Target Gas Concentration Research Articles

Gas concentration measurement by optical similitude absorption spectroscopy: methodology and experimental demonstration.

We propose a new methodology to measure gas concentration by light-absorption spectroscopy when the light source spectrum is larger than the spectral width of one or several molecular gas absorption lines. We named it optical similitude absorption spectroscopy (OSAS), as the gas concentration is derived from a similitude between the light source and the target gas spectra. The main OSAS-novelty lies in the development of a robust inversion methodology, based on the Newton-Raphson algorithm, which allows retrieving the target gas concentration from spectrally-integrated differential light-absorption measurements. As a proof, OSAS is applied in laboratory to the 2ν3 methane absorption band at 1.66 µm with uncertainties revealed by the Allan variance. OSAS has also been applied to non-dispersive infra-red and the optical correlation spectroscopy arrangements. This all-optics gas concentration retrieval does not require the use of a gas calibration cell and opens new tracks to atmospheric gas pollution and greenhouse gases sources monitoring.

Regularization and the inflection point method for sensor signal in gas concentration measurement

In this work, we focus on a transient state of sensor as a source of chemical information. This problem deserves attention for many reasons. As an example, utilization of transient states may allow for faster and more accurate information acquisition. We have considered a transient state gas sensor response to test analyte applied for quantitative gas determination. We examined the inflection point which is a promising feature of a signal resulting from transient sensor responses. This particular feature was found to be linearly dependent on the target gas concentration. However, the applicability of inflection point method is dependent on the precision of its determination. It raises a number of issues related to signal regularization. The regularization problem was studied from a theoretical point of view. A number of a-priori estimates for regularization strategies were obtained yielding some practical algorithms for calculating the inflection point in a stable way. Next, they were validated using numerical simulations and real data analysis procedure. This work provides a rigorous mathematical evaluation of the inflection point method and it may contribute to the improvement of gas sensor performance.

LED-Absorption-QEPAS Sensor for Biogas Plants.

A new sensor for methane and carbon dioxide concentration measurements in biogas plants is presented. LEDs in the mid infrared spectral region are implemented as low cost light source. The combination of quartz-enhanced photoacoustic spectroscopy with an absorption path leads to a sensor setup suitable for the harsh application environment. The sensor system contains an electronics unit and the two gas sensors; it was designed to work as standalone device and was tested in a biogas plant for several weeks. Gas concentration dependent measurements show a precision better than 1% in a range between 40% and 60% target gas concentration for both sensors. Concentration dependent measurements with different background gases show a considerable decrease in cross sensitivity against the major components of biogas in direct comparison to common absorption based sensors.

Temperature Dependence of Responses in Metal Oxide Gas Sensors

Selecting an optimum operating temperature for metal oxide gas sensors is of prime technical importance. Here, the temperature behavior of various kinds of metal oxide gas sensors in response to different levels of reducing contaminants in air is reported. The examined gas sensor samples include a Tin oxide-based resistive gas sensor and home-made diode-type Ag-TiO2-Ti gas sensors. Recorded response vs. temperature curves of all samples represent two different typical features: The responses related to the resistive gas sensor exhibit distinct maximum response at a well defined operating temperature regardless of the target gas concentration level, but the diode type samples demonstrated a continuously rising response as the operating temperature decreased to highly contaminated atmospheres. At low contaminant levels, diode type gas sensors change their behaviour and act similar to resistive gas sensors. Reported results were described by a model based on the gas diffusion theory.

Assessment and Optimization for Novel Gas Materials Through the Evaluation of Mixed Response Surface Models

In this paper, an innovative methodology aimed at improving the development of novel gas sensors through a process optimization is carried out by applying mixed response surface (RS) models. High accuracy measurements of new conductometric metal oxide gas sensors, obtained by an efficient control of the working conditions, are gathered. The response of metal–oxide–semiconductor gas sensors changes significantly when the sensors operate at different temperatures and target gas concentrations. To consider all the sources of variability there involved, the RS methodology was applied, including random effects, to improve and optimize the performance of these new gas sensors. More precisely, the optimization is performed exploiting a limited number of observations, systematically collected with an ad hoc measurement system, and it considers external sources of variability, satisfying at the same time stringent requirements. Furthermore, the statistical results and the relative assessment of novel gas materials are obtained by considering fixed as well as random effects, where random variables are considered for better controlling the optimization step.

Thermal post-annealing and gas concentration effect on liquid petroleum gas sensing characteristics of nanocrystalline zinc oxide thin films

Chemical adsorption and sensing reactions of liquid petroleum gas over the surface and between grain boundaries of nanostructured zinc oxide thin films were investigated and monitored under the influence of various post-synthesis thermal annealing and target gas concentration conditions. Prior to sensing tests, zinc oxide thin films were synthesized in three steps: at first, zinc oxide nanoparticles were synthesized by a sol–gel method using zinc acetate dihydrate, ethanol and monoethanolamine as precursor, solvent and stabilizing agent, respectively. Then, untreated zinc oxide thin films were prepared from zinc oxide sol through spin coating procedure and finally, thermal post-annealing treatment with different temperatures was performed to yield heated thin films. The formation of the synthesized nanoparticles, morphology, crystalline size, porosity and transparency of final thin films were characterized by field emission-scanning electron microscopy, X-ray diffraction and ultraviolet–visible spectroscopy techniques. Sensing tests showed the dependence of the liquid petroleum gas detection on the thermal annealing process and gas concentration and indicated a maximum response of 75 % at the operation temperature of 300 °C for the zinc oxide film pre-annealed at 600 °C and 1,000 ppm gas. Moreover, the results exhibit excellent sensitivity for the sensor via short response and recovery times of 34 and 220 s, respectively.

A selective ultrahigh responding high temperature ethanol sensor using TiO2 nanoparticles.

In this research work, the sensitivity of TiO2 nanoparticles towards C2H5OH, H2 and CH4 gases was investigated. The morphology and phase content of the particles was preserved during sensing tests by prior heat treatment of the samples at temperatures as high as 750 °C and 1000 °C. Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis were employed to characterize the size, morphology and phase content of the particles. For sensor fabrication, a film of TiO2 was printed on a Au interdigitated alumina substrate. The sensing temperature was varied from 450 °C to 650 °C with varying concentrations of target gases. Results show that the sensor has ultrahigh response towards ethanol (C2H5OH) compared to hydrogen (H2) and methane (CH4). The optimum sensing temperature was found to be 600 °C. The response and recovery times of the sensor are 3 min and 15 min, respectively, for 20 ppm C2H5OH at the optimum operating temperature of 600 °C. It is proposed that the catalytic action of TiO2 with C2H5OH is the reason for the ultrahigh response of the sensor.

Wavelet Transform Smoothing Filters for Metal Oxide Gas Sensor Signal Cleaning

Abstract This paper reports on a series of experiments to evaluate the methods for feature extraction and denoising the digital signal from thin film zinc oxide-tin dioxide composite gas sensor devices. The aim was to find a method that not only cleaned the signal but also maintained the shape, precision and resolution of the signal. It was found that the Savitzky–Golay smoothing filter method gave the best, smoothed and cleaned, approximation of the sensor response regardless of the thin film composition, target gas concentration or operating temperature.

Safety of Low Flow Sevoflurane Anesthesia for Patient with Dextrocardia and Situs Inversus Totalis: A Case Report

Introduction: Situs inversus totalis with dextrocardia also named situs inversus totalis is a rare congenital condition in which the anatomy of the abdominal and thoracic organs are arranged in a mirror image reversal of the normal positioning. Case Report: A 42-year-old Albanian female was scheduled for elective maxillofacial surgery, cyst in the left side of the neck, and previously undiagnosed dextrocardia with situs inversus was picked up in preanesthetic evaluation by an anesthesiologist, during auscultation of the heart revealed a grade 2/6 holosystolic murmur at the left parasternal border. General anesthesia was induced with propofol 3mg/kg, fentanyl 3mcg kg, rocuronium 1mg/kg to facilitate endotracheal intubation and maintained with sevoflurane 1.0- 1, 5 MAC, in O2 /N2O 50:50%. After tracheal intubation, the fresh gas flow was set to 4 L.min -1 (O 2 2 L.min, N 2O 2 L.min). When the target gas concentrations have been achieved, FGF was reduced in 1 l/min, with the relative proportion of O2 /N2O titrated to maintain the inspired oxygen fraction (FiO 2) between 0.30 and 0.35. Inhaled and exhaled gas concentrations were continuously monitored. The patient was calm after extubation (score 1-of agitation), awareness according to Awareness Categorization was pointed with score 1- no awareness. In the PACU she was hemodynamically stable and after full recovery returned to the ward. Results of arterial blood gas analyzes BUN, AST, ALT, LDH, glycemia, total bilirubin, proteinogram, haemogram and electrolytes were within normal reference range. Her postoperative course was uneventful and she was discharged on the third postoperative day. Conclusion: With particular attention in preanesthetic visit we managed to find out dextrocardia with situs inversus totalis. We realized low flow sevoflurane anesthesia, which showed to be safe.

Synthesis and characterization of NiO/TiO2 porous films and their photocurrent-enhanced mechanism in gas phase

In this work, the NiO/TiO2 porous films with different weight-ratios were prepared by the screen-printing technique. To uncover the roles of gas molecules and the junction on the carrier migration, the photocurrent measurements in methanol were employed. The corresponding experiments were performed in a high-throughput platform where the concentration of target gas and the applied bias voltage were both precisely controlled. Our results revealed that not only the p–n junction but also methanol had a distinct enhancement effect on the photocurrent. Furthermore, the synergistic effect was further confirmed by a quantitative formula of multi-factor effect (ME) based on the carrier density. In addition, a new insight into gas molecule photooxidation on the coupling catalyst was provided, which was of great significance for potential application to photocatalytic degradation and gas sensing.

First field test of linear gas sensor net for planar detection of CO2 leakages in the unsaturated zone

Gases from subsurface sources can present risks for the biosphere. To extend the lead time in managing such risks, reliable gas detection in the subsurface is required. A monitoring system was developed that is able to: (i) gather information from large areas, (ii) work efficiently within the subsurface; insensitive to changing subsurface environmental conditions while sensitive to changes in target gas concentrations, and (iii) provide a fast response. We report the first field test of linear membrane-based gas sensors: Fourteen 40m long sensors were installed in two horizontal nets one above the other within a homogenized soil and scanned from a control station. 10L/min of CO2 was injected into a point-like port 25cm below the lower sensor net. Two operation modes were successfully tested to detect the CO2 plume: an active mode, which is sensitive to the concentration, and a passive mode, which is sensitive to changes in concentration. By conversion of the monitoring results the minimum contact range of seeping gas and sensor could be determined. Based on this valuable approach, an unexpectedly high level of lateral gas spread within the soil and the formation of a gas pillow below the soil surface could be observed.

Surface-Doping Effect of InVO4 Nanoribbons and the Distinctive Behavior as Gas Sensors

Indium vanadate (InVO4) gas sensors were fabricated by depositing InVO4 nanoribbons aqueous suspension onto ceramic substrates. Their resistances distinctively increased in the detection of ammonia and propylamine, indicating an n-to-p semiconductor transition. This novel phenomenon of the InVO4-based sensor may be ascribed to the surface doping effect: electrons were trapped by H2O and O2 and produced OH(-) and O2(-) on the InVO4 surface, which resulted in holes overcompensation in the InVO4 valence band. Moreover, the sufficiently large surface-to-volume ratio of these nanoribbons enables fast carrier transfer on the sensor surface. The InVO4 nanoribbons-based sensors had optimum performance at room temperature and enjoyed good restorability. They also had great response to a wide range of target gas concentration, with ultrahigh sensitivities up to 1100% for ammonia and 760% for propylamine.

Gas sensing method applicable to real conditions**This paper was presented at the Conference on Optoelectronic and Electronic Sensors held in Karpacz, Poland, on 24–27 June 2012.

In this work, we tested the influence of various sampling techniques and modes of operation on the response of sensor systems under real conditions. The first system was based on diffusive sampling. The second one used a dynamic method of sampling. In the third system, a stop flow mode of operation was applied. A considerable error of target gas concentration assessment was obtained in the first two cases. The error resulted from sensor signal fluctuations encountered during their exposure either to the air at the measurement point (diffusive sampling) or to the air drawn from the measurement point (dynamic sampling). The fluctuations could be attributed to the temporal variation of physical and chemical parameters of air. The main merit of the third method consists in utilizing sensor exposure under the conditions of stopped flow for the purpose of pollutant concentration assessment. While using this method, the accuracy of target gas quantification under real conditions was comparable to the one achieved when measuring standard gases. We think that the proposed method is a valuable contribution, which addresses the increasing demand for sensor systems that perform quantitative pollution assessment in workplace air, under real conditions.

On-chip growth of wafer-scale planar-type ZnO nanorod sensors for effective detection of CO gas

The facile engineering of sensing devices is an important technological achievement that will lead to large-scale and cost-effective fabrication of gas nanosensors. This study demonstrates the on-chip and selective growth process of zinc oxide (ZnO) nanorods (NRs) for wafer-scale fabrication of planar-type gas sensors using combined lithography and chemical hydrothermal techniques. This facile route can effectively fabricate ZnO NR gas sensors at low temperatures without the requirement of novel metal catalyst. The gas-sensing characteristics of the ZnO NR sensors to carbon monoxide (CO) and ammonia (NH3) were evaluated as a function of growth time in correlation with operating temperature and target gas concentration. The ZnO NR sensors showed good response and recovery characteristics with a maximum response to CO and NH3 gas at 400°C. The ZnO NR sensors grown at 6h exhibited the highest response to CO and NH3 gases with sensitivities of 0.37 and 0.05%/ppm, respectively. Furthermore, the gas-sensing mechanism of the ZnO NR sensors was also discussed in terms of three types NR/NR junctions, namely, point-junctions, across-junctions, and block-junctions.

Electro-colorimetric hydrogen gas sensor based on Pt-functionalized In2O3 nanopushpins and InGaN/GaN multiple quantum wells

A novel electro-colorimetric gas sensing technique based on the catalytic metal nanoparticles decorated metal oxide nanostructures integrated with multiple quantum wells (MQWs) has been proposed. The working principle has been demonstrated by the sensing device derived from the composite consisting of In0.15Ga0.85N/GaN MQWs and Pt-functionalized In2O3 nanopushpins for the detection of hydrogen gas. The pronounced changes in emission as well as Raman scattering spectra of InGaN/GaN MQWs under different target gas concentrations clearly illustrate the feasibility of our newly designed composites for the derivative of contact-free, simple and highly sensitive gas sensors with optical detection

Remote sensing of trace gases with optical correlation spectroscopy and lidar: theoretical and numerical approach

In this paper, we propose a new active remote sensing methodology, based on laser spectroscopy, to evaluate the content of atmospheric trace gases. Its principle consists in coupling a lidar with optical correlation spectroscopy (OCS-lidar). Our theoretical and numerical studies show that OCS-lidar is a robust measurement methodology allowing trace gases environmental, agricultural, and industrial plants surveys. The novelty of this work is threefold. Firstly, we develop a new formalism to remotely evaluate the target gas concentration from optical correlation spectroscopy. Secondly, an acousto-optical programmable dispersive filter has been used to ensure that the lidar signal be spectrally correlated with the target gas of interest. It avoids using a hazardous gas reference cell, as operated in conventional OCS devices. Moreover, a clever spectral correlation is achieved since the contribution of absorption interfering species can then be minimized. Thirdly, to evaluate the performance of the OCS-lidar methodology, a numerical study of methane greenhouse gas is presented to evidence that atmospheric methane mixing ratios are retrievable over two orders of magnitude, from background level up to 100 ppb, within 100-m range resolution. Evaluation of the accuracy and the detection limit, including statistical and systematic errors assessment, are then objectively presented and discussed.

Gas sensing by tunable multimode diode laser using correlation spectroscopy

A novel method to use multimode diode lasers for gas detection is demonstrated. The relationship between the intensity decay of partially absorbed multiple modes and the target gas concentration is presented. The gas concentration is retrieved by a linear-regression-based procedure applied on simultaneously recorded absorption signals in a dual-beam wavelength-modulation configuration. Signals of non-gas origin or disturbed by interfering gases are effectively eliminated by correlation analysis procedure. Light intensity fluctuations and wavelength shifts due to temperature drift are automatically compensated for. Several absorption signals can readily be obtained within a limited tuning range by overlapping the laser multimode chart with the target gas spectrum. This scheme was applied on the detection of trace acetylene mixed in ambient air, and a sensitivity of 3 ppm was achieved with a 48.5 cm path length. The trade-off between sensitivity and reliability for retrieving gas concentration by multiple absorption signals is also presented.

Gas Sensor Using a Robust Approach under Time Multiplexing Scheme with a Twin Laser Chip for Absorption and Reference

A tunable diode laser absorption spectroscopy nitrous oxide gas sensor at 2.1 micrometres using one antimonide twin laser chip and one InGaAs photodiode is demonstrated, in which time multiplexing techniques are adopted to acquiring both the absorption and reference signal in a robust fluctuation tolerable scheme. Electronics in analogue modality is developed to extracting absorption information and compensating for fluctuations, resulting in a direct analogue voltage output corresponding to the target gas concentration in real time. The performance of the gas sensor is evaluated experimentally, the validity and feasibility of this scheme is also discussed.

나시콘 전류검출형 NO2센서의 성능개선

Many electrochemical power devices such as solid state batteries and solid oxide fuel cell have been studied and developed for solving energy and environmental problems. An amperometric gas sensor usually generates sensing signal of electric current along the proportion of the concentration of target gas under the condition of limiting current. For narrow variations of gas concentration, the amperometric gas sensor can show higher precision than a potentiometric gas sensor does. In additional, cross sensitivities to interfering gases can possibly be mitigated by choosing applied voltage and electrode materials properly. In order to improve the sensitivity to <TEX>$NO_2$</TEX>, the device was attached with Au reference electrode to form the amperometric gas sensor device with three electrodes. With the fixed bias voltage being applied between the sensing and counter electrodes, the current between the sensing and reference electrodes was measured as a sensing signal. The response to <TEX>$NO_2$</TEX> gas was obviously enhanced and suppressed with a positive bias, respectively, while the reverse current occurred with a negative bias. The way to enhance the sensitivity of <TEX>$NO_2$</TEX> gas sensor was thus realized. It was shown that the response to <TEX>$NO_2$</TEX> gas could be enhanced sensitivity by changing the bias voltage.

Surface-functionalized CdSe quantum dots for the detection of hydrocarbons

CdSe semiconductor quantum dots (QDs) were synthesized with surface modifying groups to enhance their response towards the detection of aromatic hydrocarbons. The sensing characteristics of unmodified QDs using trioctyl phosphine oxide (TOPO) plus stearic acid (SA) surface groups were compared with QDs with additional benzoic (BA) or pentafluorobenzoic acid (FBA) surface modifying groups. The unmodified and surface-modified QDs were encapsulated into poly(methyl methacrylate) (PMMA) thin films by drop coating QD-PMMA solutions onto silicon substrates. Reversible enhancement and quenching of the photoluminescence (PL) upon film exposure to toluene and xylene vapors were both observed for the QD-SA, BA and FBA systems. PL enhancement was observed at low target gas concentrations, with the onset of the quenching process for each of the films being dependent on the target gas type and its concentration. Unmodified QD/PMMA films had detection limits for xylenes and toluene of 250 and 500 ppm, respectively in a balance of nitrogen. While detection limits of 15 ppm xylenes and 50 ppm toluene in a balance of nitrogen were achieved with the FBA- and BA-modified QD/PMMA films. The surface-modified QDs were found to provide a factor of 16 and 10 sensing enhancement, respectively, for the detection of xylenes and toluene, over the unmodified QDs.