A challenge for slowly relaxing carbon monoxide (CO) molecules detection using photoacoustic spectroscopy (PAS) is to promote the vibration-translation (V-T) relaxation process. Addressing this challenge, a sensitivity enhanced photoacoustic CO sensor with sulfur hexafluoride (SF₆) as the promotor is investigated and demonstrated. A 1568 nm near-infrared (NIR) laser diode and a customized optical amplifier are used as the excitation source to generate the photoacoustic signal. A differential photoacoustic cell is simulated and designed to obtain identical laminar flow distribution in the resonant cell to suppress the flow noise. The modulation frequency and added SF₆ volume ratio are optimized experimentally to achieve optimal sensitivity. Feasibility and performance of the CO sensor with a small amount of SF₆ as promotor is discussed and evaluated, obtaining a ~ 2 times improvement of signal value compared to the one with pure N₂ background and resulting in a minimum detection limit of 467.5 ppb for CO detection.

Amplifiers and filters are essential components in microwave systems. An amplifier is used to increase the signal strength, and it is usually cascaded by filters for frequency selection or harmonic rejection [1] – [3] . Their functionalities require a precise design of the individual parts as well as the way in which they are connected. The common approach is to make individual components with a system impedance that is usually ${50}\Omega{.}$ A generic example of the filters and amplifier in a generalized receiver is shown in Figure 1(a) .

Gas pressure regulators are widely applied in natural gas pipeline networks, correspondingly, establishing an efficient fault diagnosis approach of regulators plays a critical role in optimizing the safety and reliability of pipeline network systems. In our paper, considering that the outlet pressure signals of gas regulators are nonstationary and nonlinear, we propose a fault diagnosis approach combining a complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) and fuzzy c-means (FCM) clustering to classify three typical faults of gas regulators. First, we propose to apply the CEEMDAN approach for decomposing intrinsic mode functions (IMFs). Then feature vectors of the typical faults are established by Hilbert marginal spectrum (HMS) of IMFs. Finally, we adopt cluster centers and feature clustering algorithm to distinguish the types of faults. The experimental results indicate the high performance of the present fault diagnosis approach. The membership degrees of test samples obtained from the CEEMDAN algorithm are optimized to be within 0.9 to 1.

The flow field is one of the key factors that affect the measuring accuracy of ultrasonic gas flowmeters. The present work demonstrates that the designed novel flow conditioner can enhance the measurement accuracy by homogenizing the velocity profile in the acoustic transmission area. The main objective of this study is to reduce the non-uniformity of flow on velocity distribution. To accomplish this objective, a combination of theoretical analysis and numerical simulations is implemented, and the results indicate that the 10:1 aspect ratio rectangular fluid channel is better applied in ultrasonic gas flowmeters. Thus, a Computational Fluid Dynamics (CFD) model of novel flow conditioner based on the 10:1 fluid channel is simulated and validated by an experimental study. The results verify that the ultrasonic gas flowmeter composed of designed flow conditioner can reduce indication errors by up to 6%; moreover, the repeatability of all the tested standard flow rates can meet the national standard and even be enhanced by 0.15%.

Modulation technology is the necessary means for generating periodic acoustic waves in photoacoustic gas detection, primarily including intensity modulation and wavelength modulation. In multi-gas detection, when multiple lasers employ the same modulation technique, current technologies include time-division multiplexing (TDM) for measurements at different times and frequency-division multiplexing (FDM) for simultaneous measurements; when multiple lasers employ different modulation techniques, the only available technology is TDM with measurements conducted at different times, and whether simultaneous measurement can be achieved has not yet been verified. We propose, for the first time, a multi-gas photoacoustic sensor using multi-mode demodulation. This sensor employs multi-mode frequency division multiplexing (MMFDM) technology to separate and demodulate the multi-mode photoacoustic signal, thereby enabling the simultaneous measurement of multiple gases under different modulation techniques. To demonstrate the feasibility of this method, we used SO${}_2$ and HF, the SF${}_6$ decomposition products in gas-insulated switchgear (GIS), as target gases and simultaneously detected their mixture using different modulation modes. Experimental results show that when the frequency difference is 10 Hz, multi-mode photoacoustic signal can be successfully separated, with the minimum detection limits for SO${}_2$ and HF reaching 117.9 ppb and 65.5 ppb, respectively. This study is the first to validate the separability of multi-mode photoacoustic signal and achieve multi-gas simultaneous measurement under multi-mode modulation, thereby eliminating the limitations of modulation mode in simultaneous photoacoustic multi-gas detection.

In traditional sensory array-based acoustic emission methods that are used for gas leakage localization, the localization resolution depends on the spatial aperture of the array, that is, the number of sensors. Most of the existing methods use small arrays that can only achieve low-resolution localization results because of limitations such as the amplitude and phase consistency, the complexity and cost of the system. This paper reports the first application of a virtual phased array for gas leakage detection to obtain high-resolution localization results. This method uses a virtual linear ultrasonic sensor array composed of only two sensors to acquire leakage signals. Then, we use the virtual beamforming algorithm based on the cross-power spectrum to estimate the location of the leakage source. Several experiments were conducted to evaluate the effectiveness and operability of the proposed method. The impacts of various factors on the performance of the localization technique are compared and discussed, including factors such as the number of sensors and the distance between the leak hole and virtual array. The results demonstrate that the proposed method accurately and reliably localizes gas leakages.