Chapter 5: INSTRUMENTATION FOR

BENCH- AND LARGE-SCALE

TEST FIXTURES

Kevin L. McNesby, Ph.D. 

U.S. Army Research Laboratory 

TABLE OF CONTENTS

5.1 Introduction......................................................................................................................482 

5.2 Laser Induced Breakdown Spectroscopy for Measurement of Fire Suppressants...........483 

5.3 Tunable Diode Laser Absorption Spectroscopy (TDLAS) for Measurement of 

Combustion products, Fuels, and Oxygen .......................................................................485 

5.3.1 General ..................................................................................................................485 

5.3.2 HF..........................................................................................................................486 

5.3.3 Oxygen ..................................................................................................................490 

5.3.4 Fuels ......................................................................................................................490 

5.3.5 Observations..........................................................................................................501 

5.4 Measurement of Fire Suppressant Concentration Using a Differential Infrared Rapid 

Agent Concentration Sensor (DIRRACS) .......................................................................502 

5.4.1 Introduction ...........................................................................................................502 

5.4.2 Original Instrument Design – DIRRACS I ...........................................................502 

5.4.3 Improved Hardware Design – DIRRACS II .........................................................505 

5.4.4 Data Acquisition and Analysis..............................................................................507 

5.4.5 Calibration Experiments.......................................................................................507 

5.4.6 Release Tests of HFC-125 in the TARPF Facility ................................................508 

5.4.7 HFC-125 Release Tests in a Bradley Vehicle .......................................................509 

5.4.8 Assessment ............................................................................................................513 

5.5 Conclusion .......................................................................................................................513 

5.6 References........................................................................................................................514 
482 Instrumentation for Bench- and Large-scale Test Fixtures 

5.1 INTRODUCTION 

A projectile penetrating the hull and fuel cell of a land combat vehicle may result in a fine mist of fuel 

being dispersed into the crew compartment. By military specification, the ensuing “mist fireball 

explosion” must be extinguished within 250 ms of detection, using the onboard chemical fire suppression 

system.1

 A similar hazard occurs upon the penetration of a projectile into an aircraft dry bay and adjacent 

fuel cell. The fire extinguishing agent must be distributed throughout the interior region surrounding the 

fuel tank and flame extinguishment accomplished within a time interval similar to that for ground 

vehicles. This speed is necessary to prevent structural damage to the aircraft wing or fuselage to the point 

where the plane would crash. 

Live-fire tests of ground vehicles and aircraft (or representations of sections of these) are frequently 

conducted in order to characterize the performance of a fire suppression system in quenching such fires. 

In these tests, it is particularly important to monitor the distribution of the fire suppressant agent in real 

time. This enables judgment on the extent to which the time to flame extinguishment was limited by the 

effectiveness of the agent, the agent dispersion system, or both. To obtain the desired information from 

these very short duration tests, it is necessary to use instrumentation that provides data that are resolved 

on a time scale shorter than the suppression event, generally in tens of milliseconds. 

The instruments currently used for monitoring the concentration of halon 1301 are the Statham analyzer 

and the Halonyzer.2

 Each has a time response on the order of 200 ms or longer. Clearly they are not 

capable of monitoring the rapid distribution of the agent in a dry bay or crew compartment environment. 

There are additional gases whose time-dependent concentrations reflect on the efficacy of the fire 

suppression process. These include the local oxygen and fuel concentrations, which control the extent of 

the burning. They also include the concentrations of toxic or corrosive combustion products. Chief 

among these, for both halon 1301 and most of the halocarbon replacement chemicals, is hydrogen 

fluoride (HF).3

 As various fluorine-containing suppressants emerged, minimization of HF gas production 

during fire suppression became an important design criterion.4

 The extreme reactivity of this gas dictated 

the additional requirement that the measurement be made in-situ, which, in turn, mandated the use of 

optical techniques. 

The NGP performed research to develop and adapt measurement techniques, apparatus and procedures for 

real-time, in-situ measurements of combustible gases, combustion by-products, and suppressant 

concentrations during the quenching of deflagrations in ground vehicles and aircraft. 

Each of the techniques was based on the absorption of light from infrared sources: near-infrared tunable 

diode lasers (NIR-TDL), near infrared pulsed lasers, and resistively-heated coils (or “Nernst” glowers). 

In the following text, the infrared region of the spectrum is considered to encompass wavelengths from 

1 μm to 30 μm. Table 5–1 lists the techniques used, the gases measured, the absorption wavelengths, and 

the approximate limits of detection (LoD). All LoD values listed are the best achievable laboratory 

values. Experience indicated that the LoDs in field measurement were at least one order of magnitude 

worse than in laboratory measurements. 

Limited descriptions of the methods appear in the following sections, with more extensive detail available 

in the references. In each case, the procedure was studied in a laboratory to determine the measurement 
Laser Induced Breakdown for Measurement of Fire Suppressants 483

linearity and the limits of detection. Some of the apparatus were then hardened and installed for use on a 

real-scale test fixture. 

Table 5–1. Optical Diagnostics for Fire Species. 

Method Molecule 

Wavelength 

(μm) 

LoD 

(μL/L) 

Tunable Diode Laser Absorption Spectroscopy (TDLAS) HF 1.31 0.04 

TDLAS O2 0.76 100 

TDLAS CH4 1.65 0.6 

TDLAS CO 1.55 5 

TDLAS JP-8 

DF-2 

gasoline 

1.71 100 

Laser-induced Breakdown Spectroscopy (LIBS) O2 Emission 1000 

LIBS CF3Br Emission 1000 

LIBS CF3H Emission 1000 

Differential Infrared Rapid Concentration Sensor (DIRRACS) C2HF5 8.3 to 9.0 5000 

5.2 LASER INDUCED BREAKDOWN SPECTROSCOPY FOR MEASUREMENT 

OF FIRE SUPPRESSANTS 

In this technique, a pulsed Nd:YAG laser, typically with pulse energy > 50 mJ, is focused onto a gas 

mixture with sufficient intensity that molecular breakdown takes place in the focal volume of the laser. 

The high electric field at the laser focus causes an ionization of a small number of gas molecules (usually 

air). Free electrons within this plasma then absorb laser radiation (inverse Brehmstrahlung5

), resulting in 

further ionization accompanied by collisional heating. Temperatures within the laser focal volume may 

reach 20,000 K, resulting in atomization of any molecules within the focal volume. After the laser pulse, 

rapid deceleration of the electrons within the plasma leads to an initial emission of broadband radiation 

(Brehmstrahlung5

) lasting tens to hundreds of nanoseconds, followed by emission of discrete wavelengths 

of light as the atomic species present in the plasma return to their ground states (in tens to hundreds of 

microseconds). A schematic showing the application of LIBS to laboratory measurements is shown in 

Figure 5–1. 

First, the technique was used for measurement of linearity and determination of limits of detection for 

mixtures of halogenated fire suppressants in air.6,7

 Figure 5−2 shows that the intensity of F atom emission 

near 400 nm, measured using LIBS is nearly directly proportional to vapor concentration. The limit of 

detection for each gas, using LIBS, was approximately 1000 μL/L. Following these tests, a series of 

experiments was performed to determine if the LIBS measurement process could initiate combustion in 

combustible hydrocarbon/air mixtures. Tests were performed on flowing streams of pure methane and 

pure propane in air. In most cases, the LIBS spark provided sufficient energy to cause the gas mixture to 

begin burning. Following these tests, it was decided that the LIBS technique was inappropriate for use in 

enclosed areas that may contain hydrocarbon/air mixtures approaching the lower explosion limit (LEL), 

and was therefore inappropriate for measurement of oxygen concentrations immediately following fire 

suppression, when reflash conditions may exist.
484 Instrumentation for Bench- and Large-scale Test Fixtures 

1064 nm

Pulsed Laser

R adiation

1064 nm

Pulsed Laser

R adiation

Spectrograph

G ated Intensified

C am era

Laser induced plasm a 

w ithin m easurem ent

reg io n

Focu sing 

L en s

C ollim ator / Fiber

C oupler

Figure 5–1. Schematic of Laser Induced Breakdown Spectroscopy (LIBS) 

Measurement Apparatus. 

0

0 2000 4000 6000 8000 10000 12000 14000 16000

Concentration (μL/L)

Signal Inten

sity (arbitrary units)

HFC-227ea

Halon-1301

Halon-14

HFC134a

Figure 5–2. Measured Concentrations of Fluorinated Fire Suppressants using LIBS. 
Tunable Diode Laser Absorption Spectoscopy (TDLAS) for Measurement of 

Combustion Products, Fuels and Oxygen 

485

As of the date of this book, laser induced breakdown spectroscopy (LIBS) has not been transitioned to 

field measurement of suppressant concentrations (in non-flammable environments) in armored vehicles at 

the Aberdeen Test Center. The main reason for this is that optical fibers necessary for delivering pulsed 

radiation to the measurement site were found to be unreliable, failing at fewer than 200 shots. However, 

several manufacturers are now offering fibers for delivering pulsed radiation at 1.06 μm. These high 

power delivery fibers can transmit a pulse of up to 80 mJ at 10 ns, which may enable LIBS of gases at 

remote locations. 

5.3 TUNABLE DIODE LASER ABSORPTION SPECTROSCOPY (TDLAS) FOR 

MEASUREMENT OF COMBUSTION PRODUCTS, FUELS, AND OXYGEN 

5.3.1 General 

In static laboratory environments, a widely used standard method for detection and measurement of gases 

has been Fourier transform infrared (FTIR) spectroscopy.8

 In conventional Fourier transform 

spectrometers, each “discrete” wavelength of light emitted by the glowbar source, generally a resistively 

heated tungsten coil, is power modulated by a Michelson interferometer. As a result, the output at any 

individual wavelength, as measured at the detector, is cosinusoidal, with a frequency in the kHz (acoustic) 

range. The signal measured at the detector is a superposition of cosinusoidal waveforms called an 

interferogram. The amplitude and frequency of each cosine wave composing the interferogram is a 

function of the moving mirror displacement and of the wavelength of the incident light. The 

measurement interval is ultimately determined by the velocity of the moving mirror, and may only 

approach 20 ms for a single scan. Spectral resolution is a function of the moving mirror displacement, 

and gas sensitivities may exceed parts per million. Although highly refined, the net effect of this design is 

to necessitate complicated electromechanical mirror transport systems, limit measurement applications to 

repeatable or static events, and thus limit the use of these instruments to the laboratory. 

To overcome these limitations for in-situ gas measurement during and following a ballistic event on board 

a land-based vehicle, NGP researchers at the Army Research Laboratory pursued a solid-state 

measurement system, while attempting to maintain the sensitivity and broad spectral coverage of 

interferometer-based infrared spectroscopy. The design criteria for gas detection and measurement 

employing infrared light sources and detectors were mid-range sensitivity (50 μL/L to 5000 μL/L)9

, time 

for collection of a full spectrum of 10 ms or less, and, in the near infrared, the ability to use low 

replacement cost optics, optical fibers, and detectors necessary for data sampling in hostile environments. 

For example, commercially available diode lasers, initially produced for optical communication, began to 

see use for measurement of HF gas in the early 1980s.10 By 2000, relatively low cost narrow band diode 

lasers emitting near-infrared radiation (near-infrared tunable diode lasers, NIR-TDL) at wavelengths 

absorbed by most atmospheric gases and many radical species (e.g., HF gas near a wavelength of 1.3 μm) 

were commercially available. Use of NIR-TDLs for gas measurement became a widely accepted method 

for industrial applications.10,11 Wavelength modulation techniques and the ability of inexpensive optical 

fibers to deliver light to hostile environments allowed sensitive optical components and electronics to be 

remotely located. 

There were significant challenges to be overcome for these NIR-TDLs to become practical. The first 

involved the ability to measure and store data continuously with a new data point acquired every 10 ms. 

Secondly, it was unclear how the sensor equipment would perform in the harsh environment for field 
486 Instrumentation for Bench- and Large-scale Test Fixtures 

testing, in which the sensor assembly in the vehicle was subjected to shock (explosion) and heat (fire). 

The third involved developing a method that enabled the narrow band light source (the diode laser) to 

measure a broad band absorber (JP-8 vapor), while maintaining the signal to noise enhancement 

achievable with phase sensitive detection.12,9 

The following sections describe the technique and results for the detection of HF, various fuels, and 

molecular oxygen. 

5.3.2 HF 

HF gas concentrations were measured using a stand-alone spectrometer system assembled at the Army 

Research Laboratory.13 This system used a single, distributed feedback (DFB) tunable diode laser 

emitting near infrared radiation at a wavelength near 1.31 μm. This wavelength coincides with the P2 

line of the first overtone absorption of HF gas, and is free of overlap by water lines present in most 

combustion environments [10]. Typically, the laser was modulated (wavelength tuned) at 100 Hz for a 

range of approximately 0.5 cm-1, such that the center of this tuning range coincided with the center of the 

absorption by the P2 transition in HF gas (absorption linewidth approximately 0.1 cm-1). Superimposed 

upon this “slow” modulation was a fast (50 kHz to 100 kHz) sinusoidal wavelength modulation. The 

wavelength range of the fast modulation was adjusted so that it was a small (< 0.25) fraction of the 

absorption linewidth of the P2 transition (1st overtone) in HF gas. This dual modulated light beam was 

then launched onto an optical fiber, and the fiber used to transmit the beam from a control room (located a 

safe distance from the measurement site) to the measurement site. For field work, this usually meant the 

beam was transported approximately 100 m to the measurement site. At the measurement site, the fiber 

was terminated with a gradient index (GRIN) lens, causing the light beam to be collimated while it passed 

through free space (the measurement region). After passing through free space for approximately 10 cm 

to 15 cm, the modulated light beam impinged upon a room temperature operation semiconductor light 

detector (typically InGaAs). The signal from the biased (12 V) photodetector was returned to the control 

room via a BNC cable, demodulated using a lock-in amplifier, and recorded on a laptop computer running 

LabView software. The amplitude (height) of the second derivative-like signal output by the lock-in 

amplifier was proportional to the amount of light-absorbing gas (HF) present in the measurement region. 

The steps involved in laser modulation and demodulation are shown in Figure 5–3. 

The experimental protocol, lab results, field results, and data analysis methods are detailed in several 

publications.14, , , , , 15 16 17 18 19 Early in the experimental cycle, an effort was made to use a second optical fiber 

to capture the light after it had passed through the measurement region, allowing the photodetector to 

remain in the control room. However, it was found that the alignment of the two optical fibers for the 

“pitch and catch” setup at the site (typically, the crew compartment of a Bradley Fighting Vehicle) was 

extremely sensitive. So, it was decided to place the photodetector at the measurement site, since tests 

showed that small misalignments in the open path fiber-to-detector line of sight were less important than 

small misalignments in open path fiber-to-fiber line of sight. Additionally, the costs associated with 

replacing either the fiber-to-detector or fiber-to-fiber setup within the vehicle were similar (hundreds of 

dollars). Using this system, the measurement duration was 10 ms (100 scan/s); however, the time 

between measurements was approximately 100 ms, caused by the delay in transferring information from 

the lock-in amplifier to an oscilloscope, and from the oscilloscope to the laptop computer running 

LabView software. Figure 5–4 shows the equipment used for monitoring the real-scale fires in ground 

vehicles. 
Tunable Diode Laser Absorption Spectoscopy (TDLAS) for Measurement of 

Combustion Products, Fuels and Oxygen 

487

7

8

9

10

11

12

13

14

15

16

7665.5 7665.51 7665.52 7665.53 7665.54 7665.55 7665.56

Wavenumbers (cm-1)

Signal at Detector (V)

0

0.01

0.02

0.03

0.04

0.05

0.06

Lock-in Signal I

ntensity (a.u.)

Signal at detector without

fast modulation

Signal at detector with

fast modulation

2f signal from

lock-in

Figure 5–3. Successive Signal Improvements in the Detection of HF using TDLAS. 

Laptop

Computer

Oscilloscope Lock-in

Amplifier

Gradient Index

(GRIN) Lens

InGaAs Detector

Tunable

Diode

Laser

Function

Generator

Laser

Driver

Fiber Optic

Measurement Region

(Light into free space)

(Light collection)

Figure 5–4. Schematic of TDLAS Apparatus for Measurement of Gases Produced during 

Fire Suppression in Ground Vehicles. 
488 Instrumentation for Bench- and Large-scale Test Fixtures 

This single gas stand-alone spectrometer system for HF gas measurement was used for three types of 

testing: 

1. Suppression of JP-8 pool fires, using a handheld extinguisher charged with HFC-227ea. 

2. Suppression of heptane pan fires in an enclosure, using a mechanically actuated bottle 

charged with a series of fluorinated propanes. 

3. Suppression of spark initiated or ballistically initiated JP-8 fires in the crew compartment of a 

Bradley Fighting Vehicle hull, using CF3Br or HFC-227ea. 

For these tests, measurements of HF gas (and other combustion products) were also measured using 

extractive Fourier transform infrared spectroscopy as a check. In all cases, reasonable agreement was 

obtained. For the JP-8 pool fire measurements, the objective was to obtain a measurement of the 

approximate exposure to HF experienced by a firefighter while suppressing the fire using a hand held 

extinguisher. The HF measurement location was approximately 5 m from the pool fire, allowing the 

firefighter full access to the pool fire, as he was attempting to extinguish the fire as rapidly as possible. 

For these tests, the HF gas concentration that was measured ranged from 0 μL/L to approximately 1000 

μL/L. As only one measurement site was used, the measurement was largely a function of wind 

direction, although the time-averaged exposure was always beyond acceptable levels. 

For the heptane pan fires, the objective was to understand the temporal behavior of HF gas concentration 

following release of the suppressant. These tests of approximately 30 kW fires were conducted in a cubic 

1.5 m3

 enclosure. The heptane pan was located under an 0.5 m high steel table, on which the 

NIR-TDLAS apparatus was placed. A handheld bottle extinguisher charged with the extinguisher and 

perhaps a scavenging agent was situated on top of the enclosure, with a spray nozzle protruding into the 

enclosure. The fires were extinguished by application of HFC-236 (C3F6H2), HFC-227ea (C3F7H), 

HFC-236 plus APP, or HFC-227ea plus APP. APP (ammonium polyphosphate) powder, itself an 

effective flame extinguishant, was added as both a possible synergist for the less efficient fluorocarbons 

and as a possible means of reducing HF yields. 

The heptane fire was ignited using a butane electric match and allowed to burn for 15 s before the 

contents of the extinguisher were dumped into the enclosure. A video camera monitored the fire event 

and enabled determination of the time of extinguishment. HF data were collected for 180 s after ignition 

of the fuel at rate of 1 spectrum/s. In addition, 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 a M1 land combat vehicle were performed.

The effectiveness of APP as a HF reduction and potential scavenging agent was examined by comparing 

the HF concentrations from fires extinguished with HFC-236/APP or HFC-227ea/APP mixtures and fires 

extinguished by HFC-236 or HFC-227EA alone. The two mechanisms by which HF gas concentrations 

can be decreased during and following fire extinguishment by the addition of the APP to the fluorinated 

agents are: 

• Reducing the time required to extinguish the fire, thereby minimizing the time that the 

fluorine-containing suppressant is exposed to flame temperatures, or 

• Scavenging the HF after it is produced. 
Tunable Diode Laser Absorption Spectoscopy (TDLAS) for Measurement of 

Combustion Products, Fuels and Oxygen 

489

Figure 5–5 shows the results of one such test. The application of 7 % APP clearly affects a significant 

reduction in HF. What is important is that a nominal doubling of the APP level virtually eliminated the 

HF. If this were principally a scavenging effect, one would expect that the green line in Figure 5–5 would 

rise further initially and then decline more steeply than the red curve. The observed behavior favors 

enhanced quenching of the flames and the accompanying reduction in the thermal generation of the HF. 

-200

0

200

400

600

800

1000

1200

1400

0 50 100 150 200

Time (s)

Av

e

rage HF Concentration, 

μL/

L

HFC-236

HFC-236 + 7 % APP

HFC-236 + 15 % APP

HFC-236

HFC-236 + 7 % APP

HFC-236 + 15 % APP

-200

0

200

400

600

800

1000

1200

1400

0 50 100 150 200

Time (s)

Av

e

rage HF Concentration, 

μL/

L

HFC-236

HFC-236 + 7 % APP

HFC-236 + 15 % APP

HFC-236

HFC-236 + 7 % APP

HFC-236 + 15 % APP

Figure 5–5. HF Gas Produced during Suppression of Heptane Pan Fires by HFC-236 with 

and without the Addition of Ammonium Polyphosphate). 

In the tests conducted in the ground vehicles, the HF measurement location approximated the seating 

location of a passenger within the vehicle. The results indicated that HF concentrations produced from 

fires extinguished by HFC-236 plus APP and HFC-227ea plus APP were reduced in the test facility and

that the presence of APP accelerated the reductions. Production of HF gas ranged from hundreds to 

thousands of (μL/L)-min, depending on how quickly the fire was extinguished. For successful tests (fires

extinguished within 250 ms of the suppressant being deployed), HF levels were typically in the hundreds 

of μL/L. For tests where the fire was not extinguished or suppression was not complete after 250 ms 

(resulting in suppressant being burned), the HF levels were often beyond the measurement range of both 

instruments (usually 5000 μL/L for TDLAS). Visibility reduction during extinguisher deployment was 
490 Instrumentation for Bench- and Large-scale Test Fixtures 

measured inside the combat vehicle for extinguishers containing HFC-236 plus APP and NaHCO3 plus 

N2. (HFC-227ea was not available for these tests.) 

5.3.3 Oxygen 

Measurement of the fuel and oxygen concentrations is especially important after the suppression event in 

order to predict the possibility of reignition. Minimum acceptable detection limits for fuel/oxygen gas 

mixtures were projected to be about 600 μL/L (0.06 %) for the fuel and 3000 μL/L oxygen (0.3 %), for a 

10 cm optical path length at room temperature. These values would ensure the ability to make accurate 

concentration measurements near the lean flammability limit. The oxygen measurements are described in 

this section, the fuel measurements in the next. 

A fully automated, multi-position, multi-laser instrument, built by Southwest Sciences, Inc., was 

originally designed to measure concentrations of CO, O2, HF, and NO2 gases at four different locations 

within the crew compartment of a Bradley Fighting Vehicle, achieving near continuous measurement with 

data points every 10 ms. Housed in a trailer adjacent to the control room at the test site, this instrument 

used the same technique of dual modulation spectroscopy employed by the stand alone HF measurement 

system described earlier. However, it used a 10 kHz ramp for the “slow” modulation and a 1MHz sine 

wave for the “fast” modulation. During the course of development of this instrument, it was decided to 

focus exclusively on measuring HF and O2 within the Bradley hull, during suppression by HFC-227ea of 

spark initiated JP-8 fires. 

For these fires, a heated (≈ 70 °C) spray of JP-8 liquid was begun within the crew compartment of the 

Bradley hull. The fuel nozzles were located along one interior wall. About 1 s later, this fuel spray was 

ignited by a series of spark plugs located along the wall of the interior of the vehicle. Approximately 1 s 

after ignition, two onboard bottles containing the pressurized suppressant (HFC-236, HFC-227ea, or 

FC-36, C3F8) were released. 

At the crew seating location, a sharp drop in O2 concentration occurred during the fire and suppression 

event. Following suppression, O2 levels returned to ambient in a few seconds. It was not clear whether 

the decrease in O2 levels during the fire and suppression event was due to depletion of oxygen by 

combustion or displacement of oxygen by the hot fuel vapors/expanding HFC-227ea vapor. Results from 

a typical test measuring oxygen concentration as a function of time in the crew compartment of the 

Bradley fighting vehicle are shown in Figure 5–6. 

Test difficulties related to reliability of the visible diode lasers operating near 760 nm limited the data 

obtained in spray fires in vehicles. During the course of this project, fiber pigtailed lasers became 

commercially unobtainable at this wavelength. 

5.3.4 Fuels 

The third measurement capability to be developed was for the quantification of hydrocarbon fuels in 

fuel/air mixtures: heptane (C7H16), as a model fuel, and two realistic fuels: DF-2 (a diesel fuel), and JP-8. 

The apparatus was a multi-point, field worthy, fiber-coupled, near-infrared tunable diode laser-based 

sensor. The system was to provide concentration data before (minimum five data points prior to agent 
Tunable Diode Laser Absorption Spectoscopy (TDLAS) for Measurement of 

Combustion Products, Fuels and Oxygen 

491

release), during, and after the suppression event, for a nominal event time of 250 ms, with a time 

resolution better than 10 ms. 

Oxygen Concentration

10

12

14

16

18

20

22

24

0 100 200 300 400 500 600 700 800 900

Time (ms)

% Ox

y

gen

Figure 5–6. Oxygen Concentration Measured Within the Crew Compartment of a Bradley

Fighting Vehicle During Suppression by C3F8 of a Spark-Initiated JP-8 Spray Fire. 

Field worthy detectors for hydrocarbon vapor measurements typically rely upon infrared techniques: 

DOAS (differential optical absorption spectrometry), FT-IR (Fourier transform infrared

emission/absorption)20,21,22 or electrochemical/electrothermal techniques.23 These methods give good 

results, but have found limited use for measurements in fires. FT-IR and DOAS spectrometers are usually 

bulky, require careful placement/alignment of interferometer/source optics and detector or retro-reflector, 

and are generally limited to a single line of sight. Placement of the instrument within the test 

environment (crew compartment) is not feasible (potential damage), looking across the entire

measurement space does not isolate the region of interest, and although extractive sampling has been 

shown to be useful,24 it does not provide the necessary temporal information. Additionally, the

interferometer temporal resolution is limited by the mirror scan speed (although some new designs are 

approaching scan rates near 1 kHz).25 

Nevertheless, use of FT-IR and DOAS for measurement of the fundamental C-H stretch in hydrocarbon

fuels near 3.4 μm can yield sensitivities in the nL/L (ppb) range, and because the full spectrum may be 

measured (when using FT-IR), it is often possible to determine species of hydrocarbons present in a

simple mixture. For stable, relatively safe sampling sites with an accessible line of sight, incoherent 
492 Instrumentation for Bench- and Large-scale Test Fixtures 

infrared absorption techniques (FT-IR, DOAS) are often the techniques of choice.

Electrothermal/electrochemical detectors are generally inexpensive ($2000 to $5000), are often 

transportable by a single person, and have sensitivities (≈ 100s μL/L) well below lower explosion limits 

for most hydrocarbon vapors.26,27,28,29 Time sensitivity is typically from 1 s to 15 s. Some units may have

accuracy problems when used in the presence of methane gas, and may be less sensitive to heavier 

hydrocarbon vapors.23 For measurement sites requiring full portable instrumentation with time resolution 

from one second to tens of seconds, these instruments can also provide excellent results. 

Gas detection methods using near-infrared tunable diode lasers can use optical fibers and relatively 

Figure 5–7. FT-IR Spectra of Dry Air Saturated at 294 K with Vapor from 

(The spectra have been offset for clarity. Structured absorptions 

near 1.9 and 2.7 μm are from water vapor.) 

inexpensive (room temperature operation) semiconductor detectors to separate electronics and laser 

sources from hazardous testing locations. Using modulation techniques and phase sensitive detection

methods, temporal resolution routinely exceeds 1 ms per measurement. As previously mentioned,

detection methods employed (typically wavelength modulation spectroscopy (WMS) and balanced 

ratiometric detection (BRD) techniques)8,9

 give good results for gas phase concentration measurements of

small molecules with well resolved ro-vibrational absorption lines, such as hydrogen fluoride, methane,

carbon monoxide, water, etc.30 However, large and/or heavy molecules may not possess well resolved 

ro-vibrational absorption bands because of overlap between fundamental, combination, and/or overtone 

vibrations, and because of rotational constants which may be less than typical gas phase linewidths.31 For

mixtures of heavy molecules, e.g., the middle distillate fuels, the spectra may be even less structured.

Figure 5–7 shows the near-infrared gas phase absorption spectrum of dry air saturated at 294 K by the 

hydrocarbon fuels JP-8, DF-2, and gasoline, measured using a Fourier transform spectrometer, with an

optical path of 0.75 m.

0

20

40

60

80

100

120

140

160

1.2 1.7 2.2 2.7 3.2 3.7

Unleaded

Gasoline

(regular)

JP-8

DF2

Wavelength (μm)

% Transmis

sion

0

20

40

60

80

100

120

140

160

1.2 1.7 2.2 2.7 3.2 3.7

0

20

40

60

80

100

120

140

160

1.2 1.7 2.2 2.7 3.2 3.7

Unleaded

Gasoline

(regular)

JP-8

DF2

Wavelength (μm)

% Transmis

sion

Unleaded gasoline, JP-8, and DF2. 
Tunable Diode Laser Absorption Spectoscopy (TDLAS) for Measurement of 

Combustion Products, Fuels and Oxygen 

493

As suggested by be classified as 

a gas having unstructured a sorber"). Both WMS and 

BRD, as usually employed, are not suitable for gases that exhibit broad, unstructured absorption 

 approaching the sensitivity and time response of WMS and BRD methods. The 

approach took advantage of the residual amplitude modulation (RAM) that occurs as the wavelength of a 

he 

absorption coefficient (cm2

/molecule) at the most intense region of absorption (the resonant wavelength, 

If the incident radiation at the resonant wavelength is now power modulated sinusoidally at frequency a, 

d time t, with depth of modulation tion at the resonant wavelength after 

xiting the gas becomes: 

To get a measurement of transmission off resonance (important to generate a differential signal and to 

iscriminate against scattering laser source at a wavelength 

ot absorbed by the gas, λ . This non-resonant wavelength is mixed with the resonant wavelength, 

the spectra shown in Figure 5–7, the vapors from each fuel fit criteria32 to

bsorption spectra (often called a "broad band ab

spectra.8,9,30 For both techniques, the main limitation for fuel sensing is the narrow tuning range of the 

diode laser. Because the tuning range of the DFB lasers is usually several orders of magnitude smaller 

than the width of the spectral feature in broad band absorbers (0.4 cm-1 vs. > 100 cm-1), it is not possible 

using commercially available DFB lasers to tune from the region of maximum absorption to a region of 

zero absorption. 

McNesby and co-workers developed a technique to overcome this limitation for measurement of such 

molecules, while

semiconductor tunable diode laser is varied by modulating the laser injection current. Typically (for 

WMS and BRD), RAM is a nuisance, and what is important is the wavelength modulation. For this 

method, the RAM is what enables the technique to work, and the relatively small variation of wavelength 

during the modulation is largely ignored (but will be commented upon below). For this method, two 

different DFB lasers are driven using a sinusoidally varying injection current. Each laser injection current 

is modulated at the same frequency, but the two lasers are out of phase by 180°.

32 The light output of the 

two lasers is combined to produce the probe beam used for the line-of-sight measurement. When the 

power output amplitudes of each sinusoidally modulated laser are the same, the combined output of the 

two lasers will have an amplitude that is (ideally) invariant with time. The key to the method is selection 

of one laser wavelength coincident with a wavelength absorbed by the gas of interest, and selection of the 

second laser wavelength in a region where no absorption is anticipated. When the mixed, modulated 

probe beam is passed through a gas which preferentially absorbs the laser radiation from only one of the 

lasers, the signal at the detector is modulated at the injection current modulation frequency and, for small 

absorptions, has an amplitude whose magnitude is proportional to the concentration of absorbing gas. 

The simple mathematical description begins by considering Beer's law for narrow band laser radiation 

exiting an absorbing sample gas of concentration c molecules per cm3

, for a path length of l cm. T

λR), is α(λR). I0(λR) is the intensity of the incident radiation at the resonant wavelength. I(λR) is the 

intensity of radiation at the resonant wavelength after exiting the sample gas: 

I(λR)=I0(λR) exp(-α(λR)cl) (5-1)

an DR, the intensity of the radia

e

I(λR)=I0(λR)(1+DR sin(at)) exp(-α(λR)cl) (5-2)

d by particles such as smoke), we add a second

n NR

power modulated at the same frequency but 180 degrees out of phase with the resonant wavelength 
494 Instrumentation for Bench- and Large-scale Test Fixtures 

modulation, and has a depth of modulation DNR. If the radiation exiting the gas is now focused on an 

(ideally) wavelength-insensitive detector, the power at this detector, I(λR +λNR), may be given by: 

I(λR +λNR)=[I0(λR)(1+DRsin(at)) exp(-α(λR)cl) +I0(λNR)(1+DNRsin(at+π))] (5-3)

o process this signal, we multiply by a reference sine wave of amplitude p in phase with and at the same 

C 0

π

∫ {[I0(λR) (1+DR) exp(-α(λR)cl) -I0(λNR) (1+DNR)] sin(at) psin(at)}dt (5-4)

earranging, assuming exp(-α(λR)cl) <<1 and using A (Absorbance) equal to α(λR)cl : 

2C 0 ∫ {[I0(λR) (1+DR) exp(-α(λR)cl) -I0(λNR) (1+DNR)] sin(at) psin(at)}dt (5-5)

or the special case where the power and depth of modulation of the two lasers are equal, Equation 5-5 

2C 0

π

∫ {[I0(λR) (1+DR) exp(-α(λR)cl) -I0(λNR) (1+DNR)] sin(at) psin(at)}dt (5-6)

quation 5-6 is just the output of a lock-in amplifier operating on the first harmonic for measurement of 

A diagram of the experimental apparatus employing this method is shown in Figure 5–8. The two DFB 

T

frequency, a, as the power modulation of the two lasers. We then integrate over C periods of power 

modulation: 

=(C/2)pπ [I0(λR) (1+DR) exp(-α(λR)cl) -I0(λNR) (1+DNR)] 

R

π

=Cpπ [I0 R R 0 (λ ) (1+D ) -I (λNR NR ) (1+D ) - I0 R R (λ ) (1+D )A] 

F

becomes: 

=-Cpπ [I0(λR) (1+DR)A] 

E

the amplitude of the sine wave created by the differential absorption (for small absorptions) of the mixed 

laser beam by the gas at concentration c and path length l. This output is proportional to the number of 

periods, C, over which the integration is carried, the amplitude, p, of the reference sine wave, the incident 

power, I0(λR), of the laser at the resonant wavelength, and the Absorbance, A (and hence gas 

concentration when pressure broadening of the spectral feature is not important). It is worth noting that 

Equation 5-6 also indicates that the lock-in output is increased by increasing depth of modulation (DR). 

As will also be discussed later, Equation 5-5 shows that it is important that the two laser power 

amplitudes and depths of modulation are as equal as possible, especially for small values of Absorbance. 

As mentioned previously, and implied by Equation 5-6, because the measurement relies on differential 

absorption to quantify gas concentrations, division of the raw signal at the detector by the average laser 

power should account for pseudo wavelength-independent signal intensity extinction from scatterers, such 

as smoke particles. However, it should also be mentioned that decreases in overall signal voltage will 

result in similar decreases in the signal to noise ratio of the measurement. 

laser diodes (Sensors Unlimited) used to produce the mixed wavelength probe beam have nominal 

wavelengths of 1.307 μm and 1.71 μm. The laser diodes are driven using an ILX LDC-3900 modular 

laser diode controller. The variable phase sinusoidal injection currents are supplied by two SRS Model 

DS345 function generators operating on a common time base, Sinusoidally modulated (100 kHz, 

modulation depth adjusted to near 100 %) laser diode radiation from each diode laser is mixed onto a 

common fiber using a fiber optic coupler (Gould Fiberoptics) and launched into free space using a single 

mode fiber that is terminated by a gradient index (GRIN) lens (Sentech Corp.). The output beam is 
Tunable Diode Laser Absorption Spectoscopy (TDLAS) for Measurement of 

Combustion Products, Fuels and Oxygen 

495

directed into a variable pathlength gas cell (Laser Photonics L5210) set at the minimum setting of 4 m.

Radiation exiting the 4 meter path length cell is detected using a New Focus Model 2034 InGaAs large

element photodetector (maximum responsivity near 1600 nm). The detector signal is demodulated at the 

laser injection current modulation frequency using a lock-in amplifier (SRS 830), displayed on an

oscilloscope (LeCroy 7200), and recorded on a laptop computer (Dell Latitude) running LabView 

software (National Instruments). JP-8 and DF-2 were milspec grade and were obtained from the fuel 

depot at the Aberdeen Proving Ground. The gasoline was regular grade obtained during the summer

months at a commercial vendor in Aberdeen, MD. All were used as supplied. Air was saturated with fuel 

vapor using a bubbler apparatus based upon a nitrogen sparger. Air flow through the stainless steel frit

immersed in the fuel was typically 100 cm3

/min to 250 cm3/min. Figure 5-9 shows the vapor phase 

absorption spectrum of air saturated by vapor at 294 K from JP-8, DF-2, and gasoline between 

wavelengths of 1.3 μm and 1.75 μm superimposed upon the emission from the optical fiber carrying the

mixed wavelength laser beam. The data were recorded using a Bomem DA-8 spectrometer (using a 

liquid nitrogen-cooled InSb detector) interfaced to a Wilkes 0.75 m path length cell. The near infrared

tunable diode laser radiation was transported using optical fibers to the external source port on the FT-IR

spectrometer. This figure shows that the emission of the laser at 1.71 μm falls near the most intense

region of the C-H stretch (first overtone) absorption for the three fuels studied, and that the 1.3 μm laser

falls in a region where there is no measurable absorption of the laser radiation by the fuels studied. 

Because of the narrow emission linewidth of the lasers, the reported line shape in this figure is just the 

instrument line shape (in this case, sinc) of the FT-IR spectrometer.33

Laptop

Computer Oscilloscope Lock-inAmplifier

GRIN Lens 1

InGaAs Detector

4 m optical cell 

Laser 1

1.31

microns

Function

Generator 1

Laser

Driver 1

Laser 2

1.71

microns

Function

Generator 2

Laser

Driver 2

Fiber

optic 

coupler

Laptop

Computer Oscilloscope Lock-inAmplifier

GRIN Lens 1

InGaAs Detector

4 m optical cell 

Laser 1

1.31

microns

Function

Generator 1

Laser

Driver 1

Laser 2

1.71

microns

Function

Generator 2

Laser

Driver 2

Fiber

optic 

coupler

Figure 5–8. Schematic Diagram of the Laser Mixing Apparatus Used to Measure 

Fuel Vapor Concentrations.
496 Instrumentation for Bench- and Large-scale Test Fixtures 

Figure 5–9. Absorption Spectrum of Air Saturated with JP-8, DF-2, or Gasoline Vapor at 

294 K Superimposed upon the Emission from the Optical Fiber Carrying the Mixed 

Wavelength Laser Beam. 

Figure 5–10 shows the gasoline vapor concentration (measured using the mixed laser sensor) as dry air

saturated by gasoline vapor slowly displaces (several hundred cm3

/minute) the dry air in a 13.7 L optical 

cell (4 m path length). Also shown in this figure is the change in oxygen concentration that occurs during 

the displacement (DFB laser-based oxygen sensor courtesy of Oxigraf, Inc., Mountain View, CA). This 

figure shows the sensor response as the vapor concentration approaches and exceeds the lower explosive

limit (LEL) for most hydrocarbon vapors (e.g., for JP-8, the LEL is approximately 9500 μL/L).26,34 Each

data point is the average of the lock-in amplifier output measured over 10 ms. All measurements were 

made at atmospheric pressure. Figure 5–11 shows the measured fuel vapor concentration as the dry air in

the 13.7 L optical cell is displaced by air saturated (at 294 K) with JP-8 vapor. The results for DF-2 were

similar to those for JP-8. The partial pressures at atmospheric pressure (measured using a capacitance

manometer to ± 20 %) for gasoline, JP-8, and DF-2 fuels were found to be approximately 1.7 kPa,

4.7 kPa, and 0.23 kPa, respectively. The C-H stretch first overtone absorption coefficient for the fuel 

vapors was determined to be approximately 2.7 (± 0.4) x 10-21 cm2

/molecule, which is in reasonable 

agreement with that for the first overtone of the C-H stretch for methane (8.7 x 10-22 cm2

/molecule).11 

The estimated limit of detection for each of these fuel vapors, using the mixed laser sensor, was 

approximately 100 (μL/L)-m. 

-0.1

0

0.1

0.2

0.3

0.4

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

-0.25

0.25

0.75

1.25

Absorbance

Laser Emissio

n (arb. u

nits)

aser 2

Gasoline vapor

JP-8 vapor

DF-2 vapor

Wavelength (μm)

Laser 1 L
Tunable Diode Laser Absorption Spectoscopy (TDLAS) for Measurement of 

Combustion Products, Fuels and Oxygen 

497

Air 

0

2000

4000

6000

8000

10000

12000

0 100 200 300 400 500

0

0.05

0.1

0.15

0.2

0.25

0.3

0

2000

4000

6000

8000

10000

12000

0 100 200 300 400 500

0

0.05

0.1

0.15

0.2

0.25

0.3

Time (s)

G

asoline Vap

o

r (μL/L)

Fraction Oxy

gen

gasoline

O2

Figure 5–10. Concentrations of Gasoline Vapor and Oxygen, Measured as Dry

Saturated with JP-8 Vapor Displaces the Dry Air in a 13.7 L Optical Cell. 

-0.1

0.4

0.9

1.4

1.9

2.4

2.9

0 500 1000 1500 2000 2500 3000 3500

Volume Fraction JP-8 vapor (μL/L)

(added to 13.7 liter optical celll)

Instrument Respon

s

e (arb.

Fresh sample

Sample after 5 runs

3.4

 units) -0.1

0.4

0.9

1.4

1.9

2.4

2.9

0 500 1000 1500 2000 2500 3000 3500

3.4

-0.1

0.4

0.9

1.4

1.9

2.4

2.9

0 500 1000 1500 2000 2500 3000 3500

Volume Fraction JP-8 vapor (μL/L)

(added to 13.7 liter optical celll)

Instrument Respon

s

e (arb.

Fresh sample

Sample after 5 runs

3.4

 units) Figure 5–11. Loss of Lighter Hydrocarbons from JP-8 Caused by Repeated Fills of the 

Optical Cell Used for Testing the Mixed Laser Fuel Vapor Sensor. 
498 Instrumentation for Bench- and Large-scale Test Fixtures 

Two factors led to the decision that measurement of the full absorption envelope for the C-H stretching 

vibration was necessary, and that the first generation sensor system was inadequate for general fuel vapor 

sensing: 

• Careful examination of the C-H overtone bands for each of the fuels tested (JP-8, DF-2, and 

gasoline) shows that the contours of the absorption features, when normalized to each other, 

were not superimposable. So, being limited to sampling vapors from different fuels using the 

same single wavelength would be a potential source of error. 

• Changes in temperature, storage conditions (aging) and changes in fuel composition between

vendors may cause changes in the contour of the unstructured absorption envelope. 

Therefore, it was necessary to develop a technique with the simplicity of the first generation sensor but 

with the capability of expansion to measure the full C-H absorption envelope of the fuel vapor. The 

approach taken for this second generation sensor uses an array of addressable microphotonic 

semiconductor lasers, each emitting light at a different wavelength, and power modulated at different

frequencies (typically between 100 kHz and 150 kHz). When the modulated light is combined onto a 

single path, and the combined beam brought to focus on a detector, the signal reported by the detector has 

the contour of an interferogram. This interferogram is analyzed by standard Fast Fourier Transform 

FT) techniques to yield the amplitudes of the individual component cosine waves.35 These amplitudes

are proportional to the average power of light at each wavelength reaching the detector. Figure 5–12 

hows the interferogram generated by a two laser mixed beam. 

Figure 5–12. Two Laser Interferogram Measured at the Detector for the Second 

Generation Fuel Vapor Sensor. 

(F

s

2.50E-02

2.70E-02

2.90E-02

3.10E-02

3.30E-02

3.50E-02

3.70E-02

3.90E-02

4.10E-02

4.30E-02

4.50E-02

0.00E+00 4.00E-04 8.00E-04

Signal at detector

Laser 1 (100 KHz) at 1.31μ

Laser 2 (110 KHz) at 1.71 μ

Intensit

y (a.u)

Time (s)
Tunable Diode Laser Absorption Spectoscopy (TDLAS) for Measurement of 

Combustion Products, Fuels and Oxygen 

499

L

of modulation en because it 

is not absorbed by hydrocarbon vapor or by water vapor. Laser 2 emits laser radiation near a wavelength 

f 1.71 μm and was power modulated at 110 kHz, with a depth of modulation near 95 %. Also as with 

ut from each fiber coupled laser is launched onto a single, common, single mode optical fiber 

using a 

at a differen

on a comm

the transmi ) detector. The signal at the 

detector 

manner si s. The 

time for each measurement is user controlled by varying the frequency of power modulation. Resolution 

 the demodulated signal prior to the onset 

of fuel vapor flow. The flow apparatus was characterized using the first generation sensor, and using that 

calibration the absorbance reported here is reasonably linear with concentration (within 5 %), but was not 

explicitly measured for these experiments. All of the factors that determine quality of conventionally 

measured FT-IR spectra are of importance here (modulation depth, apodization, phase, etc.).33 Most of 

the efforts for the prototype were focused on making sure the lasers were modulated in phase, using a 

common time base. Any effects of varying the modulation depth or of varying the laser power were not 

investigated. Dynamic range effects were also not investigated, although some improvement was 

anticipated relative to Michelson interferometer based instruments because the Fourier frequencies for 

each laser are no longer coupled to the laser wavelength. The time for each measurement point was 

approximately 1 s. All measurements were made at atmospheric pressure. 

The absorption coefficients for most VOCs in the near-infrared spectral region (≈ 1 μm to 2 μm 

wavelength, two (or more) vibrational quanta per transition) are typically lower by an order of magnitude 

than for the mid-infrared spectral region (≈ 2 μm – 30 μm wavelength, typically 1 vibrational quantum 

per transition).36 As indicated in Figure 5–7, the ARL team determined the near infrared absorption 

coefficients between wavelengths of 1.0 μm and 30 μm for JP-8, finding the maximum absorption 

coefficient to occur between a wavelength of 1.6 μm and 1.825 μm, with a value of about 

3 x 10-21 cm2

/molecule. Using the prototype FT-LS instrument described above, the detection limit was 

approximately 200μL/L m-1, or approximately 50 times worse than the detection limit using a commercial 

FT-IR spectrometer.37 It is worth noting that for the prototype sensor described here, the power 

modulation of each diode laser was not purely cosinusoidal (harmonics in the FFT output), even though 

aser 1 emits radiation near a wavelength of 1.31 :m and was power modulated at 100 kHz, with a depth

 near 95 %. As with the first generation sensor, this laser wavelength was chos

o

the first generation sensor, this laser wavelength was chosen because it is approximately at the center of 

the 1st overtone C-H stretching vibration of most middle distillate fuel vapors. 

The outp

commercially available fiber coupler (Gould Fiber Optics). Each laser is power modulated, each 

t frequency, using digital function generators (Stanford Research Systems DS-345) operating 

on time base. This modulated light beam is passed through a GRIN lens into free space, and 

tted light detected using an InGaAS (New Focus model 2011

 is demodulated (Agilent Infinium Oscilloscope) using a Fast Fourier Transform (FFT), in a 

milar to that of commercially available Fourier transform infrared (FT-IR) spectrometer 35

is user controlled by varying the number of periods of power modulation measured per “scan.” 

Figure 5–13 is a schematic drawing of the experimental apparatus for the Fourier transform laser 

spectroscopy (FT-LS) system. 

As with the first generation sensor, the second generation sensor was tested by measuring gasoline vapor 

concentration as dry air saturated by gasoline vapor slowly displaced (several hundred cm3

/minute) the 

dry air in a 13.7 L optical cell (4 m path length). Figure 5–14 shows the sensor response as the vapor 

concentration approaches and exceeds the lower explosive limit (LEL). For most hydrocarbon vapors 

(i.e., for JP-8 and most other middle distillate fuels), the LEL is approximately 0.1 % by volume.26,34 For 

the absorbance measurement, the background for each laser was
500 Instrumentation for Bench- and Large-scale Test Fixtures 

the dr eing

power modulated is increased, it is rom frequencies other than driver

frequencies may limit resolution. A custom driver waveform may be needed to give the best cosinusoidal

iver current was supplied by the SRS DG535 function generators. As the number of lasers b

expected the contributions to signal f

power modulation. 

Laser 1

InGaAs

detector

Laser 2

Analyte region

FFT

Laser diode controller

Function generators

GRIN

Fiber mixer lens

Fiber optic

Figure 5–13. Experimental Apparatus Used for Fourier Transform Laser Spectroscopy.

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Figure 5–14. Absorption of 1.71 μm Laser Radiation as Air is Displaced by Air Saturated 

with Gasoline Vapor in a 2 m Cell.

0

0 500 1000 1500 2000 2500

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

A

b

s

orbance

LEL (Lower

Explosion Limit)

0

0 500 1000 1500 2000 2500

Time (s)

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0

0 500 1000 1500 2000 2500

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

A

b

s

orbance

LEL (Lower

Explosion Limit)

LEL (Lower

Explosion Limit)

0

0 500 1000 1500 2000 2500

Time (s)
Tunable Diode Laser Absorption Spectoscopy (TDLAS) for Measurement of 

Combustion Products, Fuels and Oxygen 

501

At the time of publication of this book, the ARL team was developing a 10-laser FT-LS source in the near 

infrared, providing reasonable coverage of the full overtone C-H absorption envelope for hydrocarbon 

vapors. The basic idea for this sensor is illustrated in Figure 5–15. Analysis similar to PLS techniques 

already employed for FT-IR spectral data could be used to identify mixture components.38 Such an 

instrument may be readily assembled from commercially available telecommunications equipment. The 

main challenge is finding a vendor for the individual laser diodes, since there is low demand for lasers 

outside of the fiber windows near 1350 nm and 1550 nm. Provided that the cost of addressable light 

sources continues to decrease, arrays covering larger spectral windows may become available. While 

several options exist for wavelength separation, the highly developed FT-IR market may influence the 

design of new solid state spectrometers, and this approach may warrant further consideration. 

Figure 5–15. Schematic of a 10-laser FT-LS Source in the Near Infrared. 

5.3.5 Observations 

Gas sensing using near-infrared lasers will probably increase steadily in popularity. This is in large part

due to the ease of transmission of the IR signals through optical fibers, room temperature operation, high 

signal to noise ratio through phase sensitive detection techniques, and the large telecommunications 

infrastructure that continues development of electronics that may easily be adapted to gas sensing. 

Nonetheless, some changes are potentially at hand: 

• The replacement of a single laser detecting a single gas by either an addressable source or by

a single, broadly tunable laser. 

• The use of room temperature operation, mid-infrared lasers (quantum cascade and interband

cascade designs). These offer all of the advantages of phase sensitive detection available

0

0.02

0.04

0.06

0.08

0.1

0.12

1.625 1.675 1.725 1.775 1.825 1.875

Wavelength position of 10 diode laser

addressable source

Wavelength (μm)

A

bsorbance

(ai atur

ated b

y g

a

s

olin

e vapor) r s
502 Instrumentation for Bench- and Large-scale Test Fixtures 

with near-infrared DFBs, and have a huge advantage of gas sensing using the fundamental

vibrational transition. 

• Microphotonic sensors will employ suites of arrays of near- and mid-infrared devices, and 

that these devices will gradually displace conventional Michelson interferometer based 

systems in the laboratory and in the field. 

The main technical difficulties associated with this technology center on the availability of DFB lasers. 

With the contraction of the telecommunications market at the beginning of the 21st century, many 

fabrication facilities discontinued manufacture of lasers at user-specified wavelengths, producing lasers 

only within the telecommunications bands near 1330 nm and 1550 nm. Although there has been some 

supply from foreign manufacturers, obtaining lasers for gas sensing (in particular, O2 near 760 nm) 

remains challenging. This difficulty exacerbates the main challenge for gas sensing using near infrared 

DFB lasers, namely, sensing of molecules and mixtures of gases that do not present well resolved 

vibrational spectra. 

5.4 MEASUREMENT OF FIRE SUPPRESSANT CONCENTRATION USING A 

DIFFERENTIAL INFRARED RAPID AGENT CONCENTRATION SENSOR 

(DIRRACS) 

5.4.1 Introduction 

In addition to the LIBS technique described earlier, the NGP developed a second technique for 

monitoring the concentration of a fire suppressant chemical during a rapidly extinguishment event. This 

non-dispersive apparatus, the Differential Infrared Rapid Agent Concentration Sensor (DIRRACS), 

employs a resistively heated source and directly measures the broad, fundamental absorption of the 

chemical. The design goal was an instrument with a response time of 3 ms that could be used with a 

variety of fire suppression agents over a concentration range from 1 % to 20 %, with an expanded 

uncertainty of approximately 10 % of the nominal value. Following an initial design, DIRRACS I, the 

design goal was met for the fire suppressant HFC-125 (C2HF5) with the improved DIRRACS II. 

5.4.2 Original Instrument Design – DIRRACS I 

Figure 5–16 is a schematic drawing of the DIRRACS I. Figure 5–17 depicts the optics. A detailed 

description of the instrument design and theory of operation is given by Pitts et al. at the National Institute 

of Standards and Technology (NIST).

39 Briefly, IR radiation from a coil heater source heated to 500 °C is 

directed through a 2.8 cm long sample volume. The transmitted beam passes through a long- wavelength 

cut-off filter to remove IR radiation with wavelength longer than 14 μm, through a chopper operating at 

500 Hz, a narrow-band-pass filter transmitting from 8.4 to 8.9 μm, to a LiTaO3 pyroelectric sensor. As 

shown in Figure 5–18, a narrow-bandpass filter was selected that overlaps with a strong absorption 

feature for HFC-125. There is also strong IR absorption, resulting from the C-F stretch vibrational 

modes, in the same spectral region for other agents with C-F bonds including HFC-227ea (C3HF7), FC218 (C3F8), halon 1301 (CF3Br), and CF3I. The pyroelectric sensor has a built-in FET amplifier. The 

output signal was increased with a 100x, low-noise amplifier; and frequencies below about 300 Hz were 

filtered out. 
Measurement of Fire Suppressant Concentration Using a Differential Infrared Rapid 

Agent Concentration Sensor (DIRRACS) 

503

Figure 5–16. Schematic of the DIRRACS I. 

fference. A computer-controlled calibration system 

(describ l

HFC-125 c tical path length, the instrument is sensitive over a mole 

fraction n

uncertainty

systematic difference between the measured and expected values and much larger noise when compared

.

Figure 5–17. Schematic of the Optical Design Used in DIRRACS I. 

The detector output was collected at 20 kHz using a digitizing card mounted in a PC and analyzed to 

determine the peak-to-valley (Pk-Vy) voltage di

ed ater) utilizing three mass flow controllers was used for calibrating the Pk-Vy signal versus 

oncentration. For the 2.8 cm op

ra ge from 0.01 to 0.25. While the calibration measurements indicated a nominal ≈ 10 %

(coverage factor k=2), actual tests using a transient agent release facility indicated a 
504 Instrumentation for Bench- and Large-scale Test Fixtures 

to the calibration measurements. Subsequent study indicated that the high flow velocity in the facility 

caused a change in the instrument response by affecting the source temperature and inducing vibrations in

the inst

radiation ro

surround th the effect of the flow on the source temperature. 

The second method was to use a second narrow-band-pass filter at a nearby, non-absorbing wavelength. 

rument. An additional limitation of the method was the high background signal from the IR 

f m the walls. Two approaches were used to address the problems. The first approach was to 

e IR source with a cooling coil to minimize 

In this case, it was expected that both signals would be affected in the same way by a change in the source 

temperature so that the ratio would be constant. Neither method solved the flow effect problem, though

the cooling coil approach provided some improvement, and neither method addressed the problems with

vibration or background radiation from the walls. 

Figure 5–18. Absorption Spectrum of HFC-125 Superimposed with the Transmittance 

Spectrum for the Bandpass Filter. 

Even with these limitations, useful data have been obtained using the DIRRACS I. Two models of the

DIRRACS I were used in the first measurements of the dispersal time of an agent in the dry-bay facility at 

Wright-Patterson Air Force Base.39 The preliminary measurements indicated a relatively long time, on the

order of a few seconds, for dispersal of the agent throughout the dry bay. This information has important 

implications about the mechanism of extinguishment when the fire is far from the release point of the 

agent. If the fire is extinguished under these conditions, the mechanism must not involve a direct

interaction of the agent on the fire, if one assumes the presence of the fire does not affect the distribution 

of the agent. These results showed the value of a fast response agent measurement. 
Measurement of Fire Suppressant Concentration Using a Differential Infrared Rapid 

Agent Concentration Sensor (DIRRACS) 

505

5.4.3 Improved Hardware Design – DIRRACS II 

The drawbacks of the DIRRACS I necessitated major modifications to the original design. The objectives 

were to eliminate the temperature and flow sensitivity of the source, detector, and other components; 

increase the signal-to-noise ratio; and increase the frequency response. Figure 5–19 is a schematic of the 

DIRRACS II design, and Figure 5–20 is a photograph of the breadboarded instrument as set up to 

measure a transient agent release in the Transient Application, Recirculating Pool Fire Apparatus 

(TARPF), described in Chapter 6 of this book. IR radiation is emitted by the source (cylinder with 

triangle on top at bottom central) toward an elliptical mirror, reflected through the chopper and pinhole, 

collimated by a parabolic mirror (behind chopper), through the periscope (attached to the duct at left), 

reflected by a 90° mirror and a focusing mirror, through a narrow-band-pass filter, and finally detected by 

a HgCdTe detector (under the plate with the hole). 

The key design changes in the DIRRACS II are:40

• The addition of a periscope, for reducing the high noise level of the DIRRACS I at velocity 

conditions of 10 m/s to 20 m/s. The periscope also allows the sampling volume to be 

completely is e flow and temperature 

variations at th tector. 

• Reduction in system vibrations by constructing the periscope assembly from standard 

stainless steel vacuum components. This structure is much more rigid than the previous 

optical housing. 

• Decrease the sample path length by about a factor of two, increasing the practical maximum 

mole fraction measurable by the instrument from about 0.2 to about 0.5. 

• Collimating the IR bean and focusing it through an aperture just before the chopper and using 

a faster IR detector (HgCdTe), to allow operation of the chopper at frequencies as high as 

4000 Hz, while the previous design with a large, unfocussed beam and slow detector was 

limited to about 400 Hz. This IR source also provided a more stable output with a higher 

intensity at the wavelength range of interest. The performance of the HgCdTe detector 

improves with frequency up to 4000 Hz, while the pyroelectric detector response falls off for 

frequencies above about 50 Hz. The tenfold increase in frequency corresponds to about a 

tenfold improvement in time response. The new detector also produces nearly square-wave 

output well above any background signal from extraneous radiation sources. The detector 

does require liquid N2 cooling, but one filling provides at least 2 h of stable operation. 

• Placing the IR source within the instrument housing, eliminating the problems of the high 

flow affecting the temperature of the source and the movement of the source. 

• The elliptical and parabolic mirrors were used to produce an 8 mm collimated beam, which 

tle (<1 %) 

to the IR radiation reaching the detector, while in the previous design as much as 50 % of the 

IR radiation originated from blackbody radiation from the walls. 

olated from the rest of the instrument, so convectiv

e measurement location cannot affect the source or de

was ultimately focused on the detector. The background IR radiation contributed lit
506 Instrumentation for Bench- and Large-scale Test Fixtures 

Figure 5–19. Schematic of DIRRACS II. 

Figure 5–20. Photograph of the DIRRACS II with the Periscope Mounted within the Flow

Channel of the TARPF Facility.
Measurement of Fire Suppressant Concentration Using a Differential Infrared Rapid 

Agent Concentration Sensor (DIRRACS) 

507

5.4.4 Data Acquisition and Analysis 

In addition to the hardware design improvements, improvements to data acquisition and analysis were 

also made. The old data acquisition board was limited to about 20 kHz sampling frequency. The new 

board allows variable data collection rates up to 250,000 samples/s. Most often, a setting of 

128,000 samples/s is used to allow data to be collected over a longer time period for a given level of data 

storage. Either an internal or external trigger can initiate data collection. The previous method of analysis 

involved postprocessing of the data, which required at least an hour. The current software combines the 

functions of acquisition and analysis. In conjunction with a faster computer and new data acquisition 

board, the software reduces the data and displays it within 20 s after acquisition. The software performs 

preliminary filtering using Fast Fourier Transform analysis to remove DC offset, drift, and 60 Hz noise. 

The software performs the filtering using digital lock-in amplification with the frequency determined by 

the chopper output and averages over a discreet and selectable number of cycles. 

5.4.5 

Calibrations

summary o onsists of 3 mass 

flow co o

board, an a

meter the ag stem. The programming prescribes at what point the fraction of agent in the 

mixture b

which i

The pro m

flows throu

the mixture

mixture flow

agent into th

into the two

the cell was

required ab

initial calibr ts an 

average 

particula c

combined standard uncertainty

shown i

into approxi

Calibration Experiments 

 of the DIRRACS II were conducted with a facility fully described in Pitts et al.39 A 

f the apparatus and procedure are presented here. The calibration facility c

ntr llers and various solenoid and pneumatic valves controlled with a computer, data acquisition 

d nalog/digital output board. A 2 L/min mass flow controller and a 10 L/min model are used to 

ent through the sy

 is est supplied through the higher capacity controller. The third controller is a 10 L/min model 

s dedicated to air flow. 

gra uses equations based on calibrations of each flow controller for agent or air to set the mass 

gh each controller to match the user input mass fraction of agent. The system output changes 

 at the calibration cell within seconds. The sealed cell with an inlet and outlet for agent/air 

 was attached to the DIRRACS II periscope around the sampling volume. Because leakage of 

e periscope through the window seals was suspected, 120 kPa (17 psi) pressure air was blown 

 periscope ends to prevent accumulation of agent which would impact the calibration. Since 

 not particularly designed for the DIRRACS II, it had a relatively large interior volume and 

out 1 min for a mixture to completely change to a new mixture input by the operator. This 

ation of the DIRRACS II for HFC-125 is shown in Figure 5–21. Each data point represen

 of data collected over 1 s. The standard deviations for 1 s samples of data generated with 

r alibration mixtures ranged between 0.1 % and 0.25 % of the averages. The expanded 

 (2 standard deviations) among the average peak-to-valley data points 

n Figure 5–21 is about 2 % of nominal for the calibrations that were conducted, which translates 

mately a 0.005 mole fraction uncertainty. 
508 Instrumentation for Bench- and Large-scale Test Fixtures 

Figure 5–21. Calibration plot of Average Normalized Peak-to-valley Signals vs. Volume 

Fraction of HFC-125. 

5.4.6 Release Tests of HFC-125 in the TARPF Facility

The NIST TARPF facility (Chapter 9) consists of a 2.5 m long duct of 9.2 cm x 9.2 cm square cross

section which is connected to a high capacity air compressor. Variable orifice plates allow the compressor 

to generate air velocities up to 20 m/s in the duct. Various agents can be stored in 1 L and 2 L cylinders. 

Before a test, a cylinder is filled to a prescribed pressure, which is measured with a pressure transducer 

inside the cylinder. A release is controlled with a timer that determines the length of time the cylinder 

solenoid valve is opened. The normal operation of the TARPF facility consists of igniting a flame in a test

section and releasing various amounts of agent upstream to observe the extinguishment behavior. For the

DIRRACS testing, no flame was ignited, but the air flow and agent release capabilities of the system were 

utilized. The periscope assembly was mounted 0.5 m downstream of the pool fire zone and about 2 m

downstream of the region where the agent is mixed into the air stream. 

Over 40 release tests were conducted in the TARPF Facility.

40 Each test was conducted with specified 

settings of air flow, agent vessel pressure, and agent vessel valve opening period. In order to capture 

ents of less than 1 s with the data acquisition system, varying combinations of total points collected and

mpling frequency were used. The series of tests with the DIRRACS II was used to help characterize the 

of pulses of agent as they moved downstream after each release. 

ev

sa

distortion 
Measurement of Fire Suppressant Concentration Using a Differential Infrared Rapid 

Agent Concentration Sensor (DIRRACS) 

509

T

involved a pulse of 100 ms durati ent pressure. The second was a 

250 ms pulse from a 330 kPa pressure. In both cases, the air velocity through the duct was 11 m/s. The 

n ng frequency was 128 kHz. 

tion normalized by the 

average of a 1 s sample of data was typically about 0.2 %. This is a substantial improvement over the 

iation of 23 % under the same conditions. 

ere from the right, high mounting position just behind the turret. There 

was only one DIRRACS II unit, so concentration was monitored at two locations in different tests. Figure 

ter 1.5 s the volume fraction is slightly higher at 

the lower position, 0.09 compared to about 0.07. After 2 min, the HFC-125 volume fraction was in the 

range 0.03 to 0.09 compared to an estimate of 0.095 based on a well mixed distribution throughout the 

volume of the vehicle. 

wo of the HFC-125 release tests that represent the extremes of the series are described here. The first 

on from a cylinder a 30 kPa initial ag

choppi g frequency was 2 kHz and the sampli

Figure 5–22 shows the recorded peak-to-valley voltage normalized by the background value, which is 

proportional to the IR intensity incident on the detector. The abrupt decrease in the voltage is a result of 

the IR absorption by the HFC-125. The width in the dip in the voltage is slightly larger than the release 

time because of downstream mixing effects. Figure 5–23 shows the calculated evolution of the volume 

fractions of the chemical. 

The different pulse durations are apparent, although the step releases become significantly spread out. The 

test data indicate that a 0.005 volume fraction of HFC-125 can easily be detected and differentiated from 

background noise. Also, turbulent fluctuations on the order of 5 ms to 10 ms are clearly resolved due to a 

combination of the 2 kHz chopping frequency and the Fourier Transform high frequency filtering. For 

neat air flowing past the periscope in the TARPF facility, the standard devia

DIRRACS I, which had a standard dev

5.4.7 HFC-125 Release Tests in a Bradley Vehicle 

A series of 11 agent release tests were carried out in a Bradley armored personnel carrier at the Aberdeen 

Test Center in order to characterize the DIRRACS II response to temperature and flow effects during 

transient agent concentration measurements.40 The vehicle is shown in Figure 5–24. The extinguishing 

agent, HFC-125, was released in amounts ranging from about 2 kg up to about 4 kg at a fill pressure of 

5.2 MPa. All of the releases w

5–25 shows the high location, which was near head height for an occupant in the left side of the vehicle, 

about halfway between the turret and the rear bulkhead. The instrument was located between two 

rectangular structures representing the location and approximate size of two occupants of the vehicle. 

The second, low location was at about waist height. 

The agent volume fraction and the cylinder pressure are plotted versus time for the DIRRACS II in the 

high position in Figure 5–26. The HFC-125 reached the measuring position 275 ms after the release. The 

peak agent volume fraction was 0.22, and decreased to a value of about 0.07 after 1.5 s. Before the agent 

arrival, the background volume fraction is close to zero and averages about 0.0006. After arrival of the 

agent, the volume fractions range between 0.05 and 0.21, which overwhelms by two orders of magnitude 

the background level and by at least a factor of 25 the effect of pressure or vibration noise. 

Figure 5–27 compares the HFC-125 concentration in the lower location to the same upper location result 

in the prior figure. In the lower position, the arrival time increases from 300 ms to about 400 ms, and the 

peak volume fraction decreases to about 0.12; however, af
510 Instrumentation for Bench- and Large-scale Test Fixtures 

Time, s

Figure 5–22. Normalized Peak-to-valley Signals Resulting from Processing the Raw

Signal from the DIRRACS II for Two Releases of HFC125. 

Time, s 

Figure 5–23. HFC-125 Volume Fraction vs. Time for Two Releases of HFC-125 in the 

TARPF facility.
Measurement of Fire Suppressant Concentration Using a Differential Infrared Rapid 

Agent Concentration Sensor (DIRRACS) 

511

performing agent 

Figure 5–25. Interior of the Bradley 

Figure 5–24. 

Bradley armored 

personnel carrier 

modified for 

release and fire 

suppression 

studies.

Vehicle Showing the periscope of the 

DIRRACS II in the “high” location. The 

Two Rectangular Structures Simulate 

Occupants. 
512 Instrumentation for Bench- and Large-scale Test Fixtures 

Figure 5–26. HFC-125 Volume Fraction and Cylinder Pressure vs. Time at the Upper 

Figure 5–27. HFC-125 Volume Fractions vs. Time At the High and low Positions During 

Two Different Releases. 

Measurement Position. 
Conclusion 513

5.4

DIRRACS II meets the design needs for monitoring halon replacement chemicals: 

• Response time: The instrument monitored agent releases with a 4 ms time response and can 

capture the structure of distinct events that are less than 10 ms in duration. 

• Dynamic range: The instrument measured HFC-125 volume fractions from 0.01 to 0.25 with

an expanded uncertainty of 0.005. 

• Versatility: The DIRRACS II can be used for any agents with the C-F bond using the current 

filter or replacing it with one more closely matched to an agent’s absorption spectrum.

• Ruggedness: The instrument is portable and withstood the vibration from the agent release.

The DIRRACS II could be useful for monitoring agent concentration in a fire environment 

provided some protection is added to shield the periscope and other exposed components 

from the corrosive gases produced by decomposition of the agent in the fire. 

It was significant that the measured times for the HFC-125 to reach the measuring positions were longer 

than the desired time to extinguish the fire. This information could be useful for understanding the 

mechanism of fire suppression for the fire tests performed in the Bradley vehicle. 

d

gen,

hy Full

im

rem

nt in 

.8 Assessment

The design is ready to be adapted for reproduction or commercialization. A manual41 has been prepare

to assist in the fabrication of copies of DIRRACS II and to provide operation instructions to users. 

5.5 CONCLUSION 

Under the NGP, researchers at the Army Research Laboratory and the National Institute of Standards and

Technology have developed techniques for measuring the concentrations of fuel vapors, oxy

drogen fluoride, and fire suppressants during real-scale, high-speed fire suppression tests.

plementation of all of these techniques into routine testing has not yet been realized, but the discrete

aining technological barriers to this have been identified, showing the way to full measurement 

capability in future fire tests. 

The use of these instruments has already produced insights into the fluid dynamics of suppressant release 

and distribution, the potential for flame re-ignition, and the advantage of rapid flame extinguishme

containing the concentrations of noxious combustion products, all under realistic fire conditions. 
514 Instrumentation for Bench- and Large-scale Test Fixtures 

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