Compact dual-gas sensor for simultaneous measurement of atmospheric methane, and water vapor using a 3.38 μm antimonide-distributed feedback laser diode

Double-channel sensors for high precision measurement of methane based on a dual-path Herriott cell

A double-channel methane (CH4) sensor was developed using a dual-pass multipass cell (DP-MPC) and a novel method that combines averaging dual-channel concentration signals with optimized detector gain configuration. This DP-MPC features two input/output coupling holes, resulting in absorption path lengths of approximately 95.8 m and 35.8 m, respectively. By optimizing the photodetector gain configuration and averaging the dual-channel concentration signals, the detection performance of the sensor was further enhanced. Allan deviation analysis indicated that after optimizing the detector gain, the measurement precision after dual-channel averaging reaches 21 ppb with an integration time of 1 s at a concentration of 2 ppm CH4, which is approximately 1.4 times higher than the measurement precision of the long-path channel (31 ppb) and short-path channel (30 ppb). The time required to achieve a measurement precision of 21 ppb is 2.4 s for the long-path channel and 2.1 s for the short-path channel. The response speed of the dual-channel averaging is approximately 2 times that of any single channel. Meanwhile, the sensor demonstrated its stability and reliability through continuous outdoor atmospheric CH4 measurements.

Compact photoacoustic spectrophone for simultaneously monitoring the concentrations of dichloromethane and trichloromethane with a single acoustic resonator

Chlorinated hydrocarbons are frequently used as reagents and organic solvents in different industrial processes. Real-time detection of chlorinated hydrocarbons, as toxic air pollutants and carcinogenic species, is an important requirement for various environmental and industrial applications. In this study, a compact photoacoustic (PA) spectrophone based on a single acoustic resonator for simultaneous detection of trichloromethane (CHCl₃) and dichloromethane (CH₂Cl₂) is first reported by employing a low-cost distributed feedback (DFB) laser emitting at 1684 nm. In consideration of the significant overlapping of absorption spectral from trichloromethane and dichloromethane, the multi-linear regression method was used to calculate the concentrations of CHCl₃ and CH₂Cl₂ with special characterization of the absorption profile. The current modulation amplitude and detection phase in the developed PA spectrophone was optimized for high sensitivity of individual components. The measurement interference of CHCl₃ and CH₂Cl₂ on each other was investigated for accurate detection, respectively. For field measurements, all optical elements were integrated into a 40 cm × 40 cm × 20 cm chassis. This paper provides an experimental verification which strongly recommends this sensor as a compact photoacoustic field sensor system for chlorinated hydrocarbon detection in different applications.

Simultaneous Detection of Multiple Atmospheric Components Using an NIR and MIR Laser Hybrid Gas Sensing System

A compact multi-gas sensor has been developed for simultaneous detection of atmospheric carbon monoxide (CO), nitrous oxide (N₂O), and methane (CH₄). Instead of the traditional time-division multiplexing detection technique, two lasers having center emission wavelengths of 1.653 μm (near-infrared (NIR) diode feedback (DFB) laser diode) and 4.56 μm (mid-infrared (MIR) quantum cascade laser) were simultaneously coupled to a multipass cell using a dichroic mirror, which significantly decreased the complexity of the measurement and increased the temporal resolution of the spectrometer. Wavelength modulation spectroscopy (WMS) with the second-harmonic detection technique (WMS-2f) was used to improve the detection sensitivity. A LabVIEW-based digital lock-in amplifier (DLIA) algorithm and system control unit was developed to make the system more compact and flexible. Allan deviation analysis indicates that detection limits of 6.36 ppb by volume for CO, 4.9 ppb by volume for N₂O, and 23.6 ppb by volume for CH₄ are obtained at 1 s averaging time, and the sensitivity can be improved to 0.44 ppb for CO, 0.41 ppb for N₂O, and 2 ppb for CH₄ at an optimal averaging time of 900 s. Two-day real-time measurement in ambient air was performed to demonstrate the long-term stability of the sensor system.

A dual-gas sensor for simultaneous detection of methane and acetylene based on time-sharing scanning assisted wavelength modulation spectroscopy

Methane (CH₄) and acetylene (C₂H₂) are important bioscience and chemical gases. The real-time monitoring and analysis of them have important research value in industrial process control. The time-sharing scanning assisted wavelength modulation spectroscopy (WMS) technique is developed for real-time and simultaneous detection of CH₄ and C₂H₂. This system involves two near-infrared distributed feedback (DFB) lasers and a compact multipass cavity with an effective optical path of 52.2 m. The selected strong absorption lines of methane and acetylene are located at 6046.96 cm-1 and 6531.7 cm-1, respectively. The experiment environment is conducted at room temperature 23 °C and pressure 760 Torr. The sensor performance, including the minimum detection limit (MDL) and the stability, was improved by eliminating the influence of light intensity fluctuation using the WMS-2f/SAW technique. Allan deviation analysis indicates that a MDL of 0.1 ppm for CH₄ and 0.2 ppm for C₂H₂ are achieved with 1-s integration time. And the instrument response time is about 44 s through the continuous analysis of standard gases. This sensitive, simple, reliable, and lowcost dual-gas sensor is very suitable for applications in the field environment, chemical process, and many other gas-phase analysis areas.