Apparatus for measuring particles in liquid

Abstract: An apparatus for measuring particles in liquid in which a laser beam (3) is projected into the liquid containing particles (2) to be measured which are caused to flow into a measurement zone (34) in a measuring cell (1), and light scattered from the particles passing through the measurement zone is received by the light receiving means (20, 21, 22) to measure characteristics of the particles. The light receiving means are disposed with its optical axis nearly perpendicular to the axis of projection of laser light. The arrangement is such that the flow of fluid in the measurement zone (34) approximates laminar flow in a short distance within the measurement zone. This arrangement assures a stable and precise measurement of the particles.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an apparatus for measuring particles in liquid, and more particularly to an apparatus for measuring particles in liquid in which a laser beam is projected into a flowing liquid and light scattered from particles present in the liquid is evaluated to measure particle size, number of particles and other characteristics of the particles. Description of the Prior Art The prior art includes techniques in which light is projected into a measurement zone and the amount of transmitted light, scattering characteristics and the like are measured to determine the size, number and other characteristics of particles in the zone. For example, while this technique is employed in monitoring particulate impurities in pure water, since such particles in pure water tend to be small and present in low concentrations, the particles are difficult to monitor. For this reason, in order to increase the intensity of light scattered from particles, an incident beam from a laser light source or the like is conventionally collected in a small zone, creating a measurement zone of high light intensity in the measurement cell, and then the scattered light from particles passing through this zone is received (see, for example USP No. 4,830,494). In a particle measurement apparatus which projects laser light at particles and analyzes light scattered from the particles, the direction from which light scattered from the particles is received is important as is the shape of the measurement section through which the particles pass in the measurement cell. Because the distribution of laser light intensity exhibits Gaussian distribution, conventional apparatus for measuring particles in liquid employs a "90-degree lateral light receiving system" in which light scattered from particles illuminated with laser light is received in a direction nearly perpendicular to the axis of projection of the laser beam in order to improve the efficiency of particle measurement. This system employs an arrangement in which the direction of particle travel is collinear with the optical axis of a light receiving lens or in which the direction of particle travel lies in the plane defined by the optical axis of the laser and the optical axis of the light receiving lens and crosses the optical axis of the light receiving lens at an angle in the range of 20-70 degrees. Slits are disposed on the imaging surface of the light receiving lens so that scattered light from the particles which pass through areas of high light intensity is selectively received to obtain good particle size resolution. Furthermore, in order to force particles to travel in a specific direction through measurement cells employing this sort of system, there are methods in which sheath flow is employed to establish cylindrical flow through the measurement zone, e.g. GB-A-2167880 and methods in which a stirrer is used with a cylindrical wall to establish a rotating flow, e.g. US-A-4830494 In the above method using sheath flow, the high rate of flow required to establish the cylindrical flow becomes a problem. The method using rotating flow, on the other hand, disadvantageously has the lack of a way of verifying that liquid in the cylindrical section is adequately circulated through the inlet and outlet pipes in the cell with the cylindrical section used to establish the rotating flow, and has the possibility that the same particle would repeatedly pass through the measurement zone. As described above, when forcing particles to travel in a certain direction through the measurement zone, if the flow of particles assumes a turbulent flow state rather than a laminar flow state, the direction of travel of particles cannot be set to a direction along the optical axis of the light receiving lens or to a direction parallel to the plane defined by the optical axis of the laser and the optical axis of the light receiving lens, so that good particle resolution cannot be obtained. JP-A-6318243 discloses a device which includes the features of the preamble of claim 1. This device is configured to provide a uniform flow, but is not considered satisfactorily to solve the above discussed problems. SUMMARY OF THE INVENTION. Therefore, an object of the present invention is to solve these problems and provide an apparatus for measuring particles in liquid which can effectively receive scattered light from particles and perform accurate measurement of particles. According to the invention, there is provided apparatus for measuring particles in liquid, comprising a measurement cell provided with a measurement zone through which liquid can be directed, means for generating a laser beam which can be directed into a flow of liquid in the measurement zone, and light receiving means for receiving light scattered by particles in a flow of liquid passing through the measurement zone when a laser beam is directed thereinto, and for measuring characteristics of the particles; wherein the light receiving means are disposed with their optical axis nearly perpendicular to the axis of projection of the laser beam for laterally receiving light scattered from the particles, and the measuring cell causes the liquid containing the particles to flow into the measurement zone in a direction along the optical axis of the light receiving means; characterised in that means are provided in the measuring cell for forming laminar flow in the measurement zone, which means comprises: an inlet path disposed symmetrically with respect to the optical axis of the light receiving means and having a first portion allowing the fluid to flow into the measurement cell, a second portion communicating with the first portion and extending generally parallel to the optical axis of the light receiving means and having a structure comprising a central block which inhibits fluid flow, and a peripheral fluid path which allows fluid to flow; and a third portion which communicates with the second portion and extends along the optical axis of the light receiving means through the measurement zone. Preferably, the first portion comprises inlet pipes disposed symmetrically with respect to the optical axis of the light-receiving means and each having a portion for causing the liquid to flow into the measurement cell at an angle nearly perpendicular to the optical axis of the light receiving means. The diameter of the second portion is preferably set to be equal to or greater than that of the cylindrical portion, and the diameter of the first portion of the inlet pipes is set to be smaller than that of the third portion. The measuring cell further includes an outlet path communicating with the third portion for discharging the fluid to the outside. The outlet path and the inlet path are disposed symmetrically with respect to the optical axis of projection of the laser beam. In the measurement cell used in the invention, Teflon ("Teflon is a registered trade mark) or other corrosion-resistant material is used on the areas in contact with the liquid, and the inlet and outlet pipes for the liquid are disposed at points which are symmetrical with respect to the optical axis of the light receiving lens so that the liquid flows in the direction of the optical axis of the light receiving lens. A sharp increase in the sectional area of flow near the inlet pipe tends to cause turbulent flow. To prevent this, a double cylindrical section is used to smooth out the flow of liquid flowing in the direction normal to the optical axis of the light receiving lens, and a cylindrical section is also used to force particles passing through the measurement zone to travel at a fixed speed. Both the sections are formed so that there is no sharp change in sectional area between them. Furthermore, in the measurement cell of the invention, entrance and exit windows for the laser beam are formed on the sides of the cylindrical section along with a window for receiving scattered light from particles in the liquid and a transparent window formed in a position opposite to the light receiving window to prevent reflections from the wall surface. With a measurement cell of such configuration, particles in the measurement zone of the measurement cell flow in a laminar flow pattern, so stability of the particle flow in the direction of travel may be attained in a short distance. BRIEF DESCRIPTION OF THE DRAWINGS The objects and features of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: Figure 1 is a perspective view showing an arrangement of an apparatus of the invention for measuring particles in liquid; Figure 2 is an enlarged perspective view of a portion of the measuring cell of the apparatus in Figure 1; and Figure 3 is a top view of the measuring cell of the same apparatus as a cross-section along a plane containing the axis of projection of the laser and the optical axis along which scattered light is received. DETROIT DESCRIPTION OF PREFERRED EMBODIMENTS The invention will now be described in detail based on a Preferred Embodiment. In Figure 1, reference number 1 denotes a measurement cell into which a sample such as pure water or the like containing particles 2 to be measured is caused to flow as described hereafter. An incident laser beam 3 obtained from a laser light source 10 constituted by a helium-neon laser or the like is projected through a laser projection optical system onto the particles 2. The laser projection optical system is constituted by a beam expander 12 which expands the laser beam coming from the laser light source 10 and a condenser 13. The laser beam 3 from the condenser 13 is focused through an entrance window 4 formed in the measurement cell 1 at a focal point 34 which forms the center of the measurement zone. Then the laser beam exits from an exit window 5 and is absorbed by a light-absorbing trap 16. As described hereafter, a light receiving lens 20 disposed in a direction nearly perpendicular to the axis of projection of the laser is used to form an image of laser light scattered laterally from the particles 2 on a mask 21. A slit 21a is formed on the mask 21 so that scattered light limited by the slit 21a reaches a photomultiplier tube 22. The signal obtained from the photomultiplier tube 22 is provided as input to a signal processor (not shown) in which well-known methods of photon-counting are used to measure particle characteristics such as the particle distribution, particle size and the like. In addition, as shown in detail on Figures 2 and 3, the measurement cell 1 is provided with a cylindrical section 33 which acts as a measurement zone centered around focal point 34. Disposed on the periphery of this cylindrical section 33 are an entrance window 4 into which the laser beam 3 from the laser light source 10 enters, an exit window 5 through which the laser beam 3 exits, and a light receiving window 7 which receives scattered light 6 from particles 2 passing through the focal point 34. Furthermore, an anti-reflection window 8 is provided at a location opposite the light receiving window 7 on the other side of the focal point 34 to reduce the amount of reflected light which enters the light receiving optical system. Behind the exit window 5 is disposed the light-absorbing trap 16 which absorbs the laser beam as described above with reference to Figure 1. In addition, the cylindrical section 33 of the measurement cell 1 is provided with inlet pipes 30 through which a liquid containing particles 2 flows into the measurement cell 1. These inlet pipes 30, as shown in detail on Figure 3, enter the measurement cell 1 from each side at an angle nearly perpendicular to the optical axis of the light receiving lens, then bend to lie nearly parallel to the optical axis of the light receiving lens and connect to the cylindrical section 33 at the light receiving window 7. Thus a double cylindrical section 32 is formed in the liquid inlet section of the measurement cell, so that a sample liquid flows in through this double cylindrical section 32. Furthermore, outlet pipes 31 which discharge the sample liquid are disposed such that their locations and those of the inlet pipes 30 are symmetrical with respect to the axis of projection of the laser beam, and these outlet pipes 31 are formed in the same manner as the inlet pipes 30 so that a double cylindrical section 32 is also formed in the liquid outlet section. These double cylindrical sections 32 are disposed concentrically with the cylindrical section 33, and the double cylindrical section 32 and cylindrical section 33 form conical surfaces centered about an extension of the center line between them. Therefore, in the measurement cell used in the invention, both the liquid inlets 30 are disposed symmetrically with respect to the optical axis of the light receiving lens so that the liquid will flow in the direction of the optical axis of the light receiving lens, and thus the direction of flow of particles is nearly parallel to the optical axis of the optical system which receives scattered light from the particles. Note that in the measurement cell used in this preferred embodiment, Teflon (Teflon is a registered trade mark) or other corrosion-resistant material is used on the areas in contact with the liquid. Now the operation of an apparatus of such configuration will be described. A laser beam from the laser light source 10 passes through the beam expander 12 and condenser 13 to enter the entrance window 4 of the measurement cell and is focused at the focal point 34 in the measurement zone. At the same time, a sample liquid containing particles flows through the inlet pipes 30 and double cylindrical section 32 into the cylindrical section 33, or measurement zone, of the measurement cell 1. Since the two inlet pipes 30 are disposed symmetrically about and lie in directions normal to the double cylindrical section 32, the sample liquid is mixed to homogeneity in the double cylindrical section 32. In this case, if the sectional area 2A₁ of the inlet pipes 30, sectional area A₂ of the double cylindrical section 32, and sectional area A₃ of the cylindrical section 33 are related by the relation 2A₁ << A₂ >= A₃ , the change in sectional area between the double cylindrical section and the cylindrical section is reduced so that the sample liquid whose flow is smoothed by a ring-shaped cross section in the double cylindrical section will not assume a turbulent flow state when flowing into the cylindrical section 33, but rather becomes a cylindrical flow of virtually unchanged sectional area just before entering the measurement zone and thus passes through the measurement zone in laminar flow at a constant speed nearly perpendicular to the optical axis of the laser. Laser light is scattered by particles 2 passing through the measurement zone in this manner. The light receiving lens 20 forms an image on the mask 21 of that portion of the scattered light 6 which is scattered laterally in a direction perpendicular to the optical axis of the laser. The scattered light 6 then reaches the photomultiplier tube 22. The signal obtained from the photomultiplier tube 22 is provided as input to a signal processor in which well-known methods of photon-counting are used to measure particle characteristics such as the particle distribution, particle size and the like. After the sample liquid passes through the measurement zone, it is discharged to the outside through the outlet pipes 31. These outlet pipes 31 are disposed to the rear of the cylindrical section so that the effect of wake is ameliorated. Care is taken to dispose the two outlet pipes 31 in the top portion of the double cylindrical section so that any air bubbles trapped within the cell are swiftly discharged, preventing air from being retained in the cell. Note that in Figures 1, 2 and 3, the symbol D indicates the direction of flow. As described above, in the present invention, a light receiving means for receiving laterally scattered light is disposed in a direction nearly perpendicular to the optical axis of the laser, and the liquid containing particles is forced to flow in a direction nearly parallel to the optical axis of the scattered light receiving lens, while the liquid flow can reliably form a stable laminar flow in a short distance in the measurement zone, so that particle size distribution measurements can be made under optimal conditions.

Claims: Apparatus for measuring particles (2) in liquid, comprising a measurement cell (1) provided with a measurement zone through which liquid can be directed, means (10) for generating a laser beam (3) which can be directed into a flow of liquid in the measurement zone, and light receiving means (20,21,22) for receiving light scattered by particles (2) in a flow of liquid passing through the measurement zone when a laser beam (3) is directed thereinto, and for measuring characteristics of the particles (2); wherein the light receiving means (20,21,22) are disposed with their optical axis nearly perpendicular to the axis of projection of the laser beam for laterally receiving light scattered from the particles (2) and the measuring cell (1) causes the liquid containing the particles to flow into the measurement zone in a direction along the optical axis of the light receiving means; characterised in that means (30,32,33) are provided in the measuring cell for forming laminar flow in the measurement zone (34), which means (30,32,33) comprises: an inlet path disposed symmetrically with respect to the optical axis of the light receiving means (20) and having a first portion (30,31) for allowing the fluid to flow into the measurement cell; a second portion (32) communicating with the first portion and extending generally parallel to the optical axis of the light receiving means and having a structure comprising a central block which inhibits fluid flow, and a peripheral fluid path which allows fluid to flow; and a third portion (33) which communicates with the second portion and extends along the optical axis of the light receiving means through the measurement zone. An apparatus as set forth in claim 1, wherein the first portion comprises pipes (3) disposed symmetrically with respect to the optical axis of light receiving means (20) and each having a portion (30) for causing the liquid to flow into the measurement cell at an angle nearly perpendicular to the optical axis of the light receiving means. An apparatus as set forth in claim 2, wherein the diameter (A2) of the second portion (32) is set to be equal to or greater than that (A3) of the third portion (33). An apparatus as set forth in claim 2 or 3, wherein the diameter (A1) of the first portion (30) of the inlet pipes is set to be smaller than that (A3) of the third portion (33). An apparatus as set forth in any preceding claim, wherein the measuring cell further comprises an outlet path (31,32) communicating with the third portion (33) for discharging the fluid to the outside, the outlet path and inlet path being disposed symmetrically with respect to the optical axis of projection of the light receiving means.

Inventors: WAKAMURA HITOSHI, WAKAMURA, HITOSHI
Applicants: KOWA CO, KOWA COMPANY LTD.
Application year: 1990
Main classification: G01N15
IPC codes: G01N21, G01N15