Gas Sensors Classification

For gas sensor classification, various approaches can be used. For example, taking into accoimt transduction mechanisms, we can distinguish six general categories of sensors (1) optical sensors, (2) electrochemical sensors, (3) electrical sensors, (4) mass-sensitive sensors, (5) calorimetric sensors, and (6) magnetic sensors (see Table 1.11). 

We need to point out that our classification represents but one of the possible alternatives. Detection principles can also be used for gas sensor classification. According to detection principles, commonly used gas sensors can be classified into the following three groups (1) sensors based on reactivity of gas, (2) sensors based on physical properties of gas, and (3) sensors based on gas sorption (see Table 1.15). 

Another classification is based on the analytes that occur in gas or liquid phases. The CL gas sensors are applied to the analysis of compounds in gases or vapors, such as 02, 03, NO, N02, chloride, ammonia-containing compounds... 

Table 1.1 Classification of semiconductor gas sensors according to the types of transducers and receptors used...

In 2008 Borah et al. [38] proposed that Neural Network based E-Nose, comprising of an array of four tin-oxide gas sensors, can assist tea quality monitoring during quality grading, principal component analysis (PCA) was used to visualise the different aroma profiles. In addition, K-means and Kohonen s self organising map (SOM) cluster analysis was done, multi layer Perceptron (MLP) network, radial basis function (RBF) network, and constructive probabilistic neural network (CPNN) were used for aroma classification [38]. 

Sea food octopus [42] Spoilage classification of octopus treated with different concentrations of formaldehyde and water semiconductor gas sensors... 

Some recent applications of LDA include the classification of vegetable oils using the data obtained from an array of gas sensors and the use of proton magnetic resonance spectra to discriminate between normal and cancerous ovarian tissue. 

Table 1.11 The classification of gas sensors suggested in 1991 by Analytical Chemistry Division of lUPAC...

Table 1.15 The classification of gas sensors according to the detection principle...

According to technology used for gas sensor fabrication, they can be classified as ceramic, thin-fllm, and thick-flhn gas sensors. Micromachined gas sensors, which were designed during the last few decades, can also be referred to by this principle of classification. 

Table 6.1 Classification of solid electrolyte gas sensors and examples of their realization...

The principles of field effect devices with special regard to chemical sensing will be briefly described in the next section. It will be demonstrated how such devices can be used in sensor arrays eventually together with other types of gas sensors to make electronic noses for the identification, classification and analysis of gas mixtures (or odours) [5-7]. A truly microanalytical system, an "olfactory camera" based on a field effect transistor array is also suggested. 

It can be concluded that gas sensors can be made very small and with small consumption of the molecules to be detected. They can, therefore, be used to analyze gas evalution from small samples and with small gas flows with both industrial and medical applications. Furthermore, the use of sensor arrays gives the possibility to construct electronic noses for more complicated situations related to identification, classification and quantification of gas mixtures or odours. The final microanalysis system for gases may perhaps consist of microfabricated gas handling, an integrated sensor array and a dedicated chip for signal treatment and pattern recognition. 

Heat treatment of milk ensures the microbial safety of milk and increases its shelf life. Different heat treatments (UHT, pasteurized, sterilized) can be distinguished by analyzing the volatile compounds with an electronic nose. A hybrid system that uses an electronic nose combined with an SVM classification method was tested by Brudzewski, Osowski, and Markiewicz for milk recognition and classification.The electronic nose was composed of seven tin oxide-based gas sensors, and the SVM model was tested with linear and RBF kernels. In the first experiment, four brands (classes) of milk were... 

In the last years, the aim of our research was to investigate the potential of the gas and liquid sensor arrays for food authentication and quality control investigation on the freshness evolution of Italian Crescenza cheeses, characterisation and classification of honey of different botanical and geographical origin, and characterisation and classification of Italian Barbera wines. 

The adaptive spectrometer described in this paper produces either non-imaging, onedimensional or two-dimensional multispectral radiance datasets ( datacube in the case of two-dimensional spectral mapping) for gas or aerosol discrimination and classification. The spectral, temporal and spatial resolution of the data collected by the instrament are adjustable in real time, making it possible to keep the tradeoff between sensor parameters at optimum at all times. The instrument contains no macro-scale moving parts making it an excellent candidate for the development of a robust, compact, lightweight and low-power-consumption device suitable for field operation. 

Sensor systems are composed of the sensor, the transmitter, and the associated signal processing. The sensor measures certain quantities (e.g., voltage, current, or resistance) associated with devices in contact with the process such that the measured quantities correlate strongly with the actual controlled variable value. There are two general classifications for sensors continuous measurements and discrete measurements. Continuous measurements are, as the term implies, generally continuously available, whereas discrete measurements update at discrete times. Pressure, temperature, level, and flow sensors typically yield continuous measurements, whereas certain composition analyzers (e.g., gas chromatographs) provide discrete measurements. 

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