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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.