the next generation of tdlas analyzers - physical sciences inc
TRANSCRIPT
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
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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
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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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~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 – 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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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