Smoke optical density

Limits on emissions are both subjective and objective. Subjective limits are based on the visual appearance or smell of an emission. Objective limits are based on physical or chemical measurement of the emission. The most common form of subjective limit is that which regulates the optical density of a stack plume, measured by comparison with a Ringelmann chart (Fig. 25-1). This form of chart has been in use for over 90 years and is widely accepted for grading the blackness of black or gray smoke emissions. Within the past four decades, it has been used as the basis for "equivalent opacity" regulations for grading the optical density of emissions of colors other than black or gray. 

The original Ringelmann chart was a reflectance chart the observer viewed light reflected from the chart. More recently, light transmittance charts have been developed for both black (1) and white (2) gradations of optical density which correlate with the Ringelmarm chart scale. It is now common practice in the United States to send air pollution inspectors to a "smoke school" where they are trained and certified as being able to read the density of black and white plumes with an accuracy that is acceptable for court testimony. 

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 ... 

FIRE SIMULATOR predicts the effects of fire growth in a 1-room, 2-vent compartment with sprinkler and detector. It predicts temperature and smoke properties (Oj/CO/COj concentrations and optical densities), heat transfer through room walls and ceilings, sprinkler/heat and smoke detector activation time, heating history of sprinkler/heat detector links, smoke detector response, sprinkler activation, ceiling jet temperature and velocity history (at specified radius from the flre i, sprinkler suppression rate of fire, time to flashover, post-flashover burning rates and duration, doors and windows which open and close, forced ventilation, post-flashover ventilation-limited combustion, lower flammability limit, smoke emissivity, and generation rates of CO/CO, pro iri i post-flashover. 

Smoke density. Optical density measurements on the smoke evolved from burning plastic samples were carried out using an NBS Smoke Chamber. The samples, which measured 75mm x 75mm, with a thickness of 0.6 - 4mm, were burned in the flaming mode in accordance with ASTM E662-79. Specific smoke density (Ds) values reported are the averages of three independent determinations. 

Table XI presents the results of tests on the same materials in the NBS smoke chamber. It is immediately clear that these results do not correlate well with those measured on the RHR apparatuses. Furthermore, an attempt at a linear correlation between the flaming mode specific maximum optical density and the Cone calorimeter SmkPar at 20 kW/m2 yielded a correlation coefficient of ca. 1%, a coefficient of variation of 217% and statistically invalid correlations. A comparison between a Cone and OSU calorimeter correlation and one with the NBS smoke chamber is shown in Figure 4. This suggests that unrelated properties are being measured.

Table XI. Maximum Optical Density in the NBS Smoke Chamber...

Generation rate of smoke can be quantified by measuring the mass of smoke and/or the optical density of smoke, D, defined as ... 

Mass Optical Density of Smoke from NBS Smoke Chamber. Figure 5 shows the data for the mass optical density measured in the NBS smoke chamber for the flaming and nonflaming fires under static conditions. For MTL 1 and 2 samples, the MOD values in flaming fires are lower than in the nonflaming fires. For the other three samples, for which the MOD values are smaller, the trend is reversed. 

Figure 5. Mass Optical Density of Smoke Measured in the NBS Smoke Chamber for the Fiber Reinforced Composite Materials...

Figure 6. Correlation Between the Mass Optical Density Data for Fiber Reinforced Composite Materials Measured in the FMRC Small-Scale Flammability Apparatus for Flaming Fires and the NBS Smoke Chamber for Flaming and Non-Flaming Fires...

Specific Optical Density of Smoke Generated by Solid Materials [NIST (NBS) Smoke Chamber], 1993. 

Smoke opacity is another increasingly important characteristic measured by optical density. Fire-retardant additives can be halogenated or halogen-free, which reduces the corrosivity, toxicity and pollution risks. 

ISO 5659-1 1996 Plastics - Smoke generation - Part 1 Guidance on optical-density testing ISO 5659-2 1997 Plastics - Smoke generation - Part 2 Determination of optical density by a single-chamber test... 

These are similar to nephelometers, except that they measure the attenuation of a light beam due to the combined effects of absorption and scattering by the sample. The instrument consists of a light source, a collimator and a photo-detector. The most common application is the measurement of smoke density in chimney stacks. In this case the optical surfaces exposed to the smoke are kept clean by flows of clean air. The density of the smoke is expressed in terms of per cent opacity, per cent transmittance or optical density, where ... 

Indeed, radical trapping in the gas phase performed by HX is bound to increase production of CO which would otherwise be oxidized by OH radicals. Furthermore, restriction of oxidation increases the amount of nonoxidized products which may condense into droplets or particles when they leave the flame, increasing the optical density of the smoke. Finally HX and metal halides are highly corrosive. The ensuing threat to people, structures, and goods involved in the fire may discourage the use of these fire retardants in spite of their high effectiveness and versatility which... 

The NBS smoke chamber is the most commonly used bench-scale test apparatus for measuring the optical density of smoke. The apparatus and test procedure are described in ASTM E 662. The method was developed at the NBS in the 1960s.69... 

Smoke density is measured based on the attenuation of a light beam by the smoke accumulating in the closed chamber. A white light source is located at the bottom of the enclosure, and a photomultiplier tube is mounted at the top. A modified version of Equation 14.19 is used to calculate the specific optical density from the measured transmittance ... 

A test is terminated 3 min after the light transmittance value reaches a minimum value or 20 min after the start of the test, whichever occurs first. Ds calculated according to Equation 14.23 is reported as a function of time. In addition, the maximum value of the specific smoke density, Dm, is reported separately after correction for soot deposits on the windows of the optical system during the test. The correction is equal to the apparent optical density that is recorded after the test is terminated and the smoke has been cleared from the chamber. 

The smoke chamber method described in ASTM E 662 is often supplemented with toxic gas analysis. A PTFE-lined stainless steel tube is used to take a gas sample from the geometric center of the chamber at a specified time. This time can be fixed, for example V/2 or 4 min into the test, or variable, for example immediately following the maximum specific optical density. Regulations and specifications that call for these measurements require the concentration of a predefined set of gases to be determined. The product is acceptable if the concentration of every gas is within specified limits. These limits have been established from experience based on data for products that are deemed to be acceptable or not acceptable. 

ISO 5659-2 Plastics—Smoke Generation—Part 2 Determination of Optical Density by a Single-Chamber Test. International Organization for Standardization, Geneva, Switzerland. 

ISO 5659-2 Plastics—Smoke Generation—Part 2, Determination of Specific Optical Density. 

Chapter 6 Smoke test. The test is based on the National Bureau of Standards (NBS, now NIST, National Institute of Standards and Technology) smoke density chamber, which has also been standardized as ASTM E 66239 (see also Section 4.1.1). The test exposes a vertical test specimen ca. 75 mm x 75 mm (3 in. x 3 in.) to an incident radiant heat flux of 25kW/m2, from a radiant heat burner for 4 min, in the presence of an open-flame pilot burner. The test applies to the same materials as the heat release rate test. The acceptance criterion is an average maximum specific optical density of smoke that does not exceed 200 (no units). 

Tray cable, listed via UL 1581-1160 (UL tray), plus low specific optical density of smoke via ASTM E 662 (specific optical density of smoke at 4min into the test that does not exceed... 

Chapter 7 is the chapter dealing with Special Conditions and it addresses most of the cables with highly improved fire performance. Thus, Articles 725 (Class 1, Class 2, and Class 3 Remote-Control, Signaling, and Power-Limited Circuits), 760 (Fire Alarm Systems), and 770 (Optical Fiber Cables and Raceways) all use the same two schemes for fire performance of cables, as shown in Figures 21.4 and 21.5. The figures show that the best is NFPA 262,65 a cable fire test for flame spread and smoke, conducted in a modified Steiner tunnel (86 kW or 294,000 BTU/h), for which the requirements in the NEC are that the maximum peak optical density should not exceed 0.5, the maximum average optical density should not exceed 0.15, and the maximum allowable flame travel distance should not exceed 1.52m (5 ft). The next test, in the order of decreasing severity is UL 1666,64 known... 

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... 

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