Miniaturized Photoacoustic Carbon Dioxide Sensor with Integrated Temperature Compensation for Room Climate Monitoring

J. Huber; A. Ambs; J. Wöllenstein

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Procedía Engineering 120 (2015) 283 - 288

Procedía Engineering

www.elsevier.com/locate/procedia

EUROSENSORS 2015

Miniaturized Photoacoustic Carbon Dioxide Sensor with Integrated Temperature Compensation for Room Climate Monitoring

J. Huber1,2*, A. Ambs1, J. Wöllenstein1,2

1 Fraunhofer Institute for Physical Measurement Techniques IPM, Department of Gas and Process Technology, Group of Integrated Sensor

Systems, 79110 Freiburg, Germany 2Department of Microsystems Engineering - IMTEK, University of Freiburg, Germany

Abstract

A miniaturized photoacoustic spectroscopic gas sensor for room climate monitoring is presented with the measurement range of 0 -5000 ppm. The sensor consists of an innovative photoacoustic detection unit with an integrated mobile phone microphone as pressure sensor. As IR-source a broadband thermal emitter is used. The emitter is modulated with 13 Hz at a maximum temperature of about 750°C. An electronic unit with a microcontroller on a 3D-PCB design has been developed. The sensor reaches a resolution better than 50 ppm CO2. A detailed characterization of the setup has been carried out. An integrated temperature compensation is realized to improve the sensor performance under varying environmental conditions. The potential for further miniaturization and the realization as a low-cost mass market sensor is shown.

Peer-review under responsibility of the organizing committee of EUROSENSORS 2015 Keywords:

1. Introduction

The monitoring of indoor air quality in buildings or vehicles is of high importance with the target to maintain improved room climate conditions. Especially in open-plan offices and class rooms, there are a lot of persons within a small area which results in a rapid deterioration of indoor air quality. The CO2 concentration is next to humidity and temperature the main indicator for interior air quality and correlates with the number of persons inside.

Increased CO2 levels indoors cause perceptions of poor air quality, impaired work performance and can even

* Jochen Huber. Tel.: +49 761 8857-744; Fax: +49 761 8857-244 j ochen.huber@ipm. fraunhofer. de

Peer-review under responsibility of the organizing committee of EUROSENSORS 2015 doi:10.1016/j.proeng.2015.08.616

lead to acute health symptoms. This case requires controlled adequate rates of air-ventilation. Therefore a selective, low-cost sensor for CO2 is needed to maintain healthy room climate conditions. This leads to reduced energy consumption by means of non-continuous adequate ventilation rates. Due to these facts there is a high demand for low-cost, small and energy-efficient gas measuring systems for real-time and selective CO2 measurements in interior air quality monitoring applications.

CO2 as strong IR-active molecule is measured with so called NDIR-spectroscopic sensors. These sensors are often inadequate in size and underlie high manufacturing costs. These sensors typically consist of an IR-source, a measurement path and a detector. Due to the type of IR-detector, an additional interference IR-filter is used to receive improved gas selectivity.

We propose to use a miniaturized 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]. CO2 absorbs photons in the middle infrared range. If a time-modulated IR source is used, the acoustic wave can be measured as time modulated pressure wave in a closed volume [2]. The working principle and theoretical background of the effect are discussed in [3] in detail.

The system has a temperature sensing unit which allows to monitor environmental temperature. The performance of NDIR-sensors is typically strongly affected by variations of the environmental temperature. We have realized an integrated temperature processing unit to compensate the temperature drift.

2. Theory of the photoacoustic signal

Basis for selective infrared radiation absorption is a temporal changing dipole moment of an infrared-active gas. Especially CO2 is a strong IR-active gas and absorbs at characteristic wavelengths (main absorption range around 4.25 ^m). The absorption effect of infrared radiation and its dependence on absorption length, gas concentration and environmental conditions is described by Lambert-Beer-Law [4]. If the electromagnetic radiation energy equates to the energy difference of nearby energy levels of the molecule, this results in a state transition. The initial state of the molecule is changed to an excited state by stimulation through absorption. Infrared radiation photons cause IR-active molecules to vibrate and rotate at molecule specific wavelengths, which can be seen as absorption lines of the specific molecule. Inelastic collisions between excited molecules and non-excited ones result in a change of radiation energy into kinetic energy. Increased kinetic energy causes a temperature difference and a gas expansion and thus a pressure difference in a closed volume. The total absorbed power Ptotal is given by the integral of the multiplication of the absorption profile A(X) by the emission power profile of the emitter Pemitter(^). The absorption profile can be calculated from the transmission profile T(X) as follows:

For further calculations we assume ideal gas behavior and no secondary heating loss mechanisms. These assumptions mean that the total of the absorbed power is inserted as intrinsic power of the gas and can be directly calculated:

Total absorbed power is stored as thermal energy Q inside the ideal gas volume. By means of a modulation frequency f = 13 Hz and a rectangular modulation function with duty cycle v = 0.5 thermal energy is described in the following equation:

A(A) = 1 - T (1)

Poa = ¡A(X) ■ Pemitter (m

The quotient of stored thermal energy Q and gas specific thermal capacity CV results in the temperature difference AT:

aT = Q

Using the ideal gas law allows to determine the pressure difference Ap:

n • R • T

Ap = ■

The described pressure difference can be measured with a dynamic pressure sensor in the closed gas volume. Its amplitude is a qualitative measure of the absorbed radiation intensity and the absorbing gas concentration. Conventional absorption spectroscopy is based on excitation by electromagnetic radiation with intensity I0 and the measurement of reflected or transmitted light intensity I [5]. Photoacoustic sensors measure the absorbed light directly [6].

We use the described effect in a 2-chamber sensor setup to realize real-time measurements. The working principle can be seen in Figure 1. A hermetically sealed detection chamber with enclosed target gas and an integrated microphone is used. In this detection chamber a maximum photoacoustic signal is measured, if there is no absorption on the previous measurement path. If there is a high amount of target gas on this measurement path, the measured photoacoustic signal in the detection chamber is reduced.

Figure 1: Working principle of the photoacoustic 2-chamber setup. Without target gas on the absorption path there is a maximum photoacoustic signal in the detection chamber (upper picture), while the signal is reduced with target gas on the measurement path (below).

3. Experimental

In the following the sensor setup is described. The sensor consists of three main components: IR-source, detection unit and the functional sensor housing that provides the measuring path and integrates all electronic parts. As IR-source we use a broadband emitter (IR-50S, HawkEye) in a TO-5 case. The innovative photoacoustic detection unit has an integrated mobile phone microphone (SMM310, Infineon Technologies) as pressure sensor. It is hermetically sealed with nearly 100% target gas concentration (CO2) in a TO-housing and serves as gas selective detector. The functional sensor housing is designed with a microcontroller on a 3D-PCB design. All control functions like the control of the active components (emitter, detector) or the peripheral communication (UART) are implemented in the microcontroller. Furthermore the complete data processing is realized digitally in the microcontroller. A temperature sensor monitors the environmental temperature and a temperature compensation algorithm is developed. The DC-conversion of the detected AC pressure signal is done with a digitally implemented

lock-in amplifier. Figure 2 a) shows the developed photoacoustic sensor setup. Figure 2 b) shows the sensor in the measurement setup for the sensor characterization process. This setup consists of a gas test chamber with gas periphery and electrical connections to distribute defined gas concentrations to the sensor at a calibrated gas test

Figure 2: (a) picture of an assembled sensor; (b) shows an integrated sensor in an open characterization setup (1, 4) for gas test measurements

with electrical (2) and gas (3) peripheral connections..

4. Measurement results

The setup of Figure 2 b) is used to execute a detailed characterization process. The results are discussed in this chapter. Figure 3 shows a measurement from 200 - 1000 ppm CO2 in N2 in steps of 100 ppm. The sensor detects the concentration steps clearly. The sensor shows a stable zero CO2 baseline and the resolution is better than 100 ppm.

Figure 3: Measurement with the developed sensor with steps of 100 ppm CO2 in N2 from 200 - 1000 ppm.

The sensor signal shows a strong dependence on changes of environmental temperature. Due to this fact we developed a compensation strategy for variations of surrounding environmental temperature. We measure the temperature with a Pt10.000 sensor and compensate the temperature dependency of the output signal internally in the microcontroller (Figure 4). The forced temperature increase is done inside a climate chamber. The upper graph (green curve) in Figure 4 shows the change of the environmental temperature from 28.5°C to 37°C. The middle graph (black curve) shows the output signal of the sensor without the compensation algorithm. The red line shows the compensated sensor response with eliminated temperature dependency.

g 7 1,010 -

"g JO 1,005 -

S. 75 1,000

■S .§>0,995-

-0,0050

Va*—«v WJ

time [h]

Figure 4: Temperature calibration examination of the sensor in a climate chamber. The green curve shows the variation of the environmental temperature inside the climate chamber. The black curve represents the sensor output signal without the temperature compensation algorithm.

The red line shows the compensated sensor output signal.

Furthermore a calibration process is done and verified (Figure 5). The sensor system has been characterized and the sensor response has been approximated linearly over a certain measurement range (0-5000 ppm). With this measurement range the sensor can be used as indoor air quality monitoring system. A test measurement with the calibrated sensor is shown in Figure 5.

- exposed gas concentration -25 times averaged signal

2250 time [min]

Figure 5: Comparison of the sensor response (blue) with the distributed gas concentration (red) after calibration process.

The red curve indicates the gas concentration which is delivered by the gas test stand, while the blue curve represents the sensor output. The sensor shows only slight deviation between the two curves. The deviation is smaller than the resolution of the sensor, which has been approved as about 50 ppm.

5. Conclusion and Outlook

We present an innovative miniaturized selective photoacoustic CO2 sensor in non-resonant mode. The sensor consists of an IR-emitter, a selective CO2 detection chamber and a functional sensor housing with a microcontroller as electronic control unit. The resolution of the sensor is better than 50 ppm. A digital temperature compensation algorithm is realized with a Pt10.000 sensor, monitoring the environmental temperature. A calibration is done with a linear approximation of the measurement range of 0-5000 ppm.

In further characterization measurements the cross-sensitivities to other gases and the influence of further environmental condition changes (static pressure, humidity) will be investigated.

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, J. J. Barrett, D. R. Siebert, and K. V. Reddy, "Photoacoustic spectroscopy," Rev. Sci. Instruments, vol. 54, pp. 797 - 817, 1983.

[2] Z. Bozoki, A. Pogäny, and G. Szabo, "Photoacoustic Instruments for Practical Applications: Present, Potentials and Futute Challenges," Appl. Spectrosc. Rev., vol. 46, pp. 1-37, 2011.

[3] J. Huber and J. Wöllenstein, "Kompaktes photoakustisches Gasmesssystem mit Potential zur weiteren Miniaturisierung ," tm - Tech. Mess. tm - Tech. Mess. , vol. 80 , no. 12, p. 448, Jan. 2013.

[4] W. Demtröder, Experimentalphysik 2. Springer, 2008.

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

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

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