Passive Electrical Damping of a Quartz Tuning Fork as a Path to Fast Resonance Tracking in QEPAS

Abstract

In Quartz-Enhanced PhotoAcoustic Spectroscopy (QEPAS) gas sensors, the acoustic wave is detected by the piezoelectric Quartz Tuning Fork (QTF). Due to its high-quality factor, the QTF can detect very low-pressure variations, but its resonance can also be affected by the environmental variations (temperature, humidity, …), which causes an unwanted signal drift. Recently, we presented the RT-QEPAS technique that consistently corrects the signal drift by continuously measuring the QTF resonance. In this article, we present an improvement of RT-QEPAS to fasten the QTF characterization time by adding a passive electronic circuit, which causes the damping of the QTF resonance. The damping circuit is optimized analytically and through SPICE simulation. The results are supported by experimental observations, showing a 70 times improvement of the relaxation times compared to the lone QTF, which opens the way to a fast and drift-free QEPAS sensor.

Commercial and Custom Quartz Tuning Forks for Quartz Enhanced Photoacoustic Spectroscopy: Stability under Humidity Variation

“…Despite the fact that the sensitivity of QEPAS has been proven through numerous sensing demonstrations, the original frequency is not the optimal one for photoacoustic sensing: some molecular deexcitations need to be addressed with lower frequencies [ 2 ]. In addition, the time constant associated with the standard QTF complete deexcitation [ 3 ] is relatively high compared to the commonly used lock-in amplifiers (LIA) time constants in photoacoustic sensing, which implies an out-of-equilibrium detection scheme of the QTF. However, its high-quality factor (Q) (Q~100,000 in vacuum and Q~8000 in atmospheric pressure) and its quadrupole geometry renders QEPAS nearly immune to ambient noises compared to the standard microphones used in classical photoacoustic spectroscopy (PAS).…”