Detection Of Acetylene By Photoacoustic Spectroscopy With Near-infrared Fiber Laser

With the rapid development of the electric power projects and electrization of industrial corporations, more and more transformers have been used. When in operation, transformer oil may perhaps produce some malfunction gases. As one kind of malfunction gases in transformer oil, acetylene is an important indicator for grades of breakdown and degrees of severity inside the transformer. As a result, testing the concentration of acetylene accurately is vitally important to the productions and lives. With the develope of fiber sensor and spectral detection, the technique of gas monitoring with fiber has been widely explored and applied. In this paper, a new method of photoacoustic spectroscopy for acetylene detection with TEDFL (tunable erbium-doped fiber laser) as light source is demonstrated.We analyzed the transition mechanism of acetylene at 1.5μm using quantum mechanics and molecular spectroscopy, and systematically discussed absorption strength, fine structure and absorption width of this band, which provided theoretical basis for the spectral measurement of acetylene.An experimental setup designed for near-infrared absorption spectrum of acetylene is presented. This system has verified the theoretical analysis and furnish with important reference value for the design of photoacoustic measurement system. We obtain the exact locations of acetylene transitions at 1.5μm and the absorbance and FWHM (full width at half maximum) of the especial transition (P9) as functions of acetylene concentration and system pressure. Meanwhile, as a spectral measurement instrument for acetylene, this system has many practical values since the sensitivity limit can reach 36ppm.A system of photoacoustic spectroscopy for acetylene detection is demonstrated. The TEDFL is first used as light source, instead of the DFB diode laser which is relative expensive. For Acetylene, a strong and isolated absorption transition (P9) at 1530.37nm is selected for trace gas monitoring. Through combining wavelength modulation and second harmonic detection technique of lock-in amplifier, we obtain a higher SNR (signal-to-noise ratio) when the laser's power is lower. The study results show that the relation between the amplitude of second harmonic signal and acetylene concentration is linear and the sensitivity limit for acetylene detection is 15ppm at atmosphere pressure. These measurements represent what we believe to be valuable to low-cost and industrial applications of photoacoustic measurement instrument.