Abstract:
An improved structure of a gas sensor is provided which may be employed in an oxygen measuring device of an air-fuel ratio control system measuring an oxygen content in exhaust gasses of an internal combustion engine of automotive vehicles. The gas sensor includes a sensing unit which is disposed in a housing and has defined in an end portion thereof a reference gas chamber to be filed with a reference gas used in determining a given gas component content in gasses, a metallic cover installed on the housing to cover the other end portion of the sensing unit; and a cylindrical insulation porcelain disposed in the metallic cover. The insulation porcelain has a groove formed on an outer peripheral wall thereof to define a portion of a reference gas passage communicating between an air inlet formed in the metallic cover and the reference gas chamber. The outer peripheral wall is substantially circular in cross section for avoiding the deformation of the insulation porcelain arising in compressing a material of the insulation porcelain such as ceramic powder during a manufacturing process.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to an improvement on a gas sensor which may be employed in an oxygen measuring device of an air-fuel ratio control system measuring an oxygen content in exhaust gasses of an internal combustion engine of automotive vehicles. 
     2. Background Art 
     For burning control of fuel in internal combustion engines, modem automotive vehicles use a gas sensor, e.g., as an oxygen sensor which is installed in an exhaust system to measure the concentration of oxygen in exhaust gasses. 
     European Patent Application EP 0918215 A2 teaches an oxygen sensor designed to define an air gap between an insulation porcelain and a metallic cover which is large enough for admitting air used as a reference gas in determining the concentration of oxygen. FIG.  21 ( a ) illustrates the insulation porcelain disclosed in this application. The insulation porcelain  9  consists of a large-diameter portion  92  and a small-diameter portion  91 . The small-diameter portion  91  is of a rectangular shape and has formed therein through holes  30  within which lead lines are held. The insulation porcelain  9  is fitted within a metallic cover (not shown) to define the air gap between an inner wall of the metallic cover and the small-diameter portion  91 . 
     The formation of the insulation porcelain  9 , however, experiences, as shown in FIG.  21 ( b ), the deformation of the small-diameter portion  91  in compressing the ceramic powder because the interval O between an outer wall  911  of the small-diameter portion  91  and an outer wall  921  of the large-diameter portion  92  varies in a circumferential direction of the insulation porcelain  9 , thus resulting in a decreased strength of the insulation porcelain  9 . This problem is common to gas sensors of the type having a reference gas chamber admitting a reference gas used in determining the concentration of a specific gas. 
     SUMMARY OF THE INVENTION 
     It is therefore a principal object of the invention to avoid the disadvantages of the prior art. 
     It is another object of the invention to provide an improved structure of a gas sensor capable of admitting a sufficient amount of a reference gas into a reference chamber without scarifying the strength of an insulation porcelain. 
     According to one aspect of the invention, there is provided an improved structure of a gas sensor designed to measure a given component content in a gas. The gas sensor comprises: (a) a housing; (b) a sensing unit having a length disposed in the housing, the sensing unit having defined in a first end portion thereof a reference gas chamber to be filed with a reference gas used in providing a sensor signal through a lead which is employed in determining the given gas component content in the gas; (c) a first metallic cover installed on the housing to cover a second end portion of the sensing unit; (d) a second metallic cover installed on a periphery of the first metallic cover; (e) a first vent formed in the first metallic cover; (f) a second vent formed in the second metallic cover which communicates with the firs vent to admit the reference gas into the reference gas chamber through a reference gas passage; and (g) an insulating member disposed in the first metallic cover, having formed therein a hole through which the lead passes to connect with the sensing unit, the insulating member being made of a cylindrical porcelain having an outer peripheral wall which is substantially circular in cross section and which defines the reference gas passage. 
     In the preferred mode of the invention, the insulating member has a first end surface and a second end surface opposed to the first end surface in a longitudinal direction of the gas sensor parallel to the length of the sensing unit. The insulating member has a through hole extending in a direction of the first end surface to the second end surface to define a portion of the reference gas passage. 
     The insulating member is arranged in alignment with the sensor unit and has a groove formed in the outer peripheral wall which extends from the first vent to the first end surface to define a portion of the reference gas passage. 
     The insulating member has a small-diameter portion formed closer to the first end surface and a large-diameter portion continuing from the small-diameter portion. If a length of the small-diameter portion in a direction is defined as L 1 , a distance L 2  between the large-diameter portion and an upstream end of the groove facing the first vent lies within a range of L 1 /5 to L 1 /2. 
     The first vent has a diameter R in the longitudinal direction of the gas sensor. The distance between a point on a periphery of the first vent closest to the second end surface of the insulating member and an upstream end of the groove facing the first vent is greater than or equal to R/3. 
     The insulating member may alternatively have a groove formed in the outer peripheral wall which extends from the first vent to the second end surface to define a portion of the reference gas passage. 
     If a plane tangent to a periphery of the insulating member is defines as P, a plane passing through the deepest point of the groove in parallel to the plane P is defined as P 1 , and a plane passing in parallel to the plane P through the center of the through hole formed in the insulating member is defined as P 2 , a distance S 1  between the planes P and P 1  is smaller than or equal to a distance S 2  between the planes P and P 2 . 
     If a width of the reference gas passages defined on the outer peripheral wall of the insulating member is defined as H 1 , and a diameter of the insulating member is defined as H 2 , they are so selected as to meet a condition of H 1 ≦H 2 /2 1/2 . 
     The insulating member may alternatively have formed therein a plurality of lead holes through which leads pass to connect with the sensing unit. The reference gas passage may be defined at a location where a line passing through a center of the insulating member between adjacent two of the lead holes intersects the outer peripheral wall of the insulating member. 
     The reference gas passage may alternatively be defined by a hole formed in the insulating member which extends from a portion of the outer peripheral wall of the insulating member facing the first vent and communicates with the hole through which the lead passes. 
     The insulating member may have formed therein a lateral hole extending between the lead holes in communication with the through hole extending in the direction of the first end surface to the second end surface of the insulating member to define the reference gas passage. 
     The reference gas passage may alternatively be defined by a through hole formed in the insulating member which extends from a portion of the outer peripheral wall facing the first vent to the chamber through the small-diameter portion and the large-diameter portion. 
     The reference gas passage may alternatively be defined by an inner wall of the first metallic cover and a surface of the outer peripheral wall of the insulating member tapering off to the first end surface. 
     The reference gas passage may alternatively be defined by an inner wall of the first metallic cover and a first and a second annular step formed on the outer peripheral wall of the insulating member. The first annular step is smaller in diameter than the second step. 
    
    
     BRIEF DESPCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
     FIG. 1 is a longitudinal sectional view which shows an oxygen sensor equipped with an insulating holder according to the first embodiment of the invention; 
     FIG. 2 is a partially enlarged view which shows a structure of an insulating holder of the first embodiment; 
     FIG.  3 ( a ) is a horizontal sectional view taken along the line A—A in FIG.  3 ( b ); 
     FIG.  3 ( b ) is a longitudinal sectional view which shows reference gas passages defined in an insulating holder of the first embodiment; 
     FIG. 4 is a horizontal sectional view which shows an insulating holder of the first embodiment; 
     FIG. 5 is a longitudinal sectional view which shows another type of oxygen sensor equipped with an insulating holder in the first embodiment; 
     FIG.  6 ( a ) is a lateral sectional view which shows a modification of the insulating holder of FIG. 4; 
     FIG.  6 ( b ) is a lateral sectional view which shows another modification of the insulating holder of FIG. 4; 
     FIG. 7 is a partially vertical sectional view which shows reference gas passages of a gas sensor according to the second embodiment of the invention; 
     FIG. 8 is a lateral sectional view which shows reference gas passages of a gas sensor according to the third embodiment of the invention; 
     FIG. 9 shows a test machine used for measuring the strength of an insulating holder of the gas sensor in FIG. 8; 
     FIG. 10 is a graph which shows the strength of the insulating holder in FIG. 8; 
     FIG. 11 is a graph which shows the strength of the insulating holder in FIG. 8 for different values of S 1 ; 
     FIG. 12 is a graph which shows the strength of the insulating holder in FIG. 8 for different values of H 1 ; 
     FIG.  13 ( a ) is a horizontal sectional view taken along the line B—B in FIG.  13 ( b ); 
     FIG.  13 ( b ) is a longitudinal sectional view which shows reference gas passages defined in an insulating holder according to the fourth embodiment of the invention; 
     FIG.  14 ( a ) is a horizontal sectional view taken along the line C—C in FIG.  14 ( b ); 
     FIG.  14 ( b ) is a longitudinal sectional view which shows reference gas passages defined in an insulating holder of the fifth embodiment; 
     FIG.  15 ( a ) is a horizontal sectional view taken along the line D—D in FIG.  15 ( b ); 
     FIG.  15 ( b ) is a longitudinal sectional view which shows reference gas passages defined in an insulating holder which is a modification of the one shown in FIGS.  14 ( a ) and  14 ( b ); 
     FIG. 16 shows a modification of the fourth embodiment in FIGS.  13 ( a ) and  13 ( b ); 
     FIGS.  17 ( a ),  17 ( b ), and  17 ( c ) show modifications of reference gas passages, as shown in FIGS.  14 ( a ),  14 ( b ),  15 ( a ),  15 ( b ), and  16 ; 
     FIG.  18 ( a ) is a horizontal sectional view taken along the line E—E in FIG.  18 ( b ); 
     FIG.  18 ( b ) is a longitudinal sectional view which shows reference gas passages defined in an insulating holder of the sixth embodiment of the invention; 
     FIGS.  19 ( a ) and  19 ( b ) show an insulating holder according to the seventh embodiment of the invention; 
     FIG. 20 shows an insulating holder according to the eighth embodiment of the invention; and 
     FIG.  21 ( a ) is a plan view which shows a conventional insulating holder installed in an oxygen sensor; and 
     FIG.  21 ( b ) is a side view which shows the insulating holder of FIG.  21 ( a ) which is deformed during a production process. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to FIG. 1, there is shown an oxygen sensor  1  according to the first embodiment of the invention which may be employed in an air-fuel ratio control system for automotive vehicles. Note that the present invention is not limited to an oxygen sensor and may alternatively used with a variety of gas sensors such as HC, CO, and NOx sensors. 
     The oxygen sensor  1  generally includes, a housing  10 , a sensing unit  2 , and signal leads  291  and  292  connected to the sensing unit  2 . The signal leads  291  and  292  provide sensor signals to an external device which are used, as will be described later in detail, in determining the concentration of oxygen contained in a gas. The sensing unit  2  has formed therein a reference gas chamber  250  into which a reference gas (i.e., air) is admitted for use in providing the sensor signals through the signal leads  291  and  292 . This technique is well known in the art, and explanation thereof in detail will be omitted here. For instance, U.S. application Ser. No. 09/196,693, filed on Nov. 20, 1998, assigned to the same assignee as that of this application teaches a gas measuring method in this type of gas sensor, and disclosure of which is incorporated herein by reference. 
     The oxygen sensor  1  also includes a first metallic cover  11  and a second metallic cover  12 . The first metallic cover  11  covers a base portion of the sensing unit  2  and is fitted in an end of the housing  10 . The second metallic cover  12  is disposed around an upper portion of the first metallic cover  11 , as viewed in the drawing. The first and second metallic covers  11  and  12  have formed therein first and second air vents  110  and  120  in alignment with each other for admitting the reference gas into the reference gas chamber  250 . 
     An insulating holder  3  is, as clearly shown in FIG. 2, disposed inside the first metallic cover  11  which has formed therein through holes  30  into which the leads  191  and  192  are inserted. The insulating holder  3  is made of a hollow cylindrical insulation porcelain and defines reference gas passages  35  between an outer wall  311  and an inner wall of the first metallic cover  11  which lead to the reference gas chamber  250 . 
     The sensing unit  2  is, as shown in FIG. 1, retained within the housing  11 . The sensing unit  2  and the housing  11  are hermetically sealed. 
     The first metallic cover  11  consists of two cover members: outer and inner cover members  111  and  112 . The inner cover member  112  is joined at an end to an upper end of the housing  10  through a caulking ring  119 . The outer cover member  111  is joined to an upper portion of the inner cover member  112  by crimping. 
     The inner cover member  112  has an open end  116 , as shown in FIG. 2, abutting on a lower surface  328  of a large-diameter portion  32  (i.e., a flange) of the insulating holder  3  to retain the insulating holder  3  within the fist metallic cover  11  against a spring pressure of a spring  117  disposed between an upper surface  329  of the large-diameter portion  32  and a shoulder  118  of the outer cover member  111 . 
     A sealing member  14  is fitted in an upper end of the inner cover member  112  through which the leads  191 ,  192 , and  251  pass. 
     The insulating holder  3 , as clearly shown in FIGS.  3 ( a ) and  3 ( b ), has formed therein four through holes  30  through which signal pickup leads  291  and  292 , a pair of leads  259  connected to a heater  25 , as will be described later in detail, the leads  191  and  192 , and a pair of leads  251  pass. The leads  291 ,  292 , and  259  are connected to the leads  191 ,  192 , and  251  through connectors  195  within the through holes  30 , respectively. Note that another pair of leads passes through the insulating holder  3 , but it is located in an invisible area of the drawing and omitted here. 
     The insulating holder  3  has formed in an a lower portion thereof, as shown in FIG.  3 ( b ), a cavity  309  to which all the through holes  30  are exposed and in which a base portion of the sensing unit  2  is disposed. 
     The insulating holder  3 , as shown in FIGS.  3 ( a ) to  4 , includes the large-diameter portion  32 , a small-diameter portion  31 , and a tip portion  33 . The tip portion  33  projects from the large-diameter portion  32  toward the tip of the sensing unit  2  and is smaller in diameter than the large-diameter portion  32 . These portions  31 ,  32 , and  33  have circular sections, as clearly shown in FIG.  4 . The large-diameter portion  32  and the small-diameter portion  31  are arranged coaxially, so that the interval between an outer wall  321  of the large-diameter portion  32  and an outer wall  311  of the small-diameter portion  31  is kept constant in a circumferential direction of the insulating holder  3 . This eliminates the problem encountered in the prior art structure, as shown in FIGS.  21 ( a ) and  21 ( b ), that the insulating porcelain  9  is deformed during a production process. 
     The reference gas passages  35  are, as can be seen from FIGS.  3 ( a ) and  4 , defined between the inner wall of the outer cover member  111  and four grooves  160  provided in an outer wall  311  of the small-diameter portion  31  of the insulating holder  3 . The grooves  160  each have an arc-shaped cross section and are, as shown in FIG.  3 ( a ), formed at locations where lines T passing through the center of the insulating holder  3  between adjacent two of the through holes  30  intersect the outer wall  311  of the small-diameter portion  31 . This allows the small-diameter portion  31  to have wider round outer surfaces formed at regular intervals in the circumferential direction of the insulating holder  3 , thus resulting in an improved strength as compared with the prior art structure shown in FIGS.  21 ( a ) and  21 ( b ). Each of the reference gas passages  35  extends vertically, as viewed in FIG.  3 ( b ), from one of the first air vents  110  to a base end  301  of the insulating holder  3 . 
     The insulating holder  3  also has a central passage  39  extending along a longitudinal center line thereof which opens into the cavity  309 . 
     The second metallic cover  12  is installed on the periphery of the upper portion of the first metallic cover  11  and is crimped to form, as shown in FIG. 2, two annular joints  161  and  162  to the first metallic cover  11  for retaining a water-repellent filter  13  between the first and second metallic covers  11  and  12 . Specifically, the first metallic cover  11 , the second metallic cover  12 , and the water-repellent filter  13  are connected fixedly to each other through the annular joints  161  and  162 . 
     The sensing unit  2 , as shown in FIG. 1, consists of a hollow cylindrical solid electrolyte body  20  with a bottom, a measuring electrode formed on an outer wall of the body  20  exposed to a gas chamber  150 , and a reference electrode formed on an inner wall of the body  20  exposed to the reference gas chamber  250 . This structure is known, for example, in European Patent Application EP 0918215 A2 assigned to the same assignee as that of this application, disclosure of which is incorporated herein by reference. 
     Within the reference gas chamber  250 , a bar-shaped heater  25  is disposed which heats the measuring and reference electrodes up to a temperature at which the oxygen concentration can be measured correctly. The measuring and reference electrodes have conductive terminals connected to the signal pickup leads  291  and  292 . The heater  25  is supplied with power through the leads  259 . 
     In operation, the air  8  which is, as indicated by arrows in FIG.  3 ( b ), introduced from the second air vents  120  to the first air vents  110  through the water-repellent filter  13  flows upward, as viewed in the drawing, in the reference gas passages  35  and reaches the base end  301  of the insulating holder  3 . Next, the air  8  passes through a gap between the base end  301  and the bottom of the sealing member  14  and flows downward into the cavity  309  through the holes  30  and the central holes  39 . The air  8  emerging from the lower end  302  of the insulating holder  3  enters the reference gas chamber  250  at the upper end of the sensing unit  2 . 
     The oxygen sensor  1  of this embodiment is designed to measure an oxygen content in gasses using the oxygen concentration dependent electromotive force or the limiting current. Specifically, the measurement of the oxygen content using the oxygen concentration dependent electromotive force is accomplished by monitoring through the measuring and reference electrodes the electromotive force produced in the solid electrolyte body  20  which depends upon a difference in oxygen concentration between the air  8  and the gas within the gas measuring chamber  150 . The measurement of the oxygen content using the limiting current is accomplished by applying a given voltage across the measuring and reference electrodes to pick up a limiting current which depends upon the concentration of oxygen in the gasses. These techniques are known in the art, and explanation thereof in detail will be omitted here. The sensing unit  2  may alternatively be formed by laminations such as one shown in FIG. 5 in which the sensing unit  2  is made of a laminated plate having a heater layer. Further, U.S. Pat. No. 5,573,650, issued on Nov. 12, 1996 to Fukaya et al. teaches such a structure, disclosure of which is incorporated herein by reference. 
     The grooves  160  formed in the small-diameter portion  31  of the insulating holder  3  to define the reference gas passages  35  may alternatively be of generally rectangular configuration in cross section, as shown in FIG.  6 ( a ), or have parallel steps, as shown in FIG.  6 ( b ), defining an additional central groove. 
     FIG. 7 shows the second embodiment of the invention. 
     The reference gas passages  35  are, like the first embodiment, defined by the grooves  160  formed in the outer wall  311  of the insulating holder  3 , but each of the grooves  160  of this embodiment has a lower end  350  defining an inlet which leads to one of the first air vents  110  and which meets the following locational conditions. 
     Letting the length of the small-diameter portion  31  of the insulating holder  3  and the distance between the upper surface  329  of the large-diameter portion  32  and the lower end  350  of each of the grooves  160  be L 1  and L 2 , respectively, L 2  lies within a range of L 1 /5 to L 1 /2, preferably L 1 /3. For instance, L 1 =12.5 mm, and L 2 =6 mm. This allows the sensor to be decreased in size without sacrificing the strength of the small-diameter portion  31  of the insulating holder  3 . 
     The lower ends  350  face the first air vents  110 , respectively. If the diameter R of each of the first air vents  110  is defined as R, and the distance between a lowermost portion of  119  of each of the first air vents  110  and the lower end  350  of a corresponding one of the grooves  160  is defined as R 1 , then they are so selected as to meet a condition of R 1 ≦R/3. For instance, R=2 mm, and R 1  is 0.5 mm. This ensures the admission of a sufficient amount of air (i.e., the reference gas) into the sensor. 
     Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIG. 8 shows the third embodiment of the invention. 
     The reference gas passages  35  are, like the first embodiment, defined by the grooves  160  formed in the outer wall  311  of the insulating holder  3 , but the grooves of this embodiment  160  are designed so as to meet the following geometrically conditions. 
     If a plane tangent to the outer wall  311  of the small-diameter portion  31  is defines as P, a plane passing through the deepest point M of each of the grooves  160  in parallel to the plane P is defined as P 1 , and a plane passing in parallel to the plane P through the center O 1  of one of the holes  30  located closest to the plane P is defined as P 2 , the distance S 1  between the planes P and P 1  is smaller than or equal to the distance S 2  between the planes P and P 2  (S 1 ≦S 2 ). For instance, S 1 =1 mm, and S 2 =2 mm. 
     If the width of each of the reference gas passages  35  is defined as H 1 , and the diameter of the small-diameter portion  31  of the insulating holder  3  is defined as H 2 , they are so selected as to meet a condition of H 1 ≦H 2 /2 1/2 , preferably H 1 ≦(2×H 2 )/3. For instance, H 1 =3 mm, and H 2 =10 mm. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     Strength tests were performed for a comparative test piece equivalent to the insulating holder  3  not having the grooves  60  in the small-diameter portion  31 , the prior art insulation porcelain  9  shown in FIG.  21 ( a ), and the insulating holder  3  of this embodiment using a test machine as shown in FIG.  9 . The results of the tests are shown in FIG.  10 . 
     The test machine has a table  80  on which a round bar  81  having a diameter of 5 mm is retained, and a support surface  810  is formed. The insulating holder  3  is placed in contact of the small-diameter portion  31  and the large-diameter portion  32  with the round bar  81  and the support surface  810 , respectively. A round bar  82  having a diameter of 4 mm is placed on the small-diameter portion  31  of the insulating holder  3 . The pressure F which causes the insulating holder  3  to be deformed 0.05 mm per minute is applied to the round bar  82  to measure the disruptive strength. The same texts were performed for the prior art insulation porcelain  9  and the comparative test piece. 
     The graph of FIG. 10 shows that the insulating holder  3  of this embodiment has a disruptive strength greater than that of the prior art insulation porcelain  9  closer to that of the comparative text piece. 
     The strength texts were also performed on the insulating holders  3  in which H 1 =3 mm, S 2 =2 mm, and S 1  has different values. The results of the tests are indicated in a graph of FIG.  11 . As shown by the graph, the disruptive strength of the insulating holder  3  is decreased greatly when S 1  exceeds S 2 (S 1 &gt;S 2 ). 
     The strength texts were also performed on the insulating holders  3  in which S 1 =0.5 mm, H 2 =10 mm, and H 1  has different values. The results of the tests are indicated in a graph of FIG.  12 . As shown by the graph, the disruptive strength of the insulating holder  3  is decreased greatly when H 1  exceeds H 2 /2 1/2 . 
     Therefore, it is appreciated that the insulating holder  3  meeting the condition of S 1 ≦S 2  and/or the condition of H 1 ≦H 2 /2 1/2  has an increased strength. 
     FIGS.  13 ( a ) and  13 ( b ) show the fourth embodiment of the insulating holder  3 . 
     The insulating holder  3  has four grooves, similar in shape to the grooves  160  in the first embodiment, which are formed in the small-diameter portion  31  and the upper surface  329  and the side surface of the large-diameter portion  32  to define reference gas passages  36 . Each of the grooves is made up of a vertical groove  361 , a horizontal groove  362 , and a vertical groove  363 . The vertical grooves  361  are formed in the side wall of the small-diameter portion  31  at regular intervals. The horizontal grooves  362  formed in the upper surface  329  of the large-diameter portion  32 . The vertical grooves  363  are formed in the side wall of the tip portion  33 . Each of the reference gas passages  36  extends from one of the first air vents  110  to an annular gap defined between the tip portion  33  of the insulating holder  3  and the inner wall of the outer cover member  111 . Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIGS.  14 ( a ) and  14 ( b ) show the fifth embodiment of the insulating holder  3 . 
     The insulating holder  3  has four holes formed at regular intervals in the outer wall  311  thereof to define reference gas passages  37  extending horizontally, as viewed in FIG.  14 ( b ). Each of the reference gas passages  37  establishes communication between one of the first air vents  110  and one of the through holes  30 . Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     Four holes, as shown in FIGS.  15 ( a ) and  15 ( b ), which are greater in size than the through holes  30  may be formed at regular intervals in the outer wall  311  thereof to define the reference gas passages  37  extending horizontally, as viewed in FIG.  15 ( b ). 
     FIG. 16 shows a modification of the fifth embodiment in FIGS.  14 ( a ) and  14 ( b ). 
     The insulating holder  3  has four holes defining reference gas passages  38 . Each of the reference gas passages  38  extends from one of the air vent holes  110  to the central hole  39  between the adjacent two of the holes  30 . 
     Each of the reference gas passages  37  and  38  in FIGS.  14 ( a ),  14 ( b ),  15 ( a ),  15 ( b ), and  16  may have any of different sectional shapes as shown in FIGS.  17 ( a ),  17 ( b ), and  17 ( c ). 
     FIGS.  18 ( a ) and  18 ( b ) show the insulating holder  3  according to the sixth embodiment of the invention. 
     The insulating holder  3  has formed therein four vertical holes which define reference gas passages  41 . Each of the reference gas passages  41  extends from one of the first air vents  110  to the cavity  309  in the insulating holder  3 . Specifically, each of the reference gas passages  41  is made up of two sections: one is defined by a groove formed in the outer wall  311  extending from one of the first air vents  110  to a corner between the small-diameter portion  31  and the large-diameter portion  32  and the inner wall of the outer cover member  111  and the other is defined by a slant hole extending inwardly from the corner between the small-diameter portion  31  and the large-diameter portion  32  to the cavity  309 . Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIGS.  19 ( a ) and  19 ( b ) show the insulating holder  3  according to the seventh embodiment of the invention. 
     The insulating holder  3  has an annular step  42  formed around the outer wall  311  of the small-diameter portion  31  to define an upper annular passage  170  and a lower annular passage  175  between the outer wall  311  and the inner wall of the outer cover member  111 . Specifically, the upper annular passage  170  is greater in volume than the lower annular passage  175 . The lower annular passage  175  directs the air  8  admitted from the first air vents  110  to the upper annular passage  170 . The upper annular passage  170  directs the air  8  into the holes  30  and the central hole  39  through the base end  301  of the insulating holder  3 . Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIG. 20 shows the insulating holder  3  according to the eighth embodiment of the invention. 
     The insulating holder  3  has a tapered wall  43  formed on the small-diameter portion  31  to define an annular passage  180  between itself and the inner wall of the outer cover member  111 . The annular passage  180  increases in volume toward the base end  301  of the insulating holder  3  and directs the air admitted from the first air vents  110  into the holes  30  and the central hole  39  through the base end  301 . Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     The above second to eighth embodiments may be used with the oxygen sensor shown in FIG. 1 or  5 . Some of the first to eighth embodiments may be combined to form two or more types of reference gas passages in the insulating holder  3 . 
     While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims.