laser based sensor for detection of trace gases in the environment m. a. gondal, i. a. bakhtiari and...
TRANSCRIPT
LASER BASED SENSOR FOR DETECTION OF TRACE GASES
IN THE ENVIRONMENT
M. A. Gondal, I. A. Bakhtiari and Z. H. YamaniLaser Research Laboratory & Physics Department
King Fahd University of Petroleum & Minerals
Gondal
Bakhtiari
Me!!
Plan
1. Intro.: What is photoacoustic spectroscopy
2. Modifying the PA set up
3. Results for Ethylene
4. Future work
5. Conclusion
Conventional Spectroscopy
Principal:Principal: Energy absorbed is determined by the difference (Io - I) / Io.
DetectorSampleLight Source
Io I
Introduction:What is photoacoustic spectroscopy
Photoacoustic Spectroscopy (PA)
Principal: Principal: EnergyEnergy absorbed by the sample produces sound waves which are detected by sensitive microphone. The name PA is due to “Light Generated Sound”
Advantages:Advantages:Sample may be gaseous, liquid, solid opaque, powdered.Highly sensitive.
Light Source
Chopper Sample
Microphone
LASER RADIATION
EXCITATION
HEAT
(VIBRATIONAL)(ELECTRONIC)EMISSION CHEM. REACTION
RADIATION EXPANSION PHASE TRANSITION
SOUND WAVES
vent
Photoacoustic spectroscopy system using chopper for modulation
Modifying the PA set up
Stepping
Motor
P. Gauge
Valve
Vacuum Pump
Power Meter
Air FilterValve
LensTunable CO2 laser
Beam Splitter
Photoacoustic cell
MicrophoneGrating
`
Spectrum Analyzer
Newport micro stepping motor
grating controller
Variable Gain Low Noise Microphone
Amplifier
CO2 Laser Modulator. Variable
duty cycle (TTL)
Resonant Photoacoustic Cell developed at KFUPM
G a s In le t G a s O u tle t
M icro p h o n eB u ffer V o lu m e
A co u stic F iltersW in d o w s
2122
,, ])()[(2
v
L
n
Rzmn
nmn z
R = 8 mm
L = 160 mm
It turns out that: 0,0,1 ~ 3400 Hz
Don’t worry about higher resonance frequencies, because they are beyond microphone response limit
Noise Level with electronic Modulation
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
2000 2500 3000 3500 4000 4500 5000 5500 6000
Frequency (Hz)
PA
Sig
na
l (a
rb. U
nit
s)
Noise Level recorded with chopper
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
2000 2500 3000 3500 4000 4500 5000 5500 6000
Frequency (Hz)
PA
Sig
na
l In
tes
ity
(a
rb. u
nit
s)
Noise level in PA cell, filled
with pure nitrogen
Results for Ethylene
1 3026 cm-1 CH2 stretch2 1623 cm-1 CC stretch3 1342 cm-1 CH2 scissor4 1023 cm-1 CH2 twist5 3103 cm-1 CH2 stretch6 1236 cm-1 CH2 rock7 949 cm-1 CH2 wag8 943 cm-1 CH2 wag9 3106 cm-1 CH2 stretch10 826 cm-1 CH2 rock11 2989 cm-1 CH2 stretch12 1444 cm-1 CH2 scissor
Fundamental frequenciesfor Ethylene (C2H4)
Molecular weight 28.05 amuFreezing point -169 CBoiling point -104 C
10.2 10.3 10.4 10.5 10.6 10.70
20
40
60
80
100
120
10.6
9610
.675
10.6
5310
.632
10.6
1110
.591
10.5
7110
.551
10.5
3210
.513
10.4
94
10.2
8910
.274
10.2
4710
.233
10.2
2
10.1
95
lase
r po
wer
(re
lativ
e)
wavelenght (m)
Spectral distribution of CO2 linesUse the 10 P(14) of CO2
Time domain PA signal of C2H4
-40
-30
-20
-10
0
10
20
30
40
25487 25487.5 25488 25488.5 25489 25489.5 25490
Time (msec)
Am
plit
ud
e (a
rb. u
nit
)
FFT Spectra Ethlene with mechnical chopper
0
5
10
15
20
25
30
35
3000 3100 3200 3300 3400 3500 3600 3700
Frequency (Hz)
PA
Sig
na
l (a
rb. u
nit
s)
300 ppbV of ethylene under 1000 mbar of
nitrogen
Mechanical (chopper) Modulation
FFT Spectra of Ethlene with electronic moduloation
0
10
20
30
40
50
60
70
80
90
2500 3500 4500
Frequency (Hz)
Sig
na
l in
ten
sit
y (
% f
ull
sc
ale
)
Time domain spectra using electronic modulation
-100
-80
-60
-40
-20
0
20
40
60
80
100
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time (msec)
Am
pli
tu
de
(a
rb
. u
nit
s)
Electronic Modulation
300 ppbV of ethylene under 1000 mbar of
nitrogen
Chopper Modulated Spectra of Ethylene
0
5
10
15
2800 3000 3200 3400Frequency (Hz)
PA
Sig
nal
(ar
b.
Un
its)
Spectra of Ethylene with Electronic Modulation
0
20
40
60
80
2800 3000 3200 3400Frequncy (Hz)
PA
Sig
nal
(ar
b.
Un
its)
Calibration Curve for Ethylene
R2 = 0.9967
0
5
10
15
20
25
0 50 100 150 200 250 300 350
Concentration (ppbv)
PA
Sig
na
l In
ten
sit
y (
arb
. un
its
)
PA Signal vs. Laser Power
R2 = 0.9979
0
2
4
6
8
10
12
14
16
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Laser Power (watt)
PA
Sig
na
l (arb
. Un
its
)
1- SABIC funded project on detection of ozone.
2- Improve sensitivity.
3- Go out to the field
Future work
This is how the lab looks when we are
there!!!Domestically built CO2 laser
1- The PA system is up and running with good dynamic range and sensitivity for the detection of C2H4.
2- We have a plan for future work, including detection of ozone.
3- We are open for ideas and need your support (esp. graduate students).
Conclusion
KFUPM is gratefully acknowledged for supporting this work
Thank you for your attention
.. بركاته و الله رحمة و عليكم السالم و
Stepping
Motor
P. Gauge
Valve
Vacuum Pump
Power Meter
Air FilterValve
LensTunable CO2 laser
Beam Splitter
Photoacoustic cell
MicrophoneGrating
`
Spectrum Analyzer
Newport micro stepping motor
grating controller
Variable Gain Low Noise Microphone
Amplifier
CO2 Laser Modulator. Variable
duty cycle (TTL)