RECENT DEVELOPMENTS IN BUILDING FIRE DETECTION TECHNOLOGIES

Sensing technologies and their application to detecting fires in buildings continue to evolve, and recent advances are summarised in this article. Work being conducted at NIST on characterising smoke and on bench-scale evaluation of multi-element detectors and their response to emulated nuisance sources are discussed, and a full-scale test program to ascertain the performance of modern residential smoke detectors is described. Development trends are extrapolated and the research needed to move emerging sensing concepts into building applications are suggested. BACKGROUND Prior to the middle of the 20th century, few office buildings and no residences were protected by automatic fire detection systems of any type, and there was no pull from the construction industry to conduct research to change that situation. The impetus for new technology came primarily from the military and aviation sectors. The invention of the cold cathode tube in the 1940s allowed Meili to produce the forerunner of the modern ionisation smoke detector. Fire detectors are now ubiquitous (in North America and elsewhere) protecting constructed facilities of all types including single family dwellings, commercial and retail buildings, industrial installations, government complexes, and transportation systems. Recent surveys indicate that 95% of U.S. homes have at least one smoke detector installed. The ionisation smoke detector has gone from the scientist's bench to the hardware store, with a purchase cost under five U.S. dollars. Multi-sensor detectors are being installed in commercial facilities, high sensitivity aspiration systems are in use protecting critical equipment, and techniques are being Proceedings, 5 AOSFST, Newcastle, Australia, 2001 Editors: M.A. Delichatsios, B.Z. Dlugogorski and E.M. Kennedy 98 incorporated to reduce the incidence of false alarms through more intelligent signal processing and better communication systems. What do emerging technologies promise for the future, and what obstacles need to be overcome to bring the state of fire detection in constructed facilities to the next level? These questions are discussed in this paper. The state of fire sensing technologies The measurable environmental parameters that are affected the most by the presence of a fire are temperature, particulate levels, gaseous species concentrations (CO, CO2, H2O, O2, volatile organic compounds, H2, NO, HCN, HCl, etc.), and electromagnetic radiation. While the universe of options available to sense these parameters is immense, the conditions under which a fire detector must operate substantially constrain the choices of sensing technologies. Cost, reliability, and maintainability take precedence over selectivity, sensitivity, and time response for most situations. Operating principles of fire sensing devices have been reviewed a number of times in the literature, beginning with Custer and Bright in 1974. Bukowski and Jason, Meacham and Grosshandler provided reviews that expanded and updated the topic through the early 1990s. Sensing technologies specifically aimed at cooking and aircraft fire protection were more recently examined by Johnsson and Cleary and Grosshandler, respectively. Pfefferseder discussed a number of techniques useful for sensing a variety of gases produced in a fire environment. Sensors that respond to a fire within a volume, as opposed to at a point, operate on different sets of principles, as described by Willms. The red, green and blue signals from a video camera were used by Kozeki to discriminate flaming fires from other radiant sources. Kozeki extended this work thorough imaging processing techniques to detect smoldering fires, as well. A video-based smoke detection approach, specially designed for tunnel applications, was described by Wieser and Brupacher. A two wavelength, near IR sensor was developed by Lloyd et al. to respond to radiant emission either directly from a small fire or to IR reflected from the walls and ceiling, permitting the system to detect fires not in the line-of-sight or even in the same room. A prototype was constructed and shown to respond properly to the EN54 flaming fires, but to be unable to detect the EN54 smoldering fires unless viewing them directly. A fast infrared array spectrometer was used by Kim et al. to view the entire spectrum between 1.8 μm and 4.9 μm and enhance the detection of smoldering fires. Open-path FTIR methods were examined to determine the trade-offs between the quality of the optics (i.e., price) and the sensitivity of the system to detect different fire and non-fire situations. The rich amount of information in the transformed IR spectrum is sufficient to ascertain a fire from a non-fire, but the investigators have not yet come up with a practical, moderate price detection system suitable for anything other than very specialised industrial applications. A less expensive IR system approach was developed by Goldmeer, who based his sensor on a simple absorption cell using mid-IR LEDs and detectors. Kaiser and Kempka employed microwave radiation at wavelengths between 3 mm and 10 mm (frequencies between 30 GHz and 90 GHz) to increase the penetration of the beam through smoky environments, and demonstrated the ability to detect the smoldering and luminous EN54 fires. Fiber-optic coupled diode lasers have been used to sense low concentrations of combustion products (e.g., CO, CO2, O2, NO, CH4). Current research is aimed at determining the limits of multiplexing signals from hundreds of individual fibers so that a single laser can be used to monitor an entire building. This is necessary to bring the cost within reason. Fiber optics also can be used to directly Proceedings, 5 AOSFST, Newcastle, Australia, 2001 Editors: M.A. Delichatsios, B.Z. Dlugogorski and E.M. Kennedy 99 sense a fire. Through the Raman effect, it is possible to locate a hot region along a long fiber to within a few meters, which is particularly useful when monitoring tunnels (occupied or unoccupied) for fires. Micro-fabrication of tin-oxide arrays is a promising technology for sensing multiple gas species. The technique is described by Harms et al., who have demonstrated the technique's ability to sense a fire early and to discriminate fire products from confounding background species. A number of less conventional mechanisms have been proposed recently for detecting fires. For example, some devices use acoustics to sense the products of a fire. Nebiker developed a photoacoustic detector based upon a microphone coupled to an IR absorbing gas cell; and da-Silva measured the perturbation of an ultrasonic wave as it interacts with particulate matter in a smoke plume. An ion mobility spectrometer was used by Lenkeit to measure chemical products formed during degradation of a flame retarded polymer. Indoor air quality (IAQ) sensing technologies are being looked at to improve the reliability and performance of both fire detection and IAQ control systems. Multiple criteria algorithms that consider combinations of threshold values, rate of rise of signals, and statistical characteristics of signals recently have been employed and have begun to appear in commercial products. Typically, these detectors are certified via the standard test protocols developed for single-sensor detectors, which include full-scale room fire tests in EN54 part 9, and UL 268. The enhanced benefits of multisensor, multi-criteria detectors over single-sensor detectors are not completely established by such tests. Issues of concern New sensor technologies do not necessarily, in themselves, lead to new and useful fire protection systems. The usual hindrances to new technology (high first costs, high maintenance, unproven performance in the field, incompatibility with existing systems) are supplemented by the concerns associated with any life safety system. For technologies which operate on qualitatively different principles than the current state, standard test methods may not be adequate to demonstrate the value of the technology, and existing codes may not permit the benefit of the technology (even if demonstrable) to be captured. A number of issues are being tackled by NIST in the Building and Fire Research Laboratory to better enable the adoption of new fire detection technologies. These include: • better characterisation of products from fires • standardisation of nuisance sources • prediction of the transport of products from the fire to the sensor location • scientifically-based performance assessment Three specific projects that address one or more of these issues are discussed in the following sections. Proceedings, 5 AOSFST, Newcastle, Australia, 2001 Editors: M.A. Delichatsios, B.Z. Dlugogorski and E.M. Kennedy 100 Detector performance assessment The Fire-Emulator/Detector-Evaluator (FE/DE) was developed at NIST to deal with multi-sensor, multi-criteria detectors and combination IAQ/fire sensors. The FE/DE, shown schematically in Figure 1, provides a controlled environment where sensors can be exposed to fire signatures, indoor air pollutants, and nuisance sources. Increasing temperature, changing velocity, and varying particulate and chemical species concentrations at the detector location are some of the variables controlled. The device has been used to evaluate the smoke entry lag effects of detectors at low to moderate flows. Analog output detectors were exposed to square wave changes in smoke concentration at various fixed velocities and fixed ambient temperature. A two parameter model was developed to capture the time lag which was observed to be substantial at low flow velocities (< 0.05 m/s). The exposure of sensors to nuisance aerosol is also underway. Environmental signatures at the sensor location to be emulated in the FE/DE can come from full-scale experiments or numerical simulations of the smoke, gas, and heat transport from a fire or other source. Cleary et al. varied the velocity, temperature, and smoke concen