A Guide To Smoke Detectors And Fire Alarms

T.K. Hareendran

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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 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 ionisation smoke sensors 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. A residential fire-detection system may also include a heat detector.

There are two types of heat detectors: fixed-temperature and rate-of-rise. Fixed-temperature detectors are configured to wake up when air temperature exceeds the pre-set temperature. Rate-of-rise detectors raise an alert when temperature in the immediate vicinity rises higher than the pre-set rate-per-time factor.

table 1

What to look for

Photoelectric (optical) smoke detectors are quicker in detecting smoke generated by smoldering (smokey) fires. Ionisation smoke detectors are quicker in detecting smoke generated by flaming (hot) fires.

fig 6
Fig. 6: Warning labels on an ionisation smoke detector

Obscuration is a unit of measurement that has become the standard definition of smoke detector sensitivity. It is the effect that smoke has on reducing visibility (higher concentrations of smoke result in higher obscuration levels, lowering visibility). Typical smoke detector obscuration rating is shown in the table.

For each type of smoke alarm, the advantage it provides may be critical to life safety in some fire situations. Fatal fires include a large number of smoldering fires and a large number of flaming fires. As it is very difficult to predict the type of fire disaster, or when it will occur, in addition to individual photoelectric and ionisation smoke detectors, a combination of detectors that include multiple technologies in a single unit are recommended for reliable protection. Any smoke detector technology, to be acceptable, must perform tolerably for all types of fires in order to provide early warning of fire at all times of the day or night.

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T.K. Hareendran is an electronics hobbyist, freelance technical writer and circuit designer

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