A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors are usually housed in plastic enclosures, typically shaped like a disk about 150 millimetres (6 in) in diameter and 25 millimetres (1 in) 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 alarms 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 interlinked. 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 at all of the units. This happens even if household power has gone out.

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, do 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). However, some homes do not have any smoke alarms, and some homes do not have any working batteries in their smoke alarms
The first automatic electric fire alarm was patented in 1890 by Francis Robbins Upton,[2] an associate of Thomas Edison.[3] In 1902, George Andrew Darby patented the first European electrical heat detector in Birmingham, England.[4][5] In the late 1930s, Swiss physicist Walter Portable Smoke Detector attempted to invent a sensor for poison gas.[6] 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.[6] However, his device did not achieve its purpose as small concentrations of gas did not affect the sensor’s conductivity.[6] Frustrated, Jaeger lit a cigarette and was surprised to notice that a meter on the instrument had registered a drop in current.[7] Unlike poison gas, the smoke particles from his cigarette were able to alter the circuit’s current.Portable Smoke Detector experiment was one of the developments that paved the way for the modern smoke detector.[7] In 1939, Swiss physicist Ernst Meili devised an ionization chamber device capable of detecting combustible gases in mines.[8] 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.[8] In 1955, simple “fire detectors” for homes were developed,[9] which detected high temperatures.[10] In 1963, The United States Atomic Energy Commission (USAEC) granted the first license to distribute smoke detectors that used radioactive material.[6] 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.[11][12] The “SmokeGard 700″[13] was beehive-shaped, fire-resistant, and made of steel.[14] The company began mass-producing these units in 1975.[7] Studies in the 1960s determined that smoke detectors respond to fires much faster than heat detectors.[10]

The first single-station smoke detector was invented in 1970 and was brought out the next year.[10] It was an ionization detector powered by a single 9-volt battery.[10] It cost about US$125 (equivalent to $941.95 in 2022) and sold at a rate of a few hundred thousand units per year.[8] 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.[8] 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.[8] These detectors could also function with smaller amounts of radioactive source material, and the sensing chamber and smoke detector enclosure were redesigned to make operation more effective.[8] 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

In the 10-year-lithium-battery-powered smoke alarm was introduced
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 where air, which may contain smoke from a nearby fire, flows. 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.[16] In other types, typically chamber types, the light is not directed at the sensor, which is not illuminated in the absence of particles. If the air in the chamber contains particles (smoke or dust), the light is scattered and some of it reaches the sensor, triggering the alarm.[16]

According to the National Fire Protection Association Portable Smoke Detector “photoelectric smoke detection is generally more responsive to fires that begin with a long period of smoldering”. Studies by Texas A&M and the Portable Smoke Detector cited by the City of Portable Smoke Detector 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 Portable Smoke Detector recommend installing what are called combination alarms, which are alarms that either detect both heat and smoke or use both the ionization 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.
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.[18]

The smoke detector has two ionization chambers, one open to the air, and a reference chamber which 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 ageing 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 that a current difference has developed between the open and sealed chambers, and sounds the alarm.[19] The circuitry also monitors the battery used to supply or back up power, and sounds an intermittent warning when it nears exhaustion. A user-operated test button simulates an imbalance between the ionization chambers, and sounds the alarm if and only if 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 sole or backup power supply to be able to provide power for years without the need for external wiring.

Ionization smoke detectors are usually cheaper to manufacture than optical detectors. Ionization detectors may be more prone than photoelectric detectors to false alarms triggered by non-hazardous events,[20][21] and are much slower to respond to typical house fires.
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, 241
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 deployments. A smoke detector contains about 37 kBq (1,000 nCi) of radioactive element americium-
), corresponding to about 0.3 µg of the isotope.Portable Smoke Detector 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 (18 MBq), rather than the typical 241
source, in the form of reactor-grade Portable Smoke Detector
mixed with titanium dioxide onto a cylindrical alumina surface.

The amount of americium-241 contained in ionizing smoke detectors does not represent a significant radio logical hazard.[27] If the americium is left in the ionization chamber of the alarm, the radio logical risk is insignificant because the chamber acts as a shield to the alpha radiation. A person would have to open the sealed chamber and ingest or inhale the americium for the dose to be comparable to natural background radiation. The radiation risk of exposure to an ionizing smoke detector operating normally is much smaller than natural background radiation.
Photoelectric detectors and ionization detectors differ in their performance depending on the type of smoke generated by a fire.

A presentation by Siemens and the Canadian Fire Alarm Association reports that the ionization detector is the best at detecting incipient-stage fires with invisibly small particles, fast-flaming fires with smaller 0.01–0.4 micron particles, and dark or black smoke, while more modern photoelectric detectors are best at detecting slow-smouldering fires with larger 0.4–10.0 micron particles, and light-coloured white/grey smoke.[31]

Photoelectric smoke detectors respond faster to fire that is in its early, smoldering stage.[32] The smoke from the smoldering stage of a fire is typically made up of large combustion particles between 0.3 and 10.0 µm. Ionization smoke detectors respond faster (typically 30–60 seconds) to the flaming stage of a fire. The smoke from the flaming stage of a fire is typically made up of microscopic combustion particles between 0.01 and 0.3 µm. Also, ionization detectors are weaker in high air flow environments

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