The development of low-cost photoacoustic gas sensor devices for monitoring ozone and nitrogen dioxide and the quantification of gas composition through changes in the speed of sound

The dissertation aimed to develop a low-cost gas detection sensor utilizing photoacoustic spectroscopy (PAS). This technique exploits the photoacoustic (PA) effect, where target molecules absorb modulated light at a specific wavelength. When these molecules absorb optical energy, they become excited and subsequently relax to the ground state. This results in the release of the heat, generating pressure waves or sound known as the photoacoustic signal. This signal is then detected by a sensitive detector, such as a microphone, or quartz tuning fork [1].
This dissertation comprises four published articles. The first article investigated the use of a piezoelectric tube, serving as a dual function resonance tube and transducer for detecting nitrogen dioxide (NO2). In this study, a 450 nm blue laser diode with a maximum power output of 1800 mW was employed as the light source. The photoacoustic system exhibited sufficient sensitivity for measuring NO2 gas in the part per billion (ppb) range. This innovative approach offers an alternative method for gas analysis based on photoacoustic technique.
The second article presented a novel approach for measuring ozone (O3) using a visible laser diode with a wavelength of 638 nm, targeting the Chappuis bands of ozone in the spectral range of 380-800 nm [2]. Ozone absorbs light most strongly in the UV region (200-350 nm), which falls into the Hartley band [3]. However, commercially available low-power LEDs are not sensitive enough to detect ozone at low concentrations in UV band, and using lasers is expensive. An alternative method involves using a higher power red laser diode to compensate for the weaker ozone absorption in Chappuis region. Within this study, the photoacoustic cell was constructed from stainless steel, following a design similar to that previously reported by Rück et al [4]. To pick-up the generated acoustic signal, a micro-electrical mechanical system (MEMs) microphone was employed. The minimum detectable concentration of ozone using this setup was in the part per million (ppm) range, comparable to previous studies that used a Nd:YAG laser in the same wavelength region, as reported by Köhring et al. [5].
The third project aimed to contribute to the advancement of photoacoustic gas monitoring by developing a low-cost photoacoustic circuit module that could replace an expensive commercial instrument. This module, comprising a 32-bit microcontroller, waveform generator, laser diode driver, lock-in amplifier, and an analog-to-digital converter, were integrated onto a single printed circuit board (PCB). The system was operated through a Python graphical user interface (GUI) for controlling the module and acquiring data. The acquired data was collected as a text file, allowing it easy to integrate with software such as Microsoft Excel or Igor for further analysis. This device was implemented as a photoacoustic sensor for NO2 detection, with a detection limit in the ppb range. This circuitry costs twenty times less than that of commercial instruments. Its compact design also makes it favorable for future development in onsite applications.
The fourth project aimed to present a novel quantitative method for determining the composition of gas mixtures through the change in the speed of sound. The focus was on measuring the resonance frequency of binary gas mixtures (O2/N2, CO2/N2, and He/N2) at various compositions. The experiments utilized a piezoelectric transducer serving as both an acoustic resonator and a detector. A loudspeaker was utilized as an audio excitor in photoacoustic process. At different compositions in gas mixture, the resonance frequency varied due to the differences in the speed of sound in each gas sample. This analytical approach to the changes in resonance frequency make it possible to estimate the composition of the gas mixture. It provides a useful tool for understanding and quantifying gas compositions in various gas applications.