Current-Modulated Cavity Ring-Down Spectroscopy for Mobile Monitoring of Natural Gas Leaks

An improved current-modulated cavity ring-down spectroscopy(CRDS) approach is proposed and the experimental results reported in this article. The step-wise fine current modulation is imposed on the laser driving direct current (DC) component and achieves a proper wavelength matching between the incident laser beam and the resonating ringing cavity, which is then turned off to stop the laser injection. The feedback of this modulation at this matching point is used not only to monitor the status of the whole building-up process, but also to increase the measurement rate. The device exhibits a detection limit of 0.8 ppbv for CH
₄
in a 60 second averaging period and a detection limit of 10 ppbv for C
₂
H
₆
in a 16 second averaging period. In the natural gas leak field test based on a vehicle platform, the background methane concentration in the atmosphere was measured to be 1.95 ppmv with a sensitivity of 18.2 ppbv; while the background ethane concentration of 0.0787 ppmv and sensitivity of 31.2 ppbv was recorded. A typical natural gas pipeline inspection process is introduced and performed in the field test in Beijing. A leakage found during the field test has been repaired by the gas company.