Low-power Portable Laser Spectroscopic Sensor for Atmospheric CO2 Monitoring
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Low-power Portable Laser Spectroscopic Sensor for Atmospheric CO2 Monitoring

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Low-power Portable Laser Spectroscopic Sensor for Atmospheric CO2 Monitoring

Low-power Portable Laser Spectroscopic Sensor for Atmospheric CO2 Monitoring

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  • The goal of our project is to develop CO2 sensors that can be deployed into a network in the field for real-time monitoring of carbon flux over a large geographic area. The outline of this talk is as follows: I’ll talk about the requirements for a sensor to be used in such a network, I’ll overview our sensor design by talking about the optics and electronics, I”ll review sensor performance tests of SNR and Allan variance, and finally I’ll review different lab and field tests we performed in collaboration with our colleagues at JHU.
  • A sensor must ideally have all of these features to be used in a trace gas sensor network.
  • We are using TDLAS spectroscopy for CO2 detection. The optical path is 3.5 m within a Herriott multi-pass cell. The sensor uses a 2um VCSEL as the laser source and an off the shelf InGaAs photodetector. The multi-pass cell components are mostly stock commercial parts with custom Al adapters for housing the laser and detector. All of these parts are easily bought/made and assembled. The VCSEL is in a TO5 can package and has its own TEC for temp control. The detectors is in a TO18 package and is mounted on a custom PCB with an integrated pre-amp. Both the laser and the detector interface with a custom electronics board that does all the laser control and data acquisition – based on the openPhotons platform. The custom electronics board mates with a commercial wireless card. Another wireless card is plugged into a computer and the two communicate through a wireless link. All of the electronics (including real-time wireless transmission) consume approximately 300 mW of power. The system is run off of a 10Ah Li-ion polymer battery that will last for approx. 100 hours in this manner. For fieldwork, the Herriott cell and electronics were mounted inside a water-tight NEMA enclosure. The total size of the system is about that of a shoebox. In this case, a pump is used to pump outside air (that is passed through a desiccant) into the chamber and then back out. This allows for sampling of environmental CO2 while keeping the electronics protected from humidity/water/dew. With the pump running, the battery will power the system for about 10 hours.
  • Our custom control and acquisition board developed by Dr. Stephen So provides all the functionality for controlling the VCSEL and processing the data from the detector. The board is designed to communicate with a Telos mote via the standard UART protocol. Updated control board with all of the functionality integrated.TEC driver provides 0.001C stability, precision modulation frequency to match to photoacoustic or faraday rotation magnetic coils
  • The VCSEL is wavelength modulated at 10 kHz at a modulation depth that corresponds to optimum SNR when modulating over the absorption line. (amplitude/HWHM = 2.2). The 1st, 2nd, & 3rd harmonic line profiles are measured by temperature scanning about the 4987 cm-1 absorption line and a lock-in amplifier was used to select each harmonic. Harmonic SNR measurement of a calibrated 285 ppm CO2 in N2 mixture produced 1F SNR of 3247, 2F SNR of 2530, & 3F SNR of 1052.
  • The VCSEL is wavelength modulated at 10 kHz at a modulation depth that corresponds to optimum SNR when modulating over the absorption line. (amplitude/HWHM = 2.2). The 1st, 2nd, & 3rd harmonic line profiles are measured by temperature scanning about the 4987 cm-1 absorption line and a lock-in amplifier was used to select each harmonic. Harmonic SNR measurement of a calibrated 285 ppm CO2 in N2 mixture produced 1F SNR of 3247, 2F SNR of 2530, & 3F SNR of 1052.
  • Allan variance tests were performed on the long term 2nd harmonic measurements to assess the long term stability of the sensor. Two cases were test. 1st the temperature set point set to line center and the system was allowed to free-run. The ultimate minimum detectable absorption in this case was 1.4E10-6 at 25 seconds. Next, 3rd harmonic line locking was used. This is done by using a feedback loop to cause the electronics board to change the VCSEL temp such that the 3F signal is nearest zero. This corresponds to the maximum of the 2F signal. In this case, much better stability was achieved. We saw Gaussian white noise performance up to 100 seconds and ultimate minimum detectable absorption of ~6E-7 at 100 seconds. Our sensitivity with 1 sec. averaging is 0.113 ppm which corresponds to a minimum detectable absorption of 7.4E-6 at 1 sec.
  • The TDLAS CO2 sensor was placed in an environmental chamber with a commercial sensor which uses NDIR (nondispersive infrared sensor). The temperature was set to 0C and the CO2 concentration was ramped up and down by ~100 ppm. The TDLAS sensor performed exactly as the TDLAS (in terms of precision, not accuracy). Additionally, another lab test of measuring soil respiration over time was performed by the TDLAS sensor. In this case the sensor showed good responsivity to a larger change of CO2 concentration and reported a CO2 concentration increase slope consistent with soil respiration measurements. NEED REFERENCE FOR THIS…
  • Next, the TDLAS CO2 sensor was used in an experiment to detect CA isopod respiration. 10 isopods were placed into a test tube which was a part of a closed path system containing the CO2 sensor. During the course of this experiment CO2 out-gassing was observed. This is likely due to CO2 being trapped in the desiccant. Nevertheless the CO2 out-gassing rate was constant over several measurements, so it was used as a basline. When the isopods were placed in the test tube several times for different measurements of CO2 concentration increase. When this data was compared to the baseline, an increase in CO2 concentration was observed. The isopods were detectable after 2 minutes of CO2 concentration increase. This corresponds with a detected rate of increase of approx. .021 ppm/sec.
  • The sensor was then taken out in the field to the Smithsonian Environmental Research Center where measurements to study forest floor respiration were being conducted. The ambient temperature was approximately 0-5C. The first observation that was made was that the repeated thermal cycling from 25-30C to 0C caused some beamwalk in our sensor thus affecting accuracy. Nevertheless, the CO2 sensor still performed identically in taking measurements of both a control area of random forest floor composition and of measuring the respiration of an area with only Tulip Poplar leaves. Another observation was that the NEMA enclosure limited the sampling time response of the CO2 sensor. In this case it took up to 10 minutes to fully flush the container. Depending on the application, this may not matter. Nevertheless we are looking at the prospects of using an open path system to get real time sampling responsivity.Beech is otherOther slope

Transcript

  • 1. CLEO/QELS
    May 20, 2010
    Low-power Portable Laser Spectroscopic Sensor for Atmospheric CO2 Monitoring
    Clinton J. Smith1, Stephen So1, Lijun Xia2, Scott Pitz2, Katalin Szlavecz2, Doug Carlson3, Andreas Terzis3, and Gerard Wysocki1
    Dept. of Electrical Engineering, Princeton University, Princeton, NJ 08544
    2. Dept. of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, MD 21218
    3. Dept. of Computer Science, The Johns Hopkins University, Baltimore, MD 21218
    pulse.princeton.edu
  • 2. Project Goal & Outline
    The project goal:
    • Develop CO2 sensors for deployment as a real-time sensor network for carbon flux monitoring over a broad geographic area.
    • 3. Atmospheric monitoring of CO2 (fluxes, sources, and sinks)
    • 4. Soil Respiration Monitoring
    Outline
    • Requirements for a sensor to be used in trace gas sensor networks
    • 5. Overview of sensor design
    • 6. Overview of control and acquisition electronics
    • 7. Selection of laser & CO2 absorption line
    • 8. Sensor performance tests
    • 9. Lab & Field tests
    http://www.coas.oregonstate.edu/research/po/satellite.gif
    2
  • 10. Requirements for Trace Gas Sensor Networks
    A trace gas sensor for networks must provide:
    • Small size/portability
    • 11. Low unit/capital cost
    • 12. Low maintenance and operating costs
    • 13. Robust construction
    • 14. Low power consumption
    • 15. High sensitivity (ppb)
    • 16. High selectivity to trace gas species
    • 17. Wireless networking capability
    • 18. Ease of mass production
    3
    Sensors work autonomously in the field
    Base Station
    Radio Range
    Sensors
  • 19. CO2 Sensor Design & Specifications
    4
    • Tunable diode laser absorption spectroscopy (TDLAS)
    • 20. Housed within a NEMA enclosure for environmental protection
    • 21. Desiccant used to prevent condensation
    • 22. 3.5 m path Herriottmulti-pass cell
    • 23. 2 μm VCSEL & InGaAsphotodetector
    • 24. Custom electronics board (openPHOTONSplatform*)
    • 25. Powered by an integrated 10 Ah Li-ion polymer battery
    • 26. Works for 10 hours with pump/100+ hours without pump
    • 27. 300 mW power consumption without pump
    Detector
    Laser
    CO2
    Controlling Electronics
    * www.openphotons.org
  • 28. Custom Control and Acquisition Board
    Direct Digital Synthesizer
    TEC driver
    MCU
    8MHz
    Modulated Current Driver
    Lock-In Amplifier + Front End
    www.openphotons.org
    So, S., Sani, A. A., Zhong, L., Tittel, F., and Wysocki, G. 2009. Demo abstract: Laser-based trace-gas chemical sensors for distributed wireless sensor networks. In /Proceedings of the 2009 international Conference on information Processing in Sensor Networks/ (April 13 - 16, 2009). Information Processing In Sensor Networks. IEEE Computer Society, Washington, DC, 427-428
    5
  • 29. 2 μm VCSEL & CO2 Absorption Spectrum
    6
    • Low power VCSEL
    • 30. Consumes ~5 mW power
    • 31. VCSEL temperature tuning range of ~5 cm-1
    • 32. Absorption coefficients in this range correspond to ~1% absorption over 3.5 m path
    • 33. Choose 4987 cm-1 absorption line for line-locking
    • 34. Best SNR within theVCSEL drive current and temperature
    • 35. Low interference from H2O lines
    4987 cm-1
    P=1 atm
    Atmospheric
    Concentration,
    HITRAN/GEISA
    Water absorption lines have limited impact on CO2 absorption lines
    Source: HITRAN 2000 database
  • 36. TDLAS CO2 Sensor In-Lab Performance
    7
    • VCSEL is wavelength modulated at 10 kHz
    • 37. Via current modulation
    • 38. 2nd harmonic peak value will be used for CO2 concentration measurement
    • 39. Modulation depth of ~0.22 cm-1 is optimized for the 2nd harmonic
    • 40. Harmonic line profiles are measured by temperature scanning about the 4987 cm-1 absorption line
    • 41. A lock-in amplifier is used to select each harmonic
    • 42. Calibrated 285 ppm CO2 in N2 mixture yields
    • 43. 1st harmonic SNR of 3247
    • 44. 2nd harmonic SNR of 2530
    • 45. 3rd harmonic SNR of 1052
  • TDLAS CO2 Sensor 3rd Harmonic Line Locking
    8
    • Control laser temperature so that 3rd harmonic signal is near zero
    • 46. This corresponds to the maximum of the 2nd harmonic signal
    Measure the CO2 concentration by continuously monitoring the 2nd harmonic signal value at the peak
  • 47. TDLAS CO2 Sensor Long Term Stability
    9
    1.4x10-6minimum absorption
    5.1x10-7minimum absorption
    • Allan variance at constant temperature setting shows 1.4x10-6 ultimate minimum detectable absorption at 25 seconds
    • 48. Drift dominates beyond this time
    • 49. Allan variance with 3rd harmonic line locking to CO2 absorption line at 4987 cm-1 showed:
    • 50. Gaussian noise performance up to 100 seconds
    • 51. Sensitivity of 0.113 ppm in 1 second averaging time
    • 52. Minimum detectible absorption of 7.4x10-6in 1 second
    • 53. Ultimate minimum detectable absorption of 5.1x10-7 has been achieved with 100 second averaging
  • In-Lab Tests: TDLAS CO2 Sensor Measurement of Changing CO2 Concentrations
    10
    • TDLAS sensor measurements were compared with measurements of a commercial sensor
    • 54. Testing at 0 °C shows similar behavior between TDLAS and commercial sensors
    • 55. TDLAS & commercial sensor: R2 = 0.9964
    • 56. Commercial sensors compared to each other: R2 = 0.9606 - 0.9956
    • 57. Soil respiration over time
    • 58. Soil CO2 respiration at room temperature was measured to have a typical concentration increase slope of 0.24 ppm/sec
  • In-Lab Tests: TDLAS CO2 Sensor Measurement ofCalifornia Isopod Respiration
    11
    • A test tube is used to hold 10 California isopods in a closed path system with the TDLAS CO2 sensor in-line
    • 59. CO2 out-gassing is observed in control sample
    • 60. Likely from desiccant
    • 61. Repeatable out-gassing rate
    • 62. Isopod signal compared against CO2 out-gassing background shows increase in CO2 concentration
    • 63. Isopods detectable after ~2 minutes
    • 64. Approximately 0.021 ppm/sec CO2 concentration increase
  • Field Tests: TDLAS CO2 Sensor Measurement ofForest Floor Respiration
    • Soil respiration measurements were performed at the Smithsonian Environmental Research Center
    • 65. Repeated thermal cycling introduced beam walking error
    • 66. TDLAS and commercial sensor produced nearly identical measurements in the control area with random foliage makeup
    • 67. In an area with just Tulip Poplar leaves, TDLAS and commercial sensor measured soil CO2 respiration slopes of 0.18 ppm/sec. and 0.19 ppm/sec, respectively
    • 68. Random foliage area R2 = 0.8930; Tulip Poplar leaves area R2 =0.9516
    12
  • 69. Conclusion and Future Work
    13
    • A portable, wireless, low-power CO2 sensor based on TDLAS with a custom Herriott multi-pass cell was demonstrated to have:
    • 70. 0.113 ppm sensitivity with 1 second averaging
    • 71. Gaussian noise performance to 100 seconds
    • 72. Ultimate minimum detectable absorption of ~5.1x10-7
    • 73. Typical 300 mW power consumption
    • 74. Real-time transmission of spectroscopic data
    • 75. Lab and field performance tests compare well with commercial sensors
    Future Improvements
    • TDLAS sensor affected by thermal drift
    • 76. Currently developing and implementing an improved optomechanical design to ensure high thermal stability
    • 77. NEMA enclosure has reduced sensor responsivity to concentration changes
    • 78. Currently investigating the viability of an open path system for better sampling responsivity
    • 79. Further field testing of sensors to ensure reliability
    • 80. Sensor network for carbon flux measurements
  • Acknowledgements
    14
    This work was sponsored in part by:
    The National Science Foundation’s MIRTHE Engineering Research Center
    An NSF MRI award #0723190 for the openPHOTONSsystems
    An innovation award from The Keller Center for Innovation in Engineering Education
  • 81. Questions?
    15