Obscuration by smokes

The time to reach a critical smoke density, also called the obscuration time, is a measure of the time available before a typical occupant in a typical room would find his vision obscured by smoke sufficiently to hinder escape. The value of specific optical density describing this critical level is 16, based on 16 per cent light transmittance under certain specific conditions of room dimensions. 

A white-on-red EXIT sign is illuminated on the rear wall of the chamber. Its obscuration by smoke accumulation during the test procedure is recorded. Starting from the flame application, the percent light absorbance is recorded at 15-s intervals or plotted by a recorder for 4 min (Fig. 4.3). 

Light obscuration by smoke (specific optical density )... 

Table 11.8 ASTM SmaU-Scale Test Apparatuses for the Measurements of Release Rates of Heat and Fire Products and Light Obscuration by Smoke...

In the test, measurements are made for the percent light obscuration by smoke flowing through the exhaust duct, gas temperature (7.0-m/23-ft from the burner) and location of the leading edge of the flame (visual measurement) as functions of time. The measured data are used to calculate the flame spread index (FSI) and smoke developed index (SDI) from the flame spread distance-time and percent light absorption-time areas, respectively. Some typical FSI values are listed in Table 53.16 taken from Ref. 46. 

A hood attached to a sampling duct is used to capture heat and chemical compounds that are released during the test. In the sampling duct measurements are made for gas temperature, concentrations of chemical compounds released in the fire and oxygen, light obscuration by smoke, total flow of the mixture of air and chemical compounds and heat flux values at various locations in the room. Two parameters are used for ranking the products [47,48,58] ... 

Smoke release from the light obscuration by smoke in the sampling duct ... 

Most fires give off light but often this is obscured by smoke. The light comes from the candescence of carbon particles and the less yellow the flame the higher its temperature. Some fires give off very little or no light at... 

One particularly widely used test is the National Bureau of Standards (NBS) smoke chamber test. This provides a measure of the obscuration of visible light by smoke in units of specific optical density. The NBS smoke test can be run in either of two modes ... 

Smoke detection depending upon absorption of ionizing radiation by smoke particles light scattering by smoke particles light obscuration. 

Fire safety in a particular scenario is improved by decreasing the corresponding level of fire risk or of fire hazard. Technical studies will, more commonly, address fire hazard assessment. Fire hazard is the result of a combination of several fire properties, including ignitability, flammability, flame spread, amount of heat released, rate of heat release, smoke obscuration and smoke toxicity. 

In addition to phosphorus-based obscurant smokes for self-protection there are also colored smoke compositions for signaling purposes, often in a military context. It can be produced by smoke grenades, or by various other pyrotechnical devices. 

The effectiveness of screening smokes depends on obscuration by particles that reflect, refract, and scatter light rays. The effectiveness of a smoke increases with its atmospheric concentration, as shown in Table 18.1. 

Photoelectric fire sensors that detect changes in infrared energy radiated by smoke or by the smoke particles obscuring the photoelectric beam. A relay closes to complete the alarm circuit when smoke interferes with the intensity of the photoelectric beam. 

Laser detectors operate in a similar way to optical beam detectors. The laser beam is directed across the area to be protected and is deflected or obscured by either the heat, the flame or the smoke rising from a fire, thus changing the intensity of lighf at the receiver. This triggers the alarm. 

Smoke detectors are employed where the type of fire anticipated and equipment protection needs a faster response time than heat detectors can provide. A smoke detector will detect the generation of the invisible and visible products of combustion before temperature changes are sufficient to activate heat detectors. The ability of a smoke detector to sense a fire is dependent on the rise, spread, rate of burn, coagulation, and air movement of the smoke itself. Where the safety of personnel is a concern, it is crucial to detect a fire incident at its early stages because of the toxic gases, lack of oxygen that may occur, and obscuration of escape routes by smoke. Smoke detection should be considered when these factors are present. 

Fire risks and fire hazards are mainly the result of the combination of factors, inclnding ignitability, ease of extinction, flammability of volatiles generated, amonnt of heat released on burning, rate of heat release, flame spread, smoke obscuration, and smoke toxicity. The most important fire risks and Are hazards are the rates of heat release, smoke production, and toxic gas release. An early high rate of heat release causes fast ignition and flame spread, controls a fire s intensity, and is mnch more important than ignitability, smoke toxicity, or flame spread. The time for people to escape a Are is also controlled by the heat release rate. ... 

Figure 11.6 illustrates the fusion of a thermal and a visible image from a smoke trial to simulate battlefield conditions. The scene in the visible image (a) is obscured by the smoke. The thermal image (b) shows thermal events clearly but hides some of the visible details. The fused image (c) reveals both visible and thermal details with true registration. The fusion algorithm is proprietary to Waterfall Solutions. 

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