Patent Publication Number: US-2004057874-A1

Title: Light-interference fluid characteristics analyzer and frame for such analyzer

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates to an analyzer that detects difference of light refraction difference between a fluid to be analyzed and a reference fluid as a change in interference fringe and determines the calorific value, concentration and other characteristics of the fluid to be analyzed.  
       [0003] 2. Prior Art  
       [0004] A light-interference fluid characteristics analyzer comprises, as, for example, described in Japanese Provisional Utility Model Publication No. 199056 of 1988, optical elements including a measuring cell, a reference cell, a light source, lenses, a parallel plane mirror constituting a beam splitter, prisms, and an interference fringe detector that are properly positioned with respect to the optical axis and bolted to the bottom of a casing.  
       [0005] As the amount of change in interference fringe caused by a change in fluid characteristics is extremely small, displacement of the elements greatly affects detection accuracy. Therefore, installation of elements requires skill and adds up cost of manufacture. The need to secure strength to insure that external forces do not displace the elements increases the thickness of the casing and the weight of the whole analyzer.  
       SUMMARY OF THE INVENTION  
       [0006] A light-interference fluid characteristics analyzer according to this invention comprises a frame having at least three through-holes in the same cross section and cavities crossing the through-holes at a given distance. An area sandwiched between the two cavities is sealed by translucent sheets to form a cell. The frame also contains an optical means that sends two beams from a single light source into the cell through a beam splitter and forms an interference fringe by reflecting the beams from the cell to a single spot through a prism.  
       [0007] A frame for a light-interference fluid characteristics analyzer according to this invention, has at least three through-holes in the same cross section, cavities crossing the through-holes at a given distance, and a cavity to contain an optical means that forms an interference fringe by sending two beams from a single light source into an area sandwiched between said cavities.  
       OBJECT AND EFFECT OF THE INVENTION  
       [0008] An object of this invention is to provide a light-interference fluid characteristics analyzer that permits easy installation of optical elements and weight reduction.  
       [0009] Another object of this invention is to provide a frame for a light-interference fluid characteristics analyzer.  
       [0010] This invention provides high rigidity by means of vertical and horizontal walls defining the through-holes and permits easy forming of the cavities to contain the translucent sheets and optical elements constituting the cell. Fitting the elements in the defining cavities permits elements installation with high accuracy. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 is an exploded perspective view of a light-interference fluid characteristics analyzer according to this invention.  
     [0012]FIG. 2 is a perspective view showing an example of the base material forming the frame of said analyzer.  
     [0013] FIGS.  3 ( a ) and  3 ( b ) show the parallel plane mirror and prism viewed from the bottom.  
     [0014]FIG. 4 schematically illustrates a light path formed by the frame.  
     [0015] FIGS.  5 ( a ) and  5 ( b ) show other embodiments of the frame having slits to fix lenses and cavities to hold the translucent sheets to form the cell in different positions.  
     [0016] FIGS.  6 ( a ) and  6 ( b ) are perspective views showing other embodiments of the frame according to this invention.  
     [0017]FIG. 7 is a cross-sectional view of the cell area showing another embodiment of the structure to fasten the translucent sheets forming said cell. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0018]FIG. 1 shows an embodiment of a frame constituting a light-interference fluid characteristics analyzer according to this invention. A frame  1  is made of a base material such as aluminum or other metal having relatively higher rigidity and permitting easy drawing or extrusion. The frame has a first, second, third and fourth through-holes  2 ,  3 ,  4  and  5  as shown in FIG. 2.  
     [0019] The first and third through-holes  2  and  4  form reference cells S 1  and S 2 , whereas the greater through-hole  3  in the middle forms a measuring cell M. Cavities  6  and  7  reaching from the top to the bottom of the frame are formed in given positions so as to hold opposing translucent sheets  8  and  9  that forms a light path of the length conforming to the gas to be analyzed. To insure air tightness from the frame  1 , the translucent sheets  8  and  9  are pressed against opposite sides by sheet springs or other fastening means  12  and  13  through sealing materials  10  and  11  such as packing or grease layers.  
     [0020] Cavities  16  and  17  to hold a parallel plane mirror  14  constituting a beam splitter and a prism  15  are provided adjacent to and on the outside of the translucent sheets  8  and  9 .  
     [0021] A portion of the area containing a fourth through-hole  5  is cut away and a through-hole  20  is pierced at 45 degrees to the parallel plane mirror  14  and a light-source element, such as a light-emitting diode or an incandescent lamp, is fitted in the through-hole  20  so as to face the parallel plane mirror  14 .  
     [0022] A cavity  21  to pass the light reflected from the parallel plane mirror  14  is formed parallel to the light path connecting the optical element  19  and parallel plane mirror  14 . A plane mirror  22  is fastened in an area where the cavity  21  and fourth through-hole  5  cross by a bolt or other fastening means  23 . Lenses  24  and  25  to output an interference fringe at a given magnification at the exit end of the fourth through-hole  5  are disposed therein. The lenses  24  and  25  are movably fastened to parallel slits  26  and  27  formed in the fourth through-hole  5 .  
     [0023] The cavities  6 ,  7 ,  16  and  17  to hold the optical elements and the slits  26  and  27  to fasten the lenses  24  and  25  can be formed precisely by electric-discharge machining or peripheral milling.  
     [0024] As the cavities  6 ,  7 ,  16  and  17  in the frame  1  are formed with high precision, the translucent sheets  8  and  9 , parallel mirror  14  and prism  15  are precisely positioned by fastening in the cavities  6 ,  7 ,  16  and  17 , respectively.  
     [0025] Final adjustment of interference fringe interval and size is done by changing the angle of the plane mirror  22  by loosening a fastening means  23  and adjusting the lenses  24  and  25  by loosening fastening means  28  and  29 .  
     [0026] The parallel plane mirror  14  and prism  15  have projections  14   b  and  15   b  of the smallest possible cross-sectional area at the bottom thereof as shown in FIGS.  3 ( a ) and  3 ( b ). By fastening the parallel plane mirror  14  and prism  15  by forming an adhesive layer between the projections  14   b  and  15   b  and the frame  1 , strain due to difference in thermal expansion coefficient between the frame  1  and the parallel plane mirror  14  and prism  15  can be reduced to a minimum.  
     [0027] In the analyzer having the optical elements thus disposed, the parallel plane mirror  14  divides a parallel beam L 0  from the light-source element  19  into two beams L 1  and L 2  as shown in FIG. 4. The beam L 1  enters the prism  15  through a light path close to one side of the measuring cell M and then enters the parallel plane mirror  14  as a beam L 3  passing through a light path close to the other side of the measuring cell M.  
     [0028] Another beam L 2  passes through the reference cell S 1  to the prism  15 . A beam L 4  emitted from the reference cell S 2  forms an interference fringe together with the beam L 3  at point P where the beam L 3  from the measuring cell M 1  is irradiated. Then, a beam L 5  reaches the plane mirror  22 . A magnifying optical system comprising the lenses  24  and  25  magnifies a beam L 6  from the plane mirror  22  to a size suited for detection.  
     [0029] Comprising the vertical walls  1   a  to  1   e  defining the through-holes  2 ,  3 ,  4  and  5  and the top and bottom horizontal walls  1   f  and  1   g , the frame  1  has high enough rigidity to insure light weight and reduce strain caused by external forces to a minimum. Furthermore, the translucent sheets  8  and  9  elastically pressed against the frame reduce to a minimum the strain of the translucent sheets  8  and  9  due to difference in thermal expansion coefficient between the translucent sheets  8  and  9  and the frame  1 , thus permitting high-precision analysis by eliminating unnecessary movement of interference fringes.  
     [0030] While the cavities are formed from the horizontal wall if of the frame  1  in the embodiment described, slits  30  and  31  to fasten the lenses may be formed in the vertical wall  1   e  as shown in FIG. 5( a ) and through-holes  32  and  33  to accommodate the cell-forming translucent sheets  8  and  9  may be formed in the vertical walls  1   a  to  1   e  ass shown in FIG. 5( b ).  
     [0031] While the cavities  16  and  17  to accommodate the parallel plane mirror  14  and prism  15  and the cavities  6  and  7  to accommodate the cell-forming translucent sheets  8  and  9  are independently formed in the embodiment described, integral cavities  34  and  35  may be formed as shown in FIG. 6( a ) that function similarly, in conjunction with partitions  34   a  and  35   a  that may be provided as required to define both sides of the translucent sheets  8  and  9 .  
     [0032] While the through-holes in the embodiment described are rectangular in cross section, the through-holes may be of any shape offering no interference to the passage of beams, such as polygonal, circular or elliptical. The through-holes may also function similarly even if the outer cells S 1  and S 2  are filled with the fluid to be analyzed and the inner cell M with the reference fluid.  
     [0033] While the lenses  24  and  25  are fastened in the same frame  1  in the embodiment described, a frame  1 ′ having only through-holes  2 ′,  4 ′ and  3 ′ to form at least the reference cells S 1  and S 2  and the measuring cell M as shown in FIG. 6( b ) may also achieve similar function and effect as a change in the light path of the beams converted into interference fringes has no effect on measuring accuracy.  
     [0034] While air-tightness in the embodiment described is insured by pressing the back of the translucent sheets  8  and  9  with sheet springs or other fastening means  12  and  13 , the optical length of the cells can be defined with high precision by taking advantage of elasticity of the sealing materials  10  and  11  as shown in FIG. 7.  
     [0035] If the sealing members  10  and  11  are made of silicon rubber or other elastic material and the back of the translucent sheets  8  and  9  are pressed directly by the cavities  6  and  7  in the frame  1  or by spacers  36  and  37  of rigid material, the surface-to-surface distance between the translucent sheets can be kept constant regardless of elasticity of the sealing members  10  and  11 . Besides, strain of the translucent sheets  8  and  9  due to difference in thermal expansion coefficient between the translucent sheets  8  and  9  and the frame  1  is eliminated. Furthermore, the use of the sealing members of elastic material provides greater sealing force than the fastening means  12  and  13  and thus prevents leakage from the cells more effectively.