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3 NBS TECHNICAL NOTE 973

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U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

moke Detector Design

and Smoke Properties

QC

100

1IU5753

liO. 973

,1978

iC.2
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for pubHc benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

Agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities^ — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical

services to users in the public and private sectors to address national needs and to solve national problems in the public interest; conducts research in engineering and applied science

in support of objectives in these efforts; builds and maintains competence in the necessary

disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes engineering standards and code changes; develops and

proposes new engineering practices; and develops and improves mechanisms to transfer

results of its research to the utlimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering' — Mechanical

Engineering and Process Technology^ — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal Agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following divisions:

Systems and Software — Computer Systems Engineering — Information Technology.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234. ^Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
DEC 7 Sl^ .

Smoke Detector Design and Smoke Properties ^cco ^ — . (

'^ S'7^

Richard W. Bukowski

George W. Mulholland

Center for Fire Research

National Eneineerine Laboratorv

no •'-( L 01^

Errata sheet to accompany National Bureau of Standards Technical Note 973,

Smoke Detector Design and Smoke Properties, by Richard W. Bukowski and

George W. Mulholland.

The page number 8 has been Inadvertently omitted.

On page 41, the third line from the bottom now reads:

_<0.16 m diameter;

It should read:

<0.16 um diameter.

November 1978

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 Arribler, Director

Issued November 1978
National Bureau of Standards Technical Note 973

Nat. Bur. Stand. (U.S.), Tech. Note 973, 51 pages (Nov. 1978) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $2.30 Stock No. 003-003-01991-7

(Add 25 percent additional for other than U.S. mailing).
TABLE OF CONTENTS

page

LIST OF FIGURES iv

LIST OF TABLES v ABSTRACT 1

1. INTRODUCTION 1 .

2

POTENTIAL PROBLEM AREAS 3

3. PHILOSOPHY OF APPROVAL STANDARDS 5

4. REFERENCE INSTRUMENTATION. ... 5

5. TECHNICAL DEVELOPMENTS 6

6. THEORETICAL WORK 11

7. IONIZATION DETECTORS 11

7.1 Particle Size 13

7.2 High Air Velocities 15

7.3 Low Air Velocities 15

7.4 Chamber Design 16

7.4.1 Source Strength 16

7.4.2 Electrode Spacing 17

7.4.3 Electrode Bias Voltage 17

7.4.4 Outer Shell Design 17

7.5 Detector Enclosure Design 18

8. PHOTOELECTRIC DETECTORS 18

8.1 Particle Size 18

8.2 Scattering Angle 20

8.3 Particle Shape 20

8.4 Refractive Index 21

8.5 Wavelength 21

8.6 Design Parameters 21

8.6.1 Entry 21

8.6.2 Circuit Time Constants 21

9. SMOKE RESEARCH GOALS 22

10. AEROSOL INSTRUMENTATION 22

11. DETECTOR SENSITIVITY TO MONODISPERSE AEROSOLS 26

12. SMOKE PROPERTIES 30

13. SMOKE AGING 37

14. LOOKING AHEAD 38

15. REFERENCES ..... 44

111
LIST OF FIGURES

Figure No. Page

1 Smoke alarm market predictions 2

2 Comparison of smoke detector sales and average retail cost

- 1971 and 1977 3

3 Current smoke detector integrated circuits [3] 4

4 Typical variation of failure rate with time [4] 5

5 NBS reference photometer 7

6 Photometer light source 8

7 Photometer receiver 9

8 Measuring ionization chamber 10

9 Ionization chamber operation - no smoke • 12

10 Ionization chamber operation - with smoke 12

11 Bipolar ionization chamber 14

12 Unipolar ionization chamber 14

13 Influence of wind on a unipolar and a bipolar ionization

chamber 16

14 Light-scattering detector optics 19

15 Mie scattering by polydisperse spheres, refractive index

1.33, compared with diffraction, refraction and reflection 20

16 Electric aerosol analyzer 23

17 Comparison of the number concentration as determined by

the electrical aerosol analyzer and the condensation

nuclei counter for sucrose aerosols with various

geometric mean number diameters [14] 24

18 The effect of particle size on the ratio of the aerosol

volume as determined by the EAA and filter collection

method [14] 25

19 Monodisperse aerosol generation system [15] 27

20 Response of a light-scattering type detector versus number

concentration for monodisperse aerosol [15] 28

21 Detector sensitivity versus particle size for a lightscattering type detector (S-2) and for an ionization type

detector (R-2) [15] 29

22 Smoke detector evaluation chamber 31
LIST OF FIGURES, continued

Figure No. page

23 Optical density per meter versus mass concentration for

lamp wick smoke and heptane smoke [20] 33

24 Particle size distribution for lamp wick and heptane

smokes [20] 34

25 Smoke from a-cellulose under flaming and non-flaming

exposure conditions [20] 36

26 The effect of aging on smoke detector response 37

27 Aging of the punk smoke for up to 16 hours [20] 39

28 Size distribution in terms of reduced variables [20] ... 40

29 Smoke detector tester [27] 42

30 Laser doppler size spectrometer [28] 43

LIST OF TABLES

Table 1. Measured and derived parameters of smokes in the UL 217

chamber test 35

SMOKE DETECTOR DESIGN AND SMOKE PROPERTIES"*"

Richard W. Bukowski and George W. Mulholland

The importance of a reference photometer and reference ionization detector in improving the reliability of smoke detectors is

discussed. Recent developments in smoke detector technology are highlighted and theoretical as well as practical experience in

regard to detector performance is summarized. Comparison of the

theoretically predicted response of smoke detectors as a function

of particle size with measured values is given. A monodisperse

aerosol generator, an electrical aerosol analyzer with a size

sensitivity from 0.01 to 1 ym, and an optical particle counter

are described. The size distribution, mass and number concentration, optical density, and coagulation frequency for smoke from

burning heptane and smoldering cotton lamp wick are presented. It

is shown that a Junge type size distribution provides a good fit to

the measured size distribution for both fresh and aged smoke.

Key words: Aerosol generators; detector testers; fire detectors;

ionization detectors; light-scattering detectors; particle size

distribution; smoke; smoke detectors.

1. INTRODUCTION

This paper consists of two parts. The first part concentrates on smoke

detector design (sections 1-8) and the second part consists of recent research

on smoke aerosol properties pertinent to smoke detection (sections 9-14) . Smoke detectors, particularly the residential variety, have undergone an extremely rapid growth in sales to where they are currently being compared

with digital watches, pocket calculators, and citizens band radios in popularity. In the last five years, residential smoke detectors have grown from

annual sales in the U.S. of less than a few hundred thousand to a market level

of twelve to fourteen million in the calendar year 19 77. You will note in

figure 1 (which gives actual sales estimates prior to 1977 and our projections

for 1977 and beyond) that the 1977 sales were two million units ahead of our projection made in May of 1977.

Because NFPA records indicate that approximately 75% of the U.S. fire

deaths occur in residences and because studies [1]^ have indicated that almost

half of these residential fire deaths could be prevented by the widespread use

of residential smoke detectors, it is obvious that the impact of these sales

on the reduction of U.S. fire losses can be great.

The vastly increased sales volume as well as technical developments in

electronics have resulted in greatly decreased prices in the marketplace. For

example, in 1971 residential smoke detectors cost an average of $125 per unit.

Currently, a number of detectors are available for under $20 per unit retail

This paper was presented at the Third Joint Panel Meeting of the United

States and Japan Natural Resources (U.J.N.R.) Panel on Fire Research and

Safety, March 1978.

Numbers in brackets refer to the literature references listed at the

end of this paper.
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($25 average - figure 2). While decreased cost, extensive consumer interest,

and large-scale national advertising have all contributed to this increased

sales volume it can also be readily understood that the performance and

quality of the devices is critical in achieving the projected impact on fire

losses. For this reason, the National Bureau of Standards, Center for Fire

Research is deeply involved and committed to work which will provide a better understanding of the design, performance, and proper use of smoke

detectors

.

1971 1977

NO.UNITS

SOLD

50,000 12-14 MILLION

UNIT

PRICE

$ 125 $ 25

Figure 2. Comparison of smoke detector sales and average

retail cost — 1971 and 1977.

2. POTENTIAL PROBLEM AREAS

Identified problem areas with residential smoke detectors can be divided

into three broad categories. These are false alarms, performance and design,

and reliability.

The primary cause of false alarms in residential applications is cooking.

Technically, this is a difficult problem to overcome since the cooking process

produces particulates of similar size ranges and concentrations to those seen

in hostile fires. Our feeling is that, for most applications, optimum choice

of type of detector and placement within the dwelling can preclude most false

alarm problems from cooking. Also, a recent survey [2] has indicated that the

false alarm problem with residential detectors may not be as large as was originally thought. Survey data indicate that, of the people contacted who

own residential detectors, 96% rarely or never experienced a false alarm.

Also, 97% of those people contacted indicated that they were completely

satisfied with the device purchased.

With regard to design and performance, the problems are more complex.

As the market has grown, many companies have begun manufacturing residential

detectors. Some of these companies have invested little in research and have

designed detectors strictly by trial and error. This has led to marginal

designs' and marginal performance. More recently, a number of semi-conductor

manufacturers [3] have begun marketing smoke detector integrated circuits

containing all necessary electronics except for the sensing chamber, power

source, and sounding device (figure 3). Two manufacturers are producing

ionization chambers which can be connected to these smoke detector integrated

circuits, which essentially eliminates the need for any research and development on the part of the final manufacturer. Thus, more and more dependence
manufacturer

model ION PHOTO

SILICONIX SM 110 X X

GENERAL INSTRUMENT MEM 4962 X

MEM 4963 X X

MOTOROLA MC14461P/14462P X

SUPERTEX SDIA X

SD2A X X

NATIONAL SEMICONDUCTOR LM1801 X

RCA has also introduced an integrated circuit for smoke detectors,

(model CA3097)

.

Figure 3. Current smoke detector integrated circuits [3].

is being placed on the role of the approving laboratory for assurance that

marginal or poorly performing units are not marketed. Thus, the need for

strict approval standards which assure a fairly high minimum level of

performance has become increasingly important.

Another complicating

been the cost competition

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Minimizing production cos the impact of performance

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factor in the poor desi

in the marketplace. Pr

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ion volumes have increased

how up until the detector

major smoke detector recalls

sibility that additional

the hundred thousand or less

The third problem area of reliability is again a complex factor. Based

on current failure rate predictions, the average residential smoke detector

is expected to last on the order of 15 to 20 years before it has to be replaced.

It would be desirable to design a detector such that any failure which would

prevent its detection of a fire would be indicated to the homeowner by some

type of trouble signal. Taken to the extreme, this would result in an extremely expensive unit. Thus, the approach taken has been to try to

maximize the reliability of the device without significantly impacting its

retail cost. Attempts are being made to devise a realistic method by which

the failure rate of a given smoke detector design can be predicted mathematically. While this project is still in its initial stages, we hope to have

some answers to this question by the end of 1978. Figure 4 shows a general

curve of the variation of failure rate with time, valid for most electronic

products [4] . Our goal is to have point A when the unit leaves the factory

and point B at 25 to 30 years after sale.
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INFANT MORTALITY

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PERIOD

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PERIOD

WEAR OUT

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TIME

Figure 4. Typical variation of failure rate with time [4]

3. PHILOSOPHY OF APPROVAL STANDARDS

As was mentioned earlier^ the product approval standards for residential

smoke detectors are becoming increasingly important due to the large numbers

of units and designs available. In the past, the philosophy of the U.S.

approval standards has been essentially to try to duplicate the conditions

of a few specific fires to which the detector might be exposed. While this

philosophy has worked fairly well in the past, we feel that the philosophy

used by the European approval standards would be better. This philosophy is

not to duplicate a few specific fires at random but to design tests which

exaggerate the differences between the units tested and which expose the

detector to a bracketing range of conditions that might be expected but not

necessarily any specific "real life" fire type. With regard to the detection

performance testing of smoke detectors, for example, one would design a set of

full-scale tests which expose the detector to a range of particle sizes, number

concentrations, and refractive indices so that the reaction of the device to

any particular combination of these variables within the extremes tested could

be estimated.

4. REFERENCE INSTRUMENTATION

Both the increasing popularity of the use of smoke detectors and the

increasing desire for additional knowledge about their performance has resulted

in a large amount of testing being conducted by various organizations. More

and more data are being generated which can be highly useful in expanding our knowledge of the subject. But, the increase in the numbers of people conducting

tests has demonstrated the problems in comparability of the data obtained from

different sources.

5
Basically, the comparability problems arise from the large variety of

instrumentation being used to take smoke measurements in the tests. For

example, the lamp color temperature and receiver spectral sensitivity of most

photometers is not consistent or even specified. Therefore, we have developed

a specification for a reference photometer (figure 5) which we are proposing

as one such instrument. This photometer consists of a tungsten filament

light source (figure 6) of a specified color temperature transmitting a collimated beam of light through a one-meter path length to a receiving

element (figure 7) with a spectral response matching that of the hiiman eye.

By using the human eye response we hope not only to have a reference instrument by which to relate smoke detector response but also to be able to take

measurements which relate to human visibility through smoke. While these

measurements would not take into account the irritability of the smoke, we

feel that the measurements will be meaningful in estimating the amount of

time available to people in a fire area for escape or rescue.

In addition to the reference photometer, we are evaluating a reference

measuring ionization chamber developed by a Swiss firm (figure 8) . Since the

photometer is not responsive to particles smaller than about 0.3 micrometers

in diameter,- the photometer will not correlate well with the output signal

from an ionization chamber. Thus, where such correlations are necessary, some type of reference ionization chamber is necessary. This Swiss design is being

evaluated by us and others in this country and in Europe and it appears to

be the best design thus far proposed.

Another instrumentation problem that we are working on is that of reference test aerosols. Many people in fire research agree that the combustion

generated aerosols now used for small-scale test purposes are too variable.

Such parameters as material, density, moisture content, thermal history,

ambient conditions, and others, have a great effect on the characteristics of

the aerosol generated. Thus we feel that a mechanically generated aerosol is

the only practical solution to eliminating or minimizing these variables.

To this end, we are developing two types of aerosol generators. One will be

a highly precise generator for laboratory use in small-scale test compartments;

the other will be a less precise, but easy to use, portable unit for testing

installed smoke detectors in the field. The field unit could be used to

determine the sensitivity of an installed detector instead of just determining

whether or not it is operating as is now done. A description of these aerosol

generators is given in sections. 11 and 14 of this paper.

5. TECHNICAL DEVELOPMENTS

Many technical developments have been made over the last few years in

smoke detector design. Battery operated residential detectors which initially

used special batteries which were expensive and hard to obtain are being

replaced by designs using common batteries which are more available and

inexpensive. Also, new concepts in the circuitry that monitor the battery

condition have been made. Several detectors now on the market monitor the

battery for both terminal voltage and internal resistance build-up which

could prevent the operation of the sounding device in a fire situation.

Detector enclosure and chamber designs are making more use of basic

theoretical parameters with great improvements in detection performance being

seen. New types of sounding devices are being investigated which have better

frequency characteristics for audibility, higher reliability, and lower power

consumption.

Ionization detectors are being produced with less radioactive material

and a new detector using a beta source is now being marketed for commercial

applications. Designs of some newer ionization detectors exhibit increased

sensitivity to larger particles. The lack of this capability has been a performance drawback in the past.
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Photoelectric detectors are almost exclusively using long life lightemitting diode (LED) light sources, more efficient scattering angles, and

light receivers and circuit designs which minimize time constant problems

which had been encountered in earlier LED designs. More photoelectric

detectors are using electronic ambient light rejection which eliminates the

need for restrictive light-tight chambers which slow their response to fire

produced aerosols. Photoelectric detectors in particular are also making

better use of mechanical or electrical test features which allow testing of

the entire detector circuit (including basic smoke sensitivity) simply by

pushing an external button. This has come a long way from the original test

buttons which did nothing more than connect the horn to the battery.

6. THEORETICAL WORK

Considerable emphasis in the Center for Fire Research (CFR) has centered

on obtaining a better theoretical understanding of aerosol properties and design

parameters which can affect the response of smoke detectors. To this end, the

following summary of such theoretical considerations has been assembled from

open literature reference sources, from results of tests conducted by CFR

staff and others, and in some cases from observations by the authors.

7. IONIZATION DETECTORS

The general operation of an ionization chamber can be described in the

following way. The chamber consists of a source of ionizing radiation

positioned between two electrodes across which an electric potential is

maintained (figure 9) . The radiation source emits alpha particles (helium

nuclei) or beta particles (electrons) at relatively high energies (e.g. 5.48

Mev for americium 241 alpha particles) , creating positive ions by removing

electrons from gas molecules along their path. The low energy electron released

rapidly attaches to a neutral gas molecule which becomes a negative ion.

These ions are then drawn to the electrodes (positive ions to the negative

electrodes and vice versa) where they give up their charge. This charge

transfer represents a small (~10 A) current flow through the air space

between the electrodes.

There are two ways in which an ion can be prevented from reaching the

electrode and give up its charge. These are by recombination (random collision

with an oppositely charged ion) or by being carried out of the chamber by

convective airflow before reaching the electrode.

Some amount of recombination takes place in most ion chambers but is

self-limiting. That is, the recombination rate is related to the ion density;

the greater the density the greater the chance of a random collision. Thus,

any chamber will have an equilibrium condition where the recombination rate

and ion density will be constant.

Normally, the ion velocities are high enough and the convective flow

rate low enough so that most ions reach the electrodes. When smoke particles

enter the chamber, these particles capture ions, reducing their velocity by

several orders of magnitude due to the increased mass of the particle-ion

pair (figure 10) . This reduced velocity allows the pair to be carried from

the chamber before reaching the electrode, reducing the charge transfer and

thereby the chamber current. It is this reduction in current which is used

to trigger the alarm.

An alpha particle leaving the surface of americium 241 (Am) will create

an equal number of positive and negative ions for a distance of about 4 cm

(the mean range of an alpha particle of this energy in air at STP) [5] . If

the electrode spacing in the chamber is 4 cm or less, a bipolar chamber results

11
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Figure 9. Ionization chamber operation - no smoke.

CONVECTIVE FLOW

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Figure 10. Ionization chamber operation - with smoke,

12
(figure 11) . That is, ions of both polarities exist in the entire chamber.

If, however, the electrode spacing is greater than 4 cm, a unipolar chamber

is created (figure 12) . The unipolar chamber has one region where both ion

polarities are present (in this case in the first 4 cm from the source) and

a region where ions of only one polarity are present. These two regions are separated by a space charge region which acts to stabilize the ion concentration in the unipolar region.

Another way of producing a unipolar chamber is to reduce the travel

distance of the alpha particles to less than 4 cm by increasing the thickness

of the outer gold plating on the americium. A gold thickness of 6.6 ym will

reduce the alpha particles range to 0.45 cm in air [5].

7,1 Particle Size

Hosemann [6] derived a semi-empirical equation for the relative chamber

signal of a bipolar ionization chamber. Scheidweiler [7] expressed this

equation as follows:

Nd //Nd \2

S = —^ + 1 -\[—^] + 1 (1)

2n

and n = 3. = chamber constant {2]

C

where S = relative chamber signal = AI

I o

N = number of particles of size d

P. d = particle diameter

P

a = recombination coefficient

q = ion generation rate

C = Bricard capture coefficient

Litton has developed a detailed mathematical model for an ionization chamber

[8] and finds that Hosemann 's semi-empirical equation agrees within a few

percent with his model results for all values of the quantity Nd /n. For low

concentrations of a smoke aerosol equation (1) reduces to:

S <^ Nd (3)

That is, the output signal is proportional to the number of particles times

the particle diameter. In this relation, the proportionality constant is a function of the chartiber design parameters and would thus be slightly different

for different chamber designs.

Equations (1) and (3) are for monodisperse aerosols. For polydisperse

aerosols, equation (3) is simply summed over the particle size distribution

as follows:

2:«id (4)

13
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SPACE

CHARGE

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©

©

Am 241

Figure 11. Bipolar ionization chamber.

©

© © ©

© ©

©

Am 24 n

Figure 12. Unipolar ionization chamber,

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>4cm

14
When using this general relation, one must keep in mind that an aerosol

particle size distribution is dynamic, varying with time and distance from

the generation source. The particle diameter will tend to increase due to

coagulation effects which are related principally to time and concentration.

Also, particularly in combustion aerosols, the particle size distribution

being generated can change as a function of temperature of combustion,

material and its density, moisture content, and other factors.

When one looks at coagulation of liquid aerosol droplets or aerosols

composed of solid nuclei with condensed liquid exteriors it can be seen that

the number concentration for a fixed mass concentration is inversely proportional to the diameter cubed. Thus, if the aerosol diameter doubles, the

number concentration would be reduced by 1/8 so that the overall effect would

be the reduction of the relative chamber signal by a factor of 4. The effect

of coagulation will be discussed in more detail in section 13.

7.2 High Air Velocities

Since the basic principle of operation of the ionization detector involves

a small flow of current created by the transfer of charge across the chamber,

any factor which interferes with this charge transfer will affect the chamber

current and therefore the response. The effect of high air velocities is to

blow the charged ions from the chamber before they can reach the electrodes

and give up their charge. Since, under these conditions the charge transfer

would be reduced, the chamber current would also be reduced, moving it toward

alarm. This would have the effect of enhancing the sensitivity but can also cause a false alarm if the velocity is sufficient to remove enough ions. In

his convective model, Litton [8] derived the following relation for the limits

of convective flow in the chamber:

Z E P

< V^ < yE (5)

where Z = mobility of charged smoke particle

->E = average electric field

->• V = convective velocity in the chamber

y = mobility of an ion.

Design factors which can compensate for this effect would be velocity

shielding by mechanical means, increasing the ionic velocity (and therefore,

the ionic momentum) through the use of higher electrode collection potentials

or the use of the unipolar type chamber design. Since the unipolar chamber

contains a space charge region, this region tends to act as a buffer,

releasing more ions into the unipolar region when the ion concentration falls

below equilibrium. This essentially stabilizes the chamber performance over

a much broader range of air velocities than with a bipolar chamber design. A

comparison of the change in chamber current with increasing air velocity was given by Scheidweiler [7] and is shown in figure 13.

7.3 Low Air Velocities

Equation 5 also shows that the effects of low air velocities are almost

a converse of the high velocity effects. That is, low velocities can allow

charged smoke particles to reach the electrodes and discharge before the con- vective flow through the chamber can move them out of the chamber. This

15
ION. CURRENT

^^

AIR VELOCITY

Figure 13. Influence of wind on a unipolar and a bipolar

ionization chamber.

condition can result in an increase in chamber current and a corresponding

loss of sensitivity. This effect may be a partial explanation (along with

increased particle diameters and decreased number concentrations) of the

apparent loss in sensitivity of ionization detectors to smoldering fires.

7.4 Chamber Design

7.4.1 Source Strength

The chamber constant (eq. 2) is a function of the volumetric ion generation rate, q (which is established by the source strength), the recombination

coefficient, a, and Bricard's capture coefficient, C. The ion generation rate

(q) is the only parameter which is affected by the detector design, the other

parameters (a and C) being determined by the properties of the aerosol and

ions

.

From equations (1) and (2) in the limit of low particle concentrations

it is seen that the detector sensitivity is inversely proportional to the

square root of the ion generation rate and thus the source strength. In

practice the advantage of the increased sensitivity with decreased source strength must be balanced against the reduction in the signal to noise ratio

as the source strength is reduced. This effect can be seen in the new 3 source detectors which operate at very low source strength and chamber currents

(several orders of magnitude below typical a source detectors). The 6 detector

will detect the extremely small particulates from burning alcohol to which

most a detectors will not respond. The 6 detector, however, has had to

incorporate extensive RF shielding and, in addition to the normal sensing and

reference chambers a third (balance) chamber is used to obtain acceptable

stability.

16
7.4.2 Electrode Spacing

If the chamber electrodes are closer than the alpha particle path length,

the alpha particles will not produce the maximum number of ion pairs possible.

If the electrode spacing is greater than the alpha particle path length,

a unipolar chamber results. When determining proper electrode spacing, one must keep in mind that the thickness of gold outer foil plating of the ionization source greatly affects the alpha particle path length. As was explained

earlier, an unplated source will have an alpha particle path length of 4.04 cm

in air (at STP) . Addition of a 6.6 pm thickness gold foil reduces the alpha

particle path length to 0.45 cm. From this it can be seen that the plating

thickness is extremely critical and that very small nonuniformities in the

plating thickness can have great effects on the uniformity of chamber ionization. '

Simon and Axmark [9] investigated the effect of electrode spacing on the

ion chamber response both experimentally and theoretically for a parallel

plate electrode design. The detector sensitivity, AI/I , was measured as a function of electrode spacing, D, for a fixed value of the ratio of potential

difference <t> , to electrode spacing, <t>/D = constant. As the plate separation

was increased from 3 cm to 5 cm, the sensitivity increased by approximately

100%. For this experiment the range of the a particle was 3 cm so in one case the chamber is bipolar while in the other, there is a unipolar region in

the chamber. The model calculation, which includes the effect of space

charge in the field equation, was found to agree with the experimental results.

7.4.3 Electrode Bias Voltage

As shown in equation (5) , if the bias voltage on the chamber electrodes

is low, the ion velocity is low and there is more chance of recombination

with opposite ions before the electrode is reached. Also due to lower ion

momentum, the velocity at which captured ions can be blown from the chamber

is reduced.

If the electrode bias voltage is high, there is less chance of recombination and less chance that captured ions can be blown from the chamber; but

the radioactive source strength must be increased to maintain proper ion

densities. This can be a problem in countries where maximum source strength

specifications are set by law.

In the first case, the effect is an ion chamber inordinately sensitive

to air movement. In the second case, the effect is to reduce the sensitivity

of the chamber (unless the source strength is increased)

.

Litton [5] has studied the effect of the electrode bias voltage on the

chamber performance. He finds that equation (2) is valid provided the

chamber current is low compared to the saturation current (I/Ig < 0.4). As

the current increases above this ratio, the chamber response depends on the

electrode bias voltage as well as the source strength. For a given source strength and electrode geometry Litton found that there exists an optimum

bias voltage for which the current difference is a maximum. He also found

that an increase in the source strength S^^ will shift the electrode bias (f)^ at which AI is a maximum approximately as ' the square root of S .

7.4.4 Outer Shell Design

The design of the outer chamber shell relates to the high and low aerosol

velocity effects. If the shell is too open, the higher aerosol velocity

problem is enhanced and if too restricted, the low air velocity problem is

17
enhanced or the aerosol stream may take the path of least resistance and

by-pass the chamber completely.

7.5 Detector Enclosure Design

The outer enclosure of the detector can also be critical. If too open,

the unit may be susceptible to false alarm at high airflows and if too

restricted, the unit may alarm slowly due to delayed smoke entry. This

effect was demonstrated in an experiment conducted by NBS a few years ago.

Two detectors (A and B) , identical except for their enclosures, were tested

for sensitivity to smoke at varying air velocities. One unit (A) would not

alarm for any smoke concentration at a flow of 0.076 m/s (15 f pm) , while the

other (B) showed only a small decrease in alarm point at this velocity.

Conversely, one unit (B) would give a false alarm at a flow of only 1.5 m/s

(300 fpm) while the other (A) was stable to much higher velocities.

In addition to velocity effects, the following factors should also be

taken into account in a proper enclosure design.

1. The electric fields generated in the peripheral detector circuitry

can affect the chamber operation.

2. Current leakage paths on high impedence components or on insulators in the sensing chamber can cause false alarms or non-operation

.

3. Adhesion of charged aerosol particles to external plastic parts

that have acquired a static charge can reduce the number of the

small particles reaching the sensing chamber.

8. PHOTOELECTRIC DETECTORS

.Most photoelectric detectors in common use operate on the light-scattering

principle. The sensing chamber contains a source of light and a light receiver

at some angle to the light beam arranged so that the receiver does not normally

receive any of the transmitted light (see figure 14). When smoke particles

enter the scattering volume (the volume of space which intersects both the

light beam and viewing region of the receiver) , light is scattered onto the

receiver. This increase in luminous flux on the receiver is proportional to

the concentration of smoke particles and is used to trigger the alarm.

Early light-scattering detectors used short lived incandescent lamps (1-

5 year average life) and photoresistive receivers. Newer designs use lightemitting diodes as the light source (30 year or more life) and silicon

receivers (photodiodes , phototransistors , and silicon cells). Newer designs

have also eliminated the need for a darkened sensing chamber (to eliminate

effects of ambient light) by using pulsed LED ' s and electronic circuits which

reject the ambient signal. Elimination of this chamber has greatly improved

the performance by allowing much freer entry of smoke to the sensing optics.

8.1 Particle Size

The output signal from a scattering type detector optical assembly is

affected by particle diameter, complex refractive index, scattering angle,

scattering volume, light wavelength, and particle shape. In general, the

basic theory of light scattering is only well defined for spherical particles.

Some limited calculations are available for a few other shapes such as cylinders and ellipsoids.

18
PHOTO TRANSISTOR

LIGHT EMITTING DIODE

Figure 14. Light-scattering detector optics.

The effect of particle size falls into three regions defined essentially

by the ratio of particle size to light wavelength [10] . These three regions

are given below:

Rayleigh Region d < 0.1

Mie Region

Bricard Region d

O.lX < d <4a

4a

Limit varies with refractive index.

In the Rayleigh scattering region, the output signal is essentially proportional to the 6th power of the particle diameter.

The upper limit of the Mie region is not clearly defined as it varies as

a function of particle refractive index. A.t the Mie-Rayleigh boundary,

the chamber output signal is proportional to the particle diameter to the 6th

power and can oscillate in a damped sinusoidal manner until at the MieBricard boundary it is proportional to d squared. The frequency of

oscillation is a function of the refractive index of the particle.

For particles larger than about 4 A the theories of geometric optics

(Bricard, Fraunhoffer diffraction) predominate. . In this region the signal is

essentially proportional to the particle diameter squared.

19
8.2 Scattering Angle

The angular intensity distribution of light scattered by particles varies

with particle size (d ), shape, and refractive index (M) . An example of such

an angular distribution is given in figure 15 [10] . This is for Mie scattering

(particle diameter greater than one tenth of the wavelength) for a narrow band

polydisperse aerosol with M = 1.33 (water vapor). While the distribution will

vary with the above mentioned parameters for other size ranges, the same general characteristics are observed. That is, the greatest intensity is in

the forward direction, decreasing to a minimum around 90°-100° and increasing

again to a final value less than the initial value in the back scatter area. Only in the case of perfectly reflecting spheres is back scatter intensity

greater than forward scatter.

From this characteristic one can deduce that a small, forward angle

exhibits the best signal levels for most aerosols while angles around 90°

would generally give the lowest signal levels.

8.3 Particle Shape

The effects of complex aerosol shapes are largely unknown. One can empirically determine the effective scattering cross section of a complex

shaped particle but this parameter can change continuously as the particle

tumbles randomly in an aerosol stream. If the aerosol concentration is high

enough, it is valid in many cases to assume a random distribution of particle

orientations

.

CM

^ 1.0 E

^ 0.1 =

0.001

Mie theory

Diffraction+Refraction+Ref lection '^P/X = 10 to 18.5 m = 1.33

60° 100° 140° II

SCATTERING ANGLE

O

Figure 15. Mie scattering by polydisperse spheres,

refractive index 1.33, compared with diffraction,

refraction and reflection.

20
8.4 Refractive Index

Particle refractive index is a complex variable of the form:

M = M^ (1 - iK) (6)

where M is the real part of refractive index

o

K is the absorption coefficient.

One should note that absorption is the imaginary portion of the term and the

particle refractive index is only real where absorption is 0.

8. 5 Wavelength

The principle effect of wavelength is to define the boundaries of the

three scattering regions. Since the signal produced by an aerosol is strongest

in the Mie and Bricard regions, wavelengths should be as short as possible so that a majority of the aerosol sizes to be measured are in these regions.

This was not as much a problem when incandescent light sources were used since

they are polychromatic and produce light over most of the visible spectrum,

especially if driven at a relatively high color temperature. With the change

to light-emitting diodes, however, red and infrared wavelengths became more common due to decreasing quantum efficiency of LED ' s at shorter wavelengths

[11]. More recently, some improvement in yellow and green LED efficiencies have

been made but new combinations of base materials may be necessary before these

devices are usable at these shorter wavelengths.

8.6 Design Parameters

8.6.1 Entry

Ease of smoke entry into photoelectric detectors has improved greatly in

the last few years. Careful study of low velocity flow dynamics and the more

recent elimination of light-tight labyrinths by means of electronic ambient

light rejection have been the principle causes of this improvement. It is now possible to actually scatter light from an aerosol outside of the detector

enclosure. This obviously eliminates entry completely.

8.6.2 Circuit Time Constants

The change to light-emitting diode sources in photoelectric detectors

also created time constant problems. This is because the total luminous flux

from light-emitting diodes is only about 10% of that with incandescent sources.

Thus, more sensitive photoresistive (Cd S or Cd Se) cells were necessary as light receivers. But the time constant of these photoresistive cells (to a step input) is inversely proportional to its luminous flux sensitivity. ?^t normal LED light levels, time constants on the order of 5 minutes are common. These time constant problems can and have been eliminated by the use of

silicon devices such as photovoltaic cells, photodiodes or phototransistors

These devices, however, require more circuitry since they have no inherent

gain and can increase the cost of a detector by as much as 30% over those

using the photoresistive receivers. Hopefully, the cost savings associated

with the newer large-scale integrated circuits now being designed for photoelectric (and ionization) detectors may offset the additional costs and result

in better operating detectors with higher reliability.

21
9 . SMOKE RESEARCH GOALS

Above we have discussed the general behavior of smoke detectors. From

this point on we focus on properties of the smoke. In the United States there

is interest in smoke properties for several applications in the field of fire

protection

1. Early detection of smoke to alert occupants.

2. The blockage of vision caused by smoke produced by the combustion

of building materials.

3. The radiative heat transfer from smoke in the development of a fire

up to flashover.

The major emphasis in smoke research at the National Bureau of Standards has

been on the first application, which will be the focus of our discussion. The

general goal of our research is to provide some of the basic data necessary

for the development of improved smoke detectors and improved detector test

methods. The first half of our presentation on . smoke research will be con- cerned with the application of recently developed aerosol measurement techniques

and aerosol generation methods to the determination of the sensitivity of

smoke detectors as a function of particle size. The second half will be

concerned with the properties of smoke aerosols used in testing detector

performance. These properties will include size distribution, optical

density, and the aging of smoke.

10. AEROSOL INSTRUMENTATION

The measurement of the size distribution of smoke aerosols is a difficult

experimental problem. Smoke aerosols encompass a broad size range from on the

order of 0.005 ym for particulate from a propane torch to as large as 5 um for

well aged smoke generated from a smoldering source such as urethane foam. The

three orders of magnitude in this size range are equivalent to the change in

size extending from the diameter of a pin to the diameter of a beach ball.

The order of magnitude range in particle concentration is even greater. The

particle concentration of smoke drawn through a cigarette may be as high as

10 ^'^ particles/cm^ while the concentration of an aged smoke may be as low as

10 "* to 10^ particles/cm^ . in addition to the difficulty of measuring such

wide ranges in particle size and concentration, there is also the problem that

an aerosol is a dynamic, unstable suspension. As an illustration of the

dynamic nature of smoke, the number concentration of smoke will drop by a factor of ten from an initial concentration of 10^ particles/cm^ in two

minutes as a result of coagulation.

No single instrument is capable of handling the range of concentrations

and particle sizes encountered in smoke analysis. We shall describe two

instruments that we have found to be quite useful for smoke characterization

at low concentrations, 10^-10^ particles/cm^ . Special emphasis will be

placed on the calibration of these instriiments . In our work we find that

perhaps half of our time is devoted to instrument calibration. Even with this

effort, it was found that a measurement accuracy of + 30% is about the best

that can be obtained.

The principal instrument used in our study is the electrical aerosol

analyzer developed by Liu, Whitby, Pui and Clarke [12,13] to measure the size

distribution and concentration of aerosols in the size range 0.01 ym to 1 pm.

This instrument is similar in principle of operation to the ion mobility

counter described by Watanabe at the 1976 UJNR panel meeting. As shown in

figure 16 it consists of three major parts: aerosol charger, mobility

analyzer, and electrometer current sensor. During a measurement, the aerosol

22
AEROSOL J—-, r—

INLET ^"|J L_|"

MOBILITY

ANALYZER

AEROSOL

CHARGER

SHEATH AIR

HIGH VOLTAGE

PRECIPITATING

CONDENSER

ELECTROMETER

CURRENT SENSOR

Figure 16. Electric aerosol analyzer,

is first sampled into the aerosol charger to expose the particles to unipolar

positive ions produced by a corona discharge. The charged particles then

enter a mobility analyzer where they are deflected through a laminar air

stream in a cylindrical condenser. For a given voltage on the center rod,

particles above a certain critical mobility (smaller than a 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.

A series of experiments was performed at the University of Minnesota and

at the National Bureau of Standards [14] to determine the accuracy of the

electrical aerosol analyzer (EAA) for the measurements of aerosol number

concentration and volume concentration by comparison with a condensation

nuclei counter (CNC) and a filter gravimetric method, respectively. A comparison of the total number concentration as determined by the two methods

is shown in figure 17 for a polydisperse sucrose aerosol generated by an atomizer. It is seen that there is approximately a linear relationship

between the number concentration determined by the two methods over the

23
3^10^

CO

LU

3xlo3

O 0.031 ym

| 0.039 urn > EAA 1 A 0.068 |im j

0.095 m \ 0.13 m \ EAA 2 • 0.14 m j

05

Nqnc IPARTICLES/cm'

Figure 17. Comparison of the number concentration as determined by

the electrical aerosol analyzer and the condensation

nuclei counter for sucrose aerosols with various

geometric mean number diameters [14]. (Two nominally

identical electrical aerosol analyzers (EAA 1 and

EAA 2) were used for the measurements. The dashed

line corresponds to perfect agreement between the two

instruments, while the solid line represents a least

square fit of the data.)

concentration range 10^ to 10^ particles/cm^

fitted to the following functional form:

The data can be conveniently

Log NEAA Log (const) + Log NCNC (7)

From a least square fit of the data to equation (7) it is found that the

constant equals 1.3, which means that the EAA overestimates the number

concentration by about 30%.

The size dependence of the volume measurement by the electrical aerosol

analyzer is presented in figure 18 for dioctyl phthalate aerosols generated by

a nebulizer. The most significant feature about the data is the rapid decrease

in the ratio V /V for particle sizes larger than 0.4 ym. There are

two likely reasons for this discrepancy. First, particles larger than 1.0 um will contribute to the aerosol volume collected on the filter, but will not

contribute significantly to the aerosol volume measured by the electrical

aerosol analyzer. Secondly, the mobility versus particle size characteristic

of the aerosol analyzer is rather flat in the 0.4 to 1.0 um diameter size

range, precluding the possibility of making accurate size distribution measurements in this range.

24 i
0.5 -

0.2 -

0.1 -

0.04

1 III

•^ EAA 2 ^^^OOP AEROSOL.

1 1 1

ag = l.7 - •\ -

-

/ EAA 1 \ \

OOP AER0S0L,ag = 1.4 \ \

\'\

\

—

1•4 V

\

\ ~

\

\

\

\

\\_

\\

\

-< —

—

1 III 1 1 1 1 .

0.1 0.2 0.3 0.5 0.7

GEO. MEAN VOL. DIAMETER [[im)

Figure 18. The effect of particle size on the ratio of

the aerosol volume as determined by the EAA

and filter collection method [14]

Particles with diameters in the size range 0.5 to 5 ym were measured by

an optical particle counter that counts individual particles. 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 related to the size of each particle, are

then sorted and stored in channels of the multichannel analyzer of the

instrument.

The conventional method of calibrating optical particle counters is with

an aerosol formed by nebulizing a suspension of latex spheres of known particle

size. This method was used in calibrating the optical particle counter.

Later in this paper a new method for calibrating optical particle counters

involving the laser doppler shift spectrometer will be described.

25
11. DETECTOR SENSITIVITY TO MONODISPERSE AEROSOLS

As was mentioned earlier, the particle size is an important parameter in

determining the sensitivity of a smoke detector. In order to obtain accurate

data on the size sensitivity of ionization and light-scattering type detectors,

a joint National Bureau of Standards University of Minnesota study was initiated making use of the excellent capabilities at Minnesota for the

measurement and generation of aerosols [15].

A major concern in the selection of an aerosol generation system was that

it provide a stable, steady-state output to allow time for the smoke concentration in the detector to reach a steady-state and to allow time for the

measurement of the concentration and size distribution of the aerosols. The

aerosol generation system that was finally developed for the detector sensitivity measurements is illustrated in figure 19.

An atomizer was used to generate a polydisperse dioctyl phthalate aerosol,

which was then made monodisperse by passage through an evaporation - condensation column. Next, the aerosol passes through conditioning equipment

which controls the aerosol concentration, humidity, and charge, after which

it enters the smoke detector chamber.

The resulting aerosol was quite monodisperse with a geometric standard

deviation, a , about 1.25. By varying the concentration of dioctyl phthalate

in solution with isopropanol, it was possible to generate monodisperse aerosols

over the size range 0.0 5 ym to 1.3 um. The concentration range was about two

orders of magnitude (2 x lO"* to 3 x 10^ particles/cm^) for the generator. The

concentration and particle size output of the generator were determined by

the electrical aerosol analyzer, the optical counter, and by a filter gravimetric method.

The concentration dependence of the analog detector output is shown in

figure 20 for particle sizes ranging from 0.15 pm to 0.57 ym. The detector

is a light-scattering type with a nominal scattering angle of 21° and an infrared light-emitting diode with spectral peak around 940 nm. Over the

concentration range studied, the analog signal was proportional to particle

concentration and the sensitivity increased rapidly with increasing particle

size. This detector did not respond to a particle size of 0.10 ym or less

at concentrations as high as 3 x 10^ particles/cm^. The alarm voltage

labeled in figure 20 corresponds to the detector signal produced by a polydisperse smoke aerosol witli an optical density of 0.056 m as measured in

the Underwriters' Laboratories test chamber, which will be described in the

next section. This value corresponds to the maximum threshold standard set

by the Underwriters' Laboratories.

The detector sensitivity for the light-scattering type detector (S-2)

and for an ionization type detector (R-2) are plotted versus particle size

in figure 21. The detector sensitivity is defined as the detector output

minus the background reading divided by the particle concentration and is

expressed in the units yV-cm^. The uncertainty in the determination of

sensitivity is estimated to be + 30% and is primarily attributed to the

uncertainty in the measurement of the number concentration.

It is of interest to compare the experimental sensitivities with theoretical predictions. For the light-scattering type detector, the sensitivity

is found to have approximately a six power dependence on particle size for the

smaller sizes. This is in agreement with the theory of Rayleigh scattering,

which is valid for particle sizes small compared to the wavelength of light.

Qualitative agreement over the entire size range was obtained between the

experimental sensitivity and the scattered intensity as calculated by Mie

theory [15]

26
ACCUMULATOR

WATER

NEUTRALI2ER

TO FILTER TO ELECTRICAL

AEROSOL ANALYZER TO OPTICAL

PARTICLE COUNTER

Figure 19. Monodisperse aerosol generation system [15]

27
nJ rH

to • C (0

13 r-^ tji u m in •H -rH u <-\ CD +J

(U •—

.

Q. > U

iH u o -P C w o to -P Q) U 5-1 p x;

(U (U (U -P

-P fC T! •-!

QJ s

Xi (\) (U m £ CD

Q) ^^ +j y.

Q. CU >i a g

-p tn 4-) to rH tni^ to QJ C Tj W

•H C c u

s-1 (U

(U g a a

-p to to -P M CD -rH

(tJ o s-1 13 U 4-,' s-1 >i —

tn -H .

1 c u o-^

+J a.1 ^ -H 0) g

en -P en (ti -H (0 ro ^o M M -P O LTl -p iH -P o

rO C O

OJ > 13 o

4-1 U CD C g to M-l S4 O

0) u rO Oi w nH X >1 C s-1 (0 CD -P

Q) rH CuXX (U c to

u) g x: (D c

0) 3 EH £ CD p: c — 3 -C o

CM

0)u

3

&1 H

fa

(snoAl indino doi33i3a ibn

28
lO'^r

COEC3

3x10" -

10^=-

30 -

10 -

C/3

1 1^

0.3

0.1

0.03 I0.01

— o/

— / — o

/

/ — o

/ — / S-2

I / ——

/

/

/

:::- / _—-

/ /

/ "/ /''

/Q

~ /l - / 1 — uA 1 / /o — / - o/ - /

/ -

/

/

1 1 1 1 mil 1 1 1 1 Mill 1 0.01 0.03 0.1 0.3

DIAMETER (Mm]

Figure 21. Detector sensitivity versus particle size

for a light-scattering type detector (S-2)

and for an ionization type detector (R-2) [15]

29
The ionization detector sensitivity data can be correlated with particle

diameter by using a linear fit to a log-log plot as shown in figure 21. This

indicates a power law relationship between detector sensitivity and particle

size with the empirical relationship being:

S = 6.7 Dg ^-^

(8)

where S is the sensitivity and D the geometric mean number diameter. Thus

it is seen that the detector seniitivity is nearly a linear function of

particle size rather than being linearly related to the surface area of the

particle or to the volume of the particle. This result is in qualitative

agreement with Hosemann's theory [16], which predicts a linear relationship

between sensitivity and particle size for low particle concentrations (see

equation (3) )

.

It is seen in figure 21 that the ionization type detector is the more sensitive for particle sizes smaller than about 0.3 pm, while for larger

particle sizes the light-scattering detector is the more sensitive. This

difference in sensitivity has important practical implications. There is

evidence from work at Georgia Institute of Technology by Bankston et al. [17]

that smoke generated in the flaming mode of combustion for Douglas fir,

polyvinylchloride , and rigid urethane foam is generally smaller than 0.3 vm while the same materials undergoing combustion or pyrolysis in the non-flaming

mode produce particles larger than 0.3 um. Thus one would expect that the

ionization detector would be more sensitive to smoke generated from flaming

materials and that the light-scattering detector would be more sensitive to

smoke generated by non-flaming materials. This expectation has, in fact, been

demonstrated in the testing of smoke detector response to various small-scale

fires by Consumer's Union [18] and to large-scale fires by IIT Research

Institute and Underwriters ' Laboratories under contract to the ^7ational

Bureau of Standards [19].

12. SMOKE PROPERTIES

In this section we shall concentrate on the properties of those smokes

used in smoke detector testing: smokes generated from smoldering lamp wick

and punk (imported from the orient as incense sticks) and the black soot smoke

generated from the diffusion burning of heptane. The properties of interest

will be the number concentration, mass concentration, optical density, and

size distribution. The aging properties of smoke will be discussed in the

next section. A more extensive discussion of the properties of these smokes

can be found in a report by Lee and Mulholland [20].

The measurement of smoke properties was made in the Underwriters'

Laboratories (UL 217 standard) smoke detector evaluation chamber which is used

for the testing of residential smoke detectors for approval. The detector

evaluation chamber 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 22. 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 22. A constant circulation velocity of 0.18 m/s is

maintained.

A photometer which measures the optical density of the circulating smoke

has a path length of 152 cm and a beam width of 5 cm. It consists of an incandescent lamp (GE-4515) source operating on 2.4 V (2370K + 50K color

temperature) and a photovoltaic cell (Weston 594 RR) detector.

30
uco

•H

-P

;3>

0)MO

-MU

(U +J

Q) -0

Q)Od

(U

Cn •H

Cm

31
In these measurements, a smoldering lamp wick or a heptane diffusion

burner was placed in the burner end of the chamber and was withdrawn when the

smoke reached a desired optical density level, ranging from 0.005 m to

0.08 m- . After allowing about thirty seconds to insure uniform mixing, the

aerosol was sampled by the analyzers.

The dependence of the optical density on mass concentration is shown in

figure 23 for both smokes. The mass concentration for the lamp wick smoke and

heptane smoke were determined by using the quartz crystal mass monitor [21]

and the filter gravimetric method, respectively. The mass monitor was not

effective in measuring the mass concentration of the sooty heptane smoke. The

vertical bars in figure 23 represent the shift in optical density during the

three- to four-minute sampling period required for collecting sufficient

heptane smoke for precise filter weighing. Only a 20-30 second period was required for the mass monitor for the lamp wick smoke. The ratio of optical

density per meter to mass concentration, termed the particulate optical

density (POD) , is an intensive property independent of concentration, at least over a certain range of concentrations. The POD for heptane smoke is 3.4 m^/g

and for cotton lamp wick smoke the value is 1.5 m^/g. in an extensive paper

reported at the Sixteenth Symposium on Combustion [22] , Seader reports the

dependence of optical density as measured in the NBS-AMINCO smoke density

chamber [23] on mass concentration for high concentrations of smoke generated

from the following materials for both flaming and non-flaming combustion:

rigid polyvinylchloride, red oak, polystyrene, a-cellulose, Douglas fir, rigid

urethane , and polyacrylonitrile-butadiene-styrene (ABS) . All of the flaming

combustion data can be approximately correlated by a linear plot with a POD of

3.3 m^/g and the non-flaming combustion data, which appears to have somewhat more scatter, can be correlated by a linear plot with a POD of 1.9 x 10 m /g.

Thus, both from our study of low concentration smokes and the work of Seader

at high concentrations, the light-obscuring property of many smokes to white

light can be placed in two categories depending on the mode of smoke generation

flaming or non-flaming. It should be stressed that the measurements were made under specific flaming conditions and specific non-flaming conditions;

for example, the non-flaming combustion data reported by Seader are based on exposing a given size sample to a flux of 2.5 W/cm^ . It is not known whether

the POD is independent of changes in exposure conditions such as radiant flux,

sample size and ambient temperature.

The size distributions for the lamp wick smoke and the heptane smoke as determined with the electrical aerosol analyzer are presented in figure 24.

The quantity AN represents the number of smoke particles in the particle

diameter size range log D to log D + A log D . The A presentation was used

because data obtained from the measuring instruments were based on the average

within each discrete range.

Two convenient parameters for characterizing the size distribution are

the geometric mean number diameter, D , as a measure of the average particle

size and the geometric standard deviation, a , as a measure of the breadth

of the distribution. ^

n

AN. log D,

log D = J2—^—'-^-^ (9) ^ i=l

32
0.08

0.06

oo

0.04

0.02

HEPTANE

SMOKE

O/LAMP WICK

SMOKE

20 40

MASS CONCENTRATION, mg/in^

60

Figure 23. Optical density per meter versus mass concentration

for lamp wick smoke and heptane smoke [20]

.

33
107

1 I I I I I T

I I I I

106

roeo

Q. o

o»o

<

105

O 0.066 OD/m, LAMP WICK • 0.015 OD/m, LAMP WICK

A 0.017 OD/m, HEPTANE

\1

^-

10^

0.01 J \

' I I I I 1

1

\

\ J

I I I I ' 0.1 Dp,yu.m

1.0

Figure 24. Particle size distribution for lamp wick

and heptane smokes [20]

.

34
log a E

i=l

log D. - log D— |2 AN. 1/:

(10)

where N represents the total number of particles and n represents the number

of size classes. Both smokes have a D between 0.1 and 0.2 um; however, the

g heptane smoke seems to have a somewhat narrower size distribution than lamp

wick smoke. A summary of all of the smoke data is presented in table 1. The

values of cr found in our work for smoldering lamp wick smoke are significantly

larger than^those reported by Watanabe at the 1976 UJNR panel meeting (1.6 and

1.7 versus 1.4). It is known that the size sensitivity of the instrument used

in measuring the size distribution, the electrical aerosol analyzer, decreases

for particle sizes over a few tenths of a micrometer [24] and this may be

responsible for the difference in the values of a . , g

As discussed in a previous section, the electrical aerosol analyzer has

been evaluated for the measurement of the number concentration and volume

concentration. Its sensitivity to monodisperse aerosols has also been

examined [24]; however, there is no comprehensive study regarding its perfor- mance for polydisperse aerosols such as smokes. While we cannot be certain of

the size distribution in absolute terms, the instrument is very useful in

detecting relative changes in size distributions. For example, the effect of

exposure condition on the size distribution of smoke generated from the com- bustion of a-cellulose is shown in figure 25. In one case a small a-cellulose

disc was ignited by a premixed flame while in the other it was in contact with

a metal surface maintained at 500°C. The .particle size distribution peaked at

less than 0.01 ym for the flaming mode compared to 0.075 ym for the smoldering

mode. The effect of time on the size distribution of the smoke, which is

termed smoke aging, is another process that can be studied with the electrical

aerosol analyzer and will be the subject of the next section.

Table 1. Measured and derived parameters of

smokes in the UL 217 chamber test

Instrument

UL 217

Optical density

- 1 m

,a/ .b/ PMM- EAAMass concentration

mg/m^

EAA c/ CNMNumber cone

,

10^ cm"

g

ym

Lamp wick

Lamp wick

Heptane

0.066

0.015

0.017

42

10

:d/

21 3.4

6 1.4

5 0.9

2.5 0.14

1.0 0.12

0.3 0.16

1.7

1.6

1.5

a/

b/

d/

Piezoelectric mass monitor

Electrical aerosol analyzer, unit density assumed

Condensation nucleus monitor

Filter gravimetric measurement

35
T

1—I I I I I I I I I I I I TT

106

\

105

ro'eu

a. O

g. 104

103

0.001

V

a-CELLULOSE • flaming

smoldering

0.01 J I I I I 1 1

1

1.0 Dp ,^m

Figure 25. Smoke from "-cellulose under flaming

and non-flaming exposure conditions [20].

36
13. SMOKE AGING

The behavior of smoke par

transport to the walls or thai

performed by K. Mniszewski at

of aging of smoke detectors in

His data on the responses of a forward scattering angle and o time in figure 26. The optica

during the eight-minute aging

detector response decreased by

increased by about 10%.

tides is dynamic from their formation to their

r dilution in the atmosphere. An experiment

IIT Research Institute demonstrates the effect

the Underwriters' Laboratories test chamber.

light-scattering type detector with a near

f an ionization detector are plotted versus

1 density of the light beam decreased about 5%

period. During this same time, the ionization

about 25% while the light-scattering detector

In a series of similar experiments at the National Bureau of Standards

during which the number concentration and optical density were monitored

versus time, a rapid decrease in number concentration occurred in a few

minutes while the optical density remained essentially constant. This rapid

drop in number concentration can be explained by the phenomenon of coagulation,

which is simply the coalesence or attachment of aerosol particles as a result

of collisions. The basic equation describing the rate of change of number

concentration with respect to time is given by:

dN

dt = -r N' (11)

Integration yields

N/N = 1/(1 + r N t)

,

o o

(12)

where N is the initial concentration. For smoldering punk smoke, the

coagulation frequency r equals 4.0 x 10 cm^/s. Substituting this r into

equation (12) one finds that an initial concentration of 3 x 10^ particles/cm^

decreases by a factor of two in 14 minutes.

10

7

4 -

C3 2

£S 1 UJS 0.7

0.4

0.21-

0.1

- /

/

/

—

1

1 WICK REMOVED—-^

• —

1 -

LIGHT SCATTERING "

DETECTOR

1 IRHT RFAM -— IONIZATION ^DETECTOR - 7

1 1 1

SMOKE

AGING

1 1 r 16 TIME, MINUTES

24 32

Figure 26. The effect of aging on smoke detector response.

37
The coagulation phenomenon creates two opposing effects on detector

response. The decrease in the number concentration tends to decrease the

detector output while the increase in particle size accompanying particle

coalescence tends to increase the detector response. Which effect will predominate is determined by the size sensitivity characteristic for the detector.

The effect of aging on the size distribution of punk smoke is shown in

figure 27. The smoke was generated in a 1.25 m cubical chamber and was allowed

to age for up to 16 hours. The peak in the number distribution decreases by over two orders of magnitude while the particle size increases by about a factor

of two. Coagulation is the dominant mechanism during the initial aging, but

the mechanism of wall loss becomes the dominant process for the well aged, low

concentration smoke.

All of the size distribution data in the previous figures plus the size

distribution of punk smoke exposed to an air stream moving at a velocity of

2 m/s have been plotted in terms of Friedlander ' s self-preserving variables

[25] lii and n in figure 28. 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/:J) , for that particular smoke. Thus

'i' =

N

Alog N(t! (13)

n = D N(t)/V ^/^ (14)

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 different airflow conditions. There is qualitative

agreement in the shape of the size distribution curves for all the data,

especially for larger values of n- From the data for small r\ , it appears that

the heptane smoke has a narrower distribution than the others.

Also 'shown in figure 28 are a reduced Junge like size distribution and a reduced log-normal size distribution.

ip = 1.38 n^ (n^ + 0.2)"^, Junge like (15)

tjj = 1.7 3 exp - In (1.23 n ) 0.744

2

, log-normal with a=1.7. (16]

Equation (15) is a reduced version of the number distribution equation given

in the 1976 UJNR panel meeting (p. 262) . It appears that both the Junge like

distribution and the a = 1.7 log-normal distribution fit the data. It has

been shown by Mulholland, et al. [26] 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 28) . 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.

14. LOOKING AHEAD

In our previous studies of smoke aging, we considered the highly idealized

case of a perfectly mixed smoke. Our current theoretical work is concerned

with the dynamics of smoke in a buoyant plume. To date our theory includes

the effects of coagulation and of air entrainment in the plume and enables
10^ T 1 I 1 r T

106

'e

o

10^

/

PUNKS. AGED

SECONDS

120

2050

7020 • 60,000 (16.7 hrs.)

103

0.001

I I I I '

1

Figure 27,

001 0.1 Dp,/xm

A.ging of the punk smoke for up to 16 hours [20] .

1.0

39
lOtr \ I I I 1

1

1

1

1—I M I I II I I—I II II O Fig. 26, HEPTANE

D AIR STREAM ON PUNK

A Fig.27, SMOLDERING

A Fig.27, FLAMING

O Fig. 29, 120 S e Fig. 29, 7020 S • Fig. 29, 60,000 3

Q. Q

o>O

<II

10-

AL6EBRAIC

' '' 0.1 I 10 100

T7 = Dp(N/V)'^^

Figure 28. Size distribution in terms of reduced variables [20].

40
the calculation of the number concentration, mass concentration, and size

distribution of the smoke particulate as a function of height. The relative

significance of coagulation versus air entrainment on the particulate number

flux is determined by a single dimensionless constant which we term the plume

constant, A. Besides depending on the total rate of heat release and the

rate of mass release, A also depends on the coagulation frequency, height and

the number concentration at this reference height. For values of A greater

than or equal to 0.1, coagulation becomes an important effect. In the

future, we plan to include the mechanism of aerosol formation including

nucleation and condensation in our study and to initiate an experimental

study.

A second project of high priority is the development of a simplified

version of the monodisperse aerosol generator for testing the sensitivity of

installed smoke detectors. A prototype version of the tester is shown in

figure 29 [27]. Some of the major design features are a repeatable, steadystate aerosol source; adjustable aerosol concentration over a range from

10 to 60 mg/m^; mass median diameter of about 0.5 pm; and formation of the

aerosol from the atomization of the pure liquid dioctyl phthalate.

A third project in progress is the absolute calibration of our optical

particle counter using a laser Doppler size spectrometer developed by Chabay

in the NBS Analytical Chemistry Division [28] . The principal of operation for

the instrument is illustrated in figure 30. Light scattered out of a horizontally propagating laser beam by falling particles is collected at one angle

in the vertical scattering plane. Beat frequencies in the photocurrent of the

detector due to the Dopper shift of the rad"' ation scattered by the settling

aerosol are analyzed to determine particle velocities. The slip - corrected

Stokes law settling velocity gives the particles size for a known particle

density, while the amplitude of the beat frequency contains information on the number of particles of that size. The technique has a sizing accuracy of

;10.16 m diameter for measuring particle sizes over the range 2 to 20 ym.

The Berglund-Liu vibrating orifice generator [29] is being used for generating

monodisperse (cr ~ 1.05) dioctyl phthalate particles for the calibration.

41
•.« _£-M

SliOltEBtTECTOlTESTEl mFigure 29. Smoke detector tester [27]

42
k.o^

a> 0) ^— c u a>

^m. ^oa

00U

Q) VIU

+Ju

03

(UN

-au

0)

i-r ao

tJM

<U

[fl (« 1^ c

43
15. REFERENCES

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

Council, Canada (1962).

[2] Survey and analysis of occupant installable smoke detectors. The

Aerospace Corp., Report No. ATR-77 (2819) -2 (September 1977).

[3] Dobriner, R., Electronic Design, p. 24 (October 1977).

[4] Reliability Guide for Design, Management, and Procurement Personnel,

Avco Electronics Div. , Avco Corp. p. 5 (1965)

[5] Litton, Charles D. and Hertzberg, Martin, Principles of ionization

smoke detection. Development of a New Sensor for Combustion Generated

Submicrometer Particulates, U.S. Bureau of Mines Report No. 8242 (1977).

[6] Hosemann, J. P., Veber Verfahren zur Bestimmung der Korngrossenverteilung

hochkonzentrierter Polydispersionen von dielektrischen Mie-Partikeln

,

Dissertation TH Aachen (1970)

[7] Scheidweiler , Andreas, Physical aspects of ionization chamber measuring

techniques (unipolar and bipolar chambers) , Fire Detection for Life

Safety, Proceedings of a Symposium, National A.cademy of Sciences (1977) .

[8] Litton, Charles D., A mathematical model for ionization-type smoke

detectors and the reduced source approximation. Fire Technology,

Vol. 13, No. 4, NFPA (November 1977).

[9] Simon, Frank N. and Axmark, Roger E., Space charge effects in ionization

type particulate detectors. Review of Scientific Instrumentation, Vol. 48,

123 (1977)

[10] Hodkinson, J. R. , The optical measurement of aerosols, from Aerosol Science,

Ed. C. N. Davies, Academic Press, London (1966).

[11] Bukowski, R. W., An evaluation of light emitting diodes as source lamps

in photoelectric smoke detector. Fire Technology, Vol. 11, No. 3, NFPA

(August 1975)

[12] Liu, B. Y. H., Whitby, K. T. and Pui, D. Y. H., J. Air Pollut. Control

Assoc, Vol. 24, 1067 (1974).

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

[14] Mulholland, G. W. , Liu, B. Y. H. and Kapadia, A., Accuracy of the

electrical aerosol analyzer for measuring number and voliime concentration

of polydisperse aerosols, to be submitted to J. Colloid and Interface Sci.

[15] Liu, B. Y. H., Sensitivity of smoke detectors to monodisperse aerosols.

Final Report to National Bureau of Standards.

[16] Hosemann, J. P., On the theory of ionization chambers with consideration

of small-ion recombination, Staub-Reinhalf . Luft, Vol. 32, 13 (1972).

[17] Bankston, C. P., Powell, E. A., Cassanova, R. A. and Zinn, B. T.

,

Detailed measurements of the physical characteristics of smoke particulates

generated by flaming materials, J. Fire and Flamm. , Vol. 8, 391 (1977).

[18] Smoke detectors. Consumer Report, p. 555 (October 1976).

44
[19] Bukowski, R. W. , Waterman, T. E. and Christian, W. J., Detector

sensitivity and siting requirements for dwellings, NBSGCR-75-51

(August 1975)

.

[20] Lee, T. G. K. and Mulholland, G. W. , Physical properties of smokes

pertinent to smoke detector technology, NBSIR 77-1312 (November 1977)

.

[21] Olin, J. G. and Sem, G. J., Piezoelectric microbalance for monitoring

the mass concentration of suspended particles. Atmospheric Environment,

Vol. 5, 653 (1971)

.

[22] Seader, J. D. and Einhorn, I. N. , Some physical, chemical, toxicological,

and physiological aspects of fire smokes. Sixteenth Symposium (International) on Combustion, 1423, The Combustion Institute (1976).

[23] Lee T. G. K. , Interlaboratory evaluation of smoke density chamber,

NBS Technical Note 708, U.S. Government Printing Office, Washington, O.C.

(December 1971)

.

[24] Liu, B. Y. H. and Pui, D. Y. D., On the performance of the electrical

aerosol analyzer, J. Aerosol Sci, Vol. 6, 24 (1975).

[25] Wang, C. S. and Friedlander, S. K. , J. Colloid Interface Sci., Vol. 24,

170 (1967).

[26] Mulholland, G. W. , Lee, T. G. K. and Baum, H. R. , The coagulation of

aerosols with broad initial size distributions, J. Colloid Interface Sci.

Vol. 62, 406 (1977) .

[27] Lee, T. G. K. , An instrument to evaluate installed smoke detectors,

NBSIR 78-1430 (February 1978) .

[28] Chabay, I. and Bright, D. S., Measurement of the size distribution of

liquid and solid aerosols by doppler shift spectroscopy, J. Colloid

Interface Sci., Vol. 63, 304 (1978).

[29] Berglund, R. H. and Liu, B. Y. H., Environ. Sci. Technol., Vol. 7,

147 (1973).

45
.r«t^S-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-973

2. Gov't Acces.sion No.

3. Recipient's Acces.sion No.

4. TITLE AND SUBTITLE 5. Publication Date November 1978

SMOKE DETECTOR DESIGN AND SMOKE PROPERTIES 6. Performing Organization Code

7. AUTHOR(S)

Richard W. Bukowski and George W. 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. 7515678

11. Contract/Grant No.

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

Same as No . 9

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary o( most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) The importance of a reference photometer and reference ionization detector

in improving the reliability of smoke detectors is discussed. Recent developments

in smoke detector technology are highlighted and theoretical as well as practical

experience in regard to detector performance is summarized. Comparison of the

theoretically predicted response of smoke detectors as a function of particle

size with measured values is given. A monodisperse aerosol generator, an electrical aerosol analyzer with a size sensitivity from 0.01 to 1 iim, and an optical

particle counter are described. The size distribution, mass and number concentration, optical density, and coagulation frequency for smoke from burning heptane

and smoldering cotton lamp wick are presented. It is shown that a Junge type

size distribution provides a good fit to the measured size distribution for both

fresh and aged smoke.

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) ^erosol generators; detector testers; fire detectors;

ionization detectors; light-scattering detectors; particle size distribution;

smoke; smoke detectors.

18. AVAILABILITY [T Unlimited

I

' For Official Distribution. Do Not Release to NTIS

I y

^ Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No-DO'^-nnSI I Order From National Technical Information Service (NTIS)

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19. SECURITY CLASS

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51

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