Apparatus and method for measuring particle size of small particles in water by off-axis scanning light scattering

Abstract

The application provides a water body small particle size suspended matter particle size heteroaxial scanning light scattering measurement device and method, which comprises a laser, a beam expander, a small hole, a Fourier lens, a sample cell, a rotating table and a photomultiplier tube; wherein the beam expander, the small hole, the Fourier lens, a diaphragm, the rotating table and the sample cell are sequentially arranged behind the laser; the rotating arm of the rotating table is provided with the photomultiplier tube; the laser emits a single beam of light as an excitation light source of the device, which irradiates the beam expander; the beam expander is used to change the beam diameter and divergence angle of the single beam of light; the beam expander is provided with the small hole and the Fourier lens behind the beam expander, and the small hole is arranged at the common focal point of the beam expander and the Fourier lens; the Fourier lens converges the light beam filtered through the small hole; the Fourier lens is provided with the diaphragm behind the Fourier lens, and the diaphragm is used to filter the edges of the converged light beam, so that the laser beam is more uniform; the diaphragm is provided with the rotating table and the sample cell behind the diaphragm.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of ecological environment science, and particularly relates to a device and method for measuring particle size of small-particle-size suspended matter in water by anisotropic scanning light scattering. BACKGROUND

[0002] In the face of the increasing demand for detection of particle size of water suspended matter, traditional methods such as screening method, sedimentation method and electron microscope method all have great measurement limitations and are only suitable for laboratory measurement and cannot meet the real-time detection requirements in complex sampling sites. As a light scattering detection method, the static light scattering method has the advantages of real-time measurement and high measurement accuracy, can effectively break through the limitation that traditional measurement methods cannot be used for real-time measurement on site, and can quickly measure the particle size distribution of the to-be-measured particle group, so as to analyze the source, physical and chemical properties of the particulate matter, predict the influence of the particulate matter on water, and prepare solutions in advance. The static light scattering method plays an important role in lake water quality management and drinking water safety control.

[0003] The light path structure of the classical static light scattering method has a scattering light measurement blind area. Because there is a difference in refractive index between air and liquid medium, when the laser beam is expanded and filtered and then vertically irradiated on the sample cell, the particle scattering light propagates from the liquid medium to the air and is disturbed by the total reflection of the sample cell, so that part of the particle scattering light cannot normally propagate to the air and cannot be collected by the detector. When pure water is used as the liquid medium, the measurement blind area is 48.8°-131.2°, and the measurement blind area is shown in FIG. 1, taking the propagation direction of the laser main shaft as 0°. However, the scattering light in this angle range contains important characteristic information of small-particle-size particles, and is crucial for measuring the particle size distribution of small-particle-size particles. In order to realize the collection of the scattering light in this range, the British Malvern Company proposed a double light source technology, but the data splicing problem in this technology is still difficult to solve; the American Beckman Coulter Company proposed a "PIDS" technology (polarized light scattering intensity difference), but the measurement range of this technology is narrow and cannot be applied to particle systems with a wide particle size distribution range. Figure 1

[0004] The forward, side and back scattering lights of small-particle-size particles contain particle size related characteristic information, so realizing the collection of the forward, side and back large-angle range scattering lights plays an important role in the measurement of small-particle-size particles.

[0005] ​For forward light scattering signals, a ring-shaped detector or CMOS is usually used for acquisition, but due to the size of the ring-shaped photoelectric detector and the surface area of the CMOS, only forward small-angle (0-15 degrees) scattered light can be detected, and it is difficult to detect forward large-angle and lateral light scattering signals. If the size of the ring-shaped photoelectric detector and the surface area of the CMOS are increased, there are two limiting factors: ① There is no forming flow production line for processing ring-shaped photoelectric detectors, so the cost of processing ordinary ring-shaped photoelectric detectors is already very high, and if the area is further increased, the cost will increase exponentially; ② The measurement lower limit of the largest surface area COMS on the market can only be expanded to 10 μm, and it is impossible to realize the detection of particle scattering light signals in the small particle size range.

[0006] For lateral and backward scattering signals, photoelectric detector arrays are usually used for detection. At present, the commonly used detector arrays have the following two arrangement methods: ① The detectors are placed on a circular arc with the sample cell as the center, and the straight-line distance from the sample cell to the focal point of the Fourier lens as the radius; ② The detectors are placed on a circular arc with the straight-line distance from the sample cell to the focal point of the Fourier lens as the diameter. The first arrangement method requires a large space, resulting in a relatively large device volume. This method sacrifices the system volume to obtain more lateral scattered light, ensuring that the overall system has high measurement accuracy, but it is difficult to measure the weak backward signal. The second arrangement method occupies a smaller volume than the first arrangement method, but it reduces the measurement accuracy and the measurement accuracy is related to the number of detectors, resulting in a complex system structure design and high cost. SUMMARY

[0007] In order to solve the above technical problems, the present application is aimed at the full reflection of the sample cell, the light energy distribution detection problem in a large angle range, and proposes a water small particle size suspended matter particle size heteroaxial scanning light scattering measurement device and method, which adopts a heteroaxial multi-angle scanning measurement mode of oblique incidence single detector, and can meet the particle size measurement demand of small particle size suspended particulate matter. The rotating table drives the photomultiplier tube to rotate around the sample cell to detect the scattered light signal in a large angle; the distance between the detector and the sample cell is shortened, and the lateral and back scattering signal detection sensitivity is improved; the heteroaxial measurement mode is designed, that is, the photomultiplier tube is placed 30 mm below the laser main shaft, the forward light spot interference is avoided, and the forward measurement blind area problem is effectively solved; the sample cell is placed obliquely, the angle between the laser main shaft and the sample cell incident glass surface is kept at 45°, and the total reflection problem caused by the total reflection of the sample cell is effectively solved. The device can detect the effective scattered light signal in the angle range of 0-90° and 145-157° with high sensitivity, and can realize the accurate measurement of the particle size distribution of small particle size particles between 350 nm and 2 microns in combination with the Mie scattering theory model. The device has simple structure, low cost, wide measurement angle range, and adjustable measurement precision, and can effectively solve the measurement blind area of the total reflection of the sample cell and the detection problem of the weak light scattering signal in the lateral and back directions, and is a new means for rapidly measuring the particle size of water small particle size suspended matter.

[0008] The technical scheme of the present application is: a water small particle size suspended matter particle size heteroaxial scanning light scattering measurement device, comprising: a laser, a beam expander, a small hole, a Fourier lens, a sample cell, a rotating table and a photomultiplier tube; wherein the beam expander, the small hole, the Fourier lens, the diaphragm, the rotating table and the sample cell are sequentially arranged behind the laser;

[0009] The rotating arm of the rotating table is provided with a photomultiplier tube; the heteroaxial measurement mode is adopted, the photomultiplier tube is placed at a predetermined distance below the laser main shaft, the forward light spot interference is avoided, the blind area measurement is carried out, the laser emits a single beam as the excitation light source of the device, and irradiates the beam expander;

[0010] The beam expander is used to change the beam diameter and divergence angle of the single beam laser;

[0011] The beam expander is used to change the beam diameter and divergence angle of the single beam laser;

[0012] The small hole and the Fourier lens are placed behind the beam expander, and the small hole is placed at the common focal point of the beam expander and the Fourier lens;

[0013] The Fourier lens converges the light beam filtered through the small hole;

[0014] The rotating table and the sample cell are arranged behind the diaphragm, the rotating table is coaxial with the sample cell in the vertical direction, and the photomultiplier is driven to realize large-angle scanning, further, the small hole is used for filtering high-frequency noise signals generated by dust on the beam expander under laser irradiation, only low-frequency laser signals in space are passed, the detection precision of the system for scattered light of the particles to be measured is improved, and the laser spot size is controlled.

[0015] Further, the sample cell is used for storing the sample of the particles to be measured, the material is quartz glass, the sample cell has a cuboid structure, and the angle between the long axis of the sample cell and the main optical axis is 45 degrees.

[0016] Further, the photomultiplier is placed on a rotating arm with the sample cell as the center, and the scattered light signals of the particles to be measured at different angles are collected by rotation.

[0017] Further, the detector is placed on an arc with the sample cell as the center and the distance from the sample cell to the focal point of the Fourier lens being 1 / 2 of the radius.

[0018] Further, the rotating table and the sample cell are arranged behind the diaphragm, the rotating table is coaxial with the sample cell in the vertical direction, and the photomultiplier is driven to realize rotating scanning.

[0019] According to another aspect of the present application, a water body small particle size suspended particle size heteroaxial scanning light scattering measurement method is provided, comprising the following steps:

[0020] Step 1, the monochromatic light generated by the laser is first diverged by the beam expander lens;

[0021] Step 2, the diverged light beam is filtered through the small hole, and the light beam filtered by the small hole is focused again through the Fourier lens;

[0022] Step 3, the focused light beam is irradiated obliquely into the sample cell;

[0023] Step 4, the scattered light of the particles to be measured scattered from the sample cell is collected by the photomultiplier, wherein the photomultiplier is installed on a rotating arm with the sample cell center as the center, the host computer is used to control the stepping motor to drive the rotating arm to rotate, and the scattered light measurement from the front to the back is realized, and the total angle is 270 degrees;

[0024] Step 5, the angles are combined with the Mie scattering theory model to perform particle size inversion, the Chahine iterative algorithm is used for inversion algorithm, and the particle size distribution of the particles to be measured is obtained.

[0025] Advantages:

[0026] 1. The device proposed in this invention has a simple system structure, a wide measurement angle range, and adjustable measurement accuracy. It not only controls costs but also effectively solves the problems of measurement blind zone and detection of weak side-backward light scattering signals in total internal reflection of the sample cell. It can realize the particle size distribution measurement of small particles in the range of 350nm to 2μm.

[0027] 2. This invention designs a single-detector multi-angle scanning measurement method, selects a photomultiplier tube as the detector, and places the detector on an arc with the sample cell as the center and the radius as half the straight-line distance from the sample cell to the Fourier lens focal point. This shortens the distance between the detector and the sample cell, improves the detection sensitivity of lateral and backscattered signals, and drives the photomultiplier tube to rotate around the sample cell to detect scattered light signals at large angles through a rotating stage.

[0028] 3. This invention designs an off-axis measurement method for the photomultiplier tube, placing the photomultiplier tube 30mm below the laser spindle, or lower, such as... Figure 3 As shown, it avoids interference from the forward light spot, effectively solves the problem of the blind zone in forward measurement, and can obtain the complete light energy distribution in the forward and lateral directions;

[0029] 4. The present invention designs a tilted placement of the sample cell, with the laser spindle and the incident glass surface of the sample cell at an angle of 45°, so that scattered light in the range of 48.8° to 131.2° can be smoothly emitted, effectively solving the problem of total reflection caused by total reflection of the sample cell. Attached Figure Description

[0030] Figure 1 Schematic diagram of the optical path structure and the blind zone for measuring scattered light in the classical static light scattering method;

[0031] Figure 2 Schematic diagram of the off-axis scanning light scattering measurement device for small-diameter suspended solids in water according to the present invention;

[0032] Figure 3 Schematic diagram of off-axis measurement method for photomultiplier tubes;

[0033] Figure 4 : Schematic diagram of effective scattering angle;

[0034] Figure 5 Light scattering energy distribution diagrams; (a) 2μm, (b) 1.5μm, (c) 500nm, (d) 350nm;

[0035] Figure 6 Inversion results of various standards using the off-axis measurement method: (a) 2μm; (b) 1.5μm; (c) 500nm; (d) 350nm. Detailed Implementation

[0036] The technical solutions in the embodiments of the present application will be clearly and completely described with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the protection scope of the present application.

[0037] According to the embodiments of the present application, a water body small particle size suspended particle size heteroaxial scanning light scattering measurement device is provided. According to the online detection requirement of the water body small particle size suspended particle, a heteroaxial multi-angle scanning light scattering measurement device structure is adopted, and the overall structure of the device is as shown in Figure 2

[0038] The device comprises a laser, a beam expander, a small hole, a Fourier lens, a sample cell, a rotating table, a photomultiplier tube (PMT), etc. The measurement of the particle size of the water body small particle size suspended particle is realized through the cooperation of the eight components.

[0039] The laser is provided with a beam expander, a small hole, a Fourier lens, a diaphragm, a rotating table and a sample cell in sequence at the rear; and the rotating arm of the rotating table is provided with a photomultiplier tube.

[0040] The laser emits a single beam of light with a wavelength of 635 nm as the excitation light source of the device, which is irradiated to the beam expander.

[0041] The beam expander is used to change the beam diameter and divergence angle of the single beam of light.

[0042] The beam expander is provided with a small hole and a Fourier lens at the rear, the small hole has a diameter of 5 microns and is placed at the common focal point of the beam expander and the Fourier lens. The small hole has two functions: one is to filter out high-frequency noise signals generated by dust and the like on the beam expander irradiated by the laser, and only pass through low-frequency laser signals in space, thereby improving the detection accuracy of the system on the scattered light of the measured particles; and the other is to control the size of the laser spot.

[0043] The Fourier lens converges (focuses) the light beam filtered through the small hole.

[0044] The Fourier lens is provided with a diaphragm at the rear, and the diaphragm is used to filter the edges of the converged light beam, so that the laser beam is more uniform.

[0045] The diaphragm is provided with a rotating table and a sample cell at the rear; the rotating table and the sample cell are strictly coaxial in the vertical direction, and are used to drive the photomultiplier tube to realize large-angle scanning.

[0046] ​The sample cell is used for storing a sample of a particle group to be measured, is made of quartz glass, has a cuboid structure, and has a size of 40*10*60mm; the long axis of the sample cell is at an angle of 45° with the main optical axis; the initial position of the laser propagation direction is selected as 0°; and the scattered light in the range of 48.8°-131.2° can be smoothly emitted; the sample cell is designed to be placed in a tilted manner, the angle between the main axis of the laser and the incident glass surface of the sample cell is 45°, and the scattered light in the range of 48.8°-131.2° can be smoothly emitted, so that the problem of total reflection caused by total reflection of the sample cell is effectively solved.

[0047] In the embodiment of the present application, the photomultiplier tube (PMT) is placed on the rotating arm with the sample cell as the center, and the scattered light signals of the particles to be measured at different angles are collected by rotation; the position and the inclination angle of the sample cell are fixed, and only the photomultiplier tube is driven by the rotating arm to realize multi-angle measurement. According to the embodiment of the present application, the off-axis measurement mode of the photomultiplier tube is designed, the photomultiplier tube is placed 30mm below the main axis of the laser, as shown in FIG. 1, the forward spot interference is avoided, the problem of the forward measurement blind area is effectively solved, and the complete light energy distribution of the forward and lateral directions can be obtained; according to other embodiments of the present application, the photomultiplier tube can also be placed at a lower position below 30mm; optionally, the photomultiplier tube can also be placed in the range of, for example, 20-40mm, etc., as long as the forward spot interference can be avoided; Figure 3

[0048] According to another embodiment of the present application, the above device is used for off-axis scanning light scattering measurement of small particle size suspended particles in a water body, and the specific method comprises the following steps:

[0049] Step 1, the monochromatic light generated by the laser is first diverged by the beam expander lens;

[0050] Step 2, the diverged light beam is filtered by a small hole, and the light beam filtered by the small hole is focused again by the Fourier lens;

[0051] Step 3, the focused light beam is irradiated to the sample cell at an oblique incidence;

[0052] Step 4, the scattered light of the particles to be measured scattered from the sample cell is collected by the photomultiplier tube, wherein the photomultiplier tube is installed on the rotating arm with the center of the sample cell as the center, the stepping motor is driven by the upper computer to rotate the rotating arm, and the scattered light measurement from the forward to the backward angle is realized, with a total of 270°;

[0053] Step 5, but there is the influence of mechanical structure shielding and mirror reflection light interference, the effective angle is 0°-90° and 203°-214°, as shown in FIG. 2; Figure 4 ​As shown, for ease of graphical representation, the symmetric distribution based on the scattering model equates the angle range of 203° to 214° to 145° to 157°. Finally, these angles are combined with the Mie scattering theory model to perform particle size inversion. The inversion algorithm adopts the Chahine iterative algorithm to obtain the particle size distribution of the particles to be measured.

[0054] According to one embodiment of the present invention, single-particle-size national standard materials (Beijing Coast Hongmeng, polystyrene) with particle sizes of 2 μm (No.: GBW(E)120021, nominal value: 1.96 μm, uncertainty: 0.2 μm), 1.5 μm (No.: GBW(E)120136, nominal value: 1.47 μm, uncertainty: 0.11 μm), 500 nm (No.: GBW 12031, nominal value: 497.4 nm, uncertainty: 4.1 nm), and 350 nm (No.: GBW12010b, nominal value: 351 nm, uncertainty: 4 nm) were used for measurement.

[0055] The light energy distribution measurement results of the device of this invention for four particle sizes are as follows: Figure 5 As shown in (a)-(d), the particle size distribution is as follows: Figure 6 As shown in (a)-(d). In the figure, D... 50 This represents the particle size at which the cumulative volume distribution curve reaches 50%, indicating that particles with a diameter smaller than this value account for 50% of the total volume. 10 D 90 Definition and D 50 similar.

[0056] Using D 10 D 50 and D 90 Characterizing the particle size distribution of the test particles, where D 50 This refers to the particle size that constitutes 50% of the total volume, meaning that particles smaller than this diameter account for 50% of the total volume. 10 D 90 Definition and D 50 Similarly, the particle size distribution of a national single standard reference should exhibit a narrowly broadened single peak, and the broadening of its peak shape is usually expressed by a width factor (D). 90 -D 10 ) / D 50 The smaller the width coefficient, the smaller the peak broadening of the particle being tested.

[0057] The width coefficient of the particle size distribution curve of the 2 μm, 1.5 μm, 500 nm and 350 nm standard objects measured by the traditional laser particle size analyzer is respectively 0.411, 0.480, 0.746 and 0.814; the width coefficient of the particle size distribution curve of the 2 μm, 1.5 μm, 500 nm and 350 nm standard objects measured by the device is respectively 0.216, 0.379, 0.9 and 0.714. Comparing the width coefficients of the two, the particle size distribution curve of the device is better than that of the traditional laser particle size analyzer in general, which shows that the device has an advantage in measuring small particle objects.

[0058] The standard object certificate does not give the particle size D 10 , D 50 , D 90 related information, so the nominal value of the standard object is selected as the reference value, and the comparison results are shown in Table 1. The relative error of the nominal value and the uncertainty of each standard object is respectively 10.2%, 7.5%, 0.8% and 1.1%. The relative error of the D 50 of the 2 μm, 1.5 μm, 500 nm and 350 nm standard objects measured by the device is respectively 5.6%, 1.36%, 0.6% and 0.28% compared with the nominal value. The relative error of the four standard objects is less than the relative error of the uncertainty, which shows that the measurement result of the device is accurate. The relative error of the D 50 of each standard object measured by the traditional laser particle size analyzer is respectively 11.2%, 18.4%, 6.2% and 5.4%, and the comparison shows that the measurement result of the device is better than that of the traditional laser particle size analyzer.

[0059] Table 1 Particle size inversion results of 2 μm, 1.5 μm, 500 nm and 350 nm standard objects

[0060]

[0061] The above results fully show that the oblique incidence single detector off-axis multi-angle scanning device of the application can meet the technical requirements of small particle size suspended particle size detection.

[0062] Although the above describes the specific embodiments of the application for the purpose of facilitating the understanding of the application by those skilled in the art, it should be clear that the application is not limited to the scope of the specific embodiments, and for those skilled in the art, it is obvious that various changes are within the spirit and scope of the application defined and limited by the appended claims, and all the application and creation utilizing the concept of the application are within the scope of protection.

Claims

1. A device for measuring the particle size of small particles of suspended matter in a body of water by heteroaxial scanning light scattering, characterized in that it comprises: The device comprises a laser, a beam expander, a pinhole, a Fourier lens, a sample cell, a rotating table and a photomultiplier, wherein the beam expander, the pinhole, the Fourier lens, the diaphragm, the rotating table and the sample cell are sequentially arranged behind the laser. The rotating arm of the rotating table is provided with the photomultiplier. The photomultiplier is placed at a predetermined distance below the laser main shaft to avoid the interference of the forward light spot and perform blind area measurement. The beam expander is used to change the beam diameter and divergence angle of the single laser beam. The pinhole and the Fourier lens are placed behind the beam expander. The Fourier lens converges the light beam filtered by the pinhole. The diaphragm is arranged behind the Fourier lens to filter the edges of the converged light beam and make the laser beam more uniform. The rotating table and the sample cell are coaxial in the vertical direction to drive the photomultiplier to realize large-angle scanning. The pinhole is used to filter the high-frequency noise signal generated by the dust on the beam expander irradiated by the laser, only pass through the low-frequency laser signal in the space, improve the detection accuracy of the system to the scattered light of the measured particles, and control the size of the laser spot. The sample cell is used to store the sample of the measured particles, and the material is quartz glass. The photomultiplier is placed on the rotating arm with the sample cell as the center to rotate and collect the scattered light signals of the measured particles at different angles. The detector is placed on the circular arc with the sample cell as the center and the distance from the sample cell to the Fourier lens focal point as the radius. The predetermined distance is 30 mm, that is, the photomultiplier is placed 30 mm below the laser main shaft or below 30 mm. The device can detect the effective scattered light signals in the angle range of 0-90° and 145-157°, combine the Mie scattering theory model, and realize accurate measurement of the particle size distribution of small particles with a particle size of 350 nm-2 μm.

2. A method for measuring the particle size of small particles suspended in a water body by using the device according to claim 1, characterized in that, The device comprises the following steps: Step 1: The monochromatic light generated by the laser is first diverged by the beam expander; Step 2: The diverged light beam is filtered by the pinhole, and the filtered light beam is focused by the Fourier lens; Step 3: The focused light beam is obliquely incident on the sample cell; Step 4: The photomultiplier collects the scattered light of the measured particles scattered from the sample cell, wherein the photomultiplier is installed on the rotating arm with the sample cell center as the center, and the host computer controls the stepping motor to drive the rotating arm to rotate to realize the measurement of the scattered light from the forward to the backward angle of 270° in total; Step 5: Combine these angles with the Mie scattering theory model to perform particle size inversion, and use the Chahine iterative algorithm to obtain the particle size distribution of the measured particles.

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