Compact Gas Sensors

Abstract

The gas sensor (1) comprises a housing (2) capable of containing a gas (G), the housing comprising a transverse wall (3) extending around a transverse axis (Z) between a first wall (4) and a second wall (5), the sensor further comprising: a light source (10) configured to emit a light wave (11) propagating in the housing so as to form a first light cone from the light source; a measuring photodetector (20) capable of detecting the light wave emitted by the light source (10) and propagating through the housing; the sensor being such that each of the first wall and the second wall comprises at least one reflective surface forming a portion of a spheroid, each reflective surface being associated with a rank n, n being an integer greater than or equal to 1.

Description

Technical Field

[0001] The technical field of the present invention is optical gas sensors, and more particularly, non-dispersive infrared sensors. Background Art

[0002] The use of optical methods to analyze gases is very frequent. Sensors allow the composition of the gas to be determined based on the fact that the species of substances that make up the gas have different absorption spectral characteristics from each other. Thus, if the absorption spectral band of the gaseous substance is known, its concentration can be determined by estimating the absorption of light passing through the gas, using the Beer-Lambert law. This principle allows the concentration of the gaseous substance present in the medium to be estimated.

[0003] The light source is usually a source that emits infrared light, and the method used is usually specified by the English term "NDIR detection", where NDIR is an abbreviation for Non Dispersive Infra-Red. Such a principle has been frequently implemented and is described, for example, in numerous documents (such as US5026992 or WO2007064370).

[0004] According to the most commonly used methods, the gas to be analyzed is between a light source and a photodetector, called a measuring photodetector, intended to measure a light wave emitted by the light source and transmitted through the gas to be analyzed and partially absorbed by it. These methods generally consist of measuring a light wave, called a reference light wave, emitted by the light source and not absorbed by the gas to be analyzed.

[0005] The comparison between the light waves in the presence of gas and the light waves without gas allows characterization of the gas. This involves, for example, determining the amount of gas species in the gas according to a technique known under the term "absorption NDIR". This may also involve estimating the amount of particles in the gas by detecting light scattered by the gas according to a predetermined range of scattering angles.

[0006] The reference light wave is measured by a reference photodetector. This may involve a reference photodetector that is different from the measuring photodetector and is arranged facing the light source, the reference photodetector being associated with a reference filter. The reference filter defines a reference spectral band in which the gas to be analyzed has no significant absorption.

[0007] Documents EP2711687 and EP2891876 describe gas sensors comprising a housing in which one or more mirrors are arranged. The mirrors allow maximizing the path length of the light in the housing and focusing the light passing through the gas on one or more photodetectors. This allows improving the detection sensitivity while using a compact device.

[0008] Document EP3208601 describes a compact gas sensor formed by a reflective wall with an elliptical profile. The same is true for document WO2018162848.

[0009] The applicant has designed a particularly compact gas sensor which is compatible with current industrial formats. This results in a gas sensor with optimized properties, particularly with a preference for compactness and sensitivity. Summary of the invention

[0010] A first subject of the invention is a gas sensor comprising a housing capable of containing a gas, the housing comprising a transverse wall extending around a transverse axis between a first wall and a second wall, the sensor further comprising:

[0011] - a light source configured to emit light waves that propagate in the housing and form a first light cone from the light source,

[0012] - a measuring photodetector capable of detecting light waves emitted by the light source and propagating through the housing;

[0013] The sensor is such that each of the first wall and the second wall comprises at least one reflective surface forming a portion of a spheroid, each reflective surface being associated to a rank n, n being an integer greater than or equal to 1, such that the sensor comprises:

[0014] a first reflective surface of rank 1 having a first focus and a second focus, the first reflective surface being arranged facing the light source;

[0015] - at least one reflective surface of rank n strictly greater than 1, each reflective surface of rank n having:

[0016] a first focus that coincides with the second focus of the previous rank n-1 reflective surface;

[0017] a second focus which is different from the first focus and coincides with the first focus of the reflective surface of the next rank n+1;

[0018] - a last reflective surface of rank N, comprising a first focus coinciding with a second focus of a reflective surface of a preceding rank, the last reflective surface also comprising a second focus;

[0019] The light waves emitted by the light source are successively reflected by the N spheroidal reflective surfaces according to their respective ranks before converging toward the second focus of the last reflective surface to form a last light cone, and the measuring photodetector is arranged to extend into the last light cone.

[0020] The ellipsoidal first reflective surface is associated to a first ellipse. The eccentricity of the first ellipse is preferably smaller than 0.8, or even smaller than 0.7, or even smaller than 0.5.

[0021] The light source is preferably arranged at the first focal point of the first reflective surface or at a distance less than 1 mm from the first focal point of the first reflective surface. The measurement photodetector is preferably arranged at the second focal point of the last reflective surface or at a distance less than 1 mm from the second focal point of the last reflective surface.

[0022] Advantageously:

[0023] - Each reflective surface of rank n is configured to reflect the light wave it receives such that a light cone of rank n + 1 is formed, which light cone extends around the propagation axis of rank n + 1;

[0024] - When n < N, the propagation axis of rank n + 1 extends from the surface of rank n to the surface of rank n + 1 and passes through the second focal point of the surface of rank n, which second focal point coincides with the first focal point of the surface of rank n + 1;

[0025] - When n = N, the propagation axis of rank N + 1 extends from the surface of rank N to the measurement photodetector;

[0026] Such that the light wave emitted by the light source undergoes a number of reflections corresponding to the number of reflective surfaces before reaching the measurement photodetector to form the last light cone.

[0027] When n < N, each light cone of rank n + 1 can converge towards the second focal point of the reflective surface of rank n from which the light cone propagates, and then diverge towards the reflective surface of rank n + 1 towards which the light cone propagates.

[0028] According to one embodiment, the light source and the measurement photodetector are integral with the inner face of the first wall, which inner face is oriented towards the interior of the housing. According to this embodiment, the reflective surfaces of even ranks are provided on the inner face of the first wall. The reflective surfaces of odd ranks are provided on the inner face of the second wall. The housing can then be formed by assembling the second wall face-to-face with the first wall.

[0029] Preferably, the number of reflective surfaces extending between the light source and the measurement photodetector is odd. The number of reflective surfaces extending between the light source and the measurement photodetector can be equal to 5.

[0030] The sensor may include a reference photodetector configured to detect a light wave considered not to be attenuated by the gas. The sensor may then include a reference reflective surface forming a rotational ellipsoid, the reference surface including:

[0031] - A first focal point arranged at the light source or at a distance less than 1 mm from the light source;

[0032] - A second focal point arranged at the measurement photodetector or at a distance less than 1 mm from the measurement photodetector.

[0033] Preferably, the light source, the measuring photodetector and the reference photodetector are coplanar, or substantially coplanar, allowing for a tolerance of less than 1 mm or 2 mm.They may be arranged on the first wall.

[0034] Preferably, each ellipsoidal reflective surface is obtained by rotating an ellipse associated with the surface about its major axis.

[0035] A second subject of the invention is a method for analyzing a gas using a sensor according to the first subject of the invention, the method comprising the following steps:

[0036] a) introducing gas into the housing of the sensor;

[0037] b) irradiating the gas with a light source;

[0038] c) measuring, by means of a measuring photodetector, the light intensity of the light wave emitted by the light source and having propagated through the housing;

[0039] d) Based on the measured light intensity, determining the amount of gaseous species in the gas.

[0040] Step d) is generally carried out by a processing unit based on the signal generated by the measuring photodetector.

[0041] Step d) may take into account the intensity that would be measured by the measuring photodetector in the absence of absorbing substances in the housing. This intensity may be measured initially or may be estimated by using a reference photodetector that measures the intensity of a reference light wave emitted by the light source and whose absorption by the gas is considered negligible. According to one embodiment, the photodetector measures the reference light wave in a reference spectral band in which the light wave emitted by the light source is considered not to be absorbed.

[0042] The invention will be better understood by reading the description of the illustrated embodiment examples which follows in this specification in conjunction with the following drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1A and 1B are cross sections of the gas sensor in the transverse plane XZ and in the main plane XY, respectively, the main plane being perpendicular to the transverse plane.

[0044] Figure 2A The first and second walls of the sensor are schematically shown, and the positions of the main components of the sensor (i.e. the light source, the measuring photodetector, and the reflective surface) are shown. In this figure, the axis D1 corresponds to Figure 1B The circular busbar shown in FIG. 1 is a diagram showing the housing of the sensor unfolded along the busbar.

[0045] Figure 2B The first and second walls of the sensor are schematically shown. In this figure, the axis D2 corresponds to Figure 1B The circular busbar shown in FIG. 1 is a diagram showing the housing of the sensor unfolded along the busbar.

[0046] Figure 3A , 3B , 3C and 3D show the main components of the sensor. Between the different figures, the observation point is Figure 1B Rotate around the sensor in the positive direction of rotation shown in the figure.

[0047] Figure 4A and 4B are modeling results that allow estimating the performance of the sensor with respect to detecting methane and carbon dioxide, respectively. DETAILED DESCRIPTION

[0048] Figure 1A and 1B A gas sensor 1 according to the invention is schematically shown. The gas sensor 1 comprises a housing 2 delimited by a first wall 4, a second wall 5 and a transverse wall 6. The transverse wall 6 extends parallel to a transverse axis Z. The transverse wall 6 is an annular wall around the transverse axis Z. The transverse wall extends between the first wall 4 and the second wall 5. The transverse axis Z extends perpendicularly to the main plane XY. The housing 2 forms a cylindrical shell.

[0049] The first wall 4 comprises an outer face 4 e and inner face 4 i The second wall 5 comprises an outer surface 5 e and inner face 5 i The first wall 4 and the second wall 5 have respective outer surfaces 4 e , 5 e The first and second walls are perpendicular to the transverse axis Z, around which the annular wall 6 extends. The respective inner faces 4 of the first and second walls 4, 5 are parallel to the main plane XY. i , 5 i Oriented towards the interior of the housing 2. The housing 2 is connected to a connecting pin 3 parallel to the transverse axis Z.

[0050] The shape and dimensions of the sensor 1 conform to the common format in the field of gas detection, known as the 4R format. The diameter Φ of the housing 2 is 20 mm and its height h is 16.6 mm. Taking into account the thickness of the transverse wall 6, the internal diameter Φ' of the housing 2 is 17 mm. A positive direction of rotation about the transverse axis Z is defined, which is arbitrarily considered to be similar to a clockwise direction of rotation.

[0051] One advantage of the 4R format is that it is widely used in the field of electronic equipment. Thus, a gas sensor 1 conforming to this format can be easily connected to existing electronic circuits. Conformity to this format facilitates maintenance of the measurement system by facilitating replacement of sensors.

[0052] However, one difficulty associated with this format is its compactness. The useful volume of the housing is small, which requires the optimization method described below.

[0053] The housing 2 is intended to be occupied by a gas G to be analyzed. The gas to be analyzed comprises a substance whose concentration C in the gas is desired to be determined. x The gas substance G x In the absorption band Δλ x In the gas substance G x Absorbs light, especially infrared light. The sensor 1 also includes:

[0054] A light source 10 configured to emit light waves 11 in an emission band Δλ. The emission band may extend between near ultraviolet and mid infrared, for example between 200 nm and 20 μm, and most often in the infrared, the emission band Δλ being for example 1 μm to 20 μm.

[0055] · Measuring photodetector 20, which is configured to absorb the spectral band Δλ x The light wave 14 referred to as the transmitted light wave is detected in the gas. The transmitted light wave is a light wave emitted by the light source 10 and propagates through the housing and gradually attenuates as it propagates through the gas. The measuring photodetector 20 can be connected to a light source 10 that is defined as being included in the absorption spectral band Δλ. x The detection spectral band Δλ in d associated with a low-pass filter.

[0056] Reference photodetector 20 ref , which is configured to detect a reference light wave. The reference photodetector 20 ref The detected reference light wave is considered not to be attenuated by the gas G present in the housing. In this example, the reference photodetector is located in a reference spectral band Δλ in which the attenuation of the light wave emitted by the light source is considered to be negligible. ref Detection reference light wave 20 ref The reference photodetector can be associated with a defined reference spectral band Δλ ref For example, the reference spectral band Δλ ref Centered around a wavelength of 3.91 μm. The use of a reference photodetector is optional, but advantageous. Alternatively, no reference photodetector is arranged in the gas-containing enclosure 2, in which case the reference spectral band may be similar to the measurement spectral band.

[0057] Preferably, the light source 10, the measuring photodetector 20 and the reference photodetector 20ref connected to the same supporting plane. This may in particular concern an electronic board forming the first wall 4 of the housing 2. Such an arrangement simplifies the manufacture of the sensor.

[0058] The light source may be pulsed and emit light pulses of a duration of typically 50 ms to 1 s. This may in particular involve an infrared light source of the thread type formed on the suspension film, through which an electric current passes. The suspension film is thereby heated to a temperature of 400° C. to 800° C. to emit infrared light.

[0059] The measuring photodetector 20 and the reference photodetector 20 ref It can be a thermopile or a thermal detector or a photodiode. Each photodetector is configured to generate a detection signal, the amplitude A of the detection signal corresponds to the intensity I of the detected light wave.

[0060] According to the Lambert-Beer law, the intensity I of the light wave detected by the measuring photodetector 20 depends on the gas species G of interest. x The concentration of C x :

[0061]

[0062] in:

[0063] -μ(C x ) depends on the concentration C studied x The attenuation coefficient of

[0064] -l is the thickness of the gas traversed by the light wave in the shell;

[0065] -I0 corresponds to the intensity of the wave detected or to be detected by the measuring photodetector 20 in the absence of an absorbing gas in the housing.

[0066] The comparison between I and I0 in the form of the ratio I / I0 corresponds to the attenuation att due to the gas species Gx involved. During each pulse of the light source 10, μ(C x ), due to C x and μ(C x ) is known, which allows estimating the quantity C x .

[0067] The intensity I0 can be predetermined, for example, during a calibration process, or measured without gas in the housing 20. ref When the intensity I0 is based on the reference intensity I measured by the reference photodetector ref To this end, since the emission spectrum of the light source 10 is known, the expression (2) is used to allow the refEstimate the function f of I0:

[0068] I0=F(I ref ) (2)

[0069] The sensor comprises a measuring photodetector 20 and an optional reference photodetector 20 which receives ref Processing unit 30 of the measured signals. The processing unit 30 allows to carry out calculations for estimating the concentration of the gaseous species under investigation based on said signals. The processing unit 30 comprises, for example, a microprocessor.

[0070] The dimensions of the housing 2 in which the gas extends are particularly small. It is preferred that the light waves detected by the measuring photodetector 20 are sufficiently attenuated to make it easier to quantify the attenuation att, taking into account the different noises in the measurement chain. It is therefore preferred that the length l traveled by the light reaching the measuring photodetector is sufficiently large. The applicant believes that the attenuation att must be greater than 0.2, for example 0.2 to 0.3.

[0071] In order to optimize the length l that the light reaches the measuring photodetector, the inventors designed a housing 2 comprising an ellipsoidal reflective surface. Thus, the reflective surface follows a portion of a spheroid. The reflective surface allows the waves emitted by the light source to be reflected multiple times before reaching the measuring photodetector. This results in an increase in the length l that the light reaches the measuring photodetector 20 and travels in the housing 2.

[0072] Figure 2A The interior of the housing 2 is schematically shown. Figure 2A Along the middle Figure 1B The circular generatrix D1 shown in dashed lines in FIG. 2 shows the interior of the housing 2. The illustration is that the housing is unfolded along the circular generatrix D1, and the ellipsoidal reflective surface S of order n n The index n is a natural integer such that 1≤n≤N, N being the number of ellipsoidal reflective surfaces situated on the optical path of the light wave emitted by the light source 10 and propagating towards the measuring photodetector 20 .

[0073] The term ellipsoidal surface S n It means that by making the ellipse E n A spheroidal surface of revolution obtained by rotating about one of its axes. In the example described, each spheroidal surface S n Associated with the major axis a n and the short axis b n The oval E n An ellipsoidal surface associated with an ellipse is an ellipsoidal surface formed by rotating the ellipse about one of its axes. In the example shown, each ellipsoidal surface S n is obtained by making the ellipse E associated with it nabout its longitudinal axis a n obtained by rotation. Such an ellipsoidal surface is called prolate, corresponding to the English names "spheroid" and "prolate ellipsoid".

[0074] A reflective surface is a surface whose reflectance coefficient is greater than 50%, preferably greater than 80%, over the whole or part of the emission spectral band Δλ. In the infrared spectral domain, reflective materials such as metals (e.g., gold) can be used to form the reflective walls.

[0075] Each ellipsoidal reflective surface S n includes a first focus P1-n and a second focus P2-n, the two foci being different from each other. When 1 < n < N, the first focus P1-n coincides with the second focus P2-(n - 1) of the previous surface S n-1 The second focus P2-n coincides with the first focus P1-(n + 1) of the subsequent surface S n+1 When n = 1 (the first reflective surface S1), the second focus P2-1 coincides with the first focus P1-2 of the second reflective surface S2. When n = N (the last surface), the first focus P1-N coincides with the second focus P2-(N - 1) of the previous surface S N-1 The first surface S1 allows the reflection of the incident light wave 11 emitted by the light source 10, while the last surface S N allows the reflection of the light wave that has passed through the housing 10 towards the measuring photodetector 20. In Figure 2A In the embodiment shown in FIGS. 3A to 3D, N = 5. N can be from 3 to 10, but the applicant believes that N = 5 corresponds to an optimized solution.

[0076] The light waves 11 emitted by the light source are successively reflected by the different surfaces S n in increasing order according to their respective ranks. In other words, the reflective surfaces S1,..., S N extend successively between the light source 10 and the measuring photodetector 20. The term "extend successively" is explained by considering the propagation of light between the light source 10 and the measuring photodetector 20. The path of the light wave projected onto the first wall 4 or the second wall 5 describes an arc of a circle.

[0077] Taking into account the dimensional constraints regarding the compactness of the sensor, the inventor has verified that it is preferable for the first ellipsoid E1 associated with the first surface S1 to have an eccentricity less than 0.8, even less than 0.7 or 0.5. The eccentricity of an ellipse with a major axis g and a minor axis p is the following ratio:

[0078]

[0079] The eccentricity of an ellipse is strictly between 0 and 1, and an eccentricity of zero corresponds to a circle.

[0080] The light source 10 is configured to emit an emission light wave 11 so as to form a light cone Ω1, called a first cone, extending around a first axis Δ1 towards the first reflective surface S1. The light source 10 is arranged at the vertex of the first light cone Ω1. The light source 10 preferably coincides with a first focus P1-1 of the first reflective surface S1.

[0081] The term "coincide" should be interpreted with a certain tolerance and may mean "in the vicinity of...", the vicinity corresponding to an area lying at a small distance from a point. A small distance refers to a distance of less than 1 mm, or even less than 0.5 mm.

[0082] The first reflective surface S1 is configured to reflect the first light cone Ω1 to form a second light cone Ω2 extending around the second axis Δ2. The second light cone converges to a second focus P2-1 of the first reflective surface S1, which corresponds to a first focus P1-2 of the second reflective surface S2. Downstream of the first focus P1-2, the second light cone Ω2 diverges until the second reflective surface S2.

[0083] The second reflective surface S2 is configured to reflect the second light cone Ω2 to form a third light cone Ω3 extending around a third axis Δ3. The third light cone Ω3 converges toward a second focus P2-2 of the second reflective surface S2, which corresponds to a first focus P1-3 of the third reflective surface S3. Downstream of the first focus P1-3, the third light cone Ω3 diverges until the third reflective surface S3.

[0084] The third reflective surface S3 is configured to reflect the third light cone Ω3 to form a fourth light cone Ω4 extending around a fourth axis Δ4. The fourth light cone Ω4 converges toward a second focus P2-3 of the third reflective surface S3, which corresponds to a first focus P1-4 of the fourth reflective surface S4. Downstream of the first focus P1-4, the fourth light cone Ω4 diverges until the fourth reflective surface S4.

[0085] The fourth reflective surface S4 is configured to reflect the fourth light cone Ω4 to form a fifth light cone Ω5 extending around the fifth axis Δ5. The fifth light cone Ω5 converges to a second focus P2-4 of the fourth reflective surface S4, which corresponds to a first focus P1-5 of the fifth reflective surface S5. Downstream of the first focus P1-5, the fifth light cone Ω5 diverges until the fifth reflective surface S5.

[0086] The fifth reflective surface S5 is configured to reflect the fifth light cone Ω5 to form a sixth light cone Ω6 extending around a sixth axis Δ6. The sixth light cone Ω6 converges toward a second focus P2-5 of the fifth reflective surface S5. The measuring photodetector 20 is arranged in the light cone Ω6, upstream or downstream of the second focus P2-5, or at the second focus P2-5.

[0087] Thus, the light waves 11 emitted by the light source 10 are reflected on the respective reflective surfaces S1, ..., S N In this example, N = 5. n , S n+1 Between, the light wave forms from the surface S n Cone Ω n+1 The vertices of n+1 The first focus P1-n+1 coincides with the surface S n The second focus P2-n converges the cone Ω n+1 Downstream of this first focus, the cone Ω n+1 Diverges until it reaches a surface of rank n+1.

[0088] Thus, the light wave is reflected from the ellipsoidal reflective surface S of rank n. n Directly propagates to the ellipsoidal reflective surface S of rank n+1 n+1 Directly means between two ellipsoidal reflective surfaces, without undergoing reflection from the other surface. Thus, between the light source 10 and the measuring photodetector 20, the light wave emitted by the light source undergoes N reflections, i.e., a number equal to the number of ellipsoidal surfaces arranged between the light source and the measuring photodetector. This allows minimizing the signal loss at each reflection.

[0089] Advantageously, if Figure 2A As shown, the light source 10 and the measuring photodetector 20 are carried by the same wall, which in this example is the first wall 4. The ellipsoidal reflective surface S of even rank n n Supported by the same wall. Ellipsoidal reflective surface S of odd rank n n It is carried by a wall opposite to the wall carrying the light source 10 and the measuring photodetector 20. This opposite wall is in this example the second wall 5. By using an odd number N of reflective surfaces, the sensor enables the light source 10 to be arranged in a coplanar, or substantially coplanar, manner with the measuring photodetector 20.

[0090] When the sensor 1 includes a reference photodetector 20 ref , the reference photodetector may be arranged on the same wall as the light source 10. In this case, Figure 2B As shown, the reference ellipsoidal reflective surface S ref Arranged on opposite walls. Reference reflective surface associated with ellipse E ref .exist Figure 2B Along the middle Figure 1B The interior of the housing 2 is shown by a circular generatrix D2 shown in dashed lines. This illustration is to unfold the housing 2 along the circular generatrix D2. ref With the first focus P 1-ref and the second focus P 2-ref, the focal points are different from each other. The light source 10 generates a light source 10 and a reference reflective surface S ref The reference reflective surface reflects the auxiliary light cone Ω'1 to form a light beam directed toward the reference photodetector 20. ref Reference cone of convergence Ω ref The light source 10 is arranged on the reference surface S ref The first focus P 1-ref Reference photodetector 20 ref Arranged in reference cone Ω ref In the second focus P 2-ref Upstream or downstream or at the second focal point P 2-ref Like the odd-numbered reflective surfaces, the reference reflective surface is preferably composed of a light source, a measuring photodetector 20 and a reference photodetector 20. ref The wall opposite to the wall bears the load.

[0091] Figures 3A to 3D FIG. 1 shows the arrangement of various reference ellipsoidal surfaces in an example of an embodiment of the present invention. Figure 1B The positive rotation direction shown in the figure gradually surrounds the sensor three-dimensional model to obtain Figures 3A to 3D .

[0092] exist Figure 3A In the embodiment, a light source 10 and a reference photodetector 20 are distinguished. ref and the reference reflective surface S ref . A first reflective surface S1 and a second reflective surface S2 are also distinguished. Figure 3B In addition to Figure 3A In addition to the described elements, a third reflective surface, a fourth reflective surface and a fifth reflective surface can also be seen.

[0093] According to this embodiment, each reflective surface is arranged at the inner face of the first or second wall: the reflective surfaces of even order are integrated at the inner face 4 of the first wall 4 i In the embodiment, the odd-numbered reflective surface is integrated on the inner surface 5 of the second wall 5. i Integrating the reflective surface in the shape of a spheroid of revolution in each internal face can be achieved by using molding or machining manufacturing techniques. Molding is the preferred option because it is easy to implement after the mold is formed, is repetitive, and is less expensive. According to this embodiment, the housing 2 can be formed by assembling the first wall 4 simply against the second wall 5, the transverse wall 6 being integrated into the first wall 4, or the second wall 5, or partially integrated on both walls.

[0094] Figure 3C and 3D Further details of the aforementioned reflective surfaces are shown.

[0095] Table 1 describes the light source 10, the photodetector 20 and the reference photodetector 20. ref The center of source 10 is considered as the origin of the coordinate system. The orientation angle relative to the Z axis is also indicated. Table 1 also shows the main characteristic coordinates of the spheroids of revolution that form each reflective surface. For each ellipsoid, the coordinates of the center corresponding to the intersection of the major axis a and the minor axis b of the ellipse associated with each ellipsoidal surface along the X, Y and Z axes are provided. The inclination angle θ of the major axis a relative to the X, Y and Z axes is also provided. x ,θ y and θ z The dimensions of the major axis a and the minor axis b are also provided. For angles, the unit is degrees. For coordinates or dimensions, the unit is mm.

[0096] X Y Z <![CDATA[θ X ]]> <![CDATA[θ Y ]]> <![CDATA[θ Z ]]> a b 10 0.0 0.0 0 0 0 0 <![CDATA[20 ref ]]> 5.6 4.4 -1.3 0 0 76.4 <![CDATA[E ref ]]> 2.8 2.2 -0.3 90 38.2 -7.3 8.1 7.3 20 3.7 10.1 -1.3 0 0 140.3 <![CDATA[E1]]> -2.8 2.1 0.0 90 -37.5 0 6.7 5.7 <![CDATA[E2]]> -5.6 5.5 0.5 90 92.5 20 1.8 1.2 <![CDATA[E3]]> -5.0 8.1 0.9 90 65 0 3.7 3.4 <![CDATA[E4]]> -2.2 10.4 1.7 90 25 10 3.6 2.6 <![CDATA[E5]]> 1.8 10.8 0.6 90 -19.9 -38.4 5.2 4.5

[0097] Table 1

[0098] The inclination of the respective major axis of each ellipsoid, on the basis of which each ellipsoid is formed, is therefore variable, in particular relative to the transverse axis Z.

[0099] Simulations were carried out to evaluate the performance of the aforementioned configuration. Depending on the concentration of a gaseous species in the gas occupying the enclosure 2, detectable changes in the concentration of said gaseous species were evaluated. Figure 4A The results obtained for methanol (CH4) are shown. The horizontal axis shows the concentration of CH4 in the housing, while the vertical axis shows the detectable change in the CH4 concentration. Figure 4A Also shown are estimates performed by considering the gas sensor described in WO2018162848 (curve a) and according to the present invention (curve b), respectively. Figure 4B is similar to Figure 4A , the gaseous substance considered is CO 2 . In both examples it is observed that the gas sensor according to the invention allows estimating a lower gas concentration than the sensor according to the prior art.

[0100] The present invention can be applied to gas detection in environmental monitoring, or in applications related to industrial gas inspection, especially in the agricultural product industry, the petroleum industry or gas transmission and distribution.

Claims

1. A gas sensor comprising a housing configured to contain a gas, the housing comprising a transverse wall extending about a transverse axis between a first wall and a second wall, the sensor further comprising: - a light source configured to emit a light wave that propagates in said housing and forms a first light cone from said light source, - a measuring photodetector configured to detect light waves emitted by said light source and propagating through said housing; The sensor is such that each of the first wall and the second wall comprises at least one reflective surface forming a portion of a spheroid, each reflective surface being associated with a rank n , n is an integer greater than or equal to 1, such that the sensor comprises: - a first reflective surface of rank 1 having a first focus and a second focus, the first reflective surface being arranged facing the light source, the first reflective surface forming a portion of a spheroid associated with an ellipse having an eccentricity less than or equal to 0.7; - rank strictly greater than 1 n At least one reflective surface of rank n Each reflective surface has: ·The first focus, which is the same as the previous rank n -1 coincides with the second focus of the reflective surface; A second focus, which is different from the first focus and has the same rank as the next n +1 coincides with the first focus of the reflective surface; - Rank N The last reflective surface of the first order includes a first focus that coincides with the second focus of the reflective surface of the previous order, and the last reflective surface also includes a second focus, at this time, n=N; The light waves emitted by the light source are reflected by the reflective surface before converging to the second focus of the last reflective surface. N The rotating ellipsoidal reflective surfaces successively reflect according to their respective ranks to form a last light cone, and the measuring photodetector is arranged to extend into the last light cone; in: - the focal points of reflective surfaces of even rank are different from each other, - odd-rank reflective surfaces have focal points that are different from each other; Wherein, N is 5 to 10.

2. The sensor according to claim 1, wherein: The light source is arranged at a first focus of the first reflective surface, or at a distance less than 1 mm from the first focus of the first reflective surface.

3. The sensor according to claim 1, wherein: The measuring photodetector is arranged at the second focus of the last reflective surface or at a distance less than 1 mm from the second focus of the last reflective surface.

4. The sensor according to claim 1, wherein: - Rank n Each reflective surface is configured to reflect the light waves it receives, so that an order n +1 light cone, the light cone around the rank n +1 propagation axis extension; - when n < N Time, rank n The propagation axis of +1 extends from the surface of rank n to the surface of rank n +1 surface and rank n The second focus of the surface, the second focus is of rank n +1 the first focus of the surface coincides; - when n = N Time, rank N A propagation axis of +1 extends from the surface of rank N to the measuring photodetector; The light waves emitted by the light source are caused to undergo a number of reflections corresponding to the number of the reflective surfaces before reaching the measuring photodetector.

5. The sensor according to claim 4, wherein: when n < N Time, rank n +1 for each light cone towards rank n The second focus of the surface converges and then moves toward the rank n +1 for the surface divergence.

6. The sensor according to claim 1, wherein: The light source and photodetector are integral with an interior face of the first wall, the interior face being oriented toward the interior of the housing.

7. The sensor according to claim 6, wherein: - a reflective surface of even order is arranged on the inner face of the first wall; - An odd-numbered rank reflective surface is arranged on the inner face of the second wall.

8. The sensor according to claim 7, wherein: The housing is formed by assembling the first wall facing the second wall.

9. The sensor according to claim 1, wherein: The number of reflective surfaces extending between the light source and the measuring photodetector is an odd number.

10. The sensor according to claim 9, wherein: The number of reflective surfaces extending between the light source and the measuring photodetector is equal to five.

11. The sensor of claim 1, comprising a reference photodetector configured to detect light waves deemed not to be attenuated by the gas.

12. The sensor of claim 11, comprising a reference reflective surface forming a spheroid, the reference surface comprising: - a first focal point arranged at the light source or at a distance less than 1 mm from the light source; - a second focal point arranged at the measuring photodetector or at a distance of less than 1 mm from the measuring photodetector.

13. A method for analyzing gas using the sensor according to claim 1, the method comprising the following steps: a) introducing the gas into the housing of the sensor; b) irradiating the gas using the light source; c) measuring, with the measuring photodetector, the light intensity of a light wave emitted by the light source and having propagated through the housing; d) determining the amount of a gaseous substance in the gas based on the measured light intensity.

14. The method according to claim 13, wherein: Step d) is performed by a processing unit based on the signal generated by said measuring photodetector.

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