Patent Publication Number: US-2017363596-A1

Title: Gas sensor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gas sensor. 
     2. Description of the Related Art 
     An example of a known gas sensor detects the concentration of predetermined gas, such as NOx or oxygen, in measurement-object gas, such as exhaust gas of an automobile. For example, PTL 1 describes a gas sensor including an outer protective cover and an inner protective cover. The inner protective cover has a cylindrical shape with a bottom and is disposed between the outer protective cover and a sensor element so as to cover the front end of the sensor element. According to PTL 1, the inner protective cover is formed in a predetermined shape so that the sensor element has quick responsiveness in gas concentration detection and high heat retaining properties at the same time. 
     CITATION LIST 
     Patent Literature 
     PTL 1: WO 2014/192945 
     SUMMARY OF THE INVENTION 
     It is desirable that such a gas sensor has quick responsiveness in gas concentration detection. 
     The present invention has been made to solve the above-described problem, and the main object of the present invention is to increase the responsiveness in gas concentration detection. 
     To achieve the above-described object, the present invention employs the following configuration. 
     A gas sensor according to the present invention comprises: 
     a sensor element having a gas inlet through which measurement-object gas is introduced and capable of detecting a concentration of a predetermined gas in the measurement-object gas that flows into the sensor element through the gas inlet; 
     an inner protective cover that has a sensor element chamber thereinside and in which one or more element-chamber inlet and one or more element-chamber outlet are arranged, the sensor element chamber accommodating a front end of the sensor element and the gas inlet, the element-chamber inlet being an entrance to the sensor element chamber, and the element-chamber outlet being an exit from the sensor element chamber; and 
     an outer protective cover that is disposed outside the inner protective cover and in which one or more outer inlet and one or more outer outlet are arranged, the outer inlet being an entrance from outside for the measurement-object gas, and the outer outlet being an exit to the outside for the measurement-object gas, 
     wherein the outer protective cover and the inner protective cover form a first gas chamber and a second gas chamber as spaces therebetween, the first gas chamber being at least a portion of a flow channel for the measurement-object gas between the outer inlet and the element-chamber inlet, and the second gas chamber being at least a portion of a flow channel for the measurement-object gas between the outer outlet and the element-chamber outlet and not being directly connected to the first gas chamber, and 
     a cross-sectional area ratio S1/S2, which is a ratio of a total cross-sectional area S1 [mm 2 ] of the outer inlet to a total cross-sectional area S2 [mm 2 ] of the outer outlet, is more than 2.0 and 5.0 or less. 
     The measurement-object gas that flows around the gas sensor enters the gas sensor through the outer inlet in the outer protective cover, passes through the first gas chamber and the element-chamber inlet, and reaches the gas inlet in the sensor element chamber. The measurement-object gas in the sensor element chamber passes through the element-chamber outlet and the second gas chamber and flows out through the outer outlet in the outer protective cover. When the cross-sectional area ratio S1/S2 is more than 2.0, the total cross-sectional area S1 is relatively large, so that the flow rate at which the measurement-object gas enters through the outer inlet tends to increase. In addition, the total cross-sectional area S2 is relatively small, so that the flow rate at which the measurement-object gas tries to enter through the outer outlet (backflow) tends to decrease. Accordingly, the measurement-object gas in the space around the gas inlet is easily replaced by the measurement-object gas that has entered. As a result, the responsiveness in gas concentration detection increases. When the total cross-sectional area S2 is too small, the flow rate at which the measurement-object gas flows out through the outer outlet decreases, and the responsiveness may decrease accordingly. However, when the cross-sectional area ratio S1/S2 is 5.0 or less, the reduction in responsiveness can be suppressed. 
     In the gas sensor according to the present invention, the cross-sectional area ratio S1/S2 is preferably 2.5 or more, more preferably, 3.0 or more, and still more preferably, 3.4 or more. As the cross-sectional area ratio S1/S2 increases, the responsiveness in gas concentration detection tends to increase. 
     In the gas sensor according to the present invention, the total cross-sectional area S1 may be 10 mm 2  or more. The total cross-sectional area S1 may also be 30 mm 2  or less. The total cross-sectional area S2 may be 2 mm 2  or more. The total cross-sectional area S2 may also be 10 mm 2  or less. 
     In the gas sensor according to the present invention, the outer protective cover may have a cylindrical shape and include a side portion and a bottom portion. The outer outlet may not be arranged in the side portion of the outer protective cover. When there is an outer outlet formed in the side portion of the outer protective cover, the responsiveness may vary depending on the relationship between the position of the outer outlet in the side portion and the direction in which the measurement-object gas flows around the outer outlet. For example, when the outer outlet in the side portion opens parallel to, and toward the upstream side of, the direction in which the measurement-object gas flows, the flow of the measurement-object gas that tries to flow out from the space inside the outer protective cover through the outer outlet in the side portion is impeded by the measurement-object gas that flows around the outer outlet, and the responsiveness tends to decrease as a result. If the responsiveness greatly varies depending on the relationship between the position of the outer outlet in the side portion and the direction in which the measurement-object gas flows, the responsiveness may be reduced depending on, for example, the orientation in which the gas sensor is attached. When the outer outlet is not formed in the side portion, the influence of the orientation in which the gas sensor is attached on the responsiveness can be reduced. In this case, the outer outlet may be formed in at least one of the bottom portion and a corner portion between the side portion and the bottom portion. The outer outlet may be formed only in the bottom portion or only in the corner portion. 
     In the gas sensor according to the present invention, a minimum path length P from the outer inlet to the gas inlet may be 5.0 mm or more and 11.0 mm or less. When the minimum path length P is 11.0 mm or less, the measurement-object gas that has entered through the outer inlet reaches the gas inlet in a relatively short time. Accordingly, the responsiveness in gas concentration detection increases. When the minimum path length P is 5.0 mm or more, the occurrence of problems due to insufficient minimum path length P can be reduced. Such problems include, for example, the risk that external poisoning materials and water that have entered through the outer inlet will easily reach the sensor element, and the risk that the sensor element will be easily cooled by the measurement-object gas. 
     In the gas sensor according to the present invention, the minimum path length P is preferably 10.5 mm or less, more preferably 10.0 mm or less, still more preferably less than 10.0 mm, even more preferably 9.5 mm or less, and further more preferably 9.0 mm or less. As the minimum path length P decreases, the responsiveness in gas concentration detection increases. The minimum path length P may be 7.0 mm or more, or 8.0 mm or more. 
     In the gas sensor according to the present invention, the outer protective cover may include a body portion, which has a cylindrical shape and in which the outer inlet is arranged, and a front end portion, which has a cylindrical shape with a bottom and an inner diameter smaller than an inner diameter of the body portion and in which the outer outlet is arranged, the front end portion being located in front of the body portion in a forward direction, which is a direction from a back end toward the front end of the sensor element. The outer protective cover and the inner protective cover may form the first gas chamber as a space between the body portion of the outer protective cover and the inner protective cover, and the second gas chamber as a space between the front end portion of the outer protective cover and the inner protective cover. 
     In the gas sensor according to the present invention, the element-chamber inlet may be formed in the inner protective cover so that an element-side opening of the element-chamber inlet that is close to the sensor element chamber opens in a forward direction, which is a direction from a back end toward the front end of the sensor element. In this case, the measurement-object gas that has flowed out through the element-side opening is not blown against a surface of the sensor element (surface other than the gas inlet) in a direction perpendicular to the surface of the sensor element, nor does it flow a long distance along the surface of the sensor element before reaching the gas inlet. Accordingly, cooling of the sensor element can be reduced. Cooling of the sensor element is reduced by adjusting the direction in which the element-side opening opens, and not by reducing the flow rate and flow velocity of the measurement-object gas inside the inner protective cover. Therefore, the amount of reduction in the responsiveness in gas concentration detection can be reduced. As a result, the sensor element has quick responsiveness and high heat retaining properties at the same time. Here, the phrase “the element-side opening opens in the forward direction” includes a case in which the element-side opening opens parallel to the forward direction of the sensor element and a case in which the element-side opening opens obliquely to the forward direction so as to become closer to the sensor element with increasing distance in the forward direction of the sensor element. 
     In the gas sensor according to the present invention, the inner protective cover may include a first member and a second member, and the element-chamber inlet may be formed as a gap between the first member and the second member. Also, the first member may include a first cylindrical portion that surrounds the sensor element, and the second member may include a second cylindrical portion having a diameter larger than a diameter of the first cylindrical portion. The element-chamber inlet may be a tubular gap between an outer peripheral surface of the first cylindrical portion and an inner peripheral surface of the second cylindrical portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the manner in which a gas sensor  100  is attached to a pipe  20 . 
         FIG. 2  is a sectional view taken along line A-A in  FIG. 1 . 
         FIG. 3  is a sectional view taken along line B-B in  FIG. 2 . 
         FIG. 4  is a sectional view taken along line C-C in  FIG. 3 . 
         FIG. 5  is a sectional view of an outer protective cover  140  taken along line C-C in  FIG. 3 . 
         FIG. 6  is a view in the direction of arrow D in  FIG. 3 . 
         FIG. 7  is an enlarged partial sectional view taken along line E-E in  FIG. 4 . 
         FIG. 8  is a sectional view illustrating the case in which outer outlets  147   a  include a plurality of horizontal holes  147   b.    
         FIG. 9  is a perspective view illustrating the case in which the outer outlets  147   a  include a plurality of horizontal holes  147   b.    
         FIG. 10  is a sectional view illustrating the case in which the outer outlets  147   a  include corner holes  147   d.    
         FIG. 11  is a sectional view illustrating element-chamber inlets  227  according to a modification. 
         FIG. 12  is a vertical sectional view of a gas sensor  300  according to a modification. 
         FIG. 13  is a sectional view of an outer protective cover  140  according to Experimental Example 4. 
         FIG. 14  is an enlarged partial sectional view of a gas sensor  100  according to Experimental Example 5. 
         FIG. 15  is a graph showing the angular dependence of the response time of gas sensors according to Experimental Examples 1 to 5. 
         FIG. 16  is a graph showing the relationship between the flow velocity V and the response time of Experimental Examples 1 to 5. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will now be described with reference to the drawings.  FIG. 1  is a schematic diagram illustrating the manner in which a gas sensor  100  is attached to a pipe  20 .  FIG. 2  is a sectional view taken along line A-A in  FIG. 1 .  FIG. 3  is a sectional view taken along line B-B in  FIG. 2 .  FIG. 4  is a sectional view taken along line C-C in  FIG. 3 .  FIG. 5  is a sectional view of an outer protective cover  140  taken along line C-C in  FIG. 3 .  FIG. 5  illustrates the structure in which a first cylindrical portion  134 , a second cylindrical portion  136 , a front end portion  138 , and a sensor element  110  are removed from the structure illustrated in  FIG. 4 .  FIG. 6  is a view in the direction of arrow D in  FIG. 3 .  FIG. 7  is an enlarged partial sectional view taken along line E-E in  FIG. 4 . 
     As illustrated in  FIG. 1 , the gas sensor  100  is attached to the pipe  20 , which is an exhaust path from an engine of a vehicle. The gas sensor  100  detects the concentration of at least one of gas components, such as NOx and O 2 , of exhaust gas that is discharged from the engine as measurement-object gas. As illustrated in  FIG. 2 , the gas sensor  100  is fixed to the pipe  20  so that the central axis thereof is perpendicular to the flow of the measurement-object gas in the pipe  20 . Note that the gas sensor  100  may be fixed to the pipe  20  so that the central axis thereof is perpendicular to the flow of the measurement-object gas in the pipe  20  and at a predetermined angle (for example, 45°) with respect to the vertical direction. 
     As illustrated in  FIG. 3 , the gas sensor  100  includes a sensor element  110  having a function of detecting the concentration of predetermined gas in the measurement-object gas, and a protective cover  120  that protects the sensor element  110 . The gas sensor  100  also includes a metal housing  102  and a metal nut  103  having an external thread on an outer peripheral surface thereof. The housing  102  is inserted through a fixing member  22  that is welded to the pipe  20  and that has an internal screw on an inner peripheral surface thereof, and the nut  103  is screwed into the fixing member  22  so that the housing  102  is fixed to the fixing member  22 . Thus, the gas sensor  100  is fixed to the pipe  20 . The direction in which the measurement-object gas flows through the pipe  20  is the left-to-right direction in  FIG. 3 . 
     The sensor element  110  is a thin elongated plate-shaped element, and has a multilayer structure including a plurality of layers of oxygen ion conductive solid electrolyte, such as zirconia (ZrO 2 ). The sensor element  110  has a gas inlet  111  through which the measurement-object gas is introduced, and is capable of detecting the concentration of the predetermined gas (for example, NOx or O 2 ) in the measurement-object gas that flows into the sensor element  110  through the gas inlet  111 . In the present embodiment, the gas inlet  111  opens in the front end face of the sensor element  110  (bottom surface of the sensor element  110  in  FIG. 3 ). The sensor element  110  has a heater disposed therein, the heater having a function of heating the sensor element  110  to adjust the temperature thereof. The structure of the sensor element  110  and the principle of gas concentration detection are commonly known, and are described in, for example, Japanese Unexamined Patent Application Publication No. 2008-164411. The front end (bottom end in  FIG. 3 ) and the gas inlet  111  of the sensor element  110  are disposed in a sensor element chamber  124 . The direction from the back end toward the front end of the sensor element  110  (downward direction in  FIG. 3 ) is referred to as a forward direction. 
     The sensor element  110  includes a porous protective layer  110   a  that at least partially covers the surface thereof. In the present embodiment, the porous protective layer  110   a  is formed on five of the six faces of the sensor element  110 , and covers substantially the entire surface of a portion of the sensor element  110  that is exposed in the sensor element chamber  124 . More specifically, the porous protective layer  110   a  covers the entirety of the front end face (bottom face in  FIG. 3 ) of the sensor element  110  in which the gas inlet  111  is formed. In addition, the porous protective layer  110   a  covers four faces (top, bottom, left and right faces in  FIG. 4 ) of the sensor element  110  that are connected to the front end face of the sensor element  110  over areas near the front end face of the sensor element  110 . The porous protective layer  110   a  has a function of, for example, suppressing formation of cracks in the sensor element  110  due to adhesion of water or the like contained in the measurement-object gas. The porous protective layer  110   a  also has a function of suppressing adhesion of an oil component or the like contained in the measurement-object gas to electrodes (not shown) on the surface of the sensor element  110 . The porous protective layer  110   a  may be formed of a porous material, such as an alumina porous material, a zirconia porous material, a spinel porous material, a cordierite porous material, a titania porous material, or a magnesia porous material. The porous protective layer  110   a  may be formed by, for example, plasma spraying, screen printing, or dipping. Although the gas inlet  111  is also covered with the porous protective layer  110   a , the measurement-object gas can flow through the porous protective layer  110   a  and reach the gas inlet  111  because the porous protective layer  110   a  is formed of a porous material. The porous protective layer  110   a  may have a thickness of, for example, 100 μm to 700 μm; however, the thickness is not limited to this. 
     The protective cover  120  is disposed so as to surround the sensor element  110 . The protective cover  120  includes an inner protective cover  130  that has a cylindrical shape with a bottom and that covers the front end of the sensor element  110 , and an outer protective cover  140  that has a cylindrical shape with a bottom and that covers the inner protective cover  130 . A first gas chamber  122  and a second gas chamber  126  are formed as spaces defined between the inner protective cover  130  and the outer protective cover  140 , and the sensor element chamber  124  is formed as a space surrounded by the inner protective cover  130 . The gas sensor  100 , the sensor element  110 , the inner protective cover  130 , and the outer protective cover  140  have the same central axis. The protective cover  120  is made of a metal (for example, stainless steel). 
     The inner protective cover  130  includes a first member  131  and a second member  135 . The first member  131  includes a large-diameter portion  132  having a cylindrical shape, a first cylindrical portion  134  having a diameter smaller than that of the large-diameter portion  132 , and a step portion  133  that connects the large-diameter portion  132  and the first cylindrical portion  134 . The first cylindrical portion  134  surrounds the sensor element  110 . The second member  135  includes a second cylindrical portion  136  having a diameter larger than that of the first cylindrical portion  134 ; a front end portion  138  having an inverted truncated conical shape that is located in front of the second cylindrical portion  136  in the forward direction of the sensor element  110  (downward direction in  FIG. 3 ); and a connection portion  137  that connects the second cylindrical portion  136  and the front end portion  138 . A single element-chamber outlet  138   a  (also referred to as an inner gas hole) having a circular shape is formed at the center of the bottom face of the front end portion  138 . The element-chamber outlet  138   a  is connected to the sensor element chamber  124  and the second gas chamber  126 , and serves as an exit for the measurement-object gas in the sensor element chamber  124 . The diameter of the element-chamber outlet  138   a  is not particularly limited, and may be, for example, 0.5 mm to 2.6 mm. The element-chamber outlet  138   a  is located in front of the gas inlet  111  in the forward direction of the sensor element  110  (downward direction in  FIG. 3 ). In other words, the element-chamber outlet  138   a  is farther from the back end of the sensor element  110  (upper end (not illustrated) of the sensor element  110  in  FIG. 3 ) than the gas inlet  111  is (below the gas inlet  111  in  FIG. 3 ). 
     The large-diameter portion  132 , the first cylindrical portion  134 , the second cylindrical portion  136 , and the front end portion  138  have the same central axis. The inner peripheral surface of the large-diameter portion  132  is in contact with the housing  102  so that the first member  131  is fixed to the housing  102 . The outer peripheral surface of the connection portion  137  of the second member  135  is in contact with and fixed to the inner peripheral surface of the outer protective cover  140  by, for example, welding. The second member  135  may instead be fixed by forming the front end portion  138  so that outer diameter thereof is slightly larger than the inner diameter of a front end portion  146  of the outer protective cover  140  and press-fitting the front end portion  138  into the front end portion  146 . 
     A plurality of protruding portions  136   a  are formed on the inner peripheral surface of the second cylindrical portion  136  so as to protrude toward and be in contact with the outer peripheral surface of the first cylindrical portion  134 . As illustrated in  FIG. 4 , three protruding portions  136   a  are arranged at equal intervals in the circumferential direction of the inner peripheral surface of the second cylindrical portion  136 . The protruding portions  136   a  have a substantially hemispherical shape. Since the protruding portions  136   a  are provided, the positional relationship between the first cylindrical portion  134  and the second cylindrical portion  136  can be easily fixed by the protruding portions  136   a . The protruding portions  136   a  preferably press the outer peripheral surface of the first cylindrical portion  134  radially inward. In such a case, the positional relationship between the first cylindrical portion  134  and the second cylindrical portion  136  can be more reliably fixed by the protruding portions  136   a . The number of protruding portions  136   a  is not limited to three, and may instead be two, or four or more. Preferably, three or more protruding portions  136   a  are provided so that the first cylindrical portion  134  and the second cylindrical portion  136  can be stably fixed. 
     An element-chamber inlet  127  (see  FIGS. 3, 4, and 7 ) is formed in the inner protective cover  130 . The element-chamber inlet  127  is a gap between the first member  131  and the second member  135 , and serves as an entrance to the sensor element chamber  124  for the measurement-object gas. More specifically, the element-chamber inlet  127  is a tubular gap (gas flow channel) between the outer peripheral surface of the first cylindrical portion  134  and the inner peripheral surface of the second cylindrical portion  136 . The element-chamber inlet  127  includes an outer opening  128  and an element-side opening  129 . The outer opening  128  is an opening adjacent to the first gas chamber  122 , which is a space in which outer inlets  144   a  are arranged. The element-side opening  129  is an opening adjacent to the sensor element chamber  124 , which is a space in which the gas inlet  111  is arranged. The outer opening  128  is closer to the back end of the sensor element  110  (upper end in  FIG. 3 ) than the element-side opening  129  is. Therefore, in the path of the measurement-object gas from the outer inlets  144   a  to the gas inlet  111 , the element-chamber inlet  127  serves as a flow channel extending from the back-end side (upper side in  FIG. 3 ) toward the front-end side (lower side in  FIG. 3 ) of the sensor element  110 . Also, the element-chamber inlet  127  is a flow channel that is parallel to the front-back direction of the sensor element  110  (vertical flow channel in  FIG. 3 ). 
     The element-side opening  129  is preferably located so that the distance A 1  from the gas inlet  111  (see  FIG. 7 ) is −1.5 mm or more. The distance A 1  may be 0 mm or more, or more than 1.5 mm. The distance A 1  is the distance in the front-back direction of the sensor element  110  (vertical direction in  FIG. 3 ), and the front-to-back direction (upward direction in  FIG. 3 ) is defined as positive. More specifically, the distance A 1  is the distance between a portion of the opening edge of the gas inlet  111  that is closest to the element-side opening  129  and a portion of the edge of the element-side opening  129  that is closest to the gas inlet  111  in the front-back direction of the sensor element  110 . In  FIG. 3 , if the gas inlet is a horizontal hole that opens in a side surface of the sensor element  110 , and if the element-side opening  129  is located between the top and bottom ends of the opening of the gas inlet, the distance A 1  is defined as 0 mm. The upper limit of the distance A 1  is determined by the shapes of the inner protective cover  130  and the sensor element chamber  124 . Although there is no particular limitation, the distance A 1  may be 7.5 mm or less, 5 mm or less, or 2 mm or less. 
     The element-side opening  129  is located at a distance A 2  (see  FIG. 7 ) from the sensor element  110 . The distance A 2  is the distance in a direction perpendicular to the front-back direction of the sensor element  110 . More specifically, the distance A 2  is the distance between a portion of the sensor element  110  that is closest to the element-side opening  129  and a portion of the edge of the element-side opening  129  that is closest to the sensor element  110  in the direction perpendicular to the front-back direction of the sensor element  110 . As the distance A 2  increases, the element-side opening  129  becomes farther away from the sensor element  110 , so that cooling of the sensor element  110  can be further reduced. The distance A 2  is not particularly limited, and may be, for example, 0.6 mm to 3.0 mm. The element-side opening  129  opens parallel to the front-back direction of the sensor element  110  in the back-to-front direction of the sensor element  110 . In other words, the element-side opening  129  opens downward (toward the region directly below) in  FIGS. 3 and 7 . Thus, the sensor element  110  is disposed outside the region to which the element-chamber inlet  127  is virtually extended from the element-side opening  129  (region directly below the element-side opening  129  in  FIGS. 3 and 7 ). Accordingly, the measurement-object gas that flows out through the element-side opening  129  is not directly blown against the surface of the sensor element  110 , and cooling of the sensor element  110  can be reduced. 
     The outer opening  128  is located at a distance A 3  from the outer inlet  144   a  (see  FIG. 7 ). The distance A 3  is the distance in the front-back direction of the sensor element  110  (vertical direction in  FIGS. 3 and 7 ). Similar to the distance A 1 , the front-to-back direction is defined as positive. More specifically, the distance A 3  is the distance between a portion of the opening edge of the outer inlet  144   a  that is closest to the outer opening  128  and a portion of the edge of the outer opening  128  that is closest to the outer inlet  144   a  in the front-back direction of the sensor element  110 . In the present embodiment, a plurality of outer inlets  144   a  including horizontal holes  144   b  and vertical holes  144   c  are provided, and the upper ends of the horizontal holes  144   b  are closest to the outer opening  128  in the vertical direction in  FIG. 3 . Therefore, referring to  FIG. 7 , the distance A 3  is the distance between the upper end of the horizontal hole  144   b  and the outer opening  128 . When, for example, the outer opening  128  is below the lower end of the vertical hole  144   c  in the vertical direction in  FIG. 3 , the distance A 3  is the distance between the lower end of the vertical hole  144   c  and the outer opening  128  in the vertical direction. The outer opening  128  may be located so that the distance A 3  is 0 or more, or positive. Alternatively, the outer opening  128  may be located so that the distance A 3  is 0 or less, or negative. The distance A 3  is not particularly limited, and may be, for example, −3 mm or more and 3 mm or less. Alternatively, the distance A 3  may be −2 mm or more, −1 mm or more, 2 mm or less, or 1 mm or less. 
     The outer opening  128  is located at a distance A 6  from the outer inlet  144   a  (see  FIG. 7 ). The distance A 6  is the distance in the direction perpendicular to the front-back direction of the sensor element  110  (vertical direction in  FIGS. 3 and 7 ). The distance A 6  is the distance between the outer inlet  144   a  that is closest to the outer opening  128  in the front-back direction of the sensor element  110  and the outer opening  128 . In the present embodiment, the distance A 6  is equal to one-half the difference between the inner diameter of a side portion  143   a  and the inner diameter of the second cylindrical portion  136 . The distance A 6  is not particularly limited, and may be, for example, more than 0 mm and 2.5 mm or less. Alternatively, the distance A 6  may be 0.5 mm or more, 1 mm or more, 2.0 mm or less, or 1.5 mm or less. 
     The outer peripheral surface of the first cylindrical portion  134  and the inner peripheral surface of the second cylindrical portion  136  are apart from each other in the radial direction of the first and second cylindrical portions  134  and  136  by a distance A 4  at the element-side opening  129 , and by a distance A 5  at the outer opening  128 . The outer peripheral surface of the first cylindrical portion  134  and the inner peripheral surface of the second cylindrical portion  136  are apart from each other by a distance A 7  at a location where the protruding portions  136   a  are in contact with the first cylindrical portion  134  (location of the sectional view of  FIG. 4 ). The distances A 4 , A 5 , and A 7  are not particularly limited, and may be, for example, 0.3 mm to 2.4 mm. The opening areas of the element-side opening  129  and the outer opening  128  can be adjusted by adjusting the distances A 4  and A 5 . In the present embodiment, the distances A 4 , A 5 , and A 7  are equal, and the element-side opening  129  and the outer opening  128  have the same opening area. In the present embodiment, the distance A 4  (distances A 5  and A 7 ) is equal to one-half the difference between the outer diameter of the first cylindrical portion  134  and the inner diameter of the second cylindrical portion  136 . The distance between the element-side opening  129  and the outer opening  128  in the vertical direction, that is, the length L of the element-chamber inlet  127  in the vertical direction (which corresponds to the path length of the element-chamber inlet  127 ), is not particularly limited, and may be, for example, more than 0 mm and 6.6 mm or less. Alternatively, the length L may be 3 mm or more, or 5 mm or less. 
     As illustrated in  FIG. 3 , the outer protective cover  140  includes a large-diameter portion  142  that has a cylindrical shape; a body portion  143  that has a cylindrical shape, that is connected to the large-diameter portion  142 , and whose diameter is smaller than that of the large-diameter portion  142 ; and the front end portion  146  that has a cylindrical shape with a bottom and whose inner diameter is smaller than that of the body portion  143 . The body portion  143  includes the side portion  143   a , which has a side surface that extends in the direction of the central axis of the outer protective cover  140  (vertical direction in  FIG. 3 ), and a step portion  143   b  that defines the bottom of the body portion  143  and connects the side portion  143   a  and the front end portion  146 . The central axes of the large-diameter portion  142 , the body portion  143 , and the front end portion  146  coincide with the central axis of the inner protective cover  130 . The inner peripheral surface of the large-diameter portion  142  is in contact with the housing  102  and the large-diameter portion  132 , so that the outer protective cover  140  is fixed to the housing  102 . The body portion  143  is arranged so as to cover the outer peripheries of the first cylindrical portion  134  and the second cylindrical portion  136 . The front end portion  146  is arranged so as to cover the front end portion  138 , and the inner peripheral surface thereof is in contact with the outer peripheral surface of the connection portion  137 . The front end portion  146  includes a side portion  146   a , which has a side surface that extends in the direction of the central axis of the outer protective cover  140  (vertical direction in  FIG. 3 ) and whose outer diameter is smaller than the inner diameter of the side portion  143   a , and a bottom portion  146   b  that defines the bottom of the outer protective cover  140 . The front end portion  146  is located in front of the body portion  143  in the forward direction. The outer protective cover  140  has a plurality of outer inlets  144   a  (twelve outer inlets  144   a  in the present embodiment) formed in the body portion  143  and a plurality of outer outlets  147   a  (six outer outlets  147   a  in the present embodiment) formed in the front end portion  146 . The outer inlets  144   a  are entrances from the outside for the measurement-object gas, and the outer outlets  147   a  are exits to the outside for the measurement-object gas. 
     The outer inlets  144   a  are holes (referred to also as first outer gas holes) that connect the region outside the outer protective cover  140  (the outside) to the first gas chamber  122 . The outer inlets  144   a  include a plurality of horizontal holes  144   b  (six horizontal holes  144   b  in the present embodiment) formed in the side portion  143   a  at equal intervals therebetween and a plurality of vertical holes  144   c  (six vertical holes  144   c  in the present embodiment) formed in the step portion  143   b  at equal intervals therebetween (see  FIGS. 3 to 6 ). The outer inlets  144   a  (horizontal holes  144   b  and vertical holes  144   c ) are circular (perfect circular) holes. The diameters of the twelve outer inlets  144   a  are not particularly limited, and may be, for example, 0.5 mm to 2 mm. Alternatively, the diameters of the outer inlets  144   a  may be 1.5 mm or less. In the present embodiment, the horizontal holes  144   b  have the same diameter, and the vertical holes  144   c  have the same diameter. The diameter of the horizontal holes  144   b  is larger than that of the vertical holes  144   c . As illustrated in  FIGS. 4 and 5 , the outer inlets  144   a  are formed so that the horizontal holes  144   b  and the vertical holes  144   c  are alternately arranged at equal intervals in the circumferential direction of the outer protective cover  140 . In other words, in  FIGS. 4 and 5 , the line connecting the central axis of the outer protective cover  140  and the center of any horizontal hole  144   b  and the line connecting the central axis of the outer protective cover  140  and the center of one of the vertical holes  144   c  that is adjacent to that horizontal hole  144   b  form an angle of 30° (360°/12). 
     The outer outlets  147   a  are holes (referred to also as second outer gas holes) that connect the region outside the outer protective cover  140  (the outside) to the second gas chamber  126 . The outer outlets  147   a  include a plurality of vertical holes  147   c  (six vertical holes  147   c  in the present embodiment) formed in the bottom portion  146   b  of the front end portion  146  at equal intervals therebetween in the circumferential direction of the outer protective cover  140  (see  FIGS. 3, 5, and 6 ). Unlike the outer inlets  144   a , none of the outer outlets  147   a  is arranged in a side portion of the outer protective cover  140  (side portion  146   a  of the front end portion  146  in this case). The outer outlets  147   a  (vertical holes  147   c  in this example) are circular (perfect circular) holes. The diameters of the six outer outlets  147   a  are not particularly limited, and may be, for example, 0.5 mm to 2.0 mm. Alternatively, the diameters of the outer outlets  147   a  may be 1.5 mm or less. In the present embodiment, the outer outlets  147   a  have the same diameter. The diameter of the vertical holes  147   c  is smaller than the diameter of the horizontal holes  144   b.    
     The outer protective cover  140  and the inner protective cover  130  form the first gas chamber  122  as a space between the body portion  143  and the inner protective cover  130 . More specifically, the first gas chamber  122  is a space surrounded by the step portion  133 , the first cylindrical portion  134 , the second cylindrical portion  136 , the large-diameter portion  142 , the side portion  143   a , and the step portion  143   b . The sensor element chamber  124  is a space surrounded by the inner protective cover  130 . The outer protective cover  140  and the inner protective cover  130  also form the second gas chamber  126  as a space between the front end portion  146  and the inner protective cover  130 . More specifically, the second gas chamber  126  is a space surrounded by the front end portion  138  and the front end portion  146 . Since the inner peripheral surface of the front end portion  146  is in contact with the outer peripheral surface of the connection portion  137 , the first gas chamber  122  and the second gas chamber  126  are not directly connected to each other. 
     The manner in which the measurement-object gas flows inside the protective cover  120  when the gas sensor  100  detects the concentration of the predetermined gas will now be described. First, the measurement-object gas that flows through the pipe  20  enters the first gas chamber  122  through at least one of the outer inlets  144   a  (horizontal holes  144   b  and vertical holes  144   c ). Next, the measurement-object gas enters the element-chamber inlet  127  from the first gas chamber  122  through the outer opening  128 , flows through the element-chamber inlet  127 , and enters the sensor element chamber  124  through the element-side opening  129 . At least part of the measurement-object gas that has entered the sensor element chamber  124  through the element-side opening  129  reaches the gas inlet  111  of the sensor element  110 . When the measurement-object gas reaches the gas inlet  111  and enters the sensor element  110 , the sensor element  110  generates an electrical signal (voltage or current) corresponding to the concentration of the predetermined gas (for example, NOx or O 2 ) in the measurement-object gas. The gas concentration is detected on the basis of this electrical signal. The measurement-object gas in the sensor element chamber  124  enters the second gas chamber  126  through the element-chamber outlet  138   a , and flows out through at least one of the outer outlets  147   a . The output of the heater disposed in the sensor element  110  is controlled by, for example, a controller (not shown) so that the temperature of the sensor element  110  is maintained at a predetermined temperature. 
     The protective cover  120  is preferably formed so that, when the measurement-object gas flows inside the protective cover  120  in the above-described manner, a minimum path length P from the outer inlets  144   a  to the gas inlet  111  is 5.0 mm or more and 11.0 mm or less. In the present embodiment, the minimum path length P is the length of the broken line PL, that is, the bold one-dot chain line, in  FIG. 7 . The minimum path length P is the length of the shortest path for the measurement-object gas from the outer opening of the outer inlet  144   a  to the outer opening of the gas inlet  111 . When there is a plurality of outer inlets  144   a , the minimum path length P is the shortest one of the minimum path lengths from the outer inlets  144   a  to the gas inlet  111 . In the present embodiment, the outer protective cover  140  has the horizontal holes  144   b  and the vertical holes  144   c  as the outer inlets  144   a . As illustrated in  FIG. 3 , the horizontal holes  144   b  are disposed above the vertical holes  144   c , and are closer to the outer opening  128  than the vertical holes  144   c . In addition, in the present embodiment, as illustrated in  FIG. 4 , the gas inlet  111  has a rectangular opening, and is shifted upward in  FIG. 4  from the central axis of the inner protective cover  130  and the outer protective cover  140 . Accordingly, in the present embodiment, the minimum path length from one of the six horizontal holes  144   b  that is at the upper left in  FIG. 4  to the gas inlet  111  is the minimum path length P of the protective cover  120 . The minimum path length from the horizontal hole  144   b  at the upper right in  FIG. 4  to the gas inlet  111  is also the same (=minimum path length P).  FIG. 7  is an enlarged partial sectional view of a region around the horizontal hole  144   b  at the upper left in  FIG. 4  taken along line E-E. The horizontal hole  144   b  illustrated in  FIG. 7  is the horizontal hole  144   b  at the upper left in  FIG. 4 . Referring to  FIG. 7 , the minimum path length P is the length of the shortest path (broken line PL) from an end portion C 1  (upper end portion in  FIG. 7 ) of the outer opening of the horizontal hole  144   b , the end portion C 1  being closest to the outer opening  128 , to an end portion C 2  (left end portion in  FIG. 7 ) of the outer opening of the gas inlet  111 . The minimum path length P is determined without considering the porous protective layer  110   a . For example, in  FIG. 7 , a portion of the path shown by the broken line PL from the element-side opening  129  to the gas inlet  111  is determined as the combination of the straight line connecting the element-side opening  129  and the lower left corner of the sensor element  110  and the straight line connecting the lower left corner of the sensor element  110  and the left end of the opening of the gas inlet  111  without considering the porous protective layer  110   a . In the present embodiment, as described above, the shape, location, etc., of the gas inlet  111  are such that the minimum path lengths from the four horizontal holes  144   b  other than the horizontal holes  144   b  at the upper left and upper right in  FIG. 4  to the gas inlet  111  are slightly greater than the minimum path length P. When the horizontal holes  144   b  have different minimum path lengths as in this case, the minimum path lengths of as many horizontal holes  144   b  as possible are preferably 5.0 mm or more and 11.0 mm or less. In the present embodiment, not only the minimum path length P of the horizontal holes  144   b  at the upper left and upper right in  FIG. 4  but also the minimum path lengths of the other horizontal holes  144   b  are 5.0 mm or more and 11.0 mm or less. In addition to the horizontal holes  144   b , the minimum path length from at least one of the vertical holes  144   c  to the gas inlet  111  may also be 5.0 mm or more and 11.0 mm or less. Furthermore, the minimum path lengths from the vertical holes  144   c  to the gas inlet  111  may all be 5.0 mm or more and 11.0 mm or less. Furthermore, the minimum path lengths from the outer inlets  144   a  (horizontal holes  144   b  and vertical holes  144   c  in this case) to the gas inlet  111  may all be 5.0 mm or more and 11.0 mm or less. 
     The sensor element  110  included in the gas sensor  100  is preferably capable of quickly detecting a change in the concentration of the predetermined gas in the measurement-object gas. In other words, the sensor element  110  preferably has quick responsiveness in gas concentration detection. When the minimum path length P determined as described above is as small as 11.0 mm or less, the measurement-object gas that has entered through the outer inlets  144   a  reaches the gas inlet  111  in a relatively short time, and the responsiveness increases accordingly. When the minimum path length P is 5.0 mm or more, the occurrence of problems due to insufficient minimum path length P can be reduced. Such problems include, for example, the risk that external poisoning materials and water that have entered through the outer inlets  144   a  will easily reach the sensor element  110 , and the risk that the sensor element  110  will be easily cooled by the measurement-object gas or the output of the heater required to prevent cooling of the sensor element  110  will be increased. The minimum path length P is preferably 10.5 mm or less, more preferably, 10.0 mm or less, still more preferably, less than 10.0 mm, still more preferably, 9.5 mm or less, and still more preferably, 9.0 mm or less. As the minimum path length P decreases, the responsiveness in gas concentration detection increases. The minimum path length P may be adjusted by, for example, adjusting at least one of the distances A 1  to A 7  and the length L in  FIG. 7  or by adjusting the diameters of the outer inlets  144   a . The minimum path length P may be 7.0 mm or more, or 8.0 mm or more. 
     The outer protective cover  140  is structured so that a cross-sectional area ratio S1/S2, which is a ratio of the total cross-sectional area S1 [mm 2 ] of the outer inlets  144   a  to the total cross-sectional area S2 [mm 2 ] of the outer outlets  147   a , is more than 2.0 and 5.0 or less. When the cross-sectional area ratio S1/S2 is more than 2.0, the total cross-sectional area S1 is relatively large, so that the flow rate at which the measurement-object gas enters through the outer inlets  144   a  tends to increase. In addition, the total cross-sectional area S2 is relatively small, so that the flow rate at which the measurement-object gas tries to enter through the outer outlets  147   a  (backflow) tends to decrease. Accordingly, the measurement-object gas in the space around the gas inlet  111  is easily replaced by the measurement-object gas that has entered. As a result, the responsiveness in gas concentration detection increases. When the total cross-sectional area S2 is too small, the flow rate at which the measurement-object gas flows out through the outer outlets  147   a  decreases, and the responsiveness may decrease accordingly. However, when the cross-sectional area ratio S1/S2 is 5.0 or less, the reduction in responsiveness can be suppressed. The cross-sectional area ratio S1/S2 may be adjusted by, for example, adjusting the numbers of the outer inlets  144   a  and the outer outlets  147   a , or by adjusting the cross-sectional areas of the outer inlets  144   a  and the outer outlets  147   a.    
     In the present embodiment, the total cross-sectional area S1 is the sum of the total cross-sectional area of the six horizontal holes  144   b  and the total cross-sectional area of the six vertical holes  144   c . The total cross-sectional area S2 is the sum of the cross-sectional areas of the six vertical holes  147   c . The cross-sectional area of each outer inlet  144   a  is the area of the outer inlet  144   a  along a plane perpendicular to the direction in which the measurement-object gas flows through the outer inlet  144   a . In the present embodiment, the outer inlets  144   a  are holes having circular shapes, and the areas of the circular shapes serve as the cross-sectional areas thereof. This also applies to the outer outlets  147   a . When, for example, one of the outer inlets  144   a  is shaped so that the cross-sectional area thereof is not constant, for example, so that the cross-sectional area thereof differs between the entrance side (outer surface of the outer protective cover  140 ) and the exit side (inner surface of the outer protective cover  140 ), the minimum value of the cross-sectional area is defined as the cross-sectional area of that outer inlet  144   a . This also applies to the outer outlets  147   a.    
     The cross-sectional area ratio S1/S2 is preferably 2.5 or more, more preferably, 3.0 or more, and still more preferably, 3.4 or more. As the cross-sectional area ratio S1/S2 increases, the responsiveness in gas concentration detection tends to increase. The total cross-sectional area S1 may be 10 mm 2  or more. The total cross-sectional area S1 may also be 30 mm 2  or less. The total cross-sectional area S2 may be 2 mm 2  or more. The total cross-sectional area S2 may also be 10 mm 2  or less. 
     In the present embodiment, the outer protective cover  140  includes the side portion  146   a  and the bottom portion  146   b  and has a cylindrical shape with a bottom. The outer outlets  147   a  are not formed in the side portion  146   a  of the outer protective cover  140 . If the outer outlets  147   a  are formed in the side portion  146   a  of the outer protective cover  140 , the responsiveness may vary depending on the relationship between the positions of the outer outlets  147   a  in the side portion  146   a  and the direction in which the measurement-object gas flows around the outer outlets  147   a .  FIGS. 8 and 9  are a sectional view and a perspective view, respectively, illustrating the case in which the outer outlets  147   a  include a plurality of horizontal holes  147   b  (three horizontal holes  147   b  in this example) formed in the side portion  146   a . The outer protective cover  140  illustrated in  FIGS. 8 and 9  has outer outlets  147   a  including three horizontal holes  147   b  and three vertical holes  147   c . The horizontal holes  147   b  and the vertical holes  147   c  are alternately arranged at equal intervals in the circumferential direction of the outer protective cover  140 . In the outer protective cover  140  illustrated in  FIGS. 8 and 9 , when, for example, the direction in which the measurement-object gas flows is the left-to-right direction as shown by arrow D 1  in  FIG. 8 , one of the horizontal holes  147   b  (the leftmost horizontal hole  147   b  in  FIG. 8 ) opens parallel to, and toward the upstream side (leftward in  FIG. 8 ) of, the direction in which the measurement-object gas flows. In this case, the flow of the measurement-object gas that tries to flow out from the space inside the outer protective cover  140  through this horizontal hole  147   b  is impeded by the measurement-object gas that flows around this horizontal hole  147   b , and the responsiveness tends to decrease as a result. In contrast, assume that the direction in which the measurement-object gas flows is the direction shown by arrow D 2  in  FIG. 8 . The direction of arrow D 2  is the direction obtained by rotating the direction of arrow D 1  clockwise by 60° in  FIG. 8 , and is toward the middle point between the left horizontal hole  147   b  and the upper right horizontal hole  147   b  in the side portion  146   a  of the outer protective cover  140  in  FIG. 8 . In this case, the horizontal holes  147   b  are arranged only at positions that are relatively far from the region around the position at which the measurement-object gas is blown against the side portion  146   a  in a direction perpendicular to the side portion  146   a . Accordingly, the flow of the measurement-object gas that tries to flow out through the horizontal holes  147   b  is not greatly impeded, and the responsiveness is not greatly reduced. When the responsiveness greatly varies depending on the relationship between the positions of the outer outlets  147   a  in the side portion  146   a  (horizontal holes  147   b  in this example) and the direction in which the measurement-object gas flows, the responsiveness may be reduced depending on the orientation in which the gas sensor  100  is attached (angle of the outer protective cover  140  around the central axis in the rotational direction). When, for example, the gas sensor  100  is attached to the pipe  20  in such an orientation that the measurement-object gas flows in the direction of arrow D 1 , the responsiveness tends to decrease. In contrast, in the gas sensor  100  according to the present embodiment, since the outer outlets  147   a  are not formed in the side portion  146   a , the influence of the orientation in which the gas sensor  100  is attached on the responsiveness can be reduced. The influence of the orientation in which the gas sensor  100  is attached on the responsiveness is referred to as angular dependence. In the gas sensor  100  according to the present embodiment, the angular dependence can be reduced because the outer outlets  147   a  are not formed in the side portion  146   a.    
     In the gas sensor  100  according to the present embodiment described in detail above, since the cross-sectional area ratio S1/S2 is more than 2.0 and 5.0 or less, the responsiveness in gas concentration detection is increased. In addition, since no outer outlets  147   a  are arranged in the side portion  146   a , the influence of the orientation in which the gas sensor  100  is attached on the responsiveness can be reduced. As a result, the above-described effect that the responsiveness in gas concentration detection increases can be easily obtained irrespective of the attachment orientation. 
     In addition, in the gas sensor  100 , the element-chamber inlet  127  is formed in the inner protective cover  130  so that the element-side opening  129  opens in the forward direction. Therefore, the measurement-object gas that has flowed out of the element-side opening  129  is not blown against a surface of the sensor element  110  (surface other than the gas inlet  111 ) in a direction perpendicular to the surface of the sensor element  110 , nor does it flow a long distance along the surface of the sensor element  110  before reaching the gas inlet  111 . Accordingly, cooling of the sensor element  110  can be reduced. Cooling of the sensor element  110  is reduced by adjusting the direction in which the element-side opening  129  opens, and not by reducing the flow rate and flow velocity of the measurement-object gas inside the inner protective cover  130 . Therefore, the amount of reduction in the responsiveness in gas concentration detection can be reduced. As a result, the sensor element  110  has quick responsiveness and high heat retaining properties at the same time. 
     The present invention is not limited to the above-described embodiment in any way, and can be implemented in various forms within the technical scope of the present invention. 
     For example, the shape of the protective cover  120  is not limited to that in the above-described embodiment. The shape of the protective cover  120  and the shapes, numbers, arrangements, etc., of the element-chamber inlet  127 , the element-chamber outlet  138   a , the outer inlets  144   a , and the outer outlets  147   a  may be changed as appropriate. For example, although the element-chamber inlet  127  is formed as a gap between the first member  131  and the second member  135 , the element-chamber inlet is not limited to this, and may be formed in any shape as long as the element-chamber inlet serves as an entrance to the sensor element chamber  124 . For example, the element-chamber inlet may be a through hole formed in the inner protective cover  130 . Also when the element-chamber inlet is a through hole, the element-chamber inlet may serve as a flow channel extending from the back-end side toward the front-end side of the sensor element  110 . For example, the element-chamber inlet may be a vertical hole or a hole oblique to the vertical direction in  FIG. 3 . Also, the element-side opening  129  may be formed so as to open in the forward direction. The element-chamber inlet  127  is not limited to one in number, and may instead be provided in a plurality. The element-chamber outlet  138   a , the outer inlets  144   a , and the outer outlets  147   a  are not limited to holes, and may instead be gaps between members that constitute the protective cover  120 . These components may be provided in any number as long as they are provided. Although the outer inlets  144   a  include the horizontal holes  144   b  and the vertical holes  144   c , the outer inlets  144   a  may include only the horizontal holes  144   b  or only the vertical holes  144   c . Also, corner holes may be formed at the corner between the side portion  143   a  and the step portion  143   b  in place of, or in addition to, the horizontal holes  144   b  and the vertical holes  144   c . Similarly, the element-chamber inlet  127 , the element-chamber outlet  138   a , and the outer outlets  147   a  may include one or more of a horizontal hole, a vertical hole, and a corner hole. However, as described above, the outer outlets  147   a  preferably do not include horizontal holes. In other words, the outer outlets  147   a  are preferably not arranged in the side portion  146   a.    
     Examples of corner holes will now be described.  FIG. 10  is a sectional view illustrating the case in which the outer outlets  147   a  include a plurality of corner holes  147   d . As illustrated in  FIG. 10 , the outer outlets  147   a  formed in the front end portion  146  include the corner holes  147   d  formed at the corner between the side portion  146   a  and the bottom portion  146   b  in place of the vertical holes  147   c . Six corner holes  147   d  (only four corner holes  147   d  are illustrated in  FIG. 10 ) are arranged at equal intervals in the circumferential direction of the outer protective cover  140 . The corner holes  147   d  may be formed so that the angle θ between the outer openings of the corner holes  147   d  (straight line a in the enlarged view at the lower left in  FIG. 10 ) and the bottom surface (lower surface) of the bottom portion  146   b  (straight line b in the enlarged view at the lower left in  FIG. 10 ) is in the range of 10° to 80°. In  FIG. 10 , the angle θ is 45°. Also when corner holes are formed at the corner between the side portion  143   a  and the step portion  143   b  in the above-described embodiment, the angle θ between the outer openings of the corner holes and the bottom surface (lower surface) of the bottom portion  146   b  may be in the range of 10° to 80°. 
     In the above-described embodiment, the protruding portions  136   a  are formed on the inner peripheral surface of the second cylindrical portion  136 . However, the protruding portions  136   a  are not limited to this as long as a plurality of protruding portions are formed on at least one of the outer peripheral surface of the first cylindrical portion  134  and the inner peripheral surface of the second cylindrical portion  136  so as to protrude toward and be in contact with the other. In addition, in the above-described embodiment, as illustrated in  FIGS. 3 and 4 , the outer peripheral surface of the second cylindrical portion  136  is inwardly recessed at the locations where the protruding portions  136   a  are formed. However, it is not necessary that the outer peripheral surface of the second cylindrical portion  136  be recessed. The shape of the protruding portions  136   a  is not limited to a hemispherical shape, and may be any shape. Note that it is not necessary that the protruding portions  136   a  be formed on the outer peripheral surface of the first cylindrical portion  134  or the inner peripheral surface of the second cylindrical portion  136 . 
     In the above-described embodiment, the element-chamber inlet  127  is a tubular gap between the outer peripheral surface of the first cylindrical portion  134  and the inner peripheral surface of the second cylindrical portion  136 . However, the element-chamber inlet  127  is not limited to this. For example, a recess (groove) may be formed in at least one of the outer peripheral surface of the first cylindrical portion and the inner peripheral surface of the second cylindrical portion, and the element-chamber inlet may be formed as the gap defined by the recess between the first cylindrical portion and the second cylindrical portion.  FIG. 11  is a sectional view illustrating element-chamber inlets  227  according to a modification. Referring to  FIG. 11 , the outer peripheral surface of a first cylindrical portion  234  and the inner peripheral surface of a second cylindrical portion  236  are in contact with each other, and a plurality of recesses  234   a  (four recesses  234   a  in  FIG. 11 ) are formed in the outer peripheral surface of the first cylindrical portion  234  at equal intervals therebetween. The gaps between the inner peripheral surface of the second cylindrical portion  236  and the recesses  234   a  serve as element-chamber inlets  227 . 
     In the above-described embodiment, the element-chamber inlet  127  is a flow channel parallel to the front-back direction of the sensor element  110  (vertical direction in  FIG. 3 ). However, the element-chamber inlet is not limited to this, and may instead be formed as a flow channel that is oblique to the front-back direction so that the flow channel becomes closer to the sensor element  110  with increasing distance in the back-to-front direction of the sensor element  110 .  FIG. 12  is a vertical sectional view of a gas sensor  300  according to a modification in this case. In  FIG. 12 , components that are the same as those of the gas sensor  100  are denoted by the same reference numerals, and detailed description thereof is omitted. As illustrated in  FIG. 12 , the gas sensor  300  includes an inner protective cover  330  in place of the inner protective cover  130 . The inner protective cover  330  includes a first member  331  and a second member  335 . In place of the first cylindrical portion  134  of the first member  131 , the first member  331  includes a body portion  334   a  having a cylindrical shape and a first cylindrical portion  334   b  having a diameter that decreases with increasing distance in the back-to-front direction of the sensor element  110 . The back end of the first cylindrical portion  334   b  in the front-back direction of the sensor element  110  is connected to the body portion  334   a . In place of the second cylindrical portion  136  and the connection portion  137  included in the second member  135 , the second member  335  includes a second cylindrical portion  336  having a diameter that decreases with increasing distance in the back-to-front direction of the sensor element  110 . The second cylindrical portion  336  is connected to the front end portion  138 . The outer peripheral surface of the first cylindrical portion  334   b  and the inner peripheral surface of the second cylindrical portion  336  are not in contact with each other, and the gap formed therebetween serves as an element-chamber inlet  327 . The element-chamber inlet  327  has an outer opening  328 , which is an opening adjacent to the first gas chamber  122 , and an element-side opening  329 , which is an opening adjacent to the sensor element chamber  124 . The first cylindrical portion  334   b  and the second cylindrical portion  336  are shaped so that the element-chamber inlet  327  serves as a flow channel that is oblique to the front-back direction so that the flow channel becomes closer to the sensor element  110  (closer to the central axis of the inner protective cover  330 ) with increasing distance in the back-to-front direction of the sensor element  110 . Similarly, the element-side opening  329  opens obliquely to the front-back direction so as to become closer to the sensor element  110  with increasing distance in the back-to-front direction of the sensor element  110  (see the enlarged view in  FIG. 12 ). When the element-chamber inlet  327  is an oblique flow channel or when the element-side opening  329  is oblique as described above, the measurement-object gas flows into the sensor element chamber  124  through the element-side opening  329  in a direction oblique to the front-back direction of the sensor element  110 . Accordingly, an effect similar to that of the element-chamber inlet  127  and the element-side opening  129  according to the above-described embodiment can be obtained. In other words, the measurement-object gas is not blown against the surface of the sensor element  110  (surface other than the gas inlet  111 ) in a direction perpendicular to the surface of the sensor element  110 , nor does it flow a long distance along the surface of the sensor element  110  before reaching the gas inlet  111 . Accordingly, cooling of the sensor element  110  can be reduced. In  FIG. 12 , the element-chamber inlet  327  has a width that decreases with increasing distance in the back-to-front direction of the sensor element  110 . Therefore, the opening area of the element-side opening  329  is smaller than that of the outer opening  328 . In other words, the element-chamber inlet  327  is formed so that the distance A 4  described above with reference to  FIG. 7  is smaller than the distance A 5 . Accordingly, when the measurement-object gas enters through the outer opening  328  and flows out through the element-side opening  329 , the measurement-object gas flows out at a flow velocity higher than that at which the measurement-object gas enters. Therefore, the responsiveness in gas concentration detection can be increased. In  FIG. 12 , the element-chamber inlet  327  serves as a flow channel that is oblique to the front-back direction of the sensor element  110 , the element-side opening  329  opens obliquely to the front-back direction of the sensor element  110 , and the opening area of the element-side opening  329  is smaller than that of the outer opening  328 . However, one or more of these three features may be omitted. In the gas sensor  300  according to the modification, as illustrated in  FIG. 12 , the distance A 1  is the distance from the gas inlet  111  to the bottom end of the element-side opening  329  in the vertical direction. Also in the gas sensor  300  illustrated in  FIG. 12 , when, for example, the cross-sectional area ratio S1/S2 is more than 2.0 and 5.0 or less, an effect similar to that of the above-described embodiment can be obtained. 
     In the above-described embodiment, the element-side opening  129  opens in the forward direction. However, the element-side opening  129  is not limited to this, and may instead open in the sensor element chamber  124  in a direction perpendicular to the forward direction. In addition, in the above-described embodiment, the element-chamber inlet  127  is a flow channel that is parallel to the front-back direction of the sensor element  110 . However, the element-chamber inlet  127  is not limited to this. For example, the element-chamber inlet  127  may instead be a flow channel that is perpendicular to the forward direction. 
     In the above-described embodiment, the first gas chamber  122  is the only flow channel for the measurement-object gas between the element-chamber inlet  127  and the outer inlets  144   a . However, the first gas chamber  122  is not limited to this as long as the first gas chamber  122  is at least a portion of the flow channel for the measurement-object gas between the element-chamber inlet  127  and the outer inlets  144   a . For example, the protective cover  120  may include, in addition to the inner protective cover  130  and the outer protective cover  140 , an intermediate protective cover disposed between the inner protective cover  130  and the outer protective cover  140 , and the flow channel for the measurement-object gas between the element-chamber inlet  127  and the outer inlets  144   a  may include a plurality of gas chambers. Similarly, in the above-described embodiment, the second gas chamber  126  is the only flow channel for the measurement-object gas between the element-chamber outlet  138   a  and the outer outlets  147   a . However, the second gas chamber  126  is not limited to this as long as the second gas chamber  126  is at least a portion of the flow channel for the measurement-object gas between the element-chamber outlet  138   a  and the outer outlets  147   a.    
     In the above-described embodiment, the gas inlet  111  opens in the front end face of the sensor element  110  (lower surface of the sensor element  110  in  FIG. 3 ). However, the gas inlet  111  is not limited to this. For example, the gas inlet  111  may open in a side surface of the sensor element  110  (upper, lower, left, or right surface of the sensor element  110  in  FIG. 4 ). 
     In the above-described embodiment, the sensor element  110  includes the porous protective layer  110   a . However, it is not necessary that the sensor element  110  include the porous protective layer  110   a.    
     EXAMPLES 
     Examples of gas sensors that were actually manufactured will now be described. Experimental Examples 3 to 5 correspond to examples of the present invention, and Experimental Examples 1 and 2 correspond to comparative examples. The present invention is not limited to the following examples. 
     Experimental Example 1 
     A gas sensor  100  according to Experimental Example 1 was similar to the gas sensor  100  illustrated in  FIGS. 3 to 7  except that, as illustrated in  FIGS. 8 and 9 , the outer outlets  147   a  included the three horizontal holes  147   b  formed in the side portion  146   a  and the three vertical holes  147   c . The first member  131  of the inner protective cover  130  had a thickness of 0.3 mm and an axial length of 10.2 mm. The large-diameter portion  132  had an axial length of 1.8 mm and an outer diameter of 14.4 mm, and the first cylindrical portion  134  had an axial length of 8.4 mm and an outer diameter of 7.7 mm. The second member  135  had a thickness of 0.3 mm and an axial length of 11.5 mm. The second cylindrical portion  136  had an axial length of 4.5 mm and an inner diameter of 9.7 mm, and the front end portion  138  had an axial length of 4.9 mm. The bottom surface of the front end portion  138  had a diameter of 3.0 mm. With regard to the element-chamber inlet  127 , the distance A 1  was 0.59 mm, the distance A 2  was 1.7 mm, the distance A 3  was 3.1 mm, the distances A 4 , A 5 , and A 7  were 1.0 mm, the distance A 6  was 2.05 mm, and the length L was 4 mm. The element-chamber outlet  138   a  had a diameter of 1.5 mm. The outer protective cover  140  had a thickness of 0.4 mm and an axial length of 24.35 mm. The large-diameter portion  142  had an axial length of 5.85 mm and an outer diameter of 15.2 mm. The body portion  143  had an axial length of 8.9 mm (axial length from the upper end of the body portion  143  to the upper surface of the step portion  143   b  was 8.5 mm). The body portion  143  had an outer diameter of 14.6 mm. The front end portion  146  had an axial length of 9.6 mm and an outer diameter of 8.7 mm. The outer inlets  144   a  included six horizontal holes  144   b  having a diameter of 1 mm and six vertical holes  144   c  having a diameter of 1 mm. The horizontal holes  144   b  and the vertical holes  144   c  were alternately arranged at equal intervals (the adjacent holes form an angle of 30°). The outer outlets  147   a  included three horizontal holes  147   b  having a diameter of 1 mm and three vertical holes  147   c  having a diameter of 1 mm. The horizontal holes  147   b  and the vertical holes  147   c  were alternately arranged at equal intervals (the adjacent holes form an angle of 60°). The material of the protective cover  120  was SUS301S. The sensor element  110  of the gas sensor  100  had a width (length in the left-right direction in  FIG. 4 ) of 4 mm and a thickness (length in the vertical direction in  FIG. 4 ) of 1.5 mm. The porous protective layer  110   a  was an alumina porous body having a thickness of 400 μm. The minimum path length P was 11.4 mm. The total cross-sectional area S1 was 9.42 mm 2 . The total cross-sectional area S2 was 4.71 mm 2 . The cross-sectional area ratio S1/S2 was 2.00. 
     Experimental Example 2 
     A gas sensor  100  according to Experimental Example 2 was similar to the gas sensor  100  according to Experimental Example 1 except that the inner diameter of the first cylindrical portion  134  of the first member  131  was 7.88 mm, which was greater than that in Experimental Example 1. In Experimental Example 2, the distances A 4 , A 5 , and A 7  were 0.61 mm, the distance A 2  was 2.1 mm, the minimum path length P was 11.7 mm, the total cross-sectional area S1 was 9.42 mm 2 , the total cross-sectional area S2 was 4.71 mm 2 , and the cross-sectional area ratio S1/S2 was 2.00. 
     Experimental Example 3 
     A gas sensor  100  according to Experimental Example 3 was the gas sensor  100  illustrated in  FIGS. 3 to 7 . In Experimental Example 3, the outer outlets  147   a  did not include the horizontal holes  147   b , and the diameter of the vertical holes  147   c  was 1 mm as in Experimental Example 1. The horizontal holes  144   b  had a diameter of 1.5 mm, and were shifted backward so that the distance A 3  was 0.84 mm. Other dimensions were the same as those in Experimental Example 2. The minimum path length P was 10.0 mm, the total cross-sectional area S1 was 15.32 mm 2 , the total cross-sectional area S2 was 4.71 mm 2 , and the cross-sectional area ratio S1/S2 was 3.25. 
     Experimental Example 4 
     A gas sensor  100  according to Experimental Example 4 was the same as the gas sensor  100  of Experimental Example 3 except that the cross-sectional area of the vertical holes  144   c  and the cross-sectional area of the three vertical holes  147   c  were increased. More specifically, as illustrated in  FIG. 13 , the vertical holes  144   c  and the vertical holes  147   c  were formed in an arc shape that extends in the circumferential direction of the outer protective cover  140 , so that the cross-sectional areas thereof were increased. The vertical holes  144   c  and the vertical holes  147   c  were formed in an arc shape having a width of 1 mm. The six vertical holes  144   c  had a cross-sectional area of 2.4 mm 2 . The three vertical holes  147   c  also had a cross-sectional area of 2.4 mm 2 . The minimum path length P was 10.0 mm, the total cross-sectional area S1 was 25.03 mm 2 , the total cross-sectional area S2 was 7.21 mm 2 , and the cross-sectional area ratio S1/S2 was 3.47. 
     Experimental Example 5 
     A gas sensor  100  according to Experimental Example 5 was the same as the gas sensor  100  according to Experimental Example 3 except that the horizontal holes  144   b  were closer to the back end than the outer opening  128 , as illustrated in  FIG. 14 , and the distance A 3  was −0.16 mm. The minimum path length P was 9.9 mm. Referring to  FIG. 14 , in the gas sensor  100  according to Experimental Example 5, the minimum path length P was the length of the shortest path (broken line PL) from an end portion C 1  (lower end portion in  FIG. 14 ) of the outer opening of the horizontal hole  144   b , the end portion C 1  being closest to the outer opening  128 , to an end portion C 2  (left end portion in  FIG. 14 ) of the outer opening of the gas inlet  111 . The total cross-sectional area S1 was 15.32 mm 2 , the total cross-sectional area S2 was 4.71 mm 2 , and the cross-sectional area ratio S1/S2 was 3.25. 
     [Evaluation of Angular Dependence] 
     The influence of the attachment orientation of each of the gas sensors according to Experimental Examples 1 to 5 on the responsiveness (angular dependence) was evaluated. First, the gas sensor according to Experimental Example 1 was attached to a pipe in a manner illustrated in  FIGS. 1 and 2 . The attachment orientation of the gas sensor according to Experimental Example 1 was such that the measurement-object gas flowed through the pipe in the direction of arrow D 1  in  FIG. 8 . Gas obtained by mixing oxygen with atmospheric air to adjust the oxygen concentration was used as the measurement-object gas. The measurement-object gas was caused to flow through the pipe at a flow velocity of V=8 m/s. The oxygen concentration of the measurement-object gas that flowed through the pipe was changed from 22.9% to 20.2%, and a change in the output of the sensor element over time was measured. The output value of the sensor element immediately before the change in oxygen concentration was defined as 0%, and the output value of the sensor element at the time when the output of the sensor element became stable after the change in oxygen concentration was defined as 100%. The time from when the output value exceeded 10% to when the output value exceeded 90% was defined as the response time (sec) in gas concentration detection. The shorter the response time, the higher the responsiveness in gas concentration detection. The attachment orientation of the gas sensor according to Experimental Example 1 was changed to multiple orientations, and the response time was measured for each attachment orientation. More specifically, when the attachment orientation for causing the measurement-object gas to flow in the direction of arrow D 1  in  FIG. 8  was defined as 0°, the attachment orientation of the gas sensor was changed from 0° to 360° in steps of 30° by rotating the gas sensor around the central axis of the outer protective cover  140 , and the response time was measured for each attachment orientation. The attachment orientation of the gas sensor is the same for 0° and 360°. Each of the gas sensors according to Experimental Examples 2 to 5 was also attached in different orientations, and the response time was measured for each attachment orientation. In the gas sensor according to Experimental Example 2, similar to Experimental Example 1, the attachment orientation for causing the measurement-object gas to flow in the direction of arrow D 1  in  FIG. 8  was defined as 0°. In Experimental Examples 3 to 5, the attachment orientation for causing the measurement-object gas to flow from the upper left toward the lower right in  FIG. 4  in a direction parallel to the direction in which the upper left horizontal hole  144   b  in  FIG. 4  opens was defined as 0°. 
       FIG. 15  is a graph showing the angular dependence of the response time of each of the gas sensors according to Experimental Examples 1 to 5. As is clear from  FIG. 15 , in Experimental Examples 1 and 2, the response time greatly varies depending on the attachment orientation of the gas sensor. Thus, the response time had a high angular dependence. More specifically, in Experimental Examples 1 and 2, the response time periodically increased at intervals of substantially 120°. In Experimental Examples 1 and 2, the outer outlets  147   a  included three horizontal holes  147   b  formed in the side portion  146   a , and the horizontal holes  147   b  were arranged at equal intervals (120°) around the central axis of the outer protective cover  140 . Therefore, when the attachment orientation was 0°, 120°, 240°, or 360°, one of the horizontal holes  147   b  opened parallel to, and toward the upstream side of, the direction in which the measurement-object gas flowed. Accordingly, in Experimental Examples 1 and 2, when the attachment orientation was 0°, 120°, 240°, or 360°, the flow of the measurement-object gas that tried to flow out of the outer protective cover  140  through this horizontal hole  147   b  was impeded by the measurement-object gas that flowed around this horizontal hole  147   b , and the responsiveness tended to decrease as a result. In contrast, in Experimental Examples 3 to 5, as is clear from  FIG. 15 , variations in the response time depending on the attachment orientation were significantly smaller than those in Experimental Examples 1 and 2. Thus, the angular dependence was low. This is probably because no outer outlets  147   a  were formed in the side portion  146   a  in Experimental Examples 3 to 5. 
     [Evaluation of Responsiveness] 
     For each of the gas sensors according to Experimental Examples 1 to 5, the flow velocity V of the measurement-object gas that flowed through the pipe was set to 1, 2, 4, 6, 8, and 10 m/s, and the response time [sec] was measured for each flow velocity V. The response time was measured in a way similar to that for measuring the response time to evaluate the above-described angular dependence. When the flow velocity was V=8 m/s, as in the above-described case of evaluating the angular dependence, the attachment orientation was changed from 0° to 360°, and the response time was measured multiple times for each attachment orientation. In addition, the oxygen concentration in the measurement-object gas that flowed through the pipe was changed from 20.2% to 22.9% (change opposite to that in the evaluation of the angular dependence). Also in this case, the attachment orientation was similarly changed from 0° to 360°, and the response time was measured multiple times for each attachment orientation. The average of all of the response times was determined as the response time for the flow velocity V=8 m/s. In other cases (flow velocity V=1, 2, 4, 6, and 10 m/s), the attachment orientation was not changed. The response time was measured after the oxygen concentration in the measurement-object gas that flowed through the pipe was reduced (from 22.9% to 20.2%) and increased (from 20.2% to 22.9%), and the average of the response times was determined as the response time for each flow velocity V. The attachment orientation was set to 0° in Experimental Examples 1 and 2, and to 180° in Experimental Examples 3 to 5. 
     Table 1 shows the diameters and numbers of outer inlets and outer outlets in the outer protective cover, the minimum path length P, the total cross-sectional areas S1 and S2, the cross-sectional area ratio S1/S2, and the response time for each flow velocity V in Experimental Examples 1 to 5.  FIG. 16  is a graph showing the relationship between the flow velocity V and the response time in Experimental Examples 1 to 5. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                   
                   
                   
                 Total 
                   
               
               
                   
                   
                 Minimum 
                 Total cross- 
                 cross- 
                 Cross- 
               
               
                   
                   
                 path 
                 sectional 
                 sectional 
                 sectional 
               
               
                   
                 Outer protective cover 
                 length P 
                 area S1 
                 area S2 
                 area ratio 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Outer inlet 
                 Outer outlet 
                 [mm] 
                 [mm 2 ] 
                 [mm 2 ] 
                 S1/S2 
               
               
                   
               
               
                 Experimental 
                 Diameter of 1 mm × 6 
                 Diameter of 1 mm × 3 
                 11.4 
                 9.42 
                 4.71 
                 2.00 
               
               
                 example 1 
                 (horizontal hole) 
                 (horizontal hole) 
               
               
                   
                 Diameter of 1 mm × 6 
                 Diameter of 1 mm × 3 
               
               
                   
                 (vertical hole) 
                 (vertical hole) 
               
               
                 Experimental 
                 Diameter of 1 mm × 6 
                 Diameter of 1 mm × 3 
                 11.7 
                 9.42 
                 4.71 
                 2.00 
               
               
                 example 2 
                 (horizontal hole) 
                 (horizontal hole) 
               
               
                   
                 Diameter of 1 mm × 6 
                 Diameter of 1 mm × 3 
               
               
                   
                 (vertical hole) 
                 (vertical hole) 
               
               
                 Experimental 
                 Diameter of 1.5 mm × 6 
                 Diameter of 1 mm × 6 
                 10.0 
                 15.32 
                 4.71 
                 3.25 
               
               
                 example 3 
                 (horizontal hole) 
                 (vertical hole) 
               
               
                   
                 Diameter of 1 mm × 6 
               
               
                   
                 (vertical hole) 
               
               
                 Experimental 
                 Diameter of 1.5 mm × 6 
                 2.4 mm 2  × 3 
                 10.0 
                 25.03 
                 7.21 
                 3.47 
               
               
                 example 4 
                 (horizontal hole) 
                 (vertical hole) 
               
               
                   
                 2.4 mm 2  × 6 
               
               
                   
                 (vertical hole) 
               
               
                 Experimental 
                 Diameter of 1.5 mm × 6 
                 Diameter of 1 mm × 6 
                 9.9 
                 15.32 
                 4.71 
                 3.25 
               
               
                 example 5 
                 (horizontal hole) 
                 (vertical hole) 
               
               
                   
                 Diameter of 1 mm × 6 
               
               
                   
                 (vertical hole) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Response time 
                 Response time 
                 Response time 
                 Response time 
                 Response time 
                 Response time 
               
               
                   
                 (Flow velocity 
                 (Flow velocity 
                 (Flow velocity 
                 (Flow velocity 
                 (Flow velocity 
                 (Flow velocity 
               
               
                   
                 1 m/s) 
                 2 m/s) 
                 4 m/s) 
                 6 m/s) 
                 8 m/s) 
                 10 m/s) 
               
               
                   
                 [sec] 
                 [sec] 
                 [sec] 
                 [sec] 
                 [sec] 
                 [sec] 
               
               
                   
               
               
                 Experimental 
                 6.1 
                 3.7 
                 1.8 
                 1 
                 0.6 
                 0.4 
               
               
                 example 1 
               
               
                 Experimental 
                 9.6 
                 6.5 
                 2.8 
                 1.6 
                 1 
                 0.45 
               
               
                 example 2 
               
               
                 Experimental 
                 5.7 
                 3 
                 1.2 
                 0.5 
                 0.3 
                 0.2 
               
               
                 example 3 
               
               
                 Experimental 
                 5.2 
                 2.6 
                 1.1 
                 0.4 
                 0.3 
                 0.2 
               
               
                 example 4 
               
               
                 Experimental 
                 5.3 
                 2.6 
                 1.1 
                 0.5 
                 0.3 
                 0.2 
               
               
                 example 5 
               
               
                   
               
            
           
         
       
     
     Table 1 and  FIG. 16  show that, in each of Experimental Examples 1 to 5, the response time increased as the flow velocity V decreased. At each flow velocity V, the response times in Experimental Examples 3 to 5 were shorter than those in Experimental Examples 1 and 2. More specifically, the response times in Experimental Examples 3 to 5, in which the minimum path length P was 11.0 mm or less and the cross-sectional area ratio S1/S2 was more than 2.0, were shorter than those in Experimental Examples 1 and 2, in which the minimum path length P was more than 11.0 mm and the cross-sectional area ratio S1/S2 was 2.0 or less. In Experimental Examples 1 to 5, the response time decreased as the minimum path length P decreased. A comparison between Experimental Examples 3 and 5, which had the same cross-sectional area ratio S1/S2 and different minimum path lengths P, shows that the minimum path length P is preferably less than 10.0 mm. In Experimental Examples 1 to 5, the response time decreased as the cross-sectional area ratio S1/S2 increased. A comparison between Experimental Examples 3 and 4, which had the same minimum path length P and different cross-sectional area ratios S1/S2, shows that the cross-sectional area ratio S1/S2 is preferably 3.4 or more. A comparison between Experimental Examples 2 to 5, in which the first cylindrical portions  134  of the first members  131  had the same inner diameter and in which the protective covers  120  had relatively similar shapes, shows that the differences between the response time of Experimental Example 2 and the response times of Experimental Examples 3 to 5 increase as the flow velocity V decreases. This shows that when, in particular, the flow velocity V is low (4 m/s or less), the response-time reducing effect obtained by setting the minimum path length P to 11.0 mm or less or by setting the cross-sectional area ratio S1/S2 to more than 2.0 probably increases. 
     The present application claims priority from Japanese Patent Application No. 2016-121006, filed on Jun. 17, 2016, the entire contents of which are incorporated herein by reference.