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Correlation of gravimetric smoke measurement by the arapahoe chamber with optical smoke measurement by the NBS chamber for flaming combustion
Fire Research, 1 ( 1 9 7 7 / 7 8 ) 135 - 142 © Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
135
Correlation of Gravimetric Smoke Measurement by the Arapahoe Chamber with Optical Smoke Measurement by the NBS Chamber for Flaming Combustion
S. S. O U and J. D. S E A D E R
Flammability Research Center and Department of Chemical Engineering, Universityof Utah, SaltLake City,
Utah (U.S.A.)
SUMMARY
The relationship between the optical density and the particulate mass concentration of fire smoke is further developed. A correlation is obtained for relating gravimetric determinations made with the Arapahoe Smoke Chamber to light transmission measurements made with the NBS chamber. The correlation is particularly good for plastics, but less satisfactory for wood. Possible explanations for the latter deficiency are proposed.
INTRODUCTION
Fire statistics [1] indicate that smoke is the most important single cause of fire fatalities. For this reason, considerable attention is being given to the development of tests and procedures for evaluating the physical, chemical, toxicological, and physiological aspects of fire smokes [2]. The physical aspects of smoke are generally characterized by opacity to light, particulate mass or concentration, and particulate properties such as size distribution and density. The most widely used apparatus for characterizing fire smoke is the NBS-Aminco smoke density chamber [3]. With this device, measurements are made of the attenuation of a collimated light beam due to scattering and absorption by the particulate matter in the smoke, which is confined to the chamber during a test. Although a number of smoke parameters can be developed from data obtained with the NBS-Aminco chamber, the most useful is the maximum specific optical density, Din, defined by the relation:
v
(1oo I
D m =- A L loglo \ Tm!
(1)
where: V = volume of the NBS chamber occupied by smoke, A = exposed area of sample from which smoke is generated, L = length of light beam path in the chamber volume, Tm= minimum light transmittance. Measurements of the mass of smoke particulates can be made rapidly by filtering the smoke. A particularly convenient and simple apparatus for this determination is the Arapahoe smoke chamber [4]. With this device, smoke produced during flaming combustion of a small sample is continuodsiy drawn through a glass fiber filter. At the conclusion of the test, both smoke particulates and char are determined gravimetrically and reported as percentages of that portion of the original sample that was burned. Data obtained by Kracklauer et al. [4], as plotted in Fig. 1, show that Dm as measured with the Aminco-NBS chamber is almost directly proportional to wt. % particulates as measured with the Arapahoe chamber. However, separate curves are required for each of the three materials shown. The purpose of the study reported here was to develop a more generalized correlation, based on theoretical considerations and applicable to a variety of materials.
CORRELATION TECHNIQUE
Chien and Seader [5] and Seader and Ou [6] utilized the theory of radiative transport for light attenuation by scattering and absorption in conjunction with particle coagulation theory to develop an equation for relating the optical density to particulate mass concentration. The resulting expression is:
136
where: P1 = mp/ml, m l -- airborne mass loss of specimen as
DATA OF KRACKLAUER
etol (4]
(~
800
Ez~°U6ooi u~' -
measured in the Arapahoe chamber, mp = particulate mass as measured by filtration in the Arapahoe chamber, m = airborne mass loss of specimen as measured in the NBS chamber. The application of eqn. (3) in this manner involves the following two assumptions: (1) The weight of smoke particulates suspended in the NBS chamber is not reduced by settling and/or deposition. (2) The value of Fi determined from measurements with the Arapahoe chamber is identical to that obtained with the NBS chamber when operating in the flaming mode.
/ ~
h,18 400 O.Z
200
=E
0
5
I0
SMOKEPARTICULATE(wt. S %)
15
(ARAPAHOE CHAMBER)
Fig. 1. Correlation of maximum specific optical density with smoke particulates.
D -- = (POD)C, L
MATERIALS
(2)
where: D = optical density -- log10 (100/T), C, = mass concentration of particulates, POD = particulate optical density. The POD depends mainly on beam wavelength and particle size, density, and refractive index. For flaming combustion, where mainly soot particles are produced, predicted values of POD vary from a b o u t 18,000 to 40,000 cm2/g. As shown by Seader and Einhorn [ 2 ] , these values are in reasonably good agreement with calculated values for POD of 26,000 to 42,000 cm2/g based on measurements of D and Cs in NBS chambers by King [ 7 ] , Chien and Seader [ 5 ] , and Seader and Ou [6]. In this study, eqn. (2) was used to correlate gravimetric smoke measurements by the Arapahoe Smoke Chamber with optical smoke measurements by the NBS chamber for flaming combustion. Values of D / L were measured with an NBS chamber by standard procedures. In addition, the corresponding mass loss o f the sample was determined by measuring sample weight prior to and after a test. The fraction of the mass loss that was particulates was measured in the Arapahoe chamber using a much smaller sample. The predicted particulate mass concentration for the NBS chamber was c o m p u t e d from: c, -
Flm V
(3)
Ten different materials were studied. They were acrylonitrile-butadiene-styrene (ABS), low-smoke rigid poly(vinyl chloride) (LSPVC), rigid poly(vinyl chloride) (RPVC), three rigid urethane foams (RUA, RUB, RUC), two vinyl urethane foams (VU3, VU4), Douglas fir w o o d (DF), and lauan w o o d (LW). None of these materials tended to melt or drip when exposed to heat. Plastic and w o o d specimens were sawed to 2.9375 X 2.9375 X 0.125 in. and 1.5 X 0.5 X 0.125 in. for measurements with NBS and Arapahoe chambers, respectively. Corresponding dimensions for the foam specimens were 2.9375 X 2.9375 X 1 in. and 2 X 2 X 2 cm. Prior to exposure in the chambers, all specimens were dried at 140 °F for at least 12 hours and then kept at 73 + 5 OF and 50% relative humidity for at least 12 hours.
EXPERIMENTAL TECHNIQUES
Determination o f F1 with the Arapahoe chamber
An Arapahoe Chemical Company Model 705 Smoke Chamber was used. As shown in Fig. 2, the system includes a 3 in. diameter b y 15 in. high chimney m o u n t e d above a 5.5 in. diameter b y 7.5 in. high cylindrical combustion chamber and an instrument cabinet. At the top of the chimney is a filter holder for a 9 cm diameter glass fiber filter. The outlet of the filter holder is connected to a high-
137 RUBBER HOSE
F. . . . . . . . . . . . . . . . . . . . I
INSTRUMENT CABINET~
I
P
E TANK
I
- - -- -i
PRESSUREI TAPS I
I -- __-I VACUUM ~~.~.~ )=====~=~=~ / I PUMP VARIAC I III. . . . . . I~ MAGNAGAUGE HELIX 1 ~
REGULATOR
RO~PAN
"i
I
VALVE ON-OFF C ~ I. . . . . . . . . . . . . . . . . . . . .
--ORIFICE --FILTER
--CHIMNEY
CHAMBER COMBUSTION MI SPECIMEN BURNER
Fig. 2. S c h e m a t i c diagram o f t h e A r a p a h o e c h a m b e r .
capacity vacuum pump by a reinforced rubber hose. A sample holder is attached to the inside of the rear door of the combustion chamber so that the specimen can be clamped in a fixed position. A microburner is situated at the f r o n t side of the combustion chamber base plate and is preset at a 10 ° angle above the horizontal. Propane flow for the burner is metered and controlled manually at 90 cm3/min. The corresponding burner heat input is stated by Arapahoe to be 1.67 Btu/min. A Matheson Model No. 70 pressure regulator was m o u n t e d at the outlet of the propane tank to stabilize the propane flow rate. The m a x i m u m inlet pressure to the regulator was 250 lb/in 2 (gauge) and the constant metered outlet pressure range was from 0.5 to 5 lb/in 2 (gauge). Four ports on the microburner are left completely open to assure the mixing of propane with the largest possible a m o u n t of air. This generally guarantees complete combustion of propane w i t h o u t generating a significant a m o u n t of soot. The air flow is metered by an orifice and indicated by a magnahelix gauge on the instrument cabinet panel. The airflow rate is adjusted manually at the beginning of each run to 4.5 ft3/min by setting the speed of the vacuum pump with a variac located on the instrument cabinet panel. Before a run, the clean filter paper and the virgin specimen are weighed with a Mettler Model H-54 electric balance to +0.2 mg and then clamped in the filter holder and the sample holder, respectively, with the rear door of the combustion chamber left open. Then the air flow and the propane flow are turned on and the propane-air mixture is ignited manually. By closing the rear door, the speci-
men is automatically placed in touch with the flame and thereby ignited. At the same time, a reed switch is actuated causing the timer on the instrument cabinet panel to start. In most cases, the propane flow is turned off manually 30 s after the door is closed. However, the air flow is left on for another 30 s in order to collect the residual smoke particulates. Occasionally the burning time is shortened or prolonged if the quantities of smoke particulates generated are very large or very small or if ignition is slow. After a run, the filter paper and the specimen residue are weighed. The quantity F1 is calculated from the difference of the weight of the filter paper before and after the run divided by that difference for the specimen. For the foamed materials of this study, the above procedure was not suitable because the specimen often quickly shrank away from the flame. In fact, some specimens burned for only one second and, as a result, very few smoke particulates were collected. An alternative procedure suggested in the Arapahoe operating manual was found to be satisfactory. As shown in Fig. 3, a 2 cm cube specimen of foamed material is placed on a wire mesh support, 2 × 2 × 0.5 cm high. The microburner is inverted to provide a flame t h a t is directed downward. The lower edge of the specimen is placed 1.2 cm from the end of the barrel of the burner. The burning time for foamed materials is generally extended to 45 s. After a run, care must be taken to remove all of the sticky residue from the support.
S
P
E
C
I
M
E
N
~
~
INNER FLAME TIP ""'--t'gcl~ IMPINGES ON BOTTOM OF SAMPLE t ~
Fig. 3. T e s t a r r a n g e m e n t for f o a m e d materials.
For each kind of material, two to seven runs were made in order to determine an average value o f F1. In addition to the measurem e n t of smoke particulates, the a m o u n t of char formed was determined by debriding the burned specimen for 45 min in a sand mill and then reweighing the specimen.
138
Determination of chamber
Dm a n d m with the N B S
An N B S - A m i n c o Model 4-5800A smoke density chamber was used in conjunction with a Moseley Model 7001A X - Y plotter to measure specific optical density. Inside the chamber, a standard heater made o f 21-gauge nichrome wire together with a six-jet pilot burner serves as the heat source. The unexposed surface area of the specimen is wrapped with aluminum foil, inserted into a holder, and situated vertically with the exposed surface facing the furnace. A power variac and an asymptotic calorimeter (Hy-Cal Model C-1301A-120) were used to calibrate the heater to obtain an average heat flux o f 2.5 W/cm 2 at the surface of the exposed specimen. A tungsten-strip lamp, which emits white light of an average wavelength of a b o u t 0.5 pm, is used to generate the light beam. A potentiometric m i c r o p h o t o m e t e r with three decades of sensitivity provides continuous indication of the light transmittance. By removing the neutral density light filter in the light path, this range is increased five-fold. For a flaming run, the ratio of the propane flow rate to the air flow rate is adjusted to a b o u t 50:500 cm3/min so as to give horizontal blue flames a b o u t 0.125 in. long. In the standard procedure for conducting a run with the NBS chamber, the percentage light transmittance, T, is recorded as a function of time. For this study, the X - Y plotter was utilized until the minimum value of T was reached. Often, a relatively long time (from 5 min to more than 20 min) was required to obtain this value. As a result, particulate setfling and deposition effects could cause the a m o u n t of the final suspended smoke particulates to be less than that actually formed. This would cause the measured optical density to be on the low side with regard to the measured airborne mass loss. To minimize the effects of particulate setfling and deposition, a modification was made to the operating procedure. This modification also made it possible to conduct several different runs with each material. In the modified procedure, both the heater and the burner were turned o f f at an intermediate stage in the run, prior to completion of burning of the specimen. Generally only one or t w o additional minutes were then required for the smoke to spread and for the transmittance
reading to reach a steady minimum value. For each run, the airborne mass loss was determined by weighing the specimen before and after the test with a triple-beam balance to +0.01 g.
RESULTS AND DISCUSSION
The values of F1 determined from measurements with the Arapahoe chamber are listed in Table 1. Each value represents the mean of from two to seven runs. Also included in the Table are mean values of: % Smoke % Char
-
mp
ml + mc mc
(4) (5)
m1+ m c
where mc = mass of char (if any) formed. Except for RUC, mean values of r l are accompanied by the standard deviation and the coefficient of variation. For RUC, values of F1 for two runs were 0.127 and 0.118. For the plastic and foamed materials, F1 varies from 0.0888 to 0.1719. For the w o o d materials, F1 is an order of magnitude less. The percentage coefficients of variation vary from 2.89 for VU4 to a high value of 9.55 for Douglas fir. This range of values appears to be comparable to the range reported by Lee [3] for measurements of optical density with the NBS chamber on a variety of materials. Measured values of D / L and the airborne mass loss, m, as measured with the NBS chamber are listed in Table 2A for 20 runs with plastics and foamed materials and in Table 2B for 15 runs with w o o d materials. Also listed are corresponding values of the particulate mass concentration, C8, as c o m p u t e d from eqn. (3)usingvalues of F1 from Table 1, values of m from Tables 2A and 2B, and a value of V for the NBS chamber of 0.51 m 3. Values of D / L are plotted against C, in Fig. 4, which includes the correlation line for P O D = 33,000 cm2/g previously determined solely from NBS smoke chamber data by Seader and Ou [6]. Extensions of the data points indicate the estimated error. All points for plastics and foamed materials fall reasonably close to the correlation line. However, for Douglas fir and lauan woods, the data at low values of C, fall significantly below the line.
139 TABLE 1 Arapahoe chamber results F1 Material
Mean % smoke
Mean % char
Mean value
Standard deviation
Coefficient of variation (%)
ABS LSPVC RPVC RUA RUB RUC VU3 VU4 DF LW
14.82 7.50 11.36 8.1 8.37 6.51 9.81 5.70 0.861 0.492
15.66 7.47 26.9 18.1 46.8 26.4 43.5 3.8 3.91
0.1719 0.0888 0.1241 0.1102 0.1022 0.1225 0.1330 0.1009 0.00895 0.00512
0.0076 0.0033 0.0071 0.0088 0.0031 0.0109 0.0029 0.000855 0.00039
4.4 3.76 5.76 7.97 3.05 8.17 2.89 9.55 7.6
TABLE 2B
TABLE 2A
NBS chamber results and correlation variables for plastics NBS chamber results and correlation variables for wood materials and foamed materials Run No.
Material
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
ABS LSPVC RPVC RPVC RPVC RUA RUA RUA RUB RUB RUB RUC RUC RUC VU3 VU3 VU3 VU4 VU4 VU4
Experimental
m (g)
D/L (m -1)
7.87 5.8 4.33 2.57 1.75 2.25 1.86 1.06 0.674 0.634 0.847 4.66 3.15 2.76 1.3 0.435 0.32 1.34 1.09 0.329
Cs = F1 m / V
(mg/m a) 6.94 10.0 7.02 2.80 2.10 2.57 2.43 1.86 0.73 0.71 0.93 5.79 4.02 3.74 1.52 0.64 0.50 1.35 1.15 0.50
2320 1690 1520 605 454 555 524 402 156 152 199 1380 960 890 397 168 131 268 228 100
V a r i a t i o n s o f t h e d a t a p o i n t s in Fig. 4 f r o m t h e c o r r e l a t i n g l i n e are s u m m a r i z e d f o r e a c h m a t e r i a l in t e r m s o f m e a n P O D values, stand a r d d e v i a t i o n , a n d c o e f f i c i e n t o f v a r i a t i o n in T a b l e 3. T h e d e g r e e o f d e v i a t i o n o f t h e d a t a p o i n t s in Fig. 4 f r o m t h e c o r r e l a t i n g l i n e o f P O D e q u a l t o 3 3 , 0 0 0 cm2/g c a n also b e seen from this Table. With the e x c e p t i o n of the t w o w o o d m a t e r i a l s , t h e m e a n P O D v a l u e s in T a b l e 3 are e s s e n t i a l l y w i t h i n t h e p r e v i o u s l y mentioned experimental POD range of 26,000 t o 4 2 , 0 0 0 cm2/g b a s e d s o l e l y o n N B S c h a m b e r measurements.
Run No.
Material
21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
DF DF DF DF DF DF DF DF DF DF LW LW LW LW LW
Experimental
m (g)
D/L (m -1)
0.303 0.352 0.164 0.123 0.180 0.164 0.123 0.082 0.139 0.525 0.051 0.059 0.072 0.079 0.081
Cs = r 1 m / V
(mg/m 3) 8.9 6.7 7.5 3.7 4.4 4.4 5.4 5.2 5.1 7.6 4.8 4.9 5.0 5.4 5.4
163 123 138 68.1 81.0 81.0 99.4 95.7 94.0 140.0 48 49 50 54 55
A c o m p a r i s o n o f t h e average d e v i a t i o n o f e x p e r i m e n t a l D / L values f r o m v a l u e s p r e d i c t e d b y t h e c o r r e l a t i o n D / L = 3 3 , 0 0 0 C, is given in Table 4 for each of the 8 plastic and f o a m e d materials. Somewhat better agreement between e x p e r i m e n t a n d p r e d i c t i o n is g e n e r a l l y obtained with the (solid) plastic materials than with the f o a m e d (plastic) materials. The plastic m a t e r i a l s , w h e n t e s t e d in t h e N B S c h a m b e r , b u r n e d w i t h o u t changing shape. However, the f o a m e d m a t e r i a l s b u r n e d d r a s t i c a l l y f o r several s e c o n d s a n d t h e n e i t h e r e x t i n g u i s h e d or s h r a n k to a small chunk. F o r D o u g l a s fir, t h e i n h o m o g e n e i t y o f t h e c h e m i c a l c o m p o s i t i o n m a y h a v e c a u s e d irregular burning characteristics from run to run.
140
revealed a general pattern of variation of measured transmittance with respect to time. In all ten runs with Douglas fir, the measured transmittance decreased very slowly at first and then decreased rapidly. In the initial period, w o o d oil was evaporated and ignited with little generation of soot. This initial period was long in some runs and relatively short in other runs. In the second period, burning of the cellulosic c o m p o n e n t appeared to contribute much larger quantities of soot. The light transmittance dropped sharply and a very bright flame always shot out from the sample. Also, for Douglas fir, records of runs in the NBS chamber show that when the heat sources were shut off at an intermediate stage, up to 14 min was required to attain minimum light transmittance. Thus, physical aspects of coagulation, settling, and deposition could have reduced the amount of suspended smoke particulates and caused the lower values of D / L shown in Fig. 4.
I0 m
f.-
•i / .a¢.l. . .~. . . . . ....
I
41-
i,'
z hJ
I00
-r U (3.. iO-I
0~,
~e ,~
O~ _J
•
LOW SMOKE PVC
I
RIGID URETHANE (A)
x RIGID URETHANE (C) + VINYL URETHANE (~,) VINYL URETHANE (4) • DOUGLAS F i R o LAUAN WOOD
"l.J 10-2 I01
10 2
103
PARTICULATE where
Cs •
CONCENTRATION,
)0 4 Cs
(mg/m
3)
(ARAPAHOE) x m (NBS) V (NBS)
Fig. 4. Correlation of D/L from NBS chamber with calculated Cs from Arapahoe chamber. TABLE 3 Mean values o f POD Material
Mean POD (cm2/g)
Standard deviation
Coefficient of variation (%)
RPVC RUA RUB RUC VU3 VU4 DF LW
36,500 34,100 42,500 32,500 27,700 26,700 19,300 13,300
7185 7184 750 1400 4441 7345 8618 1854
19.67 21.07 1.76 4.30 16.04 27.51 45.66 13.94
I0'
/
i0 (
u ~'z I.-- .q
,11 z_~ W~ JI"-
UO.
;.m-
IO-
ol-J
TABLE 4
.
I0 i0 m
Comparison of measured and predicted values of D/L Material
Average (D/L)predicted ~
ABS LSPVC RPVC RUA RUB RUC VU3 VU4
2.9 3.8 17.5 16.8 22.3 3.1 21.2 31.6
(D/L)measured (D/L )measured
Due to different distributions of off and cellulosic-type components, values of F1 can vary. Indeed, Table 1 shows a relatively high coefficient of variation for Douglas fir. In the NBS chamber, recordings from the X - Y plotter
(%)
i
....
I
. . . . .
I
)0 z i0 3 C$, PARTICULATE CONCENTRATE
,
,
1.,, iO 4
( m g / m 3)
Fig. 5. Experimental data for lauan w o o d for flaming mode in the NBS chamber.
During the five tests with lauan w o o d in the NBS smoke chamber, simultaneous measurements of CG were made by the sampling method described by Chien and Seader [ 5 ] . The results are plotted in Fig. 5 together with the correlation line for P O D = 33,000 cm2/g. As in Fig. 4, the data for lauan wood, except for one point, fall significantly below the correlation line. Thus, at low values of C,, the deviations for w o o d in Fig. 4 may not represent a failure of correlation between the NBS and Arapahoe smoke chambers, but may be indicative of physical characteristics that are different from those at higher values of C,.
141 APPLICATION OF CORRELATION
CONCLUSIONS
Robertson [8] has recommended a m e t h o d for estimating.the potential smoke production for a room based on measured values of Dm for all finishing and furnishing materials. In his m e t h o d , effective values of Dml for each exposed material, i, are obtained by measurements in the NBS chamber with specimens of the thickness to be used in the room. The Dmi values are multiplied by their respective effective areas of exposure, Ai, in the room and summed. An effective light extinction coefficient is c o m p u t e d from:
A m e t h o d of correlating smoke particulate data from the Arapahoe Smoke Chamber to predict the optical density for the NBS Smoke Chamber was developed. The correlation is particularly good for solid and foamed plastic materials that do not drip, but is less satisfactory for woods, particularly at low levels of particulate mass concentration. The correlation allows materials to be rapidly screened by the Arapahoe Smoke chamber or any similar small-scale flow-through chamber in which a gravimetric determination of smoke is made. Modifications to such gravimetric chambers to permit materials to be exposed from above and/or in a smoldering combustion mode would make possible useful extensions of the correlation technique.
•g e f f
-
-
2.3026 f ~AiD'i Vr i
(6)
where V~= volumeof the room and f accounts for expansion and loss of smoke from the room. Robertson suggeststhat f be taken as 0.25. Results from the study of Jin [9] are applied to determine whether Keu exceeds the visibility limit. The results of the study reported here suggest the followingalternative equation for estimating Keff based on measured values of % smoke with the Arapahoe chamber and the correlation of Fig. 5.
ACKNOWLEDGEMENTS
The study reported was supported by National Science Foundation Grant GI-22650 under its RANN program and by the Society of Plastics Industry, who made available a graduate fellowship for S. S. Ou. Gratitude is expressed to Professors I. N. Einhorn and R. W. Mickelson of the Flammability Research am 2 Center of the University of Utah for their 33,000 ], 2.3026 f (% smoke)~mmi interest and helpful suggestions. ge. g / Z The NBS-Aminco Smoke Density Chamber Vr , 100 was made available by the American Instru(7) m e n t Company through Samuel Greenberg. where m m = mass of material in the room. In The Arapahoe Smoke Chamber was made this equation, C, has been estimated from the available by Arapahoe Chemicals, Inc., A relation: Syntex Company, through R. E. Legg, who also supplied the plastics and foamed materials. (% smoke) rn, c, = (8) 100 V =
where rn, = initial mass of the material. Thus, all of the material is assumed to enter into the combustion process so as to predict the maxim u m a m o u n t of smoke. One uncertainty in the use of eqn. (7) arises because of the effect of material thickness on Dm • When Dm is linear with thickness, eqn. (7) should give results comparable to eqn. (6). However, as shown by Jacobs [10], with some materials, Dm approaches a constant value beyond a certain critical thickness. In this case the value of Keu predicted by eqn. (7) may be higher than that obtained from eqn. (6).
REFERENCES
1 W. G. Berl, R. M. Fristrom and B. M. Halpin, APL Tech. Dig., The Johns Hopkins University, 14 (2) (1975) 2. 2 J. D. Seader and I. N. Einhorn, Paper presented at the 16th Int. Syrup. Combustion, MIT, Cambridge, August 15 - 21, 1976. 3 T. G. Lee, NBS Tech. Note 708, U.S. Government Printing Office, Washington, D.C. (December 1971 ). 4 J. J. Kracklauer, C. J. Sparkes and R. E. Legg, Paper presented at 32nd ANTEC of the Soc. Plast. Eng., San Francisco, May 13 - 16, 1974. 5 W. P. Chien and J. D. Seader, Fire Technol., 11 (1975) 206.
142 6 J. D. Seader and S. S. Ou, Paper presented at the Int. Symp. Toxicity and Physiology of Combustion Products, Salt Lake City, Utah, March 22 - 26, 1976. 7 T. Y. King, J. Fire Flammability, 6 (1975) 222.
8 A. F. Robertson, Fire Technol., 11 (1975) 80. 9 T. Jin, Rep. Fire Res. Inst. Japan, (33) (1971) 31 - 48. 10 M. I. Jacobs, J. Fire Flammability, 6 (1975) 347.
135
Correlation of Gravimetric Smoke Measurement by the Arapahoe Chamber with Optical Smoke Measurement by the NBS Chamber for Flaming Combustion
S. S. O U and J. D. S E A D E R
Flammability Research Center and Department of Chemical Engineering, Universityof Utah, SaltLake City,
Utah (U.S.A.)
SUMMARY
The relationship between the optical density and the particulate mass concentration of fire smoke is further developed. A correlation is obtained for relating gravimetric determinations made with the Arapahoe Smoke Chamber to light transmission measurements made with the NBS chamber. The correlation is particularly good for plastics, but less satisfactory for wood. Possible explanations for the latter deficiency are proposed.
INTRODUCTION
Fire statistics [1] indicate that smoke is the most important single cause of fire fatalities. For this reason, considerable attention is being given to the development of tests and procedures for evaluating the physical, chemical, toxicological, and physiological aspects of fire smokes [2]. The physical aspects of smoke are generally characterized by opacity to light, particulate mass or concentration, and particulate properties such as size distribution and density. The most widely used apparatus for characterizing fire smoke is the NBS-Aminco smoke density chamber [3]. With this device, measurements are made of the attenuation of a collimated light beam due to scattering and absorption by the particulate matter in the smoke, which is confined to the chamber during a test. Although a number of smoke parameters can be developed from data obtained with the NBS-Aminco chamber, the most useful is the maximum specific optical density, Din, defined by the relation:
v
(1oo I
D m =- A L loglo \ Tm!
(1)
where: V = volume of the NBS chamber occupied by smoke, A = exposed area of sample from which smoke is generated, L = length of light beam path in the chamber volume, Tm= minimum light transmittance. Measurements of the mass of smoke particulates can be made rapidly by filtering the smoke. A particularly convenient and simple apparatus for this determination is the Arapahoe smoke chamber [4]. With this device, smoke produced during flaming combustion of a small sample is continuodsiy drawn through a glass fiber filter. At the conclusion of the test, both smoke particulates and char are determined gravimetrically and reported as percentages of that portion of the original sample that was burned. Data obtained by Kracklauer et al. [4], as plotted in Fig. 1, show that Dm as measured with the Aminco-NBS chamber is almost directly proportional to wt. % particulates as measured with the Arapahoe chamber. However, separate curves are required for each of the three materials shown. The purpose of the study reported here was to develop a more generalized correlation, based on theoretical considerations and applicable to a variety of materials.
CORRELATION TECHNIQUE
Chien and Seader [5] and Seader and Ou [6] utilized the theory of radiative transport for light attenuation by scattering and absorption in conjunction with particle coagulation theory to develop an equation for relating the optical density to particulate mass concentration. The resulting expression is:
136
where: P1 = mp/ml, m l -- airborne mass loss of specimen as
DATA OF KRACKLAUER
etol (4]
(~
800
Ez~°U6ooi u~' -
measured in the Arapahoe chamber, mp = particulate mass as measured by filtration in the Arapahoe chamber, m = airborne mass loss of specimen as measured in the NBS chamber. The application of eqn. (3) in this manner involves the following two assumptions: (1) The weight of smoke particulates suspended in the NBS chamber is not reduced by settling and/or deposition. (2) The value of Fi determined from measurements with the Arapahoe chamber is identical to that obtained with the NBS chamber when operating in the flaming mode.
/ ~
h,18 400 O.Z
200
=E
0
5
I0
SMOKEPARTICULATE(wt. S %)
15
(ARAPAHOE CHAMBER)
Fig. 1. Correlation of maximum specific optical density with smoke particulates.
D -- = (POD)C, L
MATERIALS
(2)
where: D = optical density -- log10 (100/T), C, = mass concentration of particulates, POD = particulate optical density. The POD depends mainly on beam wavelength and particle size, density, and refractive index. For flaming combustion, where mainly soot particles are produced, predicted values of POD vary from a b o u t 18,000 to 40,000 cm2/g. As shown by Seader and Einhorn [ 2 ] , these values are in reasonably good agreement with calculated values for POD of 26,000 to 42,000 cm2/g based on measurements of D and Cs in NBS chambers by King [ 7 ] , Chien and Seader [ 5 ] , and Seader and Ou [6]. In this study, eqn. (2) was used to correlate gravimetric smoke measurements by the Arapahoe Smoke Chamber with optical smoke measurements by the NBS chamber for flaming combustion. Values of D / L were measured with an NBS chamber by standard procedures. In addition, the corresponding mass loss o f the sample was determined by measuring sample weight prior to and after a test. The fraction of the mass loss that was particulates was measured in the Arapahoe chamber using a much smaller sample. The predicted particulate mass concentration for the NBS chamber was c o m p u t e d from: c, -
Flm V
(3)
Ten different materials were studied. They were acrylonitrile-butadiene-styrene (ABS), low-smoke rigid poly(vinyl chloride) (LSPVC), rigid poly(vinyl chloride) (RPVC), three rigid urethane foams (RUA, RUB, RUC), two vinyl urethane foams (VU3, VU4), Douglas fir w o o d (DF), and lauan w o o d (LW). None of these materials tended to melt or drip when exposed to heat. Plastic and w o o d specimens were sawed to 2.9375 X 2.9375 X 0.125 in. and 1.5 X 0.5 X 0.125 in. for measurements with NBS and Arapahoe chambers, respectively. Corresponding dimensions for the foam specimens were 2.9375 X 2.9375 X 1 in. and 2 X 2 X 2 cm. Prior to exposure in the chambers, all specimens were dried at 140 °F for at least 12 hours and then kept at 73 + 5 OF and 50% relative humidity for at least 12 hours.
EXPERIMENTAL TECHNIQUES
Determination o f F1 with the Arapahoe chamber
An Arapahoe Chemical Company Model 705 Smoke Chamber was used. As shown in Fig. 2, the system includes a 3 in. diameter b y 15 in. high chimney m o u n t e d above a 5.5 in. diameter b y 7.5 in. high cylindrical combustion chamber and an instrument cabinet. At the top of the chimney is a filter holder for a 9 cm diameter glass fiber filter. The outlet of the filter holder is connected to a high-
137 RUBBER HOSE
F. . . . . . . . . . . . . . . . . . . . I
INSTRUMENT CABINET~
I
P
E TANK
I
- - -- -i
PRESSUREI TAPS I
I -- __-I VACUUM ~~.~.~ )=====~=~=~ / I PUMP VARIAC I III. . . . . . I~ MAGNAGAUGE HELIX 1 ~
REGULATOR
RO~PAN
"i
I
VALVE ON-OFF C ~ I. . . . . . . . . . . . . . . . . . . . .
--ORIFICE --FILTER
--CHIMNEY
CHAMBER COMBUSTION MI SPECIMEN BURNER
Fig. 2. S c h e m a t i c diagram o f t h e A r a p a h o e c h a m b e r .
capacity vacuum pump by a reinforced rubber hose. A sample holder is attached to the inside of the rear door of the combustion chamber so that the specimen can be clamped in a fixed position. A microburner is situated at the f r o n t side of the combustion chamber base plate and is preset at a 10 ° angle above the horizontal. Propane flow for the burner is metered and controlled manually at 90 cm3/min. The corresponding burner heat input is stated by Arapahoe to be 1.67 Btu/min. A Matheson Model No. 70 pressure regulator was m o u n t e d at the outlet of the propane tank to stabilize the propane flow rate. The m a x i m u m inlet pressure to the regulator was 250 lb/in 2 (gauge) and the constant metered outlet pressure range was from 0.5 to 5 lb/in 2 (gauge). Four ports on the microburner are left completely open to assure the mixing of propane with the largest possible a m o u n t of air. This generally guarantees complete combustion of propane w i t h o u t generating a significant a m o u n t of soot. The air flow is metered by an orifice and indicated by a magnahelix gauge on the instrument cabinet panel. The airflow rate is adjusted manually at the beginning of each run to 4.5 ft3/min by setting the speed of the vacuum pump with a variac located on the instrument cabinet panel. Before a run, the clean filter paper and the virgin specimen are weighed with a Mettler Model H-54 electric balance to +0.2 mg and then clamped in the filter holder and the sample holder, respectively, with the rear door of the combustion chamber left open. Then the air flow and the propane flow are turned on and the propane-air mixture is ignited manually. By closing the rear door, the speci-
men is automatically placed in touch with the flame and thereby ignited. At the same time, a reed switch is actuated causing the timer on the instrument cabinet panel to start. In most cases, the propane flow is turned off manually 30 s after the door is closed. However, the air flow is left on for another 30 s in order to collect the residual smoke particulates. Occasionally the burning time is shortened or prolonged if the quantities of smoke particulates generated are very large or very small or if ignition is slow. After a run, the filter paper and the specimen residue are weighed. The quantity F1 is calculated from the difference of the weight of the filter paper before and after the run divided by that difference for the specimen. For the foamed materials of this study, the above procedure was not suitable because the specimen often quickly shrank away from the flame. In fact, some specimens burned for only one second and, as a result, very few smoke particulates were collected. An alternative procedure suggested in the Arapahoe operating manual was found to be satisfactory. As shown in Fig. 3, a 2 cm cube specimen of foamed material is placed on a wire mesh support, 2 × 2 × 0.5 cm high. The microburner is inverted to provide a flame t h a t is directed downward. The lower edge of the specimen is placed 1.2 cm from the end of the barrel of the burner. The burning time for foamed materials is generally extended to 45 s. After a run, care must be taken to remove all of the sticky residue from the support.
S
P
E
C
I
M
E
N
~
~
INNER FLAME TIP ""'--t'gcl~ IMPINGES ON BOTTOM OF SAMPLE t ~
Fig. 3. T e s t a r r a n g e m e n t for f o a m e d materials.
For each kind of material, two to seven runs were made in order to determine an average value o f F1. In addition to the measurem e n t of smoke particulates, the a m o u n t of char formed was determined by debriding the burned specimen for 45 min in a sand mill and then reweighing the specimen.
138
Determination of chamber
Dm a n d m with the N B S
An N B S - A m i n c o Model 4-5800A smoke density chamber was used in conjunction with a Moseley Model 7001A X - Y plotter to measure specific optical density. Inside the chamber, a standard heater made o f 21-gauge nichrome wire together with a six-jet pilot burner serves as the heat source. The unexposed surface area of the specimen is wrapped with aluminum foil, inserted into a holder, and situated vertically with the exposed surface facing the furnace. A power variac and an asymptotic calorimeter (Hy-Cal Model C-1301A-120) were used to calibrate the heater to obtain an average heat flux o f 2.5 W/cm 2 at the surface of the exposed specimen. A tungsten-strip lamp, which emits white light of an average wavelength of a b o u t 0.5 pm, is used to generate the light beam. A potentiometric m i c r o p h o t o m e t e r with three decades of sensitivity provides continuous indication of the light transmittance. By removing the neutral density light filter in the light path, this range is increased five-fold. For a flaming run, the ratio of the propane flow rate to the air flow rate is adjusted to a b o u t 50:500 cm3/min so as to give horizontal blue flames a b o u t 0.125 in. long. In the standard procedure for conducting a run with the NBS chamber, the percentage light transmittance, T, is recorded as a function of time. For this study, the X - Y plotter was utilized until the minimum value of T was reached. Often, a relatively long time (from 5 min to more than 20 min) was required to obtain this value. As a result, particulate setfling and deposition effects could cause the a m o u n t of the final suspended smoke particulates to be less than that actually formed. This would cause the measured optical density to be on the low side with regard to the measured airborne mass loss. To minimize the effects of particulate setfling and deposition, a modification was made to the operating procedure. This modification also made it possible to conduct several different runs with each material. In the modified procedure, both the heater and the burner were turned o f f at an intermediate stage in the run, prior to completion of burning of the specimen. Generally only one or t w o additional minutes were then required for the smoke to spread and for the transmittance
reading to reach a steady minimum value. For each run, the airborne mass loss was determined by weighing the specimen before and after the test with a triple-beam balance to +0.01 g.
RESULTS AND DISCUSSION
The values of F1 determined from measurements with the Arapahoe chamber are listed in Table 1. Each value represents the mean of from two to seven runs. Also included in the Table are mean values of: % Smoke % Char
-
mp
ml + mc mc
(4) (5)
m1+ m c
where mc = mass of char (if any) formed. Except for RUC, mean values of r l are accompanied by the standard deviation and the coefficient of variation. For RUC, values of F1 for two runs were 0.127 and 0.118. For the plastic and foamed materials, F1 varies from 0.0888 to 0.1719. For the w o o d materials, F1 is an order of magnitude less. The percentage coefficients of variation vary from 2.89 for VU4 to a high value of 9.55 for Douglas fir. This range of values appears to be comparable to the range reported by Lee [3] for measurements of optical density with the NBS chamber on a variety of materials. Measured values of D / L and the airborne mass loss, m, as measured with the NBS chamber are listed in Table 2A for 20 runs with plastics and foamed materials and in Table 2B for 15 runs with w o o d materials. Also listed are corresponding values of the particulate mass concentration, C8, as c o m p u t e d from eqn. (3)usingvalues of F1 from Table 1, values of m from Tables 2A and 2B, and a value of V for the NBS chamber of 0.51 m 3. Values of D / L are plotted against C, in Fig. 4, which includes the correlation line for P O D = 33,000 cm2/g previously determined solely from NBS smoke chamber data by Seader and Ou [6]. Extensions of the data points indicate the estimated error. All points for plastics and foamed materials fall reasonably close to the correlation line. However, for Douglas fir and lauan woods, the data at low values of C, fall significantly below the line.
139 TABLE 1 Arapahoe chamber results F1 Material
Mean % smoke
Mean % char
Mean value
Standard deviation
Coefficient of variation (%)
ABS LSPVC RPVC RUA RUB RUC VU3 VU4 DF LW
14.82 7.50 11.36 8.1 8.37 6.51 9.81 5.70 0.861 0.492
15.66 7.47 26.9 18.1 46.8 26.4 43.5 3.8 3.91
0.1719 0.0888 0.1241 0.1102 0.1022 0.1225 0.1330 0.1009 0.00895 0.00512
0.0076 0.0033 0.0071 0.0088 0.0031 0.0109 0.0029 0.000855 0.00039
4.4 3.76 5.76 7.97 3.05 8.17 2.89 9.55 7.6
TABLE 2B
TABLE 2A
NBS chamber results and correlation variables for plastics NBS chamber results and correlation variables for wood materials and foamed materials Run No.
Material
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
ABS LSPVC RPVC RPVC RPVC RUA RUA RUA RUB RUB RUB RUC RUC RUC VU3 VU3 VU3 VU4 VU4 VU4
Experimental
m (g)
D/L (m -1)
7.87 5.8 4.33 2.57 1.75 2.25 1.86 1.06 0.674 0.634 0.847 4.66 3.15 2.76 1.3 0.435 0.32 1.34 1.09 0.329
Cs = F1 m / V
(mg/m a) 6.94 10.0 7.02 2.80 2.10 2.57 2.43 1.86 0.73 0.71 0.93 5.79 4.02 3.74 1.52 0.64 0.50 1.35 1.15 0.50
2320 1690 1520 605 454 555 524 402 156 152 199 1380 960 890 397 168 131 268 228 100
V a r i a t i o n s o f t h e d a t a p o i n t s in Fig. 4 f r o m t h e c o r r e l a t i n g l i n e are s u m m a r i z e d f o r e a c h m a t e r i a l in t e r m s o f m e a n P O D values, stand a r d d e v i a t i o n , a n d c o e f f i c i e n t o f v a r i a t i o n in T a b l e 3. T h e d e g r e e o f d e v i a t i o n o f t h e d a t a p o i n t s in Fig. 4 f r o m t h e c o r r e l a t i n g l i n e o f P O D e q u a l t o 3 3 , 0 0 0 cm2/g c a n also b e seen from this Table. With the e x c e p t i o n of the t w o w o o d m a t e r i a l s , t h e m e a n P O D v a l u e s in T a b l e 3 are e s s e n t i a l l y w i t h i n t h e p r e v i o u s l y mentioned experimental POD range of 26,000 t o 4 2 , 0 0 0 cm2/g b a s e d s o l e l y o n N B S c h a m b e r measurements.
Run No.
Material
21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
DF DF DF DF DF DF DF DF DF DF LW LW LW LW LW
Experimental
m (g)
D/L (m -1)
0.303 0.352 0.164 0.123 0.180 0.164 0.123 0.082 0.139 0.525 0.051 0.059 0.072 0.079 0.081
Cs = r 1 m / V
(mg/m 3) 8.9 6.7 7.5 3.7 4.4 4.4 5.4 5.2 5.1 7.6 4.8 4.9 5.0 5.4 5.4
163 123 138 68.1 81.0 81.0 99.4 95.7 94.0 140.0 48 49 50 54 55
A c o m p a r i s o n o f t h e average d e v i a t i o n o f e x p e r i m e n t a l D / L values f r o m v a l u e s p r e d i c t e d b y t h e c o r r e l a t i o n D / L = 3 3 , 0 0 0 C, is given in Table 4 for each of the 8 plastic and f o a m e d materials. Somewhat better agreement between e x p e r i m e n t a n d p r e d i c t i o n is g e n e r a l l y obtained with the (solid) plastic materials than with the f o a m e d (plastic) materials. The plastic m a t e r i a l s , w h e n t e s t e d in t h e N B S c h a m b e r , b u r n e d w i t h o u t changing shape. However, the f o a m e d m a t e r i a l s b u r n e d d r a s t i c a l l y f o r several s e c o n d s a n d t h e n e i t h e r e x t i n g u i s h e d or s h r a n k to a small chunk. F o r D o u g l a s fir, t h e i n h o m o g e n e i t y o f t h e c h e m i c a l c o m p o s i t i o n m a y h a v e c a u s e d irregular burning characteristics from run to run.
140
revealed a general pattern of variation of measured transmittance with respect to time. In all ten runs with Douglas fir, the measured transmittance decreased very slowly at first and then decreased rapidly. In the initial period, w o o d oil was evaporated and ignited with little generation of soot. This initial period was long in some runs and relatively short in other runs. In the second period, burning of the cellulosic c o m p o n e n t appeared to contribute much larger quantities of soot. The light transmittance dropped sharply and a very bright flame always shot out from the sample. Also, for Douglas fir, records of runs in the NBS chamber show that when the heat sources were shut off at an intermediate stage, up to 14 min was required to attain minimum light transmittance. Thus, physical aspects of coagulation, settling, and deposition could have reduced the amount of suspended smoke particulates and caused the lower values of D / L shown in Fig. 4.
I0 m
f.-
•i / .a¢.l. . .~. . . . . ....
I
41-
i,'
z hJ
I00
-r U (3.. iO-I
0~,
~e ,~
O~ _J
•
LOW SMOKE PVC
I
RIGID URETHANE (A)
x RIGID URETHANE (C) + VINYL URETHANE (~,) VINYL URETHANE (4) • DOUGLAS F i R o LAUAN WOOD
"l.J 10-2 I01
10 2
103
PARTICULATE where
Cs •
CONCENTRATION,
)0 4 Cs
(mg/m
3)
(ARAPAHOE) x m (NBS) V (NBS)
Fig. 4. Correlation of D/L from NBS chamber with calculated Cs from Arapahoe chamber. TABLE 3 Mean values o f POD Material
Mean POD (cm2/g)
Standard deviation
Coefficient of variation (%)
RPVC RUA RUB RUC VU3 VU4 DF LW
36,500 34,100 42,500 32,500 27,700 26,700 19,300 13,300
7185 7184 750 1400 4441 7345 8618 1854
19.67 21.07 1.76 4.30 16.04 27.51 45.66 13.94
I0'
/
i0 (
u ~'z I.-- .q
,11 z_~ W~ JI"-
UO.
;.m-
IO-
ol-J
TABLE 4
.
I0 i0 m
Comparison of measured and predicted values of D/L Material
Average (D/L)predicted ~
ABS LSPVC RPVC RUA RUB RUC VU3 VU4
2.9 3.8 17.5 16.8 22.3 3.1 21.2 31.6
(D/L)measured (D/L )measured
Due to different distributions of off and cellulosic-type components, values of F1 can vary. Indeed, Table 1 shows a relatively high coefficient of variation for Douglas fir. In the NBS chamber, recordings from the X - Y plotter
(%)
i
....
I
. . . . .
I
)0 z i0 3 C$, PARTICULATE CONCENTRATE
,
,
1.,, iO 4
( m g / m 3)
Fig. 5. Experimental data for lauan w o o d for flaming mode in the NBS chamber.
During the five tests with lauan w o o d in the NBS smoke chamber, simultaneous measurements of CG were made by the sampling method described by Chien and Seader [ 5 ] . The results are plotted in Fig. 5 together with the correlation line for P O D = 33,000 cm2/g. As in Fig. 4, the data for lauan wood, except for one point, fall significantly below the correlation line. Thus, at low values of C,, the deviations for w o o d in Fig. 4 may not represent a failure of correlation between the NBS and Arapahoe smoke chambers, but may be indicative of physical characteristics that are different from those at higher values of C,.
141 APPLICATION OF CORRELATION
CONCLUSIONS
Robertson [8] has recommended a m e t h o d for estimating.the potential smoke production for a room based on measured values of Dm for all finishing and furnishing materials. In his m e t h o d , effective values of Dml for each exposed material, i, are obtained by measurements in the NBS chamber with specimens of the thickness to be used in the room. The Dmi values are multiplied by their respective effective areas of exposure, Ai, in the room and summed. An effective light extinction coefficient is c o m p u t e d from:
A m e t h o d of correlating smoke particulate data from the Arapahoe Smoke Chamber to predict the optical density for the NBS Smoke Chamber was developed. The correlation is particularly good for solid and foamed plastic materials that do not drip, but is less satisfactory for woods, particularly at low levels of particulate mass concentration. The correlation allows materials to be rapidly screened by the Arapahoe Smoke chamber or any similar small-scale flow-through chamber in which a gravimetric determination of smoke is made. Modifications to such gravimetric chambers to permit materials to be exposed from above and/or in a smoldering combustion mode would make possible useful extensions of the correlation technique.
•g e f f
-
-
2.3026 f ~AiD'i Vr i
(6)
where V~= volumeof the room and f accounts for expansion and loss of smoke from the room. Robertson suggeststhat f be taken as 0.25. Results from the study of Jin [9] are applied to determine whether Keu exceeds the visibility limit. The results of the study reported here suggest the followingalternative equation for estimating Keff based on measured values of % smoke with the Arapahoe chamber and the correlation of Fig. 5.
ACKNOWLEDGEMENTS
The study reported was supported by National Science Foundation Grant GI-22650 under its RANN program and by the Society of Plastics Industry, who made available a graduate fellowship for S. S. Ou. Gratitude is expressed to Professors I. N. Einhorn and R. W. Mickelson of the Flammability Research am 2 Center of the University of Utah for their 33,000 ], 2.3026 f (% smoke)~mmi interest and helpful suggestions. ge. g / Z The NBS-Aminco Smoke Density Chamber Vr , 100 was made available by the American Instru(7) m e n t Company through Samuel Greenberg. where m m = mass of material in the room. In The Arapahoe Smoke Chamber was made this equation, C, has been estimated from the available by Arapahoe Chemicals, Inc., A relation: Syntex Company, through R. E. Legg, who also supplied the plastics and foamed materials. (% smoke) rn, c, = (8) 100 V =
where rn, = initial mass of the material. Thus, all of the material is assumed to enter into the combustion process so as to predict the maxim u m a m o u n t of smoke. One uncertainty in the use of eqn. (7) arises because of the effect of material thickness on Dm • When Dm is linear with thickness, eqn. (7) should give results comparable to eqn. (6). However, as shown by Jacobs [10], with some materials, Dm approaches a constant value beyond a certain critical thickness. In this case the value of Keu predicted by eqn. (7) may be higher than that obtained from eqn. (6).
REFERENCES
1 W. G. Berl, R. M. Fristrom and B. M. Halpin, APL Tech. Dig., The Johns Hopkins University, 14 (2) (1975) 2. 2 J. D. Seader and I. N. Einhorn, Paper presented at the 16th Int. Syrup. Combustion, MIT, Cambridge, August 15 - 21, 1976. 3 T. G. Lee, NBS Tech. Note 708, U.S. Government Printing Office, Washington, D.C. (December 1971 ). 4 J. J. Kracklauer, C. J. Sparkes and R. E. Legg, Paper presented at 32nd ANTEC of the Soc. Plast. Eng., San Francisco, May 13 - 16, 1974. 5 W. P. Chien and J. D. Seader, Fire Technol., 11 (1975) 206.
142 6 J. D. Seader and S. S. Ou, Paper presented at the Int. Symp. Toxicity and Physiology of Combustion Products, Salt Lake City, Utah, March 22 - 26, 1976. 7 T. Y. King, J. Fire Flammability, 6 (1975) 222.
8 A. F. Robertson, Fire Technol., 11 (1975) 80. 9 T. Jin, Rep. Fire Res. Inst. Japan, (33) (1971) 31 - 48. 10 M. I. Jacobs, J. Fire Flammability, 6 (1975) 347.