarm or security system. Smoke detectors with flashing lights are available for the deaf or hearing impaired. A smoke detector cannot detect carbon monoxide to prevent carbon monoxide poisoning unless it has an integrated carbon monoxide detector.

History

In 1902 George Andrew Darby, an electrical engineer of 211 Bloomsbury Street, Birmingham, England, patented the electrical Heat-Indicator and Fire Alarm. The device was a heat alarm rather than a smoke detector and indicated an increase temperature in the apartment where it was fixed. The device operated 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. This early device subsequently gave way to more modern fire and eventually smoke alarms.

In 1969, two Americans, Kenneth House and Randolph Smith patented the modern smoke detector^*.

Optical detector

An optical detector is a light sensor. When used as a smoke detector it includes a light source (infra-red LED), a lens to collimate the light into a beam like a laser, and a photodiode or other photoelectric sensor at right-angles 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 into the path of the light beam, some light is scattered by the smoke particles, and some of the scattered light is detected by the sensor. An increased input of light into the sensor sets off the alarm.

Another type of optical detector works by using a straight line infra-red beam from the sender to the receiver. When smoke enters the beam, some light is scattered which results in less light detected by the receiver. A decreased input of light into the receiver sets off the alarm.

Optical smoke detectors are quick in detecting slow burning, smoky fires.

Ionisation detector

This type of detector is cheaper than the optical detector, however it is sometimes rejected for environmental reasons. It can detect particles of smoke that are too small to be visible. It includes a tiny mass of radioactive americium-241, which is a source of alpha radiation. The radiation passes through an ionization chamber, which is an air-filled space between two electrodes, and permits a small, constant current to flow between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and interrupts this flow of current, setting off the alarm. Hot air entering the chamber changes the rate of ionization and therefore, the electric current level, which triggers an alarm.

Reliability

In the early 1990s Texas A&M University did a full scale scientific investigation into the effectiveness of optical and ionization smoke detectors in different types of fires. The study determined that in a smoldering fire, with its relatively low number of large smoke particles, optical detectors fail 4.06% of the time, while ionization detectors fail 55.8%. For flame ignition fires, which have a large number of small, energetic smoke particles, photoelectric smoke detectors had a 3.99% probability of failure while ionization smoke detector failed 19.8%.

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 has issued a a fact sheet urging the replacement of home smoke alarms every 10 years.

Batteries

Most residential smoke detectors run on 9 volt alkaline batteries. If these batteries run out, the smoke detector will become inactive. Smoke detectors are required to signal a low battery condition, but it is common for houses to have smoke detectors with dead batteries. As a result, public information campaigns have been created to remind people to change their smoke detector batteries regularly. In regions using daylight saving time, these campaigns usually suggest that people change their batteries when they change their clocks.

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 should be replaced. Though relatively expensive, user-replaceable 9 volt lithium batteries (in the same configuration as the common alkaline ones) are also available. They should only be used in a fairly new detector.

Smoke detectors with missing batteries are also a concern. As a result, many detectors sold today are designed to provide a visual indication of a missing battery. One popular brand of smoke detector will not allow the user to close the battery door until a battery has been placed in the alarm, making a missing battery situation immediately obvious. Some local governments do not permit the installation of smoke detectors with removable batteries.

In new construction, most building codes today require smoke detectors that are wired to the main electricity flow of buildings. Many of these units also include a battery backup to ensure operation during a power outage.

Rechargeable batteries should never be used in smoke detectors, since common NiMH and NiCd rechargeable batteries have a short life in between charges - in other words, they self-discharge relatively quickly. This is true even though they may provide much more power than alkaline batteries if used soon after charging (such as in a Walkman stereo). A quality alkaline battery should be installed and replaced every six months or so. The used battery will still probably have the majority of its charge, and can be reused in less critical applications such as a backup for a digital alarm clock.

Testing

Smoke detectors are required to be equipped with a "test" button. Alternatively, artificial smoke can be purchased, which has the advantage of also testing the detector itself. Many people simply wave a lit match underneath the detector to test it, however this is dangerous as it can set the smoke alarm and the rest of the house on fire. A better way is to blow out a match and wave the smoking match under the detector.

It is recommended that smoke detectors batteries should be replaced once a year and the smoke detector should be replaced every ten years.^*

Laws governing installation

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.

In older existing homes, smoke detectors are generally required on every habitable level and within the vicinity of all bedrooms. In new construction, the minimum requirements are typically much greater. All smoke detectors must be hooked directly to the electrical wiring and be interconnected. In addition, smoke detectors are required inside every bedroom.

Laws governing the installation of smoke detectors vary depending on the locality. Homeowners with questions or concerns regarding smoke detector placement are encouraged to contact their local fire marshal or building inspector for assistance.

It is recommended that smoke detectors should not be placed in kitchens and bathrooms, because the steam would often set them off.

There is also a law in Canada that every house must have a working smoke detector on every level.

References

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

External links

Firefighting | Fire detection and alarm

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