/

’ ' / A -- for information and record

purposes and is not to be reference

in any publication.

NATIONAL BUREAU OF STANDARDS REPORT

SOME OPTICAL PROPERTIES OF FINE POWDER JSED

FOR

DRY CHEMICAL FIRE EXTINGUISHMENT

by

C. S. McCamy

for

U. S. Coast Guard

Order No. CG-28-606B

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

3974
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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of the divisions and sections engaged in technical work. In general, each section is engaged in specialized research, development, and engineering in the field indicated by its title. A brief

description of the activities, and of the resultant reports and publications, appears on the inside

of the back cover of this report.

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Instrumentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Engineering Metrology.

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Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid

State Physics. Electron Physics. Atomic Physics. Nuclear Physics. Radioactivity. X-rays. Betatron. Nucleonic Instrumentation. Radiological Equipment. AEC Radiation Instruments. Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Gas Chemistry. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics.

Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Organic Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Mineral Products. Porcelain and Pottery. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstructure.

Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and Wall Coverings. Codes and Specifications.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. Components and Techniques. Digital Circuitry. Digital Systems.

Analogue Systems.

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Gas Liquefaction.

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Radio Standards. High Frequency Standards. Microwave Standards. • Office of Basic Instrumentation • Office of Weights and Measures
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

1002 - 10 -4-828 March 8, 1955

NBS REPORT

3974

SOME OPTICAL PROPERTIES OF FINE POWDER USED

FOR

DR T CHEMICAL FIRE EXTINGUISHMENT

by

Co S 0 McCamy

Fire Protection Section

Building Technology Division

The publication, reprintir Approved for public release by the ** is prohlbi,ed

for

U, S« Coast Guard

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

unless permission Is obtal

25, D.C, Such permlsslo Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

Js, Washington

as been speclfical or its own use, ly prepared If that ajj

SOME OPTICAL PROPERTIES OF FINE POWDER USED

FOR

DRY CHEMICAL FIRE EXTINGUISHMENT

ABSTRACT

In an effort to shed some light on the

mechanism of extinguishment of fires with dry

chemical, the transmittance and diffuse reflectance of commercial dry chemical dispersed

in air were measured in two spectral regions,

the visible and near infrared. The size distribution of particles in the powder was also determined.

The results indicate that the absorption of the

dispersed dry chemical is high enough to reduce

substantially the energy radiated from the flames

to the fuel.

1. INTRODUCTION

The effectiveness of dry chemical extinguishers in

fighting gasoline fires has prompted considerable speculation

concerning the mechanism by which the extinguishment is

effected (1,2, 3, 4). A dry chemical extinguisher* is a device

designed to distribute a dry powdered substance on a fire,

the substance currently employed commercially being very

finely ground sodium bicarbonate treated with one or more of

several materials to improve its flow properties. The

earliest theory of dry chemical extinguishment and probably

that which prompted its initial development was the theory

that the sodium bicarbonate decomposed in the fire and the

released carbon dioxide displaced the available oxygen.

There is considerable evidence to discredit this theory (3 ,*+) • Another emergent theory which remains plausible is that

the dispersed powder hinders the heat transfer by radiation

from the region of combustion to the unburnt gases and the

liquid fuel. The present investigation concerns this theory,

therefore, it will be stated more explicitly. In normal

combustion on the free surface of a liquid fuel, the flame

front moves downward with respect to the vaporized fuel. The

unburnt vapors move upward with respect to the liquid surface.

These two velocities are equal, on the average, causing the

flame front to have a fixed average position a short distance

above the surface. The flame front advances with respect to

the gaseous fuel at a rate determined, in part at least, by
,* .
- 2 -

the rate of heat transfer from the combustion zone to the

nearby vaporized and unvaporized fuel, The diffusion of

oxygen and fuel, the rate of intermediate reactions preceding

oxidation, and other factors presumably enter into the

establishment of the equilibrium, but it is postulated here

that the transfer of radiant energy is a primary factor and

that significant interference with that process reduces the

supply of preheated fuel, and eventually brings about

extinguishment 0 With this as a working hypothesis, it became of interest

to investigate the optical behavior of the powder in the

dispersed state 0 In particular, it became important to

determine how much of the energy in an incident beam would be

transmitted undeviated and how much would be deflected

through various angles

The energy might be deflected by either or both of two

processes, scattering and diffuse reflection, "Scattering"

is the technical term applied to a particular mechanism of

deflection which occurs when the dimensions of the suspended

particles are small compared to the wavelength of the

incident energy. The intensity of scattered radiation is

inversely proportional to the fourth power of the wavelength

and is proportional to i + cos^ of the angle

between the incident and the scattered rays. When the

dimensions of the particles are large compared to the wavelength of the incident energy, the energy is diffusely

reflected and the angular distribution and intensity depends

upon the gross properties of the material composing the

particles. The intensity of the reflected energy is inversely

proportional to the diameter of the particles. For a given

mass of material, the intensity of the deflected energy

increases as the particle diameter approaches the wavelength

of the incident energy - whether the approach is from the

longer or shorter side. These phenomena are discussed in

more detail in reference (5) and in textbooks on optics.

The determination of the prevalent mode of deflection

was of interest because it might reveal in a qualitative

fashion the relative sizes of the effective particle diameter

and the wavelength of the bulk of the radiant energy and

thereby suggest a way to improve extinguisher performance.

2, APPARATUS AND METHOD

2,1 Recirculating System

An approach which was found unsatisfactory will be

described briefly in order that the experience gained thereby

may be a matter of record, A metal cylinder about one foot

in diameter and 1 1/2 feet in height was equipped with blowers

-3-

at the top and bottom to recirculate and maintain turbulence

in the enclosed air and the sides of the enclosure were

equipped with quartz windows so that powder could be kept

dispersed in the air while being irradiated and observed

through the windows 0 It was found that the blowers changed

the form of the powder, apparently by grinding, as was

evidenced by a noticeable change in the flow properties of

the powder. In the modified form, the powder deposited on

the windows and sides of the enclosure. Although the optical effect of the powder on the windows was easily measured

in still air after the dispersed powder had been allowed to

settle to the bottom of the enclosure, the measured values

under such static conditions were not believed to represent

the values applicable to the dynamic state. In view of these

difficulties, another experimental method was sought.

2.2 Falling Powder System

It was found that commercial dry chemical would remain

on a fixed 100 mesh sieve but would flow quite readily if

the sieve were vibrating, This fact made it possible to

dispense a stream of powder which could be irradiated and

observed without windows. In the system adopted, the powder

was contained in a light metal cylinder 4 1/2 inches in

height and two inches in diameter, open at the top and bottom.

The cylinder was allowed to rest on the center of a sieve

8 inches in diameter having 100 meshes to the inch (.0059 in.

openings) and was kept from moving from the center by thin

rods attached to the cylinder and extending radially to

the rim of the sieve but fitting the rim loosely. A six

volt dc vibrator was arranged in such a manner that a clapper

attached to the armature struck the rim of the sieve,

causing the sieve to vibrate. No powder was dispensed

when the vibrator was not energized and the rate of discharge

could be varied by adjusting the position of the vibrator

with respect to the sieve rim and the potential difference

applied to the vibrator.

When falling dry chemical strikes a surface, a small

amount of the powder rebounds and drifts about in the air.

To avoid any interference from this source, an industrial

vacuum cleaner was equipped with an 8 inch conical

attachment and used to remove the powder as it fell. The

opening was fitted with a sieve having 28 meshes to the inch

to smooth and distribute the air flow. This sieve was

located 8 inches below the dispensing sieve. At this distance,

the cleaner system had no apparent effect on the free fall cf

the powder in the first three inches of fall.

The falling powder stream contracted from 50mm diameter

at the 100 mesh sieve to 40 mm diameter an inch below the

sieve. The diameter remained fairly constant over the range

from one to three inches below the sieve. Even in still air,

the stream waivered slightly about its central position.

2.3 Effective Rate of Application of Powder for Extinguishment

Commercial extinguishers have been tested at the NBS

and the discharge pattern observed. The rate of discharge

of dry chemical varies with the extinguisher and is not

constant over the entire discharge period of the extinguisher.

A typical case provides a suitable rate for experimental

purposeso A typical hand held 20 lb extinguisher directed

horizontally at a height of three feet distributes powder

over a horizontal area approximating an ellipse having axes

of 10 and 20 feet. It discharges powder at an average

rate of one pound per second. Although the center of the

stream probably contains more powder than would be necessary,

most of the outer area of this pattern must be considered

too thinly covered for effective fire fighting. Let it

be assumed that if the entire discharge were uniformly

concentrated in an ellipse having axes one-half of those of

the ellipse mentioned above, the rate of application would

be effective over this entire area. The effective ellipse

would have an area of 5650 square inches. One pound, or

b^b grams, of powder is discharged per second; therefore the

application rate is 0,08 gm/sec-in^.

The manner of application in actual fire fighting is

not predictable except that the operator approaches as close

as possible and directs the stream at the base of the

flames with a side to side sweeping motion. The flames

are swept back, extinguishment being effected over a small

segment at any one time. Over that small area, with the

extinguisher at close range, the rate of application is

instantaneously higher than that calculated above. It

has not been established whether or not the higher rate is

necessary. It seems reasonable to assume that the rate

calculated above is of the right order of magnitude for

experimental purposes and that in practice a higher rate

may be employed. The results of this study are reported

in such a form that they are applicable to various rates.

As mentioned in section 2.2, the falling powder stream

in the experimental system constricted to a diameter of

40 mm at the point of observation. This stream, then, had

an area of two square inches so, to simulate the application

rate assumed above, the powder dispenser was adjusted to

provide a flow rate of approximately 0d6 gm/sec.

2ck Optical System

For the visible region of the spectrum the optical

system consisted of a light source, a photovoltaic cell,

and a potentiometer recorder.

The light source was a 100 watt incandescent lamp

enclosed in a ventilated housing and a two lens projection

- 5

-

system designed to produce a beam of light of nearly uniform

cross section. The beam was directed horizontally with its

center line two inches below the dispensing sieve. Where

the beam traversed the powder stream it had a square cross

section 30 mm on a side. Power was supplied through a constant voltage transformer and an autotransformer so that

the lamp voltage could be controlled.

The receiver used to measure the amount of light was a photovoltaic cell having a flat circular sensitive surface

2 inches in diameter. This cell was set in the end of a steel tube 2 1/8 inches in diameter and 6 inches long.

This restricted the angle of view and precluded stray light.

The receiver was supported in such a manner that it could

be rotated about the powder stream while remaining directed

at the stream. An angular scale was provided so that the

receiver could be set at any angle with respect to the

incident beam from 0° to 150° . The use of larger angles was

impossible since this would require the source and receiver

to be in the same place.

The output of the receiver was recorded on a continuously recording potentiometer with a wide range of sensitivity.

The linearity of the response was established by measurements

of the illumination due to a small source at various

distances and the use of the inverse square law of

illumination.

The lamp was a commercial grade 100 watt, 120 volt,

tungsten filament bulb operated at rated voltage. This

provided a source with a color temperature of 271+0°K,

which was used in conjunction with the uncorrected photovoltaic cell to obtain an effective spectral distribution

similar to the relative visibility curve for the normal

human eye but shifted about 0.05 micron toward the long

wavelengths

For the infrared portion of the spectrum, the system

consisted of a heated wire source, a thermosensitive

radiation detector, and a potentiometer recorder.

The source was a specially wound conical heating

element operated at about 1200° K. The wires were wound on the

inside and outside of a porcelain cone. The cone was placed

in a metal container and the open base of the cone was

oriented in the direction of an aperture from which the

radiation emanated. Viewed through this aperture, the source

appeared to be a closely wound spiralled helix of hot wires.

A thermosensitive radiation pyrometer was used as a detector. It was a Leeds and Northrup instrument with a mica window and a concave mirror which focused the energy

on a thermopile.
'

••

.

.« ,

.
- 6 -

The output of the receiver was recorded on the same

potentiometer used in the measurements in the visible

spectrum. The linearity of the instruments were established

as before.

2.5 Energy Spectrum Emitted by Burning Hydrocarbons

Almost invariably, accidental fires result in diffusion

type flames. Previous investigations (6) have shown that

the spectral distribution of the energy emitted b^ hydrocarbon diffusion flames is similar to that of a blackbody

at l470°K. This distribution has a maximum at a wavelength

of about 2 microns and 94% of the energy is included in the

wavelength region below 9*5 microns. By means of an optical

pyrometer, it was possible to estimate the temperature of

the wire heater element. This element was adjusted to about

1200°K, giving a spectral distribution with a maximum at

about 2.4 microns. The temperature of the heater was not

increased because this might have damaged the heater element.

The spectral distribution probably was augmented slightly

on the long wavelength side of the maximum by the energy

radiated by the porcelain cone which was at a lower temperature. This enrichment at the longer wavelengths was more

or less compensated by the absorption of the mica window on

the receiver. The transmittance of a thin mica sheet is

known to drop off gradually from about 98% at 2 microns to

about 50% at 8 microns and to zero at 9 microns (and to rise

again to about 30% in a small region around 12 microns) (7)*

2.6 Experimental Procedure

A sample of powder was passed through a 48 mesh sieve

to eliminate any very large particles and a 100 gram portion

was weighed to 0.1 gram. The weighed portion was placed in

the cylinder above the sieve. The receiver was placed in

line with the incident beam and the lamp was adjusted so that

the recorder gave full scale deflection on the least sensitive

scale. The vibrator and a stopwatch were started simultaneously.

When a sufficient indication had been obtained, the receiver

was moved around to other positions, 15° at a time. After

this traverse the receiver was returned to the original

position to confirm the initial reading and check instrumental

drift. The rate was obtained by dividing the total weight

by the total time. Numerous tests showed that the rate of

discharge was nearly constant as evidenced by the constancy

of the recorded transmittance, furthermore the recorded values

were averaged over about 30 seconds; therefore, use of the

average discharge rate seems justified. A second run was

taken without powder so that any small systematic response due

to stray energy could be subtracted from the indicated values.
-
-7-

3. OBSERVATIONS

3.1 Light Deflected by Commercial Dry Chemical

The light deflected through various angles was measured

using commercial dry chemical. It was found that 85% of the

light was transmitted directly. The percentage deflected

through various angles is listed in table I. The sum of

the values shown is less than 100% because some of the incident energy was absorbed by the powder and some of it was

deflected through small or large angles at which no observations were made because of the limitations of the instrument.

Table I. Energy Deflected by Dry Chemical

at Various Angles

Angle Visible Infrared

0 Observed Integrated Observed

Relative Percent of Relative

Degrees Intensity Flux Intensity

0 85 85 81

30 0. l4 1.05

45 0. 066 0.70

60 0.059 0.77 0.23

75 0.048 0.70 0.10

90 0. 040 0.60 0.08

105 0.029 0.42 0.15

120 0.029 0.38 0.13

135 0.026 0.28

150 0.040 0.30

The method of calculating the integrated percent of

flux may be described in terms of the following coordinate

systemo Let the Z axis coincide with the center of the

incident beam and be similarly directed. Let the Y axis

coincide with the center of the powder stream and be

directed downward. Let the X axis be perpendicular to these

axes and be directed to form a right-handed system. Let the

usual corresponding spherical coordinate system be used to

indicate the direction of deflected rays, the angle 0 being

measured at the origin from the Z axis to the ray in question

and the angle 9 being measured in the XY plane from the X

axis to the projection of the ray on the XY plane.

- 8 -

The relative intensity of the deflected energy within a given solid angle was observed at various angles, 0, in the

XZ plane, i.e. with 9 equal to zero. Assuming that energy

would be deflected uniformly with respect to 9, this value

was used to integrate with respect to 9 from 0 to 21T. The

integrated values indicate the percent of the total light

deflected through various angles, 0.

3*2 Deflection of Infrared Energy by Commercial Dry Chemical

The infrared energy deflected through various angles was

measured, using commercial dry chemical. It was found that

&1% of the energy was transmitted directly through the powder

stream. The relative amounts deflected through various

angles are listed in table I. No measurements were made

at smaller or larger angles because of the inherent limitations of the instrument. The data are presented graphically

in figure 1. The angle subtended by the sensitive element

of the receiver was not sufficiently precisely known to

warrant integration with respect to 9.

3.3 Independent Determination of Fineness of Commercial Dry

Chemical

A sample of the commercial dry chemical was examined in

the fineness laboratory at the National Bureau of Standards.

Microscopic examination revealed particles ranging from less

than 10 microns to more than 50 microns in diameter. The

material was elutriated in a Roller Analyzer. The results,

shown in table II, indicate the distribution of particle

diameters with nearly half the weight of the material in

particles having diameters exceeding 40 microns.

Table II. Distribution of Particle

in Dry Chemical

Sizes

Range of Particle

Diameters Percent Percent

microns by weight by projected area

0-10

10-20

20-40

>40

3±1

14+3

38+5

45±3(by differenced

16.2

25.2

3*+-3

) 2 b .

3

-9-

b. ANALYSIS

The data concerning energy deflection in the visible and

infrared portions of the spectrum are shown in figure 1

together with the distribution to be expected if true

scattering were the principal mechanism. It may be seen

that there is little resemblance between the observed

distribution and that for scattering. Furthermore, since

the intensity of scattered light is inversely proportional

to the fourth power of the wavelength, a blue coloration

normally accompanies scattering. This coloration was not

found. It may be inferred that the mechanism is diffuse

reflectance and that the effective particle diameters are

greater than the wavelength of the radiant energy involved.

This is corroborated by the fineness determinations by

elutriation. Since the intensity of reflected energy is

inversely proportional to the diameter of the particles,

assuming the weight concentration is held constant, it may

be concluded that the use of dry chemical with a higher

percentage of fine particles would increase its effectiveness

in reflecting energy. Ideally the particles should have a diameter equal to the wavelength of the energy. In the case

of hydrocarbon flames this would be about 2 microns. It

must be borne in mind that the particles must be delivered

to the fire in this fine state of subdivision. Fine particles

are of no value if they agglomerate , for then they are

effectively larger particles.

Since reflection depends upon the cross sectional

area of particles rather than their weight, it is of some

interest to consider the relationship between the percent by

weight in particles of a certain size and the percent of total

cross sectional area in those particles. It may be shown that

WAn - W

i=n

^ H • 1=1 bp

l

where Aj is the percent of the total cross sectional area

in the particles of diameter dj (assuming particles of

spherical shape), Wj is the percent of the total weight in

such particles, and there are n different particle diameters

in the sample.

Applying this equation to the weight distribution in

table II, the area distribution is found. Where particle

diameter enters into the equation, the center of the range

was used except for the last range where 50 microns was used.

The results are given in table II.
*

.
- 10 -

This analysis emphasizes the fact that even small amounts

of material can contribute a considerable amount of area

if the particles are very fine and that a slight increase

in percentage by weight in the small sizes can greatly increase the area.

The measurements of reflected energy are primarily of

value insofar as the relative distribution reveals facts

concerning particle size. The measurements of transmittance,

on the other hand are of more significance considered as

absolute values. The transmittance was 85 percent for

visible radiation and 8l percent for the infrared region.

This difference is to be expected since the reflection increases as the wavelength approaches the particle size.

The attenuation of electromagnetic radiation of a given

wavelength may be described by the following equation

whether it is due to scattering, reflection, or absorption:

-abc

I - I 0e

where I is the intensity of the transmitted beam, I Q is

the intensity of the incident beam, e is the base of Naperian

logarithms, "a" is the absorbancy index of the material

causing attenuation (the term is here extended to include

processes other than absorption), b is the path length of

the radiation in the attenuating medium, and c is the concentration of the material causing attenuation. The

absorbancy index depends upon the wavelength of the incident

energy and upon the properties of the material, such as the

particle size discussed earlier.

The concentration of powder in a stream depends not only

upon the weight of material passing through a unit of area

per unit of time, but upon the velocity of the stream and the

deflections and turbulence of the stream. In practice, the

stream is usually directed at the base of the flame. Some

powder remains there, some rebounds or is swept up by the

turbulent gas stream into the cloud above the fuel, and

some goes past the flames and is lost. In this case, the

factors determining the concentration are too numerous for

practical analysis. In the experimental arrangement, the

particles fell downward in a laminar stream and did not

rebound. Then the concentration was simply determined by

the weight rate of discharge and the velocity. By use of

Stokes Law, it was calculated that the terminal velocity of

sodium bicarbonate particles falling in still air at standard

temnerature and pressure ranges from 0.065 in. /sec for spheres

5 microns in diameter to 6.5 in. /sec for spheres 50 microns

in diameter. The theoretical equation of motion of such

particles predicted that, for all practical purposes, the

particles could be considered to be at their

- 11 -

terminal velocities when they fell past the level of the

incident beam. Since the large particles were present in

greatest numbers and fell faster, they tended to create air

currents that swept the lighter particles along. For this

reason, the stream velocity was considered to be about

6 in. /sec. The powder concentration is the ratio of the

weight delivered per second to the volume filled per second,

i.e., 10 gm/sec divided by 12 in3/sec, or O.83 gm/in3 .

On the basis of these values, the infrared absorbancy

index of commercial dry chemical dispersed in air may be

calculated. Let I 0 = 100, I = 8l , b = 1.57 in., and c = O.83

gm/in^. Then a = 0.16 in^/gm. Using this value of "a",

the transmittance T may be calculated for various cloud

thicknesses and concentrations, as f ollows

In ^

- abc = 0.l6bc.

A range of values is given in table III.

Table III. Infrared Transmittance of

Dry Chemical Clouds of Various

Thicknesses and Concentrations

(Calculated

)

Thickness, inches

6 12 18 24

mcentration transmittance in percent

gm/in3

0.1 90 83 75 68

0.5 62 38 24 15

O. 83 * 45 20 8.9 4.0

1.0 38 15 5.6 2.1

5.0 0.83 0.0067 — —

*This is the concentration used in the experiments and it is

believed that it is somewhat lower than is actually employed

in fire fighting.

- 12 -

In an open gasoline spill fire on an area four feet

square, flames often rise six to eight feet in the air.

Dry chemical may be applied in a cloud 18 inches deep at the

base of the flame. In that event, practically all of the

energy radiated in the direction of the gasoline would be

intercepted by the cloud and the transmittance of the cloud

would probably be less than 10 percent. It seems reasonable to

conclude that this effect might account for the extinguishment

of the flame

Although the evidence thus far does not prove that the

radiation barrier effect accounts for the extinguishment, it

lends considerable support to this hypothesis.

In the light of the results of this investigation,

further experiments can be envisaged which should yield

valuable information on this subject. The most direct

approach and the one which, it seems, should give the most

conclusive evidence concerning this hypothesis would be the

direct measurement of the energy transmitted from the

flame to the gasoline before and during extinguishment.

This would require some sort of open burner with a viewing

port in the bottom arranged in such a manner that a radiometer

could be placed below the burner.

Experiments could be designed to determine whether or

not there is a correlation between particle size and

extinguishment effectiveness. A general correlation may

be observed in the results reported in reference 4. It

would be of considerable interest to try particles of much

smaller average diameter than have been used heretofore.

If this hypothesis is to be further established, it

should be possible to extinguish fires by dispersing the

powder in the fuel gases only without placing powder in the

ignition zone. It is not readily apparent how such an

experiment would be arranged, but it might be worth an

attempt

It may be of some value to attempt extinguishment

while supplying just enough heat from an external source

(such as hot wires) to vaporize the required amount of fuel.

The results of such a series of experiments should

contribute a great deal to the explanation of the

mechanism of extinguishment.

90°

Figure 1. Observed Relative Intensity of Energy Deflected

by Commercial Dry Chemical and the Pattern to

be Expected as a result of True Scattering

90°

Figure 2. Percent of Incident Light Deflected

Through Various Angles

-13-

references

(1) Crosby, Fiske, and Forster, Handbook of Fire Protection,

Tenth Edition (1948); National Fire Protection

Association.

(2) W. F. O'Connor, Safety Engineering, October 1949*

(3) C. B. White, Blazes, March-April 1950, American-LaFranceFoamite Corporation, Elmira, New York.

(4) T. G. Lee and H. Shoub, Extinguishment Efficiency of

Some Powders on Gasoline Fires, NBS Report 2310,

National Bureau of Standards, Washington, D. C.

(5) J. M. Dalla Valle, Micromeritics , Pitman Publishing

Corporation, New York, 1943*

(6) W. F. Roeser and C. S. McCamy, Principles of Fire

Detection in Aircraft Engine Spaces, Wright Air

Development Center Technical Report 54-307, June 1954.

(7) W. W. Coblentz, Investigations of Infra-Red Spectra,

Part III Infra-Red Transmission Spectra, Carnegie

Institute of Washington, Washington, D. C., December

1906 .
V
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project

or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of

Standards, Washington 25, D. C.