Enhanced Deposition, Acoustic Agglomeration, and Chladni Figures in Smoke Detectors

A full-scale house fire test was conducted to investigate the accuracy of two proposed methods for determining whether a smoke or carbon monoxide (CO) alarm sounded during a smoke exposure. One method involves examining the plastic case of the alarm's piezoelectric horn for locally enhanced soot deposition and agglomerated soot particles. The other method involves examining the metal disc of the piezoelectric horn for a Chladni figure. Pairs of detectors, each consisting of one detector with its battery properly inserted and the other with its battery disconnected, were placed throughout the house. Each of the properly charged detectors was monitored to determine if and when the detector sounded. It was found that the presence of locally enhanced deposition and soot agglomerates on the main central opening of the plastic horn enclosure of a smoke or CO detector was a strong indicator that the alarm sounded. It was also found that the use of Chladni figures on the piezoelectric discs as an indication of the smoke detector sounding was not an accurate predictor.     

Persistence of Sonic Deposition on Smoke Alarms in Forensic Fire Investigations

Smoke alarms have been shown to develop sonically-deposited regions of acoustically agglomerated soot particles when they sound in smoke-filled air. These sonic depositions can be examined forensically post-fire to determine if the smoke alarm sounded during the incident. However, it is not clear how these sonic depositions are affected by common firefighting and post-fire actions. To determine the effects of post-fire forensic smoke alarm testing and environmental conditions on the persistence of the existing sonic deposition of soot on the horns of a smoke alarm, sixty (60) smoke alarms were subjected to smoke from fires of several different fuel types and common post-fire conditions or actions. Initially, each alarm was exposed to smoke in a small-scale experimental fire to develop sonic deposition around the horn. The fuel types for the fires were smoldering wood, flaming toluene–heptane, smoldering polyurethane foam, flaming polyurethane foam, and a combination of smoldering and flaming polyurethane foam. The alarms were then subjected to four common post-fire actions: pressing the test button, exposure to synthetic canned smoke, exposure to standing water, and exposure to running water. Each detector was visually inspected before and after the post-fire action. Results varied from no soot removed to almost all soot removed depending on the fuel type and post-fire test. An objective evaluation system was used to rank the degree to which soot was removed from the alarm horns: 0 (no soot removed), 1 (some soot removed), and 2 (all soot removed) based on visual inspection. The smoldering wood and smoldering polyurethane foam fires left behind a sticky resin that was essentially unaffected by any of the post-tests. The flaming foam and flaming toluene–heptane fires left powdery soot on the horn which could be easily wiped off. This soot was almost completely washed off by running water (1.067 average degree of removal) while the canned smoke and standing water post-tests removed a significant portion of the soot (0.533 and 1.000 average degrees of removal, respectively), which could lead an investigator to an errant sounding determination. Pressing the test-button appeared to make minimal impact on the amount of soot around the alarms horns regardless of the fuel type (0.067 average degree of removal).     

Experimental Investigation of Acoustic Agglomeration and Sonic Soot Deposition on Smoke Alarms Incorporating Emerging Sounding Technologies

Traditional residential smoke alarms producing a high-frequency T-3 sound have been shown to exhibit sonic deposition of acoustically-agglomerated soot when they sound in the presence of smoke produced in a typical residential fire. The intense acoustic field generated by alarm sounders, which produce these traditional smoke alarm tones, causes colliding soot particles to adhere to one another and eventually settle onto surfaces as their mass increases. The sonic field generated by the horn creates a pulsed flow around the alarm openings that can lead to increased soot deposition in this area. The agglomerated particles then can deposit on the smoke alarm proximate to the horn. The presence of the acoustically-agglomerated soot deposition on alarms has become an accepted forensic technique used to determine if an alarm sounded in the presence of soot. In recent years, new sounding technologies have been introduced for smoke alarms in order to improve their effectiveness in alerting people to fire emergencies who may not respond reliably to traditional smoke alarm tones and patterns. Many researchers have studied the effectiveness of different alarm signals and have found that low-frequency tones and voice messages show increased effectiveness in alerting people, particularly at-risk individuals such as children and the hearing impaired. The new alarms producing low-frequency tones and voice messages generate a far different acoustic field than traditional smoke alarms and require additional understanding of the acoustic agglomeration and sonic soot deposition patterns. A low-frequency alarm sounder, a voice-incorporated alarm using one sounder to generate both the smoke alarm pattern and a voice message, and a voice-incorporated alarm using two sounders to generate the smoke alarm pattern and voice message separately were exposed while sounding to five different smoke sources representative of residential fires. When examined for the presence of enhanced soot deposition, the new alarm sounders solely producing a voice message did not exhibit sonic soot deposition but sounders that produced both the traditional smoke alarm pattern and voice message did exhibit sonic soot deposition. This indicates that the deposition is due to the traditional T-3 alarm sound rather than the voice message. Additionally, low-frequency sounders exhibited sonic soot deposition but only in the presence of the heavy, resin-like tarry deposition from the smoldering polyurethane foam source. As has been found in other studies, this study confirms that the presence of enhanced soot deposition or tarry residue deposition proximate to the horn of a smoke alarm is a reliable indicator that the alarm sounded in the presence of deposition. However, the absence of enhanced soot deposition proximate to the horn of the alarm is not necessarily a reliable indicator that the alarm did not sound in the presence of soot.

     

A Relative Time Analysis of the Performance of Residential Smoke Detection Technologies

A statistical study was conducted to compare the performance of different residential smoke detector technologies when exposed to different fire types. In order to facilitate comparisons between different fire and smoke growth rates, a non-dimensional smoke detector activation relative time was employed. Data from four major experimental studies was analyzed utilizing the relative time approach. The Common Language Effect Size, a measure of the probability that a particular detector technology will be the first to detect a fire of a particular type, was used to assess detector performance. The analysis confirmed previous results that ionization detectors, on average, respond faster to flaming fires, and that photoelectric detectors, on average, respond faster to smoldering fires. More importantly, this study also determined that the responses of ionization, photoelectric, and combination technologies are statistically equivalent for any given future residential fire That is, it cannot be determined with confidence which detector technology will alarm first to the next fire. Additionally, the analysis found that this statistical equivalence between detector technologies has not changed in the last 35 years despite increased fire growth rates associated with changes in furniture materials over that same time span.     

Method of Determining Smoke Detector Spacing in High Ceiling Applications

Simple tools for the design application of smoke detectors in commercial spaces with high ceilings and/or complex geometries do not exist due to the complexity in accurately estimating smoke densities and smoke detector activation characteristics. The response of commercial smoke detectors to UL/ULC standard flaming acceptance test fires and non-standard test fires in large open spaces with ceiling heights varying from 3 m to 21 m with radial distances from the fires up to 11.4 m is measured. Two algebraic models intended for unconfined ceilings, the two-zone computer model CFAST and the computational fluid dynamics computer model FDS are compared against each other and the experimental results. The ability to predict obscuration levels and detection times for these fires is evaluated. Recommendations are made for using the various models in commercial applications.     

Smoke Control in Hospitals

Full scale fire tests have been carried out in order to study the influence of different ventilating principles on the time point of fire detection and the smoke filling of a four-bed room. Using conventional mechanical ventilating systems as smoke exhaust systems the time difference left for evacuation of the fire room can be positively influenced. With the conventional ventilating system operating there is a significant difference between time points of detection of the ionization and optical smoke detectors, for both flaming and smoldering fire. Using the low momentum displacement ventilation this difference is reduced, resulting in possibilities for the ionization smoke detector to be optimized for both flaming and smoldering fires. Reference: Øystein Meland and Eimund Skåret, Smoke Control in Hospitals,Fire Technology, Vol. 22, No. 1, February 1986, p. 33.     

Results from a Full-Scale Smoke Alarm Sensitivity Study

A series of 24 full-scale experiments was conducted to examine the effects of alarm type (photoelectric, ionization, and dual sensor), alarm location, fabric type (100% cotton and 100% polyester), polyurethane foam density, ignition scenario, and room configuration, on smoke alarm performance. A two-level, fractional factorial design of eight experimental configurations was developed around the five factors: fabric type, foam density, fire location, ventilation, and ignition scenario. A structure, designed to represent a single-story home or apartment, was constructed inside the Large Fire Laboratory at the National Institute for Standards and Technology for the experiments. The fire source was a chair mockup consisting of a seat and back cushion of a specific cover fabric and foam density, weighing between 5.5 kg and 8.3 kg. It rested on a metal frame and was subjected to a small propane gas flame, or an electric cartridge heater to initiate smoldering. Each experimental configuration was replicated three times. Smoldering fires were allowed to progress until they naturally transitioned to flaming fires except for one test that was terminated early due to time constraints. The smoldering to flaming transition times ranged from (81 to 182) min. Each fire progressed for a time sufficient to produce multiple hazards (smoke, heat, and toxic gases). All alarms tested were purchased from retail outlets and activated at their preset levels. Photoelectric, ionization, and dual photoelectric/ionization alarms were co-located at multiple locations to facilitate comparisons of each alarm type, and different designs of the same type of alarm. For smoke alarms in the room of fire origin, it was observed that each of the five factors had an effect on the measured alarm times that was primarily a result of fire growth rate (fabric type, foam density, and ignition scenario), or smoke dilution and transport (fire location and ventilation). The photoelectric alarm responded quicker on average than ionization alarm in two of four smoldering fire configurations, responding before the ionization alarm in all 6 trials, while the ionization alarm responded before the photoelectric alarm in two of three trials for the other two configurations. The ionization alarm responded quicker on average than photoelectric alarm in all four flaming fire configurations, and responded before the photoelectric alarm in all 12 flaming fire trials. One dual alarm had the fastest average alarm time for all four smoldering fire configurations, and responded first in 11 of the 12 trials. It also yielded faster average alarm times than the other dual alarm in seven of eight configurations, and was the first dual alarm to respond in 22 out of 23 trials where dual alarms were present.     

Measurements of Smoke Characteristics in HVAC Ducts

The characteristics of smoke traveling in an HVAC duct have been observed along with the response of selected duct smoke detectors. The simulated HVAC system consists of a 9 m long duct, 0.45 m in diameter. An exhaust fan is placed at one end of the duct and is capable of inducing airflow rates that range from 0 to 1.5 m ³/s. The flow is controlled by means of a manual damper. On the upstream end of the duct there is a square exhaust hood approximately 2.2 m at the bottom and 0.3 m at the top. The bottom of the hood is approximately 2.5 m above the floor a shroud extends down to approximately 1.5 m above the floor. The test section, placed immediately downstream of the hood, is 3.5 m long duct with a square cross section of 0.4 m on a side. The instrumentation includes oxygen, carbon monoxide and carbon dioxide gas analyzers and a load cell to determine the energy release rate of the fires tested. The smoke within the duct is characterized by means of a laser light sheet and charge coupled device (CCD) camera, two white light source and photocell ensembles, a Pitot tube and an array of eight thermocouples placed on the vertical plane of symmetry. A smoke detector was placed at the downstream end of the test section. Two types of detectors were tested, ionization and photoelectric, with a single sampling probe geometry. The fires tested cover a wide range of fuels (propane, heptane, toluene, toluene/heptane mixture, shredded paper, polyurethane foam, wood cribs) with the peak energy release rates up to 800 kW. The smoke detector performance, temperature, flow field, smoke particle size and particle distributions are dependent on the fire characteristics and airflow through the duct. The different measurements could be scaled by means of the fire size and airflow rate but left a strong dependency on the fuel and burning characteristics (i.e., smoldering, flaming). The optical density and mass optical density are analyzed as metrics for characterizing smoke and smoke detector response. Detailed comparisons between the different metrics used are presented throughout this work. Clear evidence of stratification and aging of the smoke along the duct are also presented. The limitations of the present configuration and the need for a larger scale study are also discussed.     

Advanced fire detection using multi-signature alarm algorithms

The objective of this work was to assess the feasibility of reducing false alarms while increasing sensitivity through the use of combined conventional smoke detectors with carbon monoxide (CO) sensors. This was accomplished through an experimental program using both real (fire) and nuisance alarm sources. A broad selection of sources was used ranging from smoldering wood and flaming fabric to cooking fumes. Individual sensor outputs and various signal-conditioning schemes involving multiple sensors were explored.The results show that improved fire-detection capabilities can be achieved over standard smoke detectors by combining smoke measurements with CO measurements in specific algorithms. False alarms can be reduced while increasing sensitivity (i.e., decreasing the detection time for real fires). Patented alarm criteria were established using algorithms consisting of the product of smoke obscuration and the change in CO concentration. Alarm algorithms utilizing ionization detector smoke measurements proved to be more effective than measurements from photoelectric detectors.     

Laboratory evaluation of smoke detectors for use in underground mines

Laboratory experiments were conducted to determine the responses of a prototype smoke detector and a commercially available photoelectric smoke detector to smoke particles generated from various combustion sources. The prototype smoke detector combines optical scattering measurements with ionization chamber measurements in order to reduce/eliminate nuisance alarms due to the presence of airborne dusts or diesel exhaust particles. The commercially available smoke detector is designed for use in harsh environments where airborne dust represents a major problem due to both nuisance alarms and detector contamination. In the experiments, the responses of the two detectors were measured when exposed to smoke particles from the exhaust of a diesel engine and from a variety of fire sources, including wood, coal, styrene butadiene rubber, and No. 2 diesel fuel. For the solid fuels, data were obtained for both smoldering and flaming combustions. This report describes the experiments, their results, and the use of these results as they apply to early-warning fire sensors capable of the rapid and reliable detection of fires in atmospheres that may or may not be contaminated by either airborne dust or the products produced from diesel engines.     

Smoke detectors in apartments and one-family houses: Fire risk, property loss and the presence of smoke detectors

This study compared the changes in fire risks and average monetary compensation for property loss before and after the distribution of 21 000 free home smoke detectors from a local insurance company. Life safety and injuries to people were not considered. To control for changes that were not due to the large increase in smoke detector coverage, a second insurance company in the same geographical area was added as a comparison, or control group. Average monetary compensation and fire risks were compared for small, medium-sized and heavy fires. The results showed no general effect of the sharp increase in smoke detector coverage on fire risk and compensation paid by the insurance companies. For the heavy fires, though not for the small and medium-sized fires, there was a significant decrease in compensation paid by the company with the free smoke detector. The reduction in compensation paid was small compared to the yearly costs of the batteries for the smoke detectors. The results were discussed in terms of smoke detectors as a technical device and a self-selection of fire safety conscious persons to the group that install a smoke detector in their homes.     

Fire Test Comparisons of Smoke Detector Response Times

As part of an effort to develop objective smoke detector selection criteria, smoldering smoke and flammable liquid fire tests were performed on photoelectric, multicriteria (ion/heat/photo), and laser-photoelectric smoke detectors. Results showed that, when evaluated at similar sensitivities, the laser-photoelectric and multicriteria detectors had response times close to those of the photoelectric detector.     

Monitoring Multiple Aspects of Fire Signatures for Discriminating Fire Detection

Numerous researchers are exploring multisensor detection as the principal means of discriminating between fire and nuisance sources. Multisensor detectors can monitor multiple aspects of a wide variety of signatures produced by flaming fires, non-flaming fires, and nuisance sources. This paper describes one program of small- and large-scale experiments that has been conducted using a prototype advanced fire detector with multiple gas sensors. An elementary analysis is applied to demonstrate that spacing guidance can be rationally developed for multiple gas sensors to detect fires of a particular threshold fire size, i.e., heat release rate. Discriminating between flaming fires, non-flaming fires, and nuisance sources could be achieved using either a threshold concentration or CO₂ rate-of-rise to identify flaming fires and a CO or CO₂ rate-of-rise for non-flaming fires. Time to detection was also compared to commercial smoke detectors, and the reductions in time were noted.     

Using multivariate statistical methods to detect fires

Fire detectors must accurately detect fires, but they should not respond to false alarms. Contemporary smoke detectors sometimes cannot discriminate between smoke and odor sources. These detectors can also be slow in responding to smoldering fire sources. In this paper, a statistical approach for detecting fires based on fusing sensor signals from multiple sensors is presented. The multivariate statistical approach, called principal component analysis, is used to compress the sensor information down to a small number of variables that can be interpreted more easily than the raw sensor signals themselves. Experimental results presented here show that the proposed approach is more accurate than a conventional smoke alarm, particularly for early detection of smoldering fires. However, this new approach does not overcome the problem of false alarms. In spite of this current limitation, the method discussed holds great promise for future fire detection applications.     

变电站电缆沟火灾探测试验与数值仿真研究

为探讨火灾探测器灵敏度对电缆沟火灾响应的影响程度,验证火灾监控系统的准确性,应用点式温度探测器、点式烟雾探测器、电缆式感温探测器和吸入式烟雾探测器4种火灾探测器,在标准变电站电缆沟防火分区内进行了全尺寸电缆火灾试验,测试不同灵敏度火灾探测器的火灾报警序列。根据电缆沟的实际尺寸,利用FDS搭建模拟明火和阴燃两种不同火灾场景,对不同灵敏度火灾探测器的温度场、有毒气体质量浓度和火灾报警顺序进行了分析和讨论。试验结果表明,在变电站电缆沟火探测中,电缆式感温探测器、吸入式烟雾探测器、点式感烟探测器火灾平均响应时间分别为41.2,111.4,331.8 s;吸入式感烟探测器的灵敏度对探测结果影响较大,其火灾报警响应时间比电缆型线性温度探测器约延迟50%。     

柔性聚氨酯泡沫燃烧产物毒性评估

介绍了对弹性聚氨酯泡沫燃烧产物毒性的相关研究,并对其是否能用于毒性危害分析进行了评估。研究显示,暴露到聚氨酯燃烧产物中的人会导致中毒。窒息气体氮气有效剂量法能有效预测受害人因暴露于聚氨酯燃烧产物而导致的行为能力丧失情况。同时,也证明有效剂量法不能正确预测聚氨酯阴燃时情况。该方法不适用于热分解。研究发现,目前很多经验方法具有很大的局限性,对毒性危害研究造成了很多负面的影响。烟气粒子携带毒物的吸入和传播以及化学合成复杂的毒物,研究和考虑得很少。因此,很有必要对复杂毒性有机体的火灾燃烧产物进行更深入的物理和化学分析研究以及动物模型研究。     

空调通风工况地铁站台初期火灾烟气运动规律

通过在地铁站台进行的火灾实体试验,对车站各种空调通风条件下棉绳阴燃火与聚氨酯明火的烟气速度、温度进行监测与分析,研究地铁中具有格栅镂空吊顶的车站站台在不同空调通风工况下火灾初期烟气运动的规律。多点风速探头和温度记录探头设置在火源正上方以及在距火源水平距离约2 m远的4个位置,分别设置在镂空格栅吊顶的上方和下方。在空调通风工况下,送风对烟气的上升有不同程度的抑制作用,一定程度上延长了火源的燃烧时间。对于阴燃火源,烟气温度的降低导致烟气很难升至吊顶上方。     

Analysis of the Response of Smoke Detectors to Smoldering Fires and Nuisance Sources

Since the initial smoke detector was developed, research has been ongoing to reduce the response times to fire sources and improve the ability of detectors to ignore nuisance sources. Research has been conducted to analyze the signatures from cooking activities, ranging from normal cooking to the flaming ignition of food products with the intent of identifying the precursors to flaming ignition. In particular, the goal of the research is to provide an alarm sufficiently prior to flaming ignition to allow homeowners to take corrective actions to prevent a fire. For the 11 experiments analyzed, the optical density measured in the range hood was the most accurate precursor signal, having the greatest ability to predict the imminent transition to a flaming fire with a minimum of false positives. Though plagued by false positives, the temperature of the heating element and response of an ionization detector provided a faster response than the optical density measure. Given that this research only included 11 experiments, further research should be conducted on a broader range of cooking styles and items being cooked.     

Impact of Air Velocity on the Detection of Fires in Conveyor Belt Haulageways

A series of large-scale experiments were conducted in an above-ground fire gallery using three different types of fire-resistant conveyor belts and four air velocities for each belt. The goal of the experiments was to understand and quantify the effects of air velocity on the detection of fires in underground conveyor belt haulageways and to determine the rates of generation of toxic gases and smoke as a fire progresses through the stages of smoldering coal, flaming coal, and finally a flaming conveyor belt. In the experiments, electrical strip heaters, imbedded approximately 5 cm below the top surface of a large mass of coal rubble, were used to ignite the coal, producing an open flame. The flaming coal mass subsequently ignited 1.83-m-wide conveyor belts located approximately 0.30 m above the coal surface. Gas samples were drawn through an averaging probe for continuous measurement of CO, CO₂, and O₂ as the fire progressed. Approximately 20 m from the fire origin and 0.5 m below the roof of the gallery, two commercially available smoke detectors, a light obscuration meter, and a sampling probe for measurement of total mass concentration of smoke particles were placed. Two video cameras were located upstream of the fire origin and along the gallery at about 14 m and 5 m in order to detect both smoke and flames from the fire. This paper discusses the impact of ventilation airflow on alarm times of the smoke detectors and video cameras, CO levels, smoke optical densities and smoke obscuration, total smoke mass concentrations, and fire heat release rates, examining how these various parameters depend upon air velocity and air quantity, the product of air velocity, and entry cross-section.     

A first approximation method for smoke detector placement based on design fire size,critical velocity,and detector aerosol entry lag time

An analog light-scattering-type smoke detector was tested in a wind tunnel at various low velocities. The air flow in the wind tunnel contained an aerosol concentration that resulted in a high ambient optical density, simulating smoke well above threshold optical detector density. The objective of this research was to determine the lag time to alarm, t, associated with difficulty of smoke entry into a detector. A critical velocity was identified for the smoke detector, below which the lag time increased exponentially with decreasing velocity. Increased lag time results in the detector responding unacceptably late—or not at all—even when ambient obscuration is well above limits defined in UL standard tests.A preliminary method for placing smoke detectors has been developed, based on a user-defined design fire size and the detector aerosol-entry lag time. The preliminary method applies only to flaming fires producing smoke, with the detector far from a wall and mounted on smooth ceilings. The critical velocity value used in the examples in this paper applies only to the smoke detector configuration tested in this work, at the evaluated sensitivity setting, with the optical densities reached using the generated artificial smoke. Any variation in detector housing, design, operation principle, or application with different aerosols requires specific tests to determine a different critical velocity.This paper provides the basis for further development of a smoke detector placement method based on a design fire size and the proposed detector critical velocity concept. It does not presume to report a unique critical velocity for all smoke detectors, but suggests that such a value may indeed exist, but with differing values among different smoke detectors.