研究目的
To evaluate pivotal techniques in QEPAS systems for engineering applications, including optimization of scanning cycle and modulation index, long-term stability of QTF, and comparison of photoacoustic spectrophone configurations.
研究成果
The scanning cycle of 5 s was verified to be appropriate for photoacoustic energy accumulation. The optimum modulation index was found to be ~5, differing from the simulated value of 2.2. Long-term experiments revealed drifts in QTF's resonant frequency and efficiency, highlighting the need for calibration-free techniques. Dual-tubes on-beam configuration was the most efficient in standard QTF based systems.
研究不足
The long-term stability of QTF is a significant challenge for engineering applications. The initial frequency of each QTF tends to be different, and drifts of resonant frequency and acoustic-current efficiency tends to happen. This requires more attention to exploring new totally calibration-free techniques.
1：Experimental Design and Method Selection:
A standard QEPAS system was constructed to optimize scanning cycle and modulation index. Water vapor was chosen as the object gas. Slow wavelength scanning of the laser was realized by a saw-tooth ramp, and fast wavelength modulation was achieved by a sinusoidal signal. A dual-tubes based on beam acoustic model was used. A commercial lock-in amplifier was used to demodulate the photoacoustic signal in second harmonic.
2：Sample Selection and Data Sources:
Water vapor was used as the sample gas. A VAISALA humidity indicator was used for normalization to eliminate the effects from ambient water vapor fluctuation.
3：List of Experimental Equipment and Materials:
ARM7 for generating saw-tooth ramp, generator for sinusoidal signal, dual-tubes based on beam acoustic model, commercial lock-in amplifier (HF2LI, Zurich Instruments, Swiss), VAISALA humidity indicator.
4：Experimental Procedures and Operational Workflow:
The scanning cycle was set from
5：5 s to 20 s. The modulation index was varied from 4 to The second harmonic signals were measured and normalized. Data Analysis Methods:
The amplitude of second harmonic signal was calculated and plotted against scanning cycle and modulation index.
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