FR3066600A1 - SENSOR FOR MEASURING CONCENTRATION OF PARTICLES IN THE AIR - Google Patents

Abstract

One aspect of the invention relates to a sensor (1) for real-time measurement of the concentration of particles in the air comprising: - an internal channel (10) for the circulation of an air flow comprising particles to detecting, the internal channel (10) having a detection zone (115), - a device for heating the air flow circulating in the internal channel (10), the device comprising at least one resistor (R1, R2, R3) arranged on a wall of the upstream internal channel (114) of the detection zone (115).

Description

SENSOR FOR MEASURING THE CONCENTRATION OF PARTICLES IN THE AIR

TECHNICAL FIELD OF THE INVENTION

The technical field of the invention is that of atmospheric pollution sensors. The present invention relates to a sensor for measuring the concentration of particles in the air, both in an indoor, confined environment and in an outdoor, outdoor environment.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

Air quality is an essential parameter for ensuring a good quality of life, especially in cities and urban agglomerations. Among the causes of air pollution, particles a few pm in diameter are particularly dangerous for human health. These particles are produced largely by human activities linked to industry and transport. They are responsible for health risks such as impaired lung function and can lead to a decrease in life expectancy.

As episodes of intense air pollution become more and more frequent, the institutions must put in place solutions to counter these phenomena. These solutions require real-time monitoring of the atmospheric concentration of fine particles. The need to deploy a dense network of sensors is increasingly felt.

PM (Particulate Matter) fine particles are often classified according to their size. The designation PM10 is used for particles having an aerodynamic diameter of less than 10 pm; PM2.5 for particles with aerodynamic diameter less than 2.5 pm and PM1 for particles with aerodynamic diameter less than 1 pm. The aerodynamic diameter of a particle is an equivalent quantity used to describe the aeraulic behavior of particles in a gas flow such as an air flow. The aerodynamic diameter of a particle is defined as being the diameter of a sphere of unit density (1 g / cm ³ ) and having the same limiting speed of fall as said particle in a fluid at rest. In the remainder of the present application, the terms "aerodynamic diameter of a particle" and "diameter of a particle" are used interchangeably.

Today, two methods of particle measurement are mainly used:

- gravimetric techniques consist in accumulating the particles on a filter and then quantifying them by direct or indirect weighing;

- optical techniques are based on the disturbance of a light beam by the passage of particles through the light beam.

The simplest gravity devices operate by impaction of the particles on a filter which is then weighed. These devices have good size selectivity and are relatively inexpensive, but they do not allow real-time monitoring of the concentration of particles.

Beta gauge systems are based on an electron absorption technique. These systems have the advantage of being very reliable but, in addition to their high cost, they require a radioactive source which can decrease their portability.

TEOM® gravimetric systems (from the English "Tapered Element Oscillating Microbalance") use a microbalance and are used for regulatory monitoring in certain countries. These systems allow real-time monitoring of the particle concentration, but they do not allow good selectivity and remain relatively expensive.

The devices using optical techniques are compact, portable and they allow particle size selectivity. These devices are the most suitable for providing real-time information to individuals, but they suffer in practice from a lack of precision which limits their interest, by presenting a measurement uncertainty greater than or equal to 50% on the mass concentrations of PM1, PM2.5 and PM10.

SUMMARY OF THE INVENTION

The invention offers a solution to the problems mentioned above, by making it possible to measure in real time the concentration of particles in the air, with a device that is both compact, portable and selective in size for the particles, while being sufficiently precise, that is to say by achieving a measurement uncertainty of at least less than 50% for the mass concentrations of fine particles PM1, PM2.5 and PM10.

One aspect of the invention relates to a sensor for real-time measurement of the concentration of particles in the air, characterized in that it comprises:

- an internal channel for the circulation of an air flow comprising particles to be detected, the internal channel having a detection zone,

- a device for heating the air flow circulating in the internal channel, the device comprising at least one Joule effect heating element arranged on a wall of the internal channel upstream of the detection zone.

Thanks to the invention, the heating device allows a control of the hygrometry of the particles of the air flow in order to be freed from the atmospheric conditions outside. Indeed, certain types of particles swell significantly under the effect of humidity, which is likely, by modifying the size of the particles, to distort the measurements. By controlling the hygrometry of the air flow, the accuracy of the sensor is therefore improved according to one aspect of the invention.

In addition to the characteristics which have just been mentioned in the previous paragraph, the sensor according to one aspect of the invention may have one or more complementary characteristics among the following, considered individually or according to all the technically possible combinations:

- The heater has a plurality of Joule effect heaters in series.

- The heating device comprises means for controlling the at least one heating element by Joule effect as a function of a first temperature measurement and of a second humidity measurement.

- The at least one heating element by Joule effect is preferably a resistance or alternatively a linear regulator.

- The sensor preferably comprises a device for circulating an air flow in the internal channel, said device being arranged downstream of the detection zone and being configured to circulate an air flow from upstream of the detection zone and downstream of the detection zone.

- The device for circulating an air flow in the internal channel is a fan or a pump.

Advantageously, the internal channel comprises:

a first portion comprising a side wall extending between a first open end and a second closed end, the first open end extending along an entry plane;

a second portion communicating with the first portion via a first opening in the side wall of the first portion, the first opening being adjacent to the first open end;

o a storage area communicating with the first portion via a second opening in the side wall of the first portion, the second opening being adjacent to the second closed end, the first and second openings being arranged on either side of the wall lateral;

a deflection plate fixed at a junction between the first open end of the first portion and the second portion, the deflection plate extending inside the first portion and forming with a first direction normal to the entry plane an angle a such that:

0 ° <a <90 ° the deflection plate, the first portion and the first and second openings being dimensioned so that in an air flow entering the sensor through the first open end and comprising first particles of smaller diameter or equal to 10 pm and second particles with a diameter greater than 10 pm, the first particles are deflected by the deflection plate, pass through the first opening and reach the second portion of the internal channel while the second particles are deflected by the plate deflection, impacted by a part of the side wall forming an impaction plate, pass through the second opening and reach the storage area.

Thus, the internal channel of the sensor provides a filter function as input: only the first particles enter the second portion of the internal channel to be subsequently detected, while the second particles are deflected in a storage area. The first and second particles, of different sizes, are in fact of masses and therefore of different inertias, this is why they do not follow the same fluid path. This avoids fouling of the second portion of the internal channel by the second particles, which would cause a drift in the response of the sensor and the measurement artifacts. No measurement being carried out in the storage area, its fouling by the second particles is not problematic.

- The device for circulating an air flow in the internal channel is arranged at the second end of the second portion of the internal channel and is configured to circulate an air flow from the first end of the first portion from the internal channel to the second end of the second portion of the internal channel.

- Having defined a first flow surface which is the surface along the entry plane of the first open end, and a second flow surface which is the smallest flow surface of the first portion, defined between the end of the deflection plate and the side wall: the S2 / S1 ratio of the second surface on the first surface is such that:

10% <- <90%

- The second portion of the internal channel having a first open end communicating with the first portion of the internal channel and a second open end, and the second portion of the internal channel comprising a detection zone, an upstream zone between the first end and the detection and a downstream zone between the detection zone and the second end, the sensor is preferably such that:

o the second portion of the internal channel has a folded shape, and o the upstream area of the internal channel has an inclined portion widening between the first end of the internal channel and the detection zone, the inclined portion forming an angle β, measured by relative to the vertical, such as: 60 ° <β <80 °, and the inclined portion widening from 150 pm ² to 300 pm ² per mm of length.

- The storage area (120) has a volume of between 0.5 mL and 5 mL.

Advantageously, the sensor for the real-time measurement of the concentration of particles in the air comprises:

a light source configured to emit light radiation in a direction of propagation, the light radiation being focused in the detection zone of the internal channel;

a first photodetector configured to pick up a first scattering signal emitted by particles passing through the detection zone in a first direction forming a first non-zero angle with the direction of propagation of the light radiation;

a second photodetector configured to pick up a second scattering signal emitted by particles passing through the detection zone in a second direction forming a second non-zero angle with the direction of propagation of the light radiation, the second angle being different from the first angle and the first and second angles not being additional;

o a light trap configured to receive the light radiation at the output of the detection zone so as to prevent a parasitic return of the light radiation towards the detection zone.

This gives access to a diffused light intensity difference information, between the first scattering signal emitted in the first direction and the second scattering signal emitted in the second direction, which makes it possible to increase the accuracy of the particle size information. particles which pass through the detection zone and therefore increase the accuracy of the sensor according to one aspect of the invention. Indeed, two particles of different sizes each have a distinct angular diffusion.

- The difference between the first and second angles is at least 5 ° and preferably at least 15 °.

- The first direction and the direction of propagation defining a first plane, the second direction preferably belongs to a second plane distinct from the first plane, and preferably perpendicular to the first plane.

- The light trap has a black wall and a black cavity, the wall deflecting the light radiation towards the cavity. The wall is preferably smooth while the cavity is rough.

The invention and its various applications will be better understood on reading the description which follows and on examining the figures which accompany it.

BRIEF DESCRIPTION OF THE FIGURES

The figures are presented for information and in no way limit the invention.

- Figure 1 schematically shows a perspective view of a sensor for the real-time measurement of the concentration of particles in the air according to one aspect of the invention.

- Figure 2a schematically shows a view of an internal channel of the sensor of Figure 1 along a cutting plane of axes y, z.

- Figure 2b schematically shows a partial view of the internal channel of the sensor of Figure 1 along the cutting plane of axes y, z.

- Figure 3a schematically shows a view of the internal channel of the sensor of Figure 1 along a cutting plane of x, y axes.

- Figure 3b schematically shows a partial view of the internal channel of the sensor of Figure 1 along a cutting plane of x, z axes.

- Figure 4a schematically shows a view of an optical detection system of the sensor of Figure 1 along a cutting plane of x, y axes.

FIG. 4b schematically shows a partial perspective view of the optical detection system of the sensor of FIG. 1.

- Figure 5 shows schematically a view of a heating device of the sensor of Figure 1 along a cutting plane of axes x, y.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT OF THE INVENTION

Unless otherwise specified, the same element appearing in different figures has a unique reference.

The figures are defined in an orthogonal coordinate system with x, y, z axes.

FIG. 1 schematically shows a perspective view of a sensor 1 for the real-time measurement of the concentration of particles in the air according to an embodiment of the invention. Figure 1 shows a section plane A with axes y, z, a section plane B with axes x, y and a section plane C with axes x, z. The sensor 1 comprises an internal channel 10 (referenced in FIG. 2a) having a first portion 100 and a second portion 110 (referenced in FIG. 2b), the first portion having a first open end 101 forming an inlet for a flow d in the internal channel of the sensor 1, and the second portion having a second open end 112 forming an outlet for the air flow from the internal channel of the sensor 1. The air inlet 101 and the air outlet 112 s 'preferentially extend in two separate planes, and even more preferably in two planes which are not parallel to each other, so that the air flow at the inlet is not disturbed by the air flow at the outlet. In the particular example illustrated in FIG. 1, the air inlet 101 extends along a plane substantially parallel to the cutting plane B, while the air outlet 112 extends along a plane substantially parallel to the plane of section C.

FIG. 2a schematically shows a view along the cutting plane A of the internal channel 10 of the sensor 1. FIG. 2b schematically shows a partial view along the cutting plane A of the internal channel 10 of the sensor 1. FIGS. 2a and 2b particularly illustrate a fluid filter function of the sensor 1 and are described jointly.

The sensor 1 comprises an internal channel 10 having:

- The first portion 100 comprising a side wall 103 extending between the first open end 101 and a second closed end 102, the first open end 101 extending along an entry plane Pe;

- The second portion 110 communicating with the first portion 100 via a first opening 105 in the side wall 103 of the first portion 100, the first opening 105 being adjacent to the first open end 101;

a storage area 120 communicating with the first portion 100 via a second opening 107 in the side wall 103 of the first portion 100, the second opening being adjacent to the second closed end 102, the first and second openings 105, 107 being arranged on either side of the side wall 103;

a deflection plate 130 fixed at a junction between the first open end 101 of the first portion 100 and the second portion 110, the deflection plate 130 extending inside the first portion 100 and forming with a first direction D1 normal to the entry plane Pe an angle a such that:

0 ° <a <90 ° the deflection plate 130, the first portion 100 and the first and second openings 105, 107 being dimensioned so that in a flow of air entering the sensor 1 through the first open end 101 and comprising first particles with a diameter less than or equal to 10 μm and second particles with a diameter greater than 10 μm, the first particles pass through the first opening 105 and reach the second portion 110 of the internal channel while the second particles pass through the second opening 107 and reach the storage area 120.

The first and second openings 105, 107, arranged on either side of the side wall 103, are not opposite one another, that is to say that any plane d axes x, y and passing through the first opening 105 does not pass through the second opening 107, and vice versa. There is preferably a portion of side wall 103 with a length L at least equal to 2 mm between the first and second openings 105, 107. The term “the first and second openings 105, 107 are arranged on either side side wall 103 >> the fact that:

the first opening 105 is located in a first half-space, to which the deflection plate 130 and the second portion 110 of the internal channel belong, and

the second opening 107 is located in a second half-space, to which the storage area 120 belongs.

Part of the side wall 103 is an impact plate 103-i: this is the part of the side wall 103 which is arranged on the side of the first opening 105, in the first half-space. The impact plate 103-i is arranged on the side of the first opening 105.

In the particular example which is illustrated in FIGS. 2a and 2b, the first portion 100 is rectilinear in the first direction D1. However, the first portion 100 can alternatively be curved.

In the particular example which is illustrated in FIGS. 1, 2a and 2b, the first portion 100 and the second portion 110 are of section, along a plane parallel to the plane B of axes x, y, rectangular. However, the first portion 100 and the second portion 110 may alternatively be of square, polygonal, circular, elliptical section, etc. along a plane parallel to the plane B of axes x, y. In general, it is desired to define a certain speed of the air flow through the internal channel 10. To do this, the determining parameter is the area of the section of each of the first and second portions 100, 110. It is understood that several different geometries can be associated with the same surface.

First, second and third surfaces S1, S2, S3 of flow are represented in FIG. 2b:

the first surface S1 is the entry surface of the internal channel 10, that is to say the flow surface along the entry plane Pe of the first open end 101 of the first portion 100 of the internal channel 10;

the second surface S2 is the smallest flow surface of the first portion 100, defined between the end of the deflection plate 130 and the side wall 103;

the third surface S3 is the smallest flow surface defined between the end of the deflection plate 130 and the end of the impact plate 103-i delimiting the first opening 105.

The S2 / S1 ratio of the second surface S2 on the first surface S1 is advantageously such that:

10% <- <90%

Preferably such as:

30% <- <90%

And even more preferably such as:

30% <- <75%

This optimizes the filter function at the input of the internal channel 10 of the sensor 1.

According to a particular example, the first surface S1 has an area of 50 mm ² and the second surface S2 has an area of 30 mm ² , ie an S2 / S1 ratio of 60%.

The cut-off diameter d, i.e. the aerodynamic diameter such that 50% of the particles of this diameter reach the second portion 110 of the internal channel and 50% fail to do so, can be estimated by the following formula :

u d oc k-— lap where d is expressed in μιτι; k is a constant expressed in pm.s.rad; u is the speed of the air flow passing through the third surface S3; I is the length of the deflection plate 130 expressed in m; a is the angle expressed in rad formed by the deflection plate 130 with the first direction D1; p is the density of the particles considered:

density of the particles considered? reference density

The reference density is 1g / cm ³ .

The angle a, illustrated in particular in FIG. 2b, is preferably such that:

15 ° <to <75 °

The angle a is more preferably such that:

15 ° <to <45 °

The angle a is even more preferably such that:

25 ° <a <35 °

For an angle a of 30 °, a deflection plate 130 of length l = 4 mm and a speed u of the air flow passing through the third surface S3 of 5 m / s:

- a cut-off diameter d greater than 10 µm is obtained for particles of density p = 1;

- a cut-off diameter d greater than 3 μm is obtained for particles of density p = 3.

The smaller the cutoff diameter d, the smaller the diameter of the particles which actually reach the second portion 110. It is observed that the density of the particles has an important influence on the results.

The storage area 120 advantageously has a volume of between 0.5 ml and 5 ml. This guarantees a sufficient maintenance-free lifetime of the sensor 1, that is to say greater than 10,000 hours in environments highly polluted with fine particles. It is considered that an environment is very polluted with fine particles when it contains on average at least 250 pg / m ³ of fine particles. At the same time, the cumulative volume of the first portion 100 of the internal channel and of the storage area 120 is advantageously less than 10 mL. This makes it possible to limit the size of the sensor 1, and therefore its size and its mass.

FIG. 3a schematically shows a view along the cutting plane B of the sensor 1. FIG. 3a shows a cutting plane D of axes x, z. FIG. 3b schematically shows a partial view along the section plane D of the sensor 1. FIGS. 3a and 3b are described jointly.

The second portion 110 of the internal channel has a first open end 111 which communicates with the first portion 100 of the internal channel, and the second end 112 open. The second portion 110 of the internal channel comprises a detection zone 115, an upstream zone 114 between the first end 111 and the detection zone 115, and a downstream zone 116 between the detection zone 115 and the second end 112.

In order to improve the compactness of the sensor 1, the second portion 110 of the internal channel advantageously has a folded shape, with a first straight branch connected to a second straight branch by a connection substantially perpendicular to the first and second straight branches. The first and second rectilinear branches can be parallel to each other but are not necessarily parallel to each other. The upstream area 114 comprises the first straight branch, the downstream area 116 includes the second straight branch and the detection area 115 is located in the connection between the first and second straight branches. Another advantage of the folded shape is to move the detection zone 115 away from the inlet 101 and the outlet 112 of the internal channel in order to limit the risk of external stray light in the detection zone 115. At the same time, the geometry and the dimensioning of the upstream area 114 are advantageously chosen so that an air flow disturbed by the first portion 100 of the internal channel and entering the second portion 110 of the internal channel is laminar and with a homogeneous speed when it reaches the level of the detection zone 115. It is considered that the air flow has a homogeneous speed in the detection zone 115 when at each point of a section of the detection zone, the speed v according to the axis y perpendicular to said section is such that:

0.8 xv _m <v <1.2 xv _m with vm the average speed in said section.

To do this, the upstream area 114 of the internal channel has an inclined portion widening by connecting a first horizontal plane H1, along which extends substantially the first end 111 of the internal channel, to a second horizontal plane H2, along which s 'substantially extends the internal channel between its detection zone 115 and its second end 112. The inclination β of this portion, measured relative to the vertical, is such that:

60 ° <β <80 °

The widening of the inclined portion of the upstream area 114 of the internal channel is between 150 and 300 pm ² per mm of length along the axis x.

The sensor 1 preferably comprises a device 20 for circulating an air flow in the internal channel 10, referenced in FIG. 2a, which is arranged at the second end 112 of the second portion 110 of the internal channel and which is configured to circulate an air flow from the first end 101 of the first portion 100 of the internal channel to the second end 112 of the second portion 110 of the internal channel. The device 20 can advantageously be controlled in order to ensure a constant average speed of the air flow at the level of the detection zone 115. In an exemplary embodiment of the invention, the air flow through the internal channel 10 of the sensor 1 has a flow rate of 0.0032 m ³ / min and the pressure drop generated by the internal channel 10 is calculated at 9.6 Pa: in this exemplary embodiment, the device 20 is advantageously controlled in order to ensure a average speed of 1 m / s at the detection zone 115. The device 20 can in particular be a fan, a pump or a micro-pump.

FIG. 4a schematically shows a view of an optical detection system of the sensor 1 along the section plane B of axes x, y. FIG. 4b schematically shows a partial perspective view of the optical detection system of the sensor 1. FIGS. 4a and 4b particularly illustrate a function of the optical detector of the sensor 1 and are described jointly.

The optical detection system of sensor 1 comprises:

a light source 30 configured to emit light radiation in a direction of propagation Dp, the light radiation being focused in the detection zone 115 of the internal channel 10;

a first photodetector 40 configured to pick up a first scattering signal emitted in a first direction D1 by particles passing through the detection zone, the first direction D1 forming a first non-zero angle Θ1 with the direction of propagation Dp of the light radiation;

a second photodetector 50 configured to pick up a second scattering signal emitted in a second direction D2 by particles passing through the detection zone, the second direction D2 forming a second non-zero angle Θ2 with the direction of propagation Dp of the light radiation, the second angle Θ2 being different from the first angle Θ1 and the first and second angles Θ1, Θ2 not being additional;

a light trap 60 configured to receive the light radiation at the output of the detection zone 115 so as to prevent a parasitic return of the light radiation to the detection zone 115.

This gives access to a diffused light intensity difference information, between the first scattering signal emitted in the first direction and the second scattering signal emitted in the second direction, which makes it possible to increase the accuracy of the particle size information. particles which pass through the detection zone and therefore increase the precision of the sensor 1. In fact, two particles of different sizes each have a distinct angular diffusion.

The light source 30 is preferably a laser source, and in particular a laser source of the laser diode type. Alternatively, the light source 30 can be a light emitting diode, or LED. The focusing of the light radiation in the detection zone is obtained by focusing optics. The light radiation emitted by the light source 30 preferably has a power between 1 mW and 20 mW, more preferably a power between 1 mW and 10 mW, even more preferably a power between 1 mW and 5 mW. The light radiation emitted by the light source 30 has for example a light radiation of 2 mW. The light source 30 can emit visible light radiation, that is to say of wavelength in the range [400 nm; 700 nm], or near infrared radiation, that is to say of wavelength in the interval [700 nm; 900 nm]. To increase the metrological performance of the sensor according to Mie theory, visible light radiation of short wavelength, that is to say of wavelength in the interval [400 nm; 500 nm], is more efficient. However, a good performance-cost compromise is obtained in particular with visible radiation of wavelength in the range [600 nm; 700 nm], and more particularly of wavelength in the range [630 nm; 670 nm].

The first and second photodetectors 40, 50 are preferably photodiodes. Alternatively, the first and second photodetectors 40, 50 could be photomultipliers. Compared to photomultipliers, photodiodes have the advantage of being less expensive and simpler to implement in miniature systems. Each of the first and second photodetectors 40, 50 has an active surface and an axis cone normal to its active surface.

The light trap 60 has a wall 61 and a cavity 62, the wall 61 being oriented relative to the direction of propagation Dp, so as to deflect the light radiation towards the cavity 62. The wall 61 and the cavity 62 are advantageously black so as to maximize absorption of incident light radiation. Advantageously, the wall 61 is smooth while the cavity 62 is rough. The term “smooth surface” is understood to mean a surface having an average deviation in roughness Ra such that:

Ra <0.1 μτη

The term “rough surface” means a surface having an average deviation from roughness Ra such that:

Ra> 10 μτη

The average roughness deviation Ra is the standard deviation of the roughness of the surface considered, as defined in standard ISO 4287.

The difference ΔΘ is preferably chosen between the first and second angles Θ1, Θ2 so that:

dAS â0> k— where:

- k is a constant expressed in rad / W / m ^-4 ;

- d is the distance between each of the first and second photodetectors 40, 50 on the one hand, and the detection area 115 on the other hand, expressed in m; if the first and second photodetectors 40, 50 are not equidistant from the detection zone 115, d is the smallest distance between each of the first and second photodetectors 40, 50 on the one hand and the detection zone 115;

- A is the wavelength of the light radiation, expressed in m;

- S is the active surface of the first and second photodetectors, expressed in m ² ; if the first and second photodetectors do not have the same active surface, S is the largest active surface of the first and second photodetectors;

- P is the power of the light source, expressed in W.

For a given configuration of the first and second photodetectors relative to the detection zone and for a given wavelength of light radiation, the greater the power of the light source, the greater the difference ΔΘ between the first and second angles Θ1, Θ2 may be low. Generally, a difference ΔΘ of at least 5 ° and preferably at least 15 ° is chosen between the first and second angles Θ1, Θ2.

In the particular example of FIG. 4c, the first angle Θ1 between the direction of propagation Dp and the first direction D1 is such that:

Θ1 = 45 ° and the second angle Θ2 between the direction of propagation Dp and the second direction D2 is such that:

Θ2 = 90 °

The difference ΔΘ between the first and second angles Θ1, Θ2 is therefore 45 °.

Furthermore, the first and second directions D1, D2 are preferably chosen so that:

the first direction D1 and the direction of propagation Dp define a first plane P1, and

the second direction D2 belongs to a second plane P2 distinct from the first plane P1.

In the particular example of FIG. 4c, the second plane P2 is perpendicular to the first plane P1.

Still in the particular example of FIG. 4c, the axis of the detection cone of the first photodetector 40 is coincident with the first direction D1, and the axis of the detection cone of the second photodetector 50 is coincident with the second direction D2. However, the first photodetector 40 could alternatively be arranged so that the axis of its detection cone is not merged with the first direction D1, for example by using a mirror to deflect the first diffusion signal in a direction DT and in arranging the first photodetector 40 so that its detection cone is coincident with the deviated direction DT. Similarly, the second photodetector 50 could alternatively be arranged so that the axis of its detection cone is not confused with the first direction D2, for example by using a mirror to deflect the second scattering signal in a direction D2 ' and by arranging the second photodetector 50 so that its detection cone coincides with the deviated direction D2 ′.

FIG. 5 schematically shows a sectional view, along the plane B of axes x, y, of a device for heating the air flow circulating in the internal channel 10 of the sensor 1, the heater comprising at least one Joule heating element arranged on a wall of the internal channel 10, upstream 114 of the detection zone 115. FIG. 5 particularly illustrates a thermal control function of the hygrometry of the air flow circulating in the internal channel 10 of the sensor 1.

The heating device advantageously allows a control of the hygrometry of the particles of the air flow in order to overcome the external atmospheric conditions. Indeed, certain types of particles swell significantly under the effect of humidity, which is likely, by modifying the size of the particles, to distort the measurements. By controlling the humidity of the air flow, the accuracy of the sensor 1 is therefore improved.

The Joule heating element is preferably a resistor or alternatively a linear regulator.

The heating device preferably comprises a plurality of Joule effect heating elements in series, said plurality being arranged on a wall of the internal channel upstream of the detection zone. This has the advantage of using a plurality of smaller heating elements, rather than a single larger heating element, and therefore of minimizing the mechanical disturbance of the air flow within the internal channel 10. In the particular example of FIG. 5, the heating device comprises first, second and third Joule heating elements R1, R2, R3.

Thanks to the at least one Joule effect heating element, the temperature of the air flow arriving in the detection zone is preferably 3 ° C. to 10 ° C. higher than the temperature of the air flow entering the sensor 1. This temperature difference is sufficient to guarantee an absence of growth factor of the particles present in the air flow ("growth factor" in English).

The sensor 1 preferably comprises a temperature sensor and a humidity sensor and the at least one heating element by Joule effect is preferably controlled according to a first temperature measurement and a second humidity measurement of the flow of air in the sensor 1. The first temperature measurement and the second humidity measurement of the air flow can for example be performed at the inlet or outlet of the internal channel 10 sensor 1, or at any intermediate point of the internal channel 10. We thus adapt to outdoor atmospheric conditions to improve the accuracy of the sensor while optimizing its energy consumption.

The sensor 1 according to one aspect of the invention advantageously provides both the functions of fluid filter, optical detector and thermal control previously described, but the sensor 1 according to one aspect of the invention can alternatively only provide or two of these three functions, according to all possible combinations:

- fluid filter function only,

- fluid filter and optical detector functions,

- fluid filter and thermal control functions,

- optical detector only function,

- optical detector and thermal control functions,

- thermal control function only.

The sensor 1 preferably comprises a computer and a storage memory in order to perform all or part of the following functions:

control the circulation of the air flow in the sensor 1 by the circulation device 20, that is to say control the speed and the flow rate of the air flow in the internal channel 10 of the sensor 1 ;

- process the first and second diffusion signals from the first and second photodetectors 40, 50 in order to calculate in real time a concentration of fine particles in the air flow circulating in the internal channel 10 of the sensor 1;

- check the at least one resistance of the heating device as a function of the temperature and humidity measurements made by the temperature and humidity sensors.

The computer is for example a microcontroller or a microprocessor. The storage memory is preferably a reprogrammable read only memory EPROM (from the English “Erasable Programmable Read Only Memory”) but can also be a random access memory RAM (from the English “Random Access Memory”), a programmable read only memory PROM ( “Programmable Read Only Memory”), a FLASH-EPROM or any other memory chip or cartridge.

Claims (8)

1. Sensor (1) for real-time measurement of the concentration of particles in the air, characterized in that it comprises: - an internal channel (10) for the circulation of an air flow comprising particles to be detected, the internal channel (10) having a detection zone (115), a device for heating the air flow circulating in the internal channel (10), the device comprising at least one Joule effect heating element (R1, R2, R3) arranged on a wall of the internal channel upstream (114) of the detection zone (115). 2. Sensor (1) according to the preceding claim, characterized in that the heating device comprises a plurality of Joule effect heating elements (R1, R2, R3) in series. 3. Sensor (1) according to any one of the preceding claims, characterized in that the heating device comprises means for controlling the at least one heating element by Joule effect (R1, R2, R3) according to a first temperature measurement and a second humidity measurement. 4. Sensor (1) according to any one of the preceding claims, characterized in that the at least one Joule heating element is a resistance or a linear regulator. 5. Sensor (1) according to any one of the preceding claims, characterized in that it comprises a device (20) for circulating an air flow in the internal channel (10), said device (20 ) being arranged downstream (116) of the detection zone (115) and being configured to circulate an air flow from upstream (114) of the detection zone and downstream (116) of the zone detection. 6. Sensor (1) according to the preceding claim, characterized in that the device (20) for circulating an air flow in the internal channel (10) is a fan or a pump. 7. Sensor (1) according to any one of the preceding claims, characterized in that the internal channel (10) comprises: - A first portion (100) comprising a side wall (103) extending between a first open end (101) and a second closed end (102), the first open end (101) extending along an entry plane (Pe); - a second portion (110) communicating with the first portion via a first opening (105) in the side wall (103) of the first portion (100), the first opening being adjacent to the first open end; - a storage area (120) communicating with the first portion via a second opening (107) in the side wall (103) of the first portion (100), the second opening being adjacent to the second closed end, the first and second openings (105, 107) being arranged on either side of the side wall (103); - a deflection plate (130) fixed at a junction between the first open end (101) of the first portion (100) and the second portion (110), the deflection plate (130) extending inside the first portion (100) and forming with a first direction (D1) normal to the entry plane (Pe) an angle a such that: 0 ° <a <90 ° the deflection plate (130), the first portion (100) and the first and second openings (105, 107) being dimensioned so that in an air flow entering the sensor (1) by the first open end (101) and comprising first particles of diameter less than or equal to 10 pm and second particles of diameter greater than 10 pm, the first particles are deflected by the deflection plate (130), pass through the first opening (105) and reach the second portion (110) of the internal channel (10) while the second particles are deflected by the deflection plate (130), impacted by a part of the side wall (103) forming a plate impaction (103-i), pass through the second opening (107) and reach the storage area (120). 8. Sensor (1) according to any one of the preceding claims, characterized in that it comprises: - a light source (30) configured to emit light radiation in a direction of propagation (Dp), the light radiation being focused in the detection zone (115) of the internal channel; - a first photodetector (40) configured to pick up a first scattering signal emitted by particles passing through the detection zone (115) in a first direction (D1) forming a first non-zero angle (Θ1) with the direction of propagation (Dp ) light radiation; - a second photodetector (50) configured to pick up a second scattering signal emitted by particles passing through the detection zone (115) in a second direction (D2) forming a second non-zero angle (Θ2) with the direction of propagation (Dp ) light radiation, the second angle (Θ2) being different from the first angle (Θ1) and the first and second angles (Θ1, Θ2) not being additional; - a light trap (60) configured to receive the light radiation at the output of the detection zone (115) so as to prevent parasitic return of the light radiation to the detection zone (115).