RU2778205C1 - Optical absorption gas analyzer - Google Patents

Abstract

FIELD: measuring technology.

SUBSTANCE: invention relates to measuring technology and concerns an optical absorption gas analyzer. The gas analyzer contains electromagnetic radiation sources and light filters with wavelengths from the absorption region of the analyzed gases, cuvettes, photodetectors and a computer with a trained neural network. The sources of electromagnetic radiation are located in a circle of the radiation input node in the cuvettes. The cuvettes are made in the form of capillary fibers with an external reflecting surface along the length, and the lengths of the cuvettes are equal to the value of the inverse value of the absorption coefficient of the gas under study multiplied by the inverse ratio of the signal to the noise of the system. The photodetectors at the output of the cuvettes are connected to amplifiers and analog-to-digital converters, which in turn are connected to a module of a trained neural network.

EFFECT: increase in the versatility, informativeness, sensitivity and accuracy of measurements.

1 cl, 3 dwg, 1 tbl

Description

The invention relates to measuring technology, namely to devices for determining the concentration of gases, such as methane, carbon monoxide or dioxide, hydrocarbons, benzene, nitrogen oxide, etc., in the atmosphere, industrial premises, technological apparatus, etc.

Known absorption fiber-optic gas analyzer containing serially installed and optically coupled emitter, input optical fiber, multi-pass cuvette, consisting of three spherical mirrors, output optical fiber, unit for recording and processing information. A spectral integral demultiplexer is installed between the output optical fiber and the registration unit, and on the continuation of the sphere of the collective mirror in the immediate vicinity of its edge, on one side, the ends of the input and output optical fibers are installed, both mirror lenses are installed with the possibility of joint rotation relative to the center of curvature of the mirror - collective in the common meridional plane of all mirrors (RU 2091764, G 01 N 21/61, 1997).

An optical absorption gas analyzer is also known, containing an optically coupled laser source of infrared electromagnetic radiation with a wavelength from the absorption region of the analyzed gas, a multi-pass gas cell made in the form of an integrating sphere with an internal reflective coating, where the optical input and output are located asymmetrically relative to the center of the sphere, a light filter and a radiation receiver connected through an amplifier to the information signal processing and recording unit (RU 2022249, G 01 N 21/61, 1994). The inner surface of the integrating sphere can be made ellipsoidal (WO 2004/013600, G 01 N). To improve the accuracy and reliability of research, an optical absorption gas analyzer contains a broadband optical emitter, a tubular gas cell with internal reflective walls located along its radiation, and two photodetectors equipped with light filters in the absorption and transparency areas of the analyzed gas, respectively, connected to a block for differential processing and registration of information signals (US 6469303, G 01 J 005/02, 2002; US 2004/0007667, G 01 N 21/61).

However, such gas analyzers are difficult to manufacture and operate.

Among the directions of development of this type of technology, the implementation of a gas cell together with an optical focusing element can be traced. So, to control the content of gases with an infrared absorption spectrum, a cuvette is used, made in the form of a hollow reflective truncated cone with a hole in the side wall, in which an optical filter with a reference radiation receiver is installed, and the radiation source is located in close proximity to the cuvette (RU 2037809 , G 01 N 21/61, 1995). This geometry of the cuvette provides focusing and multiple reflection from its walls of light rays passing through the controlled sample.

Closest to the claimed is an optical absorption gas analyzer containing a source of electromagnetic radiation with a wavelength from the absorption region of the analyzed gas, a cuvette with internal reflective walls and a photodetector located along its radiation and connected through an amplifier to the information processing and recording unit. To improve the accuracy of control, the source and photodetector of electromagnetic radiation are made two-channel with the possibility of additional emission and reception of an optical signal with a wavelength from the region of transparency of the analyzed gas, and the block for processing and recording information is made according to the scheme of differential measurement of signals obtained at the output of the formed channels (RU 2109269, G 01 N 21/61, 1998).

However, this device has a low sensitivity due to the short path length of the light flux. The presence of one channel without a spectral instrument does not allow one to register the simultaneous presence of several studied gas components, which reduces the information content. In addition, it has low accuracy due to the possibility of direct illumination of the photodetector.

The technical task of the proposed gas analyzer is to increase the versatility, information content, sensitivity and accuracy of measurements by increasing the path of the light flux passing through a set of capillary fibers.

The solution to this technical problem lies in the fact that an optical absorption gas analyzer containing sources of electromagnetic radiation and light filters with wavelengths from the absorption region of the analyzed gases, a gas cell and photodetectors located along the radiation path, is additionally equipped with a computer with a trained neural network and additional cuvettes, while sources of electromagnetic radiation, are located around the node for inputting radiation into the cuvettes, and the cuvettes are made in the form of capillary fibers with an external reflective surface along the length, and the lengths of the cuvettes are equal to the reciprocal value of the absorption coefficient of the studied gas, multiplied by the reciprocal signal-to-noise ratio systems, and photodetectors, at the output of the cuvette, are connected to amplifiers and analog-to-digital converters, which, in turn, are connected to the trained neural network module.

In FIG. 1 shows a diagram of the proposed gas analyzer; in fig. 2 shows a neural network calibration model for CO ₂ gas; in fig. 3 shows the node input - output of radiation

In FIG. 1 introduced the following elements:

cd - radiation sources;

j, ... m - channels, including optical capillary cuvettes, arrows on which show openings for input and output of the analyzed gas; fp photodetectors; Y-amplifiers from the output of which the amplified signal is fed to the input of a neural network with a neural network calibration model.

In FIG. 2 shows a diagram of a neural network calibration model for CO _{2 gas,} to explain the essence of learning the neural network of the proposed device.

The essence of the claimed invention is as follows. A gas is selected, or a mixture of gases m, the presence of which is supposed to be determined using the claimed device, k n _absorption coefficient or optical density of each of the mentioned gases is determined, using which the length of the cuvette d is calculated for each gas.

, для m-компонентной смеси определяется по формуле

To select the optimal length of the optical path in the capillary fiber of the cell, we use the Bouguer-Lambert-Beer law, according to which the intensity of the radiation that has passed the path in the cell

, for an m -component mixture is determined by the formula

-интенсивность падающего излучения; l-длина оптического пути кюветы; kj(λi) – спектральный коэффициент поглощения j-гo компонента на i-й длине волны; сj– концентрация j-гo компонента. Здесь оптические коэффициенты являются функцией длины волны падающего излучения.where

- the intensity of the incident radiation; l is the length of the optical path of the cuvette; kj (λ i ) is the spectral absorption coefficient of the j -th component at the i -th wavelength; cj is the concentration of the j -th component. Here, the optical coefficients are a function of the wavelength of the incident radiation.

. α- величина обратная отношению сигнала к шуму измерительной системы (S/N). Поскольку каждый излучатель имеет свою ширину спектра излучения, при расчете длин пути необходимо использовать интегральный коэффициент поглощения газа k ₀, проинтегрированный в пределах полосы излучения данного источника. При заданном значении α определяем длину пути, оптимальную для регистрации данного коэффициента поглощения k ₀, который будет отличен для каждого газа.The threshold sensitivity of the method when registering the minimum absorption coefficient is determined by the path length l and the ability of the recording system to register small changes in the radiation intensity due to absorption

. α is the reciprocal of the signal-to-noise ratio of the measuring system (S/N). Since each emitter has its own width of the emission spectrum, when calculating the path lengths, it is necessary to use the integral gas absorption coefficient k ₀ integrated within the emission band of a given source. For a given value of α, we determine the path length that is optimal for registering a given absorption coefficient k ₀ , which will be different for each gas.

In this case, the optimal lengthl ₀ will be determined by the expression

l ₀ = α/ k ₀ .

Number of reflections in a light guide fiber

The length of the optical path in the fiber, taking into account reflections

l =

Accordingly, the length of the optical capillary fiber for the cuvette will be equal to

L=

After calculating the optimal length of the cell for each gas, all m cells are made, in each of which holes are made for the inlet and outlet of the analyzed gas (see Fig. 1) and the multichannel inventive device is assembled. When assembling the proposed device, for each test gas, one chooses its own source cd-radiation sources and its own fp-radiation receiver, and install the selected radiation sources and receivers around the node for inputting the analyzed gas into the corresponding channel (Fig. 3). As sources and receivers of light radiation, various optical and electronic elements operating in the appropriate frequency ranges characteristic of each gas selected for analysis can be used: lasers, LEDs, photodiodes, bolometers, etc.

Appropriate signal amplifiers are selected, coming into them from the output of photodetectors, and the output of amplified signals from the said amplifiers is connected to the input of the neural network module located in the computer (Fig. 2). The neural network, using the database entered into its memory, responds to a threshold signal corresponding to a given absorption coefficient of the component under study. The neural network can be trained and retrained after each calibration, it becomes prepared for the operation of the device.

An example of a specific implementation. The claimed device was manufactured, in which it was supposed to determine the presence of the following gases in the test sample: CO ₂ , CH ₄ , NH ₃ , NO ₂ , etc. The radiation sources used have a spectral width of 5 cm ^-1 .

For CO ₂ , CH ₄ NH ₃ NO ₂ molecules from the HITRAN database [https://hitran.org/] in the region of strong absorption bands for radiation sources with a width of 5 cm ^-1 , informative intervals were selected. For them, the table shows the centers of the intervals in frequencies (cm ^-1 ) and wavelengths (nm), also calculated the integral absorption coefficients per 1 atm, integrated over the width of the radiation spectrum of the source (5 cm ^-1 ). The selected intervals are in the IR, visible and UV regions of the spectrum. The integral absorption coefficients were 15...70 cm ^-1 . MPCs for industrial premises were taken as the minimum measured concentrations. With a given value of α=0.01 for the minimum attenuation value in the cuvette, we determine the path length l ₀ that is optimal for recording the absorption coefficient k ₀ of the xMAC corresponding to the maximum allowable concentration of each gas [https://hitran.org/]. MPC values, k ₀ x MPC, l ₀ are also given in the table.

Table

Molecule CO ₂ CH _{4 SO2} NO ₂ Frequency, cm-1 2362 3017 35000 25000 Wavelength, nm 4234 3315 285.7 400 Integral absorption coefficient for P=1 atm (for a spectrum width of 5 cm ^-1 ) k ₀ , cm ^-1 70 fifteen 19.2 16.8 MPC, ppm 10000 1000 0.3 one kxMAC, cm ^-1 0.7 0.015

10^-6 6

^10-6 1.7

^10-5 Optimal path length l ₀ , cm 0.15 0.7 1.7

10 ³ 6

10 ²

As can be seen from the table, the optimal path length l ₀ varies over a wide range of 0.15 ... 17000 cm. mixtures.

Thus, the claimed device has the following advantages over the prototype:

Increases the optical path due to reflection from the walls of the capillary column

1. Parallel and simultaneous determination of several industrial gases (no need for gas analyzers for each gas)

2. Calibration models are stored in a neural network, which can be retrained after each calibration

3. To reduce the size of the capillary columns can be presented in the form of a bundle or coiled into a spiral

Thus, the claimed device, compared with the prototype, can significantly increase the versatility and (N times) informativeness, using N-channels of registration, instead of one.

Claims (1)

An optical absorption gas analyzer containing sources of electromagnetic radiation and light filters with wavelengths from the absorption region of the analyzed gases, a gas cell and photodetectors located along the radiation path, characterized in that a computer with a trained neural network and additional cuvettes are introduced into it, while sources of electromagnetic radiation located around the node for inputting radiation into the cells, and the cells are made in the form of capillary fibers with an external reflective surface along the length, and the lengths of the cells are equal to the reciprocal value of the absorption coefficient of the gas under study, multiplied by the inverse signal-to-noise ratio of the system, and the photodetectors are connected at the output of the cells with amplifiers and analog-to-digital converters, which in turn are connected to the trained neural network module.

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