NBSIR 76-1087

On A Scattered- Light

Measuring Device for Use in

Testing Types of Smoke

Detectors

M. Pistor

Institute for Electronic Communication Technology

of the Rheinisch-Westfalian Technical College

Aachen, Germany

Translated for:

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

July 1976

Final Report

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 76-1087

ON A SCATTERED-LIGHT

MEASURING DEVICE FOR USE IN

TESTING TYPES OF SMOKE

DETECTORS

M. Pistor

Institute for Electronic Communication Technology

of the Rheinisch-Westfalian Technical College

Aachen, Germany

Translated for

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

July 1976

Final Report

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

PREFACE

This report is a translation of a paper prepared by M. Pistor and presented by him at the

Seventh International Seminar on Problems in

Automatic Fire Detection held in Aachen, Germany,

on March 5-6, 1975. The Seminar was presented by

the Institute for Electronic Communication Technology

of the Rheinish-Westfalian Technical College, Aachen,

in conjunction with the Gesamthochschule in Duisburg.

This translation has been prepared to disseminate useful information to interested fire research

personnel on a need-to-know basis and is not an

original work of the Center for Fire Research.

We wish to express our appreciation to the

author, Mr. M. Pistor, for his permission and assistance in making this information available.

We also are indebted to the Joint Publication

Research Service, Arlington, Virginia, who provided the translation of the original paper.

Richard G. Bright

Program for Fire Detection

and Control Systems

Center for Fire Research

CONTENTS

Page

PREFACE iii

LIST OF FIGURES vi

LIST OF TABLES vi

Abstract 1

1. INTRODUCTION 1

2. DIFFERENT SMOKE DETECTORS REQUIRE DIFFERENT TEST

DEVICES 2

2.1. Comparison of Scattered-Light Measurement

and Light Attenuation Measurement 3

2.2. Reasons for the Development of a ScatteredLight Measuring Device 4

2.3. Requirements Affecting the Scattered-Light

Measuring Device 5

2.4. Commercial Scattered-Light Measurement

Devices 6

3. OPERATING PRINCIPLE OF THE DEVICE 6

3.1. Technical Details 7

3.2. Various Considerations Affecting Selection

of the Photodiode 7

3.3. Discussion of the Design 9

3.4. Characteristic Equation of the Device. ... 11

3.5. Calibration of the Device 11

3.6. Errors in the Measurement of Scattered-Light 12

4. MEASUREMENT WITH TEST AEROSOLS 13

4.1. Sensitivity of the Device 17

4.2. Use in the Proof Test 18

5. REFERENCES 18

v
LIST OF FIGURES

Page

Figure 1. The Principle of Scattered-Light Measurement

and Transillumination Measurement. ...... 21

Figure 2. Block Schematic of the Laser ScatteredLight Measuring Device 22

Figure 3. Intensity Function for Soot Particles. ... 23

Figure 4. Laser Scattered-Light Device; Geometry ... 24

Figure 5. Scattered-Light Measuring Device with

Evaluating Circuit 25

Figure 6. Arrangement of the Scattered-Light Device

in the Smoke Channel 26

Figure 7. Light Attenuation as a Function of the

Scattered-Light Signal 27

Figure 8. I-Chamber Signal as a Function of the

Scattered-Light Signal 28

Figure 9. Light Attenuation as a Function of the

I-Chamber Signal 29

LIST OF TABLES

Page

Table 1. Technical Data for the Scattered-Light Device. 30

vi
ON A SCATTERED-LIGHT MEASURING DEVICE FOR THE USE

IN TESTING TYPES OF SMOKE DETECTORS

M. Pistor

Abstract

Generally, the response threshold value of

fire detectors is tested with measuring instruments which operate on the same physical principle

as the detectors to be tested. For example, this

means that the response threshold value of an ionization chamber smoke detector is checked with an

ionization measuring chamber and the response

threshold value of an optical-type smoke detector

operating on a light extinction principle is

checked using an extinction measuring instrument.

However, optical-type smoke detectors operating

on a light-scatter principle (photoelectric in

U.S. parlance) have also been checked using an

extinction measuring instrument.

Since the light-scatter type of smoke detector

is by far the most commonly used of the optical

type of smoke detector it seems appropriate to

use a light-scatter measuring instrument to check

the response threshold value of these detectors.

In addition, the need for such a measuring instrument is emphasized by the fact that both the parameters of the smoke aerosol and the design features

of the measuring instrument are affected in different ways by light scatter and light extinction.

The author describes the technical features

and design details of a newly-developed, lightscatter measuring instrument along with some experiments to determine its response to artificiallygenerated aerosols.

Key words: Fire detection; light extinction; light

scatter; light-scatter measurements; photoelectric

smoke detectors; smoke detectors.

1. INTRODUCTION

For early detection of damaging fires more and more use

is being made of those detectors which react to the fire par

meter, "smoke." Here, the optical or electrical properties

of the smoke particles are used to trigger an alarm from the

detectors

1
At the present time, the smoke detectors on the market

can be divided in two groups on the basis of their operating

principles

1. Detectors using the ionization chamber principle

(so-called I-chamber detectors) ; and

2. Detectors having an optical detection principle.

These include the so-called "light attenuation or

light extinction" and "scattered-light" detectors.

Despite their physically-differing modes of operation,

all three types of detectors have the common characteristic

that their response threshold depends simultaneously upon

particle size and smoke concentration. Thus, in dealing with

these detectors we have to deal with complicated smoke

(aerosol) detection devices whose initial signal depends, in

various ways, upon the parameters of the smoke aerosol.

Before licensing such devices for use as fire detectors one

must test their reliability and response sensitivity under

well-defined, reproducible, environmental conditions.

2. DIFFERENT SMOKE DETECTORS REQUIRE DIFFERENT TEST DEVICES

The two most important parameters of a smoke detector,

namely the response threshold and its modification by environmental influences such as, e.g., temperature, humidity, corrosion, etc., can only be suitably tested using standard

measurement devices which operate on the same physical principle as that of the detectors themselves.

If one wishes to determine how the response threshold

varies as a result of environmental influences one subjects

the detector to the basic test in the smoke test tunnel. The

magnitude of the detector's response threshold is checked by

proof-testing with test fires in the fire chamber. For

example, it is conventional in the various test laboratories

to test so-called I-chamber detectors for suitability by using

a standard ionization chamber [ 1

] 1 . So-called optical detectors — for technical reasons the models on the market are

mainly scattered-light detectors — have hitherto been tested

using an extinction measurement device. This device measures

the light attenuation resulting along a smoke-filled path

from absorption and scattering of the incident light [2,3].

Bracketed numbers refer to references listed at the end of

this paper.

2
2.1. Comparison of Scattered-Light Measurement and

Light Attenuation Measurement

Figure 1 shows the measurement principle of scatteredlight measurement in comparison with light attenuation

measurement. In the latter, one is dealing with a two-beam

process in which one light beam is attenuated by the smoke

along the measurement stretch x

Q

. The other light beam

emerging from the same source is unattenuated by smoke and

enters the same photoreceptor as the measurement beam. By

means of the thus derived comparison quantity it is possible

to compensate for errors resulting from changes in the light

source and in the receptor. To this measurement procedure,

the Lambert-Beer law generally applies:

S = S exp ( - C * z * x ) (1) o

r e o

Here SQ and S are the power densities occurring at the source

or at the receptor; z is the number of particles_per unit

volume, and xQ is the measurement path length. Ce is the

so-called mean extinction cross section of a polydispersed

aerosol. It depends upon the parameters of the particle

size distribution, upon the optical refractive index of the

particles, and upon the wavelength A of the light source.

The effect of forward scatter upon this sort of transmission

measurement may be neglected here [4]

In scattered-light measurements, the axis of the photoreceptor is placed at a scattering angle 0 relative to the

axis of the incident light beam. Only the light scattered

by the aerosol particles in the scattering volume V . at the

angle 9 is received by the receptor. For this measurement

principle, under certain assumptions, one can derive the

following formula with the aid of the Mie theory:

A • z - V

2 2

8A 1

st

(i + i ) . 1 2

(2)

I

s is the radiation intensity measurable at the receptor at

a distance 1 as scattered by the scattering volume at the

angle 9. I is the radiation intensity incident upon the

particles in the scattering volume V ^. The wavelength of

light source is denoted by A; z is, as in the light attenuation measurement, the particle concentration, and ±

l

, ±

2

are

the so-called Mie intensity functions. These are complicated

functions of_ the scattering angle 9 , of the complex refractive index m, of the particle size d and of the wavelength A.

3

cheaply than a light attenuation measuring

device of corresponding quality.

5. A more punctiform measurement of the aerosol

can be carried out. This permits determining

the extent to which the concentration in the

smoke test tunnel varies over the cross section.

6. Together with the light attenuation measuring

device, changes in the aerosol parameters can

be observed during measurements in the smoke

test tunnel.

2.3. Requirements Affecting the Scattered-Light

Measuring Device

The essential requirements which can be imposed upon a

scattered-light measuring device for the type-testing of

smoke detectors may be easily determined on the basis of tes

ing practice up to the present:

1. The device must be so compactly constructed that

it fits into the already existing smoke channel

and at the same time does not substantially constrict the cross section of 38 x 38 cm2

.

2. The design should be as open as possible so that

the smoke particles can flow undisturbed through

the measurement volume (called scattering volume)

The flow in the vicinity of the device likewise

should be disturbed as little as possible.

3. The distance 1 0 , 1 already shown in figure 1,

should be as small as possible and the light

source must be as intense as possible in order

to obtain an adequate measurement effect even

for small smoke particles or low concentrations.

4. The device should be easy to build on the basis

of design drawings without the necessity of precision adjustment.

5. The electronics for amplification and evaluation

of the scattered-light measurement should be constructible with commercially-conventional, construction elements.

6. The device should react only to smoke particles,

while being as independent as possible of environmental effects such as, foreign light, temperature, pressure, humidity, vibration, etc.

5

In the following the technical details of the structural

groups will be explained.

3.1. Technical Details

The laser diode, of type LD 23, because of the limited

heat conduction at room temperature, can only work in pulse

operation. For this application this is no disadvantage.

On the contrary it is even desirable to acquire scatteredlight pulses in order to suppress interfering effects from

daylight or from the 100-Hz radiation of artificial room

illumination. This sort of light pulse generation has the

further advantage that expensive, mechanical choppers can be

omitted. The laser diode, at a pulse-peak-power at the input

of 110 W and an efficiency of 11 percent, supplies a peak

output power of 12 W. The pulse breadth amounts to 130 ns at

a repetition frequency of 1 kHz. The power supply for the

laser diode is furnished by a traditional d-c main supply

over a special d-c converter LC 23 which can yield, at a

300-v output d-c voltage, peak currents of more than 40 amps.

The LP 23 pulse generator generates pulses lasting 130 ns

required by the laser diode. This pulse generator is controlled from the central pulse generator.

3.2. Various Considerations Affecting Selection of the

Photodiode

1. The diode must also still yield a sufficiently

measureable signal at very low intensities, i.e.,

10-100 yW/cm 2

.

2. The pulse rise time must be as low as possible

so that the height of the light pulses, which

is in fact a measure of the particle concentration, is converted into a proportional voltage

pulse

3. The maximum of the spectral sensitivity should

lie as much as possible in the range of the wavelength of the light source.

4. The angle-dependent receiver characteristic

should be as narrow as possible so that here,

too, as in the case of the light source, special

lenses can be dispensed with.

The silicon-PIN diode of type MD 2 used here meets all these

requirements

7
Diode and preamplifier are installed in a common brass

housing, because of the required shielding. The preamplifier has a voltage amplification of 6,000 (= 76 dB) in the

range 1 kHz to 1 mHz . The spectrum of the approximately

saw-toothed electrical pulses at the amplifier input suffices

nevertheless theoretically from the frequency of 1 kHz to

infinity, as can be shown by the Fourier-series development.

The individual lines of the spectrum have a spacing of 1 kHz.

There is also present a dc-component . But in order to eliminate interference from daylight or, for example, from the

100-Hz artificial, room illumination the amplifier was

decoupled with respect to d-c voltage and its 3-dB bandwidth

fixed in the range 1 kHz to 1 mHz. ' This band width suffices

for approximately errorless linear amplification of the

electrical pulses since the essential part of its spectrum

is limited to the range from 1 kHz to 1 mHz. By means of the

alternating-current coupling of the two amplifiers V 1 and V 2 , it is also possible to omit expensive offset compensation of

the operation amplifiers employed. From the output of the

amplifier V2 , the pulses arrive at the scanning holding circuit AHS . This circuit consists essentially of a capacitor

having a low-loss factor, two electronic switches and an

impedance converter. The capacitor is periodically charged

up to a voltage which is proportional to the height of the

scattered-light pulse and is again discharged before arrival

of the next pulse. The setup is controlled by the central

pulse generator which secures that the laser pulser and the

evaluation circuit operate synchronously.

Since it can be assumed that the pulses at the output

of V 2 are all of equal breadth, a short-time integrator suffices as a scanning holding circuit; this integrator is likewise constructed with an operation amplifier. The output

voltage of such a circuit is proportional to the scatteredlight pulse area divided by the pulse repetition time of 1 ms

Because of the small scanning ratio of the pulses (1/1,000),

however, the output voltage would vanish in the noise and

could no longer be measured. Hence, the integrator is controlled in such a way that it maintains the capacitor voltage

for a duration of 900 ys and is set back only for a duration of

about 100 ps. In this way the output pulse height is magnified by a factor of 9 00 as compared with normal integration.

The thus modified pulse train is delivered to a low-pass

filter having a limiting frequency of f g = 1 Hz . The limiting

frequency is obtained as follows: the total scattered-light

measuring setup can be conceived as though the laser light

source were working continuously and as though there were a

periodic scanning of the scattered-light signal which rises

with particle concentration. According to Shannon's scanning

theorem one obtains for the scanning rate T = 1 ms the condition : 8
T <_ 1/2 f (3)

where fg is the maximum frequency existing in the useful

signal. This condition is certainly met since the particle

concentration and hence the scattered-light signal varies

only slowly when other aerosol parameters are kept constant.

In order to exactly recover the useful signal the limiting

frequency of the low-pass filter must be equal to fg.

Control of the integrator is accomplished by means of

so-called monostable multivibrators. These yield a pulse

of a specific length for each trigger pulse coming from the

pulse generator TG. All the structural components for control

and evaluation which have been mentioned are, with the exception of the preamplifier V lf constructed in integrated circuitry technique and mounted in a single housing.

3.3. Discussion of the Design

We will now make a few remarks regarding determination

of the scattering angle and the space arrangement of the

structural component. As has already been mentioned, in the

apparatus the effect of the refractive index of the aerosol

particles upon the output measured value should be as low as

possible

Many authors have already calculated the intensity functions ±i, i 2 according to the Mie theory, treating them as

functions of the particle size, of the refractive index, and

of the scattering angle [7,9-12,14]. The results of the

complicated and time-consuming calculations show that the

dependence of the intensity functions upon the refractive

index at small scattering angles is least in the forward

direction. Further, the scattering intensity, as a function

of the scattering angle, is greater in the forward direction

by a factor of from 100 to 1,000 and, therefore, also oscillates much less than at a scattering angle of, for example,

90° [11,14].

Figure 3 shows, for example, the intensity function i 2

as a function of the size parameter a = n • d/X with the

scattering angle 6 as a parameter. Here d is the particle

diameter and A the wavelength of the light source. These

calculations have been carried out by Pruessmann for the

refractive index of soot particles [14] . For particles having

a different refractive index, and for the intensity function

Lit one obtains similar typical curve profiles.

9
Figure 3 also shows that the intensity function oscillates very sharply as a function of the diameter in the scattering angle range of 90°, while in the forward scatter region

it rises continuously with particle diameter d.

The marked dependence of the scattering power upon refractive index at a scattering angle of 90° seems to explain why

some scattered-light smoke detectors now on the market react

so differently to different types of smoke. These detectors

usually cover the light scattered at a mean scattering angle

of 90°.

All results mentioned above imply the desirability of a

measurement setup which operates below an angle of forward

scatter which is as small as possible.

Figure 4 shows to scale the arrangement of light source

and receptor. The radiation of the GaAs laser diode is not

as sharply collimated as, e.g., that of gas lasers. About 90

percent of the power radiates in a cone having an aperture

angle of 23 percent [sic] . In order to avoid expensive optics

for focusing the radiation, while at the same time limiting

the scattering volume, an apertured diaphragm has been placed

in front of the source. This then yields a radiation cone

having the aperture angle y = 3.8°. The power radiated into

this solid angle can be estimated, with the aid of the radiation diagram of the laser diode, to be about 2 W. The intensity in the middle of the scattering volume thus amounts to

about 20 W/cm2 as a pulse peak value.

Since because of diffraction of the light rays at the

circular diaphragms a part of the radiation reaches the photoreceptor, so-called Fresnel diffraction, it was also impossible to choose a scattering angle substantially less than

15°. This represents a compromise between attainment of a

scattered-light measuring effect which is as great as possible

and suppression of this undesirable diffraction, which moreover cannot be completely eliminated by additional smaller

apertured diaphragms in front of the light source. In order

to spatially limit the scattering volume and to provide a

field against interfering light sources, the same apertured

diaphragm was placed in front of the photoreceptor.

Limitation of the scattering volume is necessary in

order to keep as low as possible the noise voltage of the

photodiode, which is produced inter alia by scattering by air

molecules. The size of the scattering volume, together with

the particle concentration, has an influence upon the measurement effect.

10

I
The output voltage is proportional to the product

V . • z. These facts should be taken into consideration

st

whenever the measurement range of the equipment is determined

The size of the scattering volume may be computed from

the intersection of two cones. It amounts to about 0.5 cm 3

. The scattered intensity arriving at the receptor may now be

derived in accordance with equation (2) if the refractive

index, the particle diameter and the concentration of the

test aerosol are given. For a polydispersed aerosol the

intensity functions should be evaluated as a function of particle diameter by using the distribution density function of

the particle size and from this the mean value i

1 + i 2 should

be formed.

In addition, the output voltage of the device can be

calculated whenever the sensitivity of the photodiode and

the pulse transmission function of the device are known.

3.4. Characteristic Equation of the Device

The relationship between particle concentration and output d-c voltage may be expressed in a formula as:

U =K-(i +i0 )

' z+U (4)

a l 1 2 o

Here: z is the particle concentration; UQ the component

arising from diffraction at diaphragms; and i lf i 2 are the

intensity functions which, as has already been mentioned,

are dependent upon the wavelength A , the mean scattering

angle, upon the particle size, and upon the refractive index.

The constant K contains the design parameters of the arrange

ment and the transmission function of the amplifier.

Thus for an aerosol of constant refractive index and

particle diameter the output voltage of the measuring device

is directly proportional to the particle concentration.

3.5. Calibration of the Device

If finally a test aerosol is reproducible in the parameters of refractive index and particle diameter then it is

possible to calibrate the device in a simple manner. The

factor K

1 (i

1 + i ) appearing in equation (4) can be derived

by measurement whenever at the same time measurements are mad

using the above-mentioned particle counter, of the particle

concentration z and the voltage U of the scattered-light

11
measuring device. For this purpose, in principle, one measurement point suffices to obtain the calibration curve Ua

= f(z) since Ua is proportional to z. The voltage UQ is derived

from measurement of Ua in a smoke-free measurement volume.

If the device is calibrated in this way, i.e., the test

aerosol defined by Ua (z) , then at all times it will be possible in conjunction with the particle counter to easily test

whether the aerosol is constant in its properties.

3.6. Errors in the Measurement of Scattered-Light

Equation (4) still does not take into account the possible errors from scattering by air molecules and from extinction of the light beam on the path from the laser diode to the

photodiode. Scattering by air molecules can be neglected in

comparison with scattering by aerosol particles for the particle concentrations existing here which are from 10 3 to 10 6

particles per cm 3. This scattering also has no influence upon

the measurement effect since it is practically already contained in the voltage UQ in equation (4). The extinction of

the light beam in the case of a smoke-filled measurement

length may be estimated as follows:

The decadic extinction modulus is defined as:

10 , P

s . dB , C v m

A = x~ * lgi ° P" in 5T (5)

o e

P

s

is the radiation power emitted by the transmitter and Pe

is that arriving at the receiver. The measurement path length

is denoted by x . Together with the Lambert-Beer law (equation (1)) we obtain from this:

a ->* 7T . dB

m, = 4.34 • C • z in — , cs

X e m (6)

The mean extinction cross section of the test aerosol used

here has been calculated by Kraus to be 0.175 urn

2

.

For z = 10 3 cm-3 we obtain m. = 0.8-10 dB/m and

A

3 For z = 10 6 cm we obtain m^ = 0.8 dB/m.

For the last-mentioned value we obtain from this for P /P : S 6

o.oi

P /P = 10 = 1.023

s e

12
if one uses as the measurement path length the distances

between the scattering volume and the transmitter or between

the scattering volume and the receiver.

This means that for this relatively high aerosol concentration, with which traditional smoke detectors will long

before have reached the alarm threshold, there is an error of

only 2.3 percent in the output voltage of the measuring device.

Thus this error can be neglected up to concentrations of

about 10 6 cm

-3

. If it is desired to compensate for the error,

then, using a simultaneous light attenuation measurement, one

can determine the ratio P

s /P

e

and continually multiply the

reduced output voltage of the scattered-light measuring device

by this factor.

One can also compensate for the error automatically by

building a control circuit. For this, one requires a light

attenuation measurement in which the laser serves as a light

source. A second photoreceptor at the distance 1 Q

+ 1 converts the radiation, which has been attenuated by the smoke,

into a proportionate d-c voltage. This voltage serves as a

control quantity in a simple control cycle. A constant voltage serves as the reference value. The difference between

the two voltages is the control deviation, which through an

amplifier increases the supply voltage of the laser so that

the latter yields a higher power and hence compensates for

the light attenuation in the scattered-light measurement.

If this control circuit is also designed to be temperature independent then in addition one can compensate for the

effect of environmental temperature upon the output power of

the laser light source. This effect is not negligible if the

scattered-light measuring device is also to be used in the

proof test in the fire chamber.

Figure 5 shows the scattered-light device together with

the evaluating circuit. Because of the required mechanical

stability, light source and photoreceptor are firmly screwed

to a 5-mm-thick steel plate.

4. MEASUREMENTS WITH TEST AEROSOLS

Figure 6 shows the arrangement of the scattered-light

device in the smoke tunnel of the IENT. This closed test

chamber serves for testing smoke detectors in the basic test.

The aerosol is introduced at the point A through a pipe and

circulated and then uniformly distributed by a ventilator,

continually having a mean flow velocity of 0.2 m/s.

13
D is the light attenuation measurement length, I is the

standard ionization chamber. The three measuring devices

must as far as possible be so placed in the tunnel that equal

particle concentrations exist in their measurement volume,

but on the other hand this must be done in such a way that

the devices do not interfere with one another.

The test aerosol is a paraffin-oil mist. The paraffinoil, which is fluid at room temperature, is atomized under

well-defined conditions and after having been diluted with

filtered air is conducted into the smoke tunnel; here the

particle concentration rises stepwise.

If measurements of an aerosol are carried out simultaneously with the three measuring devices then the following

measured values are obtained:

1. Light attenuation measurement

From equations (5) and (6)

m = 10/x

q

• lg 10 P

s

/P

e = 4.34 C • z e

Scattered-light measurement

From equation (4)

AU = U - U

s a o

= K

(ii + i?)

Ionization chamber [1]

y = K 2

• d • z

with y x

2-x

l-x ; x = Al

i

o

(7)

x is the relative different current when smoke enters into

the measurement chamber.

All three measurements are proportional to the particle

concentration z so that between m, AU S and y there must also

exist linear relationships. Diagrams can now be drawn with

m = f(AU s ); y = f(AUs

) and m = f (y) , where the individual measurement points denote different concentrations.

In the diagram of figure 7 , m is plotted as a function

of AU S , using the particle size as a parameter.

14
Curve 1 resulted from two measurements with an aerosol

produced by the process described above. The aerosol is

polydispersed , i.e., the particles have various sizes. Curve

2 was derived with an aerosol which was produced with a

second generator of the same type at the same atomizing pressure. The curves 3 and 4 resulted from measurements with an

approximately monodispersed aerosol, i.e., the particles all

have about the same size. The aerosol is likewise paraffinoil mist, produced with a generator in accordance with the

"Lassen" principle.

Curve 3 resulted when the particle diameter was set at

0.4 pm; curve 4 at 0.3 urn. This diagram clearly confirms

the above-mentioned, theoretically established, linear relationship between m and AU

g

.

In the same way it was possible to establish by measurements a linear relationship between y and AU S

, as shown by

figure 8

Also, at higher flow velocities in the smoke tunnel of

up to 2.4 m/s, it was possible, under otherwise identical

measurement conditions, to show that the measurements m and

AU S remain proportional to one another. This is also true

in the case when the concentration does not increase stepwise

but linearly.

The deviations of individual measurement points in the

diagrams of figures 7, 8 and 9 from the sketched ideal line

arise on the one hand from measurement errors in the three

devices but probably also from the fact that the particle concentration can vary locally, e.g., from vortex formation.

The differing sensitivities of the three devices to particles of different sizes or to particle size distributions

can also be clearly shown.

Curve 4 "particle diameter 0.3 ym" is clearly steeper

than curve 3 "d = 0.4 Mm."

While in the light attenuation measurement the measurement m(Ce , z) increases approximately as the square of the

particle diameter, the scattered-light measurement for particles smaller than 1 urn in diameter is proportional to d

3 up

to d

6

. As is also apparent in figure 3, the scattered-light

effect decreases very sharply for smaller particles. This

tendency manifests itself still more clearly in the measurement results in figure 8. Curve 4 (d = 0.3 \im) is almost

twice as steep as curve 3 (d = 0.4 ym) . This is explainable

by the fact that the measurement y = K 2

• z • d in the ionization chamber is only proportional the particle diameter.

15
The curves in figure 9 were drawn in order to be able

to check the adjustment of the aerosol generators in comparison with earlier measurements. The tendency for the light

attenuation measurement at larger particle diameters to yield

a greater measurement effect in comparison with the I-chamber

measurement can also be seen here.

In comparison with earlier measurements it was finally

possible to establish that, although within a single measurement there exists a linear relationship between AU S and m

or y, nevertheless in different measurement series, clearly

differing slopes of the lines m(AU s ), y(AUs ) occurred and this

was under what were evidently the same adjustment conditions

of the same aerosol generator. If one excludes measurement

errors and equipment errors, then the differences cited can

arise as a consequence of differing aerosol parameters.

It is known that for an atomizer at higher pressure the

mean diameter of the generated particles is smaller. Some

experiments with two aerosol generators of the same type at

different pressure adjustments have confirmed this fact.

Thus in the testing of smoke detectors in the basic test,

one must make sure that, in measuring the response threshold

before and after stressing by environmental influence, the

parameters of the test aerosol are reproduced with a certain

tolerable deviation. For this purpose, use should be made

of the measurements m, y and AU S at increasing concentration.

Finally, it must also be mentioned that in the diagrams

of figure 7 the light attentuation between source and receptor

in the scattered-light measurement has still not been taken

into account. For the test aerosol employed here the extinction cross section was assumed to be 0.18 nm 2

. From this one

obtains, depending upon the particle concentration, the following measurement errors:

z/cm-3 m/dB/m Error/%

10 6 0.8 2.5

4

2 10 6

10 6

1.6

3.2 10. 0

5.0

16
4.1. Sensitivity of the Device

The smallest measurement with the scattered-light device,

expressed in terms of the light extinction produced by

paraffin-oil mist, amounted to 0.06 dB/m. In this way the

measurement range is maintained which was required for the

testing of smoke detectors in accordance with previous test

guidelines

The sensitivity of the device with respect to particle

size and concentration depends upon various quantities which

can in part be further varied. With the present device, fixed

values are preassigned to the wavelength A of the light

source, the sensitivity of the photodiode and, within certain

limits, to the scattering angle and the scattering volume.

The output power of the laser can be varied by changing

the supply voltage. The size of the scattering volume depends

upon the mean scattering angle and upon the diaphragm apertures. In contrast to the particle counter, with which individual particles are analyzed with respect to their size in a

temporal sequence one after the other, here we are dealing

with an integral measurement. This means that the output

measurement is simultaneously a function of the particle

diameter and of the mean particle concentration in the scattering volume. Hence, to estimate the largest and smallest

measurable particle diameter it is necessary in each case

also to specify the concentration, and conversely. According

to the calculation process described by Kraus for determining

the particle size parameters, with the present device it is

possible to measure particles in the size range 0.2-5 ym.

Since the scattered-light intensity at these limits depends

very sharply upon particle diameter, it is possible here to

give only an approximate measurement range from z = 10 3 to

10 6 cm-3 . These data apply in each case to the paraffin-oil

test aerosol.

By modifying the structural elements in the amplifier

the sensitivity of the device to concentration can be further

reduced by a factor ranging from 10 2 to 10 3

. If the scattering angle is enlarged then the sensitivity can also be reduced,

depending upon particle size, by the factor 10-100. However,

this has the disadvantage that then the measurement again

depends more upon the refractive index of the particles.

For the main application of the device in the basic test

the existing adjustment of the device parameters has shown

itself to be good.

17
4.2. Use in the Proof Test

We are planning future testing of the device with aerosols from actual test fires. From the results, we will know

whether for unfavorable refractive indices of these types of

smoke or for low concentrations it will become necessary to

further increase the sensitivity of the measuring equipment.

This could be accomplished, for example, by increasing the

laser power or by changing the amplifier data.

For use of the device in the proof test it is recommended

that the temperature dependence of the laser be automatically

compensated, as above-described, and that the measuring equipment, together with a more compactly-designed evaluating circuit, should be assembled on a common base plate. The power

supply and the output signal can then be brought in or conducted out over relatively long cables.

The above-described scattered-light measuring device for

the type-testing of smoke detectors has shown, through the

measurement results, that the basic design is usable for the

stated application and satisfies all requirements which need

to be imposed upon such a device for the purpose of measurement, particularly in the basic test.

No claim is made that the existing design should be proposed as a standard measuring device since this first laboratory model was also intended for studying the problem of

measuring light scattered by artificial aerosols in the smoke

tunnel. This purpose has been achieved. In addition to this,

the device is suited, together with the light attenuation

device and the ionization chamber, to determining the parameters of the size distribution of test aerosols at a prescribed refractive index.

5. REFERENCES

[1] Gilson, H. and Hosemann, J. P., Measurement Chamber

Based Upon the Small Ion Accumulation Principle, for

Quantitative Detection of Aerosol Particles,

FORSCHUNGSBERICHT DES LANDES NORDRHEIN-WESTF . (Research

Report of North Rhine-Westphalia), No. 2336, Opladen 1 s West German Publishing House (1973).

[2] Mueller, H .

, Photoelectric Smoke Density Measuring

Device Using the Double Beam Compensating Method,

STAUB-REINHALT . (Air Purification and Dust Elimination)

,

Vol. 23 , 123 (1963)

18
[3] Portscht, R. , Calculation and Measurement of the

Extinction Properties of Smoke in the Optical Spectral

Range, STAUB-REINHALT .

, Vol. 32, 277 (1972).

[4] Kraus, F. J., The Effect of Forward Scatter Upon Aerosol

Transmission Level Measurements, STAUB-REINHALT. Vol. 33,

348 (1973).

[5] Borho, K. , A Scattered-Light Measuring Device for High

Dust Concentrations, STAUB-REINHALT., Vol. 30, 479 (1970)

[6] Jacobi, W. , Eichler, J. and Stolterfoht, N., Particle

Size Spectrometry of Aerosols by Light Scattering in a

Laser Beam, STAUB-REINHALT., Vol. 28, 314 (1968).

[7] Seger, G. , The Use of Laser Light Sources in the Control

of Particulate Impurities in Air, MESSTECHNIK (Measurement Technique), No. 5, 142 (1973).

[8] Williams, I. and Hedley, A. B., The Choice, Design and

Performance of a Multichannel Aerosol Particle Counter,

AEROSOL SCIENCE, Vol. 3, 363 (1972).

[9] Gebhart, J., Bol, J., Heinze, W. and Letschert, W.

,

A Particle Size Spectrometer for Aerosols Using Small

Angle Scattering of Particles in a Laser Beam, STAUBREINHALT. , Vol. 30, 238 (1970).

[10] Hodkinson, J. R. , Particle Sizing by Means of the

Forward Scattering Lobe, APPLIED OPTICS, Vol. 5, No. 5,

839 (1966).

[11] Robock, K. , Principles of Optical Dust Measurement,

STAUB-REINHALT., Vol. 22, 80 (1962).

[12] Quenzel, H. , Influence of Refractive Index on the

Accuracy of Size Determination of Aerosol Particles

with Light-Scattering Aerosol Counters, APPLIED OPTICS,

Vol. 8 , No. 1, 165 (1969)

[13] Robock, K. , Breuer, H. , Langner, W. and Teichert, U.,

Experiments with the Photoelectronic Fine-Dust Measuring

Device Called the "Digital Tyndallometer TM, " lecture

at the Annual Congress of the Society for Aerosol

Research, Bad Soden/TS , West Germany (1974).

[14] Pruessmann, A., Optical Smoke Density Measurement for

the Evaluation of Fire Detectors FORSCHUNGSBERICHT DES

LANDES NORDRHEIN-WESTF. , No. 1801, Opladen's West German

Publishing House (1967).

19
[15] Hosemann, J. P., On Procedures for Determining the Grain

Size Distribution of Highly Concentrated Polydispersions

of Dielectric Mie Particles/ dissertation, Technical

University, Aachen (1970)

[16] Schaefer, Th. , Aspects of the Development of a ScatteredLight Measuring Device, Congress Report of the Sixth

International Lecture — Seminar on Problems of Automatic Fire Detection, IENT, Technical University,

Aachen, 294 (1971).

20
(1) DURCHLI CH TMESS UNG

.(Zweistrahlverfahren)

(2) STREUL ICH TMESS UNG

(JO

Vergleichs - strecke

Quelle (S 0 )

(5)

Mefistrecke

*o

rh

(6)

£li Empt'anger (S)

S = S0 exp(-Ce Z x0)

Ce = Ce (d,p(d). m,X)

I

(3)

V (6)

Empfanger (Is ) ^ LicMfalle (7)

A Z Vs ,

/, 2 = i(e.d.p(d).m.\)

Key: 1. Transillumination measurement (double beam

process

)

2. Scattered-light measurement

3. Source

4. Comparison stretch

5. Measurement stretch

6 . Receptor

7. Light trap

Figure 1. The Principle of Scattered-Light Measurement

and Transillumination Measurement (see table 1)

21
)— •0» 75°

DC DC

Z.C2J

T

Pulsator laserdiode

LP ?3 LD23 ; A =0,9[im

PHOTO - DIODE MD2

a~--> Preamplifier

1 V'6000

r.=/ns

r >

/^=900nS

il 7!G

f=1kHz

Of <i

TP

la,

A.HS +15V m

5V

Figure 2. Block Schematic of the Laser

Scattered-Light Measuring Device

22
Figure 3. Intensity Function for Soot Particles

23
(1) Langsschnitt des Streulichtgera ts

(6) Streuvolumen Vst = 0,5 Cm

Longitudinal section of the scattered-light

device

Transmitter

Scattered-light volume

Receiver

Scattering angle

Figure 4. Laser Scattered-Light Device; Geometry

24

Rauchdetektor

\3) A : Aerosolzufiihrung

(h-) I : Ionisations-Mefikammer

(5) D . Durchlicht messung

(6) V : VenMafor

( 2 ) Querschnitt des

Kanals >

(7) Ma/?e des

Streulichtgerats:

(8) Lange : 36 cm

(9) fire/'fe . 15 cm

(10) Wo7> e • 10 cm

(ll) ' Maximalwerte )

38

,

1

p

-- -c

•38

0

Key: 1. Smoke detector

2. Cross section of the channel

3 . Aerosol intake

4 . Ionization measuring chamber

5. Transillumination measurement

6. Ventilator

7. Dimensions of the scattered-light device

8 . Length

9. Width

10. Height

11. (Maximum values)

Figure 6. Arrangement of the Scattered-Light Device

in the Smoke Channel

26
(5) Streulichtsignal a\J,

i

Key: 1. Light attenuation

2. Parameter: particle size

3. Generator: "Draeger" — (polydispersed)

4. Generator: "Lassen" (monodispersed)

5. Scattered-light signal

Figure 7. Light Attenuation as a Function of the

Scattered-Light Signal

27
Key: 1. I-chamber signal

2. Parameter: particle size

3. Scattered-light signal

Figure 8. I-Chamber Signal as a Function of the

Scattered-Light Signal

28
Key: 1. Light attenuation

2. Parameter: particle size

3. I-chamber signal

Figure 9. Light Attenuation as a Function of the

I-Chamber Signal

29
Table 1. Technical Data for the Scattered-Light Device

1 . Dimensions

Length: 36 cm; maximum width 15 cm; maximum height 10 cm

Scattering angle: fi = 18.5°; 9 . = 11°; 7T = 15° max min

Scattering volume: = 0.4 cm 3

Distance between center of scattering volume and light source: 5.5 cm

Distance between center of scattering volume and receptor: 8.0 cm

Aperture angle of the radiation cone of the transmitter and receiver: 3.8°

Diaphragm aperture: 2 mm diameter

2 . Electrical Data

a) Light source: GaAs laser diode, type LD 23

Manufactured by Laser Diode Laboratories, New Jersey, USA or the

E. Sommer Company, Frankfurt

Wavelength: \ = 0.9 urn

Mode of operation: pulse operation, pulse breadth: 130 ns , pulse repetition frequency: 1 kHz

Peak output power: 12 W at T„ = 25 °C

Peak output power: 9 W at T u = 55 °C of which 90 percent is radiated in a cone having a 23°

aperture angle

Power supply: 28 V = + 3V, 250 mamps

Control using pulse generator, f = 1 kHz

Pulse width 1 us, TTL (expansion unknown) — level

b) Photoreceptor: Silicon PIN diode, type MD 2 Manufactured by Monsanto or A. Neye, Hamburg-Quickborn

Sensitivity: 4 amps per mW/cm 2

Dark current: 2.5 namps (Hjj 1 uW/cm ?

)

Pulse rise time: 3 ns

Maximum power loss: 300 mW

Maximum spectral sensitivity at \ = 0.9 p

c) Pulse amplifier:

Voltage amplification Vn = 6,000 = 76 dB within the 3 dB limiting frequencies from 1 kHz to 1 mHz; Vn

(100 Hz) = 58 dB

Push-pull coupling

3 . Sensitivity of the Arrangement

ua m = corresponds to a value 6 dB/m = 86%/m light attenuation in the light attenuation measurement

max

U a corresponds to 0.06 dB/m = 1.4%/m light attenuation in the light attenuation measurement

=x = 0.03; y = 0.05 in the ionization chamber measurement (test aerosol: paraffin-oil mist)

4 . Observation Error Resulting from Light Attenuation (Extinction) Between Light Source and Receptor

Error_- 2.5 percent for particle concentration of z < 10° cm-3 (applicable for paraffin-oil mist

with C = 0. 18 pm2

)

.

30
INBS-I 14A (REV. 7-73)

[ U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 76-1087

2. Gov't Accession

No.

3. Recipient's Accession No.

j4. TITLE AND SUBTITLE

ON A SCATTERED-LIGHT MEASURING DEVICE FOR THE USE IN

TESTING TYPES OF SMOKE, ULlhLlORb

5. Publication Date

July 1976

6. Performing Organization Code

7. AUTHOR(S)

M. Pistor

o* Ppri or m inc (Jroan R pnnrr No

?. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

492 8677

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 tactual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Generally, the response threshold value of fire detectors is tested with

measuring instruments which operate on the same physical principle as the detectors

to be tested. For example, this means that the response threshold value of an

ionization measuring chamber and the response threshold value of an optical-type

smoke detector operating on a light extinction principle is checked using an

extinction measuring instrument. However, optical-type smoke detectors operating

on a light-scatter principle (photoelectric in U.S. parlance) have also been

checked using an extinction measuring instrument.

Since the light-scatter type of smoke detector is by far the most commonly used

of the optical type of smoke detector it seems appropriate to use a light-scatter

measuring instrument to check the response threshold value of these detectors. In

addition, the need for such a measuring instrument is emphasized by the fact that

both the parameters of the smoke aerosol and the design features of the measuring

instrument are affected in different ways by light scatter and light extinction.

The author describes the technical features and design details of a newlydeveloped, light-scatter measuring instrument along with some experiments to determine

its response to artificially-generated aerosols.

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) Fire detection; light extinction; light scatter; lightscatter measurements; photoelectric smoke detectors; smoke detectors.

18. AVAILABILITY f~2 Unlimited

|

! For Official Distribution. Do Not Release to NTIS

C2 Order From Sup. of Doc, U.S. Government Printing Office

Washington, D.C. 20402, SD Cat. No. C13

[~xl Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

35

22. Price

$4.00

USCOMM-DC 29042-P74
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