There are two common types of smoke detectors available on the market. Each type is distinguished by its detection method. The most common smoke detector uses ionization sensors to detect smoke. The other type of smoke detector uses a photoelectric sensor to detect smoke.
Ionization detectors detect the presence of invisible particles (less than .01 micron in size) in the air. Inside the detector, there is a small ionization chamber that contains an extremely small quantity of radioactive isotope called Americium-241. The Americium-241 will decay or emit alpha particles at a fairly constant rate. The alpha particles, which travel at an extremely high rate of speed, ionize or knock off an electron (negative charge) from the atoms in the air (oxygen and nitrogen molecules) passing through the ionization chamber.
Types of Smoke Detectors
The free electron (negative charge) is then attracted to a positively charged plate, and the positively charged oxygen or nitrogen is attracted to a negatively charged plate. This creates a very small but constant current between the plates. When particles, such as combustion particles, dust, steam or fumes enter the chamber, the current is disrupted when the electrons attach to the particles. If the current drops too low, the alarm is triggered. [Note: The radioactive material generally presents a health hazard only at extremely close distances and with direct exposure (inhaling). Due to the design of the detector and limited amount of radioactive material, exposure is unlikely with proper handling.
Ionization detectors are very sensitive and are designed to detect hot and fast-moving fires that produce little smoke. The disadvantage to these sensitive detectors is possible false alarms from cooking vapors or excessive dust.
Photoelectric detectors detect the presence of visible particles (larger than 3 microns) in the air. Inside the detector, there is a light emitting diode (LED) that directs a narrow beam of infrared light across the detection chamber. When smoke or particles enter the chamber, the infrared light beam is scattered.
A photodiode or photodetector, usually placed 90 degrees to the beam, will sense the scattered infrared light and when a preset amount of light is detected, the alarm will sound. Photoelectric detectors are not as sensitive and are designed to detect cool or slow-moving (smoldering) fires that produce a lot of smoke.
It is important to properly place the smoke detectors in the home. In most locations throughout the U.S., smoke detectors are required in all new homes. The number and location are listed below:
Number and Location Requirements
The minimum standard as stated in the National Fire Prevention Association's National Fire Alarm Code (NFPA 72): There should be a smoke detector on every level of the house, including the basement and outside every bedroom.
New homes require hard-wired alarms to be interconnected so that if one alarm is activated, all alarms will sound the alarm signal.
New homes require smoke detectors in every bedroom. On floors without bedrooms, smoke alarms should be installed in or near living areas, such as family rooms and living rooms.
As stated by the NFPA: "Since smoke and deadly gases rise, alarms should be placed on the ceiling at least 4 inches from the nearest wall, or high on a wall, 4-12 inches from the ceiling. This 4-inch minimum is important to keep alarms out of possible "dead air" spaces, because hot air is turbulent and may bounce so much it misses spots near a surface. Installing alarms near a window, door or fireplace is not recommended because drafts could detour smoke away from the unit. In rooms where the ceiling has an extremely high point, such as in vaulted ceilings, mount the alarm at or near the ceiling's highest point."
Additional Guidelines for Installation
If you sleep with your bedroom doors closed, it is recommended that a smoke detector be installed inside each bedroom. Alarms should also be installed in other areas of your home where people sleep.
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- Wired systems should not be connected to a circuit that could be turned off with a wall switch.
- Plug-in systems should have a restraining device at the outlet to prevent the plug from accidentally coming loose.
- Hard-wired systems should be installed by a qualified electrician.
- Do not install the smoke detector near windows, doors or forced-air registers where air flow would interfere with the operation of the detector.
The National Fire Protection Association (NFPA) recommends testing all smoke detectors once a month to make sure they are operating. "Test each alarm by pushing the test button and listening for the alarm. If you can't reach, stand under the alarm and push the test button with a broom handle. It is vital that all smoke detectors be properly maintained and stay operational at all times.
All batteries should be replaced according to the manufacturer's instructions. The NFPA recommends "doing this at least once a year or when the alarm chirps, alerting you that the battery power is low. Replace the batteries immediately if you move into a new home."
Dust and cobwebs can interfere with the proper operation of a smoke detector. They may cause the detector to become less sensitive or even more sensitive in some cases. Smoke detectors should be cleaned regularly according to the manufacturer's instructions. Smoke detectors generally can be cleaned with a standard vacuum cleaner hose and attachment. Do NOT remove the detector's cover when cleaning.
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Based on statistical performance information, it is recommended that smoke detectors be replaced every 10 years or as monthly testing indicates.
The majority of smoke detectors are ionization detectors that detect very small particles and can be triggered by things other than combustion vapor from a fire, including cooking vapor, steam and dust. If the alarm activates, such as while cooking or dusting, do not disconnect the alarm or remove the battery. Instead, fan the air around the detector until the alarm turns off. Some newer smoke detectors come with reset buttons that will temporarily disable the alarm for approximately ten minutes. Cleaning the dust from the detector and replacing the battery may reduce the number of false alarms. If false alarms continue, replace the smoke detector or consider moving the detector to a more suitable location.
In areas that are prone to other vapors that trigger ionization detectors, photoelectric detectors may be a useful alternative. Photoelectric detectors are less sensitive to the smaller particles and they may be useful in confined areas (e.g. small apartments), in or near cooking areas, or near a heating device.
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