A Selective, Miniaturized, Low-cost Detection Element for a Photoacoustic CO2 Sensor for Room Climate Monitoring Academic research paper on "Materials engineering"

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Procedía Engineering 87 (2014) 1168 - 1171

Procedía Engineering

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EUROSENSORS 2014, the XXVIII edition of the conference series

A selective, miniaturized, low-cost detection element for a photoacoustic CO2 sensor for room climate monitoring

J. Huber*, A. Ambs1, S. Rademacher1, J. Wollenstein12

1Fraunhofer Institute for Physical Measurement Techniques IPM, 79110 Freiburg, Germany 2Department of Microsystems Engineering - IMTEK, University of Freiburg, 79110 Freiburg, Germany

Abstract

We present a miniaturized, low-cost detection element for a photoacoustic spectroscopic gas sensor system. The sensing element consists of a commercial mobile phone microphone and a standard TO socket. The elements are mounted under gas atmosphere and serve as gas selective reference cell. The system is able to detect CO2 selectively with a resolution better than 100 ppm. Further characterization included different environmental conditions and gas cross-sensitivities. The potential for further miniaturization and the realization as a low-cost mass market sensor is shown.

© 2014 PublishedbyElsevier Ltd. Thisisanopenaccess article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the scientific committee of Eurosensors 2014 Keywords: Photoacoustic, gas sensor, carbon dioxide, room climate monitoring, spectroscopy

1. Introduction

The measurement of CO2 has a high relevance on the gas sensor market. Especially monitoring indoor air quality in buildings is of high importance. Increased CO2 levels indoors cause perceptions of poor air quality, impaired work performance, and can even lead to acute health symptoms. The indoor CO2 concentration directly correlates with the number of people present and requires adequate rates of air-ventilation. Therefore a selective, low-cost sensor for CO2 is needed to maintain healthy room climate conditions with high energy conservation simultaneously.

We propose to use a photoacoustic sensor to monitor indoor CO2 levels. The photoacoustic effect describes the formation of an acoustic wave in a gas sample due to the absorption of photons [1]. The photoacoustic effect was

* Jochen Huber. Tel.: +49 761 8857-744; fax: +49 761 8857-244 E-mail address: jochen.huber@ipm.fraunhofer.de

1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi: 10.1016/j.proeng.2014.11.374

first published by A.G. Bell in 1880 [2]. Conventional absorption spectroscopy is based on excitation by electromagnetic radiation with intensity I0 and the measurement of reflected or transmitted light intensity I [3]. Photoacoustic sensors measure the absorbed light directly [4].

CO2 is IR-active in the mid-infrared range and energy is absorbed in the gas sample. At thermal equilibrium, this energy is randomly distributed into all degrees of freedom, causing an increase of thermal energy and with it a rise in temperature and pressure at a constant density in the cell [5]. If a modulated IR source is used, the pressure variations are periodic as well [6]. These pressure changes can be measured in a closed volume as periodic signal. The resulting pressure signal can be calculated from the absorption strength and the emission profile of the used IR-source as discussed in [7]. To get values for absorption strength at a specific wavelength we used data from HITRAN [8] to calculate the absorption lines with broadening and intensities. In the calculations we estimate an ideal black body radiator as broadband IR-source to describe the used thermal emitter.

2. Experimental

A photoacoustic detection unit consists of a hermetically sealed chamber with IR-optical contact and integrated pressure sensor. The chamber is filled with the target gas in order to serve as gas selective filter unit.

2.1. Detector Design

Figures 1 a) and b) show the developed photoacoustic detector setup with an integrated MEMS microphone (SMM310, Infineon) in a TO socket and an IR-transparent window.

Fig. 1: (a) technical drawing of the cross-section of a sealed microphone inside the TO-housing. (b) Photography of the mounted microphone

before the TO header is soldered to seal the detector unit.

The hermetic mounting process of the TO socket is carried out under gas atmosphere. The working principle and theoretical background of the effect are discussed in [7]. Advantages of this method are that the components of the sensing element are available in high quantities and that the assembly processes are standardized.

2.2. Characterization Setup

Figure 2 shows the measurement setup for the detailed characterization process. The setup consists of a variable measurement channel inside the tube. At one end of the measurement path the detector is placed. On the other side a broadband thermal emitter as IR-source is installed. Two peripheral gas connectors allow filling gas mixtures into the measurement cell at a gas test stand. Various characterization measurements have been performed with this setup. Long-term behavior, detection limit, resolution and cross-sensitivities to other gases are investigated.

Fig. 2: Developed characterization setup to evaluate detector unit performance. The measurement chamber length is realized variable inside

the tube.

3. Results

The results of the characterization measurements are discussed in this chapter. As IR-source we used a thermal emitter (Hawkeye, IR66). The measurement distance in the measurement chamber is set to 5 mm for all shown measurements. Results of sensitivity measurements and longterm behavior can be seen in figures 3 a) and b).

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(a) longterm measurement over 50 hours to examine stability of the zero baseline. (b) Measurement with varied CO2 concentration (0 -10000 ppm) in nitrogen to investigate resolution and detection limit of the setup.

At the beginning of the measurement there are five steps of 10.000 ppm CO2 alternating with pure nitrogen (Fig. 3 a). The system shows a stable zero signal during 50 hours of constant operation. At the beginning of the measurement the system shows a warm-up time until thermal equilibrium has been reached.

The resolution of the CO2 detection has been examined in gas measurements (figure 3 b). Steps of 100 ppm CO2 in the range between 0-10.000 ppm have been applied. The system shows a resolution better than 100 ppm over the complete measurement range. All concentration changes can be detected clearly. During the measurement, the system also shows a stable baseline after having reached the stable thermal equilibrium.

Figure 4 illustrates the investigation of cross-sensitivities to other gases. Carbon monoxide (CO) also absorbs in the range between 3 - 4 ^m. There is no signal at the detector even at high concentrations of CO up to 1000 ppm. The photoacoustic detector acts as a gas selective filter. Only the absorption of CO2 molecules results as pressure signal at the microphone. Pre-absorption of energy at other wavelengths does not affect the performance of the detector. The developed detection unit has no integrated, optical IR bandpass filter, which is usually required for selective gas measurements in the mid IR range and the usage of a broadband source.

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Fig. 4: Investigation of the cross-sensitivity of the system to CO.

4. Conclusion and outlook

The results of the characterization measurements show a stable CO2 sensor with a resolution better than 100 ppm. The detection unit shows a longtime stable signal without any baseline drift after having reached thermal equilibrium. The detection limit is lower than 100 ppm. Gas cross-sensitivities are suppressed by the 2-chamber setup. The sensor can be used as indoor air quality monitoring system. The detection unit can be mounted with standard processes and all components are cheap standard products. The potential for a low-cost, gas selective, photoacoustic CO2 sensor with a broadband IR-source without the usage of optical filters has been shown.

Acknowledgements

This work is funded by the German BMBF within the project ESEE in the ENIAC JU program of the EU. References

[1] G.A. West, et al.: Photoacoustic spectroscopy. Review of Scientific Instruments 54 (1983), 797.

[2] A. G. Bell: On the Production and Reproduction of Sound by Light. Am. J. Sci., vol. XX, no. 118, pp. 305- 324, 1880.

[3] T. Schmid: Photoacoustic spectroscopy for process analysis. Anal. Bioanal. Chem., vol. 384, no. 5, pp. 1071-86, Mar. 2006.

[4] J. Hodgkinson and R. P. Tatam: Optical gas sensing: a review. Meas. Sci. Technol., vol. 24, 2013.

[5] W. Demtroder, Laser Spectroscopy: Vol. 2: Experimental Techniques, Fourth Edi. Springer, 2008.

[6] Z. Bozoki, A. Pogany, G. Szabo: Photoacoustic Instruments for Practical Applications: Present, Potentials, and Future Challenges. Applied Spectroscopy Reviews, 46:1-37 (2011)

[7] J. Huber, J. Wollenstein: Kompaktes photoakustisches Gasmesssystem mit dem Potential zur weiteren Miniaturisierung. Technisches Messen, Band 80, Heft 12 (2013).

[8] HITRAN spectral database, 2008. [Online]. Available: http://www.cfa.harvard.eu/HITRAN.