A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors/alarms are usually housed in plastic enclosures, typically shaped like a disk about in diameter and thick, but shape and size vary. Smoke can be detected either optically (photoelectric) or by physical process (ionization). Detectors may use one or both sensing methods. Sensitive detectors can be used to detect and deter smoking in banned areas. Smoke detectors in large commercial and industrial buildings are usually connected to a central fire alarm system.
Household smoke detectors, also known as smoke alarms, generally issue an audible or visual alarm from the detector itself or several detectors if there are multiple devices interconnected. Household smoke detectors range from individual battery-powered units to several interlinked units with battery backup. With interlinked units, if any unit detects smoke, alarms will trigger all of the units. This happens even if household power has gone out.
Residential smoke alarms are usually powered with a 9-volt battery, a non-removable "10 Year Sealed" battery, or by mains electricity. Some smoke alarms use a combination of the two, usually using a battery as an extra power source should loss of mains power occur. Commercial smoke detectors issue a signal to a fire alarm control panel as part of a fire alarm system. Usually, an individual commercial smoke detector unit does not issue an alarm; some, however, have built-in sounders.
The risk of dying in a residential fire is cut in half in houses with working smoke detectors. The US National Fire Protection Association reports 0.53 deaths per 100 fires in homes with working smoke detectors compared to 1.18 deaths without (2009–2013). Smoke detectors are not suitable for every location in a building, for instance in a kitchen of a domestic property, where a heat detector would be more suitable instead.
History
The first automatic electric fire alarm was patented in 1890 by Francis Robbins Upton,. an associate of Thomas Edison. In 1902, George Andrew Darby patented the first European electrical heat detector in Birmingham, England.. In the late 1930s, Swiss physicist Walter Jaeger attempted to invent a sensor for poison gas. He expected the gas entering the sensor to bind to ionized air molecules and thereby alter an electric current in a circuit of the instrument. However, his device did not achieve its purpose as small concentrations of gas did not affect the sensor's conductivity. Frustrated, Jaeger lit a cigarette and was surprised to notice that a meter on the instrument had registered a drop in current. Unlike poison gas, the smoke particles from his cigarette were able to alter the circuit's current. Jaeger's experiment was one of the developments that paved the way for the modern smoke detector. In 1939, Swiss physicist Ernst Meili devised an ionization chamber device capable of detecting combustible gases in mines. He also invented a cold cathode tube that could amplify the small signal generated by the detection mechanism so that it was strong enough to activate an alarm.
In 1951, ionization smoke detectors were first sold in the United States. In the following years, they were used only in major commercial and industrial facilities due to their large size and high cost. In 1955, simple "fire detectors" for homes were developed, which detected high temperatures. In 1963, The United States Atomic Energy Commission (USAEC) granted the first license to distribute smoke detectors that used radioactive material. In 1965, the first low-cost smoke detector for domestic use was developed by Duane D. Pearsall and Stanley Bennett Peterson. It was an individual, replaceable, battery-powered unit that could be easily installed. The "SmokeGard 700" was beehive-shaped, fire-resistant, and made of steel. With picture of SmokeGard. The company began mass-producing these units in 1975. Studies in the 1960s determined that smoke detectors respond to fires much faster than heat detectors.
The first single-station smoke detector was invented in 1970 and was brought out the next year. It was an ionization detector powered by a single 9-volt battery. It cost about and sold at a rate of a few hundred thousand units per year. Several developments in smoke detector technology occurred between 1971 and 1976, including the replacement of cold-cathode tubes with solid-state electronics. This greatly reduced the detectors' cost and size and made it possible to monitor battery life. The previous alarm horns which required special batteries were replaced with horns that were more energy-efficient and allowed the use of widely available batteries. These detectors could also function with smaller amounts of radioactive source material, and the sensing chamber and smoke detector enclosure were redesigned to make the operation more effective. The rechargeable batteries were often replaced by a pair of AA batteries along with a plastic shell encasing the detector.
The photoelectric (optical) smoke detector was invented by Donald Steele and Robert Emmark from Electro Signal Lab and patented in 1972. The 10-year-lithium-battery-powered smoke alarm was introduced years later, in 1995.
Design
Smoke can be detected using a photoelectric sensor or an ionization process. Fire without smoke can be detected by sensing carbon dioxide. Incomplete burning can be detected by sensing carbon monoxide.
Photoelectric
A photoelectric, or optical smoke detector, contains a source of infrared, visible, or ultraviolet light—typically an incandescent light bulb or light-emitting diode (LED)—a lens, and a photoelectric receiver—typically a photodiode. In spot-type detectors, all of these components are arranged inside a chamber that admits air from the environment, including smoke if present. In large open areas such as atria and auditoriums, optical beam or projected-beam smoke detectors are used instead of a chamber within the unit: a wall-mounted unit emits a beam of infrared or ultraviolet light which is either received and processed by a separate device or reflected to the receiver by a reflector. In some types, particularly optical beam types, the light emitted by the light source passes through the air being tested and reaches the photosensor. The received light intensity will be reduced due to scattering from particulates of smoke, air-borne dust, or other substances; the circuitry detects the light intensity and generates the alarm if it is below a specified threshold, potentially due to smoke.
In other types, typically chamber types, the light source is directed away from the sensor, and detects no reflected light in the absence of particles. However, once the air in the chamber contains particles (smoke or dust), the light is scattered with some of it reflected back to the sensor, triggering the alarm.
According to the National Fire Protection Association (NFPA), "photoelectric smoke detection is generally more responsive to fires that begin with a long period of smoldering". Studies by Texas A&M and the NFPA cited by the City of Palo Alto, California state, "Photoelectric alarms react slower to rapidly growing fires than ionization alarms, but laboratory and field tests have shown that photoelectric smoke alarms provide adequate warning for all types of fires and have been shown to be far less likely to be deactivated by occupants."
Although photoelectric alarms are highly effective at detecting smoldering fires and do provide adequate protection from flaming fires, fire safety experts and the NFPA recommend installing what are called combination alarms, which are alarms that either detect both heat and smoke or use both the ionization (below) and photoelectric smoke sensing methods. Some combination alarms may also include a carbon monoxide detection capability.
The type and sensitivity of light source and photoelectric sensor and type of smoke chamber differ between manufacturers.
Ionization
An ionization smoke detector uses a radioisotope, typically americium-241, to ionize air; a difference due to smoke is detected and an alarm is generated. Ionization detectors are more sensitive to the flaming stage of fires than optical detectors, while optical detectors are more sensitive to fires in the early smouldering stage.Fleming, Jay. "Smoke Detector Technology Research" , retrieved 2011-11-07.
The smoke detector has two ionization chambers, one open to the air and one closed as a reference that does not allow the entry of particles. The radioactive source emits alpha particles into both chambers, which ionizes some air molecules. There is a potential difference (voltage) between pairs of electrodes in the chambers; the electrical charge on the ions allows an electric current to flow. The currents in both chambers should be the same as they are equally affected by air pressure, temperature, and the diminishing radioactivity of the source. If any smoke particles enter the open chamber, some of the ions will attach to the particles and not be available to carry the current in that chamber. An electronic circuit detects the difference in current between the open and sealed chambers, and sounds the alarm. The circuitry also monitors the battery used to supply or back up power. It sounds an intermittent warning when power nears exhaustion. A user-operated test button simulates an imbalance between the ionization chambers and sounds the alarm if and only if the power supply, electronics, and alarm device are functional. The current drawn by an ionization smoke detector is low enough for a small battery used as a sole or backup power supply to be able to provide power for years without the need for external wiring.
Ionization smoke detectors are usually less expensive to manufacture than optical detectors. Ionization detectors may be more prone than photoelectric detectors to false alarms triggered by non-hazardous events,Residential Smoke Alarm Performance, Thomas Cleary, Building and Fire Research Laboratory, National Institute of Standards and Technology, UL Smoke and Fire Dynamics Seminar. November, 2007. and are much slower to respond to typical house fires:"In smoldering tests, the average time to first alarm was 2489 s ± 1324 s for ionization alarms and 1927 s ± 1065 s for photoelectric alarms."Supplementary Questions and Answers Clarifying “Detector Sensitivity and Siting Requirements for Dwellings,” Phase I (NBS GCR 75-51) and Phase II (NBS GCR 77-82)
Radiation
Americium-241 is an alpha emitter with a half-life of 432.6 years. Alpha particle radiation, as opposed to beta (electron) and gamma (electromagnetic) radiation, is used for two reasons: the alpha particles can ionize enough air to make a detectable current; and they have low penetrative power, meaning they will be stopped, safely, by the air or the plastic shell of the smoke detector. During the alpha decay, emits gamma radiation, but it is low-energy and therefore not considered a significant contributor to human exposure.
The amount of elemental americium-241 in ionization smoke detectors is small enough to be exempt from the regulations applied to larger radioactive sources. A smoke detector contains about of radioactive element americium-241 (), corresponding to about 0.3 μg of the isotope. This provides sufficient ion current to detect smoke while producing a very low level of radiation outside the device. Some Russian-made smoke detectors, most notably the RID-6m and IDF-1m models, contain a small amount of plutonium, rather than the typical source, in the form of reactor-grade mixed with titanium dioxide onto a cylindrical alumina surface.