| Trace gas concentration plays an important role in the early warning of Electric-power Natura-gas and Coal mine areas.The photoacoustic spectroscopy（PAS）technology is gradually replacing the traditional non-spectral measurement method,it has the advantages of good gas selectivity,high measurement accuracy,fast response and strong anti-interference ability.However,it also has the problem of gas sensing single,that rely on the mode of"multi-sensor and multi-component detection"to realize multi-gas sensing.Therefore,it is of great social and economic values and favourable application prospect to study a kind of fiber optic sensor,which detect multiple components,wide application and high detection stability.In this paper,we proposed a multi-gas concentration photoacoustic spectral detection scheme based on the characteristics of resonant photoacoustic cell and wide-band microphone.Multi-gas concentration detecting is realized by frequency division multiplexing and time division multiplexing（FDM-TDM）,which saves the hardware resources and improves the detection speed and detection efficiency of the system.This paper introduces the core theories of photoacoustic effect,wavelength modulation and harmonic detection in photoacoustic spectroscopy.Additionally,the methods and shortcomings of multi-component gas sensing in photoacoustic spectroscopy are analyzed.A multi-component gas concentration sensing system based on frequency division multiplexing and time division multiplexing technology is constructed by NI-my DAQ.Besides,the host computer and user interface of trace multi-gas fiber sensing system were designed by Labview.In my study,a new laser driver was designed through Labview virtual instrument and NI-my DAQ programmable measurement device,and solved the problem which the signal overlap in coupled optical path after the dual-channel“sawtooth-delay”signal driven laser produced by ARM.According to the response bandwidth of the photoacoustic cell,the modulation isolation bandwidth is established through the dual-frequency phase lock effect demodulation experiment,which reduces the mutual influence of the frequency division demodulation of the mixing photoacoustic signal.At the same time,the data stream was smooth-filtered by Labview’s Savitzky-Golay filter in real time to further reduce the system random noise.In experimenting,CO2,CH4,C2H2and H2O vapor were selected as samples to test the system performance.Photoacoustic signal detection of four gas concentration was achieved at 152 Torr,23°C,and the linear fitting and minimum detection limits（MDL）for these gas concentrations were as follows:CO2:0.98821,343ppm;CH4:0.99328,82ppm;C2H2:0.98921,6.3ppm and H2O vapor:0.9979,2.4ppm,respectively.In addition,the continuous monitoring of CH4for 1 hour shows that the system has good real-time and high stability.By linear fitting the relationship between the calibrated gas concentration and the corresponding second harmonic amplitude by Origin,the inversion and on-line display of the four gas concentrations are realized in Labview.The upper computer has the functions of information storage and threshold alarm,and the positive user experience. |
