Trace Ethane Measurement Using Photoacoustic Spectroscopy With Mid-infrared LED

As overheating or discharge faults generate ethane?C₂H₆?gas in transformers,the trace concentration of C2H6 is of great significance for the early prediction of transformer faults.In view of the lack of low cost lasers in the mid-infrared band for the high precision detection requirements of C2H6,a photoacoustic spectroscopy system based on mid-infrared light emitting diode?MIR-LED?is developed.And it achieves 11.2 ppm C2H6 concentration measurement.The main work of the thesis includes:Firstly,the sensing model of the gas photoacoustic?PA?spectrum detection system was established on the structure of the designed system,which is prepared for the following experimental verification and data analysis.Secondly,the theoretical modeling and simulation of acoustic characteristics of the one-dimensional longitudinally resonant PA cell is performed using the four-terminal network method,which provides a theoretical basis for the design of the PA cell.Due to the large divergence angle of the MID-LED and miniaturization of the PA cell,the mid-infrared beam shaping,filtering and antireflection systems have been designed.The well designed mid-infrared optical shaping system and the light source control system,including the beam divergence angle,the optical path aperture and loss,leads to a good match between the MIR-LED and the sensing system.The final transmittance of the system is greater than 65%,and the output power of the LED shaping system is 0.17 mW with a diameter of 18 mm.Combined with the designed PA cell and signal acquisition and processing system,the photoacoustic experimental system with MID-LED is built.In view of the low output power of MID-LED,we discuss in detail the effects of modulation waveform,modulation depth and duty cycle of the source on the amplitude of the PA signal.The results show that the maximum amplitude of the PA signal is obtained under the modulated conditions of the LED source of square wave with maximum modulation and duty cycle of 0.4,which provides a new idea for improving the detection sensitivity of the system.In this thesis,the optimized C₂H₆ photoacoustic detection system has further verified by experiments.In the analysis of experimental data,the influence of the geometry of the PA cell and the external temperature on the photoacoustic performance is discussed.The saturation mechanism of the PA signal is also analyzed.The experimental results of different concentrations of C2H6 show the sensitivity of the system is about 10 ppm,and the average error of experimental measurement result calibration with the least squares regression method is 3.075%,which verified the feasibility of the system.