Abstract:
A gas sensor is disclosed as having a gas sensing element operative to detect a concentration of a specified gas in measuring gases, a cylindrical housing internally supporting the gas sensing element in fixed place, and a cylindrical measuring gas side cover fixedly secured to the housing at a leading end thereof so as to cover a leading end of the gas sensing element. The gas sensor has a response, ranging from 150 ms to 200 ms, which is a parameter representing a speed of detecting the concentration of the specified gas with respect to variation in a specified gas concentration in the measuring gases. The measuring gas side cover has fine holes each with an opening surface area ranging from 0.1 mm 2  to 1 mm 2 .

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to Japanese Patent Application No. 2006-209023, filed on Jul. 31, 2006, the content of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field of the Invention 
         [0003]    The present invention relates to gas sensors and, more particularly, to a gas sensor mounted on an exhaust pipe or the like of an internal combustion to be exposed to measuring gases for measuring a concentration of a specified gas. 
         [0004]    2. Description of the Related Art 
         [0005]    Gas sensors have heretofore been known as sensors to be mounted on exhaust pipes of internal combustion engines of motor vehicles and utilized for controlling an air/fuel ratio of an air fuel mixture in the engine. One example of such gas sensors is disclosed in Japanese Patent Application Publication No. 5-149914 related to a gas sensor of the type in which atmospheric air is introduced. 
         [0006]    With the gas sensor of such type mounted on the exhaust pipe of the internal combustion engine, a leading end of the gas sensor is exposed to measuring gases. Further, the gas sensor has a cover for protecting a gas sensing element operative to detect a concentration of specified gas in measuring gases. Moreover, the cover is formed with a number of gas ventilation holes to pass measuring gases therethrough to the gas sensing element for detecting variation in measuring gases with high voltage. However, during passage of measuring gases through the gas ventilation holes, water droplets prevailing in the exhaust pipe penetrate through the gas ventilation holes of the cover to an inside area of the cover. Thus, the gas sensing element, elevated at high temperatures, suffer the water droplets. This causes a damage to occur on the gas sensing element with a resultant degradation in response of the gas sensing element. 
         [0007]    Meanwhile, for addressing the tasks of a water-incursion resistance and response of the gas sensing element, it is effective to allow the cover to be formed with a large number of small gas ventilation holes. However, the provision of such a large number of ventilation holes results in the occurrence of an issue with a drop in strength of the cover. Thus, the cover encounters a difficulty in providing a large number of small ventilation holes. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention has been completed with a view to addressing the above issues and has an object to provide a gas sensor that has increased water-incursion resistance for thereby effectively preventing a gas sensing element from being damaged. 
         [0009]    To achieve the above object, a first aspect of the present invention provides a gas sensor comprising a gas sensing element operative to detect a concentration of a specified gas in measuring gases, a cylindrical housing internally supporting the gas sensing element in fixed place, and a cylindrical measuring gas side cover fixedly secured to the housing at a leading end thereof so as to cover a leading end of the gas sensing element. The gas sensor has a response, ranging from 150 ms to 200 ms, which is a parameter representing a speed of detecting the concentration of the specified gas with respect to variation in a specified gas concentration in the measuring gases. The measuring gas side cover has fine holes each with an opening surface area ranging from 0.1 mm 2  to 1 mm 2 . 
         [0010]    According to the present invention, the gas sensor has a response, ranging from 150 ms to 200 ms, which is a parameter representing a speed of detecting the concentration of the specified gas with respect to variation in a specified gas concentration in the measuring gases. Further, the measuring gas side cover has fine holes each with an opening surface area ranging from 0.1 mm 2  to 1 mm 2 . These are parameters obtained upon experimental tests conducted by the inventor of the present patent application. The experimental tests have been conducted in a sequence described below. That is, first, the gas sensor implementing the present invention was mounted on an exhaust pipe of a motor vehicle. Then, the engine was started up into a steady state with exhaust gases maintained at a fixed temperature, under which exhaust gases are altered in composition from a rich state to a lean state. Then, measurement is made on a time period in which upon altering the composition of exhaust gases, a variation takes place in an output of the gas sensor from a rich side to a lean side. 
         [0011]    With the gas sensor having a response less than 150 ms, the response of the gas sensor is impractical with a resultant difficulty of accurately detecting a concentration of the specified gas in measuring gases. With the gas sensor having a response exceeding 200 ms, further, the response of the gas sensor is adequate in practical use but has less water-incursion resistance in practical use. Therefore, a moisture penetrates through fine holes, formed in a measuring gas side cover, into an inside of the cover to adhere onto the leading end of the gas sensing element. This causes cracking to occur on the gas sensing element, resulting in a difficulty of precisely detecting the concentration of specified gas. 
         [0012]    If the fine hole, formed in the measuring gas side cover, has an opening surface area less than 0.1 mm 2 , the gas sensor has favorable water-incursion resistance. This makes it possible to effectively precluding moisture, penetrated to the inside of the cover through the fine hole formed in the measuring gas side cover, from adhering onto the gas sensing element. However, the response of the gas sensor is impractical in use with a resultant difficulty of precisely detecting the concentration of specified gas. In addition, if the fine hole, formed in the measuring gas side cover, has an opening surface area greater than 1 mm 2 , the gas sensor has a favorable response but water-incursion resistance of the gas sensor is impractical. This causes moisture, penetrated to the inside of the cover through the fine hole formed in the measuring gas side cover, to adhere onto the gas sensing element. This results in a fear of cracking occurring on the gas sensing element with a resultant difficulty of precisely detecting the concentration of specified gas. 
         [0013]      FIG. 11  is a graph representing the relationship between an opening surface area of each of and the number of fine holes, formed in a measuring gas side cover, and a response and water-incursion resistance. As shown in  FIG. 11 , in order to obtain a response in the order of 150 ms that is practical in use, the measuring gas side cover needs to have the fine holes in the number of pieces greater than 600 in case of the cover having the fine holes each with 0.1 mm 2  and have the fine holes in the number of pieces greater than 60 in case of the cover having the fine holes each with 1 mm 2  while the number of the fine holes needs to be greater 6 in case of the cover having the fine holes each with 10 mm 2 . However, with the fine holes each with 1 mm 2 , water-incursion resistance of the gas sensor is impractical in use. 
         [0014]    With the present invention, accordingly, the gas sensor is arranged to include a measuring gas side cover configured to provide a response ranging from 150 ms to 200 ms while having fine holes each with an opening surface area ranging from 0.1 mm 2  to 1 mm 2 . This enables the gas sensor to have advantages with both of a response and water-incursion resistance that are practical in use. 
         [0015]    With the gas sensor of the present embodiment, the measuring gas side cover may be preferably made of a mesh-like member composed of wire components woven with a clearance equal to or less than 1 mm, and the wire components may be made of stainless steel wires each having a diameter equal to or greater than 0.3φ. 
         [0016]    With such a structure, the mesh-like member is composed of the wire components with the clearance equal to or less than 1 mm. Thus, the gas sensor of the present embodiment has increased water-incursion resistance. This effectively precludes water droplets from penetrating from the outside into the inside area of the measuring gas side cover, enabling the gas sensor to have increased operating life while having increased reliability in operation. 
         [0017]    Further, it becomes possible to provide a gas sensor that can prevent a gas sensing element from suffering water even when used under high temperature environments. 
         [0018]    Moreover, the measuring gas side cover may be preferably and suitably formed in any one of optimum shapes. 
         [0019]    For instance, the measuring gas side cover may preferably have a saclike configuration. In forming the measuring gas side cover of such a configuration, wire components are woven into a mesh-like sheet, which in turn is pressed against a dome-shaped die, making it easy to fabricate the cover into the saclike configuration with the sheet being maintained in a uniform mesh pattern. 
         [0020]    Further, the measuring gas side cover may preferably have a cone-shaped configuration. In forming the measuring gas side cover of such a configuration, the wire components are woven into the mesh-like sheet, which in turn is wound on a cone-shaped die, making it easy to fabricate the cover into the cone-shaped configuration. 
         [0021]    Furthermore, the measuring gas side cover may preferably have a cylindrical configuration with a leading end thereof being shackled and closed. In forming the measuring gas side cover of such a configuration, the wire components are woven into the mesh-like sheet, which in turn is processed in a cylindrical shape and a leading end thereof is shackled and closed in a final shape in easy fabrication. 
         [0022]    Moreover, the measuring gas side cover may be preferably formed in a cylindrical shape and includes a cylindrical metallic plate body and a mesh-like cylindrical body, composed of woven wire components, which is connected to one end of the metallic plate body. 
         [0023]    In addition, the measuring gas side cover may be preferably formed in a cylindrical shape and include an inner cover formed in a cylindrical shape and disposed in an inside area, wherein the inner cover has a gas ventilation bore providing fluid communication between inside and outside areas, and wherein the metallic plate body acts as an outer cover that is radially spaced from the inner cover so as to cover the gas ventilation bore. With the measuring gas side cover of such a structure, the gas ventilation bore formed in the inner cover can be protected with the cylindrical metallic plate body of the outer cover, enabling the gas sensor to have increased water-incursion resistance. 
         [0024]    Further, the measuring gas side cover may preferably include a multi-layer structure formed in a cylindrical shape having two kinds of an inner cover and an outer cover, wherein the inner cover includes a mesh-like member formed by weaving wire components, and wherein the outer cover is made of a metallic plate and has a gas ventilation bore. 
         [0025]    With the gas sensor having the measuring gas side cover of such a structure, measuring gases enter the inside of the cover through the gas ventilation bore formed in the outer cover. Measuring gases then pass through the clearances among the wire components formed in the inner cover on an entire area thereof to reach the gas sensing element, causing the gas sensor to have increased response. 
         [0026]    Furthermore, the measuring gas side cover may preferably include a multi-layer structure formed in a cylindrical shape having two kinds of an inner cover and an outer cover, wherein the inner cover is made of a metallic plate and has a gas ventilation bore, and wherein the outer cover includes a mesh-like member formed by weaving wire components. 
         [0027]    With the gas sensor having the measuring gas side cover of such a structure, the heater disposed inside the inner cover develops heat that is kept with the inner cover made of the metallic plate. This enables the gas sensing element to be activated at an earlier stage. 
         [0028]    A second aspect of the present invention provides a gas sensor comprising a gas sensing element operative to detect a concentration of a specified gas in measuring gases, a cylindrical housing internally supporting the gas sensing element in fixed place, and a cylindrical measuring gas side cover fixedly secured to the housing at a leading end thereof so as to cover a leading end of the gas sensing element. The gas sensor has a response, ranging from 150 ms to 200 ms, which is a parameter representing a speed of detecting the concentration of the specified gas with respect to variation in a specified gas concentration in the measuring gases. The measuring gas side cover has a multi-layer structure at least a part of which includes a mesh-like member formed with fine holes each having an opening surface area ranging from 0.1 mm 2  to 1 mm 2 . 
         [0029]    A third aspect of the present invention provides a gas sensor comprising a gas sensing element operative to detect a concentration of a specified gas in measuring gases, a cylindrical housing internally supporting the gas sensing element in fixed place, and a cylindrical measuring gas side cover fixedly secured to the housing at a leading end thereof so as to cover a leading end of the gas sensing element. The gas sensor has a response, ranging from 150 ms to 200 ms, which is a parameter representing a speed of detecting the concentration of the specified gas with respect to variation in a specified gas concentration in the measuring gases. The measuring gas side cover has a multi-layer structure including an outer cover and an inner cover, the outer cover including a cylindrical metallic plate body and a mesh-like cylindrical body formed with fine holes, each having an opening surface area ranging from 0.1 mm 2  to 1 mm 2 , which is connected to one end of the metallic plate body. 
         [0030]    A fourth aspect of the present invention provides a gas sensor comprising a gas sensing element operative to detect a concentration of a specified gas in measuring gases, a cylindrical housing internally supporting the gas sensing element in fixed place, and a cylindrical measuring gas side cover fixedly secured to the housing at a leading end thereof so as to cover a leading end of the gas sensing element. The gas sensor has a response, ranging from 150 ms to 200 ms, which is a parameter representing a speed of detecting the concentration of the specified gas with respect to variation in a specified gas concentration in the measuring gases. The measuring gas side cover has a multi-layer structure including an outer cover and an inner cover, the outer cover including a cylindrical metallic plate body formed with a gas ventilation bore providing fluid communication between inside and outside areas, and the inner cover including a mesh-like cylindrical body formed with fine holes each having an opening surface area ranging from 0.1 mm 2  to 1 mm 2 . 
         [0031]    A fifth aspect of the present invention provides a gas sensor comprising a gas sensing element operative to detect a concentration of a specified gas in measuring gases, a cylindrical housing internally supporting the gas sensing element in fixed place, and a cylindrical measuring gas side cover fixedly secured to the housing at a leading end thereof so as to cover a leading end of the gas sensing element. The gas sensor has a response, ranging from 150 ms to 200 ms, which is a parameter representing a speed of detecting the concentration of the specified gas with respect to variation in a specified gas concentration in the measuring gases. The measuring gas side cover has a multi-layer structure including an outer cover and an inner cover, the inner cover including a cylindrical metallic plate body formed with a gas ventilation bore providing fluid communication between inside and outside areas, and the outer cover including a mesh-like cylindrical body formed with fine holes each having an opening surface area ranging from 0.1 mm 2  to 1 mm 2 . 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a longitudinal cross sectional view showing an overall structure of a gas sensor of one embodiment according to the present invention. 
           [0033]      FIG. 2A  is an external view showing one example of a measuring gas side cover forming a part of the gas sensor shown in  FIG. 1 . 
           [0034]      FIG. 2B  is an enlarged view showing an exemplified structure of the measuring gas side cover shown in  FIG. 2A . 
           [0035]      FIG. 3  is an external view showing another example of the measuring gas side cover forming the part of the gas sensor shown in  FIG. 1 . 
           [0036]      FIG. 4  is an external view showing another example of the measuring gas side cover forming the part of the gas sensor shown in  FIG. 1 . 
           [0037]      FIG. 5  is an external view showing another example of the measuring gas side cover forming the part of the gas sensor shown in  FIG. 1 . 
           [0038]      FIG. 6  is an external view showing a further example of the measuring gas side cover forming the part of the gas sensor shown in  FIG. 1 . 
           [0039]      FIG. 7  is an external view showing a further example of the measuring gas side cover forming the part of the gas sensor shown in  FIG. 1 . 
           [0040]      FIG. 8  is an external view showing a still further example of the measuring gas side cover forming the part of the gas sensor shown in  FIG. 1 . 
           [0041]      FIG. 9  is a longitudinal cross sectional view showing an overall structure of a gas sensor of another embodiment according to the present invention. 
           [0042]      FIG. 10  is a graph showing water adhesion rates of a gas sensor, implementing the present invention, and a gas sensor of the related art arising when suffered with the occurrence of water incursion. 
           [0043]      FIG. 11  is a graph showing the relationship between a surface area of a fine hole and the number of fine holes formed in the measuring gas side cover and a response and water-incursion of the gas sensor. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0044]    Now, a gas sensor of one embodiment according to the present invention and a related method of manufacturing the gas sensor are described below in detail with reference to the accompanying drawings. However, the present invention is construed not to be limited to such an embodiment described below and technical concepts of the present invention may be implemented in combination with other known technologies or the other technology having functions equivalent to such known technologies. 
         [0045]    In the following description, it is construed that a portion of the gas sensor adapted to be inserted to an exhaust pipe of an internal combustion engine of a motor vehicle is referred to as a “leading end ” or a “leading end portion” and an opposite side of the gas sensor exposed to an atmosphere is referred to as a “base end” or a “base end portion”. 
         [0046]    Also, it will be appreciated that the gas sensor of the present embodiment according to the present invention may have a wide variety of applications to an oxygen sensor, an A/F sensor, a NOx sensor, etc. 
       First Embodiment 
       [0047]    A gas sensor of one embodiment according to the present invention is described below in detail with reference to  FIGS. 1 and 2 . 
         [0048]      FIG. 1  is a longitudinal cross sectional view showing an overall structure of the gas sensor of the present embodiment according to the invention.  FIGS. 2A  is an external view showing a measuring gas side cover for covering a gas sensing element of the gas sensor shown in  FIG. 1 .  FIG. 2B  is an enlarged view showing an exemplified lattice structure of the gas measuring side cover for the gas sensing element of the present embodiment. 
         [0049]    As shown in  FIG. 1 , a gas sensor  1  of the present embodiment comprises a gas sensing element  19  for detecting a concentration of specified gas in measuring gases, a cylindrical housing  10  internally holding the gas sensing element  19 , a cylindrical measuring gas side cover  11  fixedly secured to the cylindrical housing  10  at a leading end thereof so as to cover a leading end  19   a  of the gas sensing element  19 , and a cylindrical atmospheric side cover  2  fixedly secured to the housing  10  at a base end thereof so as to cover a base end  19   b  of the gas sensing element  19 . 
         [0050]    Hereunder, these component parts with features thereof will be described below in detail. With the gas sensor  1  of the present embodiment, the measuring gas side cover  11  is formed in a mesh-like configuration by weaving wire components  11   x . The wire components  11   x  have a clearance  11   y  equal to or less than 0.5 mm. 
         [0051]    Further, the wire components  11   x  are made of stainless steel wires each with a diameter equal to or greater than 0.3φ. 
         [0052]    With the gas sensor  1  formed in such a structure, the measuring gas side cover  11 , composed of the wire components  11   x  woven into a mesh-like structure, provides the clearance  11   y  equal to or less than 0.5 mm between the wire components  11   x . Thus, water drops can be prevented from penetrating into an inside of the measuring gas side cover  11  from the outside to cause a damage to occur on the gas sensing element  19 . Accordingly, it becomes possible to provide a gas sensor with increased water-incursion resistance for preventing a gas sensing element from suffering water-incursion. 
         [0053]    Further, the wire components  11   x  employ material such as stainless steel, providing heat resistant property. This allows the gas sensor  1  to be used under severely high temperature environments such as those environments exceeding a temperature equal to or higher than 1000° C. In addition, the use of the wire components  11   x  each with the diameter equal to or greater than 0.3φ makes it possible to suppress the measuring gas side cover  11  from deforming when subjected to impact shocks applied from the outside. 
         [0054]    Hereunder, the gas sensor  1  will be described with reference to actual applications. With the gas sensor  1  of the present embodiment, in use, the cylindrical housing  10  is mounted on a wall surface of an exhaust pipe (not shown) extending from an automotive engine. Under such a mounting state, specified gas contained in exhaust gases (measuring gases) passing across the gas sensor  1  enters the inside of the measuring gas side cover  11  and is brought into contact with the leading end  19   a  of the gas sensing element  19 . When this takes place, the gas sensing element  19  measures an air/fuel ratio of specified gas emitted from the automotive engine to provide an air/fuel ratio detection signal for use in controlling an air/fuel ratio of an air/fuel mixture of the automotive engine. In use of the gas sensor I for the exhaust pipe of the engine, the cylindrical housing  10  is mounted onto the exhaust pipe so as to allow an end face  102  of a radially extending trunk section  101 , formed on a side wall of the cylindrical housing  10 , to be brought into contact with an external wall of the exhaust pipe. Under such a mounted condition, the measuring gas side cover  11  extends into the inside of the exhaust pipe to be exposed to measuring gases passing therethrough to allow the gas sensing element  19  to detect a concentration of specified gas in measuring gases. Moreover, a gasket  103  rests on the end face  102  of the housing  10  to allow the end face  102  to be fixedly secured onto the wall surface of the exhaust pipe in a gastight sealing effect. 
         [0055]    As shown in  FIG. 1 , the gas sensor  1  has a leading end region I a, extending downward from a lower end face of the gasket  103  at a boundary line L in  FIG. 1 , to be susceptible to heat of exhaust gases passing through the exhaust pipe during operation to measure the air/fuel ratio of specified gas in measuring gases. The gas sensor  1  also has a base end region  1   b  extending above the boundary line L to be susceptible to atmospheric environments. With such arrangement, the gas sensor  1  is warmed up due to heat of exhaust gases during operation such that the remoter from the boundary line L toward the base end of the gas sensor  1 , the lower will be the temperature. In this respect, an upper section of the gas sensor  1  extending above the boundary line L in  FIG. 1  is referred to as the base end region  1   b  of the gas sensor  1  and a lower section is referred to as the leading end region  1   a.    
         [0056]    The measuring gas side cover  11  is fixedly mounted to an end face of a leading end portion  10   a  of the cylindrical housing  10 . In addition, the measuring gas side cover  11  internally accommodates therein the leading end of the gas sensing element  19 . 
         [0057]    In particular, the measuring gas side cover  11  includes an inner cover  111 , having a cylindrical base portion  11   a  formed with a radially outward annular flange  11   b , and an outer cover  112  having a cylindrical base portion  112   a , fitted to an outer periphery of the cylindrical base portion  111   a  of the inner cover  111 , and a radially outward annular flange  112   b  overlapping with the annular flange  111   b  of the inner cover  111 . The annular flanges  11   b  and  112   b  of the inner cover  111  and the outer cover  112  are fixedly supported with the leading end portion  10   a  of the cylindrical housing by a caulked end  10   b  of the cylindrical housing  10  such that the measuring gas side cover  11  extends in coaxial relation with the gas sensing element  19 . 
         [0058]    The gas sensing element  19  is fixedly mounted on the housing  10  by means of an element-side insulating porcelain holder  12  having an element inserting bore  12   a  through which the gas sensing element  19  longitudinally extends to be held in a fixed place. A metallic packing element  200  rests on a tapered annular shoulder  105  formed in the housing  10  to be sandwiched between the element-side insulating holder  12  and the housing  10 . This provides a gastight sealing effect between the element-side insulating holder  12  and the housing  10 , thereby preventing fluid communication between the leading end region I a and the base end region  1   b  of the gas sensor  1 . 
         [0059]    The element-side insulating holder  12  has a cylindrical cavity  12   b  that is filled with airtight sealant  121 . Airtight sealant  121  provides a gastight sealing effect between the gas sensing element  19  and the element-side insulating holder  12  to prevent measuring gases from leaking through a clearance between the gas sensing element  19  and the element inserting bore  12   a  of the element-side insulating holder  12  to an upper area of the element-side insulating holder  12 . 
         [0060]    An atmospheric side porcelain insulator  13  is placed on the element-side insulating holder  12  in contact therewith. The atmospheric side porcelain insulator  13  has an axially extending cavity portion  130 , which accommodates therein the base end portion  19   b  of the gas sensing element  19 , and a plurality of connection holes  131  formed in an upper wall of the atmospheric side porcelain insulator  13  to provide connection between the cavity portion  130  and an end face of the atmospheric side porcelain insulator  13 . 
         [0061]    A cone-shaped disc spring  122  is disposed between an annular shoulder  2   c  of the cylindrical atmospheric side cover  2  and an annular shoulder  13   a  formed on the upper wall of the atmospheric side porcelain insulator  13  to provide a restoring force for axially pressing the atmospheric side porcelain insulator  13  toward the leading end region  1   a  of the gas sensor  1 , that is, in a direction parallel to a central axis of the gas sensor  1 . That is, the cone-shaped disc spring  122  allows the atmospheric side porcelain insulator  13  to press the element-side insulating holder  12  against the tapered annular shoulder  105  of the housing  10 , thereby compressing the packing element  200  to provide a gastight sealing effect. 
         [0062]    The axially extending cavity portion  130  of the atmospheric side porcelain insulator  13  accommodates therein a plurality of spring terminals  191 ,  191  held in electrical contact with electrode terminals (not shown) formed on the base end portion  19   b  of the gas sensing element  19  for supplying electric power thereto and extracting a detection output from the gas sensing element  19  to the outside. To this end, the spring terminals  191  are electrically connected through connecting members  192  to lead wires  16 . 
         [0063]    The lead wires  16  are taken out of the gas sensor  1  for connection to an externally located measuring device and a power supply or the like. 
         [0064]    The atmospheric side cover  2  takes a double-layer structure including an inner cover  2   a  and an outer cover  2   b . The inner cover  2   a , substantially cylindrical in cross section and made of stainless steel (SUS304), is directly fixed to a peripheral wall of a base end portion  100  of the housing  10  by welding. The outer cover  2   b , substantially cylindrical in cross section and made of stainless steel (SUS304), is fitted onto an outer circumference of a base end portion of the inner cover  2   a  and fixed thereto by caulking made at a caulked portion  2   d.    
         [0065]    The inner cover  2   a  has a base portion that accommodates therein a sealing member  17  which is fixedly retained with the caulked portion  2 d of the atmospheric side cover  2 . The sealing member  17  includes a rubber bush made of fluorine-contained rubber and has a columnar shape in cross section. The sealing member  17  has a central area formed with an axially extending atmospheric introduction bore  17   a  for introducing atmospheric air to an axially central area inside the atmospheric side cover  2 . A plurality of lead wire insertion holes  17   b ,  17   b  is formed in the sealing member  17  at plural positions around the atmospheric introduction bore  17   a.    
         [0066]    The sealing member  17  has a base end face  17   a  that carries thereon a ventilation filter  3 . The ventilation filter  3  is made of porous material such as, for instance, polytetrafluoroethylene (PTEF) and has high air ventilating capability that can permeates atmospheric air. 
         [0067]    Meanwhile, with the gas sensor I of the present embodiment, the measuring gas side cover  11  takes a double-layer structure including the inner cover  111  and the outer cover  112 . The outer cover  112  and/or the inner cover  111  are formed in mesh-like configurations by weaving the wire components  11   x  formed with a clearance  11   y  equal to or less than 0.5 mm. In addition, the wire components  11   x  are made of stainless steel wires each with a diameter equal to or greater than 0.3φ. 
         [0068]    With the gas sensor  1  of the present embodiment formed in such a structure, weaving the wire components  11   x  allows the outer cover  112  and/or the inner cover  111  to be formed in the mesh-like configurations so as to permit the clearance between the adjacent wire components  11   x  to lie in a value equal to or less than 0.5 mm. This allows the measuring gas side cover  11  to have increased water-incursion resistance to prevent water droplets from penetrating to the inside of the inner cover  111 . Accordingly, it becomes possible to provide a gas sensor that is less susceptible to water-incursion. 
         [0069]    Further, the wire components  11   x  are made of material such as stainless steel. This enables the gas sensor  1  to be used under severely high temperature environments such as those exceeding a temperature equal to or higher than 1000° C. In addition, the use of the wire components  11   x  with the diameter equal to or greater than 0.3φ enables the suppression of the measuring gas side cover  11  from deforming even when subjected to impact shocks applied from the outside. 
         [0070]    Furthermore, the measuring gas side cover  11  may take appropriately designed structure to have any suitable shape in cross section. 
       Second Embodiment 
       [0071]      FIGS. 3 to 5  are external views showing measuring gas side covers for use in gas sensors of other embodiments according the present invention. 
         [0072]      FIG. 3  shows one example of a measuring gas side cover  11 A formed in a saclike structure. In fabricating the measuring gas side cover  11 A with such a saclike structure shown in  FIG. 3 , the wire components  11   x , made of stainless steel, are woven into a mesh-like sheet. The mesh-like sheet is then pressed against a dome-shaped die (not shown) and rounded into a final saclike shape as shown in  FIG. 3  with the mesh-like sheet being maintained in a uniform mesh pattern. 
         [0073]    Further,  FIG. 4  shows another example of a measuring gas side cover  11 B formed in a cone-shaped configuration. In fabricating the measuring gas side cover  11 B with such a cone-shaped structure shown in  FIG. 4 , the mesh-like sheet, composed of the woven wire components  11   x  made of stainless steel, is wound on a cone-shaped die (not shown), making it easy to fabricate the measuring gas side cover  11 B. 
         [0074]    Furthermore,  FIG. 5  shows still another example of a measuring gas side cover  11 C composed of the stainless mesh sheet. The stainless mesh sheet is formed in a cylindrical shape with a leading end  11   s  being shackled and closed. In fabricating the measuring gas side cover  11 C, the stainless mesh sheet, composed of the woven wire components  11   x,  is pressed against the dome-shaped die and rounded into a cylindrical shape as shown in  FIG. 5 , after which the leading end  11   s  is shackled and closed, making it easy to fabricate the measuring gas side cover  11 C. 
       Third Embodiment        
       [0075]      FIGS. 6 to 8  are external views showing measuring gas side covers  11 D,  11 E,  11 F for use in gas sensors of other embodiments according the present invention. 
         [0076]    In  FIGS. 6 to 8 , right areas beyond a centerline show the measuring gas side covers in external appearances and left areas beyond the centerline represent internal structures of the measuring gas side covers. 
         [0077]    With a gas sensor  1 A shown in  FIG. 6 , the measuring gas side cover  11 D includes an outer cover  112 D. The outer cover  112 D includes a cylindrical metallic plate body  112   a , having a base end fixedly secured to the leading end portion  10   a  of the housing  10 , and a mesh-like cylindrical member  112   b , made of the woven stainless steel wire components  11   x , which is fixedly secured to a leading end of the cylindrical metallic plate body  112   a.    
         [0078]    The measuring gas side cover  111 D further includes an inner cover  111 D disposed inside the outer cover  112 D. The inner cover  111 D has a plurality of gas ventilation bores  111   a  through which measuring gases pass into an inside area of the measuring gas side cover  11 D. The cylindrical metallic plate body  112   a  is so shaped as to cover the gas ventilation bores  111   a  of the inner cover  111 D in a radial direction. This allow measuring gases to enter through the mesh-like cylindrical member  112   b  of the outer cover  112 D and pass through the gas ventilation bores  111   a  into the inside area of the inner cover  111 D. 
         [0079]    With such a measuring gas side cover  111 D, the gas ventilation bores  111   a , formed in the inner cover  111 D, can be protected with the cylindrical metallic plate body  112   a  forming the outer cover  112   a . This allows the measuring gas side cover  11 D to have increased water-incursion resistance. 
         [0080]    With a gas sensor  1 B shown in  FIG. 7 , a measuring gas side cover  11 E takes the form of a multi-layer structure formed in a cylindrical configuration. The measuring gas side cover  11 E includes two kinds of an inner cover  111 E and an outer cover  112 E. 
         [0081]    The inner cover  111 E is composed of a mesh-like sheet composed of the woven wire components  11   x.    
         [0082]    The outer cover  112 E includes a cylindrical metallic plate body, made by press forming a metallic plate into a cylindrical shape, which is formed with a plurality of gas ventilation bores  112   c.    
         [0083]    The inner cover  111 E and the outer cover  112 E are fitted to each other at both base ends thereof and fixedly secured to the leading end portion  10   a  of the housing  10 . 
         [0084]    With the gas sensor  1 B of such a structure shown in  FIG. 7 , measuring gases pass through the plurality of gas ventilation bores  112   c  formed in the outer cover  112 E to an inside area of the outer cover  112 E. Then, measuring gases, entered an internal space between the inner cover  11   b  and the outer cover  112 E, pass through the clearances  11   y  of the woven wire components  11   x , forming the inner cover  111 E, into an inside area of the inner cover  111 E to reach the leading end of the gas sensing element (not shown). Thus, the gas sensor  11 B has improved response in operation. 
         [0085]    With a gas sensor  1 C shown in  FIG. 8 , a measuring gas side cover  11 F takes the form of a multi-layer structure formed in a cylindrical configuration. The measuring gas side cover  11 F includes two kinds of an inner cover  111 F and an outer cover  112 F. 
         [0086]    Further, the inner cover  111 F internally accommodates therein the gas sensing element (not shown) and a heater (not shown) for raising a temperature of the gas sensing element. 
         [0087]    The inner cover  111 F includes a cylindrical metallic plate body, made by press forming a metallic sheet plate, and has a plurality of gas ventilation bores  111   a.    
         [0088]    The outer cover  1112  is made of a mesh-like sheet formed by weaving the wire components  11   x.    
         [0089]    The inner cover  111 F and the outer cover  112 F are fitted to each other at both base ends thereof and fixedly secured to the leading end portion  10   a  of the housing  10 . 
         [0090]    With the measuring gas side cover  11 F of such a gas sensor  1 C, a heat developed by the heater provided inside the inner cover  111 F is kept with the inner cover  111 F made of the metallic plate. This allows the gas sensing element to be activated on an earlier stage after startup of the engine. 
       Fourth Embodiment 
       [0091]      FIG. 9  is a longitudinal cross sectional view showing an overall structure of a gas sensor of a fourth embodiment according to the present invention. 
         [0092]    As shown in  FIG. 9 , the gas sensor  301  of the present embodiment comprises a hollow gas sensing element  302  with a leading end  302   a  closed and internally formed with an axial bore  302   b , and a heating element  303  embedded in the axial bore  302   b  of the gas sensing element  302  and composed of a bar-like ceramic heater. 
         [0093]    The gas sensing element  302  is made of a solid electrolyte having an oxygen ion conductivity. 
         [0094]    The gas sensing element  302  has a radially extending annular protrusion  302   c  formed at a base end of the leading portion  302   a  to have a larger diameter than that of the leading portion  302   a . An intermediate hollow portion  302   d  axially extends from the annular protrusion  302   c  in opposition to the leading end  302   a . The gas sensing element  302  has a hollow base end portion  302   e  with which a base end portion  303   a  of the heater  303  is rigidly supported. 
         [0095]    The gas sensor  301  further includes an element insulating holder  306 , made of porcelain insulating material such as ceramic, which has a hollow space  306   a  in which the intermediate hollow portion  302   d  of the gas sensing element  302  is rigidly supported. The element insulating holder  306  is accommodated in and rigidly supported with a metallic housing  309 . 
         [0096]    The metallic housing  309  includes a main housing body  309   a , acting as a gas sensing element accommodating body, which has a base end portion  309   b  having a terminal end formed with a radially inward annular flange  309   c  and a leading end portion  309   d  having an outer periphery formed with a threaded portion  309   e  adapted to be screwed onto a mounting area of an exhaust pipe of an internal combustion engine. 
         [0097]    The housing  309  has a small diameter bore  309   f  formed inside the leading end portion  309   d , an intermediate bore  309   g  formed inside the main housing body  309   a  for retaining the annular protrusion  302   c  of the gas sensing element  302 , and a large diameter bore  309   h  formed inside the main housing body  309   a  and the base end portion  309   b.    
         [0098]    A gastight sealant  308 , made of ceramic powder such as talc, is filled in an annular space between an outer periphery of the intermediate hollow portion  302   d  and the large diameter bore  309   g  of the metallic housing  309  to provide a gastight sealing effect. The element insulating holder  306  is fitted to the large diameter bore  309   g  of the metallic housing  309  so as to compact the gastight sealant  308 . In addition, the gas sensor  1  further includes an atmospheric side cover  314  having a leading end fixedly secured to the base end portion  309   b  of the metallic housing  309  by welding, and the measuring gas side cover  11  fixedly secured to a terminal end of the leading end portion  309   d.    
         [0099]    Further, a pressure ring  315  is held in pressured contact with the annular flange  309   c  of the metallic housing  309  to press the element insulating holder  306  against the gastight sealant  308 . Thus, the element insulating holder  306  and the gastight sealant  308  are fixed to the metallic housing  309  at a base end thereof. 
         [0100]    The atmospheric side cover  314  has a large diameter leading end  314   a  fitted to and fixed to the base end portion  309   b  of the metallic housing  309 . The atmospheric side cover  314  also has a small diameter base end portion  314   b  with an open end that is caulked to fixedly hold a sealing member  317  made of resilient material such as rubber or the like for providing a sealing effect. The atmospheric side cover  314  accommodates therein an insulator  318  at a position in close proximity to an annular shoulder portion  314   c  between the leading end portion  314   a  and the base end portion  314   b . The insulator  318  is held with the atmospheric side cover  314  by means of a pressure spring  316  disposed between the atmospheric side cover  314  and the insulator  318 . 
         [0101]    Further, the sealing member  317  has a ventilation bore  317   a  and a plurality of lead insertion bores  317   b , formed in areas around the ventilation bore  317   a , through which lead wires  321  extend. 
         [0102]    Meanwhile, with the gas sensor  301  of the present embodiment, the measuring gas side cover  11  takes the same double-layer structure as that of the gas sensor  1  of the first embodiment shown in  FIG. 1  and includes the inner cover  111  and the outer cover  112 . In addition, the inner cover  111  and/or the outer cover  112  are formed in the mesh-like configuration by weaving the wire components  11   x , mentioned above, which have the clearance  11   y  equal to or less than 0.5 mm. In addition, the wire components  11   x  are made of stainless steel and each of the wire components  11   x  has a diameter equal to or greater than 0.3φ. 
         [0103]    With the gas sensor  301  formed in such a structure, the measuring gas side cover  11  is formed in the mesh-like configuration by weaving the sire components  11   x  so as to provide the clearance  11   y  equal to or less than 0.5 mm. Thus, the measuring gas side cover  11 , formed in the mesh-like configuration with such a clearance, effectively prevents water droplets from entering the inside of the measuring gas side cover  11 . This makes it possible to provide a gas sensor that can prevent the gas sensing element from suffering water-incursion. 
         [0104]    Further, the wire components  11   x  are made of material such as stainless steel, providing heat resistant property. This enables the gas sensor  301  to be used under severe environments such as those exceeding a temperature equal to or higher than 1000° C. In addition, the use of the wire components  11   x  with the diameter equal to or greater than 0.3φ makes it possible to suppress the measuring gas side cover  11  from deforming even when subjected to impact shocks applied from the outside. 
       EXAMPLE  
       [0105]      FIG. 10  is a graph showing evaluated comparison results between the gas sensor of the present embodiment and the gas sensor of the related art. 
         [0106]    For comparison purposes,  30  samples of the gas sensor of the related art were manufactured each with the same dimension as that of the gas sensor of the present invention and had a measuring gas side cover formed with six gas ventilation bores each having a diameter of φ3 mm. Meanwhile, 30 samples of the gas sensor of the present invention were manufactured each having a measuring gas side cover formed in a mesh-like structure with dimensions of relevant parts mentioned above. Upon using these two types of the gas sensors, tests were conducted to obtain evaluations described below. 
         [0107]    In particular, first, powder was coated on the gas sensing elements. Then, the gas sensing elements were mounted on an exhaust pipe of an internal combustion engine and the gas sensing elements were heated to a temperature of 700° C. using a heater. Subsequently, water was poured into an inside of the exhaust pipe. Next, a blower was driven to blow off water droplets onto the gas sensing elements for a time period of three minutes. Then, the gas sensors were collected to confirm whether or not cracking occurred on the gas sensing elements. Tests were conducted on 30 samples of each of the gas sensors of the related art and the gas sensors of the present embodiment in the same sequence mentioned above. 
         [0108]    As a result of tests, among the 30 pieces of the examples of the related art, 10 samples of the gas sensor of the related art encountered with cracking occurring in the gas sensing elements with a cracking incidence rate of approximately 30% as will be apparent from the graph of  FIG. 10 . On the contrary, no cracking was found on the samples of the gas sensor of the present embodiment. 
         [0109]    While the specific embodiments of the present invention have been described in detail, the present invention is not limited to the particularly illustrated structures of the gas sensors of the various embodiment set forth above provided that the measuring gas side covers achieve the task of the present invention. It will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. For instance, the wire components are not limited to stainless steel and may be made of other heat resistant material such as Inconel or the like. In addition, measuring gases to be detected are not limited to oxygen and may include other gases such as NOx, CO and HC or the like. Moreover, the gas sensing element may include any one of a stack type, a cup type, etc. Thus, the particular arrangements disclosed are meant to be illustrative only and not limited to the scope of the present invention, which is to be given the full breadth of the following claims and all equivalents thereof.