Phase Optimized Photoacoustic Sensing of Gas Mixtures

Abstract

In this paper, we report on the progress of the auto-triggered quartz-enhanced photoacoustic spectroscopy (QEPAS) technique which operates without external frequency generators and ensures permanent locking to the current resonance frequency of the tuning fork. This is obtained by incorporating the tuning fork in an oscillator circuit that autonomously oscillates at the present resonance frequency that shifts with changing environmental conditions, e.g., density and viscosity of the surrounding gas, temperature, and pressure. Both, the oscillation amplitude as well as the frequency can be read from the oscillator circuit. The photoacoustic signal appears as an offset of the electrically induced signal amplitude. Since the sum amplitude depends on the phase relation between the electrical and photoacoustic driving forces, the phase is permanently modulated, enabling the extraction of the photoacoustic component by use of a second lock-in amplifier stage which is being referenced with the phase modulation frequency. The functionality of this method is demonstrated for methane detection in a carbon dioxide atmosphere in a concentration range from 0 to 100% and ammonia in synthetic air employing a pulsed mid infrared QCL around 1280 cm−1. The gas mixtures are motivated by the demands in biogas-analysis.

High-concentration methane and ethane QEPAS detection employing partial least squares regression to filter out energy relaxation dependence on gas matrix composition

“…The parameter representing the efficiency in photoacoustic generation is the radiation-to-sound conversion efficiency ε(P) , which is a function of the operating pressure P [27] . In addition, strong variations in the gas matrix composition result in variation of the fluid dynamics parameters, thus influencing the resonance characteristics of the QTF, i.e., resonance frequency and Q-factor [28] . Overall, the photoacoustic signal can be modelled as: S ~ α∙ P 0 ∙ e -α ∙ L ∙Q ∙ ε where α is the absorption coefficient and L is the optical path, Q is the QTF quality factor and P 0 is the laser power [29] .…”

Quartz-Enhanced Photoacoustic and Photothermal Spectroscopy

“…The effects of gas-matrix variations are also addressed in the article authored by M. Mordmueller et al, who reported on the latest results obtained with an auto-triggered QEPAS approach. This configuration works without external frequency generators, and ensures permanent locking to the current resonance frequency of the tuning fork [16]. The phase-optimized photoacoustic technique mentioned above avoids a calibration procedure and permits the continuous monitoring of a targeted trace gas.…”

An Ultra-Low-Power-Consumption Urban Sewer Methane Concentration Monitoring System Based on Ultrasound

“…Since there may be other kinds of gases in a sewer that can be oxidized, such gases will react with the sensor, which will cause a large error in the measurement results. Photoacoustic sensors are usually expensive and are suitable only for experimental projects [13]. The application of ultrasonic sensors in gas detection is relatively rare, but they offer the advantages of low power consumption and high safety, and can be used for methane gas detection [14][15][16].…”