Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/47—Scattering, i.e. diffuse reflection
  • G01N21/49—Scattering, i.e. diffuse reflection within a body or fluid
  • G01N21/51—Scattering, i.e. diffuse reflection within a body or fluid inside a container, e.g. in an ampoule
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
  • G01N15/02—Investigating particle size or size distribution
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
  • G01N15/02—Investigating particle size or size distribution
  • G01N15/0205—Investigating particle size or size distribution by optical means
  • G01N15/0211—Investigating a scatter or diffraction pattern
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
  • G01N15/06—Investigating concentration of particle suspensions
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/47—Scattering, i.e. diffuse reflection
  • G01N2021/4704—Angular selective
  • G01N2021/4707—Forward scatter; Low angle scatter
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/47—Scattering, i.e. diffuse reflection
  • G01N2021/4704—Angular selective
  • G01N2021/4709—Backscatter
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2201/00—Features of devices classified in G01N21/00
  • G01N2201/06—Illumination; Optics
  • G01N2201/061—Sources
  • G01N2201/06113—Coherent sources; lasers
  • G01N2201/0612—Laser diodes

Definitions

• the present invention relates to a method and a system for analyzing solid particles in a medium.
• the present invention relates to the field of detection and measurement of the amount of solid particles (concentration, size distribution, total mass, nature, etc.) present in the atmosphere. It applies in particular for the continuous measurement of aerosols to improve the quality of the air, for example ambient air, an industrial discharge or a motor gas.
• It relates more particularly to a system for analyzing solid particles in a medium, comprising an illumination means capable of generating a light field in the medium, a means for trapping at least a portion of the light field generated and arranged in the direction of this light field, and a main means of detecting the light field scattered by these solid particles in this medium.
• It also relates to a method for analyzing solid particles in a medium, comprising an illumination step of generating a light field in the medium, a step of trapping at least a portion of the generated light field, and disposed in the direction of this light beam, and a step of detecting the light field scattered by these solid particles in this medium.
• a first technique for measuring solid particles in the atmosphere consists of a manual gravimetric measurement, by sampling the particles with a filter and then weighing the filtered particles.
• This technique considered as the reference to the regulatory sense, remains an unsuitable technique for real-time on-site control operations due to the need for manual processing.
• a second known technique is to use an oscillating microbalance device to provide an automatic measurement suitable for on-site control operations.
• Such a measure has the disadvantage of being dependent on the ambient conditions, in particular the humidity, as well as the composition of the particles in the case where volatile compounds are present.
• empirical corrections are implemented by adding coefficients determined a posteriori, which proves to be constraining and unreliable.
• a third known technique is the absorption of beta radiation.
• This so-called "Beta gauge” solution involves the use of a radioactive source and is also unusable in real time. Indeed, depending on the concentration of solid particles, a measurement result can be obtained each hour at best. In addition, this technique sees its minimum detection degrade for small particles.
• solutions consist mainly of collecting and channeling the solid particles, in the form of aerosols, into a conduit.
• a device for transmitting a Laser radiation illuminates these solid particles, which causes the scattering of this radiation.
• a detection device is arranged with respect to the transmitting device so as to collect a part of the scattered light. This scattered light collected makes it possible to perform a quantitative measurement of the number of particles (counting) which is then converted into mass concentration and to have a classification by size range.
• a first means is a photometer for instantaneous measurement of the flux variations related to the variations in concentration of solid particles. It is then possible to deduce from photometry measurements the variation of the concentration of solid particles per unit of time.
• a second means is an aerosol counter for analyzing the presence of particles by a pulse detector. This technique makes it possible to estimate the particle concentration between a minimum size threshold and a maximum size threshold. It can also measure the particle size by the intensity of the detected luminous flux. It is also possible to combine these two means to obtain hybrid results.
• a particle analysis system comprises a first diffusion chamber, means for providing a fluid sample in the form of a laminar flow in the first diffusion chamber, and a light beam - for example laser generated - arranged to intercept the sample at right angles to the direction of sample flow at a focal point of a first concave mirror.
• This first concave mirror is used to direct the light scattered by the individual particles in the sample to at least one light collector.
• the system also includes means for converting collected light into electrical signals for processing and analysis, and means for trapping non-scattered light. It is thus possible to collect a larger scattered light flux, which improves the accuracy on the light measurements scattered by the particles.
• a second diffusion chamber comprising a second concave mirror and a light collector disposed at its near focal point and positioned so its distant focal point is at the intercept point of the light beam and the sample.
• the purpose of this second diffusion chamber is to allow detection and analysis of light scattered at low angles by individual particles. This part of The light beam provides information to determine the particle size.
• This solution then makes it possible, in real time, both to count the individual particles in a sample in order to distinguish different shapes of particles - spherical or nonspherical - and to count them separately, as well as to classify the particles by size categories.
• this solution has the disadvantage of being expensive and complex to implement. Indeed, the diffusion chambers, the collimation optics and the concave mirrors, although providing more light scattered towards the collectors, are relatively expensive and difficult to assemble.
• the object of the present invention is to overcome these technical complexities; it proposes for this purpose to have a detection means that is simple to perform and to put a work of art, including counting and photometry elements, so as to be oriented in a direction forming an angle of less than 30 ° with respect to the direction the light field generated by the illumination means.
• the measurement of the intensity of the light scattered at these angles makes it possible to estimate the number of particles per size range almost independently of their nature.
• the approach of the solution consisted in studying the behavior of the light vis-à-vis particles of different optical indexes, transparent or absorbing, and diameters between 0.3 and 30 micrometers, and to validate then to calibrate the concept by real particles of various natures. Surprisingly, it has become apparent that the detection has a higher level for diffusion angles substantially less than 20 °.
• the subject of the invention is a system for analyzing solid particles in a medium, comprising an illumination means capable of generating a light field in the medium, a means for trapping at least part of the field generated and arranged in the direction of this light field, and a main means of detecting the light field scattered by the solid particles in the medium.
• the main detection means comprises a photodetector of the light field scattered by the solid particles in the medium and a counter of these solid particles in this medium, this main detection means being oriented in a direction forming an angle substantially smaller than 30 ° with respect to the direction of said generated light field.
• the invention uses a diffusion angle for better detection of dark particles and small dimensions. This detection angle minimizes the influence of the refractive index of the particles on the measured flux, the measurement then being sensitive only to the size of the grains.
• the main detection means is oriented in a direction forming an angle substantially between 10 ° and 20 ° with respect to the direction of the light field. Measurements at a diffusion angle of less than 10 ° are indeed not optimal because of the contamination by the light source.
• the main detection means is oriented in a direction forming an angle substantially equal to 15 ° with respect to the direction of the light field, which allows an optimum count of the solid particles.
• the analysis system also comprises at least one additional means for detecting the light field scattered by the solid particles in the medium, this complementary detection means comprising a photodetector of the light field scattered by the particles. solid in the middle and a counter of these solid particles in this medium.
• this complementary detection means comprising a photodetector of the light field scattered by the particles. solid in the middle and a counter of these solid particles in this medium.
• a measurement at a second diffusion angle substantially between 40 ° and 140 ° makes the diffused flux very dependent on the index, which gives access more particularly to an estimate of the nature of the particles.
• At least one complementary detection means is oriented in a direction forming an angle of substantially between 40 ° and 140 ° with respect to the direction of the light field.
• this complementary detection means is preferably oriented in a direction forming an angle substantially equal to 100 ° by relation to the direction of the luminous field.
• At least one complementary detection means is oriented in a direction forming an angle substantially equal to 60 ° with respect to the direction of the light field.
• the analysis system also comprises at least one additional means for detecting the light field scattered by the solid particles in the medium, this complementary detection means comprising a photodetector of the light field scattered by the particles. solid in the medium and a counter of these solid particles in this medium, and being oriented in a direction forming an angle substantially equal to 160 ° with respect to the direction of the light field.
• the system according to the invention thus consisting of several detection means arranged at carefully chosen angles, allows simultaneous access to different information on the particles. Indeed, in addition to the concentration of particles provided by the detecting means e between 0 and 20 °, it is possible to differentiate the dry solid particles of hydrated those and only those liquids.
• At least one counter comprises a signal processing block generated by the corresponding detection means.
• a pulse signal generated by the detection means is rejected by the corresponding signal processing block if its duration does not exceed a threshold value depending on the speed of the solid particles in the medium, in order to eliminate false detections due to electronic noise.
• the photodetector and the counter are combined to obtain additional information on the solid particles.
• the photodetector classifies the particles by size categories, while the counter allows the solid particles to be counted by detecting the light pulses received in order to provide the total concentration of particles per unit volume, as well as the concentration per unit volume for particles by size range.
• the analysis system also comprises a polarimetric analysis means of the scattered light field.
• a polarimetric analysis means of the scattered light field By thus combining counting and photometric detection means and polarimetric analysis means, a set of complementary information is obtained that makes it possible to improve the accuracy of the results provided, in particular on the nature of the particles.
• the illumination means comprises a light source consisting of a laser diode.
• the illumination means comprises a diaphragm for selecting a part of the light field, which makes it possible to select a part of the light beam, for example the brightest or the most homogeneous part.
• the trapping means comprises an optical gun and a light trap.
• This trapping means located in the direction of the light field generated by the illumination means, makes it possible to prevent the non-scattered light from strongly interfering with the measurements made by the detection means.
• the optical gun is used to guide the non-diffused light to the light trap so that it can not reach the detection means.
• the analysis system comprises a diffusion chamber comprising a sample of solid particles and arranged to intercept at least a portion of the light field generated by the illumination means.
• This chamber makes it possible to contain a sample of particles to be analyzed, the illumination, trapping and detection means being disposed at openings in the chamber.
• the analysis system also comprises means for driving the sample of solid particles capable of driving the sample. along the diffusion chamber at a predetermined speed.
• These means make it possible to control the speed of the solid particles in the chamber and thus to know the flow rate of the medium to be analyzed.
• the analysis system also comprises means for filtering the solid particles, arranged at the inlet of the diffusion chamber so as to select these solid particles according to their dimensions. It is thus possible to filter a range of particle sizes to be analyzed. Several filter heads are available for this purpose to choose the appropriate range.
• the analysis system according to the invention is devoid of means of collection and focusing of the light scattered by the particles.
• the invention also relates to a method for analyzing solid particles in a medium, comprising an illumination step of generating a light field in the medium, a step of trapping at least a portion of the light field generated and disposed in the direction of this light beam, and a step of detecting the light field scattered by these solid particles in this medium.
• the step of detecting the scattered light field consists in photodetecting the light field scattered by the solid particles in the medium and in counting these solid particles in this medium, this detection step being carried out in a direction forming an angle substantially less than 30 ° with respect to the direction of the generated light field.
• FIG. 1 a diagram of a system for analyzing solid particles in a medium according to a first embodiment of the invention
• FIG. 2 views of a system for analyzing solid particles in a medium.
• FIG. 3 a diagram of a system for analyzing solid particles in a medium according to a second embodiment of the invention
• - Figure 4 a diagram of a counter of an analysis system according to a particular embodiment of the invention.
• a system 1 for analyzing solid particles in a medium 2 comprises an illumination means 3, a light trapping means 4, a means for 5 and a diffusion chamber 6 of the solid particles.
• This system makes it possible to obtain the particle size of the aerosols, that is to say the concentration of particles by size range as a function of their diameter.
• the illumination means 3 comprises a light source 31 and a diaphragm 32. It is arranged so that the light field that it generates is intercepted by the solid particles moving in the diffusion chamber 6, and thus the particles in motion diffract the light.
• the light source 31 may be a laser diode, whose power may typically be of the order of ten or twenty milliwatts, which poses no major risk during a fortuitous and indirect observation of the beam with the eyes.
• the beam is oblong with two Gaussian distributions at 90 ° from each other. It is also possible to consider that it is of almost rectangular shape, with a diameter of 3.5 x 1.5 millimeters.
• the beam then passes through the diffusion chamber 6 with the widest side of the beam vertically, that is to say parallel to the chamber, which provides a transit time of the particles in the longest beam possible.
• the chamber being cylindrical with a diameter of 22 millimeters, the volume of the beam in the chamber is 0.1155 cubic centimeters.
• This light source 31 emits a light beam 30 in a given direction 31.
• the diaphragm 32 is placed in front of the light source 31 so as to select only a part of the light field 30 generated by this source. It may be chosen for example the brightest part or the most homogeneous part of the light beam 30.
• the main detection means 5 comprises a photodetector 52 and a counter 53. This detection means 5 is arranged to be oriented in a direction 51 forming an angle ⁇ equal to 15 ° with respect to the direction 31 of the light field 30 generated. by the light source 31. This angle is justified by the fact that at small angles of diffusion, the effect of the absorbency of the particles has little influence. Beyond 30 °, the absorbing effect becomes significant and the diffused flow falls sharply. Making measurements for a scattering angle between 10 ° and 20 ° then has several advantages.
• the diffused stream is indeed at its maximum. Knowing that the liquid droplets are in a very small quantity for a size beyond 1 micrometer, the diffused flux comes exclusively from solid particles. At higher angles, the flux diffused by the absorbent solid particles becomes very weak and can be confused in some cases with the flux diffused by the large residual liquid particles.
• the photodetector 52 is a photodiode, therefore preferably the collecting surface is the largest possible to observe the entire luminous flux in the diffusion chamber.
• a photodiode collecting surface may typically be 3.6 millimeters square. This photodetector converts the received luminous flux into an electrical signal.
• the counter 53 realizes a detector of electrical pulses converted by the photodiode 52 from the received scattered flux.
• the counting technique for an angle of less than 30 °, and more particularly for an angle of 15 ° makes it possible to obtain, with good accuracy, the concentration of solid particles having a diameter of approximately 1 to 10 microns.
• the intensity of the flux diffused at this angle makes it possible to statistically provide a qualitative estimate of the diameter of the detected particles. It is thus possible to provide the concentration of the particles for example in 3 size ranges: less than 1 micrometer, between 1 and 2.5 micrometers and between 2.5 and 10 micrometers.
• the calibration of the instrument is performed using particles of various natures, from the lightest to the darkest possible. Thus, no use of a theoretical light diffusion calculation model (such as "Mie scattering") is necessary.
• the counter 53 must process the received signal in order to filter it and to distinguish the electric pulses which correspond to a scattered particle with respect to a signal coming from a stray light. This element must take into account the order of magnitude of the luminous flux to be received by the detector.
• the counter 53 comprises for that an analog-digital conversion block
• the flow rate being 1 cubic meter per hour, the passage time of an aerosol in the laser beam of 3.5 mm thick is about 5 meters per second.
• the analog-to-digital converter block 54 therefore operates at a frequency of at least 10 kHz in order to have sufficient sampling to see the width of the pulses as the particle passes through the beam. We should then have several tens of points that will characterize the length and intensity of the pulse.
• the signal processing block
• flow rate, light beam section, and chamber size values are given here by way of example.
• the instrument can operate at lower or higher rates, which simply requires adjustment of the beam size of the light source and optimization of the detection rate.
• the light trapping means 4 comprises an optical gun 41 and a light trap 42. It makes it possible to trap the non-diffused light, that is to say the light of which trajectory is not disturbed by the particles passing through the beam, so that it is not collected by a detector and then comes to disturb the result.
• the optical gun 41 makes it possible to minimize the parasitic light reflections along the path of the light beam.
• the light trap 42 makes it possible to avoid parasitic light reflections by the beam at the end of its path.
• a second optical gun 43 makes it possible to adjust the field of view of the detector to the size of the optical chamber and to limit the range of diffusion angle observed.
• the optical gun 41 is replaced by an optical fiber with a lens.
• the optical gun is nevertheless preferred insofar as the optical fiber requires more precise adjustments and generates a significant loss of flow.
• the diffusion chamber 6 has the shape of a cylindrical tube in which the particles are caused to move while passing through the tube. This chamber is surrounded by a dark chamber to avoid parasitic reflections on the walls of the tube and could disrupt the results of the measurement.
• a pump-type suction device (not visible) makes it possible to drive the particles inside the tube of the chamber 6.
• the flow rate of the air is typically of the order of 1 cubic meter per hour.
• a dimensional selection device of impactor type placed upstream of the diffusion chamber makes it possible to pass only the particles having a certain range of diameters, for example a diameter of less than 10 micrometers.
• FIGs. 2A to 2C show views of an implementation of a system analysis according to the embodiment previously described.
• Figures 2A and 2B show in particular profile views of the system, while Figure 2C shows a sectional top view of this system.
• the analysis system 1 is in the form of an optical module that can be integrated or connected to other modules, in particular electronic modules or display modules.
• the diffusion chamber 6, which acts as a tube for sampling solid particles, is surrounded by a dark chamber 80 which makes it possible to isolate it and thus to protect itself from the effects of parasitic light.
• the complementary detection means 7 similar to the main means 5, but oriented in a direction 71 forming an angle ⁇ substantially equal to 60 ° with respect to the direction 31 of the light field 30.
• This complementary means 7 comprises a detector 72 and a counter 73 similar to those of the main means 5.
• a third optical gun 44 makes it possible to adjust the field of view of the detector to the size of the optical chamber and to limit the range of diffusion angle observed.
• the acquired data are analyzed (levels of signals broadcast at different angles), not from theoretical calculations of light scattering, but from a database obtained previously in the laboratory with this instrument. .
• This database is open and can be supplemented according to new needs identified by the users.
• the diffused flux is almost the same for the angles beyond 60 °, whereas it continues to decrease for less absorbent particles and that it can go back beyond 140 degrees.
• the decay of the diffused flux is greater between 0 ° and 60 ° for absorbent and dark particles than for clear and / or transparent particles. Under these conditions, it is possible to define the ratio of the intensities of the scattered fluxes around 15 ° and from 60 °, this ratio being all the greater as the material considered is absorbent, and even smaller than the material is transparent.
• the counter 53 of the analysis system 1 is now more particularly described with reference to FIG. 4.
• the counter 53 comprises an analog-to-digital conversion block 54 and a signal processing block 55.
• This counter 53 is in particular in charge of ensuring that the detection coming from the pulse detector is very real, as well as of knowing the level. of stray light. It minimizes certain influencing factors, such as electronic noise, humidity, drift over time, and so on.
• the conversion block 54 must therefore perform a sampling of at least 20 kHz to separate the contribution of each particle, which occurs at the signal level in the form of a peak.
• the photodiode 52 at a scattering angle of 15 ° serves to estimate the particle concentration.
• the photodiode 72 at 60 ° is used to estimate the nature of the particles.
• the signal processing unit 55 comprises a multilevel hysteresis comparator 56 and a processing unit 57.
• the photodetector 52 and the processing unit 57 are powered by a power supply 58.
• this block 55 also comprises means for eliminating the contribution of the residual parasitic light, which can change from one instrument to another, but also change over time. Thanks to these means, the background noise of the detector is decreased, substantially improving the noise immunity of the detector / comparator system. Particle detection with more sensitivity than without filter is then possible.
• the 56 N-level hysteresis comparator allows the distinction of several sizes d ⁇ particles from the amplitude of the useful signal from the photodetector.
• the hysteresis function of the comparator 56 makes it possible to overcome the sudden changes in the logic states at the output of the comparator when the form of the useful signal is not continuous in its progression.
• the processing unit 57 of the different detection levels makes it possible to count the number of particles according to their dimensional classification, to validate the measurements by checking the values of the supply voltages of the photodetector, the output voltage level of the detector and the laser supply current and provides measurement results during continuous sampling periods.
• the significant signal is also extracted, which can be mixed with stray light. Indeed, at small scattering angles, the contribution of stray light can become very large. The signal scattered by the particles adds to the stray light. Therefore, in order to detect the smallest particles and to estimate the size of the larger ones, it is important to extract the significant signal.
• the continuous component of the signal representing the parasitic light
• this continuous component is subtracted by filtering to the total signal recorded during the peak of diffusion (it remains only the signal diffused by the particle), and
• the search for the continuous component is carried out regularly, in order to adapt to a possible temporal drift of the parasitic light.
• the stray light may thus represent up to 99.9% of the signal.
• the detection means are combined with a polarimetric analysis means of the scattered light field.
• a polarizing system requiring the use of two scattering angle detectors where the measurements are conducted can be used. It is possible to reconstruct polarimetric light scattering curves for particles in the field of view.

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• 2008
  • 2008-11-18 FR FR0806447A patent/FR2938649B1/fr active Active
• 2009
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