DIODE LASER-BASED MEASUREMENTS OF FUELS, OXIDIZERS, 

AND COMBUSTION PRODUCTS DURING FIRE SUPPRESSANT TESTING 

Kevin L. McNesby, R. Reed Skaggs, and Andrzej W. Miziolek 

Army Research Laboratory, Aberdeen Proving Ground, MD 

and 

Michael Clay, Steven H. Hoke, and Craig S. Miser 

Aberdeen Test Center, Aberdeen Proving Ground, MD 

ABSTRACT 

Near infrared tunable diode laser (NE-TDL) spectroscopy is used to quantify HF gas produced 

during fire suppressant testing of halon alternatives. Results of comparisons with other 

techniques for measuring HF gas concentrations are discussed. Measurements of HF gas 

produced in laboratory and real-scale fire suppression testing are presented. The necessity for 

time-resolved measurements during testing of suppression systems designed to scavenge HF gas 

is demonstrated. 

INTRODUCTION 

Fire and explosion suppression is of critical importance on board land combat vehicles. 

Currently, Halon 1301 (CF3Br) is employed to suppress fires and explosions in most military 

vehicles. However, due to its high ozone depletion potential (ODP), Halon 1301 was banned 

from production by international agreement [l] as of 1 January 1994. A current, service-wide 

program [2] is underway to identify near-term, environmentally acceptable halon alternative 

technologies. 

Important properties for successful Halon alternatives are high suppression efficiency, low ODP, 

low suppressant residue after use, low electrical conductivity, stability under long term storage, 

and non-toxicity prior to and during dispersion. Recent work [3] has shown that hydrogen 

fluoride gas (HF) is the principal toxic gas produced during fire suppression by Halon 1301 and 

by fluorocarbon-based fire suppressants. Therefore, for fluorocarbon-based halon alternatives, 

minimization of HF gas production during fire suppression is an important criterion. The work 

presented here describes our efforts using near-infrared tunable diode lasers (NIR-TDL) to 

measure HF gas produced during fluorocarbon-based chemical suppression of laboratory and real 

scale fires. 

BACKGROUND 

When a projectile penetrates the hull and fuel cell of a land combat vehicle, fuel cell penetration 

may result in a fine mist of fuel being dispersed into the crew compartment. The ensuing “mist 

fireball explosion” [4] is extinguished within 250 ms of detection, using the currently deployed 

Halon 1301 suppression system. Diagnostic equipment designed to measure HF gas produced 

during testing of Halon alternative systems needs to provide data on a time scale shorter than the 

suppression event. The extreme reactivity of HF gas dictates the additional requirement that the 

measurement be made in-situ. 

NIR-TDL absorption spectroscopy for HF measurement provides the mid-range sensitivity 

(50-5000 ppm) [2], speed of data collection, and ability to use low replacement cost optics and 

204 Halon Options Technical WoMng Conference 12-14 May 1998 
detectors necessary for data sampling in hostile environments. Relatively low cost, narrow band 

diode lasers emitting near-infrared radiation at wavelengths absorbed by HF gas (-1.3 micrometer wavelength-overtone transition) are commercially available. Diode lasers emitting in the 

mid-infrared region at wavelengths where HF absorbs (2.5 micron wavelength-fundamental 

transition) are not yet commercially available. 

EXPERIMENTAL 

The experimental apparatus does not appreciably change according to the type of fire being 

investigated. Figure 1 shows a schematic of the experimental setup and signal processing 

electronics. For experiments described here, an InGaAsP distributed feedback (DFB) nearinfrared tunable diode laser (Sensors Unlimited, Inc.) operating near 7665 cm- 1 (corresponding 

to P(2) of the first vibrational overtone of HF [5] ) is mounted in a temperature controlled laser 

diode mount (ILX Lightwave model LDM-4980). The temperature of the diode mount is 

controlled by a thermoelectric temperature controller (EX Lightwave model LDT-5412). Laser 

diode current and internal TEC control are provided using a laser diode controller (lLX 

Lightwave model 3700). A low frequency sawtooth modulation (-100 Hz, Tektronix FG 504 

Function Generator) is combined with a higher frequency modulation (-20 KHz, sine output 

from a SRS Model 830 DSP Lock-in Amplifier). This modulation is superimposed onto the laser 

drive current, allowing the laser wavelength to be scanned through the spectral region of interest 

(low frequency modulation) while also providing a high frequency wavelength “dither” to allow 

second harmonic signal detection. 

The wavelength modulated radiation from the laser diode is fiber coupled. This fiber is 

terminated by a gradient index (GRW) lens (Sentech Systems, Inc.), which collimates the laser 

radiation. The GRIN lens-tipped fiber is placed into a small test rig designed to maintain fiber 

and detector alignment. Laser radiation is detected using an InGaAs detector. The path length 

from grin lens to detector is typically 10 cm. The modulated detector signal is demodulated at 

twice the higher frequency modulation (SRS Model 830 DSP Lock-in Amplifier), the second 

derivative lineshape displayed on an oscilloscope (LeCroy Model 4600), and the output from the 

oscilloscope stored on a laptop computer. 

MEASUREMENTS IN PARTICULATE MATTER LADEN ENVIRONMENTS 

Normal transmission spectroscopy cannot distinguish between light attenuation (measured at the 

detector) resulting from gas absorption or light scattering such as might occur in a smoke or soot 

filled environment. Partly for this reason, we employ derivative spectroscopy to exclude 

contributions to light attenuation by scattering from particulate matter. In this method, the laser 

output wavelength is scanned through a wavelength range, which includes the wavelength of 

absorption of the gas species of interest (HF). A lock-in amplifier is used to measure the change 

in detector signal intensity with the change in wavelength. Attenuation of laser radiation by a rovibrational transition of a small gas molecule is detected because the wavelength range of the 

scan is several times the width of the spectral absorption feature (typically on the order of 0.1 cmat atmospheric pressure). Because light scattering by particulate matter is nearly constant over 

the very small wavelength range of the laser scan, the change in detector signal intensity with the 

change in Wavelength is effectively zero in the absence of any absorbing gas. In this way, the 

measurement discriminates against light attenuation by particles. 

I 

Halon Options Technical Waking Conference 12-14 May 1998 205 
Fiber Optic + GRIN lens 

laser drive current out 

Laser Controller 

Function Generator 

20 KHz modulation (lock-in reference) 

Figure 1. A schematic of the experimental apparatus used and signal processing electronics 

used in these experiments. V describes the signal prior to demodulation; 

x” describes the signal following demodulation at twice the reference frequency. 

Second harmonic detection using tunable diode lasers has been extensively discussed in the 

literature [6]. Figure 2 shows a graphical simulation of the signal processing employed in these 

experiments. Briefly, the laser output wavelength is slowly scanned through a spectral region 

where HF gas absorbs (in this case, P(2) of the first vibrational overtone). The absorption in the 

absence of any modulation may he seen in the upper trace of Figure 2. A small amplitude high 

frequency modulation is super-imposed on the laser drive current (middle trace in Figure 2). 

Demodulation at twice the frequency of the small amplitude, high frequency, laser drive 

modulation yields the second derivative signal shown in the lower trace in Figure 2. It should be 

noted that the upper trace in Figure 2 shows that the laser diode output power is a function of 

wavelength. While this is non-ideal behavior, in our experience this is a common trait of 

commercially available laser diodes. The power dependence on wavelength causes the sloping 

baseline (exaggerated here for illustration) for the second derivative signal in Figure 2. For 

measurements at extremely low concentrations or for gases with small absorption cross sections, 

laser diode output power dependence on wavelength may affect limits of detection. 

206 Halon OPlionsTechnical Working Conference 12-14 May l998 
'6 T 

15 

14 

s 13 

I 

0 

n 

;;i 11 

7 

I 

0.06 

0.05 

T 

0.04 2 

s 

UI 

v 

.- - 

0.03 c 

c 

m 

S 

ffl 

- 

- 

0.02 .? 

0.01 

0 

7665.5 7665.51 7665.52 7665.53 7665.54 7665.55 7665.56 

Wavelength (nm) 

Figure 2. A graphical simulation of signal processing for 2f measurement of HF gas 

concentrations. The slope in the baseline is caused by the change in laser 

diode output power with wavelength. 

CALIBRATION 

Calculation of HF gas concentration relies upon the Bouguer-Lambert Law 171, which states that 

for optically thin media at the absorption line center, the negative logarithm of the fraction of 

transmitted light intensity (referred to as absorbance, A) is equal to the absorption coefficient of 

the molecule of interest (a) times path length (L) times pressure (P): A = a L P. Since the 

absorption coefficient, a, is constant, and L is known, a measurement of absorbance, A, is 

sufficient to determine pressure, P. The second derivative signal peak height may be shown [SI 

to be proportional to absorbance, A: 

X"N=kA Equation P 

Were, X is the peak height of the second derivative signal (volts), V is the DC voltage measured 

by the detector in the absence of any absorbance, and k is a constant which includes the 

measuring instrument and optics function. Letting S, denoted as 2f Signal peak height (peak to 

trough), equal X'", yields: 

S=(ka)LP Equation 2 

Halon Options Technical Working Conference 12-14 May 1998 207 
The slope of a plot of LP versus S provides the value of ka. A calibration of the system using 

known concentrations of the absorbing gas must be performed to determine the value of ka. 

Once this value is known, the HF gas pressure ( P ) may be obtained directly from Equation 2. 

Care must be exercised so that HF degradation of optical surfaces during measurement does not 

affect the value of ka, since this value is instrument function dependent. For this reason, 

calibration should be performed at the beginning and end of each measurement set. Additionally, 

for HF concentrations that attenuate more than 5% of the incident light, the linear relationship 

between HF gas pressure and 2f Signal peak height may no longer hold [8]. 

COMPARISON WITH OTHER TECHNIQUES 

A flow cell was constructed to allow simultaneous measurement of HF gas mixtures (HF in N2) 

using NIR-TDL absorption spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and 

using a fluoride-ion selective electrode (ISE). The flow cell is designed with five radial sampling 

ports at four longitudinal distances from the gas inlet port. All measurements were made at 

atmospheric pressure. A schematic of the flow cell is shown in Figure 3. Results from 

measurements of HF gas using the three techniques are shown in Figure 4. The limit of detection 

(LOD) of HF gas using the NIR-TDL absorption spectroscopy apparatus described here is 

approximately 40 ppm. Calibration of the FT-IR data used the peak absorbance of the R(3) line 

of the fundamental (v = 0-1) vibrational transition. The limit of detection of HF gas using ET-IR 

absorption spectroscopy was approximately 10 ppm. The absorption techniques give similar 

limits of detection because the increased signal-to-noise ratio achieved using phase-sensitive 

detection with the NIR-TDL (v = 0-2) is offset by the higher absorption coefficient for the 

fundamental transition measured using FT-IR absorption spectroscopy. The non-linearity of the 

ET-IR absorbance versus concentration data is caused by an interpolation error, which occurs 

because the instrument resolution (4 cm-') is large compared to the linewidth being measured. 

Fitting the entire ro-vibrational envelope of the ET-IR spectrum results in improved linearity. 

The limit of detection for HF gas using the ISE was approximately 1 ppm. It should be pointed 

out that the ISE measures total fluoride ion concentration, so the technique is unable to 

distinguish between HF and CF20, which is produced in appreciable quantities during chemical 

suppression of large-scale fires [3]. 

RESULTS AND DISCUSSION 

Measurements of HF gas concentrations in well controlled, optically thin environments such as 

described in the previous section are necessary to check on instrument performance and 

calibration, but may not reveal difficulties encountered in many laboratory and real-scale tests. 

In this section we describe measurements of HF gas using NIR-TDL absorption spectroscopy in 

an atmospheric pressure opposed flow propane/air flame suppressed by C3F7H (uade name 

FM-200) and in a 30 KW heptane/air pan fire during extinguishment by a hybrid suppression 

system (C-,F6H2 (trade name FE-36) plus particulate) designed to scavenge HF gas. 

208 Halon Options Technical Wolng Conference 12-14 May 1998 

~- ~ ~ ~ ~ - ~ ~ 
baffle end cap with gas tight fitting 

flowmeter 

gas in 

to vacuum 

capacitance manometer 

optical port 

Plexiglass tube 

Figure 3. The flow tube designed to allow simultaneous 

measurement of HF gas using NIR-TDL absorption 

spectroscopy, IT-IR spectroscopy, and fluoride-ion 

selective electrode (ISE). There are five feedthrough ports 

(one for the ISE, plus four optical ports) per longitudinal 

position. 

Instrument Cali bration 

25 1 

c 

c g 2o 

* 2 e10 

E 5 15 

InQ) 

5 * ISE (10EXP(-V)) 

0 + -t- I- -1 

- 

0 1000 2000 3000 4000 5000 

HF Concentration (ppm) 

Figure 4. Results of comparative calibration of Fl-R, NIR-TDL, and 

fluoride ion selective electrode for HF gas detection. 

Halon Op180n5 Technical Working Conference 12-14 May 1998 209 
A. Laboratory Flame Suppression Studies 

Figure 5 shows a schematic of an opposed flow burner apparatus and the positioning of the fiber 

optic GRIN lens assembly and detector used to measure HF gas produced in a propane/air/ 

fluorocarbon flame. By varying the mass flow rate of fuel and oxidizer for a given flame in the 

opposed flow burner, the mole fraction of a given suppressant needed for extinguishment may be 

varied. The ability to vary the “strength” of a flame in the opposed flow burner (“strength” being 

inversely proportional to the ratio of diffusion to chemical times, referred to as the Damkohler 

number) [9] allows flames of different strengths to be investigated, and allows for a ranking of 

halon alternatives according to their flame suppression efficiency. 

Figure 6 shows the results of NIR-TDL measurement of HF gas produced in a propane& 

opposed flow flame (strain rate = 50 cm-’) to which 1% C3F7H (trade name FM-200) has been 

added to the air stream. To generate these data, the fiber optic-GRIN lens assembly and detector 

were kept fixed in position, and the burner assembly was translated vertically. In this figure, raw 

data (peak to trough distance of the 2f waveform) are represented by symbols; data corrected for 

changes in baseline DC level are shown by a line. The change in DC baseline level as different 

heights in the flame are probed is caused by density gradients that cause beam steering when the 

line-of-sight is in the vicinity of the luminous zone of the flame. Using 2f detection, these beam 

steering-induced errors may be compensated for as long as some laser radiation reaches the 

detector. The case where no correction is possible may be seen in the region from 7 to 8 mm 

from the oxidizer duct. In this region, no laser radiation was incident on the detector. 

InGaAS 

P 

Luminous Zone 

Fiber optic 

coupled 

Figure 5. Schematic of the atmospheric pressure opposed flow burner, fiber optic coupled to 

GRIN lens for delivery of laser radiation, and JnGaAs detector element. 

21 0 Halon Ophons Technical Wark~ng Conference 12-14 May 1998 

__ - 
.. .. Beam steering error not 

.. 

i+ 

Raw Dala 

0 1 2 3 4 5 6 7 8 9 10 

Distance From Air Duet (mm) 

Figure 6. HF gas absorption versus distance from the oxidizer duct for an 

atmospheric pressure opposed flow propane/air flame to which 1% C3F7H 

has been added to the air stream. The symbols are raw data, the line 

represents the data corrected for fluctuations in DC baseline signal 1. 

2000 

- 

li a 1500 

0 

L Suppression by C3F6H2 plus 1, / HF scavenging agent 

Suppression by C3F6H2 

-1 ~i 1 ~,--~~L,. LLLJ ~i_u~ - 

20 40 60 80 100 120 140 160 180 

-500 t_ , , 1 , I 1 , 1 1 , . , , , 

0 

Time (seconds) 

Figure 7. HF gas concentration versus time for heptane pan fires suppressed by C3Fs 

with and without addition of HF scavenging agent. Note the difference in 

the rate at which HF gas concentration decreases with and without 

scavenging agent. 

Halon OptionsTechnical Working Conference 12-14 May 1998 21 1 
B. Field Flame Suppr-ion Studies 

Figure 7 shows results of NIR-TDL measurement of HF gas in a test designed to measure the 

effect of particulates designed to scavenge HF. For these tests, a fiber optic-GRIN lens and 

detector assembly (pathlength = 10 cm) was placed in a 1.5-m3 cubic enclosure near an air-fed 

liquid heptane pan fire (fire size approximately 30 kW). After allowing the fire to bum 

uninhibited for 15 sec, the enclosure was flooded with 10% by volume C3FsH2 (trade name 

FE-36), with and without the particulate matter additive. A new data point was measured every 

10 ms. Figure 7 shows that the rate at which HF gas concentration decreases after peak 

concentration is reached is fastest for the fire extinguished using FE-36 to which particles have 

been added. For this test, the measurement indicates that the particulate matter added to the 

fluorocarbon-based suppressant (FE-36) successfully reduced the time-weighted average 

HF concentration. 

CONCLUSION 

NIR-TDL laser absorption spectroscopy is an extremely useful tool for measuring HF gas 

concentrations produced during fie suppression by fluorocarbon based suppressants. The low 

cost of fiber optics and detectors allows for real-time in-situ measurement of HF gas in hostile 

environments. Care must be taken to ensure calibration is maintained during testing because 

most optic surfaces are damaged during measurement. 

Future experiments will involve multiplexing of several lasers to enable multiple species to be 

measured using one fiber and one detector. Additionally, recent advances in laser fabrication 

have made diode lasers at wavelengths near 2 micrometers commercially available. This 

advance improves limits of detection for many species by decreasing the quantum number 

change in absorption, as well as enabling the measurement of species whose highest lying 

fundamental has up to now been out of reach of commercially available near infrared laser 

diodes. 

ACKNOWLEDGMENTS 

One of the authors (R.R.S.) would like to thank the American Society for Engineering Education 

(ASEE) for support. This work was partially funded by the DoD Next Generation Fire 

Suppression Technology Program. The authors would also like to thank Mr. Steve McCormick 

of the U.S. Army Tank Automotive Command (TACOM) for funding support. The authors 

would like to thank Powsus, Inc., for the use of the hybrid fire suppression system. Finally, 

thanks are expressed to William Bolt and Craig Herud of the Aberdeen Test Center for 

overseeing the full-scale suppression testing. 

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9. 

Halon Options Technical Working Conference 12-14 May 1998 21 3 
214 Halon OptionsTechnical Working Conference 12-14 May 1998