Low-power Portable Laser Spectroscopic Sensor for Atmospheric CO2 Monitoring

Clinton J. Smith1, Stephen So1, Lijun Xia2, Scott Pitz2, KatalinSzlavecz2, Doug Carlson3, Andreas Terzis3, and Gerard Wysocki1

CLEO/QELS

May 20, 2010

1. Dept. of Electrical Engineering, Princeton University, Princeton, NJ 085442. Dept. of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, MD 212183. Dept. of Computer Science, The Johns Hopkins University, Baltimore, MD 21218

pulse.princeton.edu

Project Goal & Outline

http://www.coas.oregonstate.edu/research/po/satellite.gif

The project goal:• Develop CO2 sensors for deployment as a real-time sensor network for carbon flux monitoring over a broad geographic area.

• Atmospheric monitoring of CO2 (fluxes, sources, and sinks)• Soil Respiration Monitoring

Outline• Requirements for a sensor to be used in trace gas sensor networks• Overview of sensor design

• Overview of control and acquisition electronics• Selection of laser & CO2 absorption line

• Sensor performance tests• Lab & Field tests

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Requirements for Trace Gas Sensor Networks

A trace gas sensor for networks must provide:

• Small size/portability• Low unit/capital cost• Low maintenance and operating

costs• Robust construction• Low power consumption• High sensitivity (ppb)• High selectivity to trace gas

species • Wireless networking capability• Ease of mass production

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Sensors

Base Station

Radio Range

Sensors work autonomously in the field

CO2 Sensor Design & Specifications

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• Tunable diode laser absorption spectroscopy (TDLAS)

• Housed within a NEMA enclosure for environmental protection• Desiccant used to prevent condensation

• 3.5 m path Herriott multi-pass cell• 2 μm VCSEL & InGaAs photodetector• Custom electronics board (openPHOTONS

platform*)• Powered by an integrated 10 Ah Li-ion polymer

battery• Works for 10 hours with pump/100+ hours

without pump• 300 mW power consumption without pump

* www.openphotons.org

CO2Laser Detector

Controlling Electronics

Custom Control and Acquisition Board

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TEC driverDirect Digital Synthesizer

MCU8MHz

Lock-In Amplifier + Front End

Modulated Current Driver

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

www.openphotons.org

2 μm VCSEL & CO2 Absorption Spectrum

Source: HITRAN 2000 database 6

Water absorption lines have limited impact on CO2 absorption lines

• Low power VCSEL• Consumes ~5 mW

power

• VCSEL temperature tuning range of ~5 cm-1

• Absorption coefficients in this range correspond to ~1% absorption over 3.5 m path

• Choose 4987 cm-1

absorption line for line-locking• Best SNR within the

VCSEL drive current and temperature

• Low interference from H2O lines

4987 cm-1

P=1 atm

AtmosphericConcentration,HITRAN/GEISA

TDLAS CO2 Sensor In-Lab Performance

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• VCSEL is wavelength modulated at 10 kHz• Via current modulation• 2nd harmonic peak value will be used for CO2 concentration measurement• Modulation depth of ~0.22 cm-1 is optimized for the 2nd harmonic

• Harmonic line profiles are measured by temperature scanning about the 4987 cm-1 absorption line

• A lock-in amplifier is used to select each harmonic

• Calibrated 285 ppm CO2 in N2 mixture yields • 1st harmonic SNR of 3247• 2nd harmonic SNR of 2530• 3rd harmonic SNR of 1052

TDLAS CO2 Sensor 3rd Harmonic Line Locking

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• Control laser temperature so that 3rd

harmonic signal is near zero• 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

TDLAS CO2 Sensor Long Term Stability

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1.4x10-6

minimum

absorption

5.1x10-7

minimum

absorption

• Allan variance at constant temperature setting shows 1.4x10-6 ultimate minimum detectable absorption at 25 seconds

• Drift dominates beyond this time• Allan variance with 3rd harmonic line locking to CO2 absorption line at 4987 cm-1

showed:• Gaussian noise performance up to 100 seconds• Sensitivity of 0.113 ppm in 1 second averaging time• Minimum detectible absorption of 7.4x10-6 in 1 second• 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

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• TDLAS sensor measurements were compared with measurements of a commercial sensor• Testing at 0 C shows similar behavior between TDLAS and commercial sensors • TDLAS & commercial sensor: R2 = 0.9964• Commercial sensors compared to each other: R2 = 0.9606 - 0.9956

• Soil respiration over time• 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 of California Isopod Respiration

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• A test tube is used to hold 10 California isopods in a closed path system with the TDLAS CO2 sensor in-line

• CO2 out-gassing is observed in control sample• Likely from desiccant• Repeatable out-gassing rate

• Isopod signal compared against CO2 out-gassing background shows increase in CO2 concentration• Isopods detectable after ~2 minutes• Approximately 0.021 ppm/sec CO2 concentration increase

Field Tests: TDLAS CO2 Sensor Measurement ofForest Floor Respiration

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• Soil respiration measurements were performed at the Smithsonian Environmental Research Center• Repeated thermal cycling introduced beam walking error• TDLAS and commercial sensor produced nearly identical measurements in the control

area with random foliage makeup• 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• Random foliage area R2 = 0.8930; Tulip Poplar leaves area R2 =0.9516

Conclusion and Future Work

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• A portable, wireless, low-power CO2 sensor based on TDLAS with a custom Herriott multi-pass cell was demonstrated to have:• 0.113 ppm sensitivity with 1 second averaging• Gaussian noise performance to 100 seconds• Ultimate minimum detectable absorption of ~5.1x10-7

• Typical 300 mW power consumption• Real-time transmission of spectroscopic data

• Lab and field performance tests compare well with commercial sensors

Future Improvements• TDLAS sensor affected by thermal drift

• Currently developing and implementing an improved optomechanical design to ensure high thermal stability

• NEMA enclosure has reduced sensor responsivity to concentration changes• Currently investigating the viability of an open path system for better sampling

responsivity

• Further field testing of sensors to ensure reliability• Sensor network for carbon flux measurements

Acknowledgements

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This work was sponsored in part by:

The National Science Foundation’s MIRTHE Engineering Research Center

An NSF MRI award #0723190 for the openPHOTONS systems

An innovation award from The Keller Center for Innovation in Engineering Education

Questions?

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