Electronic nose

An electronic nose is a technical system for the measurement of odors. For this purpose, generate electronic signals microelectronic gas sensors. The term electronic nose combined so that the " recognition" of odors with the technical implementation with electronic sensors. It should be noted that there can be no real sense in Electronic nose as odors must be interpreted by the brain, the technical measurement system, however, only data on gas concentrations, both odorless as the odor-active gases supplies.

Biological Background

The inspiration for the electronic nose comes from the biological model, the olfactory system. Very simplified is the smelling system of olfactory cells in the nose, which are activated by odorants. Approximately 350 different types of olfactory cells are active in humans, of each type are several 10,000 in the olfactory mucosa present. The signals from the mating olfactory cell types are summarized in mitral cells in the olfactory bulb, a part of the brain. The resulting signals are processed there, especially by the limbic system, where the strong emotional link between the sense of smell originates. The perception of an odor impression is only the last stage of the effect of odorants on the olfactory cells.

In the human sense of smell, the experience of the evolution are shown. Like all other senses provides the sense of smell necessary for survival data of the environment. To find food, their specific gas components must be detected sensitively. For maturity odors and food flavorings sense of smell is therefore particularly sensitive. The same applies to odors of hazards ( toxins ) and those associated with social functions. So each person has a very specific odor, which is genetically coded. The technical measurement systems lack these evolutionary conditions. Chemically similar is detected with comparable signal strength. Also, odorless gases, such as methane (CH4 ), carbon dioxide (CO2) or carbon monoxide (CO) can be measured.

Instead of the olfactory cells different gas sensors are used in the electronic nose. They cover the widest possible range of gaseous compounds in the air. Not the measurement of individual gas components, it is intended, but rather a metrological illustration of composition of the air sample. In fact, the analogy of the electronic nose for seeing is true. The three color channels of the visual system generate the perceptual process associated with the three intensities, the color image of the world. The few gas sensors an electronic nose ( from about 6 to 40) also generate an image of the measured sample of air. This image can be associated with additional measured parameters, in the case of odor measurement with the humansensorisch measured via olfactometry odor concentration.

Technology

A typical electronic nose consist of a number of gas sensors, whose signals are processed by means of mathematical methods in terms of a pattern recognition. As a sample here is the ratio of the signal strengths of the individual sensors meant to each other, which can be thought of as a geometric pattern in the plot around a common center point ( radar plot ) as a star with rays of different lengths. In practice, however, more abstract mathematical methods are used, such as principal component analysis, with which the pattern information are mapped onto a two-dimensional plane in which the pattern similarity manifests itself in demarcated areas. Technical gas sensors do not function as olfactory cells. In olfactory receptor cells contain molecules that specifically interact with very few gases and thus generate a nerve signal. At the same time a high signal amplification takes place, so few gas molecules, ie very low concentrations, sufficient for activation of a sensory neuron. A number of different technical sensors used in electronic noses. The main representatives are:

• Sensors based on semiconducting metal oxides, abbreviated MOX sensors
• Sensors with electrically conducting polymers, split into independent ( intrinsic) conducting polymers and in those to which a conductive component such as graphite was added.
• Sensors that utilize a mass effect, with the two groups of quartz crystal sensors ( QMB / QCM sensors ) and the surface wave sensors (SAW ) sensors.

Each of these types of sensors has its metrological characteristics. In particular, the chemical range of the measured gases is different. Preferred MOS sensors measure low molecular weight oxidizable gases, conducting polymers with polar gas components are well measured, mass-sensitive sensors measure high molecular weight substances preferred. In the industrial application but it is particularly the stability of the measuring sensors over long periods is a critical parameter, since the calibration information to be obtained. Therefore, measures for the protection and control of the sensors must be provided, depending on the operating principle of the sensors and the specific application.

The gas sensors used in electronic noses are often in the form of clustered specifically constructed partly applied on a single microchip gas sensors. One speaks here of sensor arrays or chemical sensor arrays. Instead of the term electronic nose is therefore often spoken of chemical sensor arrays.

Odor measurement with electronic noses

The measuring system of the electronic nose is not by design an odor measurement system. But the differences from the biological sense of smell are too large, the clearest is the Mitmessung also completely odorless gases through the broadband gas sensors. However, can be made into an electronic nose in many applications with a coherent methodology and a calibration measurements olfactometric a chemo -sensory measurement system.

The reference problem of electronic noses

To perform odor measurements with gas measuring systems, reference data are needed for smell. The standardized measurement technique is the olfactometry according to the European standard EN 13725th With this measuring method, the odor concentration is measured, which indicates how strong an odor sample must be diluted until it is odorless for an average nose. A sample of 3000 OU/m3 must therefore be diluted in a ratio of 1:3,000. It is shown that the olfactory measurement method only provides a very uncertain reading. The variation in repeated measurement or the comparison of several laboratories with each other are very large. The measurement uncertainty is between four times and the quarter of a measured value. A measured value of 1000 OU/m3 has a measurement uncertainty interval from 250 to 4000 OU/m3. The asymmetry is due to the logarithmic nature of the actually measured value.

For the calibration of electronic odor measurement systems, this uncertainty of the reference measurement method has great significance. In the mathematical modeling of this effect must be considered.

Applications of electronic nose

Electronic noses are suitable to continuously monitor odor sources. In contrast to the olfactometry with human Testriechern, which can only make random measurements, is possible with a carefully tailored to the use of odor measurement system for continuous monitoring. This is useful for problematic odor sources such as industrial plant, wastewater treatment and waste management. This can be achieved with the continuous monitoring contributes to the protection of nearby residents.

The first large-scale application of continuous odor monitoring can be found in upscale automobiles. This is the fan damper control, which - in a traffic jam or a tunnel - automatically shuts off the air supply when exhaust gases are sucked. In the true sense but not odors are measured, but ratios of the indicator gases, such as CO and NOx. The system is therefore also suitable for this application.

Another field of application of the electronic nose is the quality control. Food and beverages can be characterized by outgassing of flavors and perfumes and other, also odorless components. Whenever a constant product composition is required, electronic noses can be used here, which are particularly suitable for the detection of a constant composition of the outgassing.