Abstract:A detailed investigation of a set of custom quartz tuning forks (QTFs), operating in the fundamental and first overtone flexural modes is reported. Support losses are the dominant energy dissipation processes when the QTFs vibrate at the first overtone mode. These losses can be decreased by increasing the ratio between the prong length and its thickness. The QTFs were implemented in a quartz enhanced photoacoustic spectroscopy (QEPAS) based sensor operating in the near-IR spectral range and water vapor was selected as the gas target. QTF flexural modes having the highest quality factor exhibit the largest QEPAS signal, demonstrating that, by optimizing the QTF prongs sizes, overtone modes can provide a higher QEPAS sensor performance with respect to using the fundamental mode.
FIG. 4. QEPAS spectral scans of a gas mixture containing air with a 1.7% water concentration at a pressure of 75 Torr for the fundamental mode (black solid line) and for the first overtone mode (red solid line), acquired at the optimum laser modulation depth and focusing point conditions. Both scans were recorded with a 100 ms lock-in integration time and normalized to the overtone peak value. The overtone peak signal corresponds to 2.65 mV.
There are very few designs of the open photoacoustic Helmholtz cells, and most of them exhibit very strong penetration of the external acoustic noise inside the cell. So far the best values of external acoustic noise suppression obtained in such cells were reported at the level of about 40 dB to 50 dB. This paper presents an open photoacoustic Helmholtz cell design with a differential signal detection. Both Helmholtz resonator cavities are equipped with microphones and connected with the exterior via duct-buffer-duct structures. The length and diameter of the ducts as well as volume of the acoustic buffers are selected in such a way that the acoustic impedance of the duct-buffer-duct structure at the frequency of light modulation is relatively high. As a result, the resonance of the cell is not damped, while penetration of the external acoustic noise inside the cell is strongly reduced. Preliminary analysis predicts attenuation of the external acoustic noise at the resonance frequency of the cell with a single microphone to be at the level of at least 60 dB. Additional rejection of the external acoustic noise can be obtained with differential detection, which simultaneously doubles the photoacoustic signal component; as in the Helmholtz resonator, pressure changes in the cavities are in counterphase.
This paper presents the design of an open photoacoustic Helmholtz cell in which high acoustic volumes with quarter-wave ducts act as high-impedance separators between the main Helmholtz cell structure and the exterior. As a result, penetration of the external acoustic noise into the cell was substantially reduced in comparison to earlier open Helmholtz cell designs. Although the presented cell is not windowless, the photoacoustic background signal resulting from the absorption of the light by the windows is significantly lower in comparison to standard Helmholtz cells. Such an effect was obtained by locating the windows not at the relatively small sample cavity, but at the acoustic buffers, for which the volumes are two orders of magnitude higher. The proposed cell is dedicated for gas or liquid measurements, and its design allows for constant flow of the fluid. Hence, it can be used in continuous, real-time photoacoustic measurements.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations鈥揷itations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.