A Guide to Smoke Detector Types

The hazards of smoke inhalation are often underestimated, yet twice as many people die from breathing smoke from fires than die from burns, and smoke and deadly gases from a fire spread farther and more quickly than heat from flames.

Smoke and gas detectors are effective fire safety technologies that form the first lines of defence against smoke and fire by providing a precious early warning. According to the National Fire Protection Association (NFPA), 75 to 80 percent of all deaths by fire happen in the home, more than half occurring in buildings without smoke detectors. By installing a smoke detector, individuals can reduce the risk of dying by almost half.

In general, commercial, industrial, and mass residential smoke detection devices issue a signal to a fire alarm system, whereas household detectors normally issue a local audible or visual alarm from the detector itself.

Aspirating smoke detectors
Recommended for use in:

• Datacentres
• Grade-listed properties
• High-ceiling spaces (such as warehouses)
• Other locations where early warning is required

Aspirating smoke detection systems (ASDs) are extremely sensitive and can detect very small or smouldering fires far more quickly than other systems. They use a fan to draw in air from around a building via a network of sampling pipes and sampling holes. The air is then passed through a highly sensitive precision detector that analyses it and generates warning signals of potential fire when it detects smoke particles.

Expensive to install and maintain, these systems can detect cool smoke that does not rise to the ceiling, as well as smouldering fires and particles given off by overloaded electrical cables. They are therefore especially useful where early warning is required and are suited, not only for sensitive installations, such as datacentres, but also for very high ceiling spaces, such as in warehouses, and for very harsh environments.

Optical beam smoke detectors (photoelectric)
Recommended for use in:

• High-ceiling spaces (such as warehouses)

Optical beam smoke detectors, like aspirating smoke detectors, measure smoke over a large open area, rather than a single location, and are very sensitive to diluted smoke at high levels, such as in rooms with high ceilings. Expensive to install, they are very effective at detecting larger particles of smoke produced by slow-burning, smoky fires, such as smouldering foam-filled upholstery, or overheated PVC wiring.

The detectors work on the principle of light obscuration, by aiming a light source into a sensing chamber at an angle away from the sensor. When smoke enters the chamber, it reflects light onto the sensor, so triggering an alarm. Dual-path types are available with a transmitter and a receiver at one end and a reflector at the other. Beam detectors are often used in buildings where single-point smoke detectors would be uneconomical or difficult to install.

Ionization detectors
Recommended for use in:

• The majority of residences and offices

Ionization smoke detectors monitor ions, or electrically charged particles in the air. If smoke enters the device, an electrical imbalance is created. When combustion particles enter the detector, they obstruct the flow of the current, setting off an alarm when the current gets too low.

Ionization smoke detectors respond well to fast flaming fires and are best suited for areas containing highly combustible material such as cooking fat, paint, or flammable liquids. They are inexpensive and are the most popular type of smoke detector in the United States.

Ionization detectors are usually quicker to react than photoelectric detectors, but they are sometimes unable to differentiate between smoke and steam, making them prone to false alarms.

Video smoke detection
Recommended for use in:

• Large areas
• Outdoors
• Where being able to see a developing fire quickly could be important (e.g., in a tunnel)

Video smoke detection (VSD) is based on the computer analysis of video images provided by CCTV cameras, automatically identifying the particular motion patterns of smoke and alerting the system operator to its presence in the shortest possible time.

A video smoke detection system in action

VSD systems have been installed in places such as turbine halls, historic buildings, road tunnels, rail depots, warehouses, shopping malls, and aircraft hangars. Some of them only detect smoke, although others can also detect flame, as well as providing motion detection and other surveillance/security features, according to the Fire Industry Association (FIA).

Key benefits of video detection include the ability to:

• Protect a large area
• Be used outdoors (e.g., in train stations)
• Immediately view the situation
• Substance-divide images to identify fire risks
• Archive images for assessing the causes of a fire

Household smoke detectors
Recommended for use in:

• Homes
• Some small shared residential blocks

There are three available types of household smoke detectors — ionization, optical (photoelectric), and combined. They are very cheap and are available everywhere.

The combined detectors are effective at detecting slow-burning as well as flaming fires, both common types of fire in the home. The NFPA recommends a combination of both ionization and photoelectric technologies for maximum protection.

The devices are either battery or mains electricity-powered, or both. Some are inter-connectable so that smoke detected at one point can raise the alarm at all others, while others have additional facilities, such as emergency lights and silence buttons, for use where false alarms can be a nuisance, such as during cooking.

Gas detectors
Recommended for use in:

• Industrial units and factories
• Storage bins and confined spaces

A gas detector is a device which detects the presence of various gases within an area, usually as part of a safety system. Highly sensitive, they can be used to detect combustible, flammable, and toxic gases and oxygen depletion and are widely used in industry. Catalytic and infrared sensors detect combustible gases, and electrochemical and metal oxide semiconductor technologies generally detect toxic gases.

Gas detectors may be battery-operated, portable, or fixed units and work by monitoring and alerting people audibly or visibly to the presence of dangerous levels of a variety of gases. They are very efficient in confined spaces that are not continuously occupied, such as tanks, pits, vessels, and storage bins.

More fire system guides:

• A Guide to Fire Alarm System Types