Smoke Detector Types


The various smoke detector types are as follows :

Photoelectric Smoke Detectors

These aim a light source into a sensing chamber. Smoke produced by a fire affects the intensity of a light beam passing through air. Smoke can block or obscure the beam. It can also cause the light to scatter due to reflection of smoke particles. These detectors are designed to sense smoke by utilizing these effects of smoke on light.

In a photoelectric light-obscuration smoke detector, when smoke particles partially block the light beam from the inbuilt light source, reduction in light reaching the associated photosensitive device alters its output. Change in this output is sensed by the detector’s circuitry, and when the threshold is crossed, an alarm is initiated.

However, in a photoelectric light scattering smoke detector, a light source is beamed into an area not normally looked out by a photosensitive device. When smoke particles enter the light path, light strikes the particles and is reflected onto the photosensitive device causing the detector to respond.

Photoelectric smoke detectors are better suited to detect slow smoldering fires that are characterised by particulates in 0.4 to 10 micron range.

Ionization Smoke Detectors

These use a small radioactive source as a key component for detection of smoke particles. The radionuclide used in these detectors is Americium-241 (Am-241), which is bonded to a metallic foil and sealed within a small chamber. A trace of Americium creates charges particles that betray the smoke.

In principle, ionization smoke detectors have a small amount of radioactive material between two electrically-charged plates, which ionizes the air and causes current to flow between the plates. When smoke enters the chamber, it disrupts the flow of ions, thus reducing the flow of current and activating alarm.

Since reduction in ionized particles results in a decrease in chamber current, that is sensed by the electronic circuitry monitoring the chamber (when current is reduced by a predetermined amount,  a threshold is crossed and an alarm condition is established). A dual-ionization chamber is required to compensate for the possible effects of humidity and pressure changes (changes in humidity and/or pressure affect chamber current and create an effect similar to the effect of particles of combustion entering the sensing chamber).

A dual-chamber detector utilizes two ionization chambers. One is a sensing chamber, affected by particulate matter , humidity and atmospheric pressure, which is open to outside air. The other is a reference chamber, which is partially closed to outside air and is affected only by humidity and atmospheric pressure, because its tiny openings block the entry of larger particulate matter including particles of combustion.

An electronic circuitry monitors both chambers and compares their output. If humidity or atmospheric pressure changes, outputs of both chambers 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 unchanged. The resulting current imbalance is detected by the built-in electronic circuitry.

Characteristics of an ionization detector make it more suitable for detection of fast-flaming fires that are characterized by combustion particles in the 0.1 to 0.4 micron-size range.

Laser Based Smoke Detectors

Laser based smoke detectors are designed for use in areas that require extremely early warnings of fire. These are designed to detect the earliest particles of combustion, making these ideal for any area with mission-critical operations.

Laser based smoke detectors are ultra-sensitive to smoke, being as much as 100 times more sensitive than standard ones.

Aspiration Smoke Detectors

Aspiration smoke detectors use a pipe and fan system to draw smoke particulates back to the detection chamber. Most air-sampling detectors are aspiration smoke detectors that work by actively drawing air through a network of small-bore pipes laid out above or below a ceiling in parallel, 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 extremely small particles of combustion.

Air Sampling Smoke Detectors

Air sampling smoke detectors may be used to trigger an automatic fire response. Likewise, some smoke detectors use a carbon-dioxide sensor or carbon-monoxide sensor in order to detect extremely dangerous products of combustion.

Lately, designers have begun to introduce smoke detectors that combine photoelectric and ionization smoke detectors in the same unit. These units combine the advantages of both the sensors into an advanced unit that detects smoke from a broader spectrum of fires.