Data has shown that while flaming fires are not as common as smoldering fires in residential occupancies, they still comprise a large percentage of fire events. Flaming fires also tend to produce untenable conditions more quickly than smoldering fires, and ionization detectors typically perform better than other detection technologies in flaming fire situations.

Despite ion’s flaming fire prowess, there has been a distinct move in Europe and the United States during the past 15 years toward photoelectric detectors. This has been due to environmental concerns and nuisance alarm issues related to ion technologies.

Improving Photoelectric Detection’s Flaming Fire Performance

Now that the use of photoelectric sensing technology is increasing, the question is: how to create a photoelectric smoke detector that can rival the flaming fire performance of ion detectors?

The earliest attempt was the combination photoelectric-heat detector. However, these devices typically did not provide additional egress time because the heat sensor had a set point of 135 degrees Fahrenheit. Considering the thermal lag of these devices, the alarm triggered after the fire was fully developed.

In recent years, as microprocessor technology has become more cost effective, there have been efforts to make the photo-heat combination smarter by looking for rates-of-rise and other trends. The industry has, however, probably reached the limit of what can be done with photoelectric and heat sensors alone because of the limited amount of heat generated in an incipient flaming fire.

Currently, the three most promising areas in detection research are:

• Enhanced photoelectric sensing chambers

• Adding gas sensing to a photoelectric sensor

• Adding radiant energy sensing to a photoelectric sensor

Enhanced Photoelectric Sensing Chambers

One active area of research involves modifying the photoelectric sensing chamber to enhance its sensitivity to a wide range of particle sizes from different fire types. Various researchers are attempting to exploit well-known particle scattering phenomena by using multiple colors (wavelengths) of light and multiple orientations of the emitter to the sensor. Particle scattering theory demonstrates that by using shorter wavelengths of light (such as blue instead of near infrared), one obtains greater scattering from smaller particles. This, in turn, improves sensitivity to flaming fire combustion products. The reduced cost of blue LEDs is making this approach more feasible.

Similarly, one can gather more information about a particle by designing the chamber to examine how the particle scatters light, both forward and backward. Finally, combining multiple directions and wavelengths can produce better characterization of the particulate and a more accurate detection scheme.

Photoelectric and Gas Sensing

Several leading detector manufacturers have successfully combined gas sensors with photoelectric sensing chambers to increase sensitivity while lowering the probability of nuisance alarms. While most of the work to date has been with carbon monoxide sensors, the Fire Protection Research Foundation Smoke Characterization Study has yielded a wealth of information on a broad spectrum of gases that are produced as materials burn that can serve as a marker for early fire detection. This information, coupled with improved electrochemical and solid state gas sensors, is producing promising breakthroughs in fire detection using combination photo-gas detectors. 

Photoelectric and Radiant Energy

Finally, radiant energy sensors can be added to detectors to enhance their sensing ability by viewing the protected space. These sensors can pick up the infrared or visible flame signatures and combine that information with the photoelectric sensor to produce a faster and more accurate alarm decision. This technique can be deployed in isolation or combined with one or several of the approaches discussed previously.

Tags: Multi-criteria Detection

Posted in Commercial, Cover Features, Intelligent Detection, Winter 2009

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