Physical Sciences Inc. 20 New England Business Center Andover, MA 01810

VG07-187

The Next Generation of TDLAS Analyzers SPIE Paper 6765-5

M.B. Frish, M.C. Laderer, R.T. Wainner, A.O. Wright, A.H. Patel, J. Stafford-Evans,

J. R. Morency, M.G. Allen, B.D. Green

Physical Sciences Inc. 20 New England Business Center

Andover, MA 01810

Optics East 2007Boston, MA

10 September 2007

VG07-187-1

TDLAS

• It is highly-selective; generally insensitive to cross-species interference

• It is highly-sensitive, offering sub-ppm detection of many gas species– Wavelength Modulation Spectroscopy (WMS) and Balanced Ratiometric

Detection (BRD) techniques reject noise to enhance sensitivity

• It is fast, offering sub-second response time

• It is configurable as a point, open-path, or standoff sensor

• It is non-contact; only the probe beam needs to contact the analyte– Myriad analyte sampling approaches exist

Tunable Diode Laser Absorption Spectroscopy (TDLAS) is an optical method for detecting trace concentrations of one or more selected gases mixed with other gases

• Over the past decade, TDLAS has evolved from a laboratory specialty to rugged, reliable commercial industrial instrumentation

VG07-187-2

Absorption Spectroscopy Fundamentals

• Gas molecules absorb light at specific colors (“absorption lines”)

Beer-Lambert law

Iν = Iνo exp [S(T) g(ν - νo) Nl]

where:ν = optical frequency (= c/λ)νo = line center frequency

g(ν) = lineshape function l = path length

N = absorbing species number densityS(T) = temperature dependent linestrength

Iνo = unattenuated laser intensityIν = laser intensity with absorption∆I = change in intensity (= Iν0-Iν)c = speed of lightλ = wavelength of light

Absorbance = - ℓn (Iν /Iνo)

≈ ∆I/Iν0 ( with small ∆I)

2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018-0.001

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

Abs

orba

nce

Wavelength (nm)

CO2

Water

296 K, 1 atmAtmospheric Absorption (1 meter path length)

H-9703

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TDLAS System Components

• Robust TDLAS systems typically utilize near-infrared diode distributed feedback (DFB) lasers that operate at room temperature

– these are the same type of solid state lasers that are utilized for long-distance, high-speed telecommunications

• Emerging mid-IR lasers expand the range of detectable gases

Scan

Generator

LaserCurrent

Controller

LaserTemperature

Controller

LaserTransmitter

OpticalFiber

Laser BeamLaunch Optics

Laser BeamReceive Opticsand Photodetector

ElectricalWires

MeasurementPath

(a) Optional Noise ReducingOptical Comparison Signal (BRD)

(b) Optional Noise ReducingElectronic Comparison Signal (WMS)

SignalProcessor,

Display,Communications

E-5711a• Wavelength Modulation Spectroscopy (WMS) • Balanced Ratiometric Detection (BRD)

Reference Signal for Noise Reduction

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Wavelength Modulation Spectroscopy (WMS)

• Laser wavelength scans repeatedly across absorption line unique to target gas

– yields amplitude modulation at 1f (ωm)• Laser absorption by target gas produces amplitude modulation at 2f (2ωm)• Lock-in detection of 2f and 1f (rejects noise contributions at other

frequencies)– 2f signal normalized by 1f to yield path-integrated concentration

measurements independent of return signal strength

• Noise Equivalent Absorbance typically 1 – 10 x 10-5

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Practical Detection Limits for Some Gases Measured with Near-IR TDLAS

0.2C2H25.0H2CO

50.0O20.2HCl0.2NO21.0CH4

30.0NO1.0H2O40.0CO25.0NH3

40.0CO20.0H2S1.0HCN0.2HF

(ppm-m at 1 atm)

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Portable Standoff TDLAS:The Remote Methane Leak Detector (RMLD)

• Developed for manual pipeline leak surveying

• Like a flashlight, laser beam Illuminates a surface up to 100 ft distant

• Senses target gas between surveyor and illuminated surface

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RMLD Internals

• Single-Board Control Platform– Complete WMS system

• 10 kHz modulation– incorporates laser control and data processing on

battery-operated board– digital signal processor for high-speed data acquisition

and processing– embedded microcontroller for laser operation, data

reduction, communication– Serial (RS-232) data output stream and setup interface – SPI communication available for interface with other

microcontrollers

• Transceiver– lightweight, compact, rugged handheld unit– co-linear laser transmitter and receiver– rejects sunlight– integrated visible pointing laser

• User Interface – visual:

• LCD display in controller unit– audio

• variable tone: frequency = 10 x methane concentration• fluctuation algorithm: leaks indicated by rapid concentration changes

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Mobile RMLD Concept

• For city street and sidewalk survey• Side facing version of RMLD

– pan and tilt capability• Co-aligned camera to capture leak

area• Rangefinder to compensate for

ambient methane• Compatible with GPS and GIS

G-7699

RMLD View

LRF Spot

Gas Cloud

Camera Scene

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Mobile Standoff Sensor Prototype

• For natural gas leak survey, law enforcement and security applications

0

5000

10000

15000

20000

25000

30000

0 20 40 60 80 100Time (min)

Inte

grat

edC

once

ntra

tion

(ppm

-m)

G-799

1 3 4 5 P1 P2 6 P3 P4

• Data illustrates driving along Virtual Natural Gas Distribution Pipeline at Rocky Mountain Oilfield Test Center (RMOTC)

• Simulated leaks at locations over 7+ mile path

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User Interface

• Laptop Computer Interface (installed in vehicle cab) provides:

– Window showing the video image– A moving graphical display of

methane concentration history– Joystick for manual control of

pan & tilt– USB Input Ports for RMLD and

rangefinder readings– Software synthesis of rangefinder

and RMLD data– Calculation and display of range-

corrected RMLD concentration measurement

J-2267

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mRMLD Example Data

• Three successive passes by leak– 10 scfm, 5 mph, 27 – 33 ft

• This size leak is easy to identify with a simple threshold alarm

0

1000

2000

3000

4000

5000

6000

7000

8000

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0Time (0.1 s)

Concentration (ppm-m)

Aim mRMLDat leak

Drive past leak Driveby1 060821

J-2270

VG07-187-12

Airborne Standoff TDLAS

• Combining EDFA and WMS provides long-range, robust and modest cost standoff sensor

Powerswitch

Pump Laseroutput(SM fiber)

EDFA

Powerswitch

Inputpower (dc)

PCB

Data output(RS-232)

TDL

Laseroutput(SM fiber)

CONTROLLERPre-Amp /Detector

GPS

COMPUTER

Inputs:

Data GPS Video

TRANSCEIVER

CAMERA

Pump

EDFA

Powerswitch

Inputpower (dc)

Data output(RS-232)

TDL

CONTROLLERPre-Amp /Detector

GPS

COMPUTER

Inputs:

Data GPS Video

Pump

EDFA

Seed laserinput (SM fiber)

Inputpower (ac)

Pump

EDFA

Inputpower (dc)

Data output(RS-232)

TDL

CONTROLLERPre-Amp /Detector

Inputpower (dc)

Data output(RS-232)

TDL

CONTROLLERPre-Amp /Detector

GPS

COMPUTER

Inputs:

Data GPS Video

COMPUTER

Inputs:

Data GPS Video

TRANSCEIVER

CAMERA

H-5620a

Remove EDFA forlow-altitude (<100ft) survey

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aRMLD Example Data

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~0.05 m≤ 35 m

Transceiver

360 ppm x 35 m = 12,600 ppm-m 3% x 0.05 m = 1500 ppm-m

Breath Plume 3% CO2Concentration

Ambient Air:360 ppm CO2 Concentration

H-9697

Breath Detection by Standoff TDLAS

• Standoff detection of breathing patterns provides a tool for spotting, from a distance, individuals presenting abnormal emotional states due to:

– concealing weapons– posing safety or security threats, or – attempting subterfuge and deception during interrogation– injury

• Detect CO2 as marker for breath– breath plume provides easily measurable increase in CO2 above ambient contribution– 100 ft standoff range

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4 Meter Standoff Demonstration

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4 Meter Standoff – Facial Illumination

01:20 01:40 02:00 02:20 02:40 03:001000

1500

2000

2500

Normal Breathingthrough Nose

Talking, andChange in Position

Mouth

TalkingTalking Release

BreathHoldBreath

Normal Breathingthrough Nose

Con

cent

ratio

n(p

pm-m

)

Time (min:sec)

Rapid/ShallowBreathingthrough Nose

~ 900 ppm-mmagnitude

H-9706a

Panting through

~ 150 ppm-m

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35 Meter Standoff - Cardboard Target Illumination

• White cardboard surface positioned between 20 and 35 meter range

– back wall at 40 meters sampled between each cardboard measurement

– 5x breaths across surface at each position starting ~5-10 seconds into the sampling period

• Increase in detected CO2 with distance generally adheres to expectation

– wall samples indicate steady levels for fixed 40 meter distance

0 50 100 150 200 250

6000

9000

12000

15000

B

B

B

WWWW

Con

cent

ratio

n(p

pm-m

)

Time (sec)

35 m

30 m

25 m

20 m

W

W: Wall at 40 mB: Breaths across surface

B

~ 700 ppm-mMagnitude

H-9709

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Compact Configurable Standoff Sensor Heads

Standard Transceiver ModuleSampling Section

Laser launchMirror

Detector

~ 20 cm

~ 8 cm~ 2 cm

Gas inlet

Gas outlet

Sampling Section

Laser launchMirror

Detector Window

~ 20 cm

~ 8 cm~ 2 cm

Configurable and ReplaceableMeasurement Module

ExampleSampling Section

J-4919

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Application as In-situ Sensor

• Example uses– O2 analysis within fuel or chemical storage tanks for explosion protection– Measurement of contaminants or reactive gases (e.g H2O, HCl) in pipelines or

reactive chambers (e.g CVD) for quality control• Demonstrated operation over process and ambient temperatures of

-50 to +65C

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Application as Non-Contact Process Analyzer

Example Method of Use – Fuel Cell Gas Analysis• Position sensor head alignment tube against window• 1 mm diameter laser beam enters fuel cell channel

through window• Laser beam scatters from surface at channel rear

Sensor head collects scattered light and focuses it on a photodetector

• Control Unit interprets photodetector signals and computes path-integrated concentration

• Data displayed on personal computer

• Compact Sensor Head for standoff detection of process gases observed through a single window

• True non-contact method

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Measurement Examples

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Summary

• Commercial TDLAS sensors are now accepted as rugged, reliable industrial gas analyzers

• Battery-operated, hand-portable Standoff (aka Diffuse Backscatter) TDLAS sensors are gaining acceptance for natural gas pipeline leak surveying and other standoff detection applications– Vehicle and aircraft mounted leak survey versions currently in field trials

– Standoff breath sensing demonstrated

• Compact standoff sensor heads combined provide true non-contact sensing of processes observed through a single window

• Configurable compact low-cost standoff sensor heads simplify adapting TDLAS to a wide variety of applications