Then, the acoustic energy is converted to an electrical signal by the piezoelectric QTF. As in photoacoustics, a modulated laser is used to excite a gas specie in order to create a sound wave thanks to the photoacoustic effect. QEPAS is a gas sensing technique that was proposed in 2002 by Kosterev et al. Details of the characterization methods and a literature review of their performances are presented in Section 2. Various approaches are known to follow the QTF resonance, depending on the application and using different electronics circuits. The QTF output current is typically about a few fA to a few μA, amplification stages are thus needed prior to readout. The effect of these on the resonance frequency f 0 and the quality factor Q of the QTF, can be individually quantified.ĭepending on the application and the selected electronic circuit, the QTF can be operated in different modes, leading to different mesurands: QTF current amplitude, frequency shift, and Q shift, etc. The intrinsic properties are physical features (geometry, density, Young’s modulus, …) and present long-term stability, whereas the environmental conditions (temperature, pressure, humidity, gas density, …) are prone to quick variations. Any modification in those parameters affect the QTF behavior. The QTF resonance is related to the QTF intrinsic properties, as well as the environmental conditions. Gas sensing using Quartz-Enhanced Photo-Acoustic Spectroscopy (QEPAS) Finally, we integrate this characterization method in our Resonance Tracking (RT) QEPAS sensor and show the significant reduction of the signal drift compared to a conventional QEPAS sensor. Further, we demonstrate the resonance tracking of the QTF while changing the temperature and the humidity. This method proved to be fast and accurate. Then, we further detail one technique, called Beat Frequency analysis, based on the transient response followed by heterodyning. We present a review of these methods and compare them in terms of accuracy. Because the requirements vary greatly depending on the field of application, different characterization methods can be found in the literature. In most applications, regular QTF calibration is absolutely essential. The variability of the environment (temperature, humidity) can lead to a drift of the QTF resonance. The quartz tuning fork (QTF) is a piezoelectric transducer with a high quality factor that was successfully employed in sensitive applications such as atomic force microscopy or Quartz-Enhanced Photo-Acoustic Spectroscopy (QEPAS).
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