Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
  • G01N29/22—Details, e.g. general constructional or apparatus details
  • G01N29/24—Probes
  • G01N29/2418—Probes using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics
  • G01N29/2425—Probes using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics optoacoustic fluid cells therefor
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/0004—Gaseous mixtures, e.g. polluted air
  • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
  • G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
  • G01N33/0036—General constructional details of gas analysers, e.g. portable test equipment concerning the detector specially adapted to detect a particular component
  • G01N33/0047—Organic compounds
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
  • G01N2021/1704—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in gases
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N2021/3125—Measuring the absorption by excited molecules
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
  • G01N2021/396—Type of laser source
  • G01N2021/399—Diode laser
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/01—Arrangements or apparatus for facilitating the optical investigation
  • G01N21/03—Cuvette constructions
  • G01N21/05—Flow-through cuvettes
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2201/00—Features of devices classified in G01N21/00
  • G01N2201/02—Mechanical
  • G01N2201/021—Special mounting in general
  • G01N2201/0216—Vehicle borne
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2201/00—Features of devices classified in G01N21/00
  • G01N2201/06—Illumination; Optics
  • G01N2201/069—Supply of sources
  • G01N2201/0691—Modulated (not pulsed supply)

Definitions

• the present invention relates to a method and a device for detecting multiple gas traces.
• Gas analysis is one of the key technologies for the environmental and military markets and the medical and scientific fields. Among all the techniques used, the principle of optical analysis remains today limited to specific applications and niche. The main reasons are related to the complexity of its implementation, the cost of equipment and the limitation of the equipment to the analysis of a given gas.
• the photo-acoustic spectroscopy allows to solve the aspects "complexity" of the instrument and to reach competitive levels of cost with the conventional technologies.
• the advantages of photo-acoustic analysis are numerous: measurement selectivity, sensitivity, measurement accuracy and measurement range covering all gases using a wavelength suitable for excitation. laser optics.
• the curve 50 of absorption of light by a gas determined as a function of the wavelength of the light such as, for example, methane (of chemical formula CH 4 ) presents maximums for certain wavelengths ⁇ 1, ⁇ 2, A3.
• the energy absorption by a particular gas over a wavelength spectrum includes narrow bands of higher absorption spaced apart by bands of lower absorption.
• Each gas has a unique absorption spectrum that allows it to be detected and / or measured in a sample.
• the principle of photoacoustic measurement consists in the fact that the gas studied, contained in a tank, absorbs a part of the energy of the light passing through the tank. Each molecule thus increases its mechanical energy, which is manifested by an increase in temperature and pressure.
• a pressure variation 41 represented on the ordinate, detected by the signal supplied by an acousto-electric transducer, generally a microphone, varies according to the wavelength of the light passing through the tank, represented on the abscissa.
• the gas sampled is circulated in an open tank to the outside.
• the response curve of a device of the prior art non-resonant tank has the curve 42 illustrated in Figure 2B.
• One of the aims of the present invention is to propose a system that can be used both in a closed vessel and in an open vessel and that makes it possible to obtain a high detection sensitivity, and that is easily adaptable to any gas.
• the present invention aims to remedy these disadvantages.
• the present invention is directed to a photoacoustic measuring device for the quantity of at least one gas, this device comprising:
• a Helmholtz-type resonant tank consisting of at least two tubes closed at their ends and connected together, near each of their ends, by capillary tubes of diameter less than the diameter of the parallel tubes and
• This device comprises, in addition:
• each said source of radiant energy being positioned opposite a window closing a tube end
• At least one acoustoelectric transducer disposed on one of the tubes for detecting the acoustic signals produced in this tube and supplying an electrical signal representative of the concentration of the gas in the tank.
• a single tank is sufficient to have several detections and / or several gas concentration measurements each implementing one of the laser sources.
• the size and the cost of the instrument are therefore only partially increased.
• the sources of radiant laser energy at characteristic wavelengths of different gases which makes it possible to switch rapidly from the detection of traces of a gas to the detection of traces of a other gas, while using a very small volume.
• the present invention thus makes it possible to solve the problem of compactness, multiplicity of the analyzed gases and final cost of the instrument.
• the sensitivity of gas detection / measurement is improved, especially for very low concentrations, while using a simple device easily adaptable to the detection of any type of gas. gas.
• the device of the present invention mounted in a vehicle while having a high sensitivity. It is thus possible to finely measure the quality of the air over a large area, for example in the main arteries of a city.
• the device that is the subject of the present invention comprises at least two radiant laser energy sources positioned facing different windows.
• the device that is the subject of the present invention comprises at least two radiant laser energy sources positioned facing one and the same window.
• the device that is the subject of the present invention comprises at least two radiant laser energy sources whose emission wavelength corresponds to a maximum absorption wavelength for two different gases.
• the device that is the subject of the present invention comprises at least two radiant laser energy sources whose emission wavelength corresponds to two maximum absorption wavelengths for the same gas.
• the device that is the subject of the present invention comprises at least one source of laser radiant energy of the quantum cascade type.
• the device that is the subject of the present invention comprises at least three tubes forming two resonant tanks having a tube in common connected by capillary tubes to the other two tubes.
• More than two tubes forming at least two tanks of the Helmhoitz type having a tube in common can significantly reduce the size and increase the number of lasers can be integrated.
• the modulation means successively modulates the excitation energy supplied by each of the laser energy sources.
• the modulation means simultaneously modulates the excitation energy provided by at least two laser energy sources.
• the modulation means applies a phase shift of 180 ° between the excitation energies of the laser energy sources which are opposite successive windows of the device.
• said at least two laser energy sources have emission wavelengths corresponding to absorption peaks of the same gas.
• the present invention aims at a method for photoacoustic measurement of the quantity of at least one gas by implementing a resonant tank of Helmhoitz type consisting of at least two tubes closed at their ends and connected to each other, near each of their ends, by capillary tubes of diameter less than the diameter of the parallel tubes and a means for introducing the gas into said tank.
• a resonant tank of Helmhoitz type consisting of at least two tubes closed at their ends and connected to each other, near each of their ends, by capillary tubes of diameter less than the diameter of the parallel tubes and a means for introducing the gas into said tank.
• This method comprises, for each of at least two sources of radiant energy, simultaneously:
• said radiant energy source a step of modulating the excitation energy supplied by said radiant laser energy source, with a modulation frequency in correspondence with the acoustic resonance frequency of the resonant tank, said radiant energy source providing an energy of gas excitation contained in the tank, the emission wavelength of said source corresponding to a maximum absorption wavelength locally for a said gas, said source of radiant energy being positioned opposite to a window closing a tube end,
• the excitation energy supplied by at least two laser energy sources is simultaneously modulated.
• a phase shift of 180 ° is applied between the energies of excitation of the laser energy sources which are opposite successive windows of tubes.
• FIG. 1 represents the absorption spectra of light by a gas, as a function of the different wavelengths of light
• FIG. 2A represents the response of a non-resonant tank closed on the outside
• FIG. 2B represents the response of a non-resonant tank open to the outside
• FIG. 3 schematically represents a particular embodiment of the device that is the subject of the present invention
• FIG. 4 represents, in perspective, a Helmholtz-type resonator tank used in the device illustrated in FIG. 3;
• FIG. 5A represents the response of the resonant tank illustrated in FIG. 4, when it is closed on the outside,
• FIG. 5B represents the response of the resonant tank illustrated in FIG. 4, when it is open on the outside,
• FIG. 6 represents, schematically and in plan view, a particular embodiment of the device that is the subject of the present invention.
• FIG. 7 schematically and in plan view, a particular embodiment of the device that is the subject of the present invention.
• FIG. 8 is a diagrammatic and top and side view of the details of the device illustrated in FIG. 7,
• FIG. 9 represents, schematically and in plan view, a particular embodiment of the device that is the subject of the present invention.
• FIG. 10 represents a detection curve of methane and nitrous oxide in the atmosphere in the presence of water vapor
• FIG. 11 represents a signal obtained for various known gas concentrations
• FIG. 12 represents two curves of absorption of methane in air flow
• FIG. 13 represents two nitrous oxide absorption curves
• FIG. 14 represents a calculated absorption spectrum of the ambient air containing 100 ppm of nitric oxide around 5.4 microns
• FIG. 15 represents a spectrum obtained experimentally under the conditions of FIG. 14 and
• FIG. 16 represents, in the form of a logic diagram, the steps implemented in a particular embodiment of the method that is the subject of the present invention.
• the device that is the subject of the present invention comprises two laser sources 11A and 11B, for example with a diode, emitting two laser beams 13A and 13B, each having a length waveform corresponding to an absorption peak of a desired gas.
• at least one light source is used in the laser medium laser known as the "Quantum Cascade Laser".
• Quantum Cascade Laser (“QCL”) technology offers a range of lasers in the Infra-Red Medium that makes the characteristic wavelengths of a very large set of complex molecules accessible.
• Each laser beam, 13A and 13B is modulated by an electronic or mechanical modulator 12A and 12B, respectively, to be modulated in frequency at a determined frequency, for example 210 Hz, corresponding to the acoustic resonance frequency of the reactor vessel.
• Helmholtz Each laser beam 13A and 13B reaches a resonant tank 14, of the Helmholtz type, constituted, as represented in FIG. 4, by two parallel tubes, 50 and 51, closed at their ends by windows 52. These windows 52 allow the passage of each beam. laser, which thus enters the volume of a tube 50 disposed in its path.
• the two parallel tubes 50 and 51 are connected together near each of their ends by capillary tubes 53 and 54, of diameter d smaller than the diameter D of the parallel tubes 50 and 51.
• a resonant tank is produced whose acoustic resonance frequency is 210 Hz.
• parallel tubes 50 and 51 are arranged, in a central zone, with acousto-electric transducers, for example electret microphones, 20 and 21. These microphones have a flat response curve in the 100 Hz to 20 KHz range. Note that it is also possible to use condenser microphones or MEMS ("MicroElectroMechanical System" for micro-electromechanical system).
• the type of transducer used is, for example, supplied by the firm “Knowles” (registered trademark), under the reference “K 1024” or by one of the firms “Sennheiser” (registered trademark) or “Bruel &Kjaer” (registered trademark).
• the first capillary 53 is provided with an inlet tube 15.
• the second capillary 54 is provided with an outlet tube 16.
• a valve, respectively 55 and 56, is mounted to close the inlet tube 15, and the outlet tube 16. When the inlet and outlet tubes 16 are closed, the valves 55 and 56 allow the circulation gas through the capillaries from one tube to the other.
• the outlet tube of the valve 56 is connected to the inlet of a suction pump 70 so as to allow sufficient circulation of the gases to ensure a measurement in real time.
• the output signal of the microphone 20 disposed on the tube 50 receiving the laser beam 13A is sent to the positive input of a differential amplifier 18.
• the output signal of the second microphone 21, disposed on the parallel tube 51 which is not not placed in the beam of the laser beam 13A, is sent to the negative input of the differential amplifier 18.
• the output of this amplifier 18 delivers the electrical signals representative of the amount of gas detected to a central unit 19 provided with a display screen.
• the device also comprises an electronic assembly 17 which controls the modulators 12A and 12B, so that only one of the laser beams 13A and 13B is modulated during each measurement time interval.
• the modulators 12A and 12B are integrated in the sources 1 1 A and 1 1 B, respectively.
• the modulation occurs electronically by modulating the excitation current of the laser diode.
• the modulators 12A and 12B are mechanical and placed in the optical path of the laser beams leaving the sources 1 1 A and 1 1 B, respectively.
• R the response of the vessel
• Q the quality factor Q
• W the power of the laser
• L the distance traveled by the light beam in the gas.
• the quality factor Q is increased by choosing an acoustic resonance among the longitudinal, azimuthal, radial or Helmholtz type acoustic resonances.
• the laser for example with a diode, is preferably chosen with a wavelength of 1.65 micron or 7.9 micron (in particular with a QCL laser).
• the modulation frequency is chosen so that it is located at the maximum amplitude response of the resonant tank, this maximum corresponding to a response in phase opposition of the signals delivered by the second microphone 21 with respect to the signals delivered by the first microphone. 20.
• the maximum amplitude response is at the acoustic resonance frequency of the vessel.
• the signals delivered by the second microphone 21 are in phase opposition with respect to the signals delivered by the first microphone 20.
• a resonant tank 14 of very small dimensions about a square of 10 cm side, with tubes having a diameter ratio of 1 to 10 and a volume of the capillaries relative to the volume of the tubes having a volume ratio of 1 to 100, a high sensitivity of detection is obtained.
• the device thus makes it possible to detect the presence of methane with a concentration of the order of one part per million (or "ppm"), to 1.65 microns with a conventional laser diode and of the order of one part per trillion ( or "ppb”) with a quantum cascade laser.
• the photoacoustic measurement device for the presence of a gas comprises:
• a resonator tank 14 of the Helmholtz type consisting of at least two tubes 50 and 51 closed at their ends and interconnected, near each of their ends, by capillary tubes 53 and 54 of diameter d smaller than the diameter D of parallel tubes and
• At least one acoustoelectric transducer 20, 21 disposed on one of the tubes for detecting the acoustic signals produced in this tube and supplying, at the output of the differential amplifier 18, an electrical signal representative of the concentration of the gas in the tank 14.
• the device is mounted on a vehicle, the inlet tube 15 communicating with the outside of the vehicle and sucking air to perform detections of gas to be detected.
• the photoacoustic gas analysis device is adapted to simultaneously detect / measure a plurality of gases.
• the symmetry of a cell 114 is used to position at least four lasers 15, 16, 17 and 18 of different wavelengths corresponding to:
• the flexibility in coupling positioning of a laser in a photoacoustic tank 214 is implemented by assembling a plurality of lasers 215, here eight, next to FIG. least one window, here the four windows of ends of tubes, which multiplies the number of gases analyzed. As illustrated in FIG. 8, the assembly of the lasers is then made according to their geometry and that of the window, along the horizontal and the vertical (two stacks of four lasers, each in FIGS. 7 and 8).
• At least two tanks 314A and 314B having a tube in common are used, which makes it possible to reduce the space requirement substantially while increasing the number of lasers. and therefore gas analyzed.
• the various embodiments described above may be combined to form a device for measuring the amount of at least one gas comprising multiple laser sources.
• the present invention applies, in particular to scientific or industrial instrumentation concerning the following fields:
• the present invention allows the control of emissions affecting the environment (air, crops, infrastructure ).
• the present invention enables the detection of toxic agents, explosives and other illicit substances.
• the present invention relates to the detection of precursor agents of diseases (Cancer, Asthma, Glucose).
• multi-gas methane, NH 3 , ethylene, H 2 S, N 2 0 ...
• an improved detectivity is implemented by using high-performance microphones, up to 3.3 10 “10 W.cm " .
• IBSG laser registered trademark
• a liquid nitrogen cryostat is used.
• nitrous oxide N 2 0
• applications of the present invention are described for the detection of nitric oxide (NO), in particular for the fields of the environment (atmospheric chemistry, measurement of pollution, etc.), safety (the nitric oxide is a gas emitted by trinitrotoluene or TNT type explosives), medicine (nitric oxide is a marker of inflammations such as asthma).
• the fundamental band (1 -0) around 1900 cm -1 (corresponding to 5.3 ⁇ of wavelength)
• the harmonic band (2-0) around 3800 cm -1 (that is 2.6 ⁇ of wavelength).
• the inventors obtained a detection of nitric oxide with a quantum cascade laser QCL emitting at 5.4 ⁇ , operating with liquid nitrogen and having a power of 2.6 mW: 20 ppb. With the same type of laser with higher power output operating at ambient temperature: 1 ppb.
• the laser implemented operates at room temperature.
• methane and nitrous oxide can be detected in the air in the presence of water vapor, by choosing specific peaks 505 and 510, respectively.
• FIG. 11 shows the signals 515, 520 and 525 recovered at the output of an acoustoelectric transducer 20 or 21 for various known gas concentrations (103.5 ppm, 21.7 ppm and 10.1 ppm, respectively). The average amplitude of these signals makes it possible to verify the linearity between these signals and these concentrations.
• Figure 12 shows the adjustment of the 530 absorption of methane in airflow.
• the inversion 535 of this spectrum 530 recorded at 7.9 microns makes it possible to recalculate the 1.85 ppm of methane in the ambient air.
• Figure 13 shows the adjustment of nitrous oxide 540 absorption in airflow.
• the inversion 545 of this spectrum 540 recorded at 7.9 microns makes it possible to recalculate the 320 ppb of nitrous oxide in the ambient air.
• Figure 14 shows the calculated 550 spectrum of ambient air absorption containing 100 ppm nitric oxide to 5.4 microns.
• FIG. 15 represents the spectrum 555 obtained experimentally with the device and the method which are the subject of the present invention, under the conditions of FIG. 14.
• the method comprises, firstly, a step 405 for selecting at least one gas for which traces are sought.
• the gases to be detected are treated.
• the gases to be detected starts with the first gas selected in step 405.
• the gas to be treated is called, in the following description of Figure 16, "current gas".
• step 410 it is determined whether at least two sources of radiant energy of the device correspond to two characteristic absorption peaks of the gas. If yes, select the operating mode from several sources. Otherwise, the operating mode is selected at a single source.
• steps 415 to 440 are carried out. If the mono-source operating mode is selected, go directly to step 430.
• each of the sources of radiant energy corresponding to the current gas is determined.
• the respective positions of the radiating energy sources that is to say the ranks of the tubes, for example 50 and 51 in FIG. 1, in relation to which these sources are located, are determined.
• the phase differences to be applied to the different sources are determined.
• the sources lying next to tubes of the same rank have no phase difference between them.
• the sources lying opposite odd-order tubes have a phase shift of 180 ° compared to the sources lying next to tubes of even rank.
• This phase shift is to be applied by the modulation means which modulates the excitation energy supplied by each of the laser energy sources with a modulation frequency in correspondence with the acoustic resonance frequency of the resonant tank.
• the modulation is applied with, in the case of several sources, the phase differences determined during step 425, at each selected source.
• the excitation energy supplied by each selected laser radiant energy source is thus modulated, with a modulation frequency corresponding to the acoustic resonance frequency of the resonant tank, each source of laser radiant energy supplying excitation energy to the gas contained in the tank opposite which source is located, the emission wavelength of the source corresponding to a maximum absorption wavelength locally for the gas current.
• these laser sources are simultaneously selected and modulated simultaneously, possibly with different phases.
• the sound signals present in the different tubes are differentially captured and amplified.
• a step 440 as a function of this differential signal, it is determined whether the current gas is present in the tubes of the photoacoustic device and the quantity of this gas is estimated.
• a signal is processed from at least one acoustoelectric transducer disposed on one of the tubes to detect the acoustic signals produced in this tube and to provide an electrical signal representative of the concentration of the gas in the tank.
• the sources of radiant laser energy at characteristic wavelengths of different gases which makes it possible to switch rapidly from the detection of traces of a gas to the detection of traces of a other gas, while using a very small volume.

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