- Portable digital gas sensors. electrochemical analog sensitive elements

Это окончание невероятной информации про газовые датчики.

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chemical sensor . [1] It should be emphasized that the problems of the production, transportation, sale and storage of ammonia at all stages require the use of high-precision, high-speed ammonia sensors. In the microelectronic industry, ammonia is formed, for example, in the following technological processes: oxidation, deposition of silicon layers, formation of contacts and photolithography. In particular, in photolithography, monitoring the concentration of ammonia in the air is an urgent direction for reducing molecular air pollutants. It is also important to note that ammonia is an explosive and flammable gas.

When testing the experimental setup, the concentration of gaseous [2] ammonia in the air averaged no more than 200 ppm (approximately [3] 140 mg / m 3 ). [4] The minimum ammonia concentration recorded in the experiments was estimated at 5 ppm with a signal-to-noise ratio of not less than 15. In the calculations, an estimate of the effective cross section value was used ammonia absorption obtained from experimental data using the waveguide measurement technique in the wavelength range ≈ 500–750 nm [ 9 - 11 ].

For an example in fig. Figure 7 shows one of the graphs obtained for the experimental conditions of measuring the dependence of the attenuation coefficient of the waveguide due to the presence of gaseous ammonia .

This dependence characterizes the minimum sensitivity of the integrated optical sensor under consideration, depending on the length of the sensor cell L (i.e., it is assumed that L equals a certain z ). The number 1 in fig. Figure 7 shows the recorded attenuation coefficient of the waveguide mode in the presence of additive random noise (with a level ), which was calculated on a computer according to the well-known formula:

. (6)

In expression (6) - signal level in which the signal-to-noise ratio on average (from implementation to implementation of random noise) is not lower than 20.

Fig. 7. Dependence of the minimum sensitivity of the integrated optical sensor depending on the length of the sensor cell . The number 2 in fig. 7 shows the level corresponding to a concentration of gaseous ammonia in air of 0.1 ppm .

As can be seen from fig. 7 to achieve a sensitivity level of 0.1 ppm, the length of the sensor cell must be at least 4 cm .

To further increase the sensitivity of the integrated optical chemical sensor, the following methods can be used [ 6 , 9 - 12 , 14 , 17 , 18 - 19 ]:

- an increase in the length of the sensor cell (for example, the use of a substrate in the form of a cylindrical rod, Bragg reflectors, resonators, etc.);

- optimization of the parameters of the waveguide system;

- increase in signal-to-noise ratio;

- integration of sensor elements on a single substrate, including a radiation source, sensor cell and photodetector;

- and a number of others.

To increase the power fraction of the waveguide mode in the detected medium, one should use films with a large refractive index or use a thin layer on the surface of the waveguide with optimized parameters.

The signal-to-noise ratio can be increased, firstly, by optimizing the parameters of the electronic comparison circuit and, secondly, by reducing losses in the waveguide system due to scattering of laser radiation, in particular, by using a substrate with a low surface roughness. Upon reaching the limit characteristics of the integrated optical sensor, this problem will be further investigated.

Computer simulation using a model of turbulent diffusion of gaseous ammonia in air showed that the minimum concentration that can be measured using the sensor of the type considered is about 0.1 ppm with the efficiency of introducing laser radiation (visible range) into the waveguide sensor cell of about 40%, length a sensor cell of at least 4 cm and a signal-to-noise value of about 20.

4. CONCLUSIONS

Ø There is a huge variety of sensor designs.

Ø Touch technologies play, and will play, a major role in the future in various areas of life.

Ø Sensors are used in almost all branches of science and industry.

Ø Integrated optical sensors are very promising, for example, for use in infocommunication technologies: simple design, integrated performance, high accuracy, small size and weight, high resistance to environmental conditions, long service life, the ability to integrate with existing fiber optic networks, etc.

Ø According to various estimates, sales in the global sensor market today are about 150 million euros, and annual growth is approximately 15% [ 21 ].

5. CONCLUSION

The recent and growing interest in the development and use of optical chemical sensors is associated with the following most important advantages:

Ø high sensitivity;

Ø high response speed;

Ø non-contact detection capability;

Ø high noise immunity;

Ø insensitive to electromagnetic fields (not optical frequency);

Ø insensitive to radiation fields;

Ø the ability to transmit an analytical signal without distortion over long distances (for example, through optical fiber);

Ø convenience of multiplexing signals;

Ø high data transfer density;

Ø resistance to harmful environmental influences;

Ø ease of use of integrated technology.

The main disadvantages of optical chemical sensors are: a rather high, albeit selective sensitivity to light noise, as well as a certain exposure to temperature (in the case of using semiconductors in the manufacture of the sensor).

It is established that when using a highly stable miniature electronic comparison circuit based on precision operational amplifiers and computer recording and processing measurement data, the integrated optical chemical sensor demonstrates good metrological characteristics.

Sensors based on integrated optical waveguides can be used, for example, in air quality control systems. In our opinion, there is a good prospect of using sensors of this type for the study of substances dissolved in liquids, for example, in biomedical, physico-chemical and environmental studies.

LITERATURE

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3. Schmidt D., Schwartz A. Optoelectronic sensor systems . - M.: Mir, 1991.

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6. Whitenett G., Stewart G., Atherton K., Culshaw B., and Johnstone W. Optical fiber instrumentation for environmental monitoring applications // J. Opt. A: Pure Appl. Opt. 2003, V. 5, pp. S140-S145.

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fifteen. Kulyabina E.Yu., Sidorenko M.V. Lichenoindication monitoring of air quality in the Nizhny Novgorod region // Bulletin of the Samara Scientific Center of the Russian Academy of Sciences. Biology and Ecology , 2002, 4, S. 216-222.

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17. Egorov A.A., Egorov M.A., Chekhlova T.K., Timakin A.G. The study of a computerized integrated optical sensor for the concentration of gaseous substances // Quantum Electronics , 2008, T. 38, S. 787-790.

18. Egorov AA, Egorov MA, Chekhlova TK, Timakin AG Low-loss inexpensive integrated-optical waveguides as a sensitive gas sensor // ICO Topical Meeting on Optoinformatics / Information Photonics 2008. September 15-18, 2008 . St. Petersburg . Russia. St. Petersburg: ITMO. pp . 208-211 .

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[1] Ammonia is a colorless gas with a sharp characteristic odor, almost two times lighter than air, it is well soluble in water.

[2] As you know, the concepts of gas and vapor are almost completely equivalent (see, for example, [2 0 ], P. 527). In the study, for example, of the dynamics of phase transitions, the phenomenon of critical opalescence, etc., apparently, it will be necessary to clarify the state in which gaseous (vapor, gas) ammonia is located. The experiment described here did not require this.

[3] 1 μg / m 3 = (1 million -1 ∙ ) ∙ 10 3 , where - molecular weight of the gaseous substance, coefficient for temperature 25 0 С and pressure 760 mm Hg equal to 24.5.

[4] This value exceeds the maximum permissible concentration of a given substance (for the Russian Federation) both in the air of populated areas (0.2 mg / m 3 ) and in the working area (20 mg / m 3 ).

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