Patent Publication Number: US-2016223487-A1

Title: Gas sensor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gas sensor that detects water vapor and carbon dioxide of a measurement gas. 
     2. Description of the Background Art 
     For example, in combustion control of internal combustion engines such as vehicle engines, environmental control such as exhaust gas control, or various fields including medical, biotechnology, and agriculture and industry fields, there is a need for accurately obtaining the concentration of a target gas component. Various measurement and evaluation techniques and apparatuses have conventionally been researched and studied in response to such a need. 
     For example, it is well known that in theory, a limiting-current type oxygen sensor can measure a water vapor concentration and a carbon dioxide concentration, as well as an oxygen concentration (for example, see Hideaki Takahashi, Keiichi Saji, Haruyoshi Kondo, “Thin-film Limiting-current Type Oxygen Sensor,” Vol. 27 No. 2 pp. 47-57, R&amp;D Review of Toyota Central R&amp;D Labs., Inc.). 
     The apparatus that measures carbon dioxide and water is well known, which includes two sensors each including two oxygen pumping cells and obtains a carbon dioxide concentration and a water (water vapor) concentration by an inverse matrix operation based on the pumping currents in the four pumping cells (for example, see Japanese Examined Patent Application Publication No. 06-76990 (1994)). 
     The carbon dioxide sensor, which has increased specificity to carbon dioxide and decreased sensitivity to water vapor, is well known (for example, see Japanese Patent Application Laid-Open No. 09-264873 (1997)). 
     When a carbon dioxide gas of a measurement gas is a detection target, the measurement gas often contains water vapor (water). Japanese Patent Application Laid-Open No. 09-264873 (1997) first points out such a problem that typical carbon dioxide sensors are susceptible to humidity, and then discloses that a carbon dioxide sensor having low sensitivity to water vapor has been achieved. 
     Unfortunately, the carbon dioxide sensor disclosed in Japanese Patent Application Laid-Open No. 09-264873 (1997) is susceptible to temperature changes of a measurement gas, because the sensor obeys the principle of mixed potential. 
     For a measurement gas containing water vapor and carbon dioxide, both may be expected to be measured. The apparatus disclosed in Japanese Examined Patent Application Publication No. 06-76990 (1994) enables the above-mentioned measurement in theory. However, this apparatus has yet to be put to practical use because it has a complex configuration and needs to undergo a complicated calibration process. 
     In particular, the exhaust gas from the internal combustion engine such as a vehicle engine contains a number of components such as oxygen, water vapor, carbon dioxide, hydrocarbon gas, and nonflammable gases, and besides, the component ratio and temperatures thereof may vary momentarily. No technique has been established which accurately obtains the concentrations of water vapor and carbon dioxide of the exhaust gas. The concentrations are difficult to measure particularly because the condition for decomposing carbon dioxide is similar to the condition for decomposing water vapor. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a gas sensor that detects water vapor and carbon dioxide of a measurement gas. 
     According to the present invention, a gas sensor, which has a sensor element formed of an oxygen-ion conductive solid electrolyte and identifies concentrations of a water vapor component and a carbon dioxide component of a measurement gas based on a current flowing through the solid electrolyte, includes: a gas inlet through which the measurement gas is introduced from the outside; a first diffusion control part that is in communication with the gas inlet and applies a first diffusion resistance to the measurement gas; a first internal space that is in communication with the first diffusion control part, into which the measurement gas is introduced with the first diffusion resistance from the outside; a second diffusion control part that is in communication with the first internal space and applies a second diffusion resistance to the measurement gas; a second internal space that is in communication with the second diffusion control part, into which the measurement gas is introduced with the second diffusion resistance from the first internal space; a main electrochemical pumping cell formed of a main internal electrode formed to face the first internal space, a first external electrode formed on an outer surface of the sensor element, and the solid electrolyte located between the main internal electrode and the first external electrode; a first measuring electrochemical pumping cell formed of a first measuring internal electrode formed to face the second internal space, a second external electrode formed on the outer surface of the sensor element, and the solid electrolyte located between the first measuring internal electrode and the second external electrode; a second measuring electrochemical pumping cell formed of a second measuring internal electrode formed at a position opposite to the second diffusion control part relative to the first measuring internal electrode in the second internal space, a third external electrode formed on the outer surface of the sensor element, and the solid electrolyte located between the second measuring internal electrode and the third external electrode; a reference gas space into which a reference gas is introduced; and a reference electrode formed to face the reference gas space. A diffusion resistance from the gas inlet to the first internal space is 370/cm or more and 1000/cm or less. The main electrochemical pumping cell is configured and disposed to adjust an oxygen partial pressure of the first internal space to 10 −12  atm to 10 −30  atm such that substantially all of the water vapor component and the carbon dioxide component are decomposed in the first internal space. The first measuring electrochemical pumping cell is configured and disposed to adjust an oxygen partial pressure of the second internal space such that hydrogen generated by the decomposition of the water vapor component selectively burns in the second internal space and that the oxygen partial pressure of the second internal space is higher than the oxygen partial pressure of the first internal space. The second measuring electrochemical pumping cell is configured and disposed to adjust an oxygen partial pressure near a surface of the second measuring internal electrode such that carbon monoxide generated by the decomposition of the carbon dioxide component selectively burns near the surface of the second measuring internal electrode and that the oxygen partial pressure near the surface of the second measuring internal electrode is higher than the oxygen partial pressure of the second internal space. The gas sensor is configured and disposed to identify the concentration of the water vapor component contained in the measurement gas based on the magnitude of a current flowing between the first measuring internal electrode and the second external electrode when the first measuring electrochemical pumping cell supplies oxygen to the second internal space, and identify the concentration of the carbon dioxide component contained in the measurement gas based on the magnitude of a current flowing between the second measuring internal electrode and the third external electrode when the second measuring electrochemical pumping cell supplies oxygen to the surface of the second measuring internal electrode. 
     According to the present invention, a water vapor concentration and a carbon dioxide concentration can be obtained accurately in up to a high concentration range irrespective of whether a measurement gas contains only one of or both of a water vapor component and a carbon dioxide component. 
     The present invention therefore has an object to provide a gas sensor capable of accurately obtaining the concentrations of water vapor and carbon dioxide of a measurement gas containing a non-target gas component. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of the structure of a gas sensor  100 ; 
         FIG. 2  schematically shows a graph showing the functional relationship between absolute values of a water vapor detection current Ip 1  and a carbon dioxide detection current Ip 2  and the actual concentrations of water vapor and carbon dioxide; 
         FIG. 3  shows the evaluation results of water vapor sensitivity characteristics in Example 1; 
         FIG. 4  shows the evaluation results of water vapor sensitivity characteristics in Example 1; 
         FIG. 5  shows the evaluation results of carbon dioxide sensitivity characteristics in Example 1; 
         FIG. 6  shows the evaluation results of carbon dioxide sensitivity characteristics in Example 1; and 
         FIG. 7  shows the evaluations results in Example 2. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Outline Configuration of Gas Sensor 
       FIG. 1  is a schematic cross-sectional view of the structure of a gas sensor  100  according to an embodiment of the present invention. The gas sensor  100  according to this embodiment serves to detect water vapor (H 2 O) and carbon dioxide (CO 2 ) and obtain the concentrations thereof. A sensor element  101  being a main part thereof includes, as a structural material, ceramic mainly composed of zirconia that is an oxygen-ion conductive solid electrolyte. 
     Although this embodiment will be described on the premise that a measurement gas contains water vapor and carbon dioxide, the measurement gas is not necessarily required to contain both of them. 
     The sensor element  101  has the structure in which six layers, namely, a first substrate layer  1 , a second substrate layer  2 , a third substrate layer  3 , a first solid electrolyte layer  4 , a spacer layer  5 , and a second solid electrolyte layer  6 , each formed of an oxygen-ion conductive solid electrolyte, are laminated in the stated order from the bottom side of  FIG. 1 . 
     Provided between the lower surface of the second solid electrolyte layer  6  and the upper surface of the first solid electrolyte layer  4  on one-end-portion side of the sensor element  101  are a gas inlet  10 , a first diffusion control part  11 , a first internal space  20 , a second diffusion control part  30 , and a second internal space  40 . A buffer space  12  and a fourth diffusion control part  13  may be further provided between the first diffusion control part  11  and the first internal space  20 . The gas inlet  10 , the first diffusion control part  11 , the buffer space  12 , the fourth diffusion control part  13 , the first internal space  20 , the second diffusion control part  30 , and the second internal space  40  are adjacently formed so as to communicate with each other in the stated order. The part extending from the gas inlet  10  to the second internal space  40  is referred to as a gas distribution part as well. 
     The gas inlet  10 , the buffer space  12 , the first internal space  20 , and the second internal space  40  are interior spaces provided by hollowing out the spacer layer  5 . The buffer space  12 , the first internal space  20 , and the second internal space  40  are each provided, with its upper portion defined by the lower surface of the second solid electrolyte layer  6 , its lower portion defined by the upper surface of the first solid electrolyte layer  4 , and its side portion defined by the side surface of the spacer layer  5 . 
     The first diffusion control part  11 , the second diffusion control part  30 , and the fourth diffusion control part  13  are each provided as two horizontally long slits (which are openings longitudinally extending to be vertical to the sheet of  FIG. 1 ). 
     At a position that is between the upper surface of the third substrate layer  3  and the lower surface of the spacer layer  5  and is farther from the distal-end side than the gas distribution part, a reference gas introduction space  43  is provided. The reference gas introduction space  43  is an interior space with its upper portion defined by the lower surface of the spacer layer  5 , its lower portion defined by the upper surface of the third substrate layer  3 , and its side portion defined by the side surface of the first solid electrolyte layer  4 . For example, oxygen or air is introduced into the reference gas introduction space  43  as a reference gas. 
     The gas inlet  10  is a part being open to the outside. Through this gas inlet  10 , a measurement gas is taken into the sensor element  101  from the outside. 
     The first diffusion control part  11  is a part that applies a predetermined diffusion resistance to the measurement gas taken through the gas inlet  10 . 
     The buffer space  12  is provided to cancel concentration fluctuations of the measurement gas which are caused due to pressure fluctuations of the measurement gas in the outside (in the case where the measurement gas is a vehicle exhaust gas, pulsations of the exhaust gas pressure). The sensor element  101  does not necessarily need to include the buffer space  12 . 
     The fourth diffusion control part  13  is a part that applies a predetermined diffusion resistance to the measurement gas introduced from the buffer space  12  into the first internal space  20 . The fourth diffusion control part  13  is a part provided in association with the provision of the buffer space  12 . 
     The first diffusion control part  11  and the first internal space  20  directly communicate with each other if the buffer space  12  and the third diffusion control part  13  are not provided. 
     The first internal space  20  is provided as a space for adjusting an oxygen partial pressure of the measurement gas introduced through the gas inlet  10 . This oxygen partial pressure is adjusted through the operation of a main pumping cell  21 . 
     The main pumping cell  21  is an electrochemical pumping cell (main electrochemical pumping cell) formed of a main inside pump electrode  22 , an outside pump electrode  23 , and the oxygen-ion conductive solid electrolyte sandwiched between these electrodes. The main inside pump electrode  22  is provided on substantially the entire upper surface of the first solid electrolyte layer  4 , substantially the entire lower surface of the second solid electrolyte layer  6 , and substantially the entire side surface of the spacer layer  5 , those surfaces defining the first internal space  20 . The outside pump electrode  23  is provided in the region corresponding to the main inside pump electrode  22  on the upper surface of the second solid electrolyte layer  6  so as to be exposed to the outside. The main inside pump electrode  22  and the outside pump electrode  23  are each formed as a porous cermet electrode rectangular in plan view (for example, a cermet electrode made of ZrO 2  and a precious metal such as Pt containing 0.1 wt % to 30.0 wt % of Au). The main inside pump electrode  22  is typically formed with an area of 0.1 mm 2  to 20 mm 2 . 
     The main pumping cell  21  causes, upon application of a pump voltage Vp 0  by a variable power source  24  provided outside the sensor element  101 , a pump current Ip to flow between the outside pump electrode  23  and the main inside pump electrode  22 , allowing oxygen in the first internal space  20  to be pumped out to the outside or outside oxygen to be pumped into the first internal space  20 . 
     In the sensor element  101 , the main inside pump electrode  22 , a reference electrode  42  sandwiched between the upper surface of the third substrate layer  3  and the first solid electrolyte layer  4 , and the oxygen-ion conductive solid electrolyte sandwiched between these electrodes constitute a first oxygen-partial-pressure detection sensor cell  60  being an electrochemical sensor cell. The reference electrode  42  is an electrode substantially rectangular in plan view, which is formed of a porous cermet similar to, for example, the outside pump electrode. Provided around the reference electrode  42  is a reference gas introduction layer  48  that is made of porous alumina and is continuous with the reference gas introduction space  43 , so that the reference gas of the reference gas introduction space  43  is introduced to the surface of the reference electrode  42 . In the first oxygen-partial-pressure detection sensor cell  60 , an electromotive force V 0  is generated between the main inside pump electrode  22  and the reference electrode  42 , which results from the oxygen concentration difference between the atmosphere in the first internal space  20  and the reference gas of the reference gas introduction space  43 . 
     The electromotive force V 0  generated in the first oxygen-partial-pressure detection sensor cell  60  varies in accordance with the oxygen partial pressure in the atmosphere existing in the first internal space  20 . The sensor element  101  uses this electromotive force V 0  to feedback-control the variable power source  24  for the main pumping cell  21 . This allows the pump voltage Vp 0 , which is applied to the main pumping cell  21  by the variable power source  24 , to be controlled in accordance with the oxygen partial pressure in the atmosphere of the first internal space  20 . 
     The second diffusion control part  30  is a part that applies a predetermined diffusion resistance to the measurement gas introduced from the first internal space  20  into the second internal space  40 . 
     The second internal space  40  is provided as a space for performing the process for measuring the concentrations of water vapor and carbon dioxide of the measurement gas introduced through the second diffusion control part  30 . In the second internal space  40 , oxygen can be supplied from the outside through the operation of a first measuring pumping cell  50 . 
     The first measuring pumping cell  50  is an auxiliary electrochemical pumping cell (first measuring electrochemical pumping cell) formed of a first measuring inside pump electrode  51 , the outside pump electrode  23 , and the oxygen-ion conductive solid electrolyte sandwiched between these electrodes. The first measuring inside pump electrode  51  is provided on substantially the entire upper surface of the first solid electrolyte layer  4 , substantially the entire lower surface of the second solid electrolyte layer  6 , and substantially the entire side surface of the spacer layer  5 , those surfaces defining the second internal space  40 . The first measuring inside pump electrode  51  is formed as a porous cermet electrode rectangular in plan view, similarly to the outside pump electrode  23  and the main inside pump electrode  22 . The use of the outside pump electrode  23  is not necessarily required, and any other cermet electrode provided on the outer surface of the sensor element  101  may form the first measuring pumping cell  50 , in place of the outside pump electrode  23 . 
     The first measuring pumping cell  50  causes, upon application of a pump voltage Vp 1  by a variable power source  52  provided outside the sensor element  101 , a pump current (water vapor detection current) Ip 1  to flow between the outside pump electrode  23  and the first measuring inside pump electrode  51 , so that oxygen can be pumped into the second internal space  40  (particularly to near the surface of the first measuring inside pump electrode  51 ). 
     In the sensor element  101 , the first measuring inside pump electrode  51 , the reference electrode  42 , and the oxygen-ion conductive solid electrolyte sandwiched between these electrodes constitute a second oxygen-partial-pressure detection sensor cell  61  being an electrochemical sensor cell. In the second oxygen-partial-pressure detection sensor cell  61 , an electromotive force V 1  is generated between the first measuring inside pump electrode  51  and the reference electrode  42 , which results from the oxygen concentration difference between the reference gas of the reference gas introduction space  43  and the atmosphere of the second internal space  40 , particularly near the surface of the first measuring inside pump electrode  51 . 
     The electromotive force V 1  generated in the second oxygen-partial-pressure detection sensor cell  61  varies in accordance with the oxygen partial pressure in the atmosphere existing near the surface of the first measuring inside pump electrode  51 . The sensor element  101  uses this electromotive force V 1  to feedback-control the variable power source  52  for the first measuring pumping cell  50 . This allows the pump voltage Vp 1 , which is applied to the first measuring pumping cell  50  by the variable power source  52 , to be controlled in accordance with the oxygen partial pressure of the atmosphere near the surface of the first measuring inside pump electrode  51 . 
     The sensor element  101  further includes a second measuring pumping cell  47  and a third oxygen-partial-pressure detection sensor cell  41 . The second measuring pumping cell  47  is an electrochemical sensor cell (second measuring electrochemical pumping cell) formed of the outside pump electrode  23 , a second measuring inside pump electrode  44 , and the oxygen-ion conductive solid electrolyte sandwiched between these electrodes. The third oxygen-partial-pressure detection sensor cell  41  is an electrochemical sensor cell formed of the second measuring inside pump electrode  44 , the reference electrode  42 , and the oxygen-ion conductive solid electrolyte sandwiched between these electrodes. 
     The second measuring inside pump electrode  44  is an electrode substantially rectangular in plan view, which is formed of a porous cermet similarly to, for example, the outside pump electrode. The second measuring inside pump electrode  44  is formed at a position opposite to the second diffusion control part  30  relative to the first measuring inside pump electrode  51 , schematically, at a position farther from the second diffusion control part  30  than the first measuring inside pump electrode  51 . However, the second measuring inside pump electrode  44  is covered with a third diffusion control part  45 . The third diffusion control part  45  is a porous alumina layer, which is a part that applies a predetermined diffusion resistance to the measurement gas that begins to come into contact with the second measuring inside pump electrode  44  in the second internal space  40 . In other words, the third diffusion control part  45 , which is assumed to limit an amount of the inflammable gas that comes into contact with the second measuring inside pump electrode  44 , isolates the second measuring inside pump electrode  44  from the second internal space  40 . The third diffusion control part  45  also functions as an electrode protecting layer that protects the second measuring inside pump electrode  44  from, for example, particle adhesion. 
     In the third oxygen-partial-pressure detection sensor cell  41 , an electromotive force V 2  is generated between the second measuring inside pump electrode  44  and the reference electrode  42 , which results from an oxygen concentration difference between the atmosphere near the surface of the second measuring inside pump electrode  44  covered with the third diffusion control part  45  and the reference gas of the reference gas introduction space  43 . In this embodiment, the surface of the second measuring inside pump electrode  44  also includes wall portions of a number of minute holes located to be in communication with the outside in the porous cermet forming the second measuring inside pump electrode  44 , not only the portion being in contact with the third diffusion control part  45 . 
     The electromotive force V 2  to be generated in the third oxygen-partial-pressure detection sensor cell  41  varies in accordance with an oxygen partial pressure of the atmosphere existing near the surface of the second measuring inside pump electrode  44 . The sensor element  101  uses this electromotive force V 2  to feedback-control a variable power source  46  for the second measuring pumping cell  47 . This allows the pump voltage Vp 2 , which is applied to the second measuring pumping cell  47  by the variable power source  46 , to be controlled in accordance with the oxygen partial pressure of the atmosphere near the surface of the second measuring inside pump electrode  44 . 
     The sensor element  101  is also configured to measure an electromotive force V ref  generated between the outside pump electrode  23  and the reference electrode  42  to obtain the oxygen partial pressure outside the sensor element  101 . 
     In the sensor element  101 , further, a heater  70  is formed to be vertically sandwiched between the second substrate layer  2  and the third substrate layer  3 . The heater  70  generates heat by power feeding from the outside through a heater electrode (not shown) provided on the lower surface of the first substrate layer  1 . Heat generation by the heater  70  increases the oxygen-ion conductivity of the solid electrolyte forming the sensor element  101 . The heater  70  is buried over the entire area extending from the first internal space  20  to the second internal space  40  to heat a predetermined portion of the sensor element  101  to a predetermined temperature or maintain the temperature of the predetermined portion at a predetermined temperature. Formed on the upper and lower surfaces of the heater  70  is a heater insulating layer  72  made of, for example, alumina to obtain electrical insulation between the second substrate layer  2  and the third substrate layer  3  (hereinafter, the heater  70 , the heater electrode, and the heater insulating layer  72  are correctively referred to as a heater part as well). 
     Measurement of Concentrations of Water Vapor and Carbon Dioxide 
     The technique of identifying the concentrations of water vapor and carbon dioxide of a measurement gas with the gas sensor  100  having the above-mentioned configuration will be described next. 
     First, the sensor element  101  is placed in the atmosphere of the measurement gas containing oxygen, water vapor, carbon dioxide, nonflammable (inert) gases. So, the measurement gas is introduced into the sensor element  101  through the gas inlet  10 . The measurement gas that has been introduced into the sensor element  101  reaches the first internal space  20 , following the application of a predetermined diffusion resistance by the first diffusion control part  11  or further by the fourth diffusion control part  13 . 
     In the first internal space  20 , through the operation of the main pumping cell  21 , oxygen is pumped out such that the oxygen partial pressure of the inside measurement gas has a sufficiently low predetermined value (for example, 10 −11  atm to 10 −30  atm) to an extent that substantially all of the water vapor and carbon dioxide contained in the measurement gas are decomposed. Herein, “substantially all of the water vapor and carbon dioxide contained in the measurement gas are decomposed” means that the water vapor and carbon dioxide introduced into the first internal space  20  are not introduced into the second internal space  40 . 
     When oxygen is pumped out of the first internal space  20  in this manner, the decomposition reaction of water vapor (2H 2 O→2H 2 +O 2 ) and the decomposition reaction of carbon dioxide (2CO 2 →2CO+O 2 ) are promoted in the first internal space  20 , so that hydrogen and oxygen are generated from the former reaction and carbon monoxide and oxygen are generated from the latter reaction. Of those, oxygen is pumped out by the main pumping cell  21 , but hydrogen and carbon monoxide are introduced into the second internal space  40  together with the other gases. 
     Actual pumping-out of oxygen is implemented in a manner that a target value of the electromotive force V 0 , which is generated between the main inside pump electrode  22  and the reference electrode  42  in the first oxygen-partial-pressure detection sensor cell  60 , is preliminarily determined to such a predetermined value as to obtain the above-mentioned oxygen partial pressure, and then the variable power source  24  controls the pump voltage Vp 0  to be applied to the main pumping cell  21  in accordance with the difference between the actual value of the electromotive force V 0  and the target value. For example, when the measurement gas containing a large amount of oxygen reaches the first internal space  20 , the value of the electromotive force V 0  varies considerably from the target value. Therefore, the variable power source  24  controls the pump voltage Vp 0  to be applied to the main pumping cell  21  in order to reduce such variations. 
     The target value of the electromotive force V 0  is preferably set such that a (target) oxygen partial pressure in the first internal space  20  becomes lower as the oxygen partial pressure of the measurement gas that has reached the first internal space  20  becomes higher (as the difference between the measured value of the electromotive force V 0  and the latest determined target value becomes larger). This allows oxygen to be pumped out more reliably. 
     The measurement gas that has the oxygen partial pressure decreased as described above reaches the second internal space  40 , following the application of a predetermined diffusion resistance by the second diffusion control part  30 . 
     In the second internal space  40 , pumping-in of oxygen is performed through the operation of the first measuring pumping cell  50 . This pumping-in of oxygen is performed to cause, of the measurement gas that has reached a position near the surface of the first measuring pumping cell  50  in the second internal space  40  and contains hydrogen and carbon monoxide generated in the first internal space  20 , only hydrogen to selectively react with the oxygen existing at the position. That is to say, oxygen is pumped in through the first measuring pumping cell  50  such that the reaction 2H 2 +O 2 →2H 2 O is promoted to again generate an amount of water vapor correlated with the amount of water vapor introduced through the gas inlet  10 . In this embodiment, being “correlated with the amount of water vapor or carbon dioxide” means that the amount of water vapor or carbon dioxide introduced through the gas inlet  10  and the amount of the water vapor or carbon dioxide generated again by burning of the hydrogen or carbon monoxide generated by the decomposition of water vapor or carbon dioxide are equal to each other or fall within a constant error range allowable from the perspective of measurement accuracy. 
     Actual pumping-in of oxygen is implemented in a manner that a target value of the electromotive force V 1 , which is generated between the first measuring inside pump electrode  51  and reference electrode  42  in the second oxygen-partial-pressure detection sensor cell  61 , is preliminarily determined to such a predetermined value as to obtain an oxygen partial pressure high enough to burn all of the hydrogen contained in the measurement gas that has reached near the surface of the first measuring inside pump electrode  51  and to substantially cause no burning of carbon monoxide contained in the measurement gas, and then the variable power source  52  controls the pump voltage Vp 1  to be applied to the first measuring pumping cell  50  in accordance with the difference between the actual value of the electromotive force V 1  and the target value. For example, when the measurement gas containing a large amount of hydrogen reaches near the first measuring inside pump electrode  51  and reacts with oxygen, an oxygen partial pressure decreases, which causes the value of the electromotive force V 1  to vary considerably from the target value. Therefore, the variable power source  52  controls the pump voltage Vp 1  to be applied to the first measuring pumping cell  50  so as to reduce the above-mentioned variations. 
     In this case, the current (water vapor detection current) Ip 1  flowing through the first measuring pumping cell  50  is approximately proportional to the concentration of the water vapor generated by burning of hydrogen near the surface of the first measuring inside pump electrode  51  (the water vapor detection current Ip 1  and the water vapor concentration have a linear relationship). The amount of the water vapor generated through such burning has a correlation with the amount of water vapor of the measurement gas, which has been introduced through the gas inlet  10  and then decomposed once in the first internal space  20 . Thus, detecting the water vapor detection current Ip 1  allows the concentration of water vapor of the measurement gas to be obtained based on the value of the water vapor detection current Ip 1 . How to actually identify the concentration of water vapor will be described below. 
     If the measurement gas introduced through the gas inlet  10  contains no water vapor, needless to say, the decomposition of the water vapor in the first internal space  20  does not occur, and accordingly, hydrogen is not introduced into the second internal space  40 . Thus, the electromotive force V 1  keeps a target value with a slight amount of water vapor detection current Ip 1  flowing, which corresponds to an offset current OFS 2  described below. 
     The measurement gas in which hydrogen has been burned reaches the second measuring inside pump electrode  44 , following the application of a predetermined diffusion resistance by the third diffusion control part  45 . 
     On the surface of the second measuring inside pump electrode  44 , pumping-in of oxygen is performed through the operation of the second measuring pumping cell  47 . This pumping-in of oxygen is performed such that, of the measurement gas that has reached near the surface of the second measuring inside pump electrode  44  and contains the carbon monoxide generated in the first internal space  20 , only carbon monoxide selectively burns. That is to say, oxygen is pumped in to the surface of the second measuring inside pump electrode  44  such that the reaction 2CO+O 2 →2C 2 O 2  is promoted to again generate an amount of carbon dioxide correlated with the amount of carbon dioxide introduced through the gas inlet  10 . 
     Actual pumping-in of oxygen is implemented in a manner that a target value of the electromotive force V 2 , which is generated between the second measuring inside pump electrode  44  and reference electrode  42  in the third oxygen-partial-pressure detection sensor cell  41 , is preliminarily determined to such a predetermined value as to obtain an oxygen partial pressure to cause burning of carbon monoxide and cause substantially no burning of hydrocarbon gas contained in the measurement gas, and then the variable power source  46  controls the pump voltage Vp 2  to be applied to the second measuring pumping cell  47  in accordance with the difference between the actual value of the electromotive force V 2  and the target value. For example, when the measurement gas containing a large amount of carbon monoxide reaches near the surface of the second measuring inside pump electrode  44  and reacts with oxygen, an oxygen partial pressure decreases, which causes the value of the electromotive force V 2  to vary considerably from the target value. Therefore, the variable power source  46  controls the pump voltage Vp 2  to be applied to the second measuring pumping cell  47  so as to reduce the above-mentioned variations. 
     In this case, the current (carbon dioxide detection current) Ip 2  flowing through the second measuring pumping cell  47  is approximately proportional to the concentration of the carbon dioxide generated by burning of carbon monoxide on the surface of the second measuring inside pump electrode  44  (the carbon dioxide detection current Ip 2  and the carbon dioxide concentration have a linear relationship). The amount of the carbon dioxide generated through such burning has a correlation with the amount of carbon dioxide of the measurement gas, which has been introduced through the gas inlet  10  and then decomposed once in the first internal space  20 . Thus, detecting the carbon dioxide detection current Ip 2  allows the concentration of carbon dioxide of the measurement gas to be obtained based on the value of the carbon dioxide detection current Ip 2 . How to actually identify the concentration of carbon dioxide will be described below. 
     If the measurement gas introduced through the gas inlet  10  contains no carbon dioxide, needless to say, the decomposition of carbon dioxide in the first internal space  20  does not occur, and accordingly, carbon monoxide is not introduced into the second internal space  40 . Thus, the electromotive force V 2  keeps a target value with a slight amount of water carbon dioxide detection current Ip 2  flowing, which corresponds to an offset current OFS 2  described below. 
     The above-mentioned manner of controlling an oxygen partial pressure facilitates selective burning of hydrogen near the surface of the first measuring inside pump electrode  51  and selective burning of carbon monoxide on the surface of the second measuring inside pump electrode  44 . This is because, due to a difference in gas diffusion rate between hydrogen and carbon monoxide, hydrogen has a higher gas diffusion rate and is more likely to come into contact with oxygen for burning than carbon monoxide, and hydrogen is more likely to combine with oxygen, that is, is more likely to burn, than carbon monoxide. 
       FIG. 2  shows a graph (sensitivity characteristics) schematically showing the functional relationship between absolute values of the water vapor detection current Ip 1  and the carbon dioxide detection current Ip 2  and actual concentrations of water vapor and carbon dioxide.  FIG. 2  shows the values of the water vapor detection current Ip 1  and the carbon dioxide detection current Ip 2  by absolute values for brevity of description. More specifically, when oxygen is pumped in near the surface of the first measuring inside pump electrode  51  and to the surface of the second measuring inside pump electrode  44  in the gas sensor  100  of  FIG. 1 , the water vapor detection current Ip 1  and the carbon dioxide detection current Ip 2  have negative values. 
       FIG. 2  illustrates an ideal functional relationship between the water vapor detection current Ip 1  and the water vapor concentration and an ideal functional relationship between the carbon dioxide detection current Ip 2  and the carbon dioxide concentration by a solid line L. The solid line L is a straight line with a non-zero intercept on the vertical axis. Although  FIG. 2  illustrates only one solid line L for the sake of convenience, in actuality, the water vapor sensitivity characteristics and the carbon dioxide sensitivity characteristics differ from each other, and accordingly, the functional relationships thereof (the inclination and a value of the intercept) generally do not coincide with each other. Although the values of the water vapor detection current Ip 1  and the carbon dioxide detection current Ip 2  with zero water vapor concentration and zero carbon dioxide concentration (where no water vapor and no carbon dioxide exist) should be originally zero, in actuality, a slight amount of the pump-in current for obtaining a target oxygen concentration, namely, the water vapor detection current Ip 1  and the carbon dioxide detection current Ip 2  flow, even when no water vapor and no carbon dioxide exist. This is because oxygen becomes scarce compared to a target oxygen concentration (oxygen partial pressure) near the surface of the first measuring inside pump electrode  51  and near the surface of the second measuring inside pump electrode  44 , due to an influence of the main pumping cell  21  pumping out oxygen. In particular, the water vapor detection current Ip 1  and the carbon dioxide detection current Ip 2  at this time are referred to as an offset current OFS. 
     In the gas sensor  100  according to this embodiment, prior to its use, the sensitivity characteristics (specifically, the offset current OFS and the slope of the graph) as shown in  FIG. 2  are preliminarily identified for each sensor element  101  based on the values of the water vapor detection current Ip 1  and the carbon dioxide detection current Ip 2  obtained when the gas having a known water vapor concentration and a known carbon dioxide concentration is supplied to the gas sensor  100 . In actually detecting water vapor and carbon dioxide, the values of the water vapor detection current Ip 1  and the carbon dioxide detection current Ip 2  are constantly measured, to thereby obtain a water vapor concentration and a carbon dioxide concentration corresponding to each individual measured value based on the sensitivity characteristics preliminarily identified. 
     As is apparent from the above description, pumping-in of oxygen to near the surface of the first measuring inside pump electrode  51  by the first measuring pumping cell  50  and pumping-in of oxygen to near the surface of the second measuring inside pump electrode  44  by the second measuring pumping cell  47  are performed independently of each other, which allows the calculation of a water vapor concentration based on the water vapor detection current Ip 1  and the calculation of a carbon dioxide concentration based on the carbon dioxide detection current Ip 2  to be performed independently of each other. In other words, even when the measurement gas contains any one of water vapor and carbon dioxide, the gas sensor  100  can preferably obtain its concentration. 
     For accurate measurements of a water vapor concentration and a carbon dioxide concentration in this manner, it is necessary, in the determination of sensitivity characteristics as well as in actual use, that the water vapor and carbon dioxide of the measurement gas be decomposed reliably in the first internal space  20 , that carbon monoxide should not be burned near the surface of the first measuring inside pump electrode  51  but only hydrogen be burned reliably, and that carbon monoxide be burned reliably on the surface of the second measuring inside pump electrode  44 . 
     To satisfy those needs, the target values of the electromotive force V 0  in the first oxygen-partial-pressure detection sensor cell  60 , the electromotive force V 1  in the second oxygen-partial-pressure detection sensor cell  61 , and the electromotive force V 2  in the third oxygen-partial-pressure detection sensor cell  41  are preferably determined so as to satisfy such relationships that the oxygen partial pressure of the second internal space  40 , particularly near the surface of the first measuring inside pump electrode  51 , is higher than the oxygen partial pressure of the first internal space  20  and that the oxygen partial pressure near the surface of the second measuring inside pump electrode  44  is equal to or higher than the oxygen partial pressure near the surface of the first measuring inside pump electrode  51 . 
     If a little amount of oxygen is pumped out by the main pumping cell  21  due to an excessively high oxygen partial pressure in the first internal space  20 , the generation of hydrogen and carbon monoxide by decomposition of water vapor and carbon dioxide is insufficient, and the measurement gas containing oxygen, and further, the remaining water vapor and carbon dioxide that have not been decomposed are introduced into near the surface of the first measuring inside pump electrode  51 , and further, near the surface of the second measuring inside pump electrode  44 . As a result, an amount of oxygen pumped in from near the surface of the first measuring inside pump electrode  51  and from near the surface of the second measuring inside pump electrode  44  is smaller than an original amount, and thus, in this case, the sensitivity characteristics indicated by a broken line L 1  with a gentle slope, shown in  FIG. 2 , are obtained. When the gas sensor  100  is used in the partial pressure setting described above, the water vapor concentration and the carbon dioxide concentration are calculated to be excessively lower than the actual values, even if the sensitivity characteristics set in advance are correct. 
     If the oxygen partial pressure targeted is too low near the surface of the first measuring inside pump electrode  51  and near the surface of the second measuring inside pump electrode  44 , oxygen is not pumped in sufficiently, and hydrogen and carbon monoxide remain. In this case, the sensitivity characteristics indicated by the broken line L 1  with a gentle slope, shown in  FIG. 2 , are also obtained. Needless to say, also in this case, accurately calculating concentration is difficult. 
     Meanwhile, if the oxygen partial pressure at the position near the surface of the first measuring inside pump electrode  51  is excessively high, at this position, carbon monoxide is unintentionally oxidized, though only selective oxidation of hydrogen is intended. The sensitivity characteristics in this case have a larger intercept and a steeper slope, as indicated by a broken line L 2  in  FIG. 2 , than those of the solid line L indicating the actual sensitivity characteristics (the value of the interception in this case is an offset current OFS 2 ). When the gas sensor  100  is used in the above-mentioned partial-pressure setting, the water vapor concentration and the carbon dioxide concentration are calculated to be much higher than actual values even if the preset sensitivity characteristics are correct. Also for an excessively high oxygen partial pressure near the surface of the second measuring inside pump electrode  44 , the offset current becomes larger than the original sensitivity characteristics, similarly to, for example, the offset current OFS 2  in the sensitivity characteristics indicated by the broken line L 2 . 
     More specifically, to obtain the sensitivity characteristics indicated by the solid line L of  FIG. 2 , at least, the oxygen partial pressure of the first internal space  20  needs to be set to 10 −12  atm to 10 −30  atm. It is further preferable that the oxygen partial pressure of the second internal space  40 , particularly near the surface of the first measuring inside pump electrode  51 , be set to 10 −5  atm to 10 −15  atm and that the oxygen partial pressure on the surface of the second measuring inside pump electrode  44  be set to 10 0  atm to 10 −15  atm. 
     Additionally, to accurately obtain concentrations of water vapor and carbon dioxide up to the highest possible concentration values using the gas sensor  100 , in other words, to cause the sensitivity characteristics as indicated by the solid line L of  FIG. 2  to be exhibited up to the highest possible concentration, the flow rate of the measurement gas flowing into the first internal space  20  through the gas inlet  10  needs to be controlled appropriately in the gas sensor  100 , in addition to adjusting the oxygen partial pressure of the first internal space  20  so as to satisfy the above-mentioned range. 
     The reason is following. As the concentrations of the water vapor and carbon dioxide of a measurement gas flowing into the first internal space  20  become higher, an amount of oxygen generated by the decomposition reactions of them, that is, an amount of oxygen to be pumped out by the main pumping cell  21  increases. Inherently, based on the theory of chemical equilibrium, the decomposition reactions of water vapor and carbon dioxide described above are more promoted as oxygen concentration becomes lower. Therefore, if the flow rate of the measurement gas flowing into the first internal space  20  is too high, the absolute amount of flowing oxygen becomes higher. This results in insufficient pumping-out of oxygen by the main pumping cell  21 . In such a case, consequently, water vapor and carbon dioxide are more difficult to be decomposed completely in the first internal space  20  as the concentrations of water vapor and carbon dioxide of a measurement gas become higher. To prevent an occurrence of such a situation, thus, the flow rate of a measurement gas flowing into the first internal space  20  needs to be limited in such a range that sensitivity characteristics are preferably achieved to allow the measurement in up to a high concentration range. 
     When, for example, the exhaust gas from an internal engine such as a vehicle engine is a measurement gas, for the concentrations of water vapor and carbon dioxide, at least approximately 20 vol % is desired to be set as an upper limit of the value enabling measurement, and further, measurement is more preferably performed accurately up to approximately 30 vol %. 
     In the gas sensor  100  according to this embodiment, the flow rate of the measurement gas flowing into the first internal space  20  through the gas inlet  10  in the sensor element  101  depends on the diffusion resistance applied to a measurement gas between the gas inlet  10  and the first internal space  20 . More specifically, it is the first diffusion control part  11  and the fourth diffusion control part  13  that substantially control the inflow (diffusion) of the measurement gas between the gas inlet  10  and the first internal space  20 , and accordingly, the flow rate depends on the diffusion resistances applied by the first diffusion control part  11  and the fourth diffusion control part  13 . 
     For example, if the value of a larger one of the diffusion resistances of the first diffusion control part  11  and the fourth diffusion control part  13  is 370/cm or more, the linearity of sensitivity characteristics is achieved in the range of up to at least approximately 20 vol % for concentrations of water vapor and carbon dioxide, and accurate measurements are enabled for both of the concentrations. If the value of the diffusion resistance is 680/cm or more, the linearity of sensitivity characteristics can be achieved in up to the range of at least approximately 30 vol % for the concentrations of water vapor and carbon dioxide. 
     An excessively high diffusion resistance results in an excessively gentle slope of sensitivity characteristics (as indicated by the broken line L 1  of  FIG. 2 ), undesirably resulting in poor measurement accuracy. In view of the above, the diffusion resistances of the first diffusion control part  11  and the fourth diffusion control part  13  are preferably 1000/cm or less. 
     In this embodiment, the diffusion resistances of the first diffusion control part  11  and the fourth diffusion control part  13  are each defined as a value L/S where L represents the length in the element longitudinal direction of the diffusion control part and S represents a total area (a total of cross-sectional areas of two slits) perpendicular to the element longitudinal direction. 
     For example, it is preferable to set, for the slit portions of the first diffusion control part  11  and the fourth diffusion control part  13 , a length L to 0.03 cm to 0.07 cm, the thickness (the size in the lamination direction of each solid electrolyte layer) to 0.001 cm to 0.010 cm, and the width (the size in the direction perpendicular to the element longitudinal direction in the plane of the spacer layer  5 ) to 0.01 cm to 0.05 cm. Needless to say, the value of the cross-sectional area S is to be sufficiently smaller than the area of the cross section perpendicular to the element longitudinal direction of the first internal space  20 . 
     As described above, in the gas sensor according to this embodiment, the main pumping cell is operated in the first internal space, so that the measurement gas whose oxygen partial pressure is always set to a constant low value (a value that decomposes all of the water vapor and carbon dioxide contained) is supplied to the second internal space. Then, in the second internal space, particularly near the surface of the first measuring inside pump electrode, only hydrogen generated by the decomposition of water vapor in the first internal space is selectively burned. The pump current, which flows through the first measuring pumping cell for supplying oxygen in the above-mentioned burning, has a linear relationship with the concentration of the water vapor generated by burning of hydrogen, that is, the concentration of the water vapor of the measurement gas introduced through the gas inlet. Based on the linear relationship, the water vapor concentration of the measurement gas can be obtained. 
     The measurement gas after hydrogen is burned passes through the third diffusion control part to reach near the surface of the second measuring inside pump electrode. On the surface of the second measuring inside pump electrode, only carbon monoxide is selectively burned. The pump current flowing through the second measuring pumping cell for supplying oxygen in the burning has a linear relationship with the concentration of carbon dioxide generated through burning of carbon monoxide, that is, the concentration of carbon dioxide of a measurement gas introduced through the gas inlet. Based on the linear relationship, the carbon dioxide concentration of a measurement gas can be obtained. 
     The gas sensor according to this embodiment can therefore accurately obtain the concentrations of water vapor and carbon dioxide irrespective of whether a measurement gas contains any one of or both of water vapor and carbon dioxide. 
     Besides, in the gas sensor according to this embodiment, a diffusion resistance applied to the measurement gas between the gas inlet and the first internal space is appropriately determined, so that the upper limits for measuring the concentrations of water vapor and carbon dioxide are approximately 20 vol % or more. 
     Therefore, the gas sensor according to this embodiment can accurately obtain the concentration of water vapor and the concentration of carbon dioxide of a measurement gas containing various gases including oxygen, water vapor, carbon dioxide, and nonflammable (inert) gases, such as an exhaust gas from an internal combustion engine, for example, a vehicle engine. In other words, the gas sensor according to this embodiment can be preferably put into practical use as a sensor for exhaust gas in an internal combustion engine. 
     EXAMPLES 
     Example 1 
     In this example, three types of gas sensors  100 , each having a different diffusion resistance from the gas inlet  10  to the first internal space  20 , were prepared, and the sensitivity characteristics thereof were evaluated using various model gases having a known water vapor concentration and a known carbon dioxide concentration. The sensitivity characteristics were evaluated in three ways each having a different oxygen partial pressure of the first internal space  20 . Employed as the values of the diffusion resistance from the gas inlet  10  to the first internal space  20  of three types of gas sensors  100  were values of the diffusion resistance calculated from the length L in the element longitudinal direction and an area S of a cross section perpendicular to the element longitudinal direction for the first diffusion control part  11  and the fourth diffusion control part  13 . The length L in the element longitudinal direction used in calculation was common to the three types, 0.05 cm, and the area S of the cross section perpendicular to the element longitudinal direction was 0.000179 cm 2 , 0.000106 cm 2 , and 0.0000556 cm 2  for the respective types. For all the types, the area of the main inside pump electrode  22  was 16 mm 2 . 
     Table 1 shows the combinations (which are referred to as sensor conditions 1 to 12) of the oxygen partial pressure of the first internal space  20  and the diffusion resistance from the gas inlet  10  to the first internal space  20  of the gas sensor  100 . 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Oxygen partial pressure 
                 Diffusion resistance up to 
               
               
                 Sensor 
                 of first internal space 
                 first internal space 
               
               
                 condition 
                 atm 
                 /cm 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 2.1 × 10 −10   
                 280 
               
               
                 2 
                 2.1 × 10 −10   
                 470 
               
               
                 3 
                 2.1 × 10 −10   
                 900 
               
               
                 4 
                 5.7 × 10 −12   
                 280 
               
               
                 5 
                 5.7 × 10 −12   
                 470 
               
               
                 6 
                 5.7 × 10 −12   
                 900 
               
               
                 7 
                 1.0 × 10 −25   
                 280 
               
               
                 8 
                 1.0 × 10 −25   
                 470 
               
               
                 9 
                 1.0 × 10 −25   
                 900 
               
               
                 10 
                 2.5 × 10 −28   
                 280 
               
               
                 11 
                 2.5 × 10 −28   
                 470 
               
               
                 12 
                 2.5 × 10 −28   
                 900 
               
               
                   
               
            
           
         
       
     
       FIGS. 3 and 4  show the evaluation results of water vapor sensitivity characteristics. Specifically,  FIG. 3  shows a graph showing, on one coordinate plane, the relationship between the water vapor concentration of a model gas and the water vapor detection current Ip 1  per sensor condition having the same value of the diffusion resistance from the gas inlet  10  to the first internal space  20 . The gases prepared as the model gases were as follows: the oxygen concentration was fixed at 10%, the water vapor concentration was varied in the range of 0 vol % to 30 vol %, and the remaining gas was nitrogen. 
     As shown in  FIG. 3 , in all of the conditions 1 to 3 where the oxygen partial pressure of the first internal space  20  is 2.1×10 −10  atm, the water vapor detection current Ip 1  was almost the same irrespective of water vapor concentration. In other words, no sensitivity characteristics were obtained. This means that the water vapor contained in the measurement gas was not decomposed due to an excessively high oxygen partial pressure. 
     In contrast, in the sensor conditions 4 to 12 where the oxygen partial pressure of the first internal space  20  was set to be lower than those of the sensor conditions 1 to 3, the relationship between the water vapor concentration and water vapor detection current Ip 1  had linearity in at least part of the range. Specifically, in the sensor conditions 4, 7, and 10 where the value of the diffusion resistance from the gas inlet  10  to the first internal space  20  was 280/cm, linearity was obtained in the range in which the water vapor concentration was up to approximately 10 vol %. In the sensor conditions 5, 8, and 11 where the value of the diffusion resistance was 470/cm, linearity was obtained in the range in which the water vapor concentration was up to approximately 20 to 25 vol %. In the sensor conditions 6, 9, and 12 where the value of the diffusion resistance was 900/cm, linearity was obtained in the range in which the water vapor concentration was up to approximately 30 vol % that is the highest water vapor concentration of the prepared model gases. 
       FIG. 4  shows the results of examining the influence of the presence or absence of carbon dioxide on the water vapor sensitivity characteristics. Specifically,  FIG. 4  shows the results of evaluating the sensitivity characteristics of model gases in the sensor conditions 7 to 9 shown in Table 1. The gases prepared as the model gases were set as follows: the oxygen concentration and carbon dioxide concentration were each fixed at 10 vol %, the water vapor concentration was varied in the range of 0 vol % to 30 vol %, and the remaining gas was nitrogen. For easy comparison,  FIG. 4  also shows the results obtained in the sensor conditions 7 to 9 shown in  FIG. 3 , in which the model gas contains no carbon dioxide. 
     In each of the sensor conditions, as shown in  FIG. 4 , the water vapor sensitivity characteristics when water vapor and carbon dioxide coexist were almost the same as those when no carbon dioxide exists. 
       FIGS. 5 and 6  show the evaluation results of carbon dioxide sensitivity characteristics. Specifically,  FIG. 5  shows, on one coordinate plane, a graph showing the relationship between the carbon dioxide concentration of a model gas and the carbon dioxide detection current Ip 2  per sensor condition having the same value of the diffusion resistance from the gas inlet  10  to the first internal space  20 . The gases prepared as the model gases were as follows: the oxygen concentration was fixed at 10%, the carbon dioxide concentration was varied in the range of 0 vol % to 30 vol %, and the remaining gas was nitrogen. 
     As shown in  FIG. 5 , in all of the conditions 1 to 3 where the oxygen partial pressure of the first internal space  20  is 2.1×10 −10  atm, the carbon dioxide detection current Ip 2  was almost the same irrespective of carbon dioxide concentration. In other words, no sensitivity characteristics were obtained. This means that the carbon dioxide contained in the measurement gas was not decomposed due to an excessively high oxygen partial pressure. 
     In contrast, in the sensor conditions 4 to 12 where the oxygen partial pressure of the first internal space  20  was set to be lower than those of the sensor conditions 1 to 3, the relationship between the carbon dioxide concentration and carbon dioxide detection current Ip 2  had linearity in at least part of the range. Specifically, in the sensor conditions 4, 7, and 10 where the value of the diffusion resistance from the gas inlet  10  to the first internal space  20  was 280/cm, linearity was obtained in the range in which the carbon dioxide concentration was up to approximately 10 vol %. In the sensor conditions 5, 8, and 11 where the value of the diffusion resistance was 470/cm, linearity was obtained in the range in which the carbon dioxide concentration was up to approximately 20 to 25 vol %. In the sensor conditions 6, 9, and 12 where the value of the diffusion resistance was 900/cm, linearity was obtained in the range in which the carbon dioxide concentration was up to approximately 30 vol % that is the highest water vapor concentration of the prepared model gases. 
       FIG. 6  shows the results of examining the influence of the presence or absence of water vapor on the carbon dioxide sensitivity characteristics. Specifically,  FIG. 6  shows the results of evaluating the sensitivity characteristics of model gases in the sensor conditions 7 to 9 shown in Table 1. The gases prepared as model gases were set as follows: the oxygen concentration and water vapor concentration were each fixed at 10 vol %, the carbon dioxide concentration was varied in the range of 0 vol % to 30 vol %, and the remaining gas was nitrogen. For easy comparison,  FIG. 6  also shows the results in the sensor conditions 7 to 9 shown in  FIG. 5 , in which the model gas contains no water vapor. 
     In each of the sensor conditions, as shown in  FIG. 6 , the carbon dioxide sensitivity characteristics when water vapor and carbon dioxide coexist were almost the same as those when no carbon dioxide exists. 
     The results above indicate that by setting the oxygen partial pressure of the first internal space  20  to 10 −12  atm or lower, irrespective of whether water vapor and carbon dioxide coexist in a measurement gas that may contain at least one of water vapor and carbon dioxide, the concentration of water vapor and the concentration of carbon dioxide of the measurement gas can be preferably measured. 
     Further, the results above indicate that by setting the value of the diffusion resistance from the gas inlet  10  to the first internal space  20  to 370/cm or higher, the water vapor concentration and carbon dioxide concentration can be measured in the range of up to approximately 20 to 25 vol % or higher. 
     Example 2 
     In this example, the carbon dioxide detection current Ip 2  was obtained in the sensor conditions 7 to 9 of Table 1, where the exhaust gases actually emitted from various vehicle engines were measurement gases. Table 2 lists the types of engines used and the results of obtaining the carbon dioxide concentrations of the exhaust gases emitted from the engines with an engine exhaust gas analyzer. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Type of engine 
                 Co 2  concentration (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Diesel 
                 2.2 
               
               
                   
                 Diesel 
                 4.6 
               
               
                   
                 Diesel 
                 5.2 
               
               
                   
                 Diesel 
                 5.8 
               
               
                   
                 Diesel 
                 6.9 
               
               
                   
                 Diesel 
                 7.9 
               
               
                   
                 Diesel 
                 9.3 
               
               
                   
                 Natural gas 
                 9.8 
               
               
                   
                 Diesel 
                 11.2 
               
               
                   
                 Gasoline 
                 14.0 
               
               
                   
                 Gasoline 
                 14.6 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 7  shows the results of plotting the values of the carbon dioxide detection current Ip 2  in the sensor conditions 7 to 9 against the carbon dioxide concentrations shown in Table 2. 
       FIG. 7  confirms that in the sensor condition 7, linearity is found between the carbon dioxide detection current Ip 2  and the carbon dioxide concentration in the range where the carbon dioxide concentration is up to approximately 10 vol % and that in the sensor conditions 8 and 9, linearity is found between the carbon dioxide detection current Ip 2  and the carbon dioxide concentration in the rage where the carbon dioxide concentration is at least up to approximately 15 vol %. Moreover, these results including the absolute value of the carbon dioxide detection current Ip 2  almost match the results on the model gases shown in  FIGS. 5 and 6 . This means that the evaluations using the model gases in Example 1 are appropriate, and further, that the measurements using the gas sensor  100  in the embodiment above can be actually enabled.