A smoke detector is a device that senses smoke, typically as an indicator of fire. Commercial security devices issue a signal to a fire alarm control panel as part of a fire alarm system, while household smoke detectors, also known as smoke alarms, generally issue a local audible or visual alarm from the detector itself.
Smoke detectors are housed in plastic enclosures, typically shaped like a disk about 150 millimeters (6 in) in diameter and 25 millimeters (1 in) thick, but shape and size vary. Smoke can be detected either optically (photoelectric) or by physical process (ionization), detectors may use either, or both, methods. Sensitive alarms can be used to detect, and thus deter, smoking in areas where it is banned. Smoke detectors in large commercial, industrial, and residential buildings are usually powered by a central fire alarm system, which is powered by the building power with a battery backup. Domestic smoke detectors range from individual battery-powered units, to several interlinked mains-powered units with battery backup; if any unit detects smoke, all trigger even in the absence of electricity.
Smoke detectors protect people by notifying them of fire in the home, especially when people are sleeping. They usually provide an audible alarm, and some have a visual alarm consisting of a flashing light.
Ionization Type Detectors
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 smoldering stage.
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 anelectric 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. 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 draw of 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 months or years without the need for external wiring.
Carbon Dioxide Detectors
Carbon monoxide sensors detect potentially fatal concentrations of carbon monoxide gas, which may build up due to faulty ventilation where there are combustion appliances such as heaters and cookers, although there is no uncontrolled fire outwith the appliance.
High levels of carbon dioxide (CO2) may indicate a fire, and can be detected by a carbon dioxide sensor. Such sensors are often used to measure levels of CO2 which may be undesirable but not indicative of a fire; this type of sensor can also be used to detect and warn of the much higher levels generated by a fire. One manufacturer says that detectors based on CO2 levels are the fastest fire indicators, and also, unlike ionization and optical detectors, detect fires that do not generate smoke, such as those fueled by alcohol or gasoline. CO2 fire detectors are not susceptible to false alarms due to particles, making them particularly suitable for use in dusty and dirty environments.
Photoelectric detectors have a small light source that normally misses a sensor that is set up out of the target area. Smoke particles deflect the light onto the sensor, creating an alarm. Photoelectric detectors are good at sensing smoky fires.
Not Heat Detectors
Smoke detectors are not heat detectors. High temperatures will not trigger smoke detectors.
Smoke detectors require an electric power source from a battery, house voltage, or both. Batteries may be conventional alkaline type that last one to two years, or lithium type that can last 10 years. Battery powered detectors typically have a power indicator light that is permanently on or flashes intermittently. Most units emit a loud chirping sound every few seconds when the battery is low.
120 Volt Systems
Many authorities insist on smoke detectors powered by the house electrical system. These detectors don’t need batteries changed and can be connected to any junction box, like a light fixture. Smoke detectors are not usually on a dedicated circuit, but detectors should never be on a switched part of a circuit, so that they don’t get turned off accidentally.
Some detectors operate on house power but have battery backup systems. The thinking is that a fire, especially if it’s electrical, may cut power to the detector before it can sound an alarm.
Many jurisdictions call for smoke detectors to be interconnected, so if one senses smoke, all will sound an alarm. This desirable arrangement is typically only found on newer homes. The interconnection most often uses conventional 120 volt house wiring (typically with 14 gauge, 3-conductor wire), but some are on a low voltage system.
Smoke detector location is a somewhat controversial issue. Everyone agrees there should be at least one detector in each home. There is also agreement that the detector should be near the sleeping area. Some say there should be a detector on every level of the home with sleeping quarters. Others say there should be a detector on every floor level, regardless.
Detectors In Bedrooms?
Some say there should also be detectors in bedrooms, rather than just in the hall ways. This protects people sleeping from a fire that starts in the bedroom. This is particularly helpful if the door is closed, because a detector in the hall may not sound quickly enough to save the person in the bedroom. One other recommendation is to ensure the detectors near sleeping areas are between the escape path and the bedrooms (not at the other end of the hall).
Humidity Can Cause False Alarms
Smoke detectors should not be in or adjacent to kitchens or bathrooms, since high humidity levels can cause false alarms.
Not In Corners
Smoke detectors should be installed on the ceiling because smoke rises. While some say on or near the ceiling, we prefer the detector to be away from wall /ceiling intersections, where dead air may delay activation. We prefer the detector to be near the middle of the room or hall width to help ensure early warning.
Testing Smoke Detectors
Test your smoke alarms regularly by pressing the test button or by using smoke from a smoldering incense stick.
Replace batteries regularly. Install a new battery in each alarm at least once a year. All battery-operated smoke alarms are required to emit a warning sound, usually an intermittent “chirp” when the battery power is low. When warning chirp sounds, replace your battery immediately. Never wait. Change your batteries when you change your clocks in the spring and fall. Smoke alarms do wear out, so if you think your alarms are more than 10 years old, replace them with new ones. Newer smoke detectors start a dating mechanism as soon as activated. Annoying chirping will start in 10 years so ensure you replace your parents or grandparents smoke detectors to prevent getting a call in middle of night 🙂
Central Station Alarms
Some smoke detectors are connected to central station alarm monitoring facilities. These central stations will respond to an alarm, typically by phoning the home and then sending the fire department, if they don’t get an immediate response from the occupants. This kind of alarm connection typically has the detectors in the home interconnected. These monitored detectors are commonly found in homes with security systems.