Multi-pass Differential Photoacoustic Sensor for Real-Time Measurement of SF₆Decomposition Component H₂S at the ppb Level

Abstract

We designed and implemented a photoacoustic (PA) sensor for H₂S detection in SF₆ background gas based on a multi-pass differential photoacoustic cell (MDPC) and a near-infrared distributed feedback (DFB) laser. In the MDPC apparatus, two resonators with identical geometric parameters were vertically and symmetrically embedded. The differential processing algorithm of two phase-reversed signals realized the effective enhancement of the PA signal and suppressed the flow noise in the dynamic sampling process. In addition, the λ/4 buffer chamber in the MDPC was utilized as a muffler to further reduce the flow noise and realize the dynamic detection of H₂S. The collimated excitation light was reflected 30 times in a multi-pass structure constituted of two gold-plated concave mirrors, and an absorption path length of 4.92 m was achieved. Due to the high gas density of SF₆, the relationship between the signal-to-noise ratio (SNR) and the gas flow was different between SF₆ and N₂ background gases. The maximum flow rate of the characteristic gas components detected in the SF₆ background is 150 standard cubic centimeters per minute (SCCM), which is lower than 350 SCCM in N₂. The linearity property was analyzed, and the results show that the sensitivity of the sensor to H₂S in the SF₆ background was 27.3 μV/ppm. With the structure, parameters, temperature, gas flow, and natural frequency of the MDPC been optimized, a minimum detection limit (MDL) of 11 ppb was reached with an averaging time of 1000 s, which furnished an effective preventive implement for the safe operation of gas insulation equipment.