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Something you should know about a smoke alarm
2008-08-27

What is a smoke detector?

A smoke detector is a firefighting device that detects smoke and issues an alarm to alert nearby people that there is a potential fire. A household smoke detector will typically be mounted in a disk-shaped plastic enclosure about 150 mm in diameter and 25 mm thick, but the shape can vary by manufacturer. Smoke detectors are usually powered by one or more batteries, but some can be connected directly to the power mains. Detectors that are directly connected to the mains often have a battery as a power supply backup in case the main's power fails.

Types of smoke detectors and how they work

There are three main types of smoke detectors: ionization detectors, photoelectric detectors and air-sampling detectors. A smoke alarm uses one or both methods, sometimes plus a heat detector, to warn of a fire. Ionization Detectors Ionization detectors have an ionization chamber and a source of ionizing radiation. The source of ionizing radiation is a minute quantity of americium-241, which is a source of alpha particles (helium nuclei). The ionization chamber consists of two plates separated by about a centimeter. The battery applies a voltage to the plates, charging one plate positive and the other plate negative. Alpha particles constantly released by the americium knock electrons off of the atoms in the air, ionizing the oxygen and nitrogen atoms in the chamber. The positively-charged oxygen and nitrogen atoms are attracted to the negative plate and the electrons are attracted to the positive plate, generating a small, continuous electric current. When smoke enters the ionization chamber, the smoke particles attach to the ions and neutralize them, so they do not reach the plate. The drop in current between the plates triggers the alarm. Photoelectric Detectors In one type of photoelectric device, smoke can block a light beam. In this case, the reduction in light reaching a photocell sets off the alarm. In the most common type of photoelectric unit, however, light is scattered by smoke particles onto a photocell, initiating an alarm. In this type of detector there is a T-shaped chamber with a light-emitting diode (LED) that shoots a beam of light across the horizontal bar of the T. A photocell, positioned at the bottom of the vertical base of the T, generates a current when it is exposed to light. Under smoke-free conditions, the light beam crosses the top of the T in an uninterrupted straight line, not striking the photocell positioned at a right angle below the beam. When smoke is present, the light is scattered by smoke particles, and some of the light is directed down the vertical part of the T to strike the photocell. When sufficient light hits the cell, the current triggers the alarm. Air-sampling detectors An air-sampling smoke detector, sometimes called a VESDA® system, is capable of detecting microscopic particles of smoke. Most air-sampling detectors work by actively drawing air through a network of small-bore pipes lay out above or below a ceiling in parallel runs covering a protected area. Small holes drilled into each pipe form a matrix of holes (sampling points), providing an even distribution across the pipe network. Air samples are drawn past a sensitive optical device, often a solid-state laser, tuned to detect the extremely small particles of combustion. Air-sampling detectors may be used to trigger an automatic fire response, such as a gaseous fire suppression system, in high-value or mission-critical areas, such as archives or computer server rooms.

How to Shop for a Smoke Detector

When shopping for a smoke detector, consider these suggestions: Look for a laboratory seal of approval or a statement on the package or unit itself. A seal of approval indicates that the detector has been tested and certified by a recognized testing organization. Be certain that the detector has clear and detailed instructions that tell you how to install it, suggest where to place it, and provide directions for testing and maintenance.

Where Should I Install Smoke Detectors?

Put your detector as close as possible to the bedrooms or other places where people frequently sleep. If two sleeping areas are separated, each should have its own detector. The hallway next to the bedrooms or other sleeping areas should be a priority location for detectors. If bedroom doors are normally closed at night, detector locations within each bedroom might be considered, particularly if the occupant smokes in bed. In single floor homes, the detector should be placed in the hallway near the bedrooms. In a house where the bedrooms are upstairs, one detector should be near the top of the stairs to the bedroom area. The simplest rule for locating one smoke detector in your home should be "between the bedrooms and the rest of the house, but closer to the bedrooms." If you are installing multiple detectors, put one near each sleeping area. Don't put detectors within six inches of where walls and ceilings meet, or near heating and cooling ducts. In homes with more than one sleeping area on the same level or on different levels, a smoke detector should be installed to protect each separate sleeping area. In a one-floor plan with only one sleeping area, the smoke detector should be placed between the sleeping area and the rest of the house.

Furthermore

Smoke detectors don't need much attention. Regular testing and prompt replacement of batteries or bulbs is generally all that is needed. However, if you neglect these few requirements, your detector won't do its job if a fire starts. The smoke detector is one of three items of fire safety apparatus that are recommended for homes and that can be installed by the consumer. The second is a fire extinguisher, and the third is a fire blanket.

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