Multi-pass Differential Photoacoustic Sensor for Real-Time Measurement of SF Decomposition Component HS at the ppb Level.

We designed and implemented a photoacoustic (PA) sensor for HS detection in SF 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 HS. 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 SF, the relationship between the signal-to-noise ratio (SNR) and the gas flow was different between SF and N background gases. The maximum flow rate of the characteristic gas components detected in the SF background is 150 standard cubic centimeters per minute (SCCM), which is lower than 350 SCCM in N. The linearity property was analyzed, and the results show that the sensitivity of the sensor to HS in the SF 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.