Proceedings of the International Society for Optical Engineering: Photonics East, 1999

Diode Laser Measurements of HF Concentrations From

Heptane/Air Pan Fires Extinguished by FE-36 And FE-36 Plus

Ammonium Polyphosphate

RR SKAGGS, RG. DANIEL, A.W. MIZIOLEK, AND K.L. McNESBY

us. Army Research Laboratory, Aberdeen Proving Ground, MD 21005

C. HERUD, W. R BOLT, AND D. HORTON

Us. Army Aberdeen Test Center, Aberdeen Proving Ground, MD 21005

Abstract

Tunable diode laser absorption spectroscopy is used to measure the time evolution of hydrogen fluoride

concentrations produced from a series of enclosed heptane/air pan fIres extinguished by FE-36 or FE-36 plus

ammonium polyphosphate (APP). Recent advances in room temperature fIber coupled, near-infrared diode lasers

provide isolation of the laser and signal processing electronics from the hostile sampling site. 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 accelerate the dissipation ofHF from 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 powder fIre fIghting agent is released in the crew space of a M1-Abrams land combat

vehicle. Obscuration tests demonstrate that release of APP from extinguishers in an occupied space does not present

a visibility challenge to the vehicle personnel.

Keywords: Diode laser, fIre, FE-36, HF

1. Introduction

The mandatory phasing out of halons 1301 (CF3Br) and 1211 (CF2ClBr) for use as the primary fIre

extinguishing agent aboard U.S. Army combat vehicles has led to the development of candidate replacement agents

such as FE-36 (C3F6H2) and FM-200 (C3F7H). These fluorinated propanes, 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 decomposition produd. This

concern has limited the acceptance of these agents for deployment in occupied spaces.

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 conjunction 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-chekTM) with fIre retardant properties. For the tests conducted here, the APP is suspended in an extinguisher as

. a gel and released from a nitrogen pressurized extinguisher as a white powder. One of the drawbacks to the release

of a powder agent in an occupied space is the possibility of reduced visibility.

Presented here is a fIeld study of the use of hand held fIre extinguishers containing a fluorinated propane,

FE-36, and APP to extinguish liquid heptane/air pan fIres. This study has focused primarily on near infrared tunable

diode laser absorption spectroscopy (NIR- TDLAS) measurements of HF concentrations produced from a series of

enclosed heptane/air pan fIres extinguished by FE-36 or FE-36 plus APP. A recent article from this laboratory

details the use of NIR-TDLAS as the most effective diagnostic for gas phase HF measurements2• A series of

extinguishment tests were conducted with FE-36 and FE-36 plus APP. By measuring the time evolution of the HF

concentrations in fIres extinguished with FE-36 plus APP compared to fIres extinguished by FE-36 alone, the

effectiveness of APP as a HF scavenging agent will be demonstrated and evaluated. Separate measurements of light
attenuation caused by the release of a FE-36 plus APP extinguisher inside an M1-Abrams land combat vehicle will

also be described and compared against an extinguisher containing NaHC03 pressurized with Nz.

2. Experimental

Figure 1 shows a schematic diagram of the test facility containing the NIR- TDL experimental apparatus

and signal processing electronics. TDLAS has proven to be a valuable, non-intrusive optical diagnostic for species

concentrations and temperatures in combustion systems even when the local soot level is high3,4,5,6,7,8,9. The test

facility is a 1.5 m3 cubic enclosure, which contains a liquid heptane pan fIre (fIre size approximately 30 KW). HF

measurements were made using as the fIre suppressant FE-36 or one of the following combinations of FE-36 plus

APP: FE-36 plus 7 % APP in a hand held extinguisher pressurized to 150 psi by Nz; FE-36 plus 7 % APP in a hand

held extinguisher pressurized to 200 psi by N2; FE-36 plus 10 % APP in a hand held extinguisher pressurized to 250

psi by Nz; and FE-36 plus 15 % APP in a hand held extinguisher pressurized to 300 psi by Nz.

The test protocol was as follows: A 250 m1 pan fIlled with heptane fuel was placed underneath 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 a calibration cell was placed in the line-of-sight path between the GRIN lens from which the 1.3

micrometer laser radiation was emitted and the InGaAs detector. The calibration cell was used to provide a HF

spectrum to tune the laser and data collection system. The calibration cell was removed prior to fIre testing. A hand

held bottle-type extinguisher charged with 770 g FE-36 plus scavenging agent (when applicable) and pressurized

with Nz gas was situated on top of the cubic container. The extinguisher bottle output nozzle was attached via tubing

to a spray nozzle protruding into the cubic container.

The heptane pan fIre was ignited using a butane electric match and the TDLAS HF gas measurement was

initiated immediately after an internal cubic container fan was turned on and the container door was closed. The

heptane/air fIre was allowed to bum for 15 seconds at which time the hand held extinguisher's contents were

discharged into the interior of cubic container producing total flooding inhibitor conditions (10 % by volume FE-36

concentration). A video camera mounted within the cubic container recorded the fIre event during the tests. HF data

collection occurred for 180 s after ignition of the fuel, at a rate of 1 spectrum per second.

Following each test, the interior of the cubic container was rinsed with an aqueous solution of sodium

bicarbonate to neutralize any acid residues on the interior surface and the container was allowed to air dry. The

experimental details and analysis of NIR- TDLAS HF spectra have been described previouslyZ and will only be

summarized here.

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

amplitude but higher frequency modulation, ""20 KHz, taken from the sine output of a SRS Model 830 DSP Lockin AmplifIer. The application of the two modulations to the laser diode current facilitated wavelength modulation

spectroscopy (2f detection) which was needed for enhanced signal sensitivityIO,11,lZ,13,14. 2f spectroscopy is

convenient for analysis because of the linear relationship that exists between the measured absorption signal and the

analyte's concentration. Concentrations of HF were calibrated using a continuous flow cell containing various

known mixtures ofHF in N2 and measuring each mixture's unique absorption spectrum.

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 gradient index

(GRIN) lens (Sentech Systems, Inc.), which collimates the laser radiation. The GRIN lens-tipped fIber is placed into

a "pitch and catch" arrangement which directs the infrared radiation over a 14 cm open path to a 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) which acquired the 2f absorption signal. 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 a three minute time period and then sent to a Gateway 2000TM laptop computer for storage
and analysis. For experiments described here, the P(2) transition of the fIrst vibrational overtone of HF was

monitored at 7665 cm-1 5.

Figure 2 presents a schematic representation of the experimental apparatus used for attenuation testing.

The apparatus consists of a HeNe laser (Oriel Model 6611 output 632 nm), an Optical Chopper (Stanford Research

Systems Model SR540), and a fIber optic fIber coupler (Newport Optics Model F-916T). The experimental

equipment was mounted to a 46 x 46 cm optical bread board which 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 mm2

(active element) photodetector (Centro Vision Inc. OSD5-5T, 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 from exposure to the powder agent. A 18 m BNC cable and extension cord

provided remote signal communications and power to the chopper and laser while another 18 m BNC cable

transported the detector signal out to the detection electronics. The tests consisted of an occupant releasing a

KiddeTM 2.751b hand held fIre extinguisher containing either FE-36 plus APP or NaHC03 plus N2 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 positive pressure inside the crew

compartment.

Light attenuation is determined by monitoring the percent transmission of HeNe laser radiation that is

incident on the detector as chemical 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 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 8m 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 11 mm SMA fIber ferrule (Oriel Model 77670). The terminated fIber is inserted into a glass collimating beam

probe (Oriel Model 77645). The collimated light exits the beam probe and is directed into a visible NikonTM

objective lens, which apertures the beam to approximately 5-6 mm and directs it over a 0.4 m free space where the

laser radiation is incident onto the 15 mm2 photodetector. The free space through which the laser 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 voltage, which is then measured 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 using a Micron

TrekTM 233 MHz lap top computer at a rate of2 Hz.

3. Results

Figure 3 presents average HF concentration profIles from extinguishment tests with FE-36 only, FE-36

plus 7 % APP, and FE-36 plus 15 % APP. For the FE-36 plus 7 % APP and FE-36 plus 15 % APP tests, three

individual profIles for each extinguisher concentration were used to construct each test's average profIle. For the

FE-36 only test data from fIve different profIles, collected over the entire testing period, compose the average

profIle. Statistically the 1cr variances for the profIles were 41, 42, and 31 percent for FE-36 only, FE-36 plus 7%

APP, and FE-36 plus 15 % APP. The results indicate that for the fIre extinguished with FE-36 only, the average

peak HF level was 1394 ppm. For fIres extinguished using FE-36 plus 7 % APP the peak HF concentration was 996

ppm. For fIres extinguished using 15 % APP the peak HF concentration was approximately 73 ppm. The variances

in the peak values were 45,36, and 18 percent respectively.

Measurement of HF gas concentration versus time provides a monitor of the fIre history, and of the

effectiveness of any HF scavenging agent used. That is, the time from fIre suppressant release until the maximum

HF concentration occurs is a measure of the time required for fIre extinguishment (also verifIed visually using a

video recorder), while the rate at which the HF concentration decreases following extinguishment provides a

measure of effectiveness of 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. Figure 3 shows that for the fIres extinguished by FE-36 only, FE-36 plus 7 % APP, and FE-36
plus 15 % APP, the average fIre out times are 18,20, and 12 seconds respectively. Statistically the difference in fIre

out times between fIres extinguished by FE-36 and FE-36 plus 7 % APP are insignifIcant with a lcr error of 22

percent. For fIres extinguished by FE-36 plus 15 % APP, the fIre out times were reduced by approximately 35%.

The decrease in peak HF levels shown in Figure 3 for fIres extinguished using FE-36 only and fIres extinguished

using FE-36 plus 15% APP (approximately a factor of 10) is partially attributable to the fIre suppression properties

of APP (reflected by the shorter fIre out time), as well as the HF scavenging capability of the powder.

Figure 4 presents the peak HF levels versus fIre out times for all fIres extinguished by FE-36 only and with

all combinations FE-36 + APP. The FE-36 only data illustrates that as the fIre out times increase there is a

monotonic increase in the amount of HF present in the system. When the APP is added to the extinguisher, the

amount of HF measured is always less than that measured in fIres extinguished using only FE-36. In real fIres

reduction of fIre out times are diffIcult to control, and if the time duration is extended some measures must be taken

to control the HF levels. In order to select the proper concentrations of APP and operating conditions, the time rate

of change of HF concentration must be measured. The rate of change in HF concentrations can be compared

between extinguishers using data from Figure 3 starting at the maximum HF concentration time (to) and plotting the

natural logarithm of the HF concentration versus the natural logarithm of the elapsed time from the HF maximum,

as seen in Figure 5. Note: Because the HF concentrations for tests with FE-36 plus 15 % APP were well below the I

toxic exposure critical concentration of 500 pprn, no further analysis of the data was warranted as well as the fact

that the discharged APP concentration probably exceeded acceptable respiratory exposure levels [15]. With Figure

5 on a In-In scale indicates that the dissipation of HF versus time is a second order decay process. Linear regression

analysis of the dat~ in Figure 5 indicate that the FE-36 only data have a slope of -0.645 ppm/sec (R2 = 0.991) while

the FE-36 plus 7 % APP have a slope of -0.746 ppm/sec (R2 = 0.980). The difference in slopes for the two tests is

approximately 151% greater for the fIres extinguished by FE-36 plus 7 % APP, and thus faster decreases in HF

concentrations should be expected for fIres extinguished by this hybrid extinguisher.

RegardlJs of how fast HF is reduced in a given system, from a more practical standpoint the amount ofHF

in the system ovet a period of time, i.e. time weighted average, must be determined to address toxicity concerns.

Again using data from Figure 3 the time weighted average doses ofHF, calculated from the extinguisher release at

t=15 see until t=180 see, were 712, 492, and 51 ppm for fIres extinguished by FE-36 only, FE-36 plus 7% APP, and

FE-36 plus 15 % iAPP respectively. These values imply that for the tests with FE-36 only, the HF dose is toxic,

while the tests with FE-36 plus 7% APP and 15 % APP achieve minimum to very acceptable HF reductions

respectively. I

Finally Figure 6 presents the percent transmission profIles measured during and following the release of

the FE-36 plus APP and the NaHC03 plus N2 extinguishers inside the Ml-Abrams combat vehicle. The profIles

indicate that the niaxirnum light attenuations were 0 percent transmission for the NaHC03 plus N2 extinguisher and

18.04 percent trarismission for the FE-36 plus APP extinguisher. Empirical correlations using a VHS video taken

inside the vehicle 1during the extinguishers release indicate that clear visibility to the human eye correlates to an attenuation level of approximately 70 %. Thus, for all percent transmission levels recorded below 70 % corresponds

to an obscured fIJld of view at a distance of 30.48 em. The time duration that visibility is less than 70 percent

transmission for the NaHC03 extinguisher is 63 seconds and 49 seconds for the FE-36 plus APP extinguisher. The

more rapid return Ito visibility, i.e. ~ 70 % transmission, using 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.

4. Conclusions

NIR- TDLAS has been demonstrated to measure HF in a practical fIeld application. The results presented

here indicate that HF concentrations produced from fIres extinguished by FE-36 plus APP are being reduced in the

cubic test container and that the presence of APP accelerates this reduction. Thus the combination of APP in an

extinguisher containing FE-36 appears to reduce HF levels. Visibility reduction during extinguisher deployment was

measured inside an actual combat vehicle and determined to be minimal. The HF results should be met with caution

since HF and APP threshold exposure limits inside vehicles have not yet been established. From an experimental

standpoint, more tests should probably be conducted to improve statistical deviations as well as post testing analysis
of the reacted APP to understand how APP reacts with HF. Future tests will attempt to meet these concerns in order

to develop a chemical kinetic mechanism for post fIre HF activity.

5. Acknowledgments

For the light attenuation experiments, the authors would like to thank Dave Roberts (International Imaging

Systems of ATe) for VHS fIlming, Jeff Morris (ARL) for loan of the SRS optical chopper, and Edwin Lancaster

(ARL) for fabrication of the fIber optic cable. The HF testing was fmancially supported by the U.S. Army TACOM

(Steve McCormick) and POWSUS Inc. (Harry Stewart and Don MacElwee). Finally R. Skaggs would like to

acknowledge fmancial support from the Army Research Laboratory through an American Society for Engineering

Education Postdoctoral Fellowship.

6. Figures

Figure 1:

Figure 2:

Figure 3:

Figure 4:

Figure 5:

Figure 6:

Schematic diagram of experimental HF measurement apparatus

Schematic diagrams of light attenuation measurement apparatus

Average HF concentration profIles versus measurement time. The diamond symbols (.)

are HF concentrations from fIres extinguished by FE-36 only, the square symbols (.) are

HF concentrations from fITes extinguished by FE-36 plus 7 % APP, and the triangle

symbols (.) are HF concentrations from fITes extinguished by FE-36 plus 15 % APP.

Peak HF concentrations versus fITe out times for fITes extinguished by FE-36 only (~

and FE-36 + APP (.)

Rate plot of In HF concentrations versus In time for fITes extinguished by FE-36 only ( )

and FE-36 + plus 7 % APP (~). The straight solid lines represent linear regression

analysis of the data.

Percent transmission profIles collected from measurement of light attenuation while an

extinguisher containing FE-36 plus APP (solid line) and FE-36 plus NaHC03 (dashed

line) were released inside an Ml combat vehicle. The horizontal line drawn across the

graph is the 70 % transmission level which corresponds to clear visibility by the human

eye.
Lock-In Amp

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30 Hz

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Figure6
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15. POWSUS, Inc, Private Communication, 1998.