Smoke Detectors by Jeff Barker
Smoke Detectors - Prepared by Jeff Barker
Smoke detectors are not new. The technology has been around since the 1960's. The single-station, battery-powered smoke detector, similar to the one we know today, became available to consumers in the 1970's. The NFPA (National Fire Prevention Association) estimates that 93% of U.S. homes have at least one smoke detector. They save so many lives that most states have laws requiring them in residential dwellings.
But how do they work? Currently there are two common types of smoke detectors, a photoelectric type and an ionization type detector.
Photoelectric sensors work by having a source (either a white light or a low-powered laser), on one side and a photodetector that can see the light on the other side. When something passes between them it blocks off the beam. The photodector senses the lack of light and triggers an alarm. You can find this type of photodetector on most elevator doors. The photodetector senses the lack of light and triggers a switch or an alarm. This type of detector is not very sensitive and takes a lot of smoke before it goes into an alarm.
Most photoelectric smoke detectors are of the spot type and operate on the light-scattering principle. A light emitting diode (LED) is beamed into an area not normally seen by a photosensitive element, generally a photo-diode. Below is an example of a typical arraignment.
This is the normal position with no alarm present. When smoke particles enter the light path, light strikes the particles and is reflected and/or refracted onto the photodetector device causing the unit to alarm.
A typical ionization chamber consists of two electrically charged plates and a radioactive source, (typically Americium 241) for ionizing the air between the plates. The radioactive source emits particles that collide with the air molecules and dislodge their electrons. As molecules loose electrons, they become negatively charged ions. Equal numbers of positive and negative ions are created. The positively charged ions are attracted to the negatively charged electrical plate, while the negatively charged ions are attracted to the positively charged plate. This creates a small ionization current that can be measured by electronic circuitry connected to the plates.
Particles of combustion are much larger than the ionized molecules. As particles of combustion enter an ionization chamber, ionized air molecules collide and combine with them. Some particles become positively charged and some become negatively charged. As these relatively large particles continue to combine with many other ions, they become re-combination centers, and the total number of ionized particles in the chamber is reduced. This reduction in the ionized particles result in a decrease in the chamber current that is sensed by electronic circuitry monitoring in the chamber. When the current is reduced by a predetermined amount, a threshold is crossed and the unit will go into an alarm.
Changes in humidity and atmospheric pressure could affect the chamber current and create a false alarm. To compensate for these possible effects, the dual ionization chamber was developed and has become commonplace in the smoke detector market.
A dual chamber utilizes two ionization chambers: one is open to the outside air. Particulate matter, humidity, and atmospheric pressure affect his sensing chamber. The other is a reference chamber that is partially closed to the outside air and affected only by the humidity and atmospheric pressure, because its tiny opening effectively blocks the entry of larger particulate matter such as smoke. Electronic circuitry both chambers and compares their outputs. If the humidity or the atmospheric pressure changes, both chambers' outputs are affected equally and cancel each other out. When combustion particles enter the sensing chamber its current decreases while the current of the reference chamber remains virtually unchanged. The resulting current imbalance is detected by the electronic circuitry and the unit goes into an alarm.
The characteristics of an ionization detector make it more suitable for detection of fast flaming fires that are characterized by combustion particles in the 0.01 to 0.3-micron size range. Photoelectric smoke detectors are better suited to detect slow smoldering fires that are characterized by particles in the 0.3 to 10.0-micron size range. Each type of detector can detect both types of fires, but their respective response time will vary, depending on the type of fire.
Because the building or area protected will normally contain a variety of combustibles, it is often difficulty to predict what sized particles a developing fire will produce. A lighted cigarette, for example, will usually produce a slow smoldering fire if it is dropped on a sofa or bed. However, if the cigarette happens to fall upon a newspaper the resulting fire will produce more flames than smoke.
The number of detectors is more important than the type. Installing several smoke detectors of each type will provide better coverage in extreme cases of long term smoldering or fast flaming fires. But since bother types respond in time to escape, the most important thing is to install enough detectors in the proper locations. Detectors are available with both types of sensors. If the choice is between having only one of each type or having more of the same type, more detectors is the better choice.
Smoke detectors offer the earliest warning of fire possible. They have saved thousands of lives in the past and will save more in the future. Nevertheless, smoke detectors do have limitations. A first-floor detector, for example, may not detect a second-floor fire. For this reason, detectors should be located on every level of a building. In addition, detectors may not sense a fire developing on the other side of a closed door. In areas where doors are usually closed, detectors should be located on both sides of the door.
Smoke detectors have sensing limitations. Ionization detectors are better at detecting fast, flaming fires than slow smoldering fires. Photoelectric smoke detectors sense smoldering fires better than flaming fires. Because fires develop in different ways, and are often unpredictable in their growth, neither type of detector is always best. A given detector may not always provide significant advance warning of fires when protection practices are inadequate, nor when caused by violent explosions, escaping gas, improper storage of flammable liquids such as cleaning solvents, etc.
Replace the batteries according to manufacturer's recommendations, or at least once a year. Warn everyone to leave working batteries in the smoke detector, resist temptation to borrow them for other purposes. Never paint a smoke detector. Cobwebs and dust impair detectors sensitivity, vacuum the detectors at least once a week.
Brian, Marshal, "How Smoke Detectors Work", [on-line web site] [cited 1 May 1998]. Available from http://www.bygup.com
Fire Rescue Services, "Smoke Detectors", [on-line web site] [cited 1 May 1998]. Available from http://seviervilletn.org/detector.htm
International Association of Fire Chiefs, "IAFC Smoke Detector Fact Sheet", [on-line web site] [cited 1 May 1998]. Available from http://www.infocom/boston/bcfd/smkdet.htm
National Electrical Manufacturers Association, "Training Manual on Fire Alarm Systems", 1995
National Electrical Manufacturers Association, "Guide for Proper Use of System Smoke Detectors", August 1995
National Fire Protection Association, "New Information About Smoke Detectors", [on-line web site] [cited 1 May 1998]. Available from http://www.nfpa.org/detector.html