NEW Case Study on CO Detection in SF6 Using Quartz-enhanced Photoacoustic Spectroscopy

Researchers from China and Italy have developed a highly sensitive and selective optical sensor using quartz-enhanced photoacoustic spectroscopy (QEPAS) for carbon monoxide (CO) detection in sulfur hexafluoride (SF₆) decomposition. High-voltage apparatuses, utilizing SF₆ as gas-insulating medium, require rapid and continuous monitoring. Detecting CO as one of the SF₆ decomposition products allows early diagnosis of apparatus failure. With the spectrophone consisting of an 8 kHz T-shaped quartz tuning fork and a pair of resonator tubes for photoacoustic detection, a quantum cascade laser (QCL) with central wavelength of 4.61 μm is used as the light source of the optical sensor. This novel design achieved a minimum detection limit of 10 ppb at 10 seconds of signal integration time for CO detection in SF₆ decomposition.

Detecting CO gas by means of wavelength modulation, second harmonic modulation, and photoacoustic techniques requires high precision and stable control of the quantum cascade laser in the QEPAS sensor. Wavelength Electronics’ QCL driver, the QCL2000 LAB, enabled sensitive measurements with minimal electronic noise from the laser for narrow linewidth from the QCL. The driver also allows analog modulation of up to 2-3 MHz for wavelength modulation and second harmonic modulation in the QEPAS sensor. The stability of the QCL is critical for exact excitation of the gas molecules detected by the quartz tuning fork.

This benchtop QCL driver instrument exhibits noise performance of 1.3 μA RMS up to 100 kHz with an average current noise density of 4 nA/√Hz. The QCL2000 LAB QCL driver instrument enables CO detection at parts-per-billion levels with low noise and stable laser output. This makes the developed QEPAS a reliable tool for CO monitoring inside gas-insulated switchgears and other high-voltage apparatuses using SF₆ as the gas insulating medium.

The complete case study is available as CS-LD05.