Smoke detectors and alarms, Part 4

The low-cost, reliable, ubiquitous smoke detector is an example of complex physics and optics made real by semiconductors.

Technology is always improving and adding features to existing products, and that’s true of smoke detectors and alarms. At the same time, the “protection” world is very cautious about adding features and frills which may detract, impede, counter, or complicate operations to the detriment of the basic safety mission. Assuring such “fail-safe” operation is a design challenge since the units must be certified to be legal for installation.

Q: What about connectivity?

A: Today’s systems and users, as well as smart homes, require more than standalone units. Many smoke alarms now include wired or wireless features linking all alarms together such that when one sounds, they all sound. This is especially important in multi-level homes or very small multi-unit buildings when, for example, an alarm in the basement is activated while people are sleeping one or two floors above (note that larger apartment buildings have different, stricter codes.). Smoke alarms with high-intensity strobe lights are also available for the hearing impaired.

Of course, adding Bluetooth, Wi-Fi, and even cellular connectivity to a smoke alarm is also feasible. In this way, people within the building, as well as those away from it, can be alerted in various ways. Although wireless connectivity is easier to arrange, it lacks the reliability of hard-wired connections and so may not be suitable or acceptable in some situations or building codes. Again, it’s critical that these non-core functions not interfere with basic safety-related operation of the detector and alarm function or compromise performance in any way.

Q: What about new approaches to sensing?

A: Although not yet used in residential settings, there are an increasing number of aspirating smoke detectors (ASD) on the market—a more advanced, highly-sensitive technology that provides earlier warning detection.

ASD systems work by drawing in air from each room through small, flexible tubing. The air is then analyzed in a continuous process to identify the presence of minute smoke particles. The system does not rely on “casual” or haphazard room airflow, which is inconsistent or perhaps stagnant; thus, it can detect smoke before it is even visible. Aspiration systems are widely used and preferred in challenging situations where condensation is present or very early detection is required in locations such as communications and computer rooms.

Q: Can you give an example?

A: VESDA systems are laser-based ASDs that give a pre-fire warning. VESDA—Very Early Smoke Detection Apparatus—is a trademark of Honeywell, but it has also become a generic term for most air-sampling applications. They are beneficial in areas where high smoke sensitivity and easy access is required, such as computer rooms, cold rooms, and high-ceilinged buildings like warehouses and churches, because the detectors can be located at accessible levels for maintenance purposes.

Q: How does it work?

A: A VESDA detector is somewhat like a vacuum cleaner. It sucks air from the protected environment via a custom-built aspirating pipe and fittings and then samples air quality passing through the VESDA detection laser chamber (Figure 1).

Fig 1: The aspirating smoke detector actively draws air being monitored into a test and evaluation chamber located remote from the sampling location. (Image: Fire Suppression Ltd)

Q: Where and why is this beneficial?

A: VESDA smoke detection is commonly used where temperatures are not suitable for other types of smoke detection. For example, a cold room with operating temperatures ranging from -20°C to +20°C is a good environment for VESDA smoke detection. The reason is that cold temperatures will ice up the optical chambers of conventional smoke detectors, thus rendering the system useless.

In contrast, the VESDA detectors are located outside the cold environment itself, with aspirating pipe work located either inside or outside the risk area, while the smoke-sampling heads are located within the risk area. The cold air is then drawn to the smoke detector and is naturally heated before it reaches the laser chamber. If any condensation is collected at a condensate water trap, so dry air arrives for sampling by the VESDA detector.

Q: Are there any other new approaches?

A: Auto-aligning optical-beam smoke detectors are also in use. In these, the smoke detector is a laser-assisted infrared optical beam between the laser source and optical receiver (Figure 2). Rather than being single-point, localized smoke detectors, these units sense across a larger distance, somewhat analogous to free-space optical links (see EE World Related Content).

Fig 2: Optical-beam smoke detectors can cover a large area as their beam traverses an open space, in contrast to systems that monitor one or more discrete points. (Image: IFC Global/Informa Markets)

They protect large commercial and public spaces such as theatres, shopping malls, and sports centers with large skylights, lofty ceilings, or condensation issues. The optical receiver can be placed at the far end of the beam path, or it can be co-located in the same housing as the light source, with a reflecting mirror at the far end. The former approach supports a longer span but also requires wiring at the far end instead of a simple mirror.

Q: Are there any concerns or drawbacks to this approach?

A: One issue with such long-span systems is stratification, which is the development of separate layers of air and thermal barriers near the ceiling (Figure 3). As a result, it is possible for smoke-laden air from below to not reach up to the layer where the system beam is passing. To overcome this stratification-layer concern, two or more angled beams can be used in an up-and-down criss-cross pattern, with each beam typically aimed at an angle of 30 degrees across a ceiling.

Fig 3: One concern with optical-beam systems is the possibility of thermal stratification of air layers which prevents particulates from reaching the monitored zone; it can be overcome by angling two or more optical beams. (Image: IFC Global/Informa Markets)

Conclusion

The typical smoke detector is a case of principles of advanced physics made practical by application-specific ICs. It provides long-term, consistent, reliable performance for a mission-critical application at a low cost. New features for improving detection and adding connectivity enhance the performance but also must not detract from or impede the basic functionality.

Related EE World content

Smoke detector reference design and algorithm is UL217-tested and verified
SmokeBot – A Robot Serving Rescue Units
AI/ML-based sensor fusion technology platform targets home-security alarm system
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Free-space optical links, Part 1: Principles
Free-space optical links, Part 2: Technical issues
Free-space optical links, Part 3: Standard units