Abstract:
A concentration measuring method and an apparatus measures the concentration of an objective solvent in a mixed solution by flowing a mixed solution through a tube, transmitting light through the mixed solution as it flows through the tubing and measuring the quantity of light transmitted through the fluid. The concentration of the objective solvent in the mixed solution is measured on the basis of either the quantity of light transmitted, the diameter of a light beam received, or a point of convergence of the light transmitted through the mixed solution. These factors being dependent on the concentration of the objective solvent in the mixed solution.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for measuring the concentration of an objective solvent in a mixed solution. The present invention may be applied for process control in the chemical industry, the food industry and the medical and pharmaceutical industries, environmental measurement, medical diagnosis, etc. 
     2. Description of the Prior Art 
     Conventional methods of measuring the concentration of an objective solvent in a mixed solution are classified according to measurement of (1) an absorption spectrum, (2) the absorbency of the solution by coloring the objective solvent with a color coupler, (3) the electrical conductivity of the mixed solution, (4) the index of refraction of the mixed solution, and (5) the specific gravity of the mixed solution. 
     Methods (1), (3), (4), and (5) are not suitable for continuous measurement. Moreover method (1) needs a large measuring apparatus, method (2) is applicable only to the measurement of the concentration of a colorable objective solvent, and method (3) applies only to the measurement of the concentration of an objective solvent in an electrically conductive mixed solution. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is a concentration measuring method and apparatus based on the fact that light traveling from one end to the other end of a tube filled with a flowing mixed solution converges on the axis of the tube. 
     Another object of the present invention is a concentration measuring method and apparatus applicable to continuous measurement in a wide range of applications which is not susceptible to electrical noise, variations of the luminous intensity of the light source, and accidental contamination by small bubbles, etc. 
     Yet another object of the present invention is a concentration measuring method and apparatus that is useful in remote process control. 
     Still another object of the present invention is a concentration measuring apparatus that is simple, inexpensive and of compact construction. 
     These and other objects of the present invention are accomplished by a concentration measuring method comprising the steps of flowing a mixed solution in a tube, transmitting light through one end of the tube, through the mixed solution in the tube, and out of the other end of the tube, as a light beam; detecting the quantity of light received, the diameter of the light beam, or a point of convergence of the light beam; and measuring the concentration of the objective solvent in the mixed solution on the basis that the quantity of light received, the diameter of the light beam received, or the point of convergence of the light beam. These factors are dependent on the concentration of the objective solvent in the mixed solution. 
     The above and other objects of the present invention are also achieved by a concentration measuring apparatus comprising a tube through which a mixed solution flows, feed means for feeding the mixed solution into a first end of the tube, discharge means for discharging the mixed solution from the other end of the tube, light emitting means disposed at the first end of the tube for emitting a light beam into the tube, and light detecting means at the second end of the tube for measuring the quantity of light received as the light beam, the diameter of the light beam received, or a point of convergence of the light beam through the mixed solution flowing in the tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The manner by which the above and other objects, features, and advantages of the present invention are accomplished will be fully apparent from the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic illustration of a concentration measuring apparatus according to a first embodiment of the present invention; 
     FIG. 2 is an explanatory illustration of the mode of convergence of light transmitted when ethanol is used as a medium in the first embodiment; 
     FIG. 3 is an explanatory illustration of the mode of convergence of light transmitted when water is used as a medium in the first embodiment; 
     FIG. 4 is a graph showing the relation of the quantity of light received to flow rate for water and ethanol in the first embodiment; 
     FIG. 5 is a graph showing the relation of the quantity of light received to ethanol concentration in the first embodiment; 
     FIG. 6 is a sectional view of a module included in a concentration measuring apparatus in a second embodiment; 
     FIG. 7 is a diagrammatic illustration of means for detecting the point of the convergence of light in the second embodiment; 
     FIG. 8 is a diagrammatic illustration of means for detecting the point of convergence of light employing reflecting mirrors in the second embodiment; 
     FIG. 9 is an explanatory illustration of a mode in which a light emitting device is joined directly to an outside plate; 
     FIG. 10 is an explanatory illustration of a mode in which an end of an optical fiber is disposed opposite to a glass window; 
     FIG. 11 is an explanatory illustration of another mode in which an end of the optical fiber is disposed opposite to a glass window; 
     FIG. 12 is an explanatory illustration of a mode in which an end of an optical fiber is intruded into an outside plate; and 
     FIG. 13 is an explanatory illustration of another mode in which an end of an optical fiber is intruded into an outside plate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     When light is sent into one end of a tube filled with a flowing mixed solution, and is transmitted to the other end of the tube, the light converges on the axis of the tube even if the tube is not heated or cooled. The mode of convergence is dependent on the kind of solvent and mixing ratio of the mixed solution. Although the cause of the convergence of the light has not been fully elucidated, it may be empirically proven that the light converges when the mixed solution flows through the tube under prescribed conditions. 
     When the quantity of light transmitted through the mixed solution is measured by receiving the light on a light receiving area, for example, the area of the end face of an optical fiber, which is smaller than a minimum area of a light spot at a fixed position, the quantity of light or the point of convergence of the light is dependent on the concentration of the objective solvent in the mixed solution. Further, the diameter of the light beam spotted on the screen may be measured, the beam diameter being dependent on the concentration of the objective solvent in the mixed solution. 
     Accordingly, when the relation between the concentration of an objective solvent and the quantity of light received the point of convergence of the light beam, or the diameter of the light beam is determined beforehand for mixed solutions of various mixing ratios, the concentration of solvent can be determined by the measurement of the quantity of light received, etc. 
     According to the concentration measuring method and apparatus of the present invention, the quantity of light received exhibits a satisfactory relation, particularly a linear relation, with the concentration of an objective solvent in a mixed solution over a wide range of concentration. A combination of a simple optical system and a tube is used, and the concentration of an objective solvent of a mixed solution can be continuously measured with high reliability. Further, the concentration measuring method and apparatus of the present invention is not susceptible to pH and electric noise, and is able to measure the concentration stably. Furthermore, the concentration measuring method and apparatus of the present invention may be applicable to the measurement of substances which do not generate heat or do not absorb heat. Accordingly, the present invention has a wide range of applications. Moreover, measurement reliability does not depend on variations of the luminous intensity of the light source, accidental contamination by small bubbles, etc. Neither a highly accurate light receiving device nor a stable light source is required, so that a simple, inexpensive, and easy to construct and use system may be achieved. 
     A concentration measuring apparatus according to the first embodiment of the present invention may be used for measuring the ethanol concentration of a water-ethanol mixed solution. As shown in FIG. 1, the concentration measuring apparatus comprises a light transmission module M, a He-Ne laser 4, a light quantity measuring unit 5, and a constant-temperature tank 9. The module M comprises a tube 1, an inlet unit 2 as feed means and an outlet unit 3 as discharge means. 
     The tube 1 may be a copper tube with an inside diameter of 2 mm, an outside diameter of 3 mm, and a length of 120 mm. The inlet unit 2 may comprise a base 21 disposed inside, a glass window (a light transmission window) 23 disposed outside, and a cylinder 22. The cylinder 22 is disposed between the base 21 and the glass window 23 and is attached to the base 21. An inlet port 24 may be attached to the side parts of the cylinder 22. The inlet unit 2 is detachably joined to one end of the tube 1. 
     The outlet unit 3 may comprise a base 31 disposed inside, a glass window (a light transmission window) 33 disposed outside, and a cylinder 32. The cylinder 32 is disposed between the base 31 and the glass window 33 and is attached to the base 31. An outlet port 34 may be attached to the side parts of the cylinder 32. The outlet unit 3 is detachably joined to the other end of the tube 1. 
     The bases 21 and 31 form parts of the constant-temperature tank 9. Further, the bases may separate from the end walls of the constant-temperature tank 9. A mixed solution (A) is fed through the inlet port 24 and is discharged through the outlet port 34. Further, the mixed solution (A) may be fed (or discharged) in the reverse direction. The junction of the tube 1 and the inlet unit 2, and that of the tube 1 and the outlet unit 3 are sealed with appropriate sealing members. 
     The He-Ne laser 4 is disposed opposite to the glass window 23 of the inlet unit 2, and a light quantity measuring unit 5 is disposed opposite to the glass window 33 of the outlet unit 3. The positions of the He-Ne laser 4 and the light quantity measuring unit 5 may be interchanged. According to the light quantity measuring unit 5, a screen 6 is disposed in a plane perpendicular to the axis of the tube 1 at a predetermined distance, e.g., about 2000 mm, from the glass window 33. An optical fiber 52 having a core diameter of 50 μm is disposed with its extremity substantially at the center of the screen 6, and an optical power meter 51 ([Anritsu ML-910B], Anritsu Co. Ltd.), is connected to the optical fiber 52. Further, the extremity of the optical fiber 52 may be inserted in the glass window 33 (or non-light transmission window). 
     The He-Ne laser 4 (e.g., wavelength: 543 nm and output power 1 mW) serves as a light source. The He-Ne laser 4 produces a laser beam and projects the laser beam through the glass window 23 into the tube 1 substantially in parallel to the axis of the tube 1. The laser beam transmitted through the mixed solution (A) flowing in the tube 1 is received by the optical power meter 51 through the optical fiber 52. The optical power meter 51 measures the quantity of the laser beam received by the optical fiber 52. The majority of the tube 1 is disposed within the constant-temperature tank 9 (e.g., 28° C.). The mixed solution (A) is contained in another constant-temperature tank 11 (e.g., 28° C.) and is fed through the inlet port 24 into the tube 1 by a pump 10. 
     The relations between flow rate and the quantity of light received, as predetermine individually for ethanol (guaranteed reagent) and pure water through experiments, are shown in FIG. 4. Further, the mode of convergence of light through ethanol is shown in FIG. 2, and the mode of convergence of light through water is shown in FIG. 3. As is obvious from FIG. 4, the quantity of light received varies with flow rate, and the relations for ethanol and pure water are different from each other. A desirable flow rate is 0.5 ml/min, at which the difference between the quantity of light received when ethanol is fed and that of light received when pure water is fed is a maximum. 
     In the present embodiment, the distance between the screen 6 and the glass window 33 is 2000 mm, which is selected as preferable for the purpose, but an optimum distance may be determined selectively in view of measuring conditions including the kind of a medium (solvent), and the flow rate, etc. When the mixed solution was not fed, the quantity of light received was -54 db and was regular, namely the quantity of light received did not show a satisfactory linear relation, therefore a good result was not achieved. 
     Solutions of different mixing ratios were prepared and were maintained at 28° C. by the constant temperature tank 11. FIG. 5 shows the measured variation of the quantity of light received with ethanol concentration for those mixed solutions of different mixing ratios. As shown in FIG. 5, the concentration of ethanol and the quantity of light received had a satisfactory linear relation. When this analytical curve is used, and the quantity of light received through a mixed solution having an unknown ethanol concentration is measured, the unknown ethanol concentration can be readily determined. Accordingly when the ethanol concentration is measured by this apparatus, the ethanol concentration can be determined readily, accurately, quickly, and continuously without being disturbed by electrical noise. 
     According to a concentration measuring apparatus in a second embodiment as shown in FIG. 6, an inlet port 2a (feed means) and an outlet port 3a (discharge means) are attached directly to a tube 1. As shown in FIG. 7, a converging point detector 7 is disposed to another of the module M. This converging point detector 7 comprises an optical meter 72, which detects luminous flux power, an optical fiber 71 (core diameter of, for example, 50 μm), a holder 73 holding the other end of the optical fiber 71, and a vernier caliper 74 for measuring the position of a point of convergence. The concentration measuring apparatus provided with two reflecting mirrors 8a and 8b inclined at 45° to the optical axis as shown in FIG. 8 forms a concentration measuring apparatus that is particularly compact in construction. 
     A He-Ne laser beam (exemplary parameters of wavelength 543 nm, power 1 mW) is projected substantially along the axis of the tube 1. The light receiving face of the optical fiber 71 is moved along the optical axis to find a point where the quantity of the He-Ne laser beam received reaches a maximum, namely, a converging point Po of the transmitted light. 
     Since the concentration and the converging point have a satisfactory linear correlation represented by a straight line having a large gradient as in the first embodiment, the concentration measuring apparatus can accurately, quickly, and continuously measure the concentration in a wide range without being disturbed by electrical noise, and is not susceptible to variations of the luminous intensity of the light source or small bubbles contained in the mixed solution. The concentration measuring apparatus is simple and compact in construction. Particularly, the employment of the reflecting mirrors reduces the dimensions of the concentration measuring apparatus. 
     The present invention is not limited to the foregoing specific embodiments in its practical application and changes and variations may be made therein without departing from the scope of the present invention, The dimensions, general shape, and material of the tube may be selectively determined so as to meet the object and purpose of use. For example, the tube may be a straight tube or a curved tube. The sectional shape may be circular, square, hexagonal, or elliptical. The tube may have a plurality of parallel pores to flow a mixed solution, namely, a honeycomb type or a lotus root type. The material is not limited to copper, and metal, glass, etc., may be used. 
     The medium (solvent) used in the mixed solution is not limited to the above mentioned medium. The present invention may be applicable to any mixed solution wherein the light power received, etc., varies and any mixing ratio of mixed solution and the concentration can be detected. For another example, the mixed solution may be water-methanol mixed solution, water-ethyleneglycol mixed solution, etc. Optimum measuring conditions are determined selectively in consideration of the properties of the mixed solution. The number of the solvents of the mixed solution is usually two, but is not limited to two, and may be not less than three, provided that the relation between the concentration of the objective solvent and the quantity of light received can be represented by a finite curve. 
     The concentration measurement apparatus in the above mentioned embodiment was operated manually for concentration measurement and data processing. However, the concentration measurement apparatus may be operated automatically, and may be provided with a data processing unit, such as a picture processing unit, for moving the light receiving face, and for measuring and recording the measured quantity of light received at a converging point. Ordinarily, a parallel light beam is used that substantially uniformly irradiates the end face of the tube, but the irradiating method is not limited to this method. The diameter of the light beam may selectively determined according to the purpose. 
     The length, thickness, material, morphology and arrangement of the optical fiber may selectively determined. The optical fiber may be an optical glass fiber or an optical resin fiber. Optical fibers may be disposed on both sides or on side of the module or tube. This case is convenient for remote control of processes. As shown in FIGS. 10 and 11, the optical fiber may be disposed opposite to a glass window. As shown FIGS. 12 and 13, the optical fiber may be joined directly to an outside plate 25. This outside plate 25 may be made of non-transmitted material or light-transmitted material. The light emitting device may be joined directly to the outside plate 25 as shown FIG. 9. 
     Although the invention has been described in its preferred forms with a certain degree of particularity, changes and variations thereof are possible. It is therefore to be understood that the scope of the present invention is defined by the appended claims and their equivalents.