Smoke Detector

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Smoke Detector

Background

A smoke detector is a device that senses the presence of smoke in a building and warns the occupants, enabling them to escape a fire before succumbing to smoke inhalation or burns. Equipping a home with at least one smoke detector cuts in half the chances that the residents will die in a fire. In 1992 the readers of R&D Magazine selected home smoke alarms as one of the "30 Products that Changed Our Lives." Smoke detectors became widely available and affordable in the early 1970s. Prior to that date, fatalities from fires in the home averaged 10,000 per year, but by the early 1990s the figure dropped to fewer than 6,000 per year.

Two basic types of smoke detectors are currently manufactured for residential use. The photoelectric smoke detector uses an optical beam to search for smoke. When smoke particles cloud the beam, a photoelectric cell senses the decrease in light intensity and triggers an alarm. This type of detector reacts most quickly to smoldering fires that release relatively large amounts of smoke.

The second type of smoke detector, known as an ionization chamber smoke detector (ICSD), is quicker at sensing flaming fires that produce little smoke. It employs a radioactive material to ionize the air in a sensing chamber; the presence of smoke affects the flow of the ions between a pair of electrodes, which triggers the alarm. Between 80 and 90% of the smoke detectors in American homes are of this type. Although most residential models are self-contained units that operate on a 9-volt battery, construction codes in some parts of the country now require installations in new homes to be connected to the house wiring, with a battery backup in case of a power failure.

The typical ICSD radiation source emits alpha particles that strip electrons from the air molecules, creating positive oxygen and nitrogen ions. In the process, the electrons attach themselves to other air molecules, forming negative oxygen and nitrogen ions. Two oppositely charged electrodes within the sensing chamber attract the positive and negative ions, setting up a small flow of current in the air space between the electrodes. When smoke particles enter the chamber, they attract some of the ions, disrupting the current flow. A similar reference chamber is constructed so that no smoke particles can enter. The smoke detector constantly compares the current flow in the sensing chamber to the flow in the reference chamber; if a significant difference develops, an alarm is triggered.

History

The development of these life-saving appliances began in 1939 when Ermst Meili, a Swiss physicist, devised an ionization chamber device capable of detecting combustible gases in mines. The real breakthrough was Meili's invention of a cold-cathode tube that could amplify the small electronic signal generated by the detection mechanism to a strength sufficient to activate an alarm.

Although ionization chamber smoke detectors have been available in the United States since 1951, they were initially used only in factories, warehouses, and public buildings because they were expensive. By 1971 residential ICSDs were commercially available; they cost about $125 per detector and sold at a rate of a few hundred thousand per year.

A fluny of new technological developments occurred over the next five years, reducing the cost of the detectors by 80% and boosting sales to 8 million in 1976 and 12 million in 1977. By this time, solid-state circuitry had replaced the earlier cold-cathode tube, significantly reducing the size of the detectors as well as their cost. Design refinements, including more energy-efficient alarm horns, enabled the use of commonly available sizes of batteries rather than the hard-to-find specialty batteries that had previously been required. Improvements in the circuitry made it possible to monitor both the decrease in voltage and the build-up of internal resistance in the battery, either of which would trigger a signal to replace the power source. The new generation of detectors could also function with smaller amounts of radioactive source material, and the sensing chamber and smoke detector enclosure were redesigned for more effective operation.

Raw Materials

An ICSD smoke detector is composed of a housing made of polyvinylchloride or poly-styrene plastic, a small electronic alarm horn, a printed circuit board with an assortment of electronic components, and a sensing chamber and reference chamber, each containing a pair of electrodes and the radioactive source material.

Americium 241 (Am-241), a radioactive isotope, has been the preferred source material for ICSDs since the late 1970s. It is very stable and has a half-life of 458 years. It is usually processed with gold and sealed within gold and silver foils.

The Manufacturing
Process

The production of a smoke detector consists of two major steps. One is fabrication of the Am-241 into a form (typically a foil) that can be installed into the sensing and reference chambers. The other is assembly of the entire ICSD, beginning either with all of the individual components or with prefabricated sensing and reference chambers obtained from the manufacturer of the radioactive source material. The following description covers all steps, even though some may be done by different manufacturers. Tests and inspections at several stages of the assembly process ensure a reliable product.

Radioactive source

1 The process begins with the compound AmO₂, an oxide of Am-241. This substance is thoroughly mixed with gold, shaped into a briquette, and fused by pressure and heat at over 1470°F (800°C). A backing of silver and a front covering of gold or gold alloy are applied to the briquette and sealed by hot forging. The briquette is then processed through several stages of cold rolling to achieve the desired thickness and levels of radiation emission. The final thickness is about 0.008 inches (0.2 mm), with the gold cover representing about one percent of the thickness. The resulting foil strip, which is about 0.8 inches (20 mm) wide, is cut into sections 39 inches (1 meter) long.
2 Circular ICSD source elements are punched out of the foil strip. Each disc, which is about 0.2 inches (5 mm) in diameter, is mounted in a metal holder. A thin metal rim on the holder is rolled over to completely seal the cut edge around the disc.

The sensing and reference chambers

3 One disc of source material is mounted in the sensing chamber and another is mounted in the adjacent reference chamber. The electrodes are installed in both chambers and connected to external leads which project out of the bottoms of the chambers.

The circuit board

4 Printed circuit boards are prepared from design schematics by punching holes for the component leads and by laying a copper trace on the back to form the paths for electric currents. On the assembly line, the various electronic components (diodes, capacitors, resistors, etc.) are inserted into the proper holes on the board. Leads extending out the back of the board are trimmed.

5 The sensing chamber, reference chamber, and an alarm horn are installed on the printed circuit board.
6 The board then passes over a wave solder machine, which solders the electronic components into place.

Housing

7 The plastic housing consists of a mounting base and a cover. Both are made by injection molding process in which powdered plastic and molding pigments are mixed, heated, forced into a mold under pressure, then cooled to form the final pieces.

Final assembly

8 The circuit board is seated on the plastic mounting base. A test button is installed so the device can be tested periodically after installation in the home. A mounting bracket is added to the base, and the cover is added to complete the assembly.
9 The smoke detector is packaged in a cardboard box, along with a battery and an owner's manual.

New Developments

Some recent developments may make smoke detectors even more effective. One model, for example, uses a strobe light alarm to alert hearing-impaired people of danger. The remote strobe light can be mounted in a bedroom even though the detector may be located in another room or hallway, giving the same advantage of early warning available to hearing people when an alarm sounds from outside the bedroom.

In 1993 Newtron Products redesigned a traditional smoke detector to fit in the standard air filters of a central heating or air conditioning system in order to examine air that circulates through an entire building. When it detects smoke, the device shuts off the system's blower to prevent the air flow from helping spread the smoke and fire. In addition, it triggers an alarm that resonates through the duct work and is audible anywhere in the building.

Another kind of fire detector may utilize sound. Investigators at the Building and Fire Research Laboratory of the National Institute of Standards and Technology have found that various types of housing materials, such as wood, plastic, and drywall, make identifiable sounds as they expand from rapid heating. Piezoelectric transducers can detect those sounds even before the materials actually begin to burn. This would be especially helpful in detecting incipient fires caused by overheated electrical wiring within a building's walls.

Where To Learn More

Books

Belanger, R., D.W. Buckley, and J.B. Swenson. Environmental Assessment of Ionization Chamber Smoke Detectors Containing Am-241. Science Applications Inc., November 1979.

Bukowski, Richard W., and G.W. Mulhol-land. Smoke Detector Design and Smoke Properties. National Bureau of Standards, National Engineering Laboratory, Center for Fire Research, 1978.