Super Sensors Help Save Lives

This question led to NIST in 2004 publishing its research project, Performance of Home Smoke Alarms Analysis of the Response of Several Available Technologies in Residential Fire Settings, also known as Dunes II. The project was similar to Dunes I, except the furnishings were constructed of contemporary materials and more advanced instrumentation was installed to monitor and record data. Dunes II measured and analyzed smoke and toxic gas development to evaluate the current state of residential smoke detector requirements, and compared resulting escape times in the modern environment. Dunes I data served as a baseline for comparison.

The results obtained were similar to those found in Dunes I. In essence, when properly installed and maintained, current smoke detection technologies still provide enough escape time in most fire scenarios. The results also indicated that synthetic building materials burn significantly faster and hotter than those used during the testing in the 1970s. This factor greatly reduces escape times. The average times for reaching untenable conditions for flaming and smoldering furniture fires were 17-percent and 47-percent less, respectively, than those found in Dunes I.

Another interesting discovery was that the smoke obscuration sensitivity levels of the smoke detectors in both Dunes tests were statistically the same. The smoke obscuration limit was about 1.5 ±0.4 percent in Dunes I compared to 1.9 ±0.7 percent in Dunes II.

To further understand the burning characteristics of new building and furnishing materials, UL, under contract to the National Fire Protection Association-affiliated Fire Protection Research Foundation (FPRF), conducted its own smoke characterization study. Here, researchers burned synthetic materials, collected the “products of combustion,” analyzed and quantified the collected material, and developed profiles of smoke and gases emitted by the burning materials. The result of the study was an absolute characterization and relative quantification of smoke and various gases emitted by burning materials used in modern residential settings.

This smoke characterization project brings our knowledge of the chemical and physical properties of products of combustion, not just smoke, to a new level of sophistication. We now have a fingerprint for fire – a unique signature for each material being burned.

Calls for Change to Some Smokes

New information from the Smoke Characterization and NIST Dunes II projects are changing the way the fire community perceives smoke produced in fires. This enhanced information is already leading to the development of new detection technology that will further reduce the risk of injury or loss of life due to fires.

However, there are also concerns about some of the current smoke detector technologies, as well as a call for change by some members of the fire services and other interested groups, based on the significant reduction of escape time related to synthetic materials. One such call was an early proposal by a UL task group to modify the smoke obscuration limit from the current 0.5 percent sensitivity maximum and 10 percent minimum to 2-4 percent. The goal of this
change would be to limit the maximum sensitivity to reduce the incidence of false alarms and assure that detectors are still sensitive enough to detect a fire.

Such a change, if adopted, would have placed the maximum sensitivity right at the average level recorded during the Dunes I and II projects. The Dunes testing concludes that sensitivity in the range of 1-2 percent would be better, but this questions whether a simple sensitivity shift is the correct response to reduced escape times. Further consideration by the UL task group has resulted in a revised sensitivity limit proposal of 1 percent to 7 percent. 

Next Generation Technology in Use

Fire detection remains within two boundaries: It must be sensitive enough to detect a fire at its earliest stage, yet not overly sensitive to cause nuisance alarms.

However, the smoke characterization data from the FPRF/UL study has opened a new chapter for fire detection technology. For instance, recently developed sensor technology has the capability to recognize the fingerprint nature of fire identified by the study.

Thus, the technology is available today to produce fire detectors with multicriteria sensors that base the alarm decision on multiple factors, eliminating the need to balance sensitivity against potential nuisance alarms.

A detector need no longer be limited to photoelectric or ionization; rather, it could encompass multiple types of sensors. Next, incorporating a microprocessor with profiles of each fire signature identified during the smoke characterization research would empower the detector with high sensitivity without raising the incidence of false alarms by unequivocally differentiating the fire fingerprint.

Today’s multicriteria detectors, whether conventional or addressable, use signal processing embedded in the detector head to enable an alarm signal only if the composite output of the individual sensors justifies the decision. Some multicriteria detectors combine as many as four independent sensors, such as a carbon monoxide sensor, a photoelectric smoke sensor, a temperature sensor and an infrared light sensor – all managed by an embedded microprocessor running a set of sophisticated and responsive algorithms in one low-profile housing.

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