A particle counter using light obstruction method to measure true particle size

Abstract

The present invention discloses a light-blocking particle counter for measuring the true particle size, comprising a measurement area arranged in a fluid flow channel, and an illumination unit and a detector respectively located on both sides of the measurement area, a converging lens and a pinhole diaphragm are arranged in sequence between the measurement area and the detector, the illumination unit outputs a parallel light beam that passes through the measurement area, the converging lens and the pinhole diaphragm and projects onto the detector, and the detector receives the light signal and outputs a corresponding electrical signal. The present invention adds a converging lens and a pinhole diaphragm between the measurement area and the detector, so that light that affects the measurement accuracy cannot enter the detector, thereby eliminating the influence of the optical properties of the particles and the particle position on the measurement value, and significantly improving the accuracy of the measured true size of the particles.

Description

Technical Field

[0001] The invention belongs to the technical field of particle size and concentration detection, and in particular relates to a light obstruction method particle counter for measuring real particle diameters. Background Art

[0002] Light-blocking particle counters are now widely used to measure the size and quantity of micron-sized particles in fluids. Figure 2 . A beam of illumination light is emitted from the light source 1, which becomes a parallel beam after passing through the collimating lens 2 and irradiates the measuring area 4. When there are no particles in the measuring area, the light beam completely passes through the measuring area and is received by the detector 8. When there are particles 5 in the measuring area through which the light beam passes, the light beam is blocked by the particles, and the light flux received by the detector 8 is reduced. If the shape and optical properties of the particles remain unchanged and pass through the light beam along the same path, the reduction in the transmitted light flux is proportional to the square of the particle size. The particle size can be calculated from the reduction ratio of the light flux. Use appropriate methods to let the fluid flow through the measuring area perpendicular to the direction of travel of the illumination light, and continuously measure the photoelectric signal output by the photodetector. Then, when there are no particles in the fluid flowing through the measuring area, the photoelectric signal remains at a constant value; when there are particles passing through the measuring area, the photoelectric signal will decrease, and after the particles pass, the photoelectric signal returns to a constant value. Therefore, whenever a particle flows through the measuring area, the photoelectric signal output by the detector will generate a negative pulse, the pulse height is related to the size of the particle, and each pulse corresponds to a particle size. The measurement continues for a period of time, and the sizes and numbers of all measured particles are counted. At the same time, the volume of the fluid flowing through the measurement area is measured, and the size and number of particles contained in the unit volume (particle concentration) can be calculated. This is the working principle of the existing light obstruction particle counter.

[0003] But in reality, "blocking of light by particles" is essentially the absorption of light by particles (if it exists) and the change in propagation direction caused by the geometric scattering and diffraction of light by particles. Using the traditional light blocking counting optical path, for particles of the same size, changes in the refractive index, absorption coefficient, and distance of the particles relative to the detector will cause changes in the light blocking effect. In the actual measurement of fluid particles, the composition of the particles is often complex, and their optical parameters are unknown. Sometimes the path of the particles through the measurement area is also uncertain. Therefore, the true size of the particles cannot be directly determined from the reduction in light flux. In other words, the true size of the particles measured by the measurement method of the prior art deviates greatly from the actual size. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of large deviation in measuring the true size of particles in the prior art, and to provide a light obstruction method particle counter for measuring the true particle size, the true size of the particles measured can achieve high accuracy.

[0005] The technical solution adopted by the present invention to solve the above technical problems is a light-blocking particle counter for measuring the true particle size, comprising a measuring area arranged in a fluid flow channel and an illumination unit and a detector respectively located on both sides of the measuring area, a converging lens and a pinhole diaphragm are arranged in sequence between the measuring area and the detector, the illumination unit outputs a parallel light beam that passes through the measuring area, the converging lens and the pinhole diaphragm and projects onto the detector, and the detector receives the light signal passing through the measuring area and outputs a corresponding electrical signal. The present invention adds a converging lens and a pinhole diaphragm between the measuring area and the detector, so that light that affects the measurement accuracy cannot enter the detector, thereby eliminating the influence of the optical properties of the particles and the position of the particles on the measured value, and significantly improving the accuracy of the measured true size of the particles. The detector continuously measures the light signal, regardless of whether there are particles in the measuring area. The instrument can directly calculate the size of the particles based on the light flux (power) when there are no particles and the reduction in the light flux when there are particles.

[0006] Preferably, the pinhole diaphragm is arranged on the rear focal plane of the converging lens, and the light-through hole of the pinhole diaphragm is preferably a circular hole, an elliptical hole, a square hole, or a rectangular hole, or other regular shapes. The preferred scheme of the light-through hole of the pinhole diaphragm is a circular hole, or a square hole or a hole of other shapes.

[0007] Preferably, the angle of the aperture radius of the pinhole stop to the optical center of the converging lens is greater than or equal to the diffraction angle corresponding to the lower limit of the particle size measurement set by the instrument, and is less than 20°.

[0008] Preferably, the lighting unit includes a light source and a collimating lens, wherein the collimating lens is disposed between the light source and the measuring area, and the collimating lens converts the light beam emitted by the light source into a parallel light beam. The function of the collimating lens is to convert the divergent light into a parallel light beam. When the requirements are low, a simple single lens such as a plano-convex or bi-convex lens can be used, and a larger relative aperture is allowed; for applications with higher aberration requirements, a double-cemented lens can be used, and the relative aperture is appropriately reduced. The double-cemented lens can provide better imaging quality, but some light flux may be sacrificed. In addition, other lenses such as aspheric lenses and Fresnel lenses can also be used.

[0009] Preferably, a beam limiting aperture is provided on the light exiting side of the collimating lens, which can effectively limit the width of the light beam entering the measurement area and increase the sensitivity to particles by controlling the aperture of the light beam.

[0010] Preferably, the light hole of the beam-limiting aperture is circular or square; in a specific embodiment, the side length or radius of the light hole of the beam-limiting aperture can generally be selected between 0.03 and 10 mm.

[0011] Preferably, the cross-sectional length of the fluid flow channel is 0.1-500 mm, the cross-sectional width of the fluid flow channel is 0.1-400 mm, and the cross-sectional width of the fluid flow channel is not less than the diameter or side length of the light aperture of the beam-limiting aperture. The cross-sectional length of the fluid flow channel refers to the lateral width of the fluid flow channel in the direction of the parallel light beam, and the cross-sectional width of the fluid flow channel refers to the lateral width of the fluid flow channel perpendicular to the direction of the parallel light beam.

[0012] Preferably, the fluid flow direction set in the fluid flow channel is perpendicular to the path of the parallel light beam.

[0013] Preferably, the detector is a single-unit photoelectric detector, which is connected to the information processing unit and is responsible for receiving photoelectric signals, calculating particle diameters and calculating particle size distribution.

[0014] The beneficial effect of the present invention is that it solves the problem of large actual deviation when measuring the true size of particles in the light obstruction method particle counter of the prior art. The benefits brought by the present invention are: (1) According to the ratio of the light attenuation received by the detector to the incident light, the true size of the particle can be directly calculated, which basically eliminates the influence of the optical properties of the particle and the particle position on the measurement value, and the result is more accurate. (2) The instrument does not need to be calibrated with standard particles. (3) The length of the measurement area of ​​the instrument along the optical axis direction can be appropriately lengthened, increasing the cross-sectional area of ​​the fluid flow channel, thereby facilitating the increase of flow rate and improving measurement efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention is further described with the aid of the accompanying drawings, but the accompanying drawings do not constitute any limitation to the present invention. A person skilled in the art can obtain other accompanying drawings based on the following accompanying drawings without any creative work.

[0016] Figure 1 The present invention is a schematic diagram of a light path of a particle counter using a light obstruction method for measuring the true particle size.

[0017] Figure 2 It is a schematic diagram of the optical path of a light-blocking particle counter in the prior art.

[0018] Figure 3 It is a schematic diagram of the geometric scattering of particles under parallel light.

[0019] Figure 4 It is a schematic diagram of an implementation structure of a light obstruction method particle counter for measuring the true particle size of the present invention.

[0020] Figure 5 It is a schematic diagram of the structure when the light hole of the beam-limiting aperture is square and the particle is in the light beam.

[0021] In the figure: 1. light source, 2. collimating lens, 3. beam limiting aperture, 4. measuring area, 5. particles, 6. converging lens, 7. pinhole aperture, 8. detector, 9. fluid flow channel, 10. housing. DETAILED DESCRIPTION

[0022] The principle and specific implementation methods of the technical solution of the present invention are further explained below through examples and in conjunction with the accompanying drawings.

[0023] The principle of the light-blocking particle counter in the prior art has been briefly explained in the background technology, but this widely used light-blocking particle counter has a fundamental defect. Using rigorous optical theory to analyze, the mechanism of "particle blocking light" is as follows: According to the geometrical optics approximation theory of light scattering (reference: Ungut A, Grehan G, Gouesbet G..Comparisons between geometrical optics and Lorenz-Mie theory. Appl Opt. 1981; 20: 2911-8.), under the premise that the particle size is much larger than the wavelength of light, when parallel light is projected onto the particle (assuming it is spherical), two phenomena will occur: one is diffraction, and the diffraction angle is

[0024]

[0025] Where λ is the wavelength of light and D is the particle diameter. The distribution of diffracted light is only related to the particle size and has nothing to do with the optical properties inside the particle.

[0026] The second is geometric scattering, see Appendix Figure 3 , when the incident light propagates from left to right and encounters a particle, it is reflected and refracted on the surface of the particle. The reflected light (called "zero-order reflection") returns to the original fluid medium, and the direction of travel changes; the refracted light enters the interior of the particle and moves to the next surface. If the absorption coefficient of the particle is large enough, the light has decayed before reaching the next surface, and the refracted light cannot be emitted from the particle. When the particle is transparent, the refracted light is reflected and refracted again after reaching the next surface, and the reflected light returns to the interior of the particle (called "first reflection"), and the refracted light enters the fluid medium, called the first refracted light, and its direction of travel in the fluid also changes; the reflected light that occurs for the first time inside the particle travels to the next particle-fluid interface, and internal reflection and refraction occur again, and so on. The change in the propagation direction of light in the fluid medium caused by reflection and refraction on the particle surface is called "geometric scattering" of light. The geometrically scattered light is distributed in a large angular range, and its far-field distribution has nothing to do with the particle size, but is related to the particle refractive index (real part). The larger the refractive index, the larger the distribution range of the scattered light. Quantitative research shows that once refracted light accounts for the vast majority of geometrically scattered light energy, so only once refracted light is considered in the following discussion.

[0027] Under the premise that the particle size is much larger than the wavelength of light, the energy of diffracted light and geometrically scattered light (or light absorbed inside the particle) is equal to the energy of the illumination light projected onto the particle, so the extinction coefficient of large particles is 2, where "extinction" is a professional term in light scattering theory, which means that the incident light changes its propagation direction or / and is absorbed due to the presence of particles. The extinction coefficient refers to the ratio of the amount of extinction to the light energy projected onto the particle. The extinction coefficient is 2, indicating that the light energy scattered and / or absorbed is twice the light energy projected onto the particle, where the extinction coefficients brought by diffraction and geometric scattering (or absorption) are both 1.

[0028] It can be seen that in essence, the so-called "blocking of light by particles" or "light blocking by particles" is caused by the extinction effect of particles, but it is not exactly equal to extinction. In the case of strong absorption of light by particles, the extinction caused by absorption is approximately equal to light blocking; but that is only half of the extinction effect of particles, and the other half is the diffraction of light. If the particles are transparent or incompletely absorbing, the angular distribution of geometric scattered light corresponding to a single refraction will change with the change of the refractive index of the particles. The angular distribution of diffracted light changes with the size of the particles. In the traditional light-blocking particle counter, since the detector 8 has a certain width, when there are particles in the measuring area 4, the detector simultaneously receives the transmitted part of the incident light (the part that is not extinct), part of the geometric scattered light (such as the particles are incompletely absorbed or transparent) and part of the diffracted light. The existence of the latter two makes the "light blocking" measured by the instrument different from the extinction in the strict sense, and this difference changes with the change of the optical properties of the particles, the size of the particles and the distance of the particles relative to the detector. Since the detector 8 can only sense the decrease in the received optical power, that is, the light is partially blocked, and it is unable to distinguish the reason for the light blocking, for particles of the same size, the greater the absorption coefficient of the particle material, the weaker the light received by the detector, and the larger the particle "sees" from the detector; the greater the refractive index of the particle, the larger the distribution range of the geometric scattered light, the smaller the light received by the detector, and the larger the particle "sees" from the detector; when the particle size and optical properties are the same, the farther the particle is from the detector, the weaker the diffracted light received by the detector, and the larger the particle "sees" from the detector. Therefore, for particles of the same size, changes in the particle's refractive index, absorption coefficient, and the distance of the particle relative to the detector will cause changes in the light blocking effect.

[0029] In the actual measurement of particles in fluids, the composition of the particles is often complex, and their optical parameters are unknown. Sometimes the path of the particles through the measurement area is also uncertain. Therefore, the reduction in optical power caused by the particles passing through the measurement area cannot directly determine the true size of the particles. The current practice is: first, standard microspheres of several different particle sizes made of a certain material (usually polystyrene, with a refractive index of 1.59 and an absorption coefficient of 0) are mixed with the fluid medium, and the instrument is used to measure the reduction in optical power caused by particles of various particle sizes passing through the measurement area, and the corresponding relationship between the reduction and the standard particle size is obtained (the so-called "calibration" process). The data of the corresponding relationship is stored in the computer software of the instrument. When measuring particles in the actual fluid, after measuring the height of the optical power drop, according to the corresponding relationship between the diameter of the standard particle and the reduction, the particle size corresponding to the height of the measured optical power drop is calculated by interpolation method, and this particle size is used as the diameter of the actual particle. Obviously, when the actual particle refractive index is different from the refractive index of the standard particle, and more seriously, if the actual particle is an absorption type, or the position of the actual particle in the measurement area is random, it will cause the actual value of the particle size to deviate from the measured value, sometimes even seriously. Therefore, the particle size measured by various light-blocking particle counters is the equivalent value of the height of the light power drop, and not necessarily the true value of the particle size, so its accuracy cannot be guaranteed.

[0030] The present invention proposes an innovative light-blocking particle counter, which can overcome the defects of the above-mentioned traditional instruments and can measure the real and more accurate particle size of the particles. Figure 1 The illumination optical path of the present invention is the same as the conventional optical path, except for the receiving optical path after the measuring area. The light emitted from the measuring area 4 first passes through a converging lens 6, and a small aperture diaphragm 7 is placed on the rear focal plane of the converging lens 6, wherein the aperture radius of the small aperture diaphragm 7 has an opening angle of θ to the optical center of the converging lens. F ,θ F The diffraction angle θ corresponding to the particle size measurement lower limit set by the instrument is greater than or equal to A (Refer to formula (1)), and is much smaller than the geometric scattering range of the particles (for example, less than 20°); the detector 8 is placed behind the pinhole aperture 7, and the specific position and size are based on the principle that the detector can receive all the light energy passing through the pinhole aperture and the detector will not be saturated.

[0031] The working mechanism of the present invention is: no matter where the particle 5 appears in the measurement area 4, the diffracted light and geometric scattered light caused by the particle will fall on the same radius of the pinhole aperture 7 (centered on the center of the pinhole aperture 7) as long as the propagation angle is the same. When the radius of the pinhole aperture 7 is large enough, most of the diffracted light caused by the particle at any position in the measurement area 4 can pass through the pinhole aperture 7 and be projected onto the detector 8, and the light that is not affected by the particle 5 and passes through the measurement area 4 is focused at the center of the pinhole aperture 7, and can naturally pass through the small hole of the pinhole aperture 7 and reach the detector 8. In this way, the light attenuation caused by the particles in the measurement area only comes from the absorption or geometric scattering of the particles. If the particle 5 is transparent, due to the effect of the pinhole aperture 7, most of the geometric scattered light is blocked by the pinhole aperture 7 and cannot reach the detector 8. The light power drop seen by the detector 8 is approximately equal to the light energy projected onto the particle. If the particle 5 is strongly absorbing, then no refracted light can be refracted from the inside of the particle, and the light attenuation reaching the detector 8 is approximately equal to the total illumination light flux projected onto the particle (ignoring the influence of zero-order reflected light). If the particle 5 is partially absorbing, some of the refracted light will pass through the particle, and most of it will be blocked by the aperture 7. Therefore, no matter what the optical properties of the particle are and what the position of the particle in the measurement area is, the drop in the optical power received by the detector 8 is approximately equal to the light flux projected onto the particle surface, that is, the effective extinction coefficient is approximately 1.

[0032] Assume that the intensity of the incident light is uniformly distributed in the cross section of the beam, equal to 1 everywhere, and its cross-sectional area is S I , then the power of the incident light is S I ; If the particle diameter is D, then its cross-sectional area is The effective extinction is approximately If there are no particles in the measurement area, the optical power received by the detector 8 is P o ; When the particle passes through the light beam, the maximum light power received by the detector decreases (when the particle is completely in the illumination beam) by ΔP, then

[0033]

[0034] so

[0035]

[0036] It can be seen that the size of the particles can be directly calculated from the drop in light power, the power of the incident light and the cross-sectional area of ​​the incident light beam.

[0037] The present invention is further illustrated by means of specific examples below.

[0038] A specific embodiment of the present invention is shown in the attached Figure 4As shown, the housing 10 includes a measuring area 4 arranged in a fluid flow channel 9 and an illumination unit and a detector 8 respectively located on both sides of the measuring area. A converging lens 6 and a pinhole aperture 7 are arranged in sequence between the measuring area 4 and the detector. The point light source 1 of the illumination unit is at the front focus of the collimating lens 2. The light emitted from the point light source 1 passes through the collimating lens 2 and becomes a beam of parallel light, which then passes through the beam limiting aperture 3 and becomes the incident light of the measuring area 4. The parallel light beam passes through the measuring area 4, the converging lens 6 and the pinhole aperture 7 and is projected onto the detector 8. The length direction of the fluid flow channel 9 is perpendicular to the path of the parallel light beam. The detector 8 continuously receives the light passing through the measuring area and obtains the light power P when there are no particles in the measuring area. o The light power reduction ΔP when the particle 5 is completely in the measurement area 4 is calculated. Since the cross-sectional area of ​​the incident light is known, the size of the particle is calculated using formula (2). In a specific embodiment, the beam limiting aperture 8 is arranged close to the light exiting side of the collimating lens 2, and the light aperture of the beam limiting aperture 3 is a square with a side length of 0.03 to 10 mm, or a circle with a radius of 0.03 to 10 mm. This does not require creative work for technicians in this field, and other shapes of similar scales can also be used. Figure 5 This is the situation when the light hole is square and the particles are in the light beam. The area between the beam-limiting aperture 3 and the converging lens 6 is the cross-section of the fluid flow channel, and its length can be selected in the range of 0.1 to 500 mm, and the width can be selected in the range of 0.1 to 400 mm. At the same time, the cross-sectional width of the fluid flow channel is not less than the diameter or side length of the light hole of the beam-limiting aperture to ensure that the parallel light beam is within the range of the fluid flow channel. Again, this does not require creative work for technicians in this field. The fluid containing particulate matter flows along the fluid flow channel perpendicular to the direction of the parallel light beam (upward or downward). The area in the fluid flow channel illuminated by the incident light is the measuring area of ​​the instrument. Any particle will "block" the light as long as it enters the measuring area. According to the structure of this embodiment, the blocked light power is approximately equal to the light power projected on the particle by the incident light. The focal length of the converging lens 6 is 0.5 to 500 mm. The pinhole diaphragm 7 is placed on the rear focal plane of the converging lens 6. The diameter of the pinhole of the pinhole diaphragm 7 can be selected to an appropriate value according to the wavelength of the illumination light, the lower limit of the particle size measurement defined by the instrument, and the focal length of the converging lens 6. The opening angle of the pinhole radius of the pinhole diaphragm 7 to the optical center of the lens is θ F , the opening angle is greater than or equal to the diffraction angle corresponding to the lower limit of particle size measurement defined by the instrument, and is less than 20°.

[0039] In some embodiments, the detector 8 is a photoelectric conversion device that linearly converts the received light into an electrical signal, which is then sent to a back-end electrical information processing unit to calculate the particle size and calculate the particle size distribution.

[0040] The ideal goal to be achieved by the present invention is to eliminate the extinction effect caused by particle diffraction, and fully play a role in the extinction of particle absorption and / or geometric scattering, so that the effective extinction coefficient of the particle (hereinafter referred to as "light blocking coefficient") can be treated as 1. However, it is difficult for the actual optical system to fully achieve this goal, because the direction angle of the diffracted light is greater than θ. F The part (its proportion of the total diffracted light is denoted as b) is still blocked by the pinhole aperture 7, which actually enhances the light blocking effect of the particles; F The geometric scattered light passes through the pinhole aperture 7 (the proportion of the geometric scattered light is denoted as γ) and is received by the detector 8, thereby weakening the light blocking effect of the geometric scattering of the particles. Therefore, in this embodiment, the actual light blocking coefficient is

[0041] e=1+b-γ (3)

[0042] When a particle with a diameter of D is completely within the light beam, the light power drops to

[0043] ΔP ′ =ΔPe

[0044] Substituting this into formula (2), the measured particle size is

[0045]

[0046] The relative measurement error is

[0047]

[0048] The following quantitative analysis is made of the proportion b of the diffracted light blocked by the pinhole aperture 7. From the reference (M. Born, E. Wolf. Principles of Optics (Volume 1), Science Press (1978). P.519-520), it can be found that for particles with a diameter of D, the diffraction angle is less than θ F (See Appendix Figure 4 ), that is, the ratio of the light energy that can pass through the pinhole aperture to the total diffracted light energy is

[0049]

[0050] Where J0 and J1 are 0th and 1st order Bessel functions respectively. Therefore, the proportion of light energy blocked by the pinhole aperture 7 to the total light energy is

[0051]

[0052] Let's analyze the value of γ. A transparent spherical particle is equivalent to a double convex lens for paraxial light. Since the aperture angle of the pinhole on the pinhole diaphragm is very small, we only need to consider the case of small scattering angles, that is, we only need to discuss the case of paraxial parallel light incident on the particle. At this time, the focal length of the particle as a lens (reference: Zhang Yimo, ed., Applied Optics (3rd edition), Electronic Industry Press (2008). P.54-55) is

[0053]

[0054] Where n is the refractive index of the particle and r is the particle radius. The first and second principal planes of the sphere coincide with each other and are both located at the center of the sphere (see Appendix Figure 3 ). Therefore, the angle of refraction of the light incident on the particle from a position above the optical axis at a distance h from the optical axis after exiting the particle is

[0055] or

[0056] Therefore, assuming that the intensity of the illumination light is 1, the luminous flux of the refracted light distributed in the range of θ is πh 2 , and the light flux projected onto the particle is πr 2 , so the ratio of the former to the latter is

[0057]

[0058] Therefore, the proportion of geometric scattered light that can pass through the small hole and reach the detector is

[0059]

[0060] Example 1: The focal length of the converging lens 6 is 20 mm, the aperture diameter of the pinhole aperture 7 is 3 mm, the wavelength of the illumination light is 0.63 μm, the lower limit of the particle size measurement set by the instrument is 20 μm, and the fluid measured by the instrument is air, then

[0061]

[0062] Since the smaller the particle, the larger the diffraction angle, the proportion b of the diffracted light blocked by the pinhole aperture 7 takes the maximum value when the particle size is at the measurement lower limit, i.e., 20 μm. From formula (5), we can get

[0063] b=0.089

[0064] Undoubtedly, as the particle size increases, the value of b will decrease and eventually approach 0.

[0065] Let's look at the value of γ. From formula (6), we can see that since the refractive index n>1, the smaller the refractive index, the larger the value of γ, and the larger the measurement error of the light blocking coefficient caused by the small hole. The refractive index of common solid transparent substances is greater than that of water. We might as well use the refractive index of water as the minimum refractive index of solid particles, that is, n=1.33, and substitute it into formula (6) to get

[0066] γ=γ max =0.023

[0067] It can be seen from formula (3) that, since both b and γ are greater than 0, the increase in the particle light blocking coefficient caused by a portion of the diffracted light being blocked by the pinhole aperture 7 and the decrease in the particle light blocking coefficient caused by a portion of the geometric scattered light passing through the pinhole and not being blocked by the pinhole aperture 7 have a mutually offsetting effect. In order to evaluate the error range of this embodiment, only the worst case is considered below.

[0068] Case 1: If the particle is completely absorbing, then γ approaches 0, and the error caused by the partial blocking of the diffracted light by the aperture plays the greatest role. When the particle size is taken at the measurement lower limit set by the instrument, the proportion of diffracted light blocked is the highest, causing the largest error. From formula (3), we get

[0069] e=e max =1+b=1.089

[0070] From formula (4), the relative error of particle size measurement is

[0071]

[0072] This indicates that in this embodiment, the particle size measurement value calculated based on the measured light blocking ratio is larger than the true value, and the relative error is 4.4%.

[0073] Case 2: If the particle is large enough so that the diffracted light completely passes through the small hole, and the particle is completely transparent, the refractive index takes the minimum value. At this time, the error caused by the geometric scattered light passing through the small hole on the small hole diaphragm plays the largest role. From formula (3), we can get

[0074] e=1-γ=1-0.023=0.977

[0075]

[0076] This indicates that in this embodiment, the maximum negative error between the particle size measurement value calculated based on the measured light blocking ratio and the true value is 1.2%.

[0077] In summary, the error range between the measured value and the true value of the particle size measured by the light resistance method in this embodiment is -1.2% to 4.4%, which has reached a very high accuracy in monitoring particles in fluids.

[0078] Embodiment 2: The focal length of the converging lens 6 is 20 mm, the hole diameter of the pinhole aperture 7 is 5 mm, the wavelength of the illumination light is 0.63 μm, the lower limit of the particle size measurement set by the instrument is 20 μm, and the fluid measured by the instrument is air.

[0079] At this time, θ F ≈0.125rad, respectively obtained from equations (5) and (6):

[0080] b=0.049,γ=0.063

[0081] Therefore, the forward error caused by diffraction is

[0082] σ=2.4%

[0083] The negative error caused by geometric scattering is

[0084] σ=-3.2%

[0085] Therefore, the error range between the measured value and the true value of this embodiment is -3.2% to 2.4%. This has reached a very high accuracy in monitoring particles in fluids. Compared with embodiment 1, the one-way deviation is smaller.

[0086] Embodiment 3: The focal length of the converging lens 6 is 20 mm, the hole diameter of the pinhole aperture 7 is 7 mm, the wavelength of the illumination light is 0.63 μm, the lower limit of the particle size measurement set by the instrument is 20 μm, and the fluid measured by the instrument is air.

[0087] At this time, θ F ≈0.173rad, respectively obtained from equations (5) and (6):

[0088] b=0.038,γ=0.122

[0089] Therefore, the forward error caused by diffraction is

[0090] σ=1.2%

[0091] The negative error caused by geometric scattering is

[0092] σ=-9.1%

[0093] Therefore, the error range between the measured value and the true value of this embodiment is -9.1% to 1.2%. It can be seen that the error caused by geometric scattering is close to 10%, but this is an acceptable range in monitoring particles in fluids.

[0094] In the above discussion, only the case where the particle passes completely through the measurement area is considered (see Appendix Figure 5). In fact, some particles in the flow channel may not touch the incident light beam at all, while some particles may touch the incident light beam partially. The particle measurement errors caused by these two situations can be solved by subsequent calculation software, so they are not discussed in this application.

[0095] The advantages of adopting the structure of the present invention are: (1) The true size of the particle can be directly calculated based on the light attenuation received by the detector, which basically eliminates the influence of the optical properties of the particle and the particle position on the measurement value, and the result is more accurate. (2) The instrument does not need to be calibrated with standard particles. (3) The length of the instrument's measurement area along the optical axis can be appropriately lengthened, increasing the cross-sectional area of ​​the fluid flow channel, thereby facilitating the increase of flow rate and improving measurement efficiency.

[0096] It should be noted that the present invention is a further improvement on the light obstruction particle counter of the prior art. Therefore, the technology related to the light obstruction particle counter of the prior art may not be described in detail, and the technology, method and structure in the light obstruction particle counter of the prior art should be regarded as part of the specification.

[0097] In addition to the above-mentioned embodiments, under the guidance of the principles and data disclosed in the claims and specification of the present invention, the technical features or technical data of the present invention can be reselected and combined, and those skilled in the art can easily form new embodiments by changing things such as the lens structure, focal length, aperture, etc. These are all achievable by those skilled in the art without the need for creative work. Therefore, these embodiments that are not described in detail in the present invention should also be regarded as specific embodiments of the present invention and are within the protection scope of the present invention.

Claims

1. A particle counter using a light obstruction method for measuring true particle size, characterized in that: The invention comprises a measuring area arranged in a fluid flow channel and an illumination unit and a detector respectively located on both sides of the measuring area. A converging lens and a pinhole diaphragm are arranged in sequence between the measuring area and the detector. The illumination unit outputs a parallel light beam that passes through the measuring area, the converging lens and the pinhole diaphragm and projects onto the detector. The detector receives the light signal passing through the measuring area and outputs a corresponding electrical signal.

2. The light obstruction method particle counter for measuring true particle size according to claim 1, characterized in that: The pinhole diaphragm is arranged on the rear focal plane of the converging lens, and the light-through hole of the pinhole diaphragm is preferably a circular hole, an elliptical hole, a square hole, a rectangular hole or a polygonal hole.

3. The light obstruction method particle counter for measuring true particle size according to claim 1, characterized in that: The angle of the aperture radius of the pinhole aperture to the optical center of the converging lens is greater than or equal to the diffraction angle corresponding to the lower limit of the particle size measurement set by the instrument, and is less than 20°.

4. The light obstruction particle counter for measuring true particle size according to claim 1, characterized in that: The lighting unit comprises a light source and a collimating lens. The collimating lens is arranged between the light source and the measuring area. The collimating lens converts the light beam emitted by the light source into a parallel light beam.

5. The light obstruction method particle counter for measuring true particle size according to claim 4, characterized in that: A beam-limiting aperture is provided on the light-emitting side of the collimating lens.

6. The light obstruction method particle counter for measuring true particle size according to claim 5, characterized in that: The light aperture of the beam-limiting aperture is circular or square.

7. The light obstruction method particle counter for measuring true particle size according to claim 1, characterized in that: The cross-sectional length of the fluid flow channel is 0.1-500 mm, the cross-sectional width of the fluid flow channel is 0.1-400 mm, and the cross-sectional width of the fluid flow channel is not less than the diameter or side length of the light hole of the beam-limiting aperture.

8. The light obstruction method particle counter for measuring true particle size according to claim 1, characterized in that: The fluid flow direction set by the fluid flow channel is perpendicular to the path of the parallel light beam.

9. The light obstruction method particle counter for measuring true particle size according to any one of claims 1 to 8, characterized in that: The detector is a single-unit photoelectric detector, which is connected to the information processing unit. The detector is responsible for receiving the photoelectric signal, and the subsequent information processing unit calculates the particle size and calculates the particle size distribution.

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