Sensors and Actuators B 150 (2010) 513–516 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Differential, LED-excited, resonant NO 2 photoacoustic system René Bernhardt a , Guillermo D. Santiago a , Verónica B. Slezak b , Alejandro Peuriot b , Martín G. González a,* a Laboratorio Láser, Facultad de Ingeniería, Universidad de Buenos Aires, Paseo Colón 850 (1063), Buenos Aires, Argentina b Centro de Investigaciones en Láseres y Aplicaciones, CITEDEF-CONICET, J. B. LaSalle 4397 (1603), Villa Martelli, Argentina article info Article history: Received 22 June 2010 Received in revised form 31 July 2010 Accepted 5 September 2010 Available online 21 September 2010 Keywords: Photoacoustic Light-emitting-diode Differential microphone NO2 abstract We introduce the application of a differential microphone in an LED-excited photoacoustic system devoted to NO 2 measurement. The microphone ports pick up out-of-phase signals generated in two resonators, thus achieving a larger electrical signal and good common mode noise rejection. The reduced noise floor and LEDs with higher optical power, made it possible to measure NO 2 down to 60 ppbV, a figure that enables quantifying the amount of such a gas produced by gasoline engines equipped with catalytic converter. © 2010 Elsevier B.V. All rights reserved. 1. Introduction In previous articles [1,2], we reported on the development and further improvement of a blue-LED-excited, transversally illuminated, resonant photoacoustic system, devoted to NO 2 mea- surements. The interest in monitoring this substance has not decreased for it continues to be a harmful urban pollutant produced in combustion. Since light-emitting-diodes (LEDs) simplify the design of com- pact setups, suitable for mobile systems, we decided to expand the capabilities of the mentioned setups by resorting to the well known differential methods [3,4]. Generally, these setups rely on two microphones whose outputs are subtracted to remove com- mon mode signals. However, the microphones must have matched responses, a condition that raises the complexity and cost. Differential microphones give an electrical output proportional to the pressure difference between two ports. Thus, the subtrac- tion is carried out within a single sensor and the need of a matched pair is skipped. Besides the complexity reduction, such a micro- phone allows employing new illumination schemes of greater flexibility. In this paper we introduce a variation that resorts to a dif- ferential microphone in combination with a new cell design that combines multi-wavelength measurements and a novel illumina- tion pattern. We describe the rationale of the new resonator and * Corresponding author. Tel.: +54 11 4343 0893x224. E-mail address: mggonza@fi.uba.ar (M.G. González). present the experimental results which show the detection limit has been reduced to 1% of the former value. 2. Cell design The differential microphone principle suggests generating out of phase acoustic signals, fed to each port, to obtain a larger elec- trical output and Fig. 1 depicts the way we exploit this idea. Two pipes of small diameter (d) and length L/2 are connected by a large diameter (D) buffer volume of equal length. Two buffer volumes of extent L/4 enclose the glass cell. Each half of this symmetric arrangement is the well known half-wavelength resonator sur- rounded by quarter-wavelength filters. These filters acoustically isolate the two resonators. A resonance condition will be achieved if the LEDs that illuminate the small pipes are modulated out of phase at a frequency f = c s /L (c s is the speed of sound). Fig. 2 shows the pressure distribution of such a mode where the pressure at the center of each pipe is a maximum. By means of two small diameter tubes, we pick up samples that are diverted to the microphone ports. Other variables being equal, we expect double electrical output compared to a single res- onator. These coupling tubes must be carefully designed because their presence must not disturb the acoustic mode. To achieve this, the acoustic equivalent of the tube plus the microphone should be equivalent to the absent rigid wall. Since the microphone mechan- ical impedance is high, and considering the tube behaves as an acoustic transmission line, its length should be an integer multiple of half wavelength [5]. In addition, a high characteristic-impedance tube would make up for the effects of bends, section changes or 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.09.007