1、| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | BRITISH STANDARD BS 7904 : 1998 ICS 13.220
2、.40 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW Guide to Smoke measurement units Their basis and use in smoke opacity test methodsBS 7904 : 1998 This British Standard, having been prepared under the direction of the Health and Environment Sector Board, was published under
3、the authority of the Standards Board and comes into effect on 15 January 1998 BSI 1998 The following BSI references relate to the work on this standard: Committee reference FSH/21 Draft for comment 95/542183 DC ISBN 0 580 28955 9 Amendments issued since publication Amd. No. Date Text affected Commit
4、tees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee FSH/21, Reaction to fire tests, upon which the following bodies were represented: Association of Building Component Manufacturers Association of Roof Light Manufacturers Autoclave
5、d Aerated Concrete Products Association British Carpet Manufacturers Association Ltd. British Electrical Systems Association (BEAMA Ltd.) British Plastics Federation British Railways Board British Resilient Flooring Manufacturers Association British Rigid Urethane Foam Manufacturers Association Brit
6、ish Wood Preserving and Damp-proofing Association Chemical Industries Association Chief and Assistant Chief Fire Officers Association Department of Health (NHS Estates) Department of the Environment (Building Research Establishment) Department of the Environment (Construction Sponsorship Directorate
7、) Department of the Environment for Northern Ireland Eurisol (UK Mineral Wool Association) European Phenolic Foam Association Fibre Cement Manufacturers Association Limited Flat Glass Manufacturers Association GAMBICA (BEAMA Ltd.) Gypsum Products Development Association Home Office Institute of Fire
8、 Safety Institution of Fire Engineers London Fire and Civil Defence Authority Loss Prevention Council National Council of Building Material Producers National GRP Construction and Engineering Federation Polyethylene Foam Insulation Association Queen Mary and Westfield College RAPRA Technology Ltd. S
9、ociety of British Aerospace Companies Limited Specialist Ceilings and Interiors Association Ltd. Thermal Insulation Manufacturers and Suppliers Association (TIMSA) Timber Research and Development Association UK Steel Association Wood Panel Industries Federation Wood Wool Slab Manufacturers Associati
10、on The following bodies were also represented in the drafting of the standard, through subcommittees and panels: Ministry of Defence University of Edinburgh Warrington Fire Research CentreBS 7904 : 1998 BSI 1998 i Contents Page Committees responsible Inside front cover Foreword ii Guide Introduction
11、 1 1 Scope 1 2 Informative references 1 3 Symbols 2 4 Theoretical aspects and units 3 5 Analysis of units used in standard methods 7 6 Further information 9 Annex A (informative) Extinction area, visibility and standard smoke 10 Table A.1 Relationship between volume of standard smoke and values for
12、extinction area S 10 Figures 1 Visibility (v) versus extinction coefficient (k)3 2 The attenuation of light by smoke 4 3 Extinction area 4 4 Dynamic smoke measurement 7 List of references Inside back coverii BSI 1998 BS 7904 : 1998 Foreword This British Standard has been prepared by Technical Commit
13、tee FSH/21. Several British and International Standards have been published relating to measurement of smoke by optical methods. Several apparatus and series of units have been used in these publications. This document is intended to describe the origins of the units used in these standards and any
14、relationships between the units. The recent national work on fire engineering solutions within FSH/24 and the activity within ISO TC/92/SC4 demonstrates the demand for data for use in assessment of fire risk. An integral part of this assessment concerns the propensity of products to produce smoke, a
15、nd this needs to be assessed from data generated in standard smoke tests. The latter consist of different types of tests measuring different parameters and utilizing different units. There is a need for smoke to be measured and assessed in a standard way such that comparisons of the performance of p
16、roducts can be made. This is not always possible and this guide examines the available data sources and the units used to measure the smoke parameters being assessed. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprise
17、s a front cover, an inside front cover, pages i and ii, pages 1 to 10, an inside back cover and a back cover. BSI 1998 1 BS 7904 : 1998 Guide Introduction Smoke which consists of airborne particulates and aerosols reduces visibility because of light absorption and scattering. This has the potential
18、of impeding victims escape from fires because low visibility creates difficulties in locating escape routes and seeing escape signs. Visibility depends on many factors 1, but correlations have been established between visibility distances in smoke and the magnitude of the extinction coefficient of t
19、he smoke (often expressed as the log 10 analogue i.e. optical density per unit light path length) 2. Two relationships determined by Jin are shown in Figure 1 3. The production of smoke and its optical properties may be measured in stand-alone tests or simultaneously with other fire properties. The
20、former tests are usually small scale where the smoke produced is held in the chamber in which it is generated. The latter (with the notable exception of the cone calorimeter) are commonly large scale and are usually dynamic tests where the smoke is ducted away from the zone of generation. Smoke meas
21、urements made in one test may be expressed in the same units as those from another test or they may be related mathematically. Extreme care should be used when comparing results from different tests and the analyst should examine the decomposition models used in the tests as well as the parameters b
22、eing measured before comparing results from different methods. Measurements quoted using the same units may refer to very different parameters which cannot be directly compared. The same parameter measured using the same units in different test methods may show differences in results arising from di
23、fferent smoke propensities of the product or differences in the decomposition methods used in the two tests. An understanding of smoke measurement units is essential when attempting to predict full scale experimental results from small scale tests results. 1 Scope This British Standard describes the
24、 theory of optical measurement of smoke. It describes the different parameters measured, gives examples of the commonly used units, and describes relationships between different parameters. It also reviews some commonly used optical test methods for smoke measurement. 2 Informative references This B
25、ritish Standard refers to other publications that provide information or guidance. Editions of these publications current at the time of issue of this standard are listed on the inside back cover, but reference should be made to the latest editions.2 BSI 1998 BS 7904 : 1998 3 Symbols Symbol Quantity
26、 Typical units A area of sample m 2 A m smoke parameter (=D9) used in BS 7622 dimensionless A o smoke parameter used in BS 7622 m 2 (per unit of cable burned) C mass concentration of smoke particles kgm 23 D linear decadic absorption coefficient (commonly called optical density per metre) m 21 D opt
27、ical density dimensionless D mass mass optical density m 2 kg 21 D max maximum specific optical density dimensionless D s specific optical density dimensionless D sp smoke production rate (base 10) m 2 s 21 D tot total smoke production (base 10) m 2 I o /I ratio of incident light to transmitted ligh
28、t dimensionless k linear Napierian absorption coefficient (commonly called extinction coefficient) m 21 L light path length through smoke m m mass of smoke kg Dm mass of fuel burned kg m mass loss rate (fuel burning rate) kgs 21 S smoke extinction area, (also total smoke) m 2 S smoke production rate
29、 (rate of change of extinction area) m 2 s 21 t time s Dt sampling time interval s V volume of chamber in which smoke is contained m 3 V volume flow rate of smoke m 3 s 21 g the product of visibility and extinction coefficient dimensionless e fraction of fuel mass loss converted to smoke (smoke yiel
30、d) dimensionless s f specific extinction area on a fuel mass loss basis m 2 kg 21 s f,avg average value of s f m 2 kg 21 s s specific extinction area on a smoke mass basis m 2 kg 21 v visibility m NOTE 1. The quantities based on log 10 values, i.e. D, D9, D max , D mass , D s , D sp and D tot , have
31、 similar symbols but are different quantities and have different units. NOTE 2. The use of the term specific in the case of specific optical density, D s , does not denote per unit mass. NOTE 3. Within the text notes identify the recommended units to be used for each smoke parameter. BSI 1998 3 BS 7
32、904 : 1998 4 Theoretical aspects and units 4.1 Visibility Corrrelations have been established between visibility distances in smoke and the magnitude of the extinction coefficient of the smoke (often expressed as the log 10 analogue, i.e. optical density per unit light path length) 1 to 3. The gener
33、al relationship is that the product of visibility and extinction coefficient is a constant, but the value of the constant depends on the contrast and illumination of the target being viewed. Figure 1 shows two relationships established by Jin 3. NOTE. Graphs comparing visibility against extinction c
34、oefficient usually have wide error bands because several different types of light and target are used in the studies. 4.2 Fundamental principles Smoke which consists of an aerosol of particles can be measured as a function of either its gravimetric properties, or its light obscuring properties, or o
35、f a mixture of the two. Gravimetric analysis consists of methods of calculating the mass of smoke suspended in the volume of interest. The obscuring properties of the smoke are a function of the number and size of the particles in the light path. If the particles are considered as opaque, the capaci
36、ty of the smoke to obscure light is related to the sum of the cross-sectional areas of the particles in the light path. This is measured in units of area, e.g. m 2 . Optical smoke measurements are derived from Bouguers law (equation (1) which describes the attenuation of monochromatic light by smoke
37、. I/I o =e 2 kL (1) k = (1/L)l n( I o /I ) (2) NOTE 1. The units of k are reciprocal length e.g. m 21 . where I is the intensity of transmitted light; I o is the intensity of incident light; L is the light path length through the smoke; k is the linear Napierian absorption coefficient (or extinction
38、 coefficient). A useful measurement of the amount of smoke is the total effective cross-sectional area of all the smoke particles. This is known as the extinction area of the smoke, S (see figure 3). The extinction area is related both to the extinction coefficient of the smoke and to the volume tha
39、t the smoke is contained within, by the equation: S = kV (3) where V is the volume of the chamber in which the smoke is contained. 100 1 0.1 10 0.01 0.01 10 1 100 Visibilty in metres (light - reflecting sign) (light - emitting sign) Extinction coefficientin reciprocal metres g = 3 g = 8 v = g 1 k Fi
40、gure 1. Visibility (v) versus extinction coefficient (k)4 BSI 1998 BS 7904 : 1998 Transmitted light, I Incident light, I o Smoke with extinction coefficient = k L Figure 2. The attenuation of light by smoke Beam of light Smoke particles Shadows The extinction area can be thought of as the total area
41、 of the shadows of the smoke particles. Figure 3. Extinction area BSI 1998 5 BS 7904 : 1998 In some studies base 10 logarithms are used to calculate the optical density per unit light path length (D), which is properly named the linear decadic absorption coefficient. NOTE 2. D has units of reciproca
42、l length, e.g. m 21 . I/I o =1 0 2 DL (4) D = (1/L) log 10 (I o /I) (5) k = D ln 10 (6a) or k = 2.303D (6b) The extinction area of smoke (S) can also be calculated from D using the equation: S = 2.303DV (7) Bouguers law was derived using monochromatic light sources but has been applied widely for wh
43、ite (polychromatic) light conditions in smoke studies even though some deviation from it might be expected 1. Several variants of base 10 units can be found in the literature. A dimensionless optical density D9 = log 10 (I o /I) is quoted by several workers 2, 4. For a given amount of smoke D9 is pr
44、oportional to the light path length, and is thus apparatus-dependent; results from one apparatus cannot be directly compared to results from other dissimilar apparatus unless apparatus dimensions are known. Rasbash and Phillips 5 attempted to introduce a unit of light attenuation called the obscura
45、which is 1 dBm 21 . They proposed this because 1 obscura approximated to a visibility of 10 m under conditions of general illumination. This unit has not been used widely and is included here for completeness. In conditions where k = 2.303 m 21 , D=1m 21 or 10 obscura. Ames 6 has reported the amount
46、 of smoke in terms of its equivalence to a volume of standard smoke which has a D value of 1 m 21 (see annex A). If the relationship between visibility (v) and k (or D) is known, then visibility can be readily calculated if the amount of smoke (extinction area) is known and the volume occupied by th
47、e smoke is also known (see annex A). v = g(V/S) (8) where g = vk = 2.303vD. 4.3 Normalization of smoke parameters to mass of smoke produced or mass of fuel burned The extinction coefficient, as defined in equation (1) can also be expressed as: k = s s C (9) where s s is the extinction area per unit
48、mass of smoke, often called specific extinction area on a smoke mass basis; C is the mass concentration of smoke particles. Combining equations (2) and (9): s s C = (1/L) ln(I o /I) (10) but C = m/V where m is the mass of smoke; V is the volume in which the smoke is contained. therefore: s s =V /(Lm
49、) ln(I o /I) (11) NOTE 1. The units of s s are area per mass, e.g. m 2 kg 21 . The relationship between the extinction area and s s is: s s = S/m (12) s s is the effective cross-sectional area of a unit mass of smoke particulates. This value is roughly constant, about 10 000 m 2 kg 21 , for fuels which burn in a flaming mode and produce primarily graphitic carbon soot 7. In many practical applications, the mass of soot is not obtained. In these cases, s f , the specific extinction area based on the mass of the fue
