NBSIR 77-1312

Physical Properties of Smokes

Pertinent to Smoke Detector

Technology

Thomas G. K. Lee and George Mulholland

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

November 1977

Final Report

U S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 77-1312

PHYSICAL PROPERTIES OF SMOKES

PERTINENT TO SMOKE DETECTOR

TECHNOLOGY

Thomas G. K. Lee and George Mulholland

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D C. 20234

November 1977

Final Report

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
,
CONTENTS

Page

LIST OF FIGURES iv

LIST OF TABLES iv

Abstract 1

1. INTRODUCTION 1

2. FIRE SMOKE PROPERTIES 3 2.1. Definition of Fire Smoke 3 2.2. Particle Size and Its Distribution 5 2.3. Number Concentration and Mass Concentration 8

3. MEASUREMENT METHODS 8 3.1. Particle Size Distribution - 0.006 to 1 ym 9 3.2. Particle Size Distribution 0.5 to 5 ym 11

3.3. Number Concentration 300 to 10 7 cm

-3

12

3.4. Mass Concentration 13

3.5. Aerosol Chamber 14 3.6. UL 217 Smoke Detector Evaluation Chamber 15

4. AEROSOL SOURCE MATERIALS AND DILUTION . 17 4.1. Method for Aerosol Dilution 19

5. RESULTS 19

5.1. Smokes in the UL 217 Chamber 19 5.2. The Effect of Pyrolysis Conditions on Particle Sizes 27 5.3. Effect of Flaming and Non-flaming Exposure on Particle Size

of a-cellulose Smoke 27 5.4. Influence of Pyrolysis Air Temperature 30

5.5. Influence of Aging on Particle Size Distribution 30

6. DISCUSSION 33

6.1. Comparison of Measurement Methods 33

6.2. UL 217 Detector Evaluation Chamber 34

6.3. Aerosol Generation Modes 34

6.4. Aerosol Aging and Coagulation Losses 34

6.5. Universal Size Distribution 35

7. SUMMARY AND CONCLUSIONS 38

8. REFERENCES 40
LIST OF FIGURES

Page

Figure 1. Detector sensitivity for two types of detectors exposed to monodisperse aerosols 4

Figure 2. Discrete size ranges of the Electrical Aerosol Analyzer 6

Figure 3. Smoke detector evaluation chamber from UL 217 16

Figure 4. Aerosol diluter, dilution factor depends on orifice sizes 18

Figure 5. Size distribution of an aerosol before and after dilution 20

Figure 6. Mass concentration versus smoke obscuration in UL 217 22

Figure 7. Number concentration versus smoke obscuration in UL 217 23

Figure 8. Particle size distribution for lamp wick and heptane smokes 25

Figure 9. Effect of air movement (2 m/s) on particle size 26

Figure 10. Smoke from a-cellulose under flaming and non-flaming exposure

conditions 28

Figure 11. Aging of aerosol from source which progress from smoldering

to flaming 29

Figure 12. Aging of aerosols generated at two temperatures 31

Figure 13. Aging of punk aerosol up to 16 hours 32

Figure 14. Representative size distributions in terms of reduced variables 36

LIST OF TABLES

Page

Table 1. Performance of Instruments 10

Table 2. Measured and Derived Parameters of Smokes in the UL 217

Chamber Test 24

iv
PHYSICAL PROPERTIES OF SMOKES PERTINENT

TO SMOKE DETECTOR TECHNOLOGY

Thomas G. K. Lee and George Mulholland

Abstract

Several commercially available aerosol instruments including the electrical

aerosol analyzer, nuclei condensation monitor, quartz mass monitor, and optical

particle counter were used to measure particle size distribution of smokes from

burning heptane and cellulosic materials. Some limitations of these instruments

are discussed. Parameters such as mode of exposure (flaming versus smoldering),

pyrolysis temperature, air velocity at smoke emitting site, and aging were found

to have a large effect on the smoke particle size distribution. Mass and number

concentrations of smokes from cotton lamp wick as a function of smoke obscuration

in the standard UL 217 detector test chamber were determined. The maximum alarm

threshold obscuration of 0.06 OD/m (4% per ft), required for detector approval, was found to correspond to lamp wick aerosol mass concentration of 40 mg/m 3 and

particle concentration of about 4 x 10 6 cm with the peak size in the number distribution of about 0.15 um. The present UL 217 test method was shown to be affected

by smoke coagulation and did not provide a complete measure of sensitivity in smoke

detectors. An algebraic model size distribution, with number and mass concentration

the only free parameters, was shown to provide a good estimate for all the smoke

size distributions measured.

Key words: Aerosol; mass concentration; number concentration; size distribution;

smoke; smoke detector; test method; UL 217.

1 . INTRODUCTION

An effective approach in reducing fire hazard to life and property is the use of earlywarning devices such as smoke detectors. Studies have shown that fatalities might be reduced

by about 40% if detectors were installed in all houses [1] . A more recent experimental

study (1976) concluded that detectors would provide adequate life saving potential under most

real residential fire conditions [2]. Because of low cost and in some cases building code

requirements, the sale of smoke detectors has reached an estimated 8 million units in the

United States for the current year, 1977 [3].

I Numbers in brackets refer to the literature references listed at the end of this paper.

1
The purpose of this report is to provide some of the basic data necessary for the development of improved smoke detectors and improved detector test methods. The three areas treated

in this report are the physical characterization of smoke particulates, smoke detector testing

methodology, and smoke aging.

A very important area in smoke detector design technology is characterizing the smoke

in terms of the attributes sensed by the detector, i.e., particle size distribution, number

concentration, mass concentration, and refractive index of smokes. How the above parameters

are affected by the nature of the smoke source, pyrolysis temperature, and airflow at the

generating site are also important and will be examined.

To measure the smoke characteristics, some newly available commercial instruments were

used. Since these measurement techniques are relatively new, the methods used and their

limitations are described in more detail than might normally be necessary.

A second area of importance in smoke detector technology is the efficacy of certain

test methods for detector approval and specification. Currently, most state jurisdictions

will approve for sale any detector listed (approved) by the Underwriters' Laboratories (UL)

.

For acceptance testing, UL follows the test method UL 217 which specifies the minimum and

maximum triggering levels in terms of light obscuration measured by a photometer in the

specified smoke chamber [4].

Because detector sensitivity is a complex function of smoke particle size, its concentration, and refractive index, a test method based on obscuration alone cannot fully describe

detector performance. For example, two detectors with identical alarm levels in the UL

chamber may differ when exposed to other types of smoke in a real fire [5,6]. A better

method suggested for performance specification may be the use of the response curve where

the detector outputs are given as a function of particle size and concentration. Currently

neither the manufacturers nor the purchasers use this kind of performance specification in

describing smoke detectors.

A third area where information is needed to advance the art of detector design and

siting selection is the problem of smoke aging. Because aerosol particles collide and

coagulate continuously (aging) , a rapid change in size distribution occurs under the high

concentration level where the detectors operate. For example, by the time the smoke has

arrived at the detector area on the ceiling, the particle size distribution of the smoke

will have changed significantly from its original size. A model or method of calculating

the effect of aging on polydispersed smoke of various types is therefore essential to predict

smoke characteristics after it leaves its source.

Information from the study may also be applied to the false-alarm problem in the presentday detectors. A British study has shown that for every real fire there were three false

2
alarms due to ambient aerosols including moisture, cooking vapor, cigarette smoke, dust,

etc. [7] . The ambient aerosol-related false alarms are inherent in the present day detectors

because they can not differentiate among various types of particles. In general, any aerosol

containing a prerequisite concentration of particles in the size range of 0.01 to 5 pm can

trigger the detector. Although false alarm rates may be reduced by setting a higher threshold

for alarm in the detector, the resulting increase in response time to real fire, may not be

acceptable in a life safety situation. Therefore, to minimize false alarms from ambient

aerosols, a new generation of detectors capable of differentiating among types of smokes is

urgently needed.

This report will discuss the three areas mentioned above and will include some results

based on measurements to date in the smoke aerosol laboratory at the Center for Fire Research

(CFR)

2. FIRE SMOKE PROPERTIES

2.1. Definition of Fire Smoke

Smoke generated from fire-related sources consists of both gases and aerosols; however,

smoke detection technology is primarily concerned with the aerosol component of smoke.

Depending on the material burned and the condition in which it burns (smoldering or flaming)

the smoke aerosol can have a wide range of properties including the following:

1. liquid drops or solid particles (possibly including an adsorbed liquid phase)

2. shape can vary from spherical to chain agglomerates

3. optical properties vary from very absorbing for the black sooty smoke to almost transparent for some smokes from smoldering fires.

From a detection point of view, the msot significant property of the smoke aerosol is

its size distribution. The particle size of interest ranges from 0.005 pm to 5 pm. Smoke

does not consist of only a single particle size but may have a range of particle sizes with

peak concentration at a certain size. For a typical punk smoke, the peak concentration is on the order of 500 times higher than the concentration at a particle size ten times larger

than the peak size (i.e. the concentration, AN/Alog D , at 0.1 pm is 5 x 10® cm

3 compared

— 3 ^ to only 1 x 104 cm at 1 pm)

This study was confined to particle concentration ranges on the order of the detector

threshold for alarm. In the UL chamber the alarm threshold is defined in terms of smoke

obscuration by smoke generated from a smoldering lamp wick. Obscuration is expressed in

terms of optical density (OD) per meter (percent per foot in the UL 217 method).

Obscuration (log I q /I)

/

d

3

( 1 )
Figure 1. Detector sensitivity for two types of detectors exposed to monodisperse aerosols

4
where I is the intensity of the light beam (color temperature < 2000K) penetrating the smoke,

I Q is the intensity with no smoke present, and d is the path length. The maximum alarm

threshold set by the Underwriters' Laboratories (0.06 OD/m or 4% per ft) was found to correspond to a number concentration of approximately 4 x 10 6 cm with a geometric mean diameter

of 0.14 pm, a geometric standard deviation of 1.7 (to be discussed), and a mass concentration

of about 40 mg/m^ for lamp wick smoke. The desirability of expressing the alarm threshold

by using the intrinsic aerosol properties instead of the optical density will be discussed

later.

2.2. Particle Size and Its Distribution

The sensitivity of the present day smoke detectors depends strongly on the particle size

of the aerosol. These detectors will respond to particles in the size range of 0.005 to 5 pm. However, the degree of sensitivity of particle sizes within this range differs considerably

among the types of detectors and depends primarily on the physical principle on which the

detector operates. For example, the response of the light-scattering-type detector is roughly

proportional to the 6th power of the particle diameter for particles smaller than a few

tenths of a micrometer; whereas, for the ionization- type detector, the response is approximately a linear function of particle diameter. The commercial light-scattering-type detectors

are practically insensitive to particles less than 0.1 pm compared to the ionization type

which can respond to particles as small as 0.005 pm. The light-scattering types are also

sensitive to the refractive index of the smoke.

Since detector sensitivity is strongly size-dependent as illustrated in figure 1,

information on particle size of smokes and factors that control it are therefore needed for

design of better detectors. In figure 1, the response of two typical smoke detectors are plotted as a function of particle size for monodisperse dioctyl phthalate aerosols in the

number concentration range 10 4 to 10 6 cm

3 [8].

There are many ways for representing the size of a collection of particles. The choice

of the size distribution variable is mainly a choice of convenience. In this paper, data

are presented in terms of AN/Alog versus in a log-log plot, where N is a number of

aerosol particles per cubic centimeter and is particle size in micrometers.

Such a plot is necessitated by the wide range in particle size and concentration. The

quantity AN represents the number of aerosol particles in the particle diameter size range

log 0^ to log Dp + A log Dp. The A presentation was used because data obtained from the

measuring instruments were based on the average within each discrete range.

For example, the size distribution of a typical smoke is plotted in figure 2, where

A log Dp for each of the 9 discrete size ranges equals 0.25. Then the total concentration

for a given size range would equal 0.25 x (AN/Alog Dp). The dotted portion of the curve represents a large uncertainty.

5
D

p,/xm

Figure 2. Discrete size ranges of the Electrical Aerosol Analyzer

6
In many applications, the important characteristics of a size distribution may be given

by the average particle size and the spread (or shape) of the distribution. It is convenient

to use the geometric mean diameter, D , as a measure of the average size.

log D

g

ill AN

i log D£

( 2 )

D AN

1 D„ AN

2 d/N

3

, 1/N

^ an_, d

i

1 (3)

where N is the total number of particles and AN^ is the number of particles in the ith

interval. For the size distribution plotted in figure 2, = 0.072 urn.

The spread of the size distribution is measured by the geometric standard deviation,

a ,

log a g

1 1/2

Z (log D - log D ) z AN. i=l 1 g i (4)

This equation is based on log-normal distribution, which is the same as a normal or Gaussian

distribution except that log is normally distributed instead of D^. The calculated

standard deviation is log (a dimensionless number). If the aerosol were perfectly monodisperse, would have a value of 1 (log o = 0) . For a log-normal distribution, 68.3% of

the total particles are in the size range log D + log o . In the case shown in figure 2,

where D = 0.072 and a = 1.75, 68.3% of the total particles are in the range of 0.04 and

g g 0.13 pm.

The usefulness of the parameters and as characteristic parameters for size distribution depends on how close the actual size distribution is to a log-normal size distribution

given by:

dN

d log D rz— i /2ir log a explog D - log

Jl log a

(5)

Many particle size distributions are at least approximately log-normal [9].

The algebraic distribution, a modified version of the Junge distribution, which itself

has been widely used for characterizing size distributions of atmospheric aerosols, is given

by:

dN a D'

d log D

(b + d

3

) 2+g

7
In this case the exponent e is a measure of the shape of the distribution, and the parameters

a and Id can be related to the number concentration, N , and volume concentration, V, of the

aerosol. In previous work [10] it was found that the size distribution of smoke generated

from flaming a-cellulose and smoldering punk fit the above form with e = 0. The size distribution expressed in terms of the aerosol parameters is given in reference [10]:

dN

d log D 1.38

V D

3

(D 3 + 0.2 |-) 2

t

( 6 )

It should be pointed out that the distribution breaks down for large particle sizes, because

the total aerosol volume as determined by integrating eq. (6) over all particle sizes is

infinite

In this report both the log-normal distribution and the algebraic distribution will be

compared with experimental results in Section 6.5.

2.3. Number Concentration and Mass Concentration

In addition to particle size distribution total particle number concentration and mass concentration per unit volume are also important and useful characteristics of smokes. For

a given particle size, the response of both the ionization and light-scattering types of

detectors are linear functions of the particle number concentration and its associated mass

up to a limiting number density.

Because of particle coagulation in the concentration range of interest, the number

concentration of an aerosol decreases rapidly with time and creates a measurement problem

for an instrument requiring scanning type over several minutes. On the other hand, the mass concentration is much more stable and can be readily measured with good precision. Wall

loss, the primary loss mechanism for particle mass, is rather small compared to coagulation

loss. For example, the mass concentration of smoke generated from smoldering punk decreased

by less than 25% over a two-hour period during which time the number concentration dropped

by at least a factor of 10 in a 1.8 m

3 cubical aerosol chamber. Both the number and mass concentration properties of the smoke were measured to determine the mean particle diameter

for comparison with other methods.

3. MEASUREMENT METHODS

Several types of instruments were used to characterize the particle size distribution,

number concentration, mass concentration, and optical density of smokes under study. Most

of these instruments have become commercially available only recently.

8
As recently as a few years ago, the measurement of the size distribution of submicron

particles was both difficult and time consuming. As a result of research and development

support provided by the Environmental Protection Agency and other public agencies, commercial

instruments have been developed to the point that such measurements are becoming routine.

Among the most useful instruments are the electrical aerosol size analyzer, the optical

particle counter, the piezoelectric mass monitor, and the condensation nuclei counter.

Each instrument used in this work will be discussed briefly in terms of the principle

under which the instrument functions, its limitations, measurement ranges, and precision.

The reported data will be examined in light of these known limitations.

The performance of the instruments are summarized in Table 1. For a more complete

description, the reader is referred to references given in this paper. Other recent advances

in particle measurement techniques have been reported in the proceedings of a symposium at the University of Minnesota [11] and elsewhere [12],

3.1. Particle Size Distribution - 0.006 to 1 pm

The Electrical Aerosol Analyzer (EAA) , the main instrument in this study, was used to measure the size and concentration of smoke aerosol in the 0.006 to 1.0 pm diameter range.

Its operation is based on the "diffusion charging-mobility analysis" principle first described

by Whitby and Clark [13]. Such charging is characterized by ion-particle collisions resulting

from the thermal motion of the ions. The electrical mobility of an aerosol charged under

conditions closely approximating that of "diffusion" charging in the EAA is a monotonically

decreasing function of the particle size for particle sizes smaller than about one micrometer

in diameter.

The instrument samples the aerosol at a constant rate of 4 liters per minute and at concentration as high as 2 x 10 6 particles per cubic centimeter for particles with D = 0.1 pm

8

without the need for sample dilution. For smaller particle sizes, the instrument can handle

higher number concentrations. However, for smoke concentrations at detector threshold level

(106 - 10 7 cm

3

) , a tenfold or more sample dilution was necessary.

A detailed description of the instrument is given elsewhere [14,15]. Essentially,

aerosol entering the analyzer is first exposed to unipolar positive ions and becomes electrically charged. Using the relationship between particle mobility and size mentioned earlier,

the size distribution of the aerosol can be calculated from the mobility distribution measured by the mobility analyzer.

9
Table

1.

Performance

of

Instruments

10

typical

sample

at

0.03

OD/m,

midpoint

of

the

maximumminimum

alarm

threshold

limit.
The mobility analyzer is in the form of a cylindrical condenser surrounded by clean

sheath air. Aerosol outside the sheath flows down in a laminar stream. The charged aerosol

is deflected through the sheath air by the voltage applied on the center electrode. For a given voltage on the center rod, particles above a certain critical mobility (smaller than

certain critical size) are precipitated while those with lower mobility (larger particles)

escape and are sensed by the electrometer sensor. By changing the voltage on the center

rod and measuring the corresponding electrometer current, the mobility and thus the size

distribution of the aerosol can be determined. The standard operating condition of the

instrument provides appropriate voltage steps to divide the size range of the instrument

into nine equal geometrical intervals of four intervals per decade in particle size ranging

from 0.006 ym to 1.0 ym. The complete voltage sequence can be scanned in about 2 minutes.

The digital output of the instrument can either be recorded or fed directly into a minicomputer for processing.

Precision and Calibration

The absolute calibration of the instrument was performed by monodisperse aerosols of

known concentration generated from a mobility classifier having sizes corresponding to the

geometric mean diameter of each channel of the EAA [14]. The geometric standard deviation,

a , of the EAA response to each monodisperse sample varies from 1.0 for the two smallest

Ssize ranges to 1.8 for the two largest size ranges [16]. Because of the large cr^ for the

last few channels, the EAA is most reliable for measuring size distribution for polydisperse

aerosols (o^ > 1.5) and not for monodisperse or near monodisperse aerosols. For best results,

the EAA should not be used for aerosols with a peak size in the number distribution greater

than 0.2 ym.

3.2. Particle Size Distribution 0.5 to 5 ym

Particles between 0.5 and 5 ym were measured by an optical particle counter that counts

individual particles (OPC) . In this instrument, a beam of light is focused into a small

viewing volume through which the airborne particles pass one at a time. The amount of light

scattered from each individual particle is measured by a photodiode detector. The detector

signals (pulse height), which are monotonically related to the size of each particle, are then sorted and stored in channels of the multi-channel analyzer of the instrument. In instruments without the multi-channel analyzer, only the pulse heights of selected ranges

are counted and displayed one at a time. Thus the operator can manually select the size

range and record the count frequency for each range to determine size distribution of the

aerosols

.

The commercial instrument for the present study [17,18] measured scattered light falling

in the cone between 8° and 20° with respect to the direction of the incident beam. The viewing

volume was about 0.5 mm3 while sample flow rate was about 0.3 L/min. To prevent sample

deposit on the optics and wall of the cell, clean sheath air surrounds the aerosol sample

stream through the viewing volume.

11
The OPC can measure aerosol number concentrations as high as 400 cm for particles

between 0.5 and 5 pm. To avoid coincident counting error, aerosols with concentrations

_3

greater than about 400 cm are diluted prior to the measurement. High dilution (x50) was required for sampling smoke aerosol. Smoke containing a large concentration of particles

smaller than 0.5 pm, even after the prerequisite dilution, did not appear to affect the

measurement

.

Though the OPC is a convenient measurement tool, its reliability depends strongly on

its calibration. The size indicated by the counter is equivalent to the size of the calibration aerosol, usually a spherical and transparent aerosol such as polystyrene latex

spheres. The shape and refractive index, m (real and imaginary part), of the measured particles may be different from that of the calibration aerosol so that the true size may be different from the one indicated by the counter. For example, it has been predicted for an instrument similar to this that a 2 pm non-absorbing particle with refractive index m = 1.54

produces the same signal as a 4 pm absorbing particle with m = 1.54 - l.Oi, where the second

term is the imaginary or absorbing part of the refractive index [19]. This shows that all

particle size data need to be interpreted with caution and with due consideration for the

limitations of the methods used.

7 -3

3.3. Number Concentration 300 to 10 cm

To measure the number concentration of smoke aerosols with a wide range of particle

size from 0.005 to 2 pm and at moderate concentration, a condensation nuclei monitor (CNM)

[20] was used. The instrument has 6 linear scales covering the number concentration range

-3 7-3

1,000 to 300,000 cm and a non-linear range extending up to 10 cm . The sample rate is

about 3 L/min with continuous analog display of the particle concentration.

The instrument operates on the principle of a cloud chamber in which water is condensed

upon submicron particles to provide micron size droplets. A constant flow volume of sample

is periodically diverted through a humidifier and then into a cloud chamber where a fixed

volume expansion of the sample occurs. A cloud is thus produced by the 300% supersaturation

in the presence of the particles. Each large water particle grows from a single nucleus of

the smoke to a certain finite size.

The concentration of the cloud is measured by the attenuation of a light beam which

traverses the chamber and is focused on a silicon photodiode. After expansion, the cloud

chamber is pressurized and flushed out. The total measurement cycle time is approximately

one second.

The instrument has a high degree of repeatability, better than + 5% of full-scale on

the linear range (0-300K) . The accuracy of the instrument is dependent on the primary calibration standard. The manufacturer claims an accuracy of + 20% based on the Poliak counter

as the standard; however, Liu et al. found a factor of about two difference between the

nuclei counter and their standard, the mobility classifier [21].

12
3.4. Mass Concentration

Mass Monitor Method

To determine the mass concentration of smoke aerosol, both the particle mass monitor and

the filter collection method were used.

The mass monitor is based on the high sensitivity of the resonant frequency of a quartz

crystal to changes in mass due to deposition of particles on the crystal surface. In practice, the aerosol is drawn into the electrostatic precipitator chamber of the monitor at

a flow rate of 1 L/min by a vacuum source. The particles in the sample stream are deposited

onto the surface of a piezoelectric quartz crystal by a highly efficient electrostatic precipitator. The resonant frequency of the quartz crystal decreases linearly with the total mass of the particle deposition, thereby providing a measure of the mass concentration. A

second or reference quartz crystal, located downstream of the flow, compensates for possible

changes in temperature and gas composition in the flow cell. Thus the time rate-of-change

of the output frequency signal is directly proportional to the total particle mass concentration.

The commercial monitor [22] is capable of sampling mass concentration between 1 and 60

mg/m3 at size range from 0.01 to 10 ym. The frequency can be measured precisely to + 1 Hz,

and this is equivalent to a mass concentration resolution of 0.005 mg/m 3 for a one-minute

measuring period. The particle mass concentration for this instrument may be calculated

as follows:

C

1

S Q E E

c w

Af

At 333

Af

At

where C is in yg/m3

, Af in Hz, At in seconds, Q sampled aerosol flow rate in m3 /s, E^ is

the collection efficiency, and E is the efficiency for weighing the deposited particles. w

The factor 333 is obtained assuming a flow rate of 1 L/min and unity for E^ and E^. The mass sensitivity, S is given by [23,24]:

S AfAm

N b^pA

where b is the thickness of the quartz crystal, m is .mass of collected aerosol, p is the

density of the crystal (2.65 g/cm3 ), A is the area of the crystal, and N is the frequency

constant of AT - cut quartz (0.166 MHz cm). An S of 180 Hz/yg is given in the instrument

manual. As a typical example, an aerosol with a mass concentration of 40 mg/m 3 produces a frequency shift of about 1200 Hz in 10 seconds.

13
To minimize error- caused by the + 1 Hz measurement uncertainty, sampling time At has to be sufficiently long for low concentration aerosol. Fdr example, a sampling time of at least

33 seconds is required for a 1 mg/m3 aerosol in order to reduce the error to the + 1% level.

To assure 100% collection efficiency for liquid phase aerosols a trace amount of vapor

(proprietary, available from only one known supplier) was allowed to diffuse into the inlet

aerosol stream and act as a conditioning agent for the particles. This vapor adheres in one

or two monolayers to the particles. The liquid monolayer is extremely hydrophilic and conse- quently absorbs additional monolayers of water if available. These layers decrease the electrical sensitivity of the particles and make the particles adhere firmly to- the crystal. . The crystal requires cleaning when its total deposit from previous runs reached 200 to 300 yg. Collection efficiency is not necessarily 100% for aerosols of solid particulates even- with

the use of t»he conditioning vapor. In that case calibration using the filter-collection

method is needed.

Collection by Filter Method

Teflon filters (type AA or FH, Millipore) 25 mm diameter and 0.5 ym pore size, were

used in the filter-collection-weighing method. The sampling rate, for the flows regulated

by a critical orifice, was generally 4.9 L/min. The total sample mass was on the order of

0.5 mg on a filter nominally weighing 24 mg, •

'

This weighing method was inherently more accurate than the mass monitor though it

required a large volhme of sample and was less convenient. The method was used to calibrate

the mass monitor and replaced it for certain types of smoke that did not rigidly adhere to the collection surface of the monitor.

Though filter pore size was 0,5 or 0.2 ym, particles as small as 0.01 ym are also

collected with nearly 100% efficiency due to the diffusion mechanism [25],

3.5. Aerosol Chamber

In order to minimize wall losses, a relatively large chamber was constructed for the

generation and retention of smoke under study! The chamber was a 1.25 m cube with inside

surfaces consisting of smooth plastic panels. Small ports in the cube provided access for

sampling, air inlet, exhaust, viewing as well as insertion and control of the smoke generating source.

A photometer with a path length equal to the full* height of the cube was located near

the center of the chamber. The light source was a 15-watt incandescent lamp operating at a color temperature of approximately 2200K. The beam was uniformly collimated and has a width

of about 25 mm. The light detector has a P-39 phototube with S-4 sensitivity curve. The

output current from the phototube in series with 67 . 5 V dry cell battery, was measured by

14
a digital picoammeter. The photometer system was sufficiently stable that a light beam

attenuation of 0.1% could be determined.

There were provisions for control of inlet air in terms of residual particle concentrations and humidity. Prior to generation of smoke, the residue particle concentration of

filtered and dried air in the chamber was routinely maintained at less than 200 particles

per cubic centimeter as measured by the condensation nuclei monitor. The humidity could be

adjusted between 20% and 80%.

The air-tight and rigid chamber was maintained at slightly positive pressure during

sampling by feeding filtered air into it at a rate somewhat higher than the withdrawal rate

by the analysis instruments. Since the total chamber volume was 1.8 m , the effect of dilution caused by the sampling at 3 - 7 L/min for a total period of 2 to 10 minutes was less

than 5% and was neglected. A 10-cm diameter muffin fan in the chamber assured uniform

distribution of aerosol in the chamber.

The chamber was subsequently modified to include a thin plastic bag section 100 liters

in volume extending above the chamber. The purpose of the inflatable bag was to maintain a constant pressure in the chamber during sample withdrawal without the need to replace it with

air.

3.6. UL 217 Smoke Detector Evaluation Chamber

Testing of residential smoke detectors for approval is performed in the United States

by two organizations; the Underwriters' Laboratories, Inc. (UL) and Factory Mutual Research

Corp. (FM) . Each organization publishes its own test method standards and a list of approved

devices. The test method used by UL for testing both types of detectors is the UL 217 [4],

while the method used by FM for testing the ionization and photoelectric type detectors is

FM Class No. 3230-3250 [26].

Basically, these methods measure the light obscuration of a given type of smoke at the

alarm threshold of the detectors. A minimum sensitivity of 0.06 OD/m (4% obscuration per

ft for a 5-ft beam length in the UL 217 chamber) for grey-color (cellulosic) smoke and 0.14

OD/m (10% per ft) for black smokes (heptane) are required. There are no requirements based

on absorption coefficient, particle size, or concentration in the test chamber. Since it

is known that particle size as well as other properties of the smoke affect light absorption

[27] and that the sensitivity to the properties varies, depending on the type of detector,

the characteristics of the smokes used in these test methods are of particular importance

and form a principal subject of this study.

15
lli

16

Figure

3.

Smoke

detector

evaluation

chamber

from

UL

217.
The detector evaluation chamber (UL 217) is essentially a 50 x 50 x 170 cm closed

horizontal box in which the smoke detector is mounted on the center of the top surface as shown in figure 3. The chamber is partitioned into a top-and bottom-half sections by a platform extending almost the full length of the chamber. Smoke generated in the lower section

is circulated through the box in the direction indicated by the arrows in figure 3. A con- stant circulating velocity of 0.18 m/s (35 ft/min) is maintained.

A photometer which measures the light obscuration of the circulating smoke has a path

length of 152 cm (5 ft) and a beam width of 5 cm. It consists of an incandescent lamp

(GE-4515) source operating on 2.4 V and a photovoltaic cell (Weston 594 RR) detector.

In the testing, smoke from a smoldering lamp wick or heptane flame in the closed chamber

is allowed to accumulate to the triggering level of the smoke detector and was measured by

the chamber photometer in terms of percent obscuration. Since the smoke source burns continuously, the concentration of the smoke in the chamber increases with time.

4. AEROSOL SOURCE MATERIALS AND DILUTION

Punk

Punk (imported from the Orient as incense sticks) was found to be a useful source of

combustion aerosol. It consisted of a bamboo stick core with a square or rectangular cross section, 1 to 2 ram in width. A finely ground mixture of cellulosic material, binder, and

oxidant coated the stick to a final diameter of about 3 mm and a length of about 160 mm. After the igniting flame is extinguished, the punk stick smolders and self-propagates in a downward direction. The downward smoldering rate was found to be about 4 mm/min. Though

it was a convenient source for high cencentration aerosol (~10 11 particle per minute at the

peak size range between 0.1 and 0.17 pm) , large variation in size and composition of the

punk sticks were observed in single batches and resulted in a variation in the generation

rate among the sticks. However, the peak particle sizes of fresh smoke appeared to be

independent of the stick diameter.

Lamp Wick

Lamp wick, a knitted cotton strip 3 mm thick and 22 mm in width, is manufactured as wicks for kerosene lamps. In use, a strip about 4 cm in length was ignited and allowed to smolder from the bottom and burn upward. It was hung in the middle of n cylinder 5 cm in

diameter and 7 cm high, constructed from 100 mesh metallic screen. The screen cylinder

with the smoldering wick was then inserted into the UL 217 chamber to initiate the test.

17
i

CO

18

Figure

4.

Aerosol

diluter,

dilution

factor

depends

on

orifice

sizes.
a-Cellulose Disc

This material, similar to filter paper, consisted of pure os-cellulose fibers from

cotton linters. Samples 0.8 mm-thick obtained from the NBS standard reference material SRM

1006 stock were cut into 6-mm diameter discs with a mass of 8.6+0.17 mg (+ standard deviation). For some experiments, larger pieces of this material were used.

Heptane

Black sooty smoke was generated by diffusion burning of heptane from the top of a wick,

one end of which is soaked in heptane. The cotton wick bundle was imbedded inside and ex- tended slightly above the top of a 20-mm long glass tube with an inside diameter of 3.5 mm. The tubing extended about 6 mm above the surface of the cap which covered the heptane

container

4.1. Method for Aerosol Dilution

To keep within the measuring range of each instrument, aerosols above a certain concentration level were diluted prior to the measurement. A useful dilution technique was to pass

the aerosol through a 0.2 pm pore size pleated capsule filter cartridge (Gelman 12106) which

was modified to include a small by-pass orifice. For ease of mounting and for ease in changing

the orifice size, aluminum discs with different orifice sizes were constructed and cemented

to one end of pipe fittings. The fitting could easily be screwed into the threaded hole of

the plastic junction in the filter as shown in figure 4.

For a 0.71-mm orifice the dilution ratio was about 16; this means that 1/16 of the

aerosol went through the orifice and the remaining 15/16 were filtered prior to rejoining the stream. As shown in figure 5 the filter did not significantly distort the size distribution

of the aerosol. The dilution ratio for the smallest particle size (0.042 pm) was found to be 15.4 compared to 18.1 for the largest particle size (0.75 pm). No correction was made

for this small variation.

5 . RESULTS

5.1. Smokes in the UL 217 Chamber

The relationship between optical density (light obscuration), mass concentration, number

concentration, and particle size distribution of lamp wick and heptane generated smoke were

determined in the UL 217 chamber.

In these measurements, a smoldering lamp wick burning from bottom to top was held vertically in a holder and placed in the burner end of the chamber. The wick was withdrawn when

the smoke reached a desired obscuration level, ranging from 0.005 to 0.08 OD/m. After a 30-35 second waiting period to insure uniform mixing, the aerosol was sampled by the analyzers.

19
AN

/A

log

Dp,

cm

Figure 5. Size distribution of an aerosol before and after dilution.

Numbers denote dilution ratios for each size range.

20
In certain experiments, analysis was performed periodically as the aerosol continued to age

in the chamber. A nominal flow velocity of 0.18 m/s was maintained at the center of the

detector platform throughout the experiment.

Smoke Generation Rates

The average rates of change in light attenuation during the aerosol build-up period were approximately 0.02 OD/m and 0.07 OD/m per minute for lamp wick and heptane aerosols,

respectively. There were large variations in the generation rates probably caused by the

nonuniformity of lamp wick construction and heptane burning.

Mass Concentration

Figure 6 shows the mass concentration as a function of light obscuration in the UL 217

chamber for aerosols generated from smoldering cotton lamp wick and from the heptane diffusion

flame. The mass concentration for the lamp wick and the heptane aerosols were determined by

using the quartz mass monitor and the filter-weighing method, respectively. The quartz mass monitor was not effective in collecting the heptane-generated aerosol.

Because of the 3 to 4 minute sampling period required to collect sufficient heptane

particulate for precise weighing, there was a large shift in obscuration level during this

period in the chamber. The shift was caused by aging as well as by aerosol dilution from air

infusion into the small (~350 liters) chamber to replace sample withdrawal. The extent of

the shift is shown by the horizontal bars in figure 6. As a result of this longer sampling

period, in contrast to the shorter 20-30 second period used by the mass monitor, measurement

uncertainty for the filter-collection method is expected to be larger than the mass monitor

method in these experiments.

The inclusion of aerosol of various concentrations and ages has also increased the

scatter of data points shown in figure 6.

Number Concentration

The condensation nuclei monitor (CNM) with a 16 to 1 diluter was used to measure the

particle number concentration for the two types of aerosols generated in the UL 217 chamber.

Figure 7 shows the concentration as a function of smoke obscuration for aerosols of different

initial concentrations. Numbers adjacent to the data points denote the age of the aerosols,

measured from the time the burning specimen was inserted into the chamber to the time the

aerosol was sampled by the monitor. Numbers at the top of the curve also indicate the mini- mum time for generation to reach the indicated levels.

The analyses of a given aerosol as it aged are connected by the short curves. From

these curves it can be shown that the light obscuration per particle increases with aging.

For example, obscuration of lamp wick aerosols with almost identical concentration, 2.6 x

10 cm , was about 0.04 OD/m for a 2.7-minute aerosol, but over 0.07 OD/m for an 8-minute

21
22

0.02

0.04

006

0.08

SMOKE

OBSCURATION,

OD/m
5x10* £

JiJ0 *NOIlVdlN30WOO d3awnw

23

0.02

004

0.06

0.06

0.10

0.12

0.14

SMOKE

OBSCURATION,

OD/m
aerosol. This observation and its implication in the use of the UL 217 chamber will be

discussed in Section 6.2.

Size Distribution

The size distribution of aerosols generated under standard conditions for detector

evaluation in the UL 217 chamber were measured by the EAA. In figure 8 the results based

on three determinations are given for smoke aerosol generated from lamp wick at obscuration

levels of 0.066 OD/m and 0.015 OD/m, which approximately correspond to the minimum (0.06 OD/m)

and maximum (0.006 OD/m) allowable detector sensitivity used by the UL 217 method for approval

purposes. The ages of the lamp wick aerosols were 9 and 4 minutes, respectively, while the

age of the heptane smoke was 6 minutes.

In table 2 the key physical parameters for lamp wick and heptane smoke in the UL 217

chamber are given. For lamp wick smoke, the parameters given represent approximately both

the allowable minimum and maximum light obscuration levels. Two significant differences

between lamp wick smoke and heptane smoke are apparent from table 2. First, heptane smoke

has about twice the optical density per meter compared to lamp wick smoke for a given mass concentration based on particle mass monitor and filter measurements. Secondly, the size

distribution (o ) is narrower for the heptane smoke than for the lamp wick smoke (also in

fig. 8 ).

It is apparent from table 2 that optical density and mass concentration for the two lamp wick smokes are linearly related but that the optical density and number concentration

are not. As will be explained more fully later, this is because coagulation affects the

number concentration but not the mass concentration.

The relative agreement between the mass concentration as measured by the PMM (Particle

Mass Monitor) and as derived from EAA (Electrical Aerosol Analyzer) measurements (assuming

unit density for the smoke) is typical for many measurements. The ratio of the EAA number

concentration to that of the CNM (Condensation Nuclei Monitor) for the lamp wick smokes is

somewhat less than 2 , which is a typical number for many other measurements as will be

discussed in Section 6 .

Table 2. Measured and Derived Parameters of

Smokes in the UL 217 Chamber Test

Instrument UL 217

Obscuration

OD/m

PMM

Mass

EAA

Concentration

mg/m 3

EAANumber

10 6

CNM

Cone • _ 3 cm

D

8 pmg

Lamp wick 0.066 42 21 3.4 2.5 0.14 1.7

Lamp wick 0.015 10 6 1.4 1.0 0.12 1.6

Heptane 0.017 5* 5 0.9 0.3 0.16 1.5

* Filter-Collection Method

24
Figure 8. Particle size distribution for lamp wick and heptane smokes.

25
Figure 9. Effect of air movement (2 m/s) on particle size.

26
5.2 The Effect of Pyrolysis Conditions on Particle Sizes

The temperature and ambient airflow above the material undergoing pyrolysis have strong

effects on the particle size distribution of the resultant smoke aerosols. Such effects were examined using punk and a-cellulose disc in the (1.8 m

3

) aerosol chamber.

Figure 9 shows two size distribution curves. One represents the aerosol from a smoldering punk stick (glowing but not flaming) held in a vertical position with the hot tip at the

top in still air. The other curve shows a similar burning mode except that the punk stick

was exposed to an air stream moving at a velocity of 2 to 2.5 m/s. There is a considerable

difference between the two size distributions. The peak size is approximately 4 times larger

for the still air compared to the moving-air smokes. A calculation based on figure 9 shows

that the moving-air smoke was produced at a rate of 10 9 particles per second compared to only 10 8 particles per second for the still air.

5.3. Effect of Flaming and Non-flaming Exposure on Particle Size of a-cellulose Smoke

The size distribution of smoke aerosols also depends strongly on whether the material

is flaming or smoldering. In this experiment an a-cellulose disc was placed on top of a metal surface maintained at 500°C and in another, the disc was ignited by a very small

premixed pilot flame. Figure 10 shows the size distribution of the two aerosols sampled

from the 1.8 m

3 chamber after an aging and mixing period of about 5 minutes. Aerosol con- centration peaked at less than 0.01 pm under the flaming mode compared to 0.075 pm under the

smoldering mode.

Figure 11 shows an interesting mixture of aerosols generated under two distinct modes

of combustion. An a-cellulose disc was being heated at a rate of about 100°C/min from room

temperature to 600°C. Self-ignition occurred when the partially pyrolyzed specimen reached

a temperature of about 550°C.

The peaks in the size-distribution curves indicate the existence of a mixture of the

two aerosols; one characteristic of smoldering exposure, which resulted in a peak at about

0.075 pm. Another is characteristic of the flaming mode which generated much smaller particles with peak at about 0.024 pm.

As the smoke aged, the concentration of most sizes decreased with the smaller particles

decreasing at a much faster rate than the larger size particles. After 30 minutes the peak

has shifted to 0.075 pm.

27
Figure 10. Smoke from a-cellulose under flaming and non-flaming exposure

conditions

28
.Figure 11. Aging of aerosol from source which progresses from smoldering

to flaming.

29
5.4 Influence of Pyrolysis Air Temperatures

To separate the effect of pyrolysis temperature and the influence of coagulation in the

early phase of particle formation, an a-cellulose disc was exposed to high-velocity air

streams which were heated to 395°C and 485°C. Hot air at a velocity of about 0.6 m/s flowing

thru a 13-mm diameter tube impinged on the sample disc (8.6 mg) located in the end of the

tube

The resultant aerosol was discharged into the aerosol chamber, allowed to age and analyzed by the EAA. Because of the high airflow rate at the generating source, particle size

growth due to aging was effectively frozen until the aerosol began to accumulate in the

chamber. The time for complete generation of the aerosols was about 1 minute.

Figure 12 shows two sets of curves. Each set represents the particle size distribution

of the aerosol generated under a given air temperature. Peak particle size for the aerosol

formed under the 395°C condition was 0.13 pm; whereas, the peak size for the aerosol generated under the high temperature of 485°C was 0.075 pm. Because of aging the peak size gradually shifted to 0.13 pm after about 30 minutes in the chamber.

5.5. Influence of Aging on Particle Size Distribution

Particle coagulation (aging) due to Brownian collisions plays an important role in

changing the size distribution of an aerosol. Coagulation is said to occur when two particles collide and form a single particle whose volume is the sum of the volumes of the original two particles. The number of particles is thereby decreased by one while the total

volume of the particles remains the same.

The results of some aging experiments on punk and a-cellulose smoke are shown in

figures 11-13 where size distribution measurements were made at various times. The coagulation process results in a large drop in number concentration and a noticeable change in

particle size distribution for a-cellulose and punk smoke. In figure 12 the total number

concentration for the 395°C smoke drops from 2.0 x 10 6 to 1.3 x 10 6 in 10 minutes with most

of the change taking place for the small particle sizes.

The aging data in figure 13 is quite extreme with a variation of two orders of magnitude

in the total number of particles per cubic centimeter over a 16-hour period. The rate of

particle loss by coagulation is shown by the figure to be faster at high concentration. This

is consistent with coagulation theory and will be discussed in Section 6.4.

30
An/ A

log

Dp,

cm

Dp,/im

Figure 12. Aging of aerosols generated at two temperatures.

31
Figure 13. Aging of punk aerosol up to 16 hours.

32
6. DISCUSSION

6.1. Comparison of Measurement Methods

Since the instruments used in this work were based on different measurement principles

with different ranges, sensitivities and calibrations, it is not surprising that data from

different instruments do not coincide. The results were compared to determine the range of

correlation so that measurement by a single instrument may be sufficient in predicting the

response from another method of measurement.

Mass Concentration Measurement

The performance of the mass monitor was compared with that of the filter-collection

method for some punk smokes. The mass monitor results were generally about 10% lower than

the filter results. The data were taken from 10 sets of measurements during a one-month

period for aerosols with peak sizes between 0.05 and 0.12 ym in the concentration ranges of

10 to 17 mg/m3

.

The discrepancy in results may be attributed to poor collection effeciencv of the mass

monitor. Varying collection efficiency has also been observed by others for different types

of aerosols [24]. The mass monitor was not used in determining the concentration of heptanegenerated smoke because of poor collection efficiency. In subsequent work, the mass monitor

was calibrated for the specific type of smoke based on the filter-collection method to minimize error.

In estimating the relative consistency of results obtained by the quartz mass monitor

and the EAA instrument, the data on size-distribution from the EAA were converted to total mass assuming spherical particles and a density of unity for all sizes in the aerosol. The

average ratio between the mass measurement obtained by the quartz monitor and mass based on

size measurement by EAA was 1.5 + 0.3 (avg. + std. dev.) for over 20 separate punk aerosols.

The ratio appeared to increase with the peak size and age of the aerosols. This is consistent

with recent data on polydisperse aerosols where it was found that the ratio also increases

with particle size.

Particle Number Measurements

For a given aerosol, number concentration was measured by both the CNM and by the EAA. Comparison of the results in cases where both instruments were used simultaneously, showed

that the EAA registers higher values than the CNM. The ratio between EAA and CNM was 2.0 +

0.3 (avg. + std. dev.) for 15 punk and a-cellulose aerosols measured in the concentration

range of 104 to 10 6 cm

3

. This discrepancy is in the same direction and similar in magnitude

to that found by Liu, et al. [21].

33
6.2.

UL 217 Detector Evaluation Chamber

Measurements in the UL 217 standard detector chamber show the relationship of particle

number concentration, mass concentration and aging of standard smoke aerosols to light

obscuration, the criterion for approval testing. The maximum allowable alarm threshold of

0.06 OD/m was found to correspond to a mass concentration of 40 mg/m 3 for lamp wick and

18 mg/m 3 for heptane aerosols. The corresponding number concentration for fresh smokes were

about 3.5 x 106 and 1.6 x 10 6 particles/cm 3 for the aerosols, respectively.

Since detector sensitivity is fundamentally determined by aerosol concentration, particle size and in certain cases, refractive index, rather than by light obscuration, a detector

response (triggering threshold) to lamp wick smoke is unlikely to be identical to that of

another smoke, say, heptane smoke, with the same obscuration. For this reason, the UL maximum alarm threshold for the black smoke test has been established at 0.14 OD/m (10%/ft)

instead of 0.06 OD/m used for the grey smoke.

Specific deficiencies of the UL 217 test method include the repeatability in aerosol

generation and the effect of aging in the chamber as shown in figure 6. The variation in

generation rate is on the order of + 20%. The aging effect is also illustrated in figure 7. Since the residence time (aging duration) of the test smokes varies, depending on the triggering level and entrance resistance of the detector, the difference between low and high

obscuration smokes depends not only on concentration but also on the size distribution of

the aerosols. Therefore, measurements of other variables in addition to obscuration are needed for an adequate measure of detector sensitivity.

6.3.

Aerosol Generation Modes

The mode of combustion was shown to have a large effect on the size distribution of

the resultant aerosol. For a-cellulose, the flaming mode produces much smaller particles

(D * 0.01 pm) than the smoldering mode (D * 0.07). Even under smoldering conditions, the

8 S size distribution is affected by the flow conditions and the temperature. Both an increase

in temperature and an increase in flow velocity result in a decrease in particle size though

the relative importance of the two effects was not quantitatively studied. An experiment

(Section 5.4.), in which coagulation was minimized by high flow velocity, showed a noticeable reduction in particle size with increased temperature. An increase in temperature

apparently causes smaller particle size through more complete combustion while an increase

in velocity is believed to decrease the time for particle growth by coagulation.

6.4.

Aerosol Aging and Coagulation Losses

The effect of aging, which seems to show a disproportionate loss of smaller particles

with little gain in larger particles, is amply illustrated in figures 11-13. The fact that

it takes a thousand 0.1 pm particles to form a single 1.0 pm particle is a qualitative

34
explanation of the aging curves; but to determine how well this data fits the overall

coagulation theory, the average coagulation coefficient for each smoke was calculated.

The fundamental quantity for coagulation is the coagulation coefficient, T, which is

defined by:

dN

dt -r n

2 (7)

The fact that the loss rate is proportional to the square of the number concentration explains

the observation made in Section 5.5. that the rate of particle loss is much faster at higher

concentrations. The calculation of the coagulation coefficient was based on the integrated

version of the above equation:

1N 1

N

rt ( 8 )

As defined above, T is an average coagulation coefficient for a given aerosol, through F in

reality is particle size dependent. For the punk smoke data reported in figure 13 plus 2 sets of data reported by Mulholland et al. [10], T was found to be in the range between

3.9 and 4.6 x 10 cnr/s with peak diameter on the order of 0.15 pm.

- 1 0 o For flaming a-cellulose, T was found to be between 10 and 19 x 10 cm J /s with peak

diameter around 0.05 pm. These values are of the same order of magnitude as the theoretical

values reported by Zebel [28]. The fact that the smaller particle size has the larger T is

also consistent with the work reported by Zebel [28].

6.5. Universal Size Distribution

A representative sample of all the data displayed in the previous figures has been

plotted in terms of reduced variables ip and n in figure 14. The number distribution is

reduced by the total number of particles in the size distribution, N(t), and the diameter

is reduced by a quantity related to the average diameter for the size distribution, (V/N) J

for that particular smoke. Thus:

A N

A log D N (t) (9)

( 10 )

The data includes fresh smoke as well as aged smoke, smoke generated in both the flaming

and smoldering modes, whitish smoke and black sooty heptane smoke, and smoke generated under

35
Figure 14. Representative size distributions in terms of reduced variables.

36
different airflow conditions. The numerical variation in size distribution data is greatly

reduced by using the reduced variables. Based on the reduced variable, the ranges, for the

peak size and peak concentration are 0.5 - 1.0 and 1.4 - 2.0 respectively for the data

in figure 14 compared to 0.03 - 0.25 ,ym and 5 x 10 4 - 8 x 10 6 cm

3 respectively for the

unreduced data. This is with the exception of flaming a-cellulose which does not have a peak. There is a qualitative agreement in the shape of the size distribution for all the

data especially for the larger values of q. From the data for small q, it appears that the

heptane smoke has a narrower distribution than the others.

Also shown in figure 14 are the reduced versions of an algebraic size distribution

(see equation 6) and a log-normal size distribution (see equation 5).

p * 1.38n 3 (q

3 + 0.2)

2

, Junge-like (11)

r&n l.23n

|

2

p * 1.73 exp

0*744

^ log-normal with a * 1.7. (12)

It appears that both the algebraic distribution and the a * 1.7 log-normal distribution fits

o the data. It has been shown by Mulholland, et al. [10] in a coagulation calculation that

the reduced algebraic distribution is only weakly affected by aging due to coagulation as is

the case experimentally (see aging data in figure 14). This means that the algebraic distribution could be used as a model size distribution not only for a variety of smokes but also

for smokes at various stages of aging. It is not known whether the log-normal size distribution also has this property.

The observation that a number of different smokes can be represented by a simple algebraic form could have important practical applications. By measuring two macroscopic properties of a smoke aerosol, the number and volume concentration, and by using the algebraic

form, one can obtain a good estimate of the size distribution, AN/Alog D. The method for

doing this is outlined below. The practical advantage is that it is easier and less expensive in terms of instrumentation to measure the concentration of an aerosol with a nuclei

counter and the mass concentration by a filter method than to measure the full size distribution with an EAA or with an electron microscope. The danger, of course, is that the

smoke, for any of a number of reasons, might not follow this distribution.

The procedure for obtaining the physical size distribution from the reduced size

distribution is best explained by an example. Assume that the number and volume concentration have been measured and N * 8.2 x 10 5 cm

3 and V * 137 ym 3 /cm 3

. From equations (9), (10),

and (11) one finds that

AN Alog D

P

1.38 V D

3

(D

3 +

(13)

37
Substituting for V and N, one obtains the estimated size distribution:

A N

A log D

p

1.9 x 102 D 3 cm3

(D 3 + 3.3 x 10

-5

um3)

2

( 14 )

SUMMARY AND CONCLUSIONS

Commercial instruments including the electrical aerosol analyzer, condensation nuclei

monitor, optical particle counter, piezoelectric mass monitor and filter collection method

were found to have sufficient precision and sensitivity for determining the size distribution,

particle number concentration, and mass concentration of aerosols in the alarm-triggering range

of smoke detectors. More confidence is placed on the relative values rather than the absolute

values of the data. Because of the difference in operating principle and sensitivity ranges

of the instruments, experimentally determined ratios were used to correlate results from

N(EAA) M(PMM) different measurement methods. These ratios are:

jq ( c

~NM)~ "" ^

» M(EAA) ~ ^ where N * number

concentration and M = mass concentration.

The maximum allowable alarm threshold for detector approval under UL 217, 0.06 OD/m

(4%/ft), was found to correspond to a mass concentration of 40 mg/m3 and a number concentra6 — 3 tion of 4 x 10 cm with peak at about 0.15 ym for the standard lamp wick aerosol.

The UL 217 detector evaluation chamber, based only on the light attenuation (obscuration)

property of the smoke, has limitations in providing the basis for judging detectors in terms

of sensitivity. Some of the limitations are:

1. Aerosols having identical light attenuation values in the chamber

could show considerable difference in concentration or size distribution

including and and have a different effect on detector response.

2. Size distribution of the smoke changes as the smoke concentration increases

in the UL 217 chamber. The peak size increases on the order of 20-40% as obscuration changes from 0.015 to 0.06 OD/m.

The deficiencies can be removed by using a highly repeatable steady-state aerosol source such

as an aerosol generator.

It was found that many factors affect the size distribution of smoke aerosols:

38
1. The peak particle size from a cellulosic material under smoldering condition

(~0.075 pm) was several times larger than that under flaming condition

(~0 .013 ym).

2. The peak particle size from a-cellulose decreased by a factor of about

two when its exposure temperature was increased by about 100°C from 395°C.

3. The peak particle size from smoldering punk decreased by a factor of about

four when the airflow velocity was increased from near 0 to 2 m/s.

4. The size distribution for heptane smoke (o =1.5) was narrower than

§ for cellulosic smoke (a = 1 . 7 ). ©

-

0 3 The coagulation coefficient for punk smoke was found to be about 4.3 x 10 cm /s.

The number concentration of punk smoke with the above coefficient will change from 1.0 x ?—3 7—3

10' cm to 0.44 x 10 cm in 5 minutes resulting in a 30% increase in peak size.

While there was considerable variation in the peak size and concentration of the

various smokes generated, there was a general similarity in the shape of the size distribution that allowed most of the data to be reduced to a single curve. It was found that a simple algebraic function fits the data quite well for both fresh as well as aged smoke.

This universal distribution is of basic significance for smoke aerosols and also could be

of greater practical use in data correlation and smoke detector testing.

The largest particle size measured in this study was 1 ym. There is a need for size

distribution measurements up to 5 ym as well as more accurate measurements over the range

0.3 ym to 1 ym. Such information will be useful in better defining the universal distribution as well as providing needed information for predicting the response of light-scattering

type detectors.

Similar types of measurements to those given in this study, if extended to other

materials such as plastic fabrics and elastomers as well as to ambient aerosols, would

further delineate the problem of false alarms.

39
8 . REFERENCES

[1] McGuire, J. H. and Ruscoe, B. E. , Fire Study No. 9, National Research Council, Canada

(1962).

[2] Harpe, S. W. , Waterman, T. E. and Christian, W. J., Detector Sensitivity and Siting

Requirement for Dwellings, Phase 2 Nat. Bur. Stand. (U.S.), NBS-GCR 77-82 (1977).

[3] Bright, Richard G. , Nat. Bur, Stand. (U.S.) (1977), private communication.

[4] Standard for Single and Multiple Stations Smoke Detectors UL 217, Underwriters'

Laboratories, Inc. (Jan. 1976).

[5] Bukowski, Richard W. and Bright, Richard G. , Results of Full-Scale Fire Tests with

Photoelectric Smoke Detectors, Nat. Bur. Stand. (U.S.), NBSIR 75-700 (1975).

[6] Hesyestad, G. , Escape Potential from Apartments Protected by Fire Detectors in High

Rise Building, Factory Mutual Research, FMRC No. 21017 (1974).

[7] Fry, J. F. and Eveleigh, C., The Behavior of Automatic Fire Detection Systems, Fire

Research Note 810, British Fire Research Station, Borehamwood (1970).

[8] Mulholland, George W. and Liu, B. Y. H. , Smoke Detector Responses, to be published.

[9] Cadle, Richard D. , Idle Measurement of Airborne Particles, John Wiley & Sons, New York

(1975).

[10] Mulholland, George W., Lee, T. G. and Baum, H., The Coagulation of Aerosols with Broad

Initial Size Distribution, J. of Colloid Interface Sci, to be published.

[11] Liu, Benjamin Y. , Editor, Fine Particles - Aerosol Generation, Measurement, Sampling

and Analysis, Acacemic Press, Inc. (1976).

[12] Cassatt, W. A., Maddock, R. S., Editors, Aerosol Measurements, Nat. Bur. Stand. (U.S.),

Spec. Publ. 412, 193 pages (Oct. 1974).

[13] Whitby, K. T. and Clark, W. E. , Tellus, Vol. 18, 573 (1966).

[14] Liu, B. Y. H. and Pui, D. Y. D., On the Performance of the Electrical Aerosol

Analyzer, J. Aerosol Sci., Vol. 6, 24 (1975).

[15] Liu, B. Y. H. , Berglund, R. N. and Argawal, J. K. , Atm. Env. , Vol. 8, 717 (1974).

[16] Whitby, K. T. , Electrical Measurement of Aerosols, Fine Particles - Aerosol Generation,

Measurement, Sampling and Analysis, B. Y. Liu, Editor, Academic Press, 581 (1976).

[17] Willeke, Klaus and Liu, B, Y. H. , Single Particle Optical Counter, Fine Particles - Aerosol Generation, Measurement, Sampling and Analysis

,

B. Y. Liu, Editor,

Academic Press, 697 (1976).

[18] Instruction Manual Particle Counter Model 200-5, Techecology, Inc. (Met One) (1974).

[19] Cooke, D. D. and Kerker, M. , Response Calculation for Light Scattering Aerosol

Particle Counters, Applied Optics, Vol. 14, 734 (1975).

40
[20] Manual for Condensation Nuclei Monitor, Model Rich 100 (1971), Environmental One Corp.,

Schenectady, New York.

[21] Liu, B. Y. H. and Pui, D. Y. H. , A Submicron Aerosol Standard and the Primary Absolute

Calibration of the Condensation Nuclei Counter, J. of Colloid Interface Sci., Vol. 47,

155 (1974).

[22] Instruction Manual, Particle Mass Monitor Model 3200, Thermo-Systems, Inc. (1975).

[23] Olin, J. G. and Sem, G. J. , Piezoelectric Microbanance for Monitoring the Mass Concentration of Suspended Particles, Atmospheric Environment, Vol. 5, 653 (1971).

[24] Lundgren, D. A., Aerosol Mass Measurement Using Piezoelectric Crystal Sensors,

Fine Particles - Aerosol Generation, Measurement, Sampling and Analysis, B. Y. H. Liu,

Editor, Academic Press, 485 (1976).

[25] Liu, B. Y. H. and Lee, K. W. , Efficiency of Membrane and Nuclepore Filters for

Submicron Aerosols, Env. Sci, & Tech., Vol. 10, 345 (1976).

[26] Smoke Actuated Detectors for Automatic Fire Alarm Systems, Approval Standard, Factory

Mutual Research Corp. Norwood, Mass., Class No. 3230-3250 (1973).

[27] Kerker, M. , The Scattering of Light and Electromagnetic Radiation, Academic Press,

New York (1969)

[28] Zebel, G., Coagulation of Aerosol, Aerosol Science, C. N. Davies, Editor, Academic

Press, New York (1966).

41
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 77-1312

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

"Physical Properties of Smokes Pertinent to Smoke Detector

Technology"

5. Publication Date November 1977

6. Performing Organization Code

7. AUTHOR(S)

Thomas G. K. Lee and George Mulholland

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4915678

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as 9

13. Type of Report & Period Covered

Final Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Several commercially available aerosol instruments including the electrical

aerosol analyzer, nuclei condensation monitor, quartz mass monitor, and optical

particle counter were used to measure particle size distribution of smokes from

burning heptane and cellulosic materials. Some limitations of these instruments

are discussed. Parameters such as mode of exposure (flaming versus smoldering)

pyrolysis temperature, air velocity at smoke emitting site, and aging were found

to have a large effect on the smoke particle size distribution. Mass and number

concentrations of smokes from cotton lamp wick as a function of smoke obscuration in the standard UL 217 detector test chamber were determined. The maximum

alarm threshold obscuration of 0.06 OD/m (4% per ft), required for detector approval, was found to correspond to lamp wick aerosol mass concentration of 40 mg/m3 and

particle concentration of about 4 x 10® cm 3 with the peak size in the number

distribution of about 0.15 pm. The present UL 217 test method was shown to be

affected by smoke coagulation and did not provide a complete measure of sensitivity

in smoke detectors. An algebraic model size distribution, with number and mass

concentration the only free parameters , was shown to provide a good estimate for

all the smoke size distributions measured.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Aerosol; mass concentration; number concentration; size distribution; smoke;

smoke detector; test method; UL 217

18. AVAILABILITY jXXl Unlimited C For Official Distribution. Do Not Release to NTIS

1 ! Order From Sup. of Doc., U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. CU

I Xl Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

45

22. Price

$4.00

USCOMM-DC 29042- P 74