rus
Issue #2/2021
Yu. N. Snytko
Study of Optical Absorption Gas Analyser for Controlling the Concentration of Freons in the Air of Industrial Facilities
Study of Optical Absorption Gas Analyser for Controlling the Concentration of Freons in the Air of Industrial Facilities
DOI: 10.22184/1993-7296.FRos.2021.15.2.162.174
Study of Optical Absorption Gas Analyser for Controlling the Concentration of Freons in the Air of Industrial Facilities
Yu. N. Snytko
SPA “Analitpribor”, Smolensk
A comparative analysis of gas analysers that implement different physical methods for controlling the content of freons in the air is presented. A functional diagram of an optical absorption gas analyser is proposed. A method for modulating the pressure of the analysed gaseous medium in the working chamber has been developed and investigated. An emitter with a maximum luminosity density in the wavelength range from 8 to 10 μm is investigated. The design of the system of interference filters has been developed. The test results are presented.
Key words: gas analyser, freon, pressure modulation, flow booster, wave length, interference filter
Received: 11.01.2021
Accepted: 16.03.2021
In industrial refrigeration and air conditioning systems, freons are used as a refrigerant or in fire extinguishing systems. The most critical facilities that require continuous monitoring of the concentration of freons in sealed rooms are sea vessels (including icebreakers) and nuclear power plants.
Freons are air-substituting gases, therefore, accumulating in sealed rooms, they pose a high danger to the personnel working there [1]. It is clear that the control of the content of freons in such premises is of great importance [2, 3].
Industrial freons, their physical characteristics, as well as the values of maximum permissible concentrations (MPC) for working and residential areas are presented in table. 1. (based on materials [4–7]). The analysis of the MPC levels makes it possible to formulate the initial requirements for the range and measurement error of the freon gas analyser:
measurement range (NLT):
from 0 to 4000 mg / m3;
measurement error (NMT): 25% at points of 100 and 3000 mg / m3.
Due to the need for continuous monitoring, gas analysers must be stationary. The response speed should be no more than 5 minutes to ensure that the personnel can leave the gas-polluted room without harm to health. To be used in sealed rooms on sea vessels, the gas analyser must be resistant to vibration, rolling, tilt and have a service life without service and maintenance for at least 1 year.
To measure the concentration of freon in the air, optical absorption, electrochemical and semiconductor gas analysers are used. From the works [8, 9], it can be concluded that the required metrological characteristics can be provided only by an optical absorption (investigated) gas analyser [10, 11].
The principle of operation of an optical absorption gas analyser is based on the absorption of infrared radiation (2 to 20 µm) by the molecules of the substance being measured at the characteristic wavelength. Such wavelengths appear in the spectrum when certain chemical groups are present in the compound and are called characteristic [12]. These include, for example, stretching vibrations of C–H, C=O, C=C, O–H bonds, bending vibrations of C–H, N–O, vibrations of groups – NO2, – COO–, CONH2. Freons have a characteristic C–F bond in the molecule, and according to [13] powerful and sharp absorption bands of halogen derivatives are in the range of 7.15 to 10 µm (Fig. 1). It is necessary to take into account the presence of water absorption lines in the range of 5 to 8 µm, which makes it impossible to measure in this spectral range without preliminary preparation (drying) of the sample.
For clarity, let us single out a portion of the spectrum from 8 to 10 μm (Fig. 2), in which freons can be monitored without the influence of air humidity. It is obvious that the gas analyser will have cross-sensitivity to other, not measured freons, due to the superposition of their spectral characteristics. Since during installation on objects the types of controlled freons are known in advance, cross-sensitivity to other freons will not affect the measurements of their essentially absence at the object.
The design of a gas analyser operating in the wavelength range from 8 to 10 μm faces the following technical difficulties:
To achieve the required error in the entire measurement range, it was necessary to create a model of an optical-absorption gas analyser with high sensitivity and the ability to measure the target substance in a gas sample with the presence of unmeasurable components (atmospheric moisture and CO2) without preliminary sample preparation in order to avoid sample distortion.
This task is quite difficult, because it is required to measure the target component under conditions of changes in the level of atmospheric humidity, more than three orders of magnitude higher than the permissible measurement error of the gas analyser.
To achieve the required metrological characteristics of the gas analyser [15], the pneumatic modulation method [16] was applied. The functional diagram of a gas analyser operating on the principle of pneumatic modulation is shown in Fig. 3.
The gas analyser operates as follows. The gas mixture (hereinafter referred to as GM) is pumped through the inlet filter (hereinafter referred to as F) using the flow booster 1 (hereinafter referred to as FB1), which creates the pressure of the GM in reservoir 1 (hereinafter referred to as R1). With the help of the flow booster 2 (hereinafter referred to as FB2), a vacuum of the analysed GM is created in reservoir 2 (hereinafter referred to as R2). The solenoid valve (hereinafter referred to as SV) 1 opens when SV2, and pressurized GM from the reservoir R1 enters the working chamber. Overpressure builds up in the working chamber P+ = PATM + ΔP.
Then SV1 closes and SV3 opens, the pressure in the working chamber drops to atmospheric pressure. SV3 closes and SV4 opens. GM from the working chamber is fed to the reservoir R2, where the flow rate pump FB2 creates a vacuum P– = PATM – ΔP.
Then SV4 closes, after which SV2 opens. The pressure in the working chamber rises to atmospheric pressure. The stability of pressure maintenance is ensured by measuring the current pressure (excess, atmospheric and vacuum) in the working chamber using a pressure sensor (hereinafter referred to as PS) and is stabilized at the required level by controlling the operation of FB1 and FB2. The algorithm of operation of the solenoid valves is explained by the cyclogram, shown in Fig. 4. The sequence diagram of the operation of the solenoid valves is repeated with a period of 2.6 s, providing modulation of the pressure of the analysed GM in the working chamber (Fig. 4, sequence diagram 2). This algorithm of the gas analyser has been patented in the Russian Federation [17].
In this operating mode, there is practically no influence of external influencing factors on the zero readings of the gas analyser, which makes it possible to achieve high metrological characteristics of the gas analyser, stability and the absence of the need for maintenance for one year.
An emitter is installed on one side of the working chamber. On the other side of the camera, there are three optical radiation detectors (PD1, PD2, PD3, see Fig. 3), with optical filters installed in front of them, which transmit radiation along the absorption lines of the measured components in Fig. 3. Each detector picks up only the radiation that the corresponding interference filter passes through. Signals from optical detectors (hereinafter referred to as OD) are amplified by three amplifying channels, consisting of amplifiers A1-A3 (Fig. 3). A temperature sensor (TS) is designed for temperature compensation. The modulation frequency is determined by the frequency dependence of the OD sensitivity, for which the pyroelectric detectors PP‑82 are used (Fig. 9).
Pressure control is carried out using a voltage processor Uр with PS, setting the required performance FB1 and FB2.
The absorption of the flux of IR radiation occurs in the presence of the determined components in the GM, through one or two measuring channels, depending on the presence of the target substances. The third channel is a reference channel to compensate for external factors and undetectable components.
Enlightened germanium was used as the elements of the gas path [18]. The transmittance in the wavelength range from 8 to 10 microns is at least 96%. This technical solution allows increasing the sensitivity of the gas analyser and at the same time increasing the selectivity.
Currently, the most progressive and high-tech method of applying optical coatings is the method of ion-beam evaporation. The indisputable advantages of this method are high adhesion of coatings, ultra-low losses of optical radiation for scattering and absorption, precision and stability of optical parameters. In addition, during electron beam evaporation, a flux of both neutral particles and ions of the evaporated substance enters the substrate. Thus, the ion-assisted method is indirectly implemented, in which, simultaneously with the flow of atoms, a flow of ions with an energy higher than the thermal one is supplied to the substrate. Additional ion bombardment of the surface increases the adhesion of the films to the substrate and to each other and increases the density of the films.
The technology of manufacturing interference filters operating in the wavelength range from 8 to 10 µm requires the deposition of films on the substrate 10–20 times thicker than for the visible range of the spectrum [19]. As a result, the total thickness of the films of the narrow-band interference filter at a wavelength of 9.45 µm will be 10.4 µm. Therefore, the voltage and absorption in films, which could be neglected in the visible range, increase in the range from 8 to 10 μm and turn into a limiting factor in the design of structures [20]. Furthermore, interference filter substrates have high refractive indices (for the range from 8 to 10 μm, germanium with a refractive index of n = 4.0 for λ = 10 μm is used), and high-index film-forming materials are required to create effective interference structures. Therefore, materials for the visible range of the spectrum cannot be used as basic materials for the formation of optical coatings in the range from 8 to 10 μm. The main film-forming materials for the range from 8 to 10 μm are fluorides, chalcogenides, and semiconductors [21]. These materials, as a rule, have significantly worse mechanical and climatic resistance parameters than oxides and MgF2, and also poorly tolerate ion assistance. To improve the performance of coatings, in many cases it is necessary to introduce additional functional layers and interlayers [22] and to seek a compromise between strength, efficiency, and radiation resistance. The optimal pair of film-forming materials in terms of technical and economic characteristics is MgF2 and Ge. An example of a combination of interference filters fabricated on a germanium substrate by deposition of MgF2 and Ge films is shown in Fig. 6.
Analysing the relative spectral transmittance of the interference filters (see Fig. 6), one can see that it is required to suppress the parasitic region of the spectrum in the range of 3 to 7 µm. This problem is solved by installing a cut-off filter (the function of its relative spectral transmittance is shown in Fig. 7) and using an emitter having a maximum radiation intensity in the 8 to 10 µm region. The required range is provided by the application a single crystal of leucosapphire heated to a temperature of 290 °C as an emitting element.
The graphs for calculating the spectral density of the radiant luminosity of blackbody radiation (BB) and leucosapphire at a temperature of 290 °C are shown in Fig. 8. Obviously, the maximum luminosity density of the leucosapphire emitter, in comparison with the blackbody, has shifted to the long-wavelength region and is in the range of 8 to 10 µm, which achieves a high selectivity of Freon measurements. A patent for an invention [23] protects this technical solution.
Fig. 8 shows black curve – theoretical value for blackbody, green curve – Al2O3, (calculated value), red curve – Al2O3 (experimental value measured with a spectrophotometer).
Let us estimate the spectral density of the radiant luminosity of the filtered radiation flux incident on the OD when a leucosapphire rod heated to a temperature of 290 °C is used as a radiation source (Fig. 9). It is obvious that the use of narrow-band interference filters together with a cut-off filter and an emitter having a maximum radiant luminosity in the range of 8 to 10 μm makes it possible to achieve the selectivity of the gas analyser (no influence of unmeasured components).
The determination of the zero line drift was carried out in normal climatic conditions for 48 hours. In fig. 10 is a graph of the drifting baseline of the gas analyser. The zero line drift in 48 hours was no more than ±2.5 mg / m3 (±0.23 ppm) for the measurement channel of Freon 114B2 (green line) and ±1.5 mg / m3 (±0.42 ppm) for the measurement channel freon 22 (blue line).
The main metrological characteristics obtained when testing a sample of an optical absorption analyser of freons are given in table. 2.
CONCLUSIONS
Due to the use of interference filters, the region of the radiation spectrum is allocated, which is necessary for the selective measurement of freons. The use of a three-channel circuit makes it possible to measure two freons simultaneously, using the third channel as a reference. In addition, the main and reference ODs are located side by side, in the same block, which excludes the temperature drift of one receiver relative to the other.
The use of the method of modulating the pressure of the analysed GM in the working chamber achieves almost complete absence of the influence of external influencing factors on the zero readings of the gas analyser, and the use of an emitter with a maximum radiant luminosity density in the range from 8 to 10 μm allows achieving the required selectivity of the gas analyser (elimination or significant reduction of the influence of unmeasured components).
For comparison, table 3 shows the parameters of gas analysers of various types. The data indicates that the analysed gas analyser provides the required parameters, ahead of other devices on the industrial market.
Thus, due to the selected technical solutions, the developed optical absorption gas analyser [24] differs from the existing analogues in high selectivity, stability of readings, low signal-to-noise ratio, the ability to measure low concentrations of two freons simultaneously (less than 1 ppm), does not require service maintenance during a year.
Yu. N. Snytko
SPA “Analitpribor”, Smolensk
A comparative analysis of gas analysers that implement different physical methods for controlling the content of freons in the air is presented. A functional diagram of an optical absorption gas analyser is proposed. A method for modulating the pressure of the analysed gaseous medium in the working chamber has been developed and investigated. An emitter with a maximum luminosity density in the wavelength range from 8 to 10 μm is investigated. The design of the system of interference filters has been developed. The test results are presented.
Key words: gas analyser, freon, pressure modulation, flow booster, wave length, interference filter
Received: 11.01.2021
Accepted: 16.03.2021
In industrial refrigeration and air conditioning systems, freons are used as a refrigerant or in fire extinguishing systems. The most critical facilities that require continuous monitoring of the concentration of freons in sealed rooms are sea vessels (including icebreakers) and nuclear power plants.
Freons are air-substituting gases, therefore, accumulating in sealed rooms, they pose a high danger to the personnel working there [1]. It is clear that the control of the content of freons in such premises is of great importance [2, 3].
Industrial freons, their physical characteristics, as well as the values of maximum permissible concentrations (MPC) for working and residential areas are presented in table. 1. (based on materials [4–7]). The analysis of the MPC levels makes it possible to formulate the initial requirements for the range and measurement error of the freon gas analyser:
measurement range (NLT):
from 0 to 4000 mg / m3;
measurement error (NMT): 25% at points of 100 and 3000 mg / m3.
Due to the need for continuous monitoring, gas analysers must be stationary. The response speed should be no more than 5 minutes to ensure that the personnel can leave the gas-polluted room without harm to health. To be used in sealed rooms on sea vessels, the gas analyser must be resistant to vibration, rolling, tilt and have a service life without service and maintenance for at least 1 year.
To measure the concentration of freon in the air, optical absorption, electrochemical and semiconductor gas analysers are used. From the works [8, 9], it can be concluded that the required metrological characteristics can be provided only by an optical absorption (investigated) gas analyser [10, 11].
The principle of operation of an optical absorption gas analyser is based on the absorption of infrared radiation (2 to 20 µm) by the molecules of the substance being measured at the characteristic wavelength. Such wavelengths appear in the spectrum when certain chemical groups are present in the compound and are called characteristic [12]. These include, for example, stretching vibrations of C–H, C=O, C=C, O–H bonds, bending vibrations of C–H, N–O, vibrations of groups – NO2, – COO–, CONH2. Freons have a characteristic C–F bond in the molecule, and according to [13] powerful and sharp absorption bands of halogen derivatives are in the range of 7.15 to 10 µm (Fig. 1). It is necessary to take into account the presence of water absorption lines in the range of 5 to 8 µm, which makes it impossible to measure in this spectral range without preliminary preparation (drying) of the sample.
For clarity, let us single out a portion of the spectrum from 8 to 10 μm (Fig. 2), in which freons can be monitored without the influence of air humidity. It is obvious that the gas analyser will have cross-sensitivity to other, not measured freons, due to the superposition of their spectral characteristics. Since during installation on objects the types of controlled freons are known in advance, cross-sensitivity to other freons will not affect the measurements of their essentially absence at the object.
The design of a gas analyser operating in the wavelength range from 8 to 10 μm faces the following technical difficulties:
- the need to use optical materials with a high transmittance (more than 95%) in the wavelength range from 8 to 10 microns;
- the need to use interference filters [14] in the wavelength range from 8 to 10 microns requires a certain approach in terms of the materials used;
To achieve the required error in the entire measurement range, it was necessary to create a model of an optical-absorption gas analyser with high sensitivity and the ability to measure the target substance in a gas sample with the presence of unmeasurable components (atmospheric moisture and CO2) without preliminary sample preparation in order to avoid sample distortion.
This task is quite difficult, because it is required to measure the target component under conditions of changes in the level of atmospheric humidity, more than three orders of magnitude higher than the permissible measurement error of the gas analyser.
To achieve the required metrological characteristics of the gas analyser [15], the pneumatic modulation method [16] was applied. The functional diagram of a gas analyser operating on the principle of pneumatic modulation is shown in Fig. 3.
The gas analyser operates as follows. The gas mixture (hereinafter referred to as GM) is pumped through the inlet filter (hereinafter referred to as F) using the flow booster 1 (hereinafter referred to as FB1), which creates the pressure of the GM in reservoir 1 (hereinafter referred to as R1). With the help of the flow booster 2 (hereinafter referred to as FB2), a vacuum of the analysed GM is created in reservoir 2 (hereinafter referred to as R2). The solenoid valve (hereinafter referred to as SV) 1 opens when SV2, and pressurized GM from the reservoir R1 enters the working chamber. Overpressure builds up in the working chamber P+ = PATM + ΔP.
Then SV1 closes and SV3 opens, the pressure in the working chamber drops to atmospheric pressure. SV3 closes and SV4 opens. GM from the working chamber is fed to the reservoir R2, where the flow rate pump FB2 creates a vacuum P– = PATM – ΔP.
Then SV4 closes, after which SV2 opens. The pressure in the working chamber rises to atmospheric pressure. The stability of pressure maintenance is ensured by measuring the current pressure (excess, atmospheric and vacuum) in the working chamber using a pressure sensor (hereinafter referred to as PS) and is stabilized at the required level by controlling the operation of FB1 and FB2. The algorithm of operation of the solenoid valves is explained by the cyclogram, shown in Fig. 4. The sequence diagram of the operation of the solenoid valves is repeated with a period of 2.6 s, providing modulation of the pressure of the analysed GM in the working chamber (Fig. 4, sequence diagram 2). This algorithm of the gas analyser has been patented in the Russian Federation [17].
In this operating mode, there is practically no influence of external influencing factors on the zero readings of the gas analyser, which makes it possible to achieve high metrological characteristics of the gas analyser, stability and the absence of the need for maintenance for one year.
An emitter is installed on one side of the working chamber. On the other side of the camera, there are three optical radiation detectors (PD1, PD2, PD3, see Fig. 3), with optical filters installed in front of them, which transmit radiation along the absorption lines of the measured components in Fig. 3. Each detector picks up only the radiation that the corresponding interference filter passes through. Signals from optical detectors (hereinafter referred to as OD) are amplified by three amplifying channels, consisting of amplifiers A1-A3 (Fig. 3). A temperature sensor (TS) is designed for temperature compensation. The modulation frequency is determined by the frequency dependence of the OD sensitivity, for which the pyroelectric detectors PP‑82 are used (Fig. 9).
Pressure control is carried out using a voltage processor Uр with PS, setting the required performance FB1 and FB2.
The absorption of the flux of IR radiation occurs in the presence of the determined components in the GM, through one or two measuring channels, depending on the presence of the target substances. The third channel is a reference channel to compensate for external factors and undetectable components.
Enlightened germanium was used as the elements of the gas path [18]. The transmittance in the wavelength range from 8 to 10 microns is at least 96%. This technical solution allows increasing the sensitivity of the gas analyser and at the same time increasing the selectivity.
Currently, the most progressive and high-tech method of applying optical coatings is the method of ion-beam evaporation. The indisputable advantages of this method are high adhesion of coatings, ultra-low losses of optical radiation for scattering and absorption, precision and stability of optical parameters. In addition, during electron beam evaporation, a flux of both neutral particles and ions of the evaporated substance enters the substrate. Thus, the ion-assisted method is indirectly implemented, in which, simultaneously with the flow of atoms, a flow of ions with an energy higher than the thermal one is supplied to the substrate. Additional ion bombardment of the surface increases the adhesion of the films to the substrate and to each other and increases the density of the films.
The technology of manufacturing interference filters operating in the wavelength range from 8 to 10 µm requires the deposition of films on the substrate 10–20 times thicker than for the visible range of the spectrum [19]. As a result, the total thickness of the films of the narrow-band interference filter at a wavelength of 9.45 µm will be 10.4 µm. Therefore, the voltage and absorption in films, which could be neglected in the visible range, increase in the range from 8 to 10 μm and turn into a limiting factor in the design of structures [20]. Furthermore, interference filter substrates have high refractive indices (for the range from 8 to 10 μm, germanium with a refractive index of n = 4.0 for λ = 10 μm is used), and high-index film-forming materials are required to create effective interference structures. Therefore, materials for the visible range of the spectrum cannot be used as basic materials for the formation of optical coatings in the range from 8 to 10 μm. The main film-forming materials for the range from 8 to 10 μm are fluorides, chalcogenides, and semiconductors [21]. These materials, as a rule, have significantly worse mechanical and climatic resistance parameters than oxides and MgF2, and also poorly tolerate ion assistance. To improve the performance of coatings, in many cases it is necessary to introduce additional functional layers and interlayers [22] and to seek a compromise between strength, efficiency, and radiation resistance. The optimal pair of film-forming materials in terms of technical and economic characteristics is MgF2 and Ge. An example of a combination of interference filters fabricated on a germanium substrate by deposition of MgF2 and Ge films is shown in Fig. 6.
Analysing the relative spectral transmittance of the interference filters (see Fig. 6), one can see that it is required to suppress the parasitic region of the spectrum in the range of 3 to 7 µm. This problem is solved by installing a cut-off filter (the function of its relative spectral transmittance is shown in Fig. 7) and using an emitter having a maximum radiation intensity in the 8 to 10 µm region. The required range is provided by the application a single crystal of leucosapphire heated to a temperature of 290 °C as an emitting element.
The graphs for calculating the spectral density of the radiant luminosity of blackbody radiation (BB) and leucosapphire at a temperature of 290 °C are shown in Fig. 8. Obviously, the maximum luminosity density of the leucosapphire emitter, in comparison with the blackbody, has shifted to the long-wavelength region and is in the range of 8 to 10 µm, which achieves a high selectivity of Freon measurements. A patent for an invention [23] protects this technical solution.
Fig. 8 shows black curve – theoretical value for blackbody, green curve – Al2O3, (calculated value), red curve – Al2O3 (experimental value measured with a spectrophotometer).
Let us estimate the spectral density of the radiant luminosity of the filtered radiation flux incident on the OD when a leucosapphire rod heated to a temperature of 290 °C is used as a radiation source (Fig. 9). It is obvious that the use of narrow-band interference filters together with a cut-off filter and an emitter having a maximum radiant luminosity in the range of 8 to 10 μm makes it possible to achieve the selectivity of the gas analyser (no influence of unmeasured components).
The determination of the zero line drift was carried out in normal climatic conditions for 48 hours. In fig. 10 is a graph of the drifting baseline of the gas analyser. The zero line drift in 48 hours was no more than ±2.5 mg / m3 (±0.23 ppm) for the measurement channel of Freon 114B2 (green line) and ±1.5 mg / m3 (±0.42 ppm) for the measurement channel freon 22 (blue line).
The main metrological characteristics obtained when testing a sample of an optical absorption analyser of freons are given in table. 2.
CONCLUSIONS
Due to the use of interference filters, the region of the radiation spectrum is allocated, which is necessary for the selective measurement of freons. The use of a three-channel circuit makes it possible to measure two freons simultaneously, using the third channel as a reference. In addition, the main and reference ODs are located side by side, in the same block, which excludes the temperature drift of one receiver relative to the other.
The use of the method of modulating the pressure of the analysed GM in the working chamber achieves almost complete absence of the influence of external influencing factors on the zero readings of the gas analyser, and the use of an emitter with a maximum radiant luminosity density in the range from 8 to 10 μm allows achieving the required selectivity of the gas analyser (elimination or significant reduction of the influence of unmeasured components).
For comparison, table 3 shows the parameters of gas analysers of various types. The data indicates that the analysed gas analyser provides the required parameters, ahead of other devices on the industrial market.
Thus, due to the selected technical solutions, the developed optical absorption gas analyser [24] differs from the existing analogues in high selectivity, stability of readings, low signal-to-noise ratio, the ability to measure low concentrations of two freons simultaneously (less than 1 ppm), does not require service maintenance during a year.
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