Smoke detector

A smoke detector or smoke alarm is a safety device 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 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 indicated any change of 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, the patent for this invention was claimed by Kenneth House and Randolph Smith.

Optical detector

Optical Smoke Detector
1: optical chamber
2: cover
3: case moulding
4: photodiode (detector)
5: infrared LED

This type of detector includes a light source, a lens to collimate the light into a beam, and a photodiode or other photoelectric sensor at right-angles to the beam. In the absence of smoke, the light passes in front of the detector but does not fall on it. When visible smoke enters the beam, some light is scattered by the smoke particles, and some of the scattered light is detected by the sensor. An increased output from the sensor sets off the alarm.

Optical detectors need not be confined inside a single module; there are some optical detectors that span the width of an arena. In this case, the detector works by using a straight line infared 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 output from the receiver sets off the alarm.

Ionization 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. 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.

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, to hold a lit cigarette under the smoke detector, or, for smokers, to exhale cigarette smoke directly into the smoke detector.

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.

External links

Sources

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

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