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For almost 50 years, fire-alarm system designers have used both photoelectric and ionization devices because each ensures reliable operation. Photoelectric detectors were preferred for smoldering fires because they could easily detect specific types of smoke, including burning plastics. Ionization detectors were used to sense clean-burning fires such as paper and wood.


For almost 50 years, fire-alarm system designers have used both photoelectric and ionization devices because each ensures reliable operation. Photoelectric detectors were preferred for smoldering fires because they could easily detect specific types of smoke, including burning plastics. Ionization detectors were used to sense clean-burning fires such as paper and wood. Because a single device could not reliably detect both types of fires, designers were forced to opt for one type of detector over the other. Rather than install both types in one area, the less expensive ionization detector often won out.

Eventually, manufacturers and system designers realized that by combining both elements into a single initiating device, the effectiveness of two individual fire-detection methods was maximized, and thus, the genesis of multi-criteria detection. Working in unison, the two sensing technologies created a device that was not only more reliable, but also more sensitive to a wider range of fire stimuli. Another unintended benefit also arose—the combined input signals improved response speed.

The next logical step was to determine whether adding a thermal heat detector would further enhance the device. In theory, this third element would create a device with faster response times and greater stability for rejecting false alarms. When heat-rise output is recorded into a microprocessor, the smoke-detection element outputs are amplified proportionally through the use of software algorithms. Based on this concept, it was thought that the rise in heat-element output could accelerate the response of the smoke elements. However, due to the low ratio of heat-to-smoke in slow-growing incipient fires, some experts have questioned whether there would be any real response-time improvement. Some smoke models suggest that in an incipient fire, the amount of smoke generated prior to the rise of the room temperature should create a reaction in the smoke-detection elements long before the heat element responds, except in very small enclosures. Therefore, the effectiveness of adding a heat element remains unclear.

Despite this debate, the use of dual smoke-sensing elements can provide a distinct advantage over traditional single-detection devices for slow-growing incipient fires. While smoke in the early stages of a fire may not be of significant enough concentration or obscuration to set off one sensing element, the output from the combined signals of two detection elements forms a single, more stable signal. This means a positive reaction to smoke can be reliably obtained earlier in the fire timeline, even without a heat element.

Historically, attempts to increase sensitivity in detectors have resulted in a substantial increase of nuisance alarms. While combining two smoke-sensing technologies into a single detection unit created a superior device, it was still possible for both technologies to react jointly to a single false stimulus, such as cigarette smoke—one of the biggest causes of nuisance alarms. This results from each detection element sensing the products of combustion in a single puff of smoke and reacting similarly to the same source.

Distance the key

Adding one more key element to the mix—distance—corrected this problem. A cooperative multi-sensing system identifies a fire as quickly as the best multi-criteria sensors, yet it is less susceptible to nuisance alarms because of the distance between the individual sensors in the "group." For example, a mix of standard photoelectric and ionization sensors is installed according to the spacing specified by fire codes to create a multi-sensing system. This multi-sensor approach will analyze the smoke levels of all devices over the entire protected area. Multi-criteria technologies are still used, but instead of being housed in a single enclosure, the signals from adjacent devices are used as inputs, creating a more intelligent alarm system. This design makes it very difficult, if not impossible, for a single puff of nuisance smoke to be reported by two sensors simultaneously. In addition, by combining values of adjacent sensors, the fire-alarm panel should respond faster to uniformly distributed smoke than any other technology.

Based on the identification of a fourth fire signature called uniform smoke spread, cooperative multi-sensor detection incorporates two distinct principles: First, smoke, by virtue of the laws of physics, acts like a gas, dispersing uniformly; second, sampling smoke over a wider area is more reliable than spot sampling.

Using uniform smoke spread as its foundation, cooperative multi-sensor technology is recognized as a highly effective approach to fire detection. A fire-alarm panel's software can analyze the input from three or more separate standard sensors to create a more reliable and often faster signal.

Each sensor measures the ambient smoke conditions at its location and communicates the values to the main fire-alarm panel. The panel views several sensor values simultaneously to accurately identify whether a true fire condition exists. This is not detector voting, cross-zoning or double-knock technology. This new technology actually uses statistical analysis of data collected from several different points—again, employing the element of distance—to identify whether a true fire condition exists. Whereas older technologies require more than one sensor to be at 100% of their alarm sensitivity levels in order to read true fire conditions, this information can now be ascertained before any individual sensor approaches its alarm threshold.

Uniform smoke spread

With cooperative multi-sensing and uniform smoke spread, the combined alarm threshold can be as low as 58% sensitivity at each device. The algorithm in the fire-alarm panel software uses values from several different sensors in a group to generate a distinctly new signal. Some panels can accommodate as many as 1,000 such analyses while checking for uniform smoke spread.

This type of system permits the creation of very sophisticated methods for detecting fire schemes that are more complex than one could create with traditional multi-criteria sensors. For example, consider four physical sensors spaced and reporting normally in a room. Theoretically, the fire-alarm panel could be programmed to run a combination of several different data interrogations from those devices to search for incipient signs of fire. This is a significant breakthrough that can lead directly to more reliable, faster fire detection. In addition, by adjusting the alarm thresholds for the individual sensors, manufacturers can lower the overall susceptibility of their equipment to nuisance alarms while maintaining a faster response. Since the devices are single-technology sensors, they still respond individually—at the normal rate of response—to a fire alarm situation when smoke spread is restricted. In other words. any device reaching the designated alarm threshold will still initiate an alarm.

Cooperative multi-sensor detection is ideally suited for many environmental situations. For example, a hotel elevator lobby often presents a significant fire detection problem related to single-source transient smoke such as cigarette smoke. Through the use of selectable sensitivity settings and this technology, the protection around the elevator can be designed to be less susceptible to nuisance smoke, but still capable of responding rapidly to uniform smoke distribution.

Ultimately, cooperative multi-sensor technology offers three advantages:

  • Faster reaction to uniform smoke distribution, a reliable signature of real fires.

  • More intelligent alarm decisions that can be less susceptible to nuisance alarms, thanks to the dimension of distance.

  • The capability to combine mixed-sensor technology with single-technology sensors so as not to increase the price of the system.

A vote for sensors

So which is better—detectors or sensors? Detectors include the electronics to make the fire/no-fire decision in the smoke head and report the fire decision to the alarm panel. Sensors, on the other hand, do not make the fire/no-fire decision in the smoke head. Instead, they report an ambient value to the fire-alarm panel where the decision is made. Detectors are not capable of providing cooperative multi-sensing, because they process and make the fire decision at the initiating device level, not at the system level. Each detector makes decisions independent from other detectors in the immediate area. Only systems employing the use of multiple types of fire sensors, each reporting values to the control panel for processing, can achieve the advantages offered through cooperative, multi-sensing technologies.

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