RU2262684C1 - Optical absorption gas analyzer - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: gas analyzer has electromagnet radiation source with wavelength belonging to spectrum of absorption of analyzed gas, tubular gas tray with internal light-reflecting walls and photoreceiver, both disposed along the path of radiation. Photoreceiver is connected through amplifier to data processing and registration unit. Tubular gas tray is made in form of spiral to increase path of light flow and to eliminate direct exposure of photoreceiver. It's advised to make tray in form of spiral of Archimedes. To perform exchange of tray with outer medium, the through holes can be made in its walls at acute angle to plane of symmetry of tray at the side of electromagnet radiation source. Sensitivity of measurement of CO, CO₂ and methane is no lower than 20ppm.

EFFECT: improved precision; higher sensitivity of measurement.

2 dwg, 1 tbl

Description

The invention relates to measuring technique, and in particular to devices for determining the concentration of gases, for example methane, carbon monoxide or carbon dioxide, hydrocarbons, benzene, nitric oxide, etc., in the atmosphere, industrial premises, technological apparatuses, etc.

Known absorption fiber-optic gas analyzer containing sequentially mounted and optically coupled emitter, input optical fiber, multi-way cuvette, consisting of three spherical mirrors, output optical fiber, information recording and processing unit. A spectral integral demultiplexer is installed between the output optical fiber and the recording unit, and on the continuation of the collective mirror sphere in the immediate vicinity of its edge, the ends of the input and output optical fibers are installed on one side, both lens mirrors are mounted 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).

Also known is an optical absorption gas analyzer 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 cuvette made in the form of an integrating sphere with an internal reflective coating, where the optical input and output are located asymmetrically with respect to the center of the sphere, a filter and a radiation receiver connected through an amplifier to the processing unit and recording the information signal (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 increase the accuracy and reliability of the studies, the optical absorption gas analyzer contains a broadband optical emitter located along its radiation with a tubular gas cuvette with internal reflective walls and two photodetectors equipped with light filters in the region of absorption and transparency of the analyzed gas, respectively, connected to the differential processing unit and recording 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 along with an optical focusing element can be traced. So, to control the content of gases having an infrared absorption spectrum, a cuvette made in the form of a hollow retroreflective truncated cone with an aperture in the side wall is used, 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). Such a cell geometry provides focusing and multiple reflection from its walls of light rays passing through a controlled sample.

Closest to the claimed one is an optical absorption gas analyzer containing a source of electromagnetic radiation with a wavelength from the absorption region of the analyzed gas, a tubular gas cuvette with internal reflective walls and a photodetector connected through the amplifier to the information processing and recording unit located along its radiation. To increase the control accuracy, the source and photodetector of electromagnetic radiation are made two-channel with the possibility of additional radiation and receiving an optical signal with a wavelength from the transparency region of the analyzed gas, and the information processing and recording unit is made according to the differential measurement scheme of the signals received 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. In addition, it has low accuracy due to the possibility of direct exposure to the photodetector.

The technical task of the inventive gas analyzer is to increase the sensitivity and accuracy of measurements by increasing the path of the light flux passing through the gas cell.

The solution to this technical problem lies in the fact that in the design of an optical absorption gas analyzer containing a source of electromagnetic radiation with a wavelength from the absorption region of the analyzed gas, a tubular gas cuvette with internal reflective walls and a photodetector connected through the amplifier to the processing unit and located recording information, a tubular gas cuvette is made spiral-shaped to increase the path of the light flux and exclude direct illumination opriemnika.

For gas exchange with the environment without attenuating the received photo signal, through holes at an acute angle to the plane of symmetry of the tubular gas cell from the side of the electromagnetic radiation source can be made in the walls of the tubular gas cell.

In a spiral-shaped cuvette, direct illumination of the photodetector is excluded due to the shading of direct radiation by the bends of the formed spiral-shaped fiber, which results in an increase in the measurement accuracy. The spiral shape of the cuvette also contributes to an increase in the path of the light flux in equivalent areas of the area.

A gas cell can be made in the form of cylindrical, conical and other spiral shapes. The most appropriate implementation of a gas cell in the form of a spiral of Archimedes. In this embodiment, the cuvette has a minimum volume for a given path length of the light flux. In addition, as the light beam approaches the center of the Archimedes spiral, the number of reflections per unit length of the tubular cuvette increases, which results in an additional increase in the sensitivity and accuracy of measurements.

The proposed design of a gas cell allows the use of both single-channel and multi-channel sources of electromagnetic radiation and a photodetector. In particular, in order to increase the accuracy of measurements, the source of electromagnetic radiation and / or the photodetector can be made dual-channel with the possibility of additionally emitting and receiving an optical signal with a wavelength from the transparency region of the analyzed gas. In this case, the information processing and recording unit is made according to the differential measurement scheme for the signals of the reflection and transparency regions obtained at the output of the formed channels. The use of multichannel measuring systems in the proposed device is advisable when controlling several components in the analyzed gas environment.

As a source of electromagnetic radiation, LEDs, resistive type emitters, incandescent lamps, semiconductor injection lasers (in a multi-channel version), etc. can be used. Photo-resistors, photodiodes, semiconductor bolometers, pyrodetectors, thermocouple batteries can be used as photodetectors.

As an amplifier and a unit for processing and recording information, any well-known constructions of this purpose can be used.

Figure 1 shows a drawing of a tubular gas cell of the proposed form; figure 2 is a diagram of the location and relationship of structural elements.

Table 1 shows the technical characteristics of the gas analyzer variant.

The tubular gas cuvette (FIGS. 1, 2) contains a pipe laid in a spiral of Archimedes, which is formed by connecting two parts 1 and 2 symmetrical with respect to the plane of the spiral of Archimedes. A spiral groove is made on the inner end surfaces of elements 1 and 2, into which the cuvette is inserted ribbon insert 3. This insert performs the following functions:

a) provides an accurate combination of parts 1 and 2;

b) improves the mechanical strength of the cell;

c) prevents parasitic illumination of the adjacent turn of the tubular cuvette through possible slots at the place of alignment of parts 1 and 2.

The stiffness of the cuvette is also provided by the presence of stiffening ribs 4 made outside the cuvette. On the outer surfaces of the elements 1 and 2, eyelets 5 are also provided for fastening the cuvette.

The working surfaces of elements 1 and 2 in this embodiment are coated with an aluminum reflective film.

For gas exchange with the environment without attenuating the received photo signal, through holes 6 are made in the walls of the reduced tubular gas cuvette at an acute angle to its plane of symmetry from the side of the electromagnetic radiation source 7 mounted on the outer end of the tubular part of the cuvette. This source emits light with a wavelength of at least from the absorption region of the analyzed gas. In the center of the gas cell, a chamber 8 is made, in which a photodetector 9 and an amplifier 10 are placed. In this case, the photodetector 9 is optically connected to the inner end of the tubular part of the cell. The electrical output of the photodetector 9 is connected to the processing unit and recording information 11 through the amplifier 10.

The described tubular gas cuvette in the form of an Archimedes spiral can be made of plastic by stamping its parts, obtained by cutting in a plane of symmetry, followed by applying a reflective coating (for example, from Ag or Al) on the inner surfaces of parts 1 and 2 and gluing end surfaces in the plane symmetry of the cell. Before gluing, a ribbon-like insert 3 is inserted into the groove between parts 1 and 2.

Reflecting repeatedly from the surface of the inner wall, the light flux in the absorption region of the analyzed gas is attenuated depending on the concentration of the analyzed gas molecules that it encounters, which is detected by the photodetector and taken into account by the processing and recording unit.

The proposed gas analyzer with a gas cuvette in the form of an Archimedes spiral with an inner diameter and length of the tubular part of 9 mm and 1.2 m, respectively, with a differential measurement scheme, was tested on air models containing CO, CO ₂ and methane at concentrations of 100, 1000 and 10000 ppm. We took into account the accuracy (average value) and the error of the average value. Additionally, the sensitivity threshold was determined - the lowest concentration of gas that can be recorded by the device. The test results (10 measurements in each mode) are given in table 1.

As can be seen from the table, the average measurement values differ from the reference data (set manometrically) by no more than 4%, and the maximum error of the mean does not exceed 12%. Sensitivity of measurements makes from 10 to 20 ppm.

The test results obtained indicate high accuracy and sensitivity of the claimed gas analyzer. Since a tubular gas cuvette of the indicated length (1.2 m) cannot be performed, it can be stated that a positive effect, derived from the achieved, is a decrease in the overall size of the cuvette. It is important to emphasize that the largest relative increase in the path length of the light beam takes place in the center of the cell, which indicates the most appropriate form of its execution in the form of a conical spiral or Archimedes spiral.

Table 1
Accuracy and sensitivity of a gas analyzer with a tubular cuvette in the form of a spiral of Archimedes Controlled gas Wavelength (nm) in channels: The average value and its error in the gas content, ppm: Sensitivity, ppm takeover transparency 100 1000 10,000 With 4.66 4.0 98 ± 11 990 ± 40 9600 ± 120 fifteen CO ₂ 4.20 4.0 103 ± 9 1040 ± 60 10200 ± 140 twenty Methane 3.40 4.0 96 ± 12 970 ± 10 9900 ± 90 10

Claims (1)

An optical absorption gas analyzer containing a source of electromagnetic radiation with a wavelength of at least from the absorption region of the analyzed gas, a tubular gas cuvette with internal reflective walls located along the radiation and a photodetector connected through an amplifier to the information processing and recording unit, characterized in that the tubular gas the cuvette is spiral-shaped and through-holes are made in its walls at an acute angle to its plane of symmetry from the side of the electric source electromagnetic radiation.

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