Measurement of smoke obscuration

The Cone calorimeter yields smoke results which have been shown to correlate with those from full scale fires [10, 15-18]. The concept of a combined heat and smoke release measurement variable for small scale tests has been put into mathematical terms for the cone calorimeter smoke parameter (SmkPar) [10]. It is the product of the maximum rate of heat release and the average specific extinction area (a measure of smoke obscuration). The correlation between this smoke parameter and the smoke obscuration in full scale tests has been found to be excellent [10]. The corresponding equation is ... 

ASTM E 1995 Standard Test Method for Measurement of Smoke Obscuration Using a Conical Radiant Source in a Single Closed Chamber, With the Test Specimen Oriented Horizontally. ASTM International, West Conshohocken, PA. 

Specific extinction area The measure of smoke obscuration averaged over the whole test period m 7kg... 

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 ... 

This secondary effect of materials is illustrated by the difficulties encountered, in a recent study [54], when attempts were made to correlate CO concentrations measured in small scale and full scale fire tests. The same small scale equipment (typically the cone calorimeter rate of heat release test) could predict adequately a number of very important full scale fire properties, including ignitability, rate of heat release, amount of heat release and smoke obscuration. It could not, however, be used to... 

The traditional way in which smoke obscuration has been measured is by determining the maximum smoke density (or the specific maximum smoke density) by means of a smoke density chamber developed by the National Bureau of Standards (NBS smoke chamber, ASTM E662). This instrument measures the obscuration inside a static 500 L chamber, after a sample has been exposed, vertically, to a 2.5 W/cm2 radiant source. 

The measurement of screening performance of smokes is important because smoke screens are one of the countermeasures for IR surveillance systems. The performance of smoke formulations is decided in terms of total obscuring power (TOP), yield factor (Y), mass extinction coefficient (a) followed by calculation of obscuration effectiveness (a. Y. p). These parameters are defined in the following manner. 

The quality of a smoke produced by a smoke formulation can be measured in terms of its TOP under standard laboratory conditions. The TOP of a smoke is obtained by dividing the volume of smoke (in cubic meters) produced per kilogram of material by the standard smoke (smoke layer [in meters] necessary to obscure the filament of a 40-watt lamp) and is expressed by Equation 5.27 ... 

Despite the understanding that smoke obscuration ought to be measured in a large scale, or by a method which can predict large-scale smoke release, the most common small-scale test method for measuring smoke from burning products is the traditional smoke chamber in the vertical mode (ASTM E 662)39 (Figure 21.14). The test results are expressed in terms of a quantity called specific optical density, which is defined in the test standard. This test has now been shown to have some serious deficiencies. The most important problem is misrepresentation of the smoke... 

Hirschler, M.M., Smoke in fires Obscuration and toxicity, Plenary Fecture, Business Communications Company Conference on Recent Advances in Flame Retardancy of Polymeric Materials, May 15-17, Stamford, CT, Eds. G.S. Kirshenbaum and M. Lewin, pp. 70-82, Norwalk, CT, 1990 Hirschler, M.M., How to measure smoke obscuration in a manner relevant to fire hazard assessment Use of heat release calorimetry test equipment, J. Fire Sci., 9, 183-222 (1991). 

Hirschler, M.M. and Janssens, M.L., Smoke obscuration measurements in the NFPA 265 Room-Comer Test, Proceedings of the 6th Fire and Materials Conference, February 22-23, Ed. S.J. Grayson, San Antonio, TX Interscience Commun., London, U.K., pp. 179-198, 1999. 

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. 

The ASTM test requires to report smoke obscuration as the average specific extinction area (m /kg) for each specimen. The average specific extinction area (<7, m / kg) is calcnlated as the volume exhaust flow rate (V, mVs), measured at the location of the laser photometer, multiplied by the smoke extinction coefficient k, m ) and by the sampling time interval (At, s), divided by the specimen mass loss (Am, kg), and averaged for repeated tests. 

Since the purpose of signaling smokes is quite different from that of screening smokes, the formerly-described test method designed to measure obscuring power is probably not, or only conditionally, applicable to colored smokes. On the other hand, an accurate measurement of color is possible, as evident from a U. S. patent for a smoke colorimeter. 

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

Smoke detectors are particularly useful in those situations where the fire is likely to generate a substantial amount of smoke before temperature changes are sufficient to actuate a heat-detection system and before a fire eye will detect a flame. Smoke detectors use a photoelectric beam between a receiving element and hght source. If smoke obscures the beam, an alarm is sounded. There are also refraction-type models that measure the light changes that occur within the instrument when smoke particles enter it. 

Two approaches are available for estimating the surface emitted power the point source and solid plume radiation models. The point source is based on the total combustion energy release rate while the solid plume radiation model uses measured thermal fluxes from pool fires of various materials (compiled in TNO, 1979). Both these methods include smoke absorption of radiated energy (that process converts radiation into convection). Typical measured surface emitted fluxes from pool fires arc given by Raj (1977), Mudan (1984), and Considine (1984). LPG and LNG fires radiate up to 250 kW/m (79,000 Btu/hr-ft ). Upper values for other hydrocarbon pool fires lie in the range 110-170 kW/m (35,000-54,000 Btu/hr- ), but smoke obscuration often reduces this to 20-60 kW/m ( 6300-19,000 Btu/hr-ft ). 

Blake wrote his report immediately after emerging from the mine. His viewpoint is limited. Smoke obscured his vision. He measured distance from moment to moment as he headed into the air flow and observed patterns of smoke coming around a comer. Stunned and shocked by the events he describes, he does not provide a coherent account of the origins of the fire or the technical problems that contributed to the deaths of 27 miners. 

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