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
- 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
- 3. Atmospheric monitoring of CO2 (fluxes, sources, and sinks)
- 5. Overview of sensor design
- 6. Overview of control and acquisition electronics
- 7. Selection of laser & CO2 absorption line
- 9. Lab & Field testshttp://www.coas.oregonstate.edu/research/po/satellite.gif 2
- 12. Low maintenance and operating costs
- 16. High selectivity to trace gas species
- 18. Ease of mass production3 Sensors work autonomously in the field Base Station Radio Range Sensors
- 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 pumpDetector 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
- 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 lines4987 cm-1 P=1 atm Atmospheric Concentration, HITRAN/GEISA Water absorption lines have limited impact on CO2 absorption lines Source: HITRAN 2000 database
- 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
- 46. This corresponds to the maximum of the 2nd harmonic signalMeasure the CO2 concentration by continuously monitoring the 2nd harmonic signal value at the peak
- 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
- 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
- 59. CO2 out-gassing is observed in control sample
- 62. Isopod signal compared against CO2 out-gassing background shows increase in CO2 concentration
- 63. Isopods detectable after ~2 minutes
- 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 =0.951612
- 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
- 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
- 81. Questions? 15
Hinweis der Redaktion
- 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