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In this study, we developed and validated an improved tuning method for a nonlinear scanning laser diode in methane gas sensing, using wavelength modulation spectroscopy - tunable diode laser absorption spectroscopy (WMS-TDLAS). This approach combines cubic nonlinear scanning with high-frequency modulation to precisely target the methane absorption peak at 6046.96 cm−1. The method enhances absorption time, thus increasing system sensitivity and stability. Our results show that this nonlinear scanning, optimized for peak absorption, significantly improves the slope of the methane concentration relationship curve by 76.9%. It also reduces the standard deviation by 61.1% and the limit of detection by 60% compared to linear scanning. The limit of detection the system is 4.2 ppm, and the optimal integration time is 48s. This novel modulation strategy significantly enhances the WMS-TDLAS system's efficiency, offering significant benefits for various applications. • Cubic nonlinear scanning method (CNSM) was proposed and CH 4 detection was verified. • Scanning central wavenumber of LD obtained a longer absorption time. • CNSM significantly improved the slope of the methane concentration curve. • The sensing sensitivity and stability were improved by 45% and 49%, respectively. [ABSTRACT FROM AUTHOR]

A new optical diagnostic method that predicts the global fuel–air equivalence ratio of a swirl combustor using absorption spectra from only three optical paths is proposed here. Under normal operation, the global equivalence ratio and total flow rate determine the temperature and concentration fields of the combustor, which subsequently determine the absorption spectra of any combustion species. Therefore, spectra, as the fingerprint for a produced combustion field, were employed to predict the global equivalence ratio, one of the key operational parameters, in this study. Specifically, absorption spectra of water vapor at wavenumbers around 7444.36, 7185.6, and 6805.6 cm–1 measured at three different downstream locations of the combustor were used to predict the global equivalence ratio. As it is difficult to find analytical relationships between the spectra and produced combustion fields, a predictive model was a data-driven acquisition. The absorption spectra as an input were first feature-extracted through stacked convolutional autoencoders and then a dense neural network was used for regression prediction between the feature scores and the global equivalence ratio. The model could predict the equivalence ratio with an absolute error of ±0.025 with a probability of 96%, and a gradient-weighted regression activation mapping analysis revealed that the model leverages not only the peak intensities but also the variations in the shape of absorption lines for its predictions. [ABSTRACT FROM AUTHOR]

Hydroxyl-terminated polybutadiene (HTPB) is a fuel proposed for use in hypersonic rockets, specifically in solid fuel ramjets (SFRJs), where it combines with air and undergoes combustion processes that range from fuel pyrolysis to fuel lean-combustion. A better understanding and the improvement of SFRJ combustion can be accomplished through fundamental experiments and computational fluid dynamics modeling. The current chemical-kinetic models need data to capture the reaction kinetics at the conditions under which SFRJs operate. This work focuses on providing combustion data to assess the predictive capabilities of state-of-the-art mechanisms at conditions relevant to SFRJs. The University of Central Florida shock-tube facility was used to conduct experiments of various combinations of HTPB products in air near 5 atm for temperatures ranging from 1100 to 1800 K and Φ=0.5, 1.0, 4.76. Nonintrusive, time-resolved laser absorption spectroscopy was used to record carbon-monoxide-concentration time histories and compared against the predictions of AramcoMech3.0 and HyChem mechanisms. For each mixture, the mid- and high-temperature conditions matched the models well, whereas there was a significant discrepancy for the low-temperature condition. A sensitivity analysis was then conducted to determine which reactions warranted further investigation to address the discrepancy at low temperatures. [ABSTRACT FROM AUTHOR]

High-sensitivity solutions for real-time and on-line multi-component gas detection are required in coal mine safety, petrochemical and storage and transportation industries, atmospheric and environmental monitoring, combustion diagnosis, and industrial process monitoring and control. In optical technology, Tunable Diode Laser Absorption Spectroscopy (TDLAS) has been proven to be an advanced technology for detection of multi-component gas, which benefits from the high sensitivity, good selectivity, fast response and anti-disturbance of TDLAS sensors. In detection of multi-component gas, the presence of non-negligible cross-interference, cost and performance requirements become a primary concern in the design of many detection schemes. In this review article, the application of TDLAS technology in multi-component gas detection will be discussed in two categories: (I) Multiple lasers for detection of multi-component gas without cross-interference. (II) A single laser for detection of multi-component gas. This article outlines the advantages and disadvantages of applying various schemes in the two categories when TDLAS is applied to multi-component detection, with the aiming of providing ready-made guidance for multi-component detection under various applications. [ABSTRACT FROM AUTHOR]

We report on sensitive tunable laser absorption spectroscopy using a multipass gas cell and a solid-state photoacoustic optical power detector. Unlike photoacoustic spectroscopy (PAS), this method readily allows a low gas pressure for high spectral selectivity and a free gas flow for continuous measurements. Our photoacoustic optical power detector has a large linear dynamic range and can be used at almost any optical wavelength, including the middle infrared and THz regions that are challenging to cover with traditional optical detectors. Furthermore, our approach allows for compensation of laser power drifts with a single detector. As a proof of concept, we have measured very weak CO2 absorption lines at 9.2 µm wavelength and achieved a normalized noise equivalent absorption (NNEA) of 2.35·10−9 Wcm−1Hz−1/2 with a low-power quantum cascade laser. The absolute value of the gas absorption coefficient is obtained directly from the Beer-Lambert law, making the technique calibration-free.

Here is described an instrument for ammonia determination utilizing near-infrared laser absorption suitable for high-temperature applications. The laser wavelength is scanned across the ammonia absorption line at 1.512 µm and wavelength-modulation spectroscopy is simultaneously performed to provide robust and accurate measurements. The thermal effects and beam-steering effects are highlighted in the design. An aluminum baffle is employed to alleviate the adverse impact of elevated temperature on the electronic components and evaluated by an infrared thermographic camera. The beam-steering effects were quantitatively assessed through a combination of two-dimensional computational fluid dynamics and ray tracing analysis. Reference standards from 1 to 20 ppm ammonia were used to verify the system performance. The determined ammonia concentrations exhibit good consistency with the reference values, as demonstrated by a coefficient of determination (R2) of 0.999. Furthermore, a detection limit of 0.02 ppm was achieved using an integration time of 60 s. Pprecision of 0.11 ppm was achieved using 1 s time resolution. [ABSTRACT FROM AUTHOR]

The emission of carbon dioxide (CO2) by thermal power plants is the main factor leading to the greenhouse effect. The measurement of CO2 is of great significance to carbon emission data statistics. In this paper, a measurement system based on tunable diode laser absorption spectroscopy (TDLAS) was built to measure the spectrum of CO2 near 1579 nm. To simplify the experimental system, a direct absorption signal fitting method is proposed to replace etalons for time‐frequency conversion. The method was applied to the measurement of four different concentrations of CO2 in the range of 7%–16%, and the measurement results were compared with the multipeak method, another etalon‐free method. The results show that the mean relative error of the measurement results is 4.23%, which is 1.76% lower than the measurement results of the multipeak method. The experimental research provides a feasible idea to simplify the CO2 measurement system based on TDLAS. [ABSTRACT FROM AUTHOR]

A compact absorption spectrometer with a narrowband continuous tunable diode laser is created; it provides sensitivity on the order of 1 × 10−6 cm−1 in terms of the absorption coefficient. The design of the spectrometer, the measurement technique, and the ozone generation and control procedure are described. The absorption spectrum of the ozone molecule is recorded for a system of Wulf bands in the near-IR range 11 900–12 800 cm−1, which correspond to rovibronic transitions from the ground to excited triplet electronic states above the main dissociation threshold of the molecule. The absorption coefficient is simulated and predissociation broadening of spectral lines is estimated in the spectral range under study based on the simulation results. Ozone absorption cross sections in this range are recommended for atmospheric applications; they have been derived using statistically weighted averaging of the new measurements and published laboratory experimental data. [ABSTRACT FROM AUTHOR]

The temperature measurement of Sulfur dioxide (SO2) is a crucial parameter for accurately determining its concentration, particularly in high‐temperature environment. Herein, it is realized by using three different techniques, which are direct absorption spectroscopy, wavelength modulation spectroscopy and non‐calibration wavelength modulation spectroscopy based on the bispectral lines temperature measurement method. The bispectral lines of SO2 locates at 1396.844 and 1396.921 cm−1, respectively. The gas temperature in the absorption cell was also measured with K‐type thermocouple and compared with these three spectral measurements. The linearities corresponding to these three techniques are 99.71%, 99.8%, and 99.88%, respectively. Their average relative error between the spectral measurement results and the linear fitting value of the three techniques is 1.15%, 0.86%, and 0.57%, respectively. It is illustrated the feasibility of the bispectral lines method for high‐temperature measurement. These investigations provide methods for temperature measurement in high‐temperature coal‐fired boilers, potentially for investigating other sulfide gas molecules such as SO3 and H2S. [ABSTRACT FROM AUTHOR]

Methane (CH4) is the main pollutant in oil and gas production. The detection and accounting of CH4 is an important issue in the process of greenhouse gas control and emission reduction in oil and gas industry. In this study, a portable CH4 detector based on tunable diode laser absorption spectroscopy (TDLAS) technology was deployed. The three-dimensional distribution of CH4 in a natural gas purification plant in Sichuan was obtained through vertical unmanned aerial vehicle (UAV) flight observations and ground mobile observations. According to the mass balance method, the emission of CH4 on 30 m above ground level (AGL) and 60 m AGL in this site was about 0.012 kg/s (±42% at 1σ) and 0.034 kg/s (±47% at 1σ), respectively, in one day. The vertical distribution showed that the CH4 concentration reached the maximum (2.75 ± 0.19 ppm) with height of 0 to 100 m AGL. The CH4 concentration from 100 to 300 m AGL showed a downward trend with height. Atmospheric instability at high altitude and high wind speed promoted the diffusion of CH4. The CH4 concentrations of horizontal distribution on 30 m AGL and 60 m AGL were 2.48 ± 0.11 ppm and 2.76 ± 0.34 ppm. In the observation of mobile campaigns, the connecting equipment of natural gas treatment facilities was prone to leakage, such as in valves and flanges. CH4 leakage was also detected at the torch mouth, especially when there was an open flame at the torch mouth. During the mobile movement investigation, the downwind measurement (OTM-33A) was applied to determine the overall CH4 emission rate shortly after patrolling the site. This work plays a vital role in optimizing the operation and maintenance of natural gas production stations pipe network, ensuring human safety and minimizing greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

Combustion diagnosis is crucial for quantifying exhaust gas and calculating combustion efficiency. Temperature, concentration, and velocity measurements allow for the accurate identification of combustion conditions. However, the current diagnostic equipment used to assess the combustion state has various issues, such as flow disturbance caused by probes and measurement delays caused by sampling. Furthermore, using multiple measurement devices can be particularly inconvenient. This study confirmed the potential application of an exhaust gas emission and engine operation monitoring system by simultaneously measuring temperature, concentration, and velocity using tunable diode laser absorption spectroscopy (TDLAS) technology. To determine the combustion chamber of methane combustion, multiple laser beams that are separated using a beam splitter were arranged in various forms to measure temperature, concentration, and velocity. The reconstructed temperature and concentration distribution results for each position of the measurement cell case showed similar trends at identical measurement positions. Additionally, experiments confirmed that the velocity measured by laser crossing at a 45-degree angle from the center increased as the supply gas flow rate increased. [ABSTRACT FROM AUTHOR]

We propose a novel technique to accurately predict carbon dioxide (CO2) concentrations even in flow fields with temperature gradients based on a single laser path absorption spectrum measurement and machine learning. Concentration measurements in typical tunable diode laser absorption spectroscopy are based on a ratio of two integrated absorbances, each from a spectral line with different temperature dependence. However, the inferred concentrations can deviate significantly from the actual concentrations in the presence of temperature gradients. Furthermore, it is also difficult to find an analytical expression to compensate for the effect of nonuniform temperature profiles on concentration measurements. In this study, the entire absorption feature was considered since its shape and peak intensities vary with temperature and concentration. Specifically, a predictive model is obtained in a data-driven manner that can identify and compensate for the effect of a nonuniform temperature field on the spectrum. Despite a very detailed understanding of the CO2 absorption spectrum, it is nearly impossible to collect sufficient spectra for model acquisition by varying all temperature gradient conditions. Therefore, the model was obtained using only simulated data, much like the concept of a "digital twin". Finally, the predictive performance of the acquired model was verified using experimental data. In all test cases, the predictive performance of the model was superior to that of the two-line method. Additionally, a gradient-weighted regression activation mapping analysis confirmed that the model utilizes both the peak intensities as well as the change in the shape of absorption lines for prediction. Graphical Abstract This is a visual representation of the abstract. [ABSTRACT FROM AUTHOR]

This study presents an absorption-area-based method for the measurement of gas flow velocity using diode laser absorption spectroscopy. In the proposed method, the gas flow velocity is determined using the area difference between the absorption lines. To verify the proposed method, flow velocity measurements are conducted for test conditions with a velocity range of 5–30 m/s in a low-speed wind tunnel, and the results are compared with those of the conventional method using the Doppler-shifted peak. Oxygen in the air is selected as the target gas. By comparing the flow velocity measurement results of the conventional method and the proposed area-based method, it is confirmed that the measurement precision is improved when the proposed area-based method is used. Moreover, the measurement accuracy of the proposed area-based method is significantly improved at a relatively low-speed range of 5–10 m/s compared to the conventional method. [ABSTRACT FROM AUTHOR]

The use of optical circular multipass absorption cells (CMPAC) in an open-path configuration enables the sampling free analysis of cylindrical gas flows with high temporal resolution and only minimal disturbances to the sample gas in the pipe. Combined with their robust unibody design, CMPACs are a good option for many applications in atmospheric research and industrial process monitoring. When deployed in an open-path configuration, the effects of inhomogeneities in the gas temperature and composition have to be evaluated to ensure that the resulting measurement error is acceptable for a given application. Such an evaluation needs to consider the deviations caused by spectroscopic effects, e.g., nonlinear effects of temperature variations on the intensity of the spectral line, as well as the interaction of the temperature and concentration field with the characteristic laser beam pattern of the CMPAC. In this work we demonstrate this novel combined evaluation approach for the CMPAC used as part of the tunable diode laser absorption spectroscopy (TDLAS) reference hygrometer in PTB's dynH2O setup for the characterization of the dynamic response behavior of hygrometers. For this, we measured spatially resolved, 2D temperature and H2O concentration distributions, and combined them with spatially resolved simulated spectra to evaluate the inhomogeneity effects on the line area of the used H2O spectral line at 7299.43 cm−1. Our results indicate that for dynH2O, the deviations caused by the interaction between large concentration heterogeneities and the characteristic sampling of the beam pattern of the CMPAC are three orders of magnitude larger than deviations caused by small temperature heterogeneity induced spectroscopic effects. We also deduce that the assumption that the "path-integrated" H2O concentration derived with the open-path CMPAC setup represents an accurate H2O area average in the flow section covered by the CMPAC in fact shows significant differences of up to 16% and hence does not hold true when large H2O concentration gradients are present. [ABSTRACT FROM AUTHOR]

This article presents a new linear calibration‐free wavelength modulation spectroscopy (LCF‐WMS) technique for gas sensing. By combining the natural logarithms of the transmitted intensity and background signals, this technique eliminates the effects of the intensity, intensity modulation, and the modulation phase to obtain integrable harmonic signals along the line‐of‐sight and thus realize calibration‐free measurements. To illustrate the validity of the LCF‐WMS technique, it was applied to measurement of the water vapor concentration in air at ambient temperature. Because of its specific features of line‐of‐sight integrability and baseline independence, this method will be advantageous in absorption tomography and high‐pressure applications. [ABSTRACT FROM AUTHOR]

This study presents a single-line absorption spectroscopy method for the simultaneous measurement of gas pressure and temperature through diode laser absorption spectroscopy using a single absorption line. In the proposed method, the absorbance loss of an absorption line that may occur during baseline subtraction is predicted, and the gas properties are determined by matching a calculated absorbance to a recovered absorbance using the predicted loss. To verify the proposed method, gas property measurements are conducted for air conditions with a pressure range of 10–100 kPa and a temperature range of 300–500 K. An absorption line in the oxygen A band is selected to measure the gas properties of air. Using the proposed method, it is confirmed that the gas pressure and temperature could be measured within errors of 25 % and 18 %, respectively. When the gas pressure is predetermined using a pressure gauge, the gas temperature can be measured within an error of 14 %. [ABSTRACT FROM AUTHOR]

A tunable diode laser absorption spectroscopy (TDLAS) device has been developed to study long-path atmospheric transmission near diode pumped alkali laser (DPAL) emission wavelengths. By employing a single aperture and retro reflector in a mono-static configuration, the noise associated with atmospheric and platform jitter were reduced by a factor of ∼30 and the open-air path length was extended to 4.4 km and over a very broad spectral range, up to 120 cm−1. Water vapor absorption lines near the rubidium (Rb) and cesium (Cs) variants of the DPAL near 795 and 894 nm, oxygen lines near the potassium (K) DPAL near 770 nm, and water vapor absorption in the vicinity of the neodymium-doped yttrium aluminum garnet (Nd:YAG) laser 1.064 μm and chemical oxygen iodine laser (COIL) 1.3 μm lines were studied. The detection limit for path absorbance increases from ΔA = 0.0017 at 100 m path length to 0.085 for the 4.4 km path. Comparison with meteorological instruments for maritime and desert environments yields agreement for the 2.032 km path to within 1.5% for temperature, 4.5% for pressure, and 5.1% for concentration, while agreements for the 4.4 km path are within 1.4% for temperature, 7.7% for pressure, and 23.5% for concentration. An intra cavity output spectroscopy (ICOS) device was also used as a spectral reference to verify location of atmospheric lines. Implications of TDLAS collection system design on signal-to-noise (S/N) are discussed as well as the effect of path turbulence on baseline noise and inform the selection of the DPAL variant least affected by molecular absorption. Graphical Abstract This is a visual representation of the abstract. [ABSTRACT FROM AUTHOR]

A new version of a sensor for temperature measurements in the case of strong laser intensity fluctuation was developed. It was based on tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation, logarithmic conversion of the absorption signal, and detection of the first harmonic of the modulation frequency. The efficiency of the technique was demonstrated under experimental conditions with excess multiplicative noise. Temperature was evaluated from the ratio of integrated absorbance of two lines of the water molecule with different lower energy levels. Two algorithms of data processing were tested, simultaneous fitting of two spectral ranges with selected absorption lines and independent fitting of two absorption lines profiles. The correctness of the gas temperature evaluation was verified by simultaneous measurements with a commercial thermocouple. An error in temperature evaluation of less than 40 at 1000 K was achieved even when processing a single scan of the diode lasers. [ABSTRACT FROM AUTHOR]

Summary: Use of a complete dynamic model of NADP‐malic enzyme C4 photosynthesis indicated that, during transitions from dark or shade to high light, induction of the C4 pathway was more rapid than that of C3, resulting in a predicted transient increase in bundle‐sheath CO2 leakiness (ϕ).Previously, ϕ has been measured at steady state; here we developed a new method, coupling a tunable diode laser absorption spectroscope with a gas‐exchange system to track ϕ in sorghum and maize through the nonsteady‐state condition of photosynthetic induction.In both species, ϕ showed a transient increase to > 0.35 before declining to a steady state of 0.2 by 1500 s after illumination. Average ϕ was 60% higher than at steady state over the first 600 s of induction and 30% higher over the first 1500 s.The transient increase in ϕ, which was consistent with model prediction, indicated that capacity to assimilate CO2 into the C3 cycle in the bundle sheath failed to keep pace with the rate of dicarboxylate delivery by the C4 cycle. Because nonsteady‐state light conditions are the norm in field canopies, the results suggest that ϕ in these major crops in the field is significantly higher and energy conversion efficiency lower than previous measured values under steady‐state conditions. [ABSTRACT FROM AUTHOR]

Highly sensitive standoff methane detection is vital for atmospheric science, environmental protection, and production safety. We develop a mid-infrared methane standoff sensor using a cooperative target based on tunable diode laser absorption spectroscopy (TDLAS). To enhance sensor sensitivity, we propose a one-dimensional frequency channel attention-based convolutional neural network (1D-FCACNN) filter, which can effectively denoise the methane absorbance signals and its second harmonic signals. The filter model is adequately trained on a simulated spectrum dataset, which we constructed based on data augmentation. In comparison with reported filtering algorithms, the proposed filter shows the best performance in both measuring modes and evaluation metrics. Real-time measurements show that the measuring accuracy and limit of detection (LOD) of the proposed sensor reach a minimum of 30.40 ppb and 5.04 ppb over a 10-meter optical range, a significant improvement compared to previous reports of methane standoff sensors. The proposed methane standoff sensor proves the feasibility of enhancing the performance of TDLAS gas sensors with the attention mechanism, bringing a new option for high-sensitivity measurements of methane and other atmospheric trace gases. • A TDLAS-based mid-IR standoff sensor is developed for atmospheric CH 4 detection. • CNNs and FCA mechanisms are used to construct filters for 1D spectral denoising. • A straightforward data augmentation-based dataset construction method is proposed. • The filter-assisted sensor shows sensitivity and stability beyond existing studies. [ABSTRACT FROM AUTHOR]

• Simultaneous and interference-free measurements of T and C 2 H 4 are performed using differential absorption spectroscopy. An Interband Cascade Laser near 3.3 μm is employed to improve the detection sensitivity of this system. The interference from non-target absorption lines and background radiation is suppressed. The effects of particle or droplet extinction and scattering in the flame are also reduced. In diagnostic applications based on tunable diode laser absorption spectroscopy, the measurement of target substances can be influenced by factors such as background thermal radiation in the combustion environment, extinction caused by solid or liquid particles, and other interfering absorptions. In this work, we developed a differential absorption diagnostic technique based on wavelength pairs, utilizing an interband cascade laser near 3.3 μm to simultaneously measure temperature and C 2 H 4 concentration in hydrocarbon flames. Based on a detailed study of the C 2 H 4 spectrum in this region and considering the optimal standard for spectral lines, two wavelength pairs were selected. The temperature is determined by the ratio of the absorption cross-sections of two wavelength pairs, and the C 2 H 4 concentration is inferred based on the wavelength pair with higher differential absorption. In the initial stage, the system's accuracy was verified in high-temperature static conditions (T = 300–800 K, P = 1 atm), and continuous time series measurements demonstrated the system's stability. The limit of detection achieved by Allan-Werle variance analysis is 2.5 ppm at the optimal average time of 100 s. Subsequently, measurements were taken in a hydrocarbon flame. The obtained results indicate an average deviation of 1.021 % between the measured temperature in the flame and the reference value, with a standard deviation of 1.381 % for concentration measurement. All the measurements show that the system can be potentially applied to combustion diagnosis. [ABSTRACT FROM AUTHOR]

The second harmonic signal serves as the informational foundation for the inversion of the gas concentration in the tunable diode laser absorption spectroscopy (TDLAS) detection method. However, due to the impact of noise on the second harmonic signal, the accuracy and stability of the detection system are decreased. In this paper, a digital denoising method based on Long Short-Term Memory Denoising Autoencoder (LSTM-DAE) is proposed to address the issue of multiple noise interference in TDLAS detection system. In LSTM-DAE, the conventional DAE is combined with LSTM structure. The second harmonic signal has time correlation, and the LSTM structure can selectively remember the past node state, which helps the DAE reconstruct the original data. Additionally, CO 2 absorption spectrum at 2004 nm as an example, the effectiveness of this method is confirmed through simulated and real experiments. The experimental results demonstrate that LSTM-DAE, with the signal-to-noise ratio of 46.65 dB, the correlation coefficient of 0.9983, and the system detection limit of 6.1 ppb, outperforms other competitive methods in both qualitative and quantitative evaluations. [ABSTRACT FROM AUTHOR]

• Multitask learning enables simultaneous C 2 H 2 concentration and gas pressure sensing. • Stable performance maintains under varying spectral-overlap deformation. • Zero-absorption strategy simplifies dataset preparation for model training. Spectral deformation from the overlap of nonconstant impurities, as well as system noises, has become one of the critical factors that limit the wide applications of tunable diode laser absorption spectroscopy, particularly when multiparameter sensing highly relies on the accuracy of retrieved spectroscopy. Herein, we demonstrate a joint retrieval model that can simultaneously measure analyte concentration and gas pressure. The model is composed of expert module, gating module and tower module, which are responsible for feature extraction, expert selection and concentration/pressure output, respectively. An alternative method of introducing zero-absorption system noise into spectral simulation prepares datasets for model training, achieving measurement of acetylene concentration (50–500 ppm) and gas pressure (0.1–1 atm) with low relative deviations maintained, even with concentration-varying ammonia. This method, in a simple manner, enables accurate multiparameter sensing in many applications, where potential spectral deformation from impurities exists. [ABSTRACT FROM AUTHOR]

Methane (CH4) is the main pollutant in oil and gas production. The detection and accounting of CH4 is an important issue in the process of greenhouse gas control and emission reduction in oil and gas industry. In this study, a portable CH4 detector based on tunable diode laser absorption spectroscopy (TDLAS) technology was deployed. The three-dimensional distribution of CH4 in a natural gas purification plant in Sichuan was obtained through vertical unmanned aerial vehicle (UAV) flight observations and ground mobile observations. According to the mass balance method, the emission of CH4 on 30 m above ground level (AGL) and 60 m AGL in this site was about 0.012 kg/s (±42% at 1σ) and 0.034 kg/s (±47% at 1σ), respectively, in one day. The vertical distribution showed that the CH4 concentration reached the maximum (2.75 ± 0.19 ppm) with height of 0 to 100 m AGL. The CH4 concentration from 100 to 300 m AGL showed a downward trend with height. Atmospheric instability at high altitude and high wind speed promoted the diffusion of CH4. The CH4 concentrations of horizontal distribution on 30 m AGL and 60 m AGL were 2.48 ± 0.11 ppm and 2.76 ± 0.34 ppm. In the observation of mobile campaigns, the connecting equipment of natural gas treatment facilities was prone to leakage, such as in valves and flanges. CH4 leakage was also detected at the torch mouth, especially when there was an open flame at the torch mouth. During the mobile movement investigation, the downwind measurement (OTM-33A) was applied to determine the overall CH4 emission rate shortly after patrolling the site. This work plays a vital role in optimizing the operation and maintenance of natural gas production stations pipe network, ensuring human safety and minimizing greenhouse gas emissions.

This article describes the process of integrating one of the most commonly used laser methane detectors, the Laser Methane mini (LMm), and a multi-rotor unmanned aerial vehicle (UAV) based on the Pixhawk flight controller to create an unmanned aerial system designed to detect methane leakages from the air. The integration is performed via the LaserHub+, a newly developed device which receives data from the laser methane detector, decodes it and transmits it to the flight controller with the protocol used by the ArduPilot platform for laser rangefinders. The user receives a single data array from the UAV flight controller that contains both the values of the methane concentrations measured by the detector, and the co-ordinates of the corresponding measurement points in three-dimensional space. The transmission of data from the UAV is carried out in real time. It is shown in this project that the proposed technical solution (the LaserHub+) has clear advantages over not only similar serial commercial solutions (e.g., the SkyHub complex by SPH Engineering) but also experimental developments described in the scientific literature. The main reason is that LaserHub+ does not require a deep customization of the methane detector or the placement of additional complex devices on board the UAV. Tests using it were carried out in aerial gas surveys of a number of municipal solid waste disposal sites in Russia. The device has a low cost and is easy for the end user to assemble, connect to the UAV and set up. The authors believe that LaserHub+ can be recommended as a mass solution for aerial surveys of methane sources. Information is provided on the approval of LaserHub+ for aerial gas surveys of a number of Russian municipal waste disposal facilities.

A highly sensitive dual-gas sensor based on a two-channel multipass cell (MPC) was designed and developed for simultaneous detection of atmospheric methane (CH4) and carbon dioxide (CO2) by using two distributed feedback lasers emitting at 1653 nm and 2004 nm. The nondominated sorting genetic algorithm was applied to intelligently optimize the MPC configuration and accelerate the dual-gas sensor design process. A compact and novel two-channel MPC was used to achieve two optical path lengths of 27.6 m and 2.1 m in a small volume of 233 cm3. Simultaneous measurements of CH4 and CO2 in the atmosphere were performed to demonstrate the stability and robustness of the gas sensor. According to the Allan deviation analysis, the optimal detection precision for CH4 and CO2 was 4.4 ppb at an integration time of 76 s and 437.8 ppb at an integration time of 271 s, respectively. The newly developed dual-gas sensor exhibits superior characteristics of high sensitivity and stability, cost-effectiveness and simple structure, which make it well-suited for multiple trace gas sensing in various applications, including environmental monitoring, safety inspections and clinical diagnosis.

In the Ruhrstahl–Heraeus (RH) vacuum degassing process, we propose a real-time prediction model for the carbon content in molten steel, and show that the decarburization endpoint can be accurately determined using this model. Firstly, we applied a novel off-gas analyzer that can measure the carbon oxide concentration produced in the decarburization reaction faster and more accurately. Next, we generate decarburization curves using the off-gas components measured by the new analyzer. The decarburization curve describes the carbon content profile well during operation, and shows good agreement with the actual carbon content. In order to predict the carbon content during operation in real time, we create an artificial neural network (ANN) using the decarburization curves and operation data. By comparing the endpoint carbon content measured at the end of the operation with the predicted values, we confirmed the excellent predictive performance of the ANN model. Finally, we show that it is possible to accurately determine the decarburization endpoint using the prediction model. We expect that the proposed real-time prediction model can increase the productivity of the RH process. [ABSTRACT FROM AUTHOR]

In the quantitative analysis of mixed gases by tunable diode laser absorption spectroscopy, the overlapping of absorption spectra and mutual interference of multi-component gases can lead to problems of large measurement errors and low analysis accuracy. In this paper, an improved firefly algorithm is proposed and applied to the support vector machine regression model to solve this problem. The specific method includes introducing an adaptive step size to balance the local and global searches and using the gradient descent method to accelerate the parameter optimization process so as to improve the model's generalization ability and prediction accuracy. The experimental results show that the maximum errors of the improved algorithm in the prediction of CH 4 and CO gas concentrations are no more than 0.0443 % and 2 ppm, with coefficients of determination, R 2 , of 0.9994 and 0.99815. The promising results obtained by the system provide theoretical support for the realization of high-precision detection of multicomponent gases with a single source of light, and also demonstrate the high efficiency and feasibility of the method in practical detection., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Water vapour plays a crucial role in atmospheric processes. Hence, monitoring the altitude-related variations in water vapour properties is important to decipher atmospheric processes. Direct tunable diode laser absorption spectroscopy (dTDLAS) measures the concentration and temperature of gas molecules by scanning the rotation-vibration absorption lines using a high-spectral-resolution laser. In this study, we devised an integrated measurement and data processing method (integrative measurement and processing method for hygrometry, IMPMH) to enhance the in-situ airborne measurement capability of dTDLAS. We measured a wide range (240-18,000 ppm) of water vapour concentrations, aiming for atmospheric measurements in a highly water-saturated regime, called the "optically thick condition". For recovering the full absorption spectra, the "integrative area" was defined and a difference factor D, which is the distance between two spectral regions with width corresponding to the half width of half maximum of the Voigt profile, was used to calculate the area. From the data, the low-bound concentration was measured to be 244 ppm. At D = 1.8, the transition concentration to the "optically thick condition" was measured to be 5,800 ppm. By increasing D from 1.8 to 2.8, the measurable upper-bound concentration increased to 17,993 ppm. IMPMH was applied to the measured data to estimate the final absorber density or water vapour concentration. The estimation was well-fitted with the measured detector signal with signal-to-noise ratio (SNR) of ∼ 300 of the residual spectrum, promising its applicability to in-situ airborne measurements. To validate IMPMH, the water vapour concentration range was divided into two regimes: (1) optically thick (5,800 < c < 18,000 ppm) and (2) optically thin (c < 5,800 ppm) conditions. Under the optically thick condition, IMPMH was validated by comparing the results between the short and long-path cells. In the optically thin condition, IMPMH was validated through comparison with the general dTDLAS method. Lastly, long-term stability of the dTDLAS system was validated by measuring 10 different concentrations (240-18,000 ppm) for 1000 s by characterising the precision and SNRs of the residual. The results demonstrate that IMPMH significantly enhances the in-situ airborne measurement capability of dTDLAS under both optically thick and thin conditions. Furthermore, requirements for the implementation of IMPMH in airborne measurement were investigated considering four aspects-sampling, low-pressure measurement, accuracy and precision, and multiplex detection. The results were examined with regard to atmospheric implications., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier B.V. All rights reserved.)

We report on sensitive tunable laser absorption spectroscopy using a multipass gas cell and a solid-state photoacoustic optical power detector. Unlike photoacoustic spectroscopy (PAS), this method readily allows a low gas pressure for high spectral selectivity and a free gas flow for continuous measurements. Our photoacoustic optical power detector has a large linear dynamic range and can be used at almost any optical wavelength, including the middle infrared and THz regions that are challenging to cover with traditional optical detectors. Furthermore, our approach allows for compensation of laser power drifts with a single detector. As a proof of concept, we have measured very weak CO 2 absorption lines at 9.2 µm wavelength and achieved a normalized noise equivalent absorption (NNEA) of 2.35·10 -9 Wcm -1 Hz -1/2 with a low-power quantum cascade laser. The absolute value of the gas absorption coefficient is obtained directly from the Beer-Lambert law, making the technique calibration-free., Competing Interests: The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Jussi Rossi reports financial support was provided by Research Council of Finland. Jussi Rossi reports financial support was provided by Finnish Cultural Foundation. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Authors.)

Accurate monitoring of ammonia has decisive significance for the environmental control of poultry housing. Existing sensors based on semiconductor or electrochemistry have the defects of short life, severe baseline drift and delayed response when facing the harsh environment of poultry housing. In this work, we developed a portable sensor based on tunable diode laser absorption spectroscopy with a micro circular absorption cell for sensitive detection of ammonia in poultry housing. The micro circular absorption cell has a volume of only 25 ml, but the effective absorption path is up to 5 m, which allows the sensor to achieve the ability of less than 15 s response time and 0.2 ppm measurement accuracy. The results of continuous monitoring for 6 days showed that the ammonia concentration in the range of 0–6 ppm was accurately detected in a poultry house with 36 roosters. Through analyzing dynamic changes in ammonia concentration, we successfully identified some abnormal activity caused by humans or weather. Therefore, our sensor has performances of accurate, stable, real-time measurement of ammonia and can provide strong technical support for environmental control of poultry housing.

This paper develops a size-adaptive hybrid meshing scheme for Chemical Species Tomography (CST) that is driven by the customized spatial resolution of the sensing region. Traditionally, the entire sensing region in CST is uniformly discretized with the empirically determined density of the meshes. Such a discretization results in a) waste of computational efforts on the less spatially resolved location; and b) much severer rank deficiency. To solve the above-mentioned issues, we introduce, for the first time, a size-adaptive hybrid meshing scheme for CST. Driven by the spatial resolution, dense meshes are deployed in the region of interest (RoI) to detail the target flow field while sparse ones are deployed out of the RoI to fully consider the physically existing laser absorption. The proposed scheme is numerically validated using a CST sensor with 128 laser beams. The visual and quantitative metric comparisons show that the proposed hybrid-size meshing scheme outperforms the traditionally uniform-size meshing scheme, giving 35% lower image error and 38% less significant dislocation at a typical 35 dB signal-to-noise ratio in the RoI. The proposed hybrid-size meshing scheme significantly facilitates the reconstruction of the industrial combustion processes where the combustion zone is bypassed by cooling flows. In these scenarios, the proposed scheme can adapt a finer resolution to detail the combustion zone, while maintaining the integrity of the physical model by less resolved reconstruction of the bypass flows. [ABSTRACT FROM AUTHOR]

[Display omitted] • IMPMH, an integrative measurement and processing method for hygrometry is proposed. • IMPMH significantly increases the upper-bound of water-vapour concentration. • It enables concentration measurements in both optically thick and thin conditions. • Thus, the study improves the airborne measurement capability and range of dTDLAS. Water vapour plays a crucial role in atmospheric processes. Hence, monitoring the altitude-related variations in water vapour properties is important to decipher atmospheric processes. Direct tunable diode laser absorption spectroscopy (dTDLAS) measures the concentration and temperature of gas molecules by scanning the rotation-vibration absorption lines using a high-spectral-resolution laser. In this study, we devised an integrated measurement and data processing method (integrative measurement and processing method for hygrometry, IMPMH) to enhance the in-situ airborne measurement capability of dTDLAS. We measured a wide range (240–18,000 ppm) of water vapour concentrations, aiming for atmospheric measurements in a highly water-saturated regime, called the "optically thick condition". For recovering the full absorption spectra, the "integrative area" was defined and a difference factor D , which is the distance between two spectral regions with width corresponding to the half width of half maximum of the Voigt profile, was used to calculate the area. From the data, the low-bound concentration was measured to be 244 ppm. At D = 1.8, the transition concentration to the "optically thick condition" was measured to be 5,800 ppm. By increasing D from 1.8 to 2.8, the measurable upper-bound concentration increased to 17,993 ppm. IMPMH was applied to the measured data to estimate the final absorber density or water vapour concentration. The estimation was well-fitted with the measured detector signal with signal-to-noise ratio (SNR) of ∼ 300 of the residual spectrum, promising its applicability to in-situ airborne measurements. To validate IMPMH, the water vapour concentration range was divided into two regimes: (1) optically thick (5,800 < c < 18,000 ppm) and (2) optically thin (c < 5,800 ppm) conditions. Under the optically thick condition, IMPMH was validated by comparing the results between the short and long-path cells. In the optically thin condition, IMPMH was validated through comparison with the general dTDLAS method. Lastly, long-term stability of the dTDLAS system was validated by measuring 10 different concentrations (240–18,000 ppm) for 1000 s by characterising the precision and SNRs of the residual. The results demonstrate that IMPMH significantly enhances the in-situ airborne measurement capability of dTDLAS under both optically thick and thin conditions. Furthermore, requirements for the implementation of IMPMH in airborne measurement were investigated considering four aspects—sampling, low-pressure measurement, accuracy and precision, and multiplex detection. The results were examined with regard to atmospheric implications. [ABSTRACT FROM AUTHOR]

The solid-to-gas phase transition of potassium during biomass combustion significantly impacts ash-related issues in bioenergy systems, affecting operational efficiency and equipment longevity. However, the specific mechanisms and kinetics of this transition process remain inadequately understood. This work investigates the time-resolved transition of solid-phase potassium to the gas phase during the combustion of rice husk and wheat straw pellets, combining experimental measurements with theoretical modeling. Tunable diode laser absorption spectroscopy (TDLAS) was employed to measure atomic potassium concentrations 15 mm above burning pellets tray, where gas-phase equilibrium is approached. Key combustion characteristics including thermogravimetric profiles, spectral radiation, and temperature were simultaneously monitored. A novel multi-step model was developed to describe the transition of different forms of solid-phase potassium (organic, exchangeable, and inorganic) to the gas phase. This model integrates TDLAS measurements, observed combustion characteristics, and biomass physicochemical properties. Thermodynamic equilibrium calculations were used to estimate the atomic potassium fraction from total gaseous potassium. The results showed that the solid-to-gas phase transition of organic potassium synchronizes with volatiles release. In contrast, the maximum emission rates of inorganic and exchangeable potassium occurred at the onset of char combustion. The developed model agrees well with the online detection experiments and were further validated by offline ICP analysis of residual ash. While not directly simulating gas-solid interface reactions near the particle surface, this work lays groundwork for future multi-scale modeling of particle-laden flows and reactor-scale phenomena in biomass combustion systems. [ABSTRACT FROM AUTHOR]

• Design of the transceiver telescope for TDLAS system of the remote sounding in the atmosphere. • Coaxial sensing scheme with minimization of the TDLAS system shadow zone. • Possibility of detecting an echo-signal on a horizontal sounding path 0–1000 m. In this paper, we report on stages of the development of a transceiving telescope for atmospheric gas analysis TDLAS systems. Design of transmitters and receivers for active remote sounding systems is an important problem when creating instruments for studying atmospheric parameters. Transmitter and receiver aperture size directly influences the minimum permissible and maximum practicable sounding ranges. The work describes the stages of designing the receiving and transmitting parts of an atmospheric gas analysis tunable diode laser absorption spectroscopy (TDLAS) system (with anthropogenic methane as a target gas). Calculations show that a backscattered signal is reliably detected over an optical path 0–1000 m long with the use of a transceiving telescope with aperture 50–80 mm diameter and magnification of no less than 5. This design enables minimizing the dead zone of the coaxial TDLAS system within the 1645–1655 nm spectral range, which is informative for anthropogenic methane sounding. The use of a transceiving telescope will ensure resistance to misalignment, namely, resistance to mismatch between the mutual overlap of the device field of view and the laser radiation spot on the topographic target. Results of this work can be useful when designing and manufacturing telescopes for TDLAS systems methane (CH 4) and also other atmospheric gas analysis. [ABSTRACT FROM AUTHOR]

In this article, a field deployable sensor was developed using a self-developed 4.58-µm continuous wave quantum cascade laser (CW-QCL) for the simultaneous detection of carbon monoxide (CO) and nitrous oxide (N2O), both of which have strong fundamental absorption bands in this waveband. The sensor is based on tunable diode laser absorption spectroscopy (TDLAS) technology, which combined a multi-pass gas cell (MPGC) with a 41 m optical path length to achieve high-precision detection. Meanwhile, the particle swarm optimization-kernel extreme learning machine (PSO-KELM) algorithm was applied for CO and N2O concentration prediction. In addition, the self-designed board-level QCL driver circuit and harmonic signal demodulation circuit reduce the sensor cost and size. A series of validation experiments were conducted to verify the sensor performance, and experiments showed that the concentration prediction results of the PSO-KELM algorithm are better than those of the commonly used back propagation (BP) neural networks and partial least regression (PLS), with the smallest root mean square error (RMSE) and linear correlation coefficient closest to 1, which improves the detection precision of the sensor. The limit of detection (LoD) was assessed to be 0.25 parts per billion (ppb) for CO and 0.27 ppb for N2O at the averaging time of 24 and 38 s. Field deployment of the sensor was reported for simultaneous detection of CO and N2O in the air.

Based on our original development of a new laser absorption spectroscopy chamber (LASC) system, we further report ammonia emission concentration measurement using the LASC system based on deep belief networks (DBNs), aiming to present an effective approach for the retrieval of the gas concentration to increase the measurement accuracy of the LASC system and expand its application to monitoring ammonia emission in farmland. Surrounding the LASC system, an experimental system was constructed, and a DBN algorithm was introduced for gas concentration retrieval. The absorption spectroscopy obtained by the experimental system was first pretreated by an empirical wavelet transform algorithm and principal component analysis method, which greatly improved the signal-to-noise ratio of the signal and reduced the dimensionality of the processed signal to meet the need of the training of DBN model. The results showed that the measured gas concentrations were close to true values with small errors, and the mean relative error obtained by the DBN algorithm (0.37%) was much smaller than those obtained by the back-propagation neural network algorithm (0.97%) and absorbance peak method (2.37%) in a wide range of NH3 standard concentrations. Field experiments verified the effectiveness and reliability of the LASC system when it was applied to ammonia emission measurement in farmland with the concentration retrieval based on the DBN algorithm, which is of importance for its applications in air pollution detection. [ABSTRACT FROM AUTHOR]

The use of optical circular multipass absorption cells (CMPAC) in an open-path configuration enables the sampling free analysis of cylindrical gas flows with high temporal resolution and only minimal disturbances to the sample gas in the pipe. Combined with their robust unibody design, CMPACs are a good option for many applications in atmospheric research and industrial process monitoring. When deployed in an open-path configuration, the effects of inhomogeneities in the gas temperature and composition have to be evaluated to ensure that the resulting measurement error is acceptable for a given application. Such an evaluation needs to consider the deviations caused by spectroscopic effects, e.g., nonlinear effects of temperature variations on the intensity of the spectral line, as well as the interaction of the temperature and concentration field with the characteristic laser beam pattern of the CMPAC. In this work we demonstrate this novel combined evaluation approach for the CMPAC used as part of the tunable diode laser absorption spectroscopy (TDLAS) reference hygrometer in PTB’s dynH2O setup for the characterization of the dynamic response behavior of hygrometers. For this, we measured spatially resolved, 2D temperature and H2O concentration distributions, and combined them with spatially resolved simulated spectra to evaluate the inhomogeneity effects on the line area of the used H2O spectral line at 7299.43 cm−1. Our results indicate that for dynH2O, the deviations caused by the interaction between large concentration heterogeneities and the characteristic sampling of the beam pattern of the CMPAC are three orders of magnitude larger than deviations caused by small temperature heterogeneity induced spectroscopic effects. We also deduce that the assumption that the “path-integrated” H2O concentration derived with the open-path CMPAC setup represents an accurate H2O area average in the flow section covered by the CMPAC in fact shows significant differences of up to 16% and hence does not hold true when large H2O concentration gradients are present.

In order to realize real-time capnographic monitoring, a compact carbon dioxide sensor based on tunable diode laser spectroscopy technology was developed by means of a 2004 nm distributed feedback laser and a circular gas cell. The toroidal cell can achieve an optical path length of 3.49 m in a volume of 38 ml, which enables the tunable diode laser spectroscopy carbon dioxide sensor to achieve a detection limit of 175 ppm at a measurement rate of 12.5 Hz. The actual exhaled carbon dioxide gas detection experiments were implemented, including the breath analysis of a normal subject under different conditions and another subject with pneumothorax. It indicates that the sensor has good effectiveness and practicability for carbon dioxide metabolic monitoring and related disease diagnosis. [ABSTRACT FROM AUTHOR]

Water vapor temperatures in a heating device were measured by ultra-low sampled and high-precision tunable diode laser spectroscopy (TDLAS) Bichromatic distributed feedback (DFB) lasers were used as light sources. Sampled data at an ultra-low rate was collected after laser passing through a low-pass filter and served as the inputs of an artificial neural network. Classical direct absorption spectroscopy using the line-shape fitting method provided the training dataset, i.e., the integrated absorbances. The proposed method required an ultra-low sampling rate, i.e., only 1/50 of the classical method, but its calculation speed was nearly 14,000 times faster. Also, the proposed method yielded satisfying estimates at temperatures uncovered by the training dataset and was insensitive to random noises.

Hydrogen chloride (HCl) monitoring during combustion/gasification of biomass fuels and municipal solid waste, such as polyvinyl chloride (PVC) and food residues, is demanded to avoid the adverse effect of HCl to furnace operation and to improve the quality of the gas products. Infrared tunable diode laser absorption spectroscopy (IR-TDLAS) is a feasible nonintrusive in-situ method for HCl measurements in harsh environments. In the present work, the measurement was performed using the R(3) line of the ν2 vibrational band of HCl at 5739.25 cm–1 (1742.4 nm). Water vapor is ubiquitous in combustion/gasification environments, and its spectral interference is one of the most common challenges for IR-TDLAS. Spectral analysis based on the current well-known databases was found to be insufficient to achieve an accurate measurement. The lack of accurate temperature-dependent water spectra can introduce thousands parts per million (ppm) HCl overestimation. For the first time, accurate spectroscopic data of temperature-dependent water spectra near 5739.3 cm–1 were obtained based on a systematic experimental investigation of the hot water lines in a well-controlled, hot flue gas with a temperature varying from 1100 to 1950 K. With the accurate knowledge of hot water interference, the HCl TDLAS system can achieve a detection limit of about 100 ppm⋅m at around 1500 K, and simultaneously the gas temperature can be derived. The technique was applied to measure the temporally resolved HCl release and local temperature over burning PVC particles in hot flue gas at 1790 K. Graphical Abstract [ABSTRACT FROM AUTHOR]

A new version of a sensor for temperature measurements in the case of strong laser intensity fluctuation was developed. It was based on tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation, logarithmic conversion of the absorption signal, and detection of the first harmonic of the modulation frequency. The efficiency of the technique was demonstrated under experimental conditions with excess multiplicative noise. Temperature was evaluated from the ratio of integrated absorbance of two lines of the water molecule with different lower energy levels. Two algorithms of data processing were tested, simultaneous fitting of two spectral ranges with selected absorption lines and independent fitting of two absorption lines profiles. The correctness of the gas temperature evaluation was verified by simultaneous measurements with a commercial thermocouple. An error in temperature evaluation of less than 40 at 1000 K was achieved even when processing a single scan of the diode lasers.

A non-intrusive method for determining the time-resolved fuel mass flow rate in a hybrid rocket engine is proposed based on tunable diode laser absorption spectroscopy (TDLAS). The present work performed experimental tests using oxygen/paraffin as the propellant in a laboratory-scale hybrid rocket engine, applying oxidizer-to-fuel ratios in the range of 2.5–2.9 corresponding to combustion chamber pressures of 1.68–2.46 MPa. Variations in temperature and H 2 O partial pressure at the nozzle outlet were determined adopting TDLAS based on H 2 O absorption near 2.5 μm. Three H 2 O absorption lines at 4029.5, 4030.6 and 4030.7 cm−1 were simultaneously measured using only one diode laser at 2.0 kHz repetition rate and adopting a scanned-wavelength direct absorption strategy. The airflow velocity at the nozzle outlet was ascertained in two-dimensional numerical simulations. In this manner, the time-resolved fuel mass flow rate participating in combustion can be derived from the ideal state equation. A detailed discussion of the measurement uncertainty was provided herein considering the influence from determination of temperature, H 2 O partial pressure and velocity. The effectiveness of the novel method was validated by comparing its results with that acquired with the traditional weight-loss technique. Finally, it is initially analyzed that the combustion efficiency of pure paraffin fuel gradually increases during the combustion process according to the measurement results. • An optical fuel mass flow measurement method for hybrid rocket engines is proposed. • The quantitative fuel mass flow rate participating in combustion is obtained. • A compact tunable diode laser absorption spectroscopy system was designed. • Variations of temperature and H 2 O partial pressure at the nozzle exit were diagnosed. • Combustion characteristics of the oxygen/paraffin hybrid rocket were discussed. [ABSTRACT FROM AUTHOR]

In the Ruhrstahl–Heraeus (RH) vacuum degassing process, we propose a real-time prediction model for the carbon content in molten steel, and show that the decarburization endpoint can be accurately determined using this model. Firstly, we applied a novel off-gas analyzer that can measure the carbon oxide concentration produced in the decarburization reaction faster and more accurately. Next, we generate decarburization curves using the off-gas components measured by the new analyzer. The decarburization curve describes the carbon content profile well during operation, and shows good agreement with the actual carbon content. In order to predict the carbon content during operation in real time, we create an artificial neural network (ANN) using the decarburization curves and operation data. By comparing the endpoint carbon content measured at the end of the operation with the predicted values, we confirmed the excellent predictive performance of the ANN model. Finally, we show that it is possible to accurately determine the decarburization endpoint using the prediction model. We expect that the proposed real-time prediction model can increase the productivity of the RH process.

In order to understand the evaporation and particle formation processes of sprays in technical applications such as fuel injectors or drying processes in the food and pharmaceutical industries in detail, single droplet drying experiments, for example, acoustic levitation, are widely used as model experiments. We combined acoustic levitation and tunable diode laser absorption spectroscopy (TDLAS) to measure the absolute H2O concentration in the exhaust gas of a levitation chamber to investigate drying and particle formation processes from single droplets of pure water and protein–water solutions. To that end, we designed and developed a non-invasive, calibration-free TDLAS-based hygrometer to analyze the 1.4 µm overtone band. To increase the detection range of the developed hygrometer and to track the complete drying process of protein solution droplets even after the critical point of drying, the absorption length was extended to a path length of 18 m using an astigmatic multipass cell of the Herriott type. The setup was validated by drying pure water droplets, resulting in a determination of the water mole fraction in a range from 73 ppm to 1314 ppm, with a single scan resolution of 1.7 ppm. For protein solution droplets, the entire drying process, even beyond the critical point of drying, can be tracked and the different phases of the drying process can be characterized at different drying temperatures. [ABSTRACT FROM AUTHOR]

• Beam-steering-effect immune DAS method is proposed. • A cost-effective and compact distortion detection scheme is introduced. • The SNR of the raw transmitted laser signal increases by 35 dB. • 4、Temperature successful fitting rate up to 100 % from 93.7 % over the 1000 repetitions. • 5、Temperature standard deviations have decreased from 296.6 K to 61.8 K. Tunable diode laser absorption spectroscopy is widely used for temperature measurement in combustion monitoring due to its merits of rapidity, non-contact capability, and high precision. In actual applications, the beam-steering effect will cause time-varying distortion in the laser intensity signal, resulting in a low signal-to-noise ratio and inaccurate extraction of the absorption spectrum. This work proposes a cost-effective distortion detection scheme and adaptive distortion filter and compensation algorithm for the extraction of the beam-steering effect immune laser absorption spectrum. Turbulent flame monitoring experiment results prove the proposed method can improve the signal-to-noise ratio of the raw transmitted absorptive laser signal by 35 dB after the beam-steering distortion has been compensated. When compared with the temperature results calculated from the distorted absorptive laser signal, the results from the distortion-compensated absorptive laser signal show a significant improvement in both robustness and precision. The experimental results demonstrate that the successful temperature fitting rate has increased from 93.7 % to 100 % (over 1000 repetitions), and the average standard deviations of temperature have decreased from 296.6 K to 61.8 K. The proposed method is cost-effective and compact, which will potentially play an important role in dynamic combustion monitoring and diagnosis. [ABSTRACT FROM AUTHOR]

[Display omitted] • Simultaneously measuring FeNO concentrations and ETCO 2 in real-time. • The ability to differentiate between healthy people, asthmatics, and COPD patients. • Real-time detection were made on volunteers with different levels of FeNO and ETCO 2. • Taking into account the effect of exhalation flow rate on the concentrations of FeNO and ETCO 2. The simultaneous detection of fractional exhaled nitric oxide (FeNO) and end-tidal carbon dioxide (ETCO 2) is of great importance for the distinguishing and diagnosis of asthma and chronic obstructive pulmonary disease (COPD), providing more comprehensive information on respiratory disorders. This work demonstrates a simultaneous ETCO 2 and FeNO detection system based on quantum cascade laser absorption spectroscopy (QCLAS) technology was presented. The system employs wavelength modulation spectroscopy (WMS) technology and the Herriott multi-pass cell, achieving a detection limit of 2.82 ppb for nitric oxide (NO) and 0.05 % for carbon dioxide (CO 2). Real-time exhalation measurements were performed on volunteers with varying ETCO 2 and FeNO levels, and the results of the test can accurately distinguish whether the corresponding volunteer was healthy, had asthma or COPD. The effect of exhalation flow rate on the concentration of the two gases was explored. A range of expiratory flow rates were tested in the flow rate interval from 1 to 4 L/min, and there was always an inverse relationship between expiratory flow rate and FeNO concentration, but flow rate changes did not affect ETCO 2 concentration. The results indicate that this detection system can simultaneously and effectively measure ETCO 2 and FeNO concentrations in real-time. [ABSTRACT FROM AUTHOR]