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 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 SpectroscopicSensors for Atmospheric CO2 Monitoring Clinton J. Smith Advisor: Gerard Wysocki PECS Student Dinner Talk 10/23/2011
Requirements for Trace Gas Sensor NetworksA trace gas sensor for networks Sensors work autonomouslymust provide: in the field Base Station•Small size/portability•Low unit/capital cost•Low maintenance and operatingcosts•Robust construction•Low power consumption•High sensitivity (ppb) Radio•High selectivity to trace gas species Range•Wireless networking capability•Ease of mass production Sensors 2
Wireless Sensor Network & Deployment350 m range directionalantennas are used Test Sight: Crop field in Princeton, NJ Three locations selected to monitor coupled local environments: 1. Adjacent to the local road: car traffic 2. In the inner courtyard: local vegetation 3. On the roof of the building CO2 sensor-node. The total size is less than that of a shoebox. 3
Determine detection methodDirect absorbance• current scan across absorption feature, relate I/Io to conc. (+) absolute measurement, straightforward (-) less sensitive, difficult to determine baselinePhotoacoustic• high power laser modulated as it is tuned across absorption line; sound waves generatedwith an amplitude proportional to concentration (+) zero baseline, high precision measurements (-) closed cell, not proportional to pathlength, long-term reproducibilityCavity ringdown• light enters high-finesse cavity, observe light decay with time (+) extremely high sensitivity (long pathlengths) (-) closed-path cell, high reflectivity mirrorsWavelength modulation spectroscopy• current scan modulated at high frequency to reduce 1/f noise (+) high sensitivity, zero baseline method (-) requires calibration, more complex electronics circumstances/conditions determine method! Slide compliments of Mark A. Zondlo
Slide compliments of Mark A. Zondlo Wavelength modulation spectroscopy• high sensitivity detection (absorbance 10-6 to 10-5); needs calibration - limited by optical interference fringes (étalons), not shot noise• scan current over absorption feature at 1 kHz
Slide compliments of Mark A. Zondlo Wavelength modulation spectroscopy• add sinusoidal variation to current scan at 250 kHz
Slide compliments of Mark A. Zondlo Wavelength modulation spectroscopy• Fourier transform photosignal to obtain 3rd component (2f spectra)• low noise limit (1 x 10-5 min. absorbance for 1 Hz)
resulting signal has 2nd derivative shape Slide compliments of Mark A. Zondlo
CO2 Sensor Design & Specifications•Tunable diode laser absorption spectroscopy(TDLAS)•Housed within a NEMA enclosure forenvironmental protection •Desiccant used to prevent condensation•3.5 m path Herriott multi-pass cell•2 μm VCSEL & InGaAs photodetector•Custom electronics board (openPHOTONSplatform*)•Powered by an integrated 10 Ah Li-ion polymer Laser CO2 Detectorbattery •Works for 10 hours with pump/100+ hours without pump •300 mW power consumption without pump Controlling Electronics* www.openphotons.org 9
Custom Control and Acquisition Board Direct Digital Synthesizer TEC driver MCU 8MHz Modulated Current Lock-In Amplifier + Front End Driver www.openphotons.orgSo, S., Sani, A. A., Zhong, L., Tittel, F., and Wysocki, G. 2009. Demo abstract: Laser-based trace-gas chemical sensors for distributed wirelesssensor 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 10
2 μm VCSEL & CO2 Absorption Spectrum •Low power VCSEL 4987 cm-1 •Consumes ~5 mW power •VCSEL temperature tuning P=1 atm 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 Water absorption lines have limited •Low interference from H2O impact on CO2 absorption lines linesSource: HITRAN 2000 database 11
TDLAS CO2 Sensor 3rd Harmonic Line Locking •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 12
In-Lab Tests: TDLAS CO2 Sensor Measurement of Changing CO2 Concentrations• 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 13
In-Lab Tests: TDLAS CO2 Sensor Measurement of California Isopod Respiration•A test tube is used to hold 10 California isopods in a closed path system withthe 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 increasein CO2 concentration •Isopods detectable after ~2 minutes •Approximately 0.021 ppm/sec CO2 concentration increase 14
Field Tests: TDLAS CO2 Sensor Measurement of Forest Floor Respiration•Soil respiration measurements were performed at the Smithsonian EnvironmentalResearch 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 15
Multi-Node Long-Term Cross-Correlation Performance Base Station Node Node Node 1 2 3
CO2 Sensor Allan Deviation – What’s the bigproblem? 29
Examples of environmental influence on data 30
Let’s audit the composition of CO2 sensor 24 cmLeft: CO2 sensor as seen from top. The total size is less than that of a shoebox.Right: Schematic of optical configuration and electricalcontrol systems. 31
Use a temperature controlled environment to find source of driftTemperature Controlled VesselBoth sensor board and optical The sensor board is placed outsidesystem are placed in the while the optical system is placedtemperature controlled inside the temperature controlledenvironment. environment. 32
Allan variances from different environments… All Inside, Line- LockingAll Outside, Gimbal ~0.64 ppm 1x10-5 UMDL Cell Inside, Constant ~0.29 ppm TemperatureAll Outside, Fixed Cell Inside, 2x Over-Modulation All Outside, 2x Over-Modulation 1.5x10-6 UMDL Cell Inside, Line- Locking 33
How to fix?•Control the laser environment•Correct for drift influence? 35
AcknowledgementsThis work was sponsored in part by:Nanotechnology for Clean Energy IGERTThe National Science Foundation’s MIRTHE Engineering Research CenterAn NSF MRI award #0723190 for the openPHOTONS systemsAn innovation award from The Keller Center for Innovation in EngineeringEducation 36