EP2596331A1

EP2596331A1 - Method and device for detecting trace amounts of many gases

Info

Publication number: EP2596331A1
Application number: EP11752269.8A
Authority: EP
Inventors: Regis Hamelin; Virginie Zeninari; Bertrand Parvitte; Lilian Joly; Georges Durry; Ronan Le Loarer; Jean Charles Garcia
Original assignee: Universite de Reims Champagne Ardenne URCA; AEROVIA
Current assignee: Universite de Reims Champagne Ardenne URCA; AEROVIA
Priority date: 2010-07-21
Filing date: 2011-07-21
Publication date: 2013-05-29
Also published as: FR2963102B1; WO2012010806A1; FR2963102A1; US20130205871A1

Abstract

The photoacoustic device for measuring the amount of at least one gas comprises: a Helmholtz resonant cell (14, 114, 214, 314A, 314B), consisting of at least two tubes closed at their ends and connected together, close to each of their ends, by capillary tubes having a diameter smaller than the diameter of the parallel tubes, and a means (15, 55) for introducing a gas into said cell. This device further includes: at least two physically separate radiant laser energy sources (11A, 11B, 215) each designed to deliver excitation energy to the gas contained in the cell, a different emission wavelength corresponding to a local maximum absorption wavelength for said gas, each said radiant energy source being positioned facing a window (52) closing off one end of the tube; a modulation means (17), which modulates the excitation energy delivered by each of the laser energy sources with a modulation frequency corresponding to the acoustic resonant frequency of the resonant cell; and at least one acoustic-electric transducer (20, 21) placed on one of the tubes so as to detect the acoustic signals produced in this tube and deliver an electrical signal representative of the concentration of the gas in the cell.

Description

METHOD AND DEVICE FOR DETECTING MULTIPLE GAS TRACES

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.

Among the optical techniques, the photo-acoustic spectroscopy allows to solve the aspects "complexity" of the instrument and to reach competitive levels of cost with the conventional technologies. In addition, 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.

It is known, as represented in FIG. 1, that 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 λ1, λ2, A3. Generally, 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.

As illustrated in FIG. 2A, in a non-resonant closed tank, 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.

When it is desired to detect or measure the concentration of a gas in various locations and in real time, the gas sampled is circulated in an open tank to the outside. In this case, the response curve of a device of the prior art non-resonant tank has the curve 42 illustrated in Figure 2B. We observe in this Figure that it is difficult to extract the signal that corresponds to the presence of the gas in question, in the noise. 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.

However, in many applications it is desirable to analyze several gases in the same sample. But the multiplication of single-gas analysis instruments multiplies the size and the final cost. In addition, it is desirable to increase the accuracy and reliability of the detection of each gas, even for the detection of a single gas.

The article "Design and characteristics of a differential helmholtz resonant photoacoustic cell for infrared gas detection" Infrared Physics & Technology Elsevier, Netherlands and the international application WO 03/083455, which describe photoacoustic devices are known. However, these devices have a limited sensitivity and can detect only one type of gas.

The present invention aims to remedy these disadvantages.

For this purpose, according to a first aspect, 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

a means for introducing gas into said tank.

This device comprises, in addition:

at least two physically dissociated laser radiant energy sources each adapted to supply excitation energy to the gas contained in the vessel at a different emission wavelength corresponding to an absorption wavelength maximum locally for a said gas, each said source of radiant energy being positioned opposite a window closing a tube end,

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 and

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.

Thanks to these provisions, 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.

According to the modes of operation of this device: at least two sources of laser radiant energy at two characteristic wavelengths of the same gas are simultaneously implemented, which increases the detection sensitivity of this gas,

or one uses, successively, 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.

In addition, one can easily switch from one to the other of these modes of operation by providing radiant laser energy sources that correspond to different absorption peaks of the same gas and sources of radiant laser energy. which correspond to different absorption peaks of different gases. It then suffices to switch between the first and the second to switch from the first operating mode described above to the second.

The present invention thus makes it possible to solve the problem of compactness, multiplicity of the analyzed gases and final cost of the instrument.

Thanks to the implementation of a Helmholtz-type resonator tank, 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. In addition, one can implement 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.

According to particular features, the device that is the subject of the present invention comprises at least two radiant laser energy sources positioned facing different windows.

According to particular features, 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.

According to particular features, 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.

According to particular features, 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.

According to particular features, the device that is the subject of the present invention comprises at least one source of laser radiant energy of the quantum cascade type. According to particular features, 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.

According to particular features, the modulation means successively modulates the excitation energy supplied by each of the laser energy sources.

According to particular features, the modulation means simultaneously modulates the excitation energy provided by at least two laser energy sources.

According to particular features, 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.

According to particular characteristics, said at least two laser energy sources have emission wavelengths corresponding to absorption peaks of the same gas.

According to a second aspect, 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.

This method comprises, for each of at least two sources of radiant energy, simultaneously:

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,

- A step of processing a signal from at least one acousto-electric transducer disposed on one of the tubes for detecting the acoustic signals produced in this tube and provide an electrical signal representative of the concentration of the gas in the tank.

According to particular characteristics, during the modulation step, the excitation energy supplied by at least two laser energy sources is simultaneously modulated. According to particular characteristics, during the modulation step, a phase shift of 180 ° is applied between the energies of excitation of the laser energy sources which are opposite successive windows of tubes.

Since the advantages, aims and particular characteristics of this process are similar to those of the device that is the subject of the present invention, as briefly described above, they are not recalled here.

Other advantages, aims and features of the present invention will emerge from the description which follows, for an explanatory and non-limiting purpose, with reference to the appended drawings, in which:

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.

Figures 1, 2A and 2B have already been described in the preamble of this document.

As illustrated in FIG. 3, in a particular embodiment, 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. Preferably, 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.

Thus, for example, by choosing tubes of 10 cm in length and a ratio of the diameter of the capillaries to the diameter of the tubes equal to 1/10, 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.

Pumping downstream improves laminar flow and avoids pollution by the pump itself (traces of the previous sample).

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.

In an alternative embodiment, 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. In other variants, 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.

In the tank 14, the photoacoustic signal, in the case of low absorptions (a L "1) is given by the following equation:

S _PA = RW a

Where R, the response of the vessel, is proportional to the quality factor Q, W is the power of the laser, has the absorption coefficient of the gas and L is the distance traveled by the light beam in the gas.

Preferably, to improve the photoacoustic signal, the quality factor Q is increased by choosing an acoustic resonance among the longitudinal, azimuthal, radial or Helmholtz type acoustic resonances.

Among the advantages of the Helmholtz type photoacoustic tank, mention may be made of:

high sensitivity making low concentrations detectable,

a small volume, - an efficiency at atmospheric pressure,

- a great dynamic of measurement: 5 to 6 decades,

- a low time constant of measurement and

- a great robustness and a limited cost.

An example of an application will now be made explicit for the detection of methane.

To detect this gas, 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. For this frequency value, the signals delivered by the second microphone 21 are in phase opposition with respect to the signals delivered by the first microphone 20. These signals are therefore added to the amplifier 18 and output a signal of higher amplitude as well closed vessel on the outside, as represented by the signal 61 of Figure 5A, that tank open on the outside, as represented by the signal 62 of Figure 5B.

Thus, with 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.

Thus, 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

a means for introducing 55, 56 and 70 of the gas into said tank,

at least two sources 1 1 A and 1 1 B of energy radiating lasers adapted to provide excitation energy to the gas contained in the tank 14, whose emission wavelength corresponds to a wavelength of maximum absorption for said gas, each said source of radiant energy being positioned opposite a window closing a tube end, a modulation means 12A, 12B, 17 which modulates the excitation energy supplied by each of the laser energy sources 1 1 A and 1 1 B with a modulation frequency in correspondence with (preferably equal to) the acoustic resonance frequency of the resonant tank 14 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.

In another embodiment, 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.

Preferably, by the choice of the wavelengths of the different laser sources, the photoacoustic gas analysis device is adapted to simultaneously detect / measure a plurality of gases.

In the embodiment illustrated in FIG. 6, the symmetry of a cell 114 is used to position at least four lasers 15, 16, 17 and 18 of different wavelengths corresponding to:

at different gas absorption peaks, which makes it possible to detect and / or measure the concentration of a plurality of different gases and / or

at different absorption peaks of the same gas, which allows a finer analysis of detection and / or measurement of the concentration of this gas than if a single absorption peak were treated.

In embodiments, such as that illustrated in FIGS. 7 and 8, 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).

In embodiments, such as that illustrated in FIG. 9, 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:

- oil, gas, agro-food, semiconductors ...

- control of industrial processes and the proper functioning of installations In the field of the environment, the present invention allows the control of emissions affecting the environment (air, crops, infrastructure ...).

In the field of defense and security, the present invention enables the detection of toxic agents, explosives and other illicit substances.

In the medical field, the present invention relates to the detection of precursor agents of diseases (Cancer, Asthma, Glucose).

Thanks to the implementation of the present invention, it is possible:

detect 1 molecule per billion, or ppb (acronym for "share per billion"), or even 1 molecule per thousand billion, or ppt (1 ppt = 0.001 ppb),

- detect variations of the same order of magnitude,

with good selectivity, multi-gas: methane, NH ₃ , ethylene, H ₂ S, N ₂ 0 ...

and

- obtain portable or integrated instruments at low or lower cost.

In addition, the operation of the flow system, with a low time constant, allows very simple optical adjustments and avoids multipassages of laser beams that are found in direct spectroscopy.

Preferentially, an improved detectivity is implemented by using high-performance microphones, up to 3.3 10 ^"10 W.cm ^" .

Applications of the present invention are described below for the detection of methane (CH ₄ ), in particular for the mining industry and the analysis of urban gases. For these applications, it is possible to implement the fundamental bands n ₄ and n ₂ at 1400 cm ^-1 (ie a wavelength slightly greater than 7 μηι), the fundamental bands n ₁ and n ₃ at 3000 cm ^-1 (either a wavelength of approximately 3.3 μηι), the harmonic band (n ₄ or n ₂ ) + (ni or n ₃ ) to 4400 cm ^-1 (ie a wavelength of approximately 2.3 μηι) ), the harmonic band 2n ₃ to 6000 cm ^-1 (corresponding to 1.65 μηι).

For the detection of methane with a laser diode, the inventors obtained the following results:

with an IBSG laser (registered trademark) emitting at 1.65 μηι: 300 ppm,

with a Sensor Unlimited laser (registered trademark) emitting at 1.65 μιτι: 1 ppm, with a laser from the University of Montpellier emitting at 2.3 μηι: 50 ppm,

with a laser mounted in Sacher (registered trademark) external cavity emitting at 1.65 μηι: 0.3 ppm and

- with a laser lasers quantum laser lasers (registered trademark) emitting at 7.9 μηι: 17ppb and 3 ppb (with cryostat).

In embodiments, a liquid nitrogen cryostat is used.

It is observed that nitrous oxide (N ₂ 0) can also be detected and quantified. In the following, 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). For these applications, it is possible to implement the fundamental band (1 -0) around 1900 cm ^-1 (corresponding to 5.3 μηι of wavelength), the harmonic band (2-0) around 3800 cm ^-1 ( that is 2.6 μηι of wavelength).

The inventors obtained a detection of nitric oxide with a quantum cascade laser QCL emitting at 5.4 μηι, 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.

In particular to constitute a portable gas analysis instrument, preferably, the laser implemented operates at room temperature.

It is noted that with the practice of the present invention, all infrared absorbing gases are accessible for detection and / or concentration measurement.

It is observed in FIG. 10 that 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.

All these figures demonstrate that the device is suitable regardless of the wavelength and whatever the detectable gas. As illustrated in FIG. 16, in a particular embodiment, the method comprises, firstly, a step 405 for selecting at least one gas for which traces are sought.

Then, successively, the gases to be detected are treated. For example, it 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".

For the current gas, during 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.

If the multi-source operating mode is selected, steps 415 to 440 are carried out. If the mono-source operating mode is selected, go directly to step 430.

During step 415, each of the sources of radiant energy corresponding to the current gas is determined. During a step 420, 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.

During step 425, 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. In addition, 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.

During step 430, the modulation is applied with, in the case of several sources, the phase differences determined during step 425, at each selected source. During step 430, 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. In the case where at least two laser sources provide light at the same wavelength, these laser sources are simultaneously selected and modulated simultaneously, possibly with different phases. During a step 435, the sound signals present in the different tubes are differentially captured and amplified.

During 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. Thus, 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.

Then we go to the next gas and go back to step 410.

As can be understood from reading the description of FIG. 16, according to the modes of operation of this device:

at least two sources of laser radiant energy at two characteristic wavelengths of the same gas are simultaneously implemented, which increases the detection sensitivity of this gas,

In addition, we switch from one to the other of these modes of operation according to the sources of radiant laser energy that correspond to different absorption peaks of the same gas and radiant laser energy sources that correspond at different absorption peaks of different gases. It then suffices to switch between the first and the second to switch from the first operating mode described above to the second.

Claims

1. A photoacoustic measuring device for the quantity of at least one gas, said device comprising:

- A resonant tank (14, 1 14, 214, 314A, 314B) Helmholtz type consisting of at least two tubes closed at their ends and interconnected, near each of their ends, by capillary tubes of smaller diameter to the diameter of parallel tubes and

means (15, 55) for introducing gas into said tank,

characterized in that it further comprises:

at least two physically dissociated laser radiant energy sources (1 1 A, 1 1 B, 215) each adapted to supply excitation energy to the gas contained in the vessel at an emission wavelength different corresponding to a maximum absorption wavelength locally for a said gas, each said source of radiant energy being positioned opposite a window (52) closing a tube end,

modulation means (17) 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 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 an electrical signal representative of the concentration of the gas in the tank.

2. Device according to claim 1, wherein the modulation means (17) is adapted to simultaneously modulate the excitation energy provided by at least two laser energy sources (1 1 A, 1 1 B, 215).

3. Device according to claim 2, wherein the modulation means (17) applies a phase shift of 180 ° between the excitation energies of the laser energy sources which are opposite the successive tube windows of the device.

4. Device according to one of claims 2 or 3, wherein said at least two laser energy sources (1 1 A, 1 1 B, 215) have emission wavelengths corresponding to peaks of absorption of the same gas.

5. Device according to one of claims 1 to 4, which comprises at least two sources of radiant laser energy (1 1 A, 1 1 B) positioned opposite different windows.

6. Device according to one of claims 1 to 5, which comprises at least two radiant laser energy sources (215) positioned opposite a same window.

7. Device according to one of claims 1 to 6, which comprises at least two sources of radiant laser energy (1 1 A, 1 1 B, 215) whose emission wavelength corresponds to a length of maximum absorption wave for two different gases.

8. Device according to one of claims 1 to 7, which comprises at least two sources of radiant laser energy (1 1 A, 1 1 B, 215) whose emission wavelength corresponds to two lengths of maximum absorption wave for the same gas.

9. Device according to one of claims 1 to 8, which comprises at least one source of laser radiant energy (1 1 A, 1 1 B, 215) of quantum cascade type.

10. Device according to any one of claims 1 to 9, which comprises at least three tubes forming two resonant tanks (314A, 314B) having a tube in common connected by capillary tubes to the other two tubes.

1 1. Device according to any one of claims 1 to 10, wherein the modulation means (17) successively modulates the excitation energy supplied by each of the radiant laser energy sources (1 1 A, 1 1 B, 215) .

12. A method for photoacoustic measurement of the quantity of at least one gas by using a resonant tank (14, 1 14, 214, 314A, 314B) of Helmholtz 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 (15, 55) into said tank,

characterized in that it comprises, for each of at least two sources of radiant energy, simultaneously:

a step (410 to 430) for modulating the excitation energy supplied by said source of radiant laser energy (11A, 215), with a modulation frequency in correspondence with the acoustic resonance frequency of the vessel; resonant, said source of laser radiant energy supplying an excitation energy to the gas 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 a window (52) closing a tube end,

a step (435, 440) for processing a signal coming from at least one acoustoelectric transducer arranged on one of the tubes to detect the acoustic signals produced in this tube and to supply an electrical signal representative of the concentration of the gas in the tank.

The method according to claim 12, wherein, during the modulation step, the excitation energy provided by at least two laser energy sources (1 1 A, 1 1 B, 215) is simultaneously modulated. .

14. The method of claim 13, wherein, during the modulation step, a phase shift of 180 ° is applied between the excitation energies of the laser energy sources which are opposite successive tube windows.