NBSIR 78-1430

An Instrument to Evaluate

Installed Smoke Detectors

Thomas G. K. Lee

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D C. 20234

February 1 978

Final Report

U S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 78-1430

AN INSTRUMENT TO EVALUATE

INSTALLED SMOKE DETECTORS

Thomas G. K. Lee

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D C. 20234

February 1978

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

Abstract 1

1. INTRODUCTION 2

2. INTERACTION BETWEEN SMOKE AND DETECTORS 4

2.1 Fire Smoke and Its Characterization 4

2.2 Particle Size and Detector Response 5 2.3 Standard for Detector Sensitivity Evaluation 6

3. THE DETECTOR TESTER 7 3.1 Control of Aerosol Concentration 8

3.2 Dioctyl Phthalate (DOP) , [Bis (2-ethylhexyl)phthalate]

Kodaflax 8

4. RESULTS AND DISCUSSION 9

4.1 Effect of Temperature Pressure and Reservoir Height

on Concentration 9

4.2 Particle Size Distribution 10

4.3 Aerosol Optical Density, Number Concentration and

Mass Concentration 12

4.4 Concentration Range of the Tester 13

4.5 Alarm Threshold of Some Detectors 14

5. SUMMARY AND CONCLUSION 15

6. ACKNOWLEDGMENTS 16

APPENDIX A. CHARACTERISTICS OF THE COMPONENTS 17

A. 1 Compressor, Nebulizer and Impactor 17

A. 2 Blower, Plumbing and Shroud 19

APPENDIX B. CHARACTERISTICS OF THE PARTICLE MASS MONITOR .... 21

iii

REFERENCES 22
LIST OF FIGURES

Figure No. Page

1 Sensitivity of two types of detectors exposed

to monodisperse aerosols 25

2 Schematic of the smoke detector tester 26

3 A smoke detector and the tester 27

4 Interior view of the tester 28

5 Impactor and dilution system 29

6 Size distribution of test aerosol and lamp wick

smoke as measured by EAA 30

7 Relative number concentration of test aerosol at

selected size ranges as measured by the particle

optical counter 31

8 Logarithmic probability plots of the cummulative mass distribution of the test aerosol 32

9 Optical density of test aerosol as a function of

aerosol mass and number concentration 33

10 Mass concentration of test aerosol at various

rotameter settings 34

iv
AN INSTRUMENT TO EVALUATE INSTALLED SMOKE DETECTORS

Thomas G. K. Lee

Abstract

An inexpensive portable instrument has been constructed

to test the sensitivity response of installed fire smoke

detectors. The test stream consists of impactor-selected

small particles from aerosol generated by a pneumatic

nebulizer and dilution air. The flow rate of the test stream

is 140 L/min and its aerosol concentration can be varied from

10 to 60 mg/m 3

, sufficient to cover the sensitive ranges of

the nine typical detectors tested. The aerosol mass median

particle diameter is 0.46 ym and the breadth of the size

distribution in terms of geometric standard deviation, is

1.5. The size distribution of the polydisperse aerosol at

the output is independent of the aerosol concentration in the

range of interest and is comparable to some fire smokes.

Aerosol light obscuration as measured by a standardized

photometer is linear with respect to the aerosol mass con- centration (mg/m 3

) and has a ratio 2.3 m

2 /g for this aerosol

compared to 1.5 m2 /g for cotton lampwick smoke in the UL 217

chamber.

Key words: Aerosol generators; detector sensitivity; detector

testers; light obscuration; particle size distribution;

polydisperse aerosol; smoke detector.

1
1. INTRODUCTION

An effective approach in reducing fire hazard to life and property

is the use of early-warning devices such as smoke detectors. Studies

have shown that fatalities might be reduced by about 40% if detectors

were installed in all houses [l]"*

-

. 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, advertising, and in some cases building code requirements,

smoke detector sales reached an estimated 8 million units in the United

States in 1977 [3] and is expected to continue at the same rate for the

next few years.

The reliability and life expectancy of the installed detectors are

not known, although component design requirements for detectors listed

by the Underwriters’ Laboratories (UL) include a theoretical expected

life of 12-15 years at 50% failure rate. The best estimate by the

manufacturers of the useful life of current detectors is about ten years

[4].

Very little field data based on valid sampling of detector failure

rate is available. A recent (1976) inspection of 186 apartments with

single 12 to 18-month old smoke detectors of one model by the Fire

Marshall’s Office of a large city showed that 18% were inoperative [5].

The tests were based on blowing cigarette smoke into the units. A

majority of the failures was attributed to burned-out incandescent bulbs

in the light-scattering-type units. The example may not be typical

because some apartment occupants simply silenced the detector to stop the

trouble signal instead of attempting to repair the detector which included

with it two spare lamps and instructions for such replacement [4,5]. The

occupants might have failed to receive or have ignored instructions on maintenance of these types of detectors. Certainly detectors based on

Numbers in brackets refer to the literature references listed at the

end of this paper.

2
different design characteristics such as that of ion-chamber, or the LED

light source for the light-scattering type, are not expected to have

such problems in such short periods of time. Nevertheless the detectors

in the above example are listed by UL and the Factory Mutual Research

Corporation (FM) and conform to their standards.

Clearly, there is a need for periodic sampling surveys of installed

smoke detectors of various types and manufacturers to determine their

failure rate and sensitivity shift under different environmental con- ditions. Such information would be useful to consumers and manufacturers

as well as the public agencies (i.e. local fire departments). The field

data not only would encourage design of more reliable detectors, but it

would also help define the problem, if any, for the agencies responsible

for public fire safety.

To perform detector reliability surveys or periodic maintenance

tests in large installations with hundreds of installed detectors, a portable and inexpensive tester is needed. A tester of this type is not

available today.

The testing instrument should produce an aerosol of known and

reproducible particle size characteristics with adjustable concentrations

at the proper ranges. Limits for both repeatability (for a single

instrument) and reproducibility (among instruments of the same design)

in terms of aerosol concentration and particle size should be known and

acceptable

.

This report describes the design, construction, and calibration of

a prototype of such a detector tester. It also gives some preliminary

results on the particle size distribution of the aerosol, the adjustable

concentration range in terms of optical density and mass concentration

as well as some repeatability data. Finally, the sensitivity responses

of some common smoke detectors are also compared using the tester.

Several prototype instruments of this design are being built for field

trial to pinpoint any field use problems.

3
Before describing the tester, a brief discussion of fire smokes

and smoke detectors in general would be helpful in understanding the

instrument

.

2. INTERACTION BETWEEN SMOKE AND DETECTORS

2.1 Fire Smoke and Its Characterization

Part of the combustion products from a fire are generated in the

form of an aerosol, commonly called smoke. It consists of solid particulates, or liquid condensates with or without a solid kernel, of various

sizes. Smoke does not consist of only a single particle size but has a range of particle sizes with a peak concentration at a certain size.

Particle sizes may range between 0.005 and 5 micrometers.

The number size-distribution (defined as the number of particles in

the aerosol within a certain discrete diameter range) of various smokes

have been found to fit at least qualitatively, a log-normal distribution.

The count (geometric mean) median diameter D and the geometric standard

g

deviation, o (the breadth of size distribution), may be derived from

S

such a distribution [6]. These two parameters are commonly used to

characterize the polydisperse aerosols. For example cigarette smoke

collected by a smoking machine simulating a single inhalation by a smoker

may have a D = 0.29 pm and a = 1.4 [71.

g g

Neither D nor the concentration of an aerosol remain stable with

8

6 -3

time, especially at the high concentration level (>1 x 10 cm ) required

for detector alarm. The quantity D^ will increase with time because

of particle growth due to coagulation during particle collision, while

the number concentration decreases rapidly. (It takes many small particles to form a single large particle.) The mean diameter of punk (incense

stick) smoke, for example, will increase by about 30% in five minutes 7-3 7-3 while its concentration decreases from 1 x 10 cm to 0.44 x 10 cm (particles per cubic centimeter) during the same period. Coagulation

rate is proportional to the square of concentration, thus coagulation

losses become important only at high concentrations.

4
For a given material, smokes generated under smoldering exposure

conditions are generally larger in mean particle size compared to that

generated under active flaming. Under smoldering conditions, aerosols

are generated at a high rate and are convected away from the source

rather slowly; both conditions favor particle coagulation and growth.

2.2 Particle Size and Detector Response

The sensitivity (defined as signal output per unit change of

aerosol concentration) of present day smoke detectors depends strongly

on the particle size of the aerosol. These detectors will respond to

particles in the size range of 0.01 to 5 pm. However, the degree of

sensitivity to particle sizes within this range differs considerably

between 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 with diameter to wavelength

of light ratio smaller than a few tenths; whereas, for the ionizationtype 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 ym compared to the

ionization type which can respond to particles as small as 0.01 ym.

The light-scattering types are also sensitive to the refractive index of

the smoke.

Figure 1 illustrates the strong size-dependence of detector response.

In this figure, the sensitivities of an ion-chamber and a light-scattering

detector are plotted as a function of particle size for monodisperse

dioctyl phthalate aerosols in the number concentration range of 10 *+ to

10

6 cm"

3 [8].

The difference in mean particle size between the flame-generated

and smolder-generated smokes mentioned earlier, combined with the variation of particle size sensitivity of the two types of detectors shown in

5
.

\ Figure 1, resulted in the common observation that ion-chamber detectors

are more sensitive to smoke from flaming combustion and that lightscattering detectors are more sensitive to smoke from a smoldering

source [9,10].

In addition to particle size and particle concentration, the

refractive index (scattering and absorption coefficients) are also

important characteristics of an aerosol affecting a light-scattering

detector. Detector output can be 2 to 5 times higher when exposed to

white colored or highly scattering smoke compared to black sooty smoke

of the same concentration and particle size.

2.3 Standard for Detector Sensitivity Evaluation

Because of the large variation in concentration, particle sizes and

refractive indices of potential fire smokes, aerosols selected for detector

evaluation standards must necessarily be arbitrary. For convenience,

UL uses an aerosol generated from a smoldering cotton lamp wick for their

detector sensitivity test in the UL 217 standard [11].

The sensitivity requirement of the standard applies to all types of

smoke detectors. It requires that the detector under test must alarm

when the aerosol concentration reaches a level corresponding to an optical density of 0.06 m

1 (4 percent/ft) for minimum sensitivity and

0.006 m

1 (0.4 percent/ft) for maximum sensitivity as measured by a given photometer with a 1.5 m-pathlength in the UL standard smoke chamber.

The mass concentration of lamp wick smoke matching the 0.06 m

1 optical

density is about 40 mg/m 3

. The number or mass concentration required

to attain the same obscuration level depends strongly on the particle

size and refractive index of the smoke. A more detailed discussions on

the UL 217 standard may be found in [12].

6
3. THE DETECTOR TESTER

The purpose of the tester is to provide a polydisperse aerosol to

evaluate the sensitivity of installed smoke detectors. Unlike instruments

used for research which require high precision and accuracy, the tester

design must also consider other important factors. These include component

reliability, ruggedness, convenience as well as cost and safety to the

operator

.

Though precision is not of primary importance, sufficient test data

from several testers plus sufficient field results should be evaluated

to provide for an estimate of the precision. Such documentation would

help the operator to judge the significance of any perceived shift in

sensitivity of the detectors tested.

Figure 2 shows the schematic of the tester. The generator is based

on a commercially available nebulizer, which generates a polydisperse

aerosol with a relatively large mass median diameter. An impactor

reduces the median diameter by selectively removing the large-diameter

particles. The remaining aerosol is diluted by a large volume of room

air at a constant rate from the blower.

A diaphragm pump which serves as a compressor as well as a vacuum

source, provides the air pressure necessary for the operation of the

nebulizer. Aerosol output and its particle size distribution from the

nebulizer is kept constant by maintaining a constant pressure from the

compressor. To vary the aerosol concentration of the test stream, part

of the aerosol from the generator is diverted to the vacuum source through an adjustable valve. The diverted aerosol is filtered to prevent

system contamination before being measured by the rotameter.

Figures 3 and 4 present the exterior and interior views, respectively,

of the prototype tester. (Figure 4 does not include later improvements

such as a much larger filter and impactor manifold.) Figure 5 is a drawing of the impactor and dilution system of the tester. Details on

7
the design and component characteristics of the tester are given in

appendix A of this report.

3.1 Control of Aerosol Concentration

The concentration of the test aerosol is regulated by the removal

rate of the undiluted aerosol at the impactor manifold. The rate is

controlled by the vacuum valve and monitored by the rotameter. To calculate the true (STP) flow rate, the line vacuum (pressure) at locations

immediately upstream and downstream of the rotameter as well as the

rotameter reading itself must be known. The upstream vacuum is particularly sensitive to the filter size and loading, which changes with time.

On a short term basis and assuming constant filter and vacuum pump

characteristics, a rotameter reading alone can be used to indicate an apparent (uncorrected) flow rate. Tester calibration can be based on this apparent rate.

3.2 Dioctyl Phthalate (DOP)

,

2 [Bis (2-ethylhexyl) phthalate] , Kodaflax

Technical grade DOP is used as a liquid in the nebulizer. The

3 nebulizer will also work using other liquids though the size distribution of the resulting aerosol could be different. DOP was selected __

Certain commercial equipment, instruments, or materials are identified

in this paper in order to adequately specify the experimental procedure.

In no case does such identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply that the material

or equipment identified is necessarily the best available for the

purpose

.

3 Such as peanut oil.

8
because it is a very stable, colorless, oily liquid with low vapor

pressure. It is used in large quantity in the plastics industry and its

properties are well documented.

The air contaminant exposure limit for DOP aerosol under OSHA

regulation (29-CFR 1910.1000) for an eight-hour time-weighted average

concentration in work places is 5 mg/m 3

. In detector testing, aerosol

concentrations, between 10 and 30 mg/m 3 are normally used. Aerosol

leaving the test shroud is quickly diluted by the room air. However,

appropriate precaution should be taken by personnel who use the tester

continuously and in confined spaces.

4. RESULTS AND DISCUSSION

4.1 Effect of Temperature Pressure and

Reservoir Height on Concentration

Since the testers are to be used in the field where conditions may

not be ideal, some of the variables that may affect the generator output

rate were examined.

Temperature

It is known that the viscosity of the fluid in the nebulizer has

some effect on its aerosol characteristics. Tests show that a change of

temperature by 10°C from 23 to 33°C in the DOP liquid result in an average of a 7% increase in mass concentration in the test stream, an increase from an average of 28.0 to 32.0 mg/m 3

. It is believed that the

percentage increase would be similar for other concentrations.

Pressure

Though pressure to the nebulizer is standardized and regulated to

0.103 MN/m2 (15.0 psig) , pressure variations due to poor meter calibration

or operator control may introduce error. Limited data from 0.100 to 0.106

MN/m2 (14.5 to 15.5 psig) indicated that the concentration increases with

9
pressure at a rate of about 15% per each psi increase. Therefore

precision in pressure control is important for good repeatability.

Liquid Level

The level of liquid in the reservoir of the nebulizer may also

affect the generation rate and thus the final aerosol concentration.

Data based on three sets of experiments show that concentration in the

test stream increases as the liquid level in the reservoir increases.

The level is defined and measured after all the liquid has been emulsified by 2 to 3 minutes of operation of the nebulizer and recirculated

back to the reservoir. The increase in output concentration was on the

order of 6-9% when the reservoir content was increased from 2 to 8 mL.

To minimize this effect, reservoir loadings between 5 and 8 mL were used

throughout this work.

4.2 Particle Size Distribution

The particle size distribution of the test aerosol was measured by

three different commonly used methods. These are: (1) the electrical

aerosol analyzer (EAA) ; (2) the optical particle counter, and (3) the

cascade impactor. The first two instruments give particle diameters

based on count numbers whereas the last one is based on size derived

from its collected mass. The electrical aerosol analyzer is based on a mobility analysis of charged particles. The charge acquired by "diffusion" charging is a function of the particle size [13] • The optical

particle counter counts each particle and measures its forward scattered

light intensity, which is a function of particle size [14]. The cascade

impactor classifies the aerosol into size fractions according to particle

momentum, and each collected size fraction is weighed to determine the

distribution [15].

10
The result based on the electrical aerosol analyzer (EAA) is shown

in figure 6. In this figure, comparison is made between lamp wick smoke

generated in the UL 217 standard test chamber and the DOP test aerosol

at different concentrations. For the test aerosol, D = 0.17 p and a =

§ & 1.7 compares to lamp wick smoke of D = 0.12 ym and a =1.6. The drop

S 6

in concentration as particle size increased was not as steep for the test

aerosol as the lamp wick smoke.

The distribution 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 the particle size in micrometers. The quantity AN

represents the number of aerosol particles in the particle diameter size

range log to log + A log D^. The A presentation was used because

data obtained from the measuring instruments were based on the average

within each discrete range.

Several EAA measurements at different test stream concentration

levels between 40 and 70 mg/m 3 show that the size distribution is

independent of concentration.

To measure aerosol particles larger than 0.5 ym for which the EAA

is not efficient, an optical particle counter was used. The measurements

based on a light-scattering type optical counter coupled to a multichannel

pulse height analyzer (PHA) , are shown in figure 7. This figure shows

the relative particle concentration at various size ranges for samples

before and after going thru the impactor. The cut-off near the 0.5 ym

region is relatively sharp. The size range corresponding to each channel

was on the order of 0.02 ym. The counter is insensitive to sizes smaller

than 0.5 ym. Since the two curves are not based on the same dilution

ratio, only the relative magnitude of each curve is significant.

11
The third technique for size distribution measurement is that based

on the mass distribution of the aerosol. An Anderson (7 stage) cascade

impactor^ model 1ACFM [16] with a sampling rate 28.3 L/min was used for

this purpose. Figure 8 shows the results plotted on log-probability

paper which plots the cumulative mass distribution as percent less than

the stated size as function of the particle diameter. The 50% point or the mass median diameter (MMD) was 0.46 ym according to the plot with a calculated o =1.5. Four determinations at various concentration levels

g

gave results in MMD to within +0.1 ym based on the Anderson instruments.

The calculated D based on In D = In (MMD) - 3(lna ) 2 is 0.28 ym.^

g _ g g

This is higher than the D^ = 0.17 ym based on the EAA measurements.

The discrepancy may be due to differences in sensitivity ranges in the

two instruments and questionable log-normal assumption of the test aerosol

conditioned by the impactor.

The essential point here is not necessarily that good agreement

between two methods of measurement exist, but rather that replicate

measurements at several aerosol concentrations give consistent results

based on a single method. Results indicate that this is the case for

particle size distribution in the test stream.

4.3 Aerosol Optical Density, Number Concentration

and Mass Concentration

Underwriters' Laboratory standard UL 217 for smoke detector approvals

evaluations is based on optical density measurement. This is a simplified

method to measure aerosol concentration. Light obscuration measurements

as a function of mass concentration for the test aerosol at various

concentrations were made by a standardized 1-m path length photometer

which has an incandescent light source (color temperature 2425 K) and a

4 Calibrated according to mass diameter

In refers to the natural logarithm.

12
S-4 response phototube corrected with a photopic filter for CIE human

eye response [17]. The photometer is housed in a long narrow chamber^

for rapid stream flow- thru in order to minimize aerosol aging. At the

standard flow rate of 140 L/min at least 3 minutes were needed to com- pletely change the aerosol concentration in the chamber. In addition to

optical density measurements, samples were also monitored continuously

from the end of the chamber by the EAA and the particle mass monitor.

q The sensitivity of the mass monitor is on the order of 0.01 mg/m . Calibration by the weighted filter method using DOP showed good collection

efficiency at our range of interest. Appendix B describes the mass monitor in more detail. Figure 9 shows the optical density as a function

of measured number concentration and mass concentration.

Because of the difference in particle size and refractive index,

the optical density to mass concentration ratio for the generated aerosol

(2.3 m /g) is about 50% higher than the lamp wick smoke (1.5 m /g) used

in the UL 217 test [12]. As a result light-scattering type detectors

will be somewhat more sensitive to these aerosols than ion-chamber

detectors if both detectors have equal sensitivity to the lamp wick

smoke. However, the difference becomes insignificant because there is a wide difference in sensitivity setting among detectors of both types.

4.4 Concentration Range of the Tester

Under the present design configuration, without any change in

dilution air rate, the concentration range of the tester is between

q 10 and 60 mg/m . Because of its limited capacity, the vacuum source was

not sufficient to remove all the generated aerosol. As a result, the

minimum concentration achieved by the system was only 10 mg/m instead

2 Cross-sectional area is 929 cm

13
of zero. It is very unlikely, however, for a detector to alarm at

aerosol concentrations below 10 mg/m 3

. If needed, the concentration

may be reduced by a factor of 2 (from 10 to 5 mg/m 3

) by a corresponding

increase in the dilution airflow to 280 L/min. This requires only

partial closing of the blower vent.

Figure 10 is a calibration curve showing the concentration of the

test aerosol sampled at the shroud by the mass monitor as a function of

rotameter reading. The data also shows the degree of scatter. The

calculated standard deviation for a single test relative to the calibration

3 3 curve at a given setting is 1.0 mg/m in the 10 to 40 mg/m range; and

slightly higher in the 40 to 60 mg/m range. Repeatability for a single test is 2.8 mg/m . Repeatability defined as the difference

between the average of n tests and that shown by the calibration curve

in a given setting, expected in 95% of the time, is 1.92’^2 (cr/v^n);

where a is the standard deviation and n = number of tests.

4.5 Alarm Threshold of Some Detectors

Table 1 presents alarm thresholds on nine typical smoke detectors

of various types exposed to test aerosols of different concentrations at

the shroud. Numbers in the table denote exposure time to cause alarm or fail to alarm. The detectors, including ion-chambers and light-scattering

types, were mounted on a wall while the operator covered the detector

with the tester shroud by holding the rigid plastic connection in a horizontal position.

Alarm threshold levels of detectors tested varied from about 10 to

30 mg/m 3. If the detector failed to alarm after a 50 second exposure,

additional time did not change the result. Depending on the design

A different vacuum pump was able to reduce the concentration to zero.

14
entrance characteristics, the detectors generally responded in less than

25 seconds. Response time appears shorter if the aerosol concentration

is much higher than the threshold value.

Detectors that responded to an aerosol concentration change at a rotameter increment of five (about 1.7 mg/m 3

) were definite and repeatable

(3 tests per detector) except in detector E where there was overlapping

within an increment. The 1.7 mg/m 3 increment corresponds to a change

ranging from 6 to 17% for an initial concentration of 30 to 10 mg/m3

. Smaller incremental changes to obtain better resolution of the threshold

has not been investigated.

5. SUMMARY AND CONCLUSION

A portable tester based on commercially available components for

evaluating the sensitivity of installed smoke detectors was designed

and a prototype was constructed. It is based on selectively retaining

the small-size fraction of a DOP aerosol generated by the Rectec nebulizer

using a unique impactor. The final aerosol has a mass median diameter

of 0.46 pm, comparable to smoke in the UL 217 chamber. Particle size

distribution was very stable.

The flow rate of -the test stream is 140 L/min and the concentration

of the aerosol ranges between 10 and 60 mg/m3

, sufficient to activate

all the detectors tested. The concentration range can be extended or reduced by a factor of 2 if necessary.

Variables such as air pressure, liquid temperature and liquid level

contributed to the uncertainty. The greatest source of uncertainty is

in the method adopted to vary the aerosol concentration. This relies on

flow rate differences to determine the proportion of the aerosol removed

from the output before dilution. Direct monitoring of the aerosol with

a light-scattering or ion-chamber instrument could provide a direct

15
measure of relative change in concentration and greatly improve the

precision. However, the present precision is sufficient for most

applications

.

Laboratory tests for alarm threshold of nine typical smoke detectors

using the developed tester showed good repeatable results to within

1.7 mg/m 3. Testers based on this design should undergo field trial to

determine problems encountered by field operators and to obtain data on

the reproducibility among testers.

6 . ACKNOWLEDGMENTS

The author wishes to thank Dr. George Mulholland for his many

suggestions and assistance so that this study could be completed within

the time constraint.

16
APPENDIX A. CHARACTERISTICS OF THE COMPONENTS

A.l Compressor, Nebulizer and Impactor

Compressor

The electric compressor selected to power the nebulizer utilizes a diaphragm pump. These reliable, low-noise units have been produced

commercially for other applications for many years. It can deliver up

to 12 L/min with a working pressure of 20 psig.

To maintain a nebulizer pressure of 15 psig, the standard adopted

for this design, excessive air from the compressor is vented thru

an adjustable valve. Experience showed that the pressure stayed constant

after initial adjustment. However, a small adjustment is needed when

the vacuum setting is changed to vary the concentration of the test

aerosol. An accurate (+0.1 psig) 15-cm diameter Bourdon-tube gage

monitors the pressure.

A pleated filter (2 ym pore size) with a dead volume of about 200 cm 3

not only cleans the compressed air but also dampens pressure oscillations

from the pump.

Another outlet from the diaphragm pump is the vacuum source. It

is utilized thru a valve to control the amount of aerosol being dumped

so as to change the concentration of the test aerosol. A vent with an attached filter is also connected to the vacuum source to provide a minimum flow to avoid pump damage caused by excessive vacuum.

Nebulizer

The heart of the generator is the Retec nebulizer, which has been

used for many years in inhalation therapy [18]. A nebulizer is an atomizer used to provide aerosols of fine particles by atomization of

liquid to produce droplets, the majority of which are less than 10 m

in diameter. In this particular design, high pressure air from a nozzle

17
picks up a fluid (gas and liquid) and spreads it onto a close-coupled

spherical target as a film. The liquid component becomes progressively

thinner as it is forced at high velocity over an increasing surface.

The liquid surface tension eventually overcomes and breaks the film into

small particles. The principle and construction of various nebulizers

have been reviewed by Raabe [19].

The small, all plastic Retec X-70/N unit has a liquid holding

capacity of 10 mL. At an operating pressure of 15 psig, the gas flow

rate was about 4.7 L/min. The liquid output rate based on DOP (dioctyl

phthalate) was 0.2 mL/min, thus allowing at least 20 minutes of operation

between refills. The 22-mm OD outlet of the nebulizer is coupled to the

impactor manifold by a rubber 0-ring seal; the nebulizer may easily be

removed for liquid refill.

Before reaching the impactor, the output number concentration of

7 3 the aerosol from the nebulizer is on the order of 10 particles per cm , including large particles.

To determine the reproducibility of the flow rate, 12 different

nebulizers were examined. A flow rate range of 4.7 + .25 L/min was

found. To minimize error introduced by such variability, only selected

units with a tolerance of 4.7 + 0.05 L/min were used in the construction

of the generator. The plastic orifice that governs the flow rate may be

purchased as a part at nominal cost from the manufacturer.

Possible deterioration of the nebulizer after long use, which may

alter the aerosol characteristic, has not been investigated. However, a one-month immersion of the plastic orifice from a nebulizer in DOP

showed no observable change in aerosol characteristic.

18
Impactor

An impactor is used to control the upper particle-size limit of the

aerosol from the nebulizer. Because of its large momentum, the largesize particles cannot follow the air stream in the sharp turns of the

impactor. As a result these particles hit the vertical impaction plate,

form a liquid film and drop away after sufficient accumulation.

By adjusting parameters such as the diameter and number of holes,

the impactor can be designed to selectively change the upper size characteristic of the aerosol. The design parameters for inertia impactors

have been reviewed by Marple [20]. The present design is based on those

criteria for a sharp 50% cut-off point at 0.5 pm for a 5 L/min total flow

rate with Reynolds number of 3000 thru each hole. The impactor consists

of 5 holes with 0.53 mm ID drilled in a stainless steel disc 1.3 mm thick

and 22 mm in diameter. The distance between the impaction plate and the

hole plate is 0.78 mm. The impactor changed the DOP aerosol MMD from

2.0 ym to 0.46 ym.

Figure 5 is a drawing of the impactor and dilution system of the

tester

.

A. 2 Blower, Plumbing and Shroud

Blower

The squirrel cage blower rated at 1.7 m

3 /min free air and based on

a 115 VAC shaded-pole motor, is a rugged inexpensive and commonly used

exhaust blower. This normally high capacity blower is necessary to overcome the flow pressure drop of about 124 N/m 3 (0.5 cm of H2 0) thru the

2.5 m-length of hose connecting the generator and the shroud of the

tester. The excessive blower capacity is vented back to the room thru a tee connection. The magnitude of venting maintains the constant rate of

dilution air to the generator.

19
Dilution airflow is monitored by an orifice meter with a thin 19 mm

ID orifice plate. The differential pressure of the orifice is on the

order of 200 N/m2 (0.8 cm H2 O) for the standard flow rate of 140 L/min in

a 27 mm ID smooth copper pipe including the connector and shroud. A

Magnehelic gage with a 0-1 cm H 2 O range measures the differential pressure

from two radius taps. To minimize error, the complete system should be

calibrated with a known flow rate source.

Plumbing

Copper piping 28.6 mm (1-1/8") OD connects the output from the

blower and the generator. A corrigated flexible vacuum cleaner hose

with a 35 mm ID plus a short section of rigid ABS plastic pipe (intended

for vacuum cleaner use) with similar size are also connected to direct

the test aerosol to a shroud.

Shroud

The circular plastic shroud is about 20 cm in diameter and 16 cm

deep. It is large enough to completely cover almost all known commercial

smoke detectors. Small diameter holes located at the periphery of the

shroud allow the test aerosol to exhaust out while the shroud covers

the detector under test.

To avoid direct high velocity aerosol impingment on the test detector

from the pipe, a diffuser cap covers the pipe and directs the flow to the

side. The flow velocity parallel to the shroud is on the order of 0.5

m/min.

20
APPENDIX B. CHARACTERISTICS OF THE PARTICLE MASS MONITOR

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 a 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

deposited, thereby providing a measure of the mass concentration. The

time rate-of-change of the output frequency signal is directly proportional to the total particle mass concentration.

21
REFERENCES

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

Research Council, Canada (1962).

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

Sensitivity and Siting Requirements for Dwellings, Nat. Bur. Stand.

(U.S.), NBS-GCR 75-51 (1975).

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

communication.

[4] Bright, Richard G. , Status Report on Residential Smoke Detectors,

NFPA Annual Meeting, Washington, D.C. (May 1977).

[5] Boyd, H. , Test of Smoke Detectors, Fire Journal, Vol. 71, p. 83-84,

(Jan. 1977).

[6] Cadle, R. R. , The Measurement of Airborne Particles, John Wiley & Son, New York (1976).

[7] Phalen, R. F., Cannon, W. C. and Esparza, D., Comparison of

Impaction, Centrifugal Separation and Electron Microscopy for

Sizing Cigarette Smoke, Fine Particles - Aerosol Generation,

Measurement, Sampling and Analysis, Liu, B.Y., Editor, Academic

Press, Inc. (1976).

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

Electrical Aerosol Analyzer for Measuring Number and Volume

Concentration of Polydisperse Aerosols, to be submitted to J. Colloid

and Interface. Sci.

[9] 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).

[10] Smoke Detector, Consumer Report, p. 555 (Oct. 1976).

[11] Standard for Single and Multiple Stations Smoke Detectors, UL 217,

Underwriters' Laboratories, Inc. (Jan. 1976).

22
[12] Lee, Thomas G. K. and Mulholland, George, Physical Properties of

Smokes Pertinent to Smoke Detector Technology, Nat. Bur. Stand.

(U.S.), NBSIR 77-1312 (Nov. 1977).

[13] Liu, B. H. H. and Pui, D. Y. D., On the Performance of the

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

[14] Willeke, Klaus, and Liu, B. Y. H. , Single Particle Counter,

Fine Particles - Aerosol Generation, Measurement, Sampling and

Analysis, Liu, B. Y. , Editor, Academic Press, 697 (1976).

[15] Newton, G. J. , Raabe, 0. G. and Mokler, B. V., Cascade Impactor

Design and Performance, J. Aerosol Sci., Vol. 8, 337 (1977).

[16] Andersen, A. A. , New Sampler for Collection, Sizing and Enumeration

of Viable Airborne Particle, J. Bacteriol., Vol. 76, 471-484

(1958).

[17] Bukowski, R. W. , Standardized Instrumentation for Smoke Measurements in Large-and Small-Scale Testing, NFPA Annual Meeting,

Washington, D.C. (May 1977).

[18] Mercer, T. T. , Goddard, R. F. and Flores, R. L. , Output Characteristic of Several Commercial Nebulizers, Annals of Allergy,

Vol. 23 (1965).

[19] Raabe, 0. G. , The Generation of Aerosols of Fine Particles, Fine

Particles - Aerosol Generation, Measurement, Sampling and Analysis,

Liu, B. Y. , Editor, Academic Press, Inc. (1976).

[20] Marple, V. A. and Willeke, K. , Inertial Impactors: Theory, Design

and Use Fine Particles - Aerosol Generation, Measurement, Sampling

and Analysis, Liu, Y. Y. , Editor, Academic Press, New York (1976).

23
Table 1. Alarm Threshold of Typical Smoke Detectors

Rotameter

Setting

Concentration

mg/m3 A B C

Dete

D

ctors

E F G H I

110 10.5 o 50

105 11.8 •30

100 13.0 • 21 o50 o50

95 14.5 •15 o50

90 16.0 • 18 •20 • 50 o 60 o50

85 17.5 •11 o50 o50 o50 o55 o50

80 19.0 •30 • 37 •25

o55

•30 • 4 o50 o50

75 20.5 • 12 • 16

70 22.5 • 9 • 10 •13 o 50

65 24.0 o50

60 26.0 • 25 • 15 • 40 o50

55 27.7 o50

50 30.5 • 12

45 32.3

40 35.0 •16

35 37.7

30 40.5

25 43.5

20 46.5

10 53.0

0 60.0

* Number denotes response time in second

o Failed to alarm

•Alarm

24
Figure 1. Sensitivity of two types of detectors exposed

to monodisperse aerosols [8].

25
SCHEMATIC

OF

SMOKE

DETECTOR

TESTER

26

Figure

2.

Schematic

of

the

smoke

detector

tester.
COPPER

PIPING

ORIFICE

FLANGE &

PLATE

Figure

5.

Impactor

and

dilution

system.
AN/A

log

D

n

,

cm

D

p

, pm

Figure 6. Size distribution of test aerosol

and lamp wick smoke as measured by EAA.

30
RELATIVE

NUMBER

CONCENTRATION

D

p

, pm

Figure 7. Relative number concentration of test aerosol

at selected size ranges as measured by the

particle optical counter.

31
PERCENT

LESS

THAN

THE

STATED

SIZE

Figure 8. Logarithmic probability plots of the cumulative

mass distribution of the test aerosol.

32
OPTICAL

DENSITY,

1/m

NUMBER CONCENTRATION. 1/cm 3 0123456 7 * 10

6

Figure 9. Optical density of test aerosol as a function of aerosol mass and number

concentration.

33
I

34

ROTAMETER

SETTING,

mm
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 78-1430

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE 5. Publication Date February 1978

AN INSTRUMENT TO EVALUATE INSTALLED SMOKE DETECTORS 6. Performing Organization Code

7. AUTHOR(S)

Thomas G. K. Lee

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. 491 5678

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 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.) An inexpensive portable instrument has been constructed to test the

sensitivity response of installed fire smoke detectors. The test

stream consists of impactor-selected small particles from aerosol

generated by a pneumatic nebulizer and dilution air. The flow rate

of the test stream is 140 L/min and its aerosol concentration can be

varied from 10 to 60 mg/m 3

, sufficient to cover the sensitive ranges

of the nine typical detectors tested. The aerosol mass median particle

diameter of 0.46 pm and the breadth of the size distribution in terms

of geometric standard deviation, is 1.5. The size distribution of the

polydisperse aerosol at the output is independent of the aerosol con- centration in the range of interest and is comparable to some fire

smokes. Aerosol light obscuration as measured by a standardized

photometer is linear with respect to the aerosol mass concentration

(mg/m 3

) and has a ratio 2.3 m

2 /g for this aerosol compared to 1.5 m

2 /g

for cotton lampwick smoke in the UL 217 chamber.

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 generators; detector sensitivity; detector testers;

light obscuration; particle size distribution; polydisperse aerosol; smoke

detector.

18. AVAILABILITY [^J 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. Cl

3

I I Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

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(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

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UNCLASSIFIED

21. NO. OF PAGES

38

22. Pric $4.50

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