Smoke density measurement, optical method

This method is basically the NBS Smoke Density Chamber Test. It covers the determination of the specific optical density of smoke generated by solid materials and assemblies mounted in a vertical position in thickness up to 1 inch (2.54 cm). It measures attenuation of a light beam by smoke accumulating within a closed chamber due to nonflaming pyrolytic decomposition and flaming combustion. 

Several methods have been proposed for the determination of the smoke density. Reproducible values are obtained with the laboratory method worked out by the National Bureau of Standards, whereby a specimen of 7.6 X 7.6 cm is heated by radiation. The smoke density is then measured optically. 

Conventional laboratory methods for testing the smoke formation, such as the NBS chamber, involve an area (A) of specimen, a chamber of fixed volume (V) and a measured optical density (D) over a fixed path length (L). In these measurements, the specific optical density is obtained according to the above relation ... 

The most common optical method for measuring the density of smoke particles is to make use of the Beer-Lambert law that is described by the following equations ... 

Optical methods are favored for measuring smoke density because they provide information relevant to visibility through smoke. Other methods involve weighing particulates and calculating the weight loss on combustion relative to the weight loss ofthe combustible. This method correlates with smoke measurements by optical methods [26-28]. 

Optical methods of measuring smoke density can be carried out under static conditions (e.g. NBS smoke density chamber ASTM E-662-83, see Figure 2), or dynamic conditions, an open system where smoke is... 

Black smoke (BS) is a particulate measure that typically contains at least 50% respirable particulates smaller than 4.5 mm in aerodynamic diameter, sampled by the British smokeshade (BS) method. The reflectance of light is measured by the darkness of the stain caused by particulates on a white filter paper. The result of BS sampling depends on the density of the stain and the optical properties of the particulates. Because the method is based on reflectance from elemental carbon, its use is recommended in areas where coal smoke from domestic fires is the dominant component of ambient particulates. 

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... 

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. 

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... 

ASTM E 906 Heat and Visible-Smoke-Release Rate Test (based on Ohio State University Release Rate Apparatus). In this test the specimen is injected into an environmental chamber through which a constant flow of air passes. A radiant-heat source is used to produce the desired heat flux in the specimen, which way be tested horizontally or vertically. Combustion may be initiated by a number of different methods. The changes in temperature and optical density of the gas leaving the chamber are monitored, and from the resultant data the heat-release rate and visible anoke-release rate are calculated. Toxic-gas release and oxygen consumption rates may also be measured. 

BS 6401 [103] and ASTM E662 [104] are based on the NBS smoke test and are essentially the same test although some differences occur see Fig. 18. A 75 mm square specimen of up to 25 mm thick is combusted in a vertical orientation at 25 kW m incident heat flux. Tests are carried out with and without a scries of pilot flames along the lower edge of the specimen. The smoke is contained in a cabinet of 0.51 m and measured using a vertical photomultiplier lamp system. The results are typically expressed as specific optical density, which relates the optical density of the smoke to the volume of the cabinet, the length of the smoke-measuring path, and the area of the specimen exposed in the test. Other methods have been used in which specific optical density is related to the mass of the specimen combusted and/or to time. 

ISO 5659-2 [108] determines the optical density of smoke generated and measured in a single test chamber. The test cabinet and smoke tneasuring equipment is that of the earlier NBS test defined as ASTM E662 and BS 6401. This ISO test uses a horizontal fire model in which the standard test specimen (75 mm square) is supported on a load cell. The specimen is exposed to radiant heat from a conical radiator positioned above the specimen holder see Fig. 19. Although a range of heat flux values can be used, the standard specifies that tests should be carried out at 25 kW.nr with and without a pilot ignition flame, at 50 kW m without a pilot flame. Calculation of results is by the method defined in the earlier tests. 

BS 6401 (1987). Method of measurement in the laboratory of specific optical density of smoke generated by materials. 

The XP-2 chamber was further developed by the US National Bureau of Standard (NBS). This NBS chamber as standardized in ASTM E 662-1979 has become the most wide-spread test apparatus in the United States. In this method, the light transmittance is again measured for calculating the specific optical density of the smoke generated on the following theoretical basis ... 

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