Gas sensors for climate research

The availability of datasets providing information on the spatial and temporal evolution of greenhouse gas concentrations is of high relevance for the development of reliable climate simulations. However, current gas detection technologies do not allow for obtaining high-quality data at intermediate spatial scales with high temporal resolution. In this regard the deployment of a wireless gas sensor network equipped with in situ gas analysers may be a suitable approach. Here we present a novel, non-dispersive infrared absorption spectroscopy (NDIR) device that can possibly act as a central building block of a sensor node to provide high-quality data of carbon dioxide (CO2) concentrations under field conditions at a high measurement rate. Employing a gas-based, photoacoustic detector we demonstrate that miniaturized, low-cost, and low-power consuming CO2 sensors may be built. The performance is equal to that of standard NDIR devices but at a much reduced optical path length. Because of the spectral properties of the photoacoustic detector, no cross-sensitivities to humidity exist.</p

MID-IR LED-based, Photoacoustic CO2 Sensor

AbstractThe technology used to implement CO2 sensors depends on the requirements in terms of sensitivity, price and robustness. The most common technology for highly sensitive tasks are based on tunable diode laser spectroscopy, while so-called non-dispersive infrared (NDIR) photometers [2] are used in less demanding scenarios such as control air conditioning systems. Most NDIR systems use thermal emitters as light source which are readily available at low cost but require compensation for cross-sensitivities toward other gas species. The detector technology employed in these systems ranges from photodiodes to thermopiles and pyroelectric detectors, all of which require the use of spectral filters to avoid cross sensitivities. Here we present a low-cost photoacoustic-based detector comprised of a microphone in a hermetically sealed chamber filled with CO2. To excite sound waves a MID-IR LED emitting radiation in the strong CO2 absorption region around 4.2μm is used for the first time

Compact silicon-based attenuated total reflection (ATR) sensor module for liquid analysis

Infrared attenuated total reflection (ATR) spectroscopy is a common laboratory technique for the analysis of highly absorbing liquids and solids, and a variety of ATR accessories for laboratory FTIR spectrometers are available. However, ATR spectroscopy is rarely found in industrial processes, where compact, robust, and cost-effective sensors for continuous operation are required. Here, narrowband photometers are more appropriate than FTIR instruments. We show the concept and implementation of a compact Si-based ATR module with a four-channel microelectromechanical systems (MEMS) detector. Measurements of liquid mixtures demonstrate the suitability for applications in the chemical industry. Apart from sapphire (for wavelengths below 5 µm) and diamond (extending to the far-infrared region), most materials for ATR elements do not have either high enough infrared transmission or sufficient mechanical and chemical stability to be exposed to process fluids, abrasive components, or aggressive cleaning agents. However, using diamond coatings on Si improves the stability of the sensor surface. In addition, by proper choice of incidence angle and coating thickness, an enhancement of the ATR absorbance is theoretically expected and demonstrated by first experiments using a compact sensor module with a diamond-coated Si ATR element.</p

Resonant photoacoustic cells for laser-based methane detection

Against the background of the steady increase in greenhouse gases in the atmosphere, a fast and inexpensive method for detecting methane is required. This applies to the direct measurement of the background concentration of methane in the atmosphere and also to the detection of leaks in natural gas pipelines. Photoacoustic (PA) sensors offer the possibility of highly sensitive gas detection and cost-effective design at the same time. In this work, we investigated a photoacoustic sensor for methane in low concentrations, focusing on a special cell design, the so-called T-cell. Different cylinder geometries of six T-cells and the influence on the sensor performance were examined. An interband cascade laser (ICL) with a central wavelength of 3270 nm was used for excitation and a micro-electromechanical systems (MEMS) microphone as detector. The detection limits achieved were below the methane background concentration in air of 1.8 ppm.</p

Colorimetric sensor for bad odor detection using automated color correction

Colorimetric sensors based on color-changing dyes offer a convenient approach for the quantitative measurement of gases. An integrated, mobile colorimetric sensor can be particularly helpful for occasional gas measurements, such as informal air quality checks for bad odors. In these situations, the main requirement is high availability, easy usage, and high specificity towards one single chemical compound, combined with cost-efficient production. In this contribution, we show how a well stablished colorimetric method can be adapted for easy operation and readout, making it suitable for the untrained end user. As an example, we present the use of pH indicators for the selective and reversible detection of NH3 in air (one relevant gas contributing to bad odors) using gas-sensitive layers dip coated on glass substrates. Our results show that the method can be adapted to detect NH3 concentrations lower than 1 ppm, with measure-to-result times in the range of a few minutes. We demonstrate that the color measurements can be carried out with the optical signals of RGB sensors, without losing quantitative performance

Simulation model for the evaluation and design of miniaturized non-resonant photoacoustic gas sensors

This publication reports the derivation and the implementation of a simulation model that describes non-resonant photoacoustic gas sensors. The photoacoustic effect is modelled in detail for the successive steps of radiation emission, stimulation of molecules, collisional relaxation processes and finally the pressure formation in a closed gas cell. The photoacoustic effect offers great potential in the development of selective, miniaturized gas sensor systems. We verify and discuss the results of our model assuming typical parameters and values in indoor CO₂ sensing applications. We set up a sensor system for experimental verification of the simulated data and discuss the results. The results of the simulation model are in good accordance with the experimental data and can therefore be used as a novel and efficient tool for the development of non-resonant photoacoustic gas sensor systems

Einsatz von Metalloxid-Gassensoren zur Luftqualitätssensorik für Raumluftregelung

In einer Machbarkeitsstudie wurde der Einsatz von Metalloxid-Gassensoren zur Luftqualitätssensorik für die Raumluftregelung untersucht. Die Langzeitmesskampagne wurde mit unterschiedlichen Metalloxid-Gassensoren in drei verschiedenen anwendungsrelevanten Szenarien, Büro, Besprechungszimmer und Diskothek, durchgeführt. Im Büro und Besprechungszimmer wurden als Referenz zusätzlich Infrarot-CO2-Messgeräte eingesetzt. In der Diskothek ist eine zusätzliche wichtige Leitsubstanz für die Luftgüte das im Zigarettenrauch enthaltene Kohlenmonoxid, deshalb wurde hier als Referenz ein elektrochemischer CO-Sensor verwendet. Außerdem wurde jeweils die Temperatur und die Luftfeuchte aufgenommen. Zusätzlich zu den Sensorsignalen wurde die Anzahl der sich im Büro aufhaltenden Personen aufgezeichnet. Die experimentell ermittelten Ergebnisse bestätigten die prinzipielle Eignung der kostengünstigen Sensoren. Allgemein gilt unter den Gasen CO2 als die maßgebliche Leitsubstanz für die Luftgüte, welches aber keine Sensorreaktion hervorruft. Auf andere gasförmige Komponenten als Indikator für die Belegung und damit letztendlich für die Luftgüte kann zurückgegriffen werden. Alle Sensoren zeigten eine mehr oder weniger gute Reaktion auf die steigende Frequentierung der Räume. Hauptnachweisgas in der Diskothek ist das durch Zigarettenrauch verursachte CO. Im Büro und im Besprechungsraum sind es sehr wahrscheinlich flüchtige organische Bestandteile verbunden mit der Abnahme der Sauerstoffkonzentration, die direkt mit dem Anstieg der CO2-Konzentration im Raum korreliert

The influence of light on the gas sensitive properties of microstructured metal oxide thin films

Metal oxide gas sensors are widely used for different applications and operate normally at high temperatures between 300°C and 600°C. Such high temperatures are mainly needed to speed up the desorption of molecules from the gas sensor surface. Goal of the reported investigations is the reduction of the operating temperatures of such devices by the influence of radiation on the gas adsorption/desorption process. Therefore, the influences of radiation on the gas sensing mechanisms at surfaces of different metal oxides (SnO 2, ZnO, WO 3 and Cr 2-x Ti x O 3+z) have been studied for different wavelengths. The experiments were carried out at an operating temperature of 130°C as well as at room temperature. As radiation sources LEDs emitting at different wavelength were used. The sensor response to NO 2, CO, NH 3 and H 2 has been measured with and without illumination. The investigations have shown that light mainly influences the photo-activation of electron-hole pairs, which results in an increasing of the electrical conductivity of the illuminated metal oxide. The observed influences of photoadsorption and photocatalytic effects are small compared to the photoelectric effect. Only a weak increase of the NO 2 sensitivity during illumination has been measured

Colorimetric gas sensors based on optical waveguides made on plastic foil

We report on the realization of a low-cost polymeric optical waveguide made on a plastic foil and used for colorimetric gas detection. Low-temperature processes have been used for the realization of polymeric micro-mirrors and the depositions of the chemochromic reagent on PET foil. The planar waveguide configuration aims at improving the transducer sensitivity and simplifies its fabrication. Indeed, the device is currently produced by standard microfabrication techniques but its configuration and the materials involved make the sensor compatible with large scale and low-cost fabrication techniques. The operation of the optical transducer was applied to a colorimetric gas sensor and validated for the detection of ammonia