BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an electrochemical gas sensor, and more particularly to a method for clamping the same. 
         [0003]    2. Description of Related Art 
         [0004]    The present inventors have developed electrochemical gas sensors in which the bottom of a gas sensor body is disposed in a metal container, a cover is disposed on the top surface of the gas sensor body, and the cover and the container are clamped via a gasket (Patent Document 1: JP 2004-226346A and Patent Document 2: JP 2006-84319A). Patent Document 1 discloses a configuration in which the tip end of the container is bent at a right angle from the axial direction of the container so as to press the gasket downward. Patent Document 2 discloses a configuration in which the gasket is in contact with the tip end of the container but is not provided near the gas sensor body. Patent Document 3 (U.S. Pat. No. 5,650,054) discloses a configuration in which a cover is received in the recess of a gasket. 
         [0005]    Electrochemical gas sensors exhibit variation in their gas sensitivity, and those equipped with a water reservoir exhibit variation in the water evaporation amount as well. The variation in the gas sensitivity requires accurate adjustment of attached circuitry or reduces the yield of the gas sensor. Also, the variation in the water evaporation amount brings about variation in the service life of the gas sensor. 
         [0006]    Patent Document 1: JP 2004-226346A 
         [0007]    Patent Document 2: JP 2006-84319A 
         [0008]    Patent Document 3: U.S. Pat. No. 5,650,054 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the present invention to reduce the variation in the gas sensitivity of electrochemical gas sensors. 
         [0010]    The present invention provides an electrochemical gas sensor including: a disc-shaped metal bottom member; a cylindrical metal side member that extends in at least a first direction along an axial direction of the bottom member so as to surround the bottom member; a ring-shaped polymer gasket that includes an opening in the center and in which both sides of the opening each have an L-shaped member in cross section, with one section of the L-shaped member being in contact with an inner side of the side member and the other section of the L-shaped member being in contact with the bottom member; a gas sensor body that is located in the opening of the gasket and whose bottom surface is in contact with the bottom member, the gas sensor body including at least a pair of electrodes and a solid electrolyte membrane or a separator retaining a liquid electrolyte; and 
         [0011]    a metal cover whose bottom surface is circular and is in contact with a top surface of the gas sensor body, 
         [0012]    wherein the cover includes a ring-shaped protrusion provided around periphery of the cover and extending to a side opposite the bottom surface along the axial direction of the cover, and a tip end of the side member and a tip end of the one section of the L-shaped member of the gasket are bent at an acute angle toward the bottom member from the first direction, the tip end of the side member, the tip end of the one section of the L-shaped member of the gasket and a tip end of the protrusion of the cover are in hermetic contact with each other, and none of the three tip ends is parallel to a surface of the bottom member. 
         [0013]    A method for clamping an electrochemical gas sensor according to the present invention is a method for clamping an electrochemical gas sensor including: a disc-shaped metal bottom member; a cylindrical metal side member that extends in at least a first direction along an axial direction of the bottom member so as to surround the bottom member; a ring-shaped polymer gasket that includes an opening in the center and in which both sides of the opening each have an L-shaped member in cross section; a gas sensor body including at least a pair of electrodes and a solid electrolyte membrane or a separator retaining a liquid electrolyte; and a metal cover having a circular bottom surface, the cover including a ring-shaped protrusion provided around periphery of the cover and extending to a side opposite the bottom surface along the axial direction of the cover, the method including the steps of: disposing the gasket, the gas sensor body and the cover on the bottom member such that the one section of the L-shaped member of the gasket is surrounded by the side member and the other section is in contact with the bottom member, that the gas sensor body is located in the opening of the gasket and a bottom surface of the gas sensor body is in contact with the bottom member, and that the bottom surface of the cover covers a top surface of the gas sensor body and the ring-shaped protrusion of the cover extends to a side opposite the bottom member; and bringing a surface of a hole of a mold tapered to have a larger diameter at front and a smaller diameter at rear into contact with a tip end of the side member, whereby the tip end of the side member and a tip end of the one section of the L-shaped member of the gasket are bent at an acute angle toward the bottom member from the first direction, the tip end of the side member, the tip end of the one section of the L-shaped member of the gasket and a tip end of the protrusion of the cover are in hermetic contact with each other, and the three tip ends are clamped such that none of them is parallel to a surface of the bottom member. 
         [0014]    According to the present invention, the three tip ends can be hermetically clamped to each other. Also, the gap between the other section of the L-shaped member of the gasket and the bottom member and the gap between the other section of the L-shaped member of the gasket and the cover is eliminated. As a result, the constituent members of the electrochemical gas sensor are accurately positioned, and the variation in gas sensitivity is reduced. When the present embodiment is compared to a comparative example in which, for example, the angle of the tip end of the side member from the first direction is set to 90°, in the present embodiment, the distribution of gas sensitivity can be narrowed as compared to that of the comparative example ( FIGS. 13 and 14 ). The angle of the tip end of the side member is preferably 10° to 60° from the first direction, and particularly preferably 20° to 50°. 
         [0015]    Preferably, the side member extends in a direction opposite to the first direction from a position of the bottom member and constitutes a bottomed cylindrical container, the bottom member is held by a waist portion of the cylindrical container, water is stored in a bottom space of the cylindrical container that is below the bottom member, and the bottom member is provided with a hole. The water may be liquid water or water gelled with silica or the like. Besides plain water, water containing an electrolyte such as a sulfonic acid compound may be used. According to the present invention, as shown in  FIG. 12 , the variation in the amount of water evaporated from the water reservoir can be reduced, and thus the variation in the service life of the gas sensor can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a vertical cross-sectional view of an electrochemical gas sensor according to an embodiment of the present invention. 
           [0017]      FIG. 2  is an enlarged cross-sectional view of a relevant part of the electrochemical gas sensor shown in  FIG. 1 . 
           [0018]      FIG. 3  is a vertical cross-sectional view of a container and a bottom plate of the electrochemical gas sensor shown in  FIG. 1 . 
           [0019]      FIG. 4  is a vertical cross-sectional view of a gasket of the electrochemical gas sensor shown in  FIG. 1 . 
           [0020]      FIG. 5  is a cross-sectional view of a gas sensor body and a diffusion control plate of the electrochemical gas sensor shown in  FIG. 1 . 
           [0021]      FIG. 6  is a vertical cross-sectional view of a cover of the electrochemical gas sensor shown in  FIG. 1 . 
           [0022]      FIG. 7  is a diagram illustrating a method for clamping an electrochemical gas sensor according to an embodiment of the present invention. 
           [0023]      FIG. 8  is a vertical cross-sectional view of an electrochemical gas sensor according to a first variation of the embodiment. 
           [0024]      FIG. 9  is a vertical cross-sectional view of an electrochemical gas sensor according to a second variation of the embodiment. 
           [0025]      FIG. 10  is a photograph showing a vertical cross section of a relevant part of the electrochemical gas sensor according to the embodiment. 
           [0026]      FIG. 11  is a photograph showing a vertical cross section of a relevant part of an electrochemical gas sensor according to a comparative example. 
           [0027]      FIG. 12  is a characteristic diagram showing the distribution of water evaporation amount (at 70° C. for 10 days) between the embodiment and the comparative example. 
           [0028]      FIG. 13  is a characteristic diagram showing the distribution of CO sensitivity obtained in the embodiment. 
           [0029]      FIG. 14  is a characteristic diagram showing the distribution of CO sensitivity obtained in the comparative example. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    Hereinafter, best modes for carrying out the present invention will be described. 
       Embodiment 
       [0031]      FIGS. 1 to 14  show an embodiment of the present invention and characteristics thereof.  FIGS. 1 to 6  show a structure of an electrochemical gas sensor  2  of the present embodiment. Reference numeral  3  indicates a bottomed cylindrical container made of metal, and reference numeral  4  indicates a bottom plate that is made of a metal disc and provided with a hole  5 . The bottom plate  4  is supported by a waist  6  formed in the container  3 . A lower part of the container  3  that is below the waist  6  constitutes a water reservoir  7  in which water  8  is stored. The water may be plain water, or may be water containing an electrolyte, water gelled with silica or the like, water containing deliquescent salt, or water held by water absorbent polymer or the like. 
         [0032]    An upper part of the container  3  that is above the bottom plate  4  is referred to as a side portion  10 , and the side portion  10  has a cylindrical shape. A gasket  12  is disposed on the bottom plate  4 , and a gas sensor body  14  is placed in a center opening of the gasket  12 . An atmosphere to be detected is supplied via a circular diffusion control plate  16  as viewed from above. A metal cover  18  is disposed so as to cover the diffusion control plate  16 . The cover  18  is rotationally symmetric about an axial direction D. Clamping the tip end (upper part in the diagrams) of the side portion  10  provides a hermetic seal between the side portion  10  and the gasket  12  and between the gasket  12  and the cover  18  and causes the cover  18  to be attached to the container  3 . 
         [0033]      FIG. 2  shows a major part of the present embodiment. Reference letter D indicates the axial direction of the gas sensor  2 . The axial direction D is the axial direction of the bottom plate  4 , the cover  18  and the container  3 , and corresponds to the first direction recited in the claims. Reference letter F indicates the direction of the tip end of the side portion  10 , and the tip end direction F is angled obliquely with respect to the axial direction D and points toward the center of the bottom plate  4 . The angle between the directions D and F is defined as an angle of inclination θ. The angle of inclination θ can range from 0° to 90°, and is preferably 10° to 60° and more preferably 20° to 50°. As used herein, the terms “above” and “up” refer to the direction in which the axial direction D points, and the terms “below” and “down” refer to the opposite direction to the axial direction D. The term “right and left” refers to the direction that is parallel to the surface of the bottom plate  4  and perpendicular to the axial direction D. The shape of the respective members of the present embodiment does not change even when they are rotated about the axial direction as the axis of rotation, and is circular as viewed from above. 
         [0034]    By the tip end of the side portion  10  being clamped, the tip end is bent obliquely inwardly with respect to the axial direction D, as a result of which the tip end of the gasket  12  is also bent obliquely inwardly and the tip end of the cover  18  is also bent obliquely inwardly. Reference numeral  20  indicates a contact portion between the side portion  10  and the gasket  12 , and reference numeral  21  indicates a contact portion between the gasket  12  and the cover  18 . The contact portions  20  and  21  need to be hermetically sealed, and they are preferably parallel to each other. Reference numeral  22  indicates a contact portion between the bottom surface of the gasket  12  and the bottom plate  4 , and it is preferable that there is no gap in the contact portion. Reference numeral  23  indicates a contact portion between the top surface of the gasket  12  and the bottom surface of the cover  18 , and it is preferable that they are in contact without a gap therebetween. 
         [0035]      FIG. 3  shows the shape of the container  3  before the gasket  12  and the like are attached. The side portion  10  extends in a ring upward from the bottom plate  4 .  FIG. 4  shows a structure of the gasket  12 . The gasket  12  is circular as viewed from above and has a circular opening  24  in the center. The right and left sides of the gasket  12  are L-shaped membered in vertical cross section and are composed of a ring-shaped bottom portion  25  and a protrusion  26  protruding in a ring upward from the periphery of the bottom portion  25 . The gasket  12  is preferably made of plastic having plasticity and elasticity such as polyethylene, polypropylene, nylon or tetrafluoroethylene, or may be natural rubber, synthetic rubber or the like. 
         [0036]      FIG. 5  shows the gas sensor body  14  and the diffusion control plate  16 , which are circular in shape. Reference numeral  28  indicates a separator in the form of a film made of porous paper or plastic and retaining a liquid electrolyte. A sensing electrode  29  and a counter electrode  30  are provided, for example, on the top and bottom surfaces of the separator  28 . The sensing electrode  29  and the counter electrode  30  are each made of a porous carbon sheet or the like carrying electrode materials such as platinum and carbon. It is also possible to use a solid electrolyte membrane or the like, instead of the separator  28 . It is also possible to dispose a porous carbon fiber sheet or the like between the sensing electrode  29  and the diffusion control plate  16  and between the counter electrode  30  and the bottom plate  4 . The diffusion control plate  16  is a thin plate made of titanium or the like and includes a diffusion control hole  17 . The diffusion control plate  16  controls supply of the atmosphere to the sensing electrode  29  and electrically connects the sensing electrode  29  and the cover  18 . The counter electrode  30  is electrically connected to the container  3  via the bottom plate  4 . 
         [0037]      FIG. 6  shows a structure of the cover  18 . Reference numeral  32  indicates a lower metal member, and reference numeral  34  indicates an upper metal member. These metal members are welded to each other. The space between the metal members  32  and  34  is filled with a filter member  38  such as activated carbon, and retaining sheets  39  are provided to prevent the filter member from spilling. The atmosphere is supplied toward the diffusion control plate  16  via holes  36  and  37 . The metal member  32  provided on the diffusion control plate  16  side (lower side) includes a ring-shaped flange portion  33  on its outer periphery, and a ring-shaped protrusion  40  is formed by bending the outer periphery of the flange portion  33  upward. The lower metal member  32  is disc-shaped, the flange portion  33  is in contact with the top surface of the bottom portion  25  of the L-shaped member of the gasket  12 , and the protrusion  40  is brought into hermetic contact with the inner surface of the protrusion  26  of the L-shaped member of the gasket  12  by clamping. The outer periphery of the upper metal plate  34  serves as a flange portion  35  by being bent so as to be parallel to the bottom plate  4 , and the flange portions  33  and  35  are welded to each other. 
         [0038]      FIG. 7  illustrates a method for clamping the electrochemical gas sensor  2 . In the state shown in  FIG. 3 , the gasket  12  is placed on the bottom plate  4 , the gas sensor body  14  and the diffusion control plate  16  are placed in the opening of the gasket  12 , and the cover  18  is placed so as to cover the diffusion control plate. It should be noted that the order of placing these constituent elements can be set as appropriate. Reference numeral  42  indicates a press mold having a conical hole  43  in a center portion thereof, and clamping is performed by pressing and deforming the side portion  10  with a conical surface  44 . In  FIG. 7 , the angle between the surface  44  and the axial direction D is substantially equal to the angle θ between the tip end of the side portion  10  and the axial direction D. 
         [0039]    In the present embodiment, the diffusion control plate  16  is provided in order to make the amount of atmosphere supplied to the gas sensor body  14  constant, but it may be omitted. Also, the gas sensor body  14  may be disc-shaped as viewed from above, and the structure and material thereof can be selected as appropriate. A reference electrode may be provided in addition to the sensing electrode and the counter electrode. The present invention is provided to clamp the cover  18 , the gasket  12  and the container  3 , and therefore parts other than the parts described above can be modified according to known techniques. 
         [0040]      FIG. 8  shows a gas sensor according to a variation, in which a container  46  without a water reservoir is used instead of the container  3  with a water reservoir. Accordingly, the gasket  12  and the gas sensor body  14  are disposed on a bottom portion  47  of the container  46 , and the side portion  10  is replaced by a ring-shaped protrusion  48  provided on the outer periphery of the container  46 . Other than those changes, the gas sensor according to this variation is the same as that of the present embodiment. 
         [0041]      FIG. 9  shows a second variation in which the filter member  38  is not used. Accordingly, a container  50  having a hole  54  is used, and the gasket  12  and the gas sensor body  14  are disposed on a bottom portion  52  of the container  50 . In this example, the diffusion control plate  16  is not provided because the thickness of the gas sensor body  14  is increased. Also, a metal cover  58  is used, and the bottom surface of the cover  58 , serving as a cover  59 , is brought into contact with the gasket  12  and the gas sensor body  14 . A ring-shaped upward protrusion  60  is provided around periphery of the cover  59 , and the protrusions  56  and  60  are clamped via the gasket  12 . Other than those changes, the gas sensor according to this variation is the same as that of the present embodiment. 
         [0042]    Besides the gas sensor  2  of the present embodiment, a gas sensor was produced by clamping such that the tip end of the side portion  10  was horizontal and parallel to the surface of the bottom plate  4 , and the produced gas sensor is referred to as a comparative example.  FIG. 10  shows a photograph showing a cross section of the gas sensor of the present embodiment and  FIG. 11  shows a photograph showing a cross section of the gas sensor of the comparative example. Referring to the gas sensor of the comparative example, there is a gap between the tip end of the side portion and the gasket so that the range in which the tip end of the side portion and the gasket are hermetically sealed is very small. Also, the bottom surface of the gasket has come away from the bottom plate, and the inner periphery side is tilted upward and the outer periphery side is tilted downward. There is also a gap between the top surface of the bottom portion of the gasket and the outer periphery of the bottom surface of the cover  18 . These factors cause variation in hermetic seal property between the inside and the outside of the gas sensor  2  and suggest, for example, the water in the water reservoir may evaporate without passing through the hole  5  of the bottom plate. These factors also suggest that the atmosphere may bypass the cover  18  and arrive at the gas sensor body  14 , and it is therefore highly likely that there is also variation in the pressure of the cover  18  pressing the diffusion control plate  16 . 
         [0043]    Referring to the present embodiment shown in  FIG. 10 , the tip end of the side portion  10  and the tip end of the gasket  12 , as well as the tip end of the gasket  12  and the tip end of the cover  18 , are hermetically sealed. Obliquely clamping the tip end of the side portion  10  causes the tip end of the protrusion  26  of the gasket and the tip end of the cover to be obliquely bent, as a result of which these are hermetically clamped to each other. Although the bottom surface of the gasket  12  is slightly tilted from the bottom plate  4 , the gap therebetween is small. No gap is observed between the top surface of the gasket  12  and the outer periphery of the bottom surface of the cover  18 . Also, it seems that the cover  18  is pressing the diffusion control plate  16  with a uniform downward force by receiving an obliquely downward force, or in other words, a downward force as a whole, from the surrounding elements. Due to these factors, a hermetic seal can be secured between the inside and the outside of the gas sensor  2 , and the variation in the shape, the compressive force and the like is small. 
         [0044]      FIG. 12  shows the amount of water evaporated from the water reservoir when the gas sensors of the present embodiment and the comparative example were stored in a dry atmosphere at 70° C. for 10 days. With the comparative example, the evaporation amount varied over a wide range from 0.12 g to 0.17 g, whereas with the present embodiment, the evaporation amount was in a small range of 0.12 g±0.007 g. Wide variations in the evaporation amount result in wide variations in the time required for the water in the water reservoir to be depleted. The water reservoir is provided to supply water vapor to the gas sensor body  14  so as to cause the electrolyte to maintain its conductivity. The variation in the service life of the water reservoir results in variation in the service life of the gas sensor. 
         [0045]      FIGS. 13 and 14  respectively show the distributions of CO sensitivity of the gas sensors of the present embodiment ( FIG. 13 ) and the comparative example ( FIG. 14 ). The horizontal axis represents the output current per CO concentration, and the unit is arbitrary. In the present embodiment, the standard deviation of the CO sensitivity distribution is approximately 1/3 of that of the comparative example. Accordingly, the gas sensor of the present embodiment provides uniform gas sensitivity. The electrochemical gas sensor is capable of detecting various types of gases including hydrogen, ethanol and hydrogen sulfide, other than CO. 
         [0046]    The present embodiment provides the following effects. 
         [0047]    (1) The variation in the water evaporation amount from the water reservoir can be reduced, and as a result the variation in the service life of the gas sensor can be reduced ( FIG. 12 ). 
         [0048]    (2) The variation in gas sensitivity can be reduced ( FIGS. 13 and 14 ). 
         [0049]    The tip end of the side portion  10  and the protrusion  26  of the gasket  12  are deformed into hermetic contact with the tip end of the protrusion  40  during clamping. The protrusion  40  may be bent during clamping, or it may be initially bent so that the degree of bending during clamping is very small. 
       DESCRIPTION OF THE NUMERALS 
       [0050]      2  Electrochemical Gas Sensor 
         [0051]      3  Container 
         [0052]      4  Bottom Plate 
         [0053]      5  Hole 
         [0054]      6  Waist 
         [0055]      7  Water Reservoir 
         [0056]      8  Water 
         [0057]      10  Side Portion 
         [0058]      12  Gasket 
         [0059]      14  Gas Sensor Body 
         [0060]      16  Diffusion Control Plate 
         [0061]      17  Diffusion Control Hole 
         [0062]      18  Cover 
         [0063]      20  to  23  Contact Portion 
         [0064]      24  Opening 
         [0065]      25  Bottom Portion 
         [0066]      26  Protrusion 
         [0067]      28  Separator 
         [0068]      29  Sensing Electrode 
         [0069]      30  Counter Electrode 
         [0070]      32 ,  34  Metal Member 
         [0071]      36 ,  37  Hole 
         [0072]      38  Filter member 
         [0073]      39  Retaining Sheet 
         [0074]      40  Protrusion 
         [0075]      42  Mold 
         [0076]      43  Hole 
         [0077]      44  Surface 
         [0078]      46  Container 
         [0079]      47  Bottom Portion 
         [0080]      48  Protrusion 
         [0081]      50  Container 
         [0082]      52  Bottom Portion 
         [0083]      54  Hole 
         [0084]      56  Protrusion 
         [0085]      58  Cover 
         [0086]      59  Cover 
         [0087]      60  Protrusion 
         [0088]    D Axial Direction 
         [0089]    F Tip End Direction 
         [0090]    θ Angle of Inclination
An electrochemical gas sensor includes: a disc-shaped metal bottom member; a cylindrical metal side member that extends along the axial direction of the bottom member to surround the bottom member; a ring-shaped polymer gasket that includes an opening in the center and in which both sides of the opening each have an L-shaped member in cross section, with one section of the L-shaped member being in contact with the inner side of the side member and the other section of the L-shaped member being in contact with the bottom member; a gas sensor body that is located in the opening of the gasket and whose bottom surface is in contact with the bottom member and that includes a pair of electrodes and a solid electrolyte membrane or a separator retaining a liquid electrolyte; and a metal cover that is in contact with the top surface of the gas sensor body.