TABLE Il. Slope (pprn/second) values from linear regression analysis of HF dissipation rates.

FE-36 + 7% FM-200 + 15%
FE-36 APP ° FM-200 APP.
Slopes (pprv/second) 0.65 -0.75 -0.52 0.55

attributable to the fire suppression properties of APP (re-
flected by the shorter fire-out times).

Figure 3 and Table I indicate that the presence of APP,
regardless of the percentage in the extinguisher, helps re-
duce the overall HF levels with respect to those fires ex-
tinguished by only neat agents. A possible explanation
for the HF reductions is that, without APP in the enclo-
sure, fires extinguished by the neat agents are able to
produce HF more rapidly than fires extinguished by neat
agents plus APP. This statement is supported by Fig. 4,
which plots HF concentrations for each extinguisher from
¢ = 16s until ¢ = 31 s, which are the time locations from
the extinguisher release to just before the HF concentra-
tions reach their respective maximums in Fig. 3. Linear
regression analysis of these data indicates that FM-200
produces HF 39% faster than FE-36. For the fires extin-
guished by FE-36, HF is formed 21% faster than the fires
extinguished by FE-36 plus 7% APP, while fires extin-
guished by FM-200 form HF 12% quicker than those
fires extinguished by FM-200 plus 15% APP. Obviously
the data in Fig. 4 are dependent on the fire-out times, and
previous studies"? have shown that as the fire-out times
increase, this trend is accompanied by an increase in the
amount of HF present in the system. Nevertheless, the
presence of APP appears to slow HF production.

In real fires, reduction of fire-out times is difficult to
control, and if the time duration is extended, some mea-
sures must be taken to control the HF levels. APP is
added to these extinguishers because it is believed that
APP can heterogeneously scavenge HF from the post
flame gases of a fire situation extinguished by a fluori-
nated fire-fighting agent. To evaluate APP’s scavenging
abilities, one must measure the time rate of change of the
HF concentration.

w
=
&
®
"
»
=

y=

i 35

to

is

z 0

g§

0
=
x
s
*
$
ke
oe Oe wo No to WO 300

Time, seconds

Percent transmission profiles collected from measurement of
ight attenuation while an extinguisher containing either FE-36 plus
APP (solid line) or FE-36 plus NaHCO, (dashed line) was released
inside an MI combat vehicle. The horizontal line drawn across the
graph is the 70% transmission level, which corresponds to the minimal
Ievel of clear visibility to the human eye,

The rate of change in HF concentration can be com-
pared between extinguishers with and without APP by
using data from Fig. 3 starting at the maximum HF con-
centration time (f)) and plotting the natural logarithm of
the HF concentration vs. the natural logarithm of the
elapsed time from the HF maximum, as seen in Fig. 5.
As the HF concentrations for tests with FE-36 plus 15%
APP were well below the target concentration of 500
ppm, no further analysis of the data was warranted.'? Re-
sults from linear regression analysis of the data in Fig. 5
are presented in Table III. The difference in slopes be-
tween the FE-36 and the FE-36 plus 7% APP tests is
approximately 13% greater for the fires extinguished by
FE-36 plus 7% APP, while the difference between the
FM-200 and FM-200 plus 15% APP is approximately 5%
greater. Thus, the faster decreases in HF concentrations
from fires extinguished by FE-36 plus APP and FM-200
plus APP vs. FE-36 and FM-200, respectively, are attri-
buted to the presence of APP.

The primary concern with combining a powder sub-
stance like APP in a hand-held extinguisher with the
fluorinated agents is that, when the extinguisher contents
are released, the powder is temporally suspended in the
air, forming a visibly dense ‘‘cloud” that could be dif-
ficult to see through. To address and quantify this situ-
ation, we conducted a series of obscuration measure-
ments. Figure 6 presents results from the obscuration
measurements with temporally resolved percent trans-
mission profiles measured during and following the re-
lease of the FE-36 plus APP and the NaHCO, plus N,
extinguishers inside an M1-Abrams combat vehicle. The
profiles indicate that the maximum light attenuations
were 0% transmission for the NaHCO, plus N, extin-
guisher and 18% transmission for the FE-36 plus APP
extinguisher. Empirical correlations found by using a
VHS video taken inside the vehicle during the extin-
guishers’ discharge indicate that clear visibility to the
human eye correlates with an attenuation level of ap-
proximately 70% transmission. Thus, all percent trans-
mission levels recorded below 70% transmission corre-
spond to an obscured field of view at a distance of 30.48
cm. The time duration during which visibility is less
than 70% transmission for the NaHCO, extinguisher is
63 s while the FE-36 plus APP extinguisher experiences
less than 70% transmission for 49 s. The more rapid
return to visibility, i.e, =70% transmission, with the use
of the FE-36 plus APP extinguisher, is attributed to the
fact that the powder is: not released in a dry state; rather,
it is ‘wet’? and thus falls faster to’ the vehicle floor.

CONCLUSION

NIR-TDLAS has been demonstrated to measure HF in
a practical field application. The results presented here
indicate that HF concentrations produced from fires ex-
tinguished by FE-36 plus APP and FM-200 pius APP are
being reduced in the cubic test container and that the

APPLIED SPECTROSCOPY 1147


TABLE 1. Maximum HF concentrations (ppm) and time weighted average HF concentrations (ppm).
FE-36 + 7% FE-36 + 15% : FM-200 + 15%
FE-36 APP APP FM-200 APP
HF maximum, ppm 1394 996 B 2667 1638
HF TWA, ppm 712 493 23 1626 646
TABLE Il. Fire-out times (seconds) from average profiles in Fig. 3.
FE-36 + 7% FE-36 + 15% FM-200 + 15%
FE-36 APP APP FM-200 APP.
Fire-out times 18 20 12 20 12

not as evident in the FE-36 tests. Table I lists the max-
imum HF concentrations and time weighted averages
from each profile in Fig. 3. The results from Fig. 3 and
‘Table I indicate that fires extinguished by FM-200 pro-
duce almost twice as much HF as fires extinguished by
FE-36. For fires extinguished by FE-36 plus APP or
FM-200 plus APP, the data show that HF is reduced with
respect to fires extinguished by the neat agents alone.
Table I also lists the time weighted average HF values,
which represent the average dose of HF one would be
exposed to from the time of extinguisher release at t =
15 s until the end of the measurement period, t = 180 s.
The time weighted average is very important because the
primary toxicity concern for HF exposure is not the max-
imum HF one experiences but the average concentration
one is expoged to over a period of time. It should be
noted that the target nontoxic HF TWA level for these
tests was 500 ppm or less. From Table I the HF TWA
values imply that, for the tests with FE-36, FM-200, and
FM-200 plus 15% APP, the HF dose is toxic, while the
tests with FE-36 plus 7% APP and FE-36 plus 15% APP
achieve minimum to very acceptable HF reductions, re-
spectively.

2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
-200

HF Concentration, ppm

16 «6t8)|= 2022 HH BHD 2
Time, seconds

Fic. 4. Plot of HF concentrations vs. measurement time from ¢ = 16
stor = 31 8 to illustrate HF production rates. The [] symbols are HF
concentrations from fires extinguished by FE-36: the © symbols are HF
Concentrations from fires extinguished by FE-36 plus 7% APP; the A
symbols are HF concentrations from fires extinguished by FE-36 plus
15% APP; the V symbols are HF concentrations from tires extinguished
by FM-200; and the © symbols are HF concentrations trom fires ex-
tinguished by FM-200 plus 15% APP. The straight solid lines represent
lincar regression analysis of the data

1146 Volume 53, Number 9, 1999

Measurement of HF gas concentration vs. time pro-
vides a monitor of the fire history, and of the effective-
ness of any HF reduction agent used. That is, the time
from fire suppressant release until the point where the
maximum HF concentration occurs is a measure of the
time required for fire extinction (also verified visually by
using a video recorder), while the rate at which the HF
concentration decreases following extinguishment pro-
vides a measure of the effectiveness of the scavenging
agent (when used), or a measure of the rate at which HF
gas reacts with the walls of the enclosure. Reduction in
fire-out times reduces HF levels by reducing the time the
agent is exposed to flame temperatures that cause agent
decomposition to HE Table II lists the fire-out times from
the data in Fig. 3. Statistically the difference in fire-out
times between fires extinguished by FE-36 and FE-36
plus 7% APP is insignificant, with a lo error of 22%.
For fires extinguished by FE-36 plus 15% APP and FM-
200 plus 15% APP, the fire-out times were reduced by
approximately 33 to 40% relative to the fires extinguished
by the respective neat agents. The decrease in the TWA
HF levels in Table I for fires extinguished by using FE-
36 and FM-200 compared to fires extinguished by FE-36
plus 15% APP and FM-200 plus 15% APP (approxi-
mately a factor of 30 and 2.5, respectively) is partially

In (HF, ppm)

54 Lt 1
3 4 5
In (Time, seconds)
Fic. 5. Rate plot of In (HF) concentrations vs. In (time) for fires ex-

tinguished by FE-36 only ([1); FE-36 plus 7% APP (Oy, FM-200 (4
and FM-200 plus 15% APP (V7). The straight solid lines represent linear
regression analysis of the data,


presence of APP accelerates this reduction. Thus the
combination of APP in an extinguisher containing FE-36
or FM-200 appears to reduce HF levels. Visibility reduc-
tion during extinguisher deployment was measured inside
an actual combat vehicle for extinguishers containing FE-
36 plus APP and NaHCO, plus N,. From an experimental
standpoint, more tests should probably be conducted to
analyze the reacted APP to understand how APP reacts
with HE Future tests will attempt to meet this concer in
order to develop a chemical kinetic mechanism for post-
fire HF activity.

ACKNOWLEDGMENTS

For the light attenuation experiments, the authors would like to
thank Dave Roberts (International Imaging Systems of ATC) for
VHS filming, Jeff Morris (ARL) for loan of the SRS optical chopper,
and Edwin Lancaster (ARL) for fabrication of the fiber-optic cable.
Testing was financially supported by the U.S. Army TACOM (Steve
McCormick. HF and Obscuration) and POWSUS Ine. (Harry Stewart
and Don MacElwee, HF). Finally, R. Skaggs would like to acknowl-
edge financial support from the Army Research Laboratory through
an American Society for Engineering Education Postdoctoral Fel-
lowship.

1. K. L. MeNesby, R. G. Daniel, J. M. Widder, and A. W. Miziolek,
Appl. Spectrosc, 50, 126 (1996).

1148 Volume 53, Number 9, 1999

2. K. L. McNesby, R. R. Skaggs. A. W. Miziolek, M. Clay, $. Hoke,
and C. S. Miser, App. Phys. B 67, 443 (1998).
3. R. K. Hanson, Appl. Opt. 19, 482 (1980).
4. PL. Varghese and R. K. Hanson, J. Quant. Spectrosc. Radiat.
Transfer 24, 479 (1980).
5. S. M. Schoenung and R. K. Hanson, Combust. Sci. Technol. 24,
227 (1981).
6, J. H. Miller, S. Elreedy, B. Ahvazi, F Woldu, and P. Hassanzadeh,
Appl. Opt. 32, 6082 (1993).
7. QV. Nguyen, B. L. Edgar, R. W. Dibble, and A. Gulati, Combust.
Flame 100, 395 (1995). at
. R. R. Skaggs and J. H. Miller, Combust. Flame 100, 430 (1995).
). R. R. Skaggs and J. H. Miller. in Twenty-Sixth (Inil.) Symposium
on Combustion (The Combustion Institute, Pittsburgh, 1996), p.
1181.
10. J. Reid and D. Labrie, Appl. Phys. B 26, 203 (1981).
11. D. T. Cassidy and J. Reid, Appl. Opt. 21, 1186 (1982).
12. D. M. Bruce and D. T. Cassidy, Appl. Opt. 29, 1327 (1990).
13. J. A. Silver, D. S. Bomse, and A. C. Stanton Appl. Opt. 30, 1505,
(1991).
14. D. S. Bomse, A. C. Stanton, and J. A. Silver, Appl. Opt. 31, 718
(1992).
15. R. K. Hanson, P. A. Falcone, and C. H. Kruger, Appl. Opt. 16, 2045
(197).
16. P. L. Varghese and R. K. Hanson, J. Quant. Spectrosc, Radiat.
Transfer 24, 479 (1980).
17. S. M. Schoenung and R. K. Hanson, Appl. Opt. 21, 1767 (1981).
18. R. R. Skaggs, R. G. Daniel, A. W. Miziolek, K. L. McNesby, C.
Herud, W. Bolt, and D. Horton, Proceedings of the SPIE Fail Meet-
ing: Photonics East Environmental and Industrial Monitors, (SPIE,
Billingham, Washington, 1998), p. 156.
19, POWSUS, Inc.; private communication (1998).

Oo


Diode Laser Measurements of HF Concentrations Produced
from Heptane/Air Pan Fires Extinguished by FE-36, FM-
200, FE-36 Plus APP, or FM-200 Plus APP

R. R. SKAGGS,* R. G. DANIEL, A. W. MIZIOLEK, K. L. MCNESBY, C. HERUD,

W. R. BOLT, and D. HORTON

U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005 (R.R.S., R.G.D., A.W.M., K.L.M.); and
U.S. Army Aberdeen Test Center, Aberdeen Proving Ground, Maryland 21005 (C.H., W.R.B., D.H.)

Tunable diode laser absorption spectroscopy (TDLAS) Is used to
measure the time evolution of hydrogen fluoride (HF) concentra-
tions produced from a series of enclosed heptane/air pan fires ex-
tinguished by either FE-36, FM-200, FE-36 plus ammonium poly-
phosphate (APP), or FM-200 plus APP. For the fires studied, the
change in HF gas concentration with time is dependent upon the
fire-fighting chemical used to extinguish the fire. The presence of
APP is observed to reduce HF concentrations in the fire enclosure.
Visible attenuation spectroscopy is also used to measure the amount
of light attenuation (obscuration) that occurs as a hand-held fire
extinguisher containing powdered fire fighting agent is released in
the crew space of a M1-Abrams land combat vehicle.

Index Headings: Diode laser; HF; FE-36; FM-200.

INTRODUCTION

Fire protection on military platforms, including ground
fighting vehicles, is being challenged by the impending
loss of the ubiquitous fire-fighting agent halon 1301
(CF,Br) due to environmental concerns related to the de-
struction of the stratospheric ozone layer. Replacement
fire extinguishment agents need to be found that will sat-
isfy numerous criteria including fast fire suppression,
minimum production of toxic gases when used, low tox-
icity, compatibility with storage materials, and environ-
mental acceptability.

The U.S. Army’s search for halon replacement agents
has largely involved an empirical approach of testing and
evaluation of commercially available compounds/sys-
tems. A description of the testing and evaluation of two
candidate halon replacement agents, FE-36 (C,F,H,) and
FM-200 (C,F,H), is presented here from full-scale fire
suppression tests conducted at the Aberdeen Test Center
(ATC). The fluorinated propane agents, though effective
at extinguishing fires, are deficient in terms of increased
weight and volume requirements needed for fighting the
most difficult military fires. The increased amounts of
replacement agent required for extinguishment, relative
to halons 1301 and 1211, pose a concern with the levels
of hydrogen fluoride (HF) formed as the primary decom-
position product. Thus, the tests presented here measure
the concentration of gas-phase HF with the use of near-
infrared tunable diode laser absorption spectroscopy
(NIR-TDLAS).' A recent article from this laboratory de-
tails the use of NIR-TDLAS as the most effective diag-
nostic for gas-phase HF measurements.*

Received 5 January 1999; accepted 12 April 1999.
* Author to whom correspondence should be sent

0003-7028/9

Volume 53, Number 9, 1999

'5309-1143$2,00/0
© 1999 Society for Applied Spectroscopy

The two mechanisms by which HF concentrations can
be decreased during and following fire extinguishment by
FE-36 or FM-200 are by (1) reducing the time required
to extinguish the fire, thereby minimizing the time that
the fluorine-containing suppressant is exposed to flame
temperatures, or (2) releasing a scavenging agent in con-
junction with the fire suppressant chemical to remove HF
after it is produced. A potential chemical scavenging
agent of gaseous HF is ammonium polyphosphate (APP),
which is a commercially available chemical (Phos-
chek®) with fire-retardant properties. One of the draw-
backs to the release of a powder agent in an occupied
space is the possibility of reduced visibility.

HF is produced here from a series of enclosed heptane/
air pan fires extinguished by either FE-36 (C;F,H,), FM-
200 (C,F,H), FE-36 plus APP, or FM-200 plus APP. By
measuring how much the HF concentrations are reduced
in fires extinguished with FE-36/APP or FM-200/APP
mixtures compared to fires extinguished by FE-36 or FM-
200 alone, we can demonstrate the effectiveness of APP
as an HF reduction and potential scavenging agent. In
addition, we will discuss spectroscopic measurements of
the amount of light attenuation (obscuration) that occurs
as a hand-held fire extinguisher containing powder fire-
fighting agent is released in the crew space of an M1
land combat vehicle.

EXPERIMENTAL

Figure 1 shows a schematic diagram of the test facility
containing the NIR-TDLAS experimental apparatus and
signal processing electronics. TDLAS has proven to be a
valuable, nonintrusive optical diagnostic for species con-
centrations and temperatures in combustion systems even
when the local soot level is high.*° The test facility is a
1.5 m cubic enclosure, which contains a liquid heptane
pan fire (fire size approximately 30 kW). The fire sup-
pressants used were FE-36 or FM-200. For the extin-
guishers with APP added, the following combinations of
FE-36 plus APP and FM-200 plus APP were tested: FE-
36 plus 7% APP, FE-36 plus 15% APP, and FM-200 plus
15% APP.

The test protocol was as follows: A 250 mL pan filled
with heptane fuel was placed underneath a 0.5 m high
steel table within the cubic container. The NIR-TDLAS
HF gas measurement apparatus was situated on top of the
metal table where an HF gas cell was placed in the line-
of-sight path between the gradient index (GRIN) lens
from which the 1.3 ym laser radiation was emitted and

APPLIED SPECTROSCOPY 1143


Fire Extinguish

sb ns
—>/
Fiber Opie f awe

NIR-TOL
LASER

NX
i

1eubis

Laser !
Control Trig a
WWW | Wawa 1
Laser Modulation | [Laver Moduaton | L_oaSila%on
0K monte

Lock-In Amp]

Schematic diagram of experimental HF measurement appara-

Fic. 1.
tus.

the InGaAs detector. The HF gas cell was used to provide
an HF spectrum to tune the laser and data collection sys-
tem. The HF gas cell was removed prior to fire testing.
A hand-held bottle-type extinguisher charged with FE-36
or FM-200 plus scavenging agent (when applicable) and
pressurized with N, gas was situated on top of the cubic
container. The extinguisher bottle output nozzle was at-
tached via tubing to a spray nozzle protruding into the
cubic container.

The heptane pan fire was ignited by using a butane
electric match, and the NIR-TDLAS HF gas measure-
ment was initiated immediately after an internal cubic
container fan was turned on and the container door was
closed. The heptane pan fire was allowed to burn for 15 s,
at which time the hand-held extinguisher’s contents were
discharged into the interior of the cubic container, pro-
ducing total flooding inhibitor conditions (10% by vol-
ume). A video camera mounted within the cubic contain-
er recorded the fire event during the tests. HF data col-
lection occurred for 180 s after ignition of the fuel, at a
rate of one spectrum per second,

Following each test, the interior of the cubic container
was rinsed with an aqueous solution of sodium bicarbon-
ate to neutralize any acid residues on the interior surface,
and the container was allowed to air dry. The experi-
mental details and analysis of NIR-TDLAS HF spectra
have been described previously? and will only be sum-
marized here as described below.

A sawtooth modulation (~100 Hz) supplied from a
function generator (Tektronix Model FG 504) was used
to rapidly scan over the desired spectral frequency range.
Coincident with the 100 Hz modulation was a small-am-
plitude but higher frequency modulation, ~20 kHz, taken
from the sine output of an SRS Model 830 DSP lock-in
amplifier. The application of the two modulations to the
laser diode current facilitated wavelength modulation
spectroscopy (2f detection) that was needed for enhanced
signal sensitivity.'-'" Wavelength modulation spectros-
copy (WMS) is convenient for analysis because of the
linear relationship that exists between the measured ab-
sorption signal and the analyte’s concentration.

When WMS is employed, the second derivative of the

1144 Volume 53. Number 9. 1999

diode laser probe beam intensity with respect to wave-
length is usually measured, since the wavelength at which
the second derivative is a maximum coincides with the
wavelength of maximum light absorption. To quantify a
2f absorption signal, denoted as x”, the Bouguer-Lam-
bert law is written as:6

a = Hylv ~ P)-S(T)- 80%) P-L. a)

From Eq. 1, H; is the second Fourier component of
the modulated absorption coefficient, S(T) is the temper-
ature-dependent line strength for the transition, g(v®) is
the line shape factor evaluated at the line center, P, is the
partial pressure of the absorbing analyte, LZ is the path-
length, and 7° is the incident laser intensity. The imple-
mentation of Eq. 1 for the quantification of measured
absorption signals requires knowledge of the quantities
H,, S, and g. For TDL measurements, Hanson et al.'5-!7
demonstrated that the product of the line strength S (cm?
atm~') and the line shape factor g(v — v°) (cm) yields the
absorption coefficient, a, = S - g(v — v°), which is nor-
mally used when quantifying a measured signal in terms
of absorbance, i.e., In(//I°), = a, » P, - L. To quantify a
2f absorption signal, Reid and Labrie’ determined that
the modulation of the absorption signal is accounted for
in the Bouguer-Lambert law with the addition of the sec-
ond Fourier component of the modulated absorption co-
efficient, H,, which describes the modification of the
spectral line shape. Hence, the absorption coefficient for
a 2f absorption signal can be rewritten as a, = H,-S-
g(v — v°). The values for a, are easily determined from
signal calibrations of the 2f signal measured in an at-
mospheric pressure continuous flow cell, pathlength of
=12 cm, containing known concentrations of HF in N,.
For calibrations, Eq. 1 is simplified to

x 2 a,-P-L. (2)

From Eq. 2, the slope obtained from a plot of x"/J°,
the normalized 2f signal peak height (peak-to-trough
height, the distance between the maximum and minimum
points on the 2f signal), vs. (P, - L) provides the value
for a,. Once the a, value is known, the unknown gas
pressure (P,) may be obtained directly from Eq. 2. Cali-
brations are performed at the beginning and end of each
measurement set so that changes in optical surfaces dur-
ing measurement (which may occur when measuring cor-
rosive gases such as HF) do not affect the value of a,.

The emitted infrared light from the diode was launched
into a fiber coupled to the laser housing. Fiber-optic patch
cables were used to deliver infrared light into the test -
facility. The fiber is terminated by a GRIN lens (Sentech
Systems, Inc.) that collimates the laser radiation. The
GRIN lens-tipped fiber is placed into a “pitch and catch”
arrangement that directs the infrared radiation over a 14
cm open path to an InGaAs detector (Epitaxx Model ETX
1000 T). The signal observed at the detector was directed
to a phase-sensitive lock-in amplifier (Stanford Research
Systems Model 830) that acquired the 2f absorption sig-
nal. The 2f output signal from the SRS lock-in amplifier
was then sent to a digital oscilloscope (LeCroy Model
9654). Resulting spectra were acquired every second for


Hele Laser
(632 am)

Chopper Fiber Collimating Objective

tm ise

Oscilloscope

Detector Output

JUUL

Laptop Computer

Fic. 2.

a three-minute time period and then sent to a Pentium®-
based laptop computer for storage and analysis. For ex-
periments described here, the P(2) transition of the first
vibrational overtone of HF was monitored at 7665 cm='.*

Figure 2 presents a schematic representation of the ex-
perimental apparatus used for attenuation testing. The ap-
paratus consists of a HeNe laser (Oriel Model 6611 out-
put 632 nm), an optical chopper (Stanford Research Sys-
tems Model SR540), and a fiber-optic fiber coupler (New-
port Optics Model F-916T). The experimental equipment
was mounted to a 46 X 46 cm optical bread board that
was placed outside the crew compartment on top of the
vehicle. A separate 41 cm optical rail supported the fiber
collimating/projection optics and a 15 mm? (active ele-
ment) photodetector (Centro Vision Inc. OSDS-ST, 350—
1100 nm). The optical rail was placed inside the vehicle
in the driver’s seat. Not shown is a plastic enclosure that
was placed over the fiber collimating/projection optics to
minimize the optic devices exposure to the powder agent.
A 18 m BNC cable and extension cord provided remote
signal communications and power to the chopper and la-
ser while another 18 m BNC cable transported the detec-
tor signal out to the detection electronics. The tests con-
sisted of an occupant releasing a KiddeTM 2.75 Ib hand-
held fire extinguisher containing either FE-36 plus APP
or NaHCO, plus N} at the personnel heater while inside
the vehicle, and all outside hatches were closed. With the
hatch doors closed, the vehicle’s nuclear and biological
containment (NBC) system was operated to create a pos-
itive pressure inside the crew compartment.

Light attenuation is determined by monitoring the per-
cent transmission of HeNe laser radiation that is incident
on the detector as powdered agent is dispersed over the
measurement region. Thus, if 100% transmission occurs,
all the light that exits the laser source is incident upon
the detector and no attenuation has occurred. The laser
radiation is passed through an optical chopper, which
modulates the laser radiation at ~1000 Hz. After passing
through the chopper, the laser radiation is launched into
a 8 m long visible fiber-optic cable (3M® multi-mode,
1000 mm diameter core, glass-substrate optical fiber).
Light passes out the opposite end of the fiber, where it is
terminated into a 1| mm SMA fiber ferrule (Oriel Model

Schematic diagram of light attenuation measurement apparatus.

2800 F ° 7
2600
2400 F
2200
2000 F
1800
1600 |
1400 F
1200
1000 f
800 +

FM-200 + 15% APP
FE-36 + 7% APP

HF Concentration, ppm

400
200

200 L 1 1 1 L
0 30 60 90 120, 150180

Time, seconds

Fic. 3. Average HF concentration profiles vs. measurement time. The
Cl symbols are HF concentrations from fires extinguished by FE-36; the
O symbols are HF concentrations from fires extinguished by FE-36 plus
7% APP; the A symbols (bottom trace) are HF concentrations from
fires extinguished by FE-36 plus 15% APP: the V symbols are HF
concentrations from fires extinguished by FM-200; and the 0 symbols
are HF concentrations from fires extinguished by FM-200 plus 15%
APP. :

77670). The terminated fiber is inserted into a glass col-
limating beam probe (Oriel Model 77645). The collimat-
ed light exits the beam probe and is directed into a visible
Nikon® objective lens, which apertures the beam to ap-
proximately 5-6 mm and directs it over a 0.4 m free
space where the laser radiation is incident onto the 15
mm? photodetector. The free space through which the la-
ser radiation passes is the region in which the powdered
agent, if present, will attenuate the laser radiation. The
light intensity reaching the detector is converted to a volt-
age, which is then measured by using a lock-in amplifier
referenced to the chopper frequency. A dc voltage (0-5
V full scale), proportional to the detector signal, is output
to the digital oscilloscope (LeCroy Model 9654), which
is also triggered by the optical chopper, and is processed
and recorded by using a Pentium®-based laptop com-
puter at a rate of 2 Hz.

RESULTS AND DISCUSSION

Figure 3 presents average HF concentration profiles
from extinguishment tests with FE-36, FE-36 plus 7%
APP, FE-36 plus 15% APP, FM-200, and FM-200 plus
15% APP. For the FE-36 plus 7% APP and FE-36 plus
15% APP tests, three individual profiles for each extin-
guisher concentration were used to construct each test’s
average profile. For the FE-36 profile, test data from five.
different profiles, collected over the entire testing peri-
od, compose the average profile. The FM-200 average
profile is constructed from 10 separate tests, while the
FM-200 plus 15% APP profile consists of four separate
tests. Statistically the | o variances for the profiles were
41, 42, 31, 56, and 54% for FE-36, FE-36 plus 7% APP,
FE-36 plus 15% APP, FM-200, and FM-200 plus 15%
APP. For the FM-200 and FM-200 plus APP tests, the
large statistical deviations are attributed to inconsistent
fire extinguishment times from test to test, which was

APPLIED SPECTROSCOPY 1145