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

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