Abstract:
A gas sensor which includes walls delimiting a gas detection chamber, and having an introduction port (or an inlet) through which an observed gas is introduced into the gas detection chamber, a measuring element disposed in the gas detection chamber and measuring concentration of a subject gas contained in the observed gas, and a heater constituting at least a portion of the walls, the portion facing the gas detection chamber.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a gas sensor such as a catalytic combustion hydrogen sensor which is provided in a fuel-cell vehicle. 
   Priority is claimed on Japanese Patent Application No. 2005-107806, filed Apr. 4, 2005 and Japanese Patent Application No. 2005-176228, filed Jun. 16, 2005, the contents of which are incorporated herein by reference. 
   2. Description of Related Art 
   Conventionally, a known solid polymer membrane fuel cell has a stack including a plurality of cells which are constituted by a solid polymer electrolyte membrane being sandwiched by a fuel electrode and an oxygen electrode (below, simply referred to as the “fuel cell”). Hydrogen is supplied to the fuel electrode as fuel and oxygen is supplied to the oxygen electrode as oxidizing gas. Hydrogen ions generated by a catalytic reaction at the fuel electrode move to the oxygen electrode through the solid polymer electrolyte membrane. Electricity is generated by an electrochemical reaction of the hydrogen ions and oxygen at the oxygen electrode. 
   In fuel cells including the aforementioned one, a protection device having a hydrogen detector (a gas sensor) provided in an exhaust system of the oxygen electrode of the fuel cell, wherein the protection device stops fuel supply when the gas sensor detects that hydrogen of the fuel electrode has leaked to the oxygen electrode through the solid polymer electrolyte membrane is conventionally known (see, for example, Japanese Unexamined Patent Application, First Publication No. H06-223850). 
   As the hydrogen detector, a catalytic combustion hydrogen detector having a gas detection element made of a catalytic substance such as platinum and a temperature compensation element is known. When hydrogen contacts the catalytic substance such as platinum, it combusts and heat is generated. The gas detection element reaches high temperature due to the heat and the temperature compensation element, which is at under ambient temperature, is caused to be at relatively low temperature. This hydrogen detector measures a concentration of hydrogen gas in accordance with a difference in electric resistance between the gas detection element and the temperature compensation element generated by the aforementioned process. 
   In addition, a gas sensor having a tube-shaped case which houses the gas detection element and the temperature compensation element and a tube-shaped heater provided in the case is also known. The inner surface of the heater releases heat and surrounds the gas detection element and the temperature compensation element (PCT International Publication No. WO2003/042678). 
   Incidentally, in the aforementioned fuel cells, offgas of the fuel cell, especially from the oxygen electrode reaches high humidity because water (hereinbelow, simply referred to as “humidification water”) is mixed into the reaction gas (such as hydrogen or oxygen) which is supplied to the fuel cell by a humidification device or the like, in order to maintain the ion-conductivity of the solid polymer electrolyte membrane. Furthermore, water is produced by an electrochemical reaction (so-called “produced water”) when the fuel cell operates. 
   Therefore, in the protection devices disclosed in the aforementioned publications, condensation may occur on the hydrogen detection element provided in a flow path of the offgas from the fuel cell, which has high humidity. In this case, the hydrogen detection element may deteriorate or be damaged. Especially, the water in the offgas readily condenses in the aforementioned solid polymer membrane fuel cell because the fuel cell usually operates at a temperature lower than the one where water becomes vapor and offgas having high humidity is exhausted from the fuel cell. Therefore, when the catalytic combustion hydrogen detector is provided in the exhaust system of the oxygen electrode of the fuel cell, the gas detection element may be broken or decline in sensitivity in a case where topically inhomogeneous temperature of the surface of the gas detection element caused when current is applied to the gas detection element with the humidification water, produced water or the like being deposited on the surface thereof. 
   When a heater provided for lowering a humidity of the case is provided in the case which houses the gas detection element and the temperature compensation element to solve the aforementioned problem as the above publication discloses, the number of parts of the gas sensor increases and the gas sensor must be large to ensure a detection chamber of a desired size. Furthermore, a notch is made on the wall of the tube-shaped heater along the axial direction to connect two leads for applying current. Therefore, a traverse sectional shape of the heater becomes almost a C-shape and a temperature near the notch becomes relatively low. This may cause inhomogeneous temperature of the surroundings of the gas detection element and the temperature compensation element and may lead to condensation. 
   SUMMARY OF THE INVENTION 
   The present invention was made considering the aforementioned circumstances and thus it is an object to provide a gas sensor which is able to prevent damage, degradation, and decline in detection accuracy. 
   The first aspect of the present invention is a gas sensor including: walls delimiting a gas detection chamber, and having an introduction port through which an observed gas is introduced into the gas detection chamber; a measuring element disposed in the gas detection chamber and measuring concentration of a subject gas contained in the observed gas; and a heater constituting at least a portion of the walls, the portion facing the gas detection chamber. 
   The heater may be provided facing the gas detection chamber. In this case, relative humidity can be lowered by the heater even when an observed gas of high relative humidity brings water into the gas detection chamber. 
   The gas sensor may further include a first dehumidifier absorbing water in a reversible manner, and disposed on a portion of the walls that is located opposite the introduction port with respect to the measuring element. In this case, the produced water can be absorbed by the first dehumidifier. Therefore, decline in accuracy of detection of the subject gas, and damage or degradation caused when current is applied to the measuring element with water being deposited on the surface thereof can be prevented. 
   The heater may include a first heater which is tube-shaped and a second heater which is ring-shaped, and the first dehumidifier be provided to be sandwiched between the first heater and the second heater. In this case, the first dehumidifier can be maintained to absorb a desired amount of water which is deposited at a position where condensation is likely to occur and develop. 
   The gas sensor may further including a second dehumidifier absorbing water in a reversible manner, and disposed between the measuring element and the introduction port. In this case, a relative humidity of the observed gas to which the measuring element is exposed can be lowered and condensation on the surface of the measuring element can be prevented. 
   The heater may be provided to be sandwiched between the first dehumidifier and the second dehumidifier. In this case, the first dehumidifier and the second dehumidifier can be heated and dried (that is, to release water) by the heater, and be maintained to absorb a desired amount of water. 
   The heater may constitute the entirety of the walls. In this case, a temperature and a humidity of the gas detection chamber and a temperature of the surface of the measuring element can be maintained uniform. At the same time, a desired temperature and a desired humidity can be easily set. Therefore, condensation due to inhomogeneous temperature or humidity in the gas detection chamber can be prevented. 
   In addition, since there is no need to provide an extra heater in the gas detection chamber, a detection space of the gas detection chamber can be enlarged without an enlargement of the gas sensor itself. This leads to prevention of drastic changes in temperature and humidity of the detection space and the temperature of ambient gas and the measuring element in the detection space rapidly lowering below dew point. In addition, the accuracy of detection of the observed gas can be improved because the amount of the observed gas introduced to the gas detection chamber can be increased. 
   The gas sensor may further include a third dehumidifier absorbing water in a reversible manner, and being disposed on the heater. In this case, a temperature of the third dehumidifier can be directly controlled. Therefore, the third dehumidifier can be secured to absorb a desired amount of water by being heated and dried (that is, to release water) by the heater. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a fuel cell system having a gas sensor of a first embodiment of the present invention. 
       FIG. 2  is a sectional view of the gas sensor shown in  FIG. 1 . 
       FIG. 3  is a schematic sectional view along line A-A of  FIG. 2 . 
       FIG. 4  is a detailed sectional view of the gas detection chamber shown in  FIG. 3 . 
       FIG. 5  is a diagram showing a circuit of the gas sensor shown in  FIG. 1 . 
       FIG. 6  is a block diagram of a fuel cell system having a gas sensor of a second embodiment of the present invention. 
       FIG. 7  is a sectional view of the gas sensor shown in  FIG. 6 . 
       FIG. 8  is a schematic sectional view along line A-A of  FIG. 7 . 
       FIG. 9  is a detailed sectional view of the gas detection chamber shown in  FIG. 8 . 
       FIG. 10  is a diagram showing a circuit of the gas sensor shown in  FIG. 6 . 
       FIG. 11  is a sectional view of a gas sensor according to a variation of an embodiment of the present invention. 
       FIG. 12  is a schematic sectional view along line A-A of  FIG. 11 . 
       FIG. 13  is a detailed sectional view of the gas detection chamber shown in  FIG. 12 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A gas sensor of a first embodiment of the present invention is described hereinbelow, with reference to the drawings. 
   A gas sensor  1  of this embodiment is, for example, a hydrogen sensor which measures hydrogen. The gas sensor  1  is provided in a fuel cell system  10  including a control device  2 , a memory device  3 , an alarm device  4 , a fuel cell  5  which drives a vehicle, and pipes  6 ,  7 ,  8  and  9  (inlet pipes  6  and  7 , outlet pipes  8  and  9 ) which are connected to the fuel cell  5  and supply reaction gas thereto. The gas sensor  1  is provided above (in terms of gravity) the outlet pipe  9  extending from the oxygen electrode, to monitor whether or not hydrogen is exhausted from the outlet pipe  9 . 
   The control device  2  is connected to the gas sensor  1  provided on the outlet pipe  9  and determines whether or not there is an abnormality in accordance with, for example, a comparison result of a detection signal output from the gas sensor  1  with a predetermined threshold value stored in the memory device  3 . When the control device  2  determines that there is an abnormality, it makes the alarm device  4  outputs an alarm. The memory device  3  stores a map of predetermined threshold values of the detection value of the gas sensor  1  in accordance with operational conditions of the fuel cell  5  such as a pressure difference between electrodes and operation pressure or the like. 
   The fuel cell  5  is provided in a vehicle such as an electric vehicle as a drive source. The fuel cell  5  is constituted by a plurality of cells (not illustrated), each cell consisting of an electrolyte electrode structure, in which a solid polymer electrolyte membrane is held between a fuel electrode and an oxygen electrode, and sandwiched by separators. 
   When fuel gas such as hydrogen is supplied from the inlet pipe  6  to the fuel electrode, the hydrogen is ionized at a catalyst electrode of the fuel electrode. The hydrogen ions move to the oxygen electrode through the solid polymer electrolyte membrane which is properly humidified. Generated electrons are taken out to an outer circuit and used as direct current energy. Since oxidizing gas such as oxygen or air is supplied to the oxygen electrode via the inlet pipe  7 , water is generated at the oxygen electrode as a result of a reaction of hydrogen ions, electrons, and oxygen. Offgas remaining after the reaction is exhausted from the outlet pipes  8  and  9  from the fuel electrode and the oxygen electrode respectively. 
   Since the gas sensor  1  is provided on the upper side of the outlet pipe  9  from the oxygen electrode, generated water is exhausted on the bottom even when the offgas of the oxygen electrode is cooled and condensation occurs. 
   The gas sensor  1  has a rectangular case  21  extending along the outlet pipe  9 , that is, in the horizontal direction, for example, shown in  FIGS. 2 to 4 . The case  21  is made of, for example, polyphenylene sulfide and has flange portions  22  at both ends in the longitudinal direction. Collars  23  are attached to the flange portions  22 . The flange portions  22  are adapted to be clinched and fixed on a fixing base  25  provided on the outlet pipe  9  of the oxygen electrode by inserting bolts  24  into the collars  23  as shown in  FIG. 3 . 
   A tube portion  26  is provided at an end in the thickness direction of the case  21  as shown in  FIGS. 3 and 4 . The tube portion  26  is constituted for example of a tube-shaped tube end portion  26   a  having a bottom and a tube-shaped tube tip portion  26   b , being connected along the same axis. The inner space of the tube portion  26  is constituted as a gas detection chamber  27 . A flange portion  28  is provided on the inner surface of the tube portion  26  toward the inside thereof and a space inside the flange portion  28  is constituted to open as a gas introduction portion  29 . 
   A sealing member  35  is attached to the outer surface of the tube portion  26  and assures airtightness by adhering to the inner wall that defines a hole  9   a  of the outlet pipe  9 . 
   A circuit board  30  sealed in a resin is provided in the case  21  and a gas detection element  31  and a temperature compensation element  32  provided in the gas detection chamber  27  are connected to the circuit board  30  by a plurality of (for example, four) stays  33  for power distribution and a lead  33   a  which are connected to the circuit board  30 . 
   In the gas detection chamber  27 , a base portion  34  of the tube end portion  26   a,  a first heater  36   a,  a second heater  36   b,  a sensor  37  which measures a temperature and a humidity of the gas detection chamber  27 , a first dehumidifier  38   a,  a second dehumidifier  38   b,  a structure member  39 , a sintered filter  40 , and a water-repellent filter are disposed with the gas detection element  31  and the temperature compensation element  32 . 
   The base portion  34 , the first dehumidifier  38   a,  and the structure member  39  are disposed from the bottom surface  27 A of the gas detection chamber  27  to the tip portion of the gas sensor  1  along the thickness direction thereof in series. The four stays  33  for power distribution penetrate the disk-shaped base portion  34 , the disc-shaped first dehumidifier  38   a,  and the disc-shaped structure member  39 , and are fixed by the base portion  34 . 
   Appropriate covers for preventing corrosion are provided on the surfaces of the stays  33 . The structure member  39  is made of polyphenylene sulfide or phenol resin and has a recess portion  39   a  provided at the tip portion of the structure member  39  toward the base portion thereof. An annular stay cover  33   b  is disposed to entirely cover an exposed portion of the four stays  33  which are exposed in the recess portion  39   a.    
   A set of the gas detection element  31  and the temperature compensation element  32  is disposed by the lead  33   a  connected to the stays  33  at a predetermined distance, in a space which is delimited by the recess portion  39   a  and the stay cover  33   b  and communicates with the gas detection chamber  27 , and a predetermined distance apart from the base portion  34  in the thickness direction of the gas sensor  1 . 
   As shown in  FIG. 4 , the first heater  36   a  made of, for example, an annular ceramic heater is disposed on the bottom surface  27 A of the gas detection chamber  27  and covers at least a portion inside the gas detection chamber  27 . The inner wall of the first heater  36   a  is provided to face the outer portion of the base portion  34  which is disposed on the bottom surface  27 A of the gas detection chamber  27 . 
   The second heater  36   b  made of, for example, a tube-shaped ceramic heater is disposed on the inner wall  27 B of the gas detection chamber  27  and covers at least a portion inside the gas detection chamber  27 . The outer wall of the second heater  36   b  is provided to cover the inner wall  27 B of the gas detection chamber  27 . The inner wall of the second heater  36   b  is provided to face the outer wall of the structure member  39  and the stay cover  33   b.    
   One side of the first heater  36   a  abuts the bottom surface of the gas detection chamber  27 . A first dehumidifier  38   a  is sandwiched by the opposite side of the first heater  36   a  and one end of the second heater  36   b . An annular second dehumidifier  38   b  is provided to abut the opposite side of the second heater  36   b.    
   The first heater  36   a  and the second heater  36   b  are ceramic heaters, for example, ceramic heaters having conductive ceramics as an exothermic body or ceramic heaters having a resistance body made of a metal provided in a nonconductive ceramics, These heaters are adapted to heat the inside of the gas detection chamber  27 , the gas detection element  31 , the temperature compensation element  32 , the first dehumidifier  38   a , and the second dehumidifier  38   b  when powered by the circuit board  30  which is connected to the first heater  36   a  and the second heater  36   b  in a parallel manner. For example, in this embodiment, the first heater  36   a  heats the first dehumidifier  38   a , and the second heater  36   b  heats the first dehumidifier  38   a  and the second dehumidifier  38   b.    
   The second dehumidifier  38   b , an annular sintered filter  40 , and a water-repellent filter  41  which covers the gas introduction portion  29  are disposed between the opposite side of the second heater  36   b  and the gas introduction portion  29 , from a base portion to the tip portion of the gas sensor  1  along the thickness direction thereof in series. 
   The first dehumidifier  38   a  and the second dehumidifier  38   b  are made of materials which absorb water in accordance with ambient humidity such as silica gel, zeolite, activated charcoal, activated alumina, water-absorbing polymer, or oxidized zirconium. These dehumidifiers can release water which they absorb into the ambient gas as vapor when heated by the first heater  36   a  and the second heater  36   b  and are constituted to absorb water in a reversible manner. 
   The sintered filter  40  and the water-repellent filter  41  are adapted to be permeable to the observed gas and the gas detection chamber  27  is adapted for the offgas which flows through the outlet pipe  9  to enter the gas detection chamber  27  through at least the sintered filter  40  and the water-repellent filter  41 . 
   As shown in  FIG. 5 , the gas detection element  31  is a known element and is made of a coil  31   a  made of metals such as platinum. The gas detection element  31  is constituted of the coil  31   a , the surface thereof being covered with catalyst carrier such as alumina, which supports a catalyst  31   b  made of a noble metal active for the subject gas of hydrogen. 
   The temperature compensation element  32  is made inactive to the subject gas and is constituted of a coil  32   a , the coil  32   a  itself being identical to that of the gas detection element  31  and the surface thereof being covered with catalyst carrier such as alumina. 
   The gas sensor  1  can measure a concentration of hydrogen by measuring a difference in electric resistance between the gas detection element  31 , when it reaches high temperature due to the reaction heat generated when the subject gas of hydrogen contacts the catalyst  31   b  of the gas detection element, and the temperature compensation element  32  which is at a lower temperature than the gas detection element  31  because of not reacting with the subject gas, while canceling a change in electric resistance due to a change in ambient temperature. 
   As shown in  FIG. 5 , a bridge circuit is constituted by a branch in which the gas detection element  31  (resistance value: R 4 ) being connected to the temperature compensation element  32  (resistance value: R 3 ) in series and a branch in which a fixed resistor  51  (resistance value: R 1 ) being connected to a fixed resistor  52  (resistance value: R 2 ) in series being connected to a reference voltage generation circuit  44  which applies a predetermined voltage based on the voltage supplied from an outer power supply  43  in parallel. 
   In this bridge circuit, a detection circuit  45  is connected between a connecting point PS of the gas detection element  31  and the temperature compensation element  32 , and a connecting point PR of the fixed resistor  51  and the fixed resistor  52 . The detection circuit  45  measures a voltage between the connecting points PS and PR. An output circuit  46  is connected to the detection circuit  45 . 
   When the subject gas of hydrogen is not present in the observed gas introduced to the gas detection chamber  27 , the bridge circuit is in a balanced state where an expression R 1 ×R 4 =R 2 ×R 3  is true and the output of the detection circuit  45  becomes zero. On the other hand, when hydrogen is present it combusts at the catalyst  31   b  of the gas detection element  31 . As a result, the temperature of the coil  31   a  becomes higher and the resistance value R 4  increases. Since hydrogen does not combust at the temperature compensation element  32 , the resistance value R 3  does not change. Thereby, the balance of the bridge circuit is lost and a voltage which increases in accordance with the concentration of hydrogen is applied to the detection circuit  45 . The voltage value is output from the detection circuit  45  to the output circuit  46  and the output circuit  46  outputs the voltage value to the control device  2 . The concentration of hydrogen is computed in the control device  2  based on the map thereof which is predetermined in accordance with the voltage value. 
   A heater voltage generating circuit  47  which applies suitable voltage to the first heater  36   a  and the second heater  36   b  is connected to the outer power supply  43  to which the reference voltage generation circuit  44  is connected. The first heater  36   a  and the second heater  36   b  are connected to the heater voltage generating circuit  47  in parallel to each other. 
   The control device  2  is connected to the sensor  37 , the first heater  36   a , and the second heater  36   b  in the gas detection chamber  27 , and controls operational conditions of the gas detection element  31 , the temperature compensation element  32 , the first heater  36   a , and the second heater  36   b , for example, timing of start and stop of power distribution and amount thereof, in accordance with conditions of temperature and humidity of the ambient gas in the gas detection chamber  27  output from the sensor  37 , conditions on loading and operation of the fuel cell  5  or the like. At this time, the control device  2  controls the amount of power distribution to the first heater  36   a  and the second heater  36   b  by feedback control in accordance with the current values distributed to the first heater  36   a  and the second heater  36   b  and chopper control (that is, switching on and off of the power distribution) based on on/off operations of a switching element, or the like. 
   For example, the control device  2  controls the power distribution to the first heater  36   a  and the second heater  36   b  based on the temperature measured by the sensor  37  so that the temperature of the gas detection chamber  27  is within a predetermined range which is at least more than a dew-point temperature. The control device  2  also controls the timing of start and stop of power distribution and amounts thereof to the first heater  36   a  and the second heater  36   b  so that the relative humidity of the gas detection chamber  27  which is measured by the sensor  37  becomes identical to an index value listed in the predetermined relative humidity map in accordance with the temperature of the gas detection chamber  27 . 
   Furthermore, the control device  2  controls amounts of power distribution to the first heater  36   a  and the second heater  36   b  in accordance with various parameters such as operating conditions of the fuel cell  5  (including start and stop), conditions of the loading of the fuel cell  5  during operation, such as a command to generate electricity sent to the fuel cell  5  (FC output command value), a current value of the fuel cell  5  measured by an output current sensor (not illustrated), and power generating conditions of the fuel cell  5  calculated based on a flow volume of the air supplied from an air compressor (not illustrated) to the fuel cell  5  measured by a flow volume sensor (not illustrated). 
   When the loading of the fuel cell  5  becomes higher and there is a risk of the temperature inside the detection chamber  27  of the gas sensor  1  lowering due to an increase of the offgas which flows through the outlet pipe  9  of the oxygen electrode and cools the gas sensor  1 , or of the relative humidity inside the gas detection chamber  27  increasing due to an increase of the produced water which is generated in the fuel cell  5  and included in the offgas, the control device  2  prevents condensation in the gas detection chamber  27  by raising the temperature inside the gas detection chamber  27 . On the other hand, the control device  2  reduces energy consumption by lowering the amounts of power distribution to the first heater  36   a  and the second heater  36   b  when the loading of the fuel cell  5  becomes lower. 
   In addition, when a purge is conducted to drain water left in the fuel cell system due to an increase of the offgas flowing through the outlet pipes  8  and  9  when stopping operation of the fuel cell  5 , the control device  2  increases the volume of the saturated water vapor of the ambient gas in the gas detection chamber  27  by raising the temperature of the gas detection chamber  27  by increasing power distribution to the first heater  36   a  and the second heater  36   b . Thereby, condensation in the gas detection chamber is prevented. 
   The control device  2  starts power distribution to the gas detection element  31 , the temperature compensation element  32 , the first heater  36   a , and the second heater  36   b  before starting offgas flow in the outlet pipe  9  of the oxygen electrode when starting the fuel cell  5 , and stops power distribution to the gas detection element  31 , the temperature compensation element  32 , the first heater  36   a , and the second heater  36   b  after stopping offgas flow in the outlet pipe  9  of the oxygen electrode when stopping the fuel cell  5 . 
   Functions of the gas sensor  1  of the aforementioned embodiment are described. 
   The first dehumidifier  38   a  and the second dehumidifier  38   b  provided in the gas detection chamber  27  absorb and release water in accordance with the humidity of the ambient gas in the gas detection chamber  27 . For example, relative humidity in the gas detection chamber  27  decreases when power is distributed to the first heater  36   a  and the second heater  36   b  during operation of the gas sensor  1 , due to the rise of the temperature inside the gas detection chamber  27 . The first dehumidifier  38   a  and the second dehumidifier  38   b  release water, and thus, more water can be absorbed therein. When the power distribution to the first heater  36   a  and the second heater  36   b  is stopped after offgas flow in the outlet pipe  9  is stopped when the fuel cell  5  is stopped, the relative humidity in the gas detection chamber  27  increases due to the decline of the temperature inside the gas detection chamber  27 . At this time, the first dehumidifier  38   a  and the second dehumidifier  38   b  absorb water including water vapor in the ambient gas in the gas detection chamber  27  in accordance with their capacities. Thereby conditions in which condensation occurs such as the relative humidity in the gas detection chamber  27  becoming 100% and the temperature of the gas detection chamber becoming equal to or less than the dew-point temperature can be prevented from occurring. In addition, even if condensation occurs when a temperature of a portion of the gas detection chamber  27  becomes equal to or less than the dew-point temperature, the water generated by condensation can be absorbed by the first dehumidifier  38   a  and the second dehumidifier  38   b.    
   In particular, in the gas detection chamber  27 , the region between the base portion  34  and the gas detection element  31  and the temperature compensation element  32  is likely to be the situations of condensation and generated condensation is likely to remain. The first dehumidifier  38   a  provided opposite of the introduction portion  29  with respect to the gas detection element  31  and the temperature compensation element  32  is dried (that is, releases water) by the first heater  36   a  and the second heater  36   b  to absorb a desired amount of water surely. Thereby condensation at an upper region of the gas detection element  31  and the temperature compensation element  32  or the like can be prevented. 
   The second dehumidifier  38   b  provided nearer the gas introduction portion  29  in relation to the second heater  36   b  and between the gas detection element  31  and the temperature compensation element  32 , and the gas introduction portion  29  can decrease the relative humidity of the ambient gas, to which the gas detection element  31  and the temperature compensation element  32  are exposed, and prevent condensation on the surface of the gas detection element  31  and the temperature compensation element  32 . 
   Since the second heater  36   b  is provided to cover the inner wall  27 B of the gas detection chamber  27 , condensation on the inner wall  27 B near the tube portion where condensation is likely to occur due to being made of a metal of relatively high heat conductivity can be prevented. 
   As described above, in the gas sensor  1  of the present embodiment, condensation in the gas detection chamber  27  can be prevented by the first dehumidifier  38   a  and the second dehumidifier  38   b  which absorb water included in the ambient gas in the gas detection chamber  27 , decrease the relative humidity thereof, and prevent the temperatures of the ambient gas and the surfaces of the gas detection element  31  and the temperature compensation element  32  from declining to be equal to or less than the dew-point temperature, even when the temperatures of the ambient gas and the surfaces of the gas detection element  31  and the temperature compensation element  32  decline when the first heater  36   a  and the second heater  36   b  which maintain the temperature and humidity of the ambient gas and the temperatures of the surface of the gas detection element  31  and the temperature compensation element  32  are stopped. 
   In addition, even when condensation occurs due to a local decrease in temperature or the like because of a stoppage of the first heater  36   a  and the second heater  36   b , water generated by condensation can be absorbed by the first dehumidifier  38   a  and the second dehumidifier  38   b . Thereby a condition where condensation on the gas sensor  1  remains when restarting can be prevented and the relative humidity in the gas detection chamber  27  can be decreased in advance for the next operation of the gas sensor  1 . 
   The relative humidity of the gas detection chamber  27  can be decreased by the second heater  36   b  covering the inner wall  27 B of the gas detection chamber  27  even when water included in the offgas of high relative humidity or the like entered the gas detection chamber  27 . 
   Since the first dehumidifier  38   a  is provided in the region opposite to the introduction portion  29  with respect to the gas detection element  31  and the temperature compensation element  32  on which condensation is likely to form and remain, water generated by condensation can be absorbed. Thereby deterioration and degradation of the gas detection element  31  and the temperature compensation element  32  when they are powered with condensation remaining can be prevented. 
   Furthermore, since the outer power supply  43  supplies power to a detection portion including the gas detection element  31 , the temperature compensation element  32 , the fixed resistors  51  and  52 , the first heater  36   a , and the second heater  36   b , the structure of the circuit can be simplified. 
   Although the gas sensor  1  is a hydrogen sensor in the aforementioned embodiment, the gas sensor of the present invention is not limited thereto. The gas sensor may be one which measures other gases such as a combustible gas of carbon monoxide and methane or the like. 
   In addition, although the gas sensor  1  is a catalytic combustion sensor in the aforementioned embodiment, the gas sensor of the present invention is not limited thereto. The gas sensor may be of another type such as a semiconductor type. 
   Furthermore, although the bridge circuit to which the gas detection element  31  and the temperature compensation element  32  are connected is provided in the gas sensor  1  of the aforementioned embodiment, the circuit is not limited thereto. The circuit may be of another type such as a series circuit and values of voltage or current between predetermined connecting points may be output to the control device  2  as parameters related to the resistance value R 4  of the gas detection element  31 . 
   A gas sensor according to the second embodiment of the present invention is described hereinbelow with reference to the drawings. A gas sensor  101  of the present embodiment is, for example, a hydrogen sensor which measures hydrogen. The gas sensor  101  is provided in a fuel cell system  110  including a control device  2 , a memory device  3 , an alarm device  4 , a fuel cell  5  which drives a vehicle, and pipes  6 ,  7 ,  8  and  9  (inlet pipes  6  and  7 , outlet pipes  8  and  9 ) which are connected to the fuel cell  5  and supply reaction gas thereto. The gas sensor  101  is provided above (in terms of gravity) the outlet pipe  9  extending from the oxygen electrode, to monitor whether or not hydrogen is exhausted from the outlet pipe  9 . Since the control device  2 , the memory device  3 , the alarm device  4 , the fuel cell  5 , and the pipes  6 ,  7 ,  8 , and  9  are identical to those of the first embodiment, redundant descriptions are omitted. 
   The gas sensor  101  has a rectangular case  121  extending along the outlet pipe  9 , that is, in the horizontal direction, for example, shown in  FIGS. 7 to 9 . The case  121  is made of, for example, polyphenylene sulfide and has flange portions  122  at both ends in the longitudinal direction. Collars  123  are attached to the flange portions  122 . The flange portions  122  are adapted to be clinched and fixed on a fixing base  125  provided on the outlet pipe  9  of the oxygen electrode by inserting bolts  124  in the collars  123  as shown in  FIG. 8 . 
   A tube portion  126  made of ceramic is provided at an end in the thickness direction of the case  121  as shown in  FIGS. 8 and 9 . The tube portion  126  is constituted, for example, of a tube-shaped tube main portion  126   a  and a tube-shaped tube tip portion  126   b , being connected along the same axis in the thickness direction of the gas sensor  101 . 
   The inner space of the tube portion  126  is constituted as a gas detection chamber  127 . A base flange portion  128   a  made of ceramic is provided on the inner surface of the tube main portion  126   a  in an integrated manner toward the inside thereof and the inner wall of the base flange portion  128   a  is constituted to open as a stay providing portion  129 . 
   A tip flange portion  128   b  made of ceramic is provided on the inner surface of the tube tip portion  126   b  in an integrated manner toward the inside thereof and an inner wall of the tip flange portion  128   b  is constituted to open as a gas introduction portion  127   a.    
   Among the components of the tube portion  126  made of ceramics, the tube base portion  126   a  and the base flange portion  128   a  formed as a single member is a ceramic heater powered from a circuit board  130  in the case  121  via a wiring  130   a . This ceramic heater is, for example, one having conductive ceramic as an exothermic body or one having a resistance body made of a metal provided in a nonconductive ceramic. 
   In other words, the gas detection chamber  127  is defined by the tube main portion  126   a  which is a ceramic heater and the inner wall of the gas detection chamber  127  is formed by the inner surface  126 A of the tube main portion  126   a.    
   The circuit board  130  is sealed in a resin and is provided in the case  121 , and a gas detection element  131  and a temperature compensation element  132  provided in the gas detection chamber  127  are connected to the circuit board  130  by a plurality of (for example, four) stays  133  for power distribution and a lead  133   a  which are connected to the circuit board  130 . 
   The four stays  133  for power distribution penetrate a waterproof plug  134  which seals the stay providing portion  129  of the tube main portion  126   a  and an annular base portion  135  which is provided in the gas detection chamber  127 , and are fixed, for example, by the base portion  135 . 
   The waterproof plug  134  is, for example, made of rubber and adheres to the inner surface of the stay providing portion  129  and the base flange portion  128   a . The water proof plug  134  also adheres to the surfaces of the stays  133  which penetrate the waterproof plug  134  and assures airtightness and waterproof of the circuit board  130 . 
   The annular base portion  135  is provided at the base of the gas detection chamber  127  and the outer surface of the base portion  135  contacts the inner surface  126 A of the tube main portion  126   a  which constitutes the inner wall of the gas detection chamber  127 . 
   Appropriate covers for preventing corrosion are provided on the surfaces of the stays  133 . An annular stay cover  136  for preventing degradation due to such as corrosion is disposed to entirely cover an exposed portion of the four stays  133  which penetrate the waterproof plug  134  and the base portion  135  and are exposed in the gas detection chamber  127 , and connection portions of the lead  133   a  which are connected to a set of the gas detection element  131  and the temperature compensation element  132  and the stays  133 . 
   A set of the gas detection element  131  and the temperature compensation element  132  is disposed by the lead  133   a  connected to the stays  133  at a predetermined distance, in a space which is delimited by a hole  135   a  of the annular base portion  135  and a hole  136   a  of the stay cover  136  and communicates with the gas detection chamber  127 , and a predetermined distance apart from the base portion  135  in the thickness direction of the gas sensor  101 . 
   A dehumidifier  137  which is, for example, tube-shaped and absorbs in a reversible manner is provided and the outer surface  137 A of the dehumidifier  137  contacts the inner surface  126 A of the tube main portion  126   a  which constitutes the inner wall of the gas detection chamber  127 . The dehumidifier  137  is provided to be sandwiched by the stay cover  136  and the tip flange portion  128   b , the base end  137 B of the dehumidifier  137  contacts the stay cover  136 , and the tip end  137 C of the dehumidifier  137  contacts the tip flange  128   b.    
   The dehumidifier  137  is made of materials which absorb water in accordance with ambient humidity such as silica gel, zeolite, activated charcoal, activated alumina, water-absorbing polymer, and oxidized zirconium. The dehumidifier  137  can release water which it absorbed in the ambient gas as vapor when heated by the tube main portion  126   a , which is a ceramic heater and is constituted to absorb water in a reversible manner. 
   A tube-shaped outer case  138  which houses the tube portion  126  inside is provided along the thickness direction of the gas sensor  101  at a position a predetermined distance apart from an outer surface  126 C of the tube portion  126 , for example as shown in  FIG. 8 . A flange portion  138   a  is provided on the inner surface of the outer case  138  in an integrated manner toward the inside thereof and the inner portion of the flange portion  138   a  is constituted to open as a gas introduction port  138   b.    
   A disc-shaped sintered filter  139  and a disc-shaped water-repellent filter  140  are disposed inside the tip of the outer case  138  from a base portion to the tip portion of the gas sensor  101  along the thickness direction thereof in series, and the water-repellent filter  140  covers the gas introduction portion  138   b.    
   The sintered filter  139  and the water-repellent filter  140  are adapted to be permeable to the observed gas and the gas detection chamber  127  is adapted for the offgas which flows through the outlet pipe  9  to enter the outer case  138  and the gas detection chamber  127  through at least the sintered filter  139  and the water repellent filter  140 . 
   Condensation around the gas detection element  131  and the temperature compensation element  132  can be prevented by a gap between the sintered filter  139  and the base flange portion  128   a.    
   A sealing member  138   c  is attached on the outer surface of the outer case  138  and assures airtightness by adhering to the inner wall of a hall  9   a  of the outlet pipe  9 . 
   As shown in  FIG. 10 , the gas detection element  131  is a known element and is made of a coil  131   a  made of metals such as platinum. The gas detection element  131  is constituted of the coil  131   a , the surface thereof being covered with catalyst carrier such as alumina, which supports a catalyst  131   b  made of a noble metal active for the subject gas of hydrogen. 
   The temperature compensation element  132  is made inactive to the subject gas and is constituted of a coil  132   a , the coil  132   a  itself being identical to that of the gas detection element  131  and the surface thereof being covered with catalyst carrier such as alumina. 
   The gas sensor  101  can measure a concentration of hydrogen by measuring a difference in electric resistance between the gas detection element  131 , when it reaches high temperature due to the reaction heat generated when the subject gas of hydrogen contacts the catalyst  131   b  of the gas detection element, and the temperature compensation element  132 , which is at a lower temperature than the gas detection element because of not reacting with the subject gas, while canceling a change in electric resistance due to a change in ambient temperature. 
   As shown in  FIG. 10 , a bridge circuit is constituted by a branch in which the gas detection element  131  (resistance value: R 14 ) being connected to the temperature  compensation element  132  (resistance value: R 13 ) in series and a branch in which a fixed resistor  151  (resistance value: R 11 ) being connected to a fixed resistor  152  (resistance value: R 12 ) in series being connected to a reference voltage generation circuit  144  which applies a predetermined voltage based on the voltage supplied from an outer power supply  143  in parallel. 
   In this bridge circuit, a detection circuit  145  is connected between a connecting point PS 1  of the gas detection element  131  and the temperature compensation element  132 , and a connecting point PR 1  of the fixed resistor  151  and the fixed resistor  152 . The detection circuit  145  measures a voltage between the connecting points PS 1  and PR 1 . An output circuit  146  is connected to the detection circuit  145 . 
   When the subject gas of hydrogen is not present in the observed gas introduced to the gas detection chamber  127 , the bridge circuit is in a balanced state where an expression R 11 ×R 14 =R 12 ×R 13  is true and the output of the detection circuit  145  becomes zero. On the other hand, when hydrogen is present it combusts at the catalyst  131   b  of the gas detection element  131 . As a result, the temperature of the coil  131   a  becomes higher and the resistance value R 14  increases. Since hydrogen does not combust at the temperature compensation element  132 , the resistance value R 13  does not change. Thereby, the balance of the bridge circuit is lost and a voltage which increases in accordance with the concentration of hydrogen is applied to the detection circuit  145 . The voltage value is output from the detection circuit  145  to the output circuit  146  and the output circuit  146  outputs the voltage value to the control device  2 . The concentration of hydrogen is computed in the control device  2  based on the map thereof which is predetermined in accordance with the voltage value. 
   A heater voltage generating circuit  147  which applies suitable voltage to the tube main portion  126   a  and the base flange portion  128   a  formed by the ceramic heater is connected to the outer power supply  143  to which the reference voltage generation circuit  144  is connected. 
   The control device  2  is connected to the sensor (not illustrated) and the tube main portion  126   a  and the base flange portion  128   a , which are formed by the ceramic heater, in the gas detection chamber  127  and controls powering conditions of the gas detection element  131  and the temperature compensation element  132 , and the tube main portion  126   a  and the base flange portion  128   a  for example, timing of start and stop of power distribution and amount thereof, in accordance with conditions of temperature and humidity of the ambient gas in the gas detection chamber  127  output from the sensor, conditions of loading and operation of the fuel cell  5  or the like. At this time, the control device  2  controls the amount of power distribution to the tube main portion  126   a  and the base flange portion  128   a  by feedback control in accordance with the current values distributed to the tube main portion  126   a  and the base flange portion  128   a  and chopper control (that is, switching on and off of the power distribution) based on on/off operations of a switching element, or the like. 
   For example, the control device  2  controls the power distribution to the tube main portion  126   a  and the base flange portion  128   a  based on the temperature measured by the sensor so that the temperature of the gas detection chamber  127  is within a predetermined range which is at least more than the dew-point temperature. The control device  2  also controls the timing of start and stop of power distribution and amounts thereof to the tube main portion  126   a  and the base flange portion  128   a  so that the relative humidity of the gas detection chamber  127  which is measured by the sensor becomes identical to an index value listed in the predetermined relative humidity map in accordance with the temperature of the gas detection chamber  127 . 
   Furthermore, the control device  2  controls amounts of power distribution to the tube main portion  126   a  and the base flange portion  128   a  in accordance with various parameters such as operating conditions of the fuel cell  5  (including start and stop), conditions of the loading of the fuel cell  5  during operation such as a command to generate electricity sent to the fuel cell  5  (FC output command value), a current value of the fuel cell  5  measured by an output current sensor (not illustrated), and power generating conditions of the fuel cell  5  calculated based on a flow volume of the air supplied from an air compressor (not illustrated) to the fuel cell  5  measured by a flow volume sensor (not illustrated). 
   When loading of the fuel cell  5  becomes higher and there is a risk of the temperature inside the gas detection chamber  127  of the gas sensor  101  lowering due to an increase of the offgas which flows through the outlet pipe  9  of the oxygen electrode and cools the gas sensor  101 , or of the relative humidity inside the gas detection chamber  127  increasing due to an increase of the produced water which is generated in the fuel cell  5  and included in the offgas, the control device  2  prevents condensation in the gas detection chamber  127  by raising the temperature inside the gas detection chamber  127 . On the other hand, the control device  2  reduces energy consumption by lowering amounts of power distribution to the tube main portion  126   a  and the base flange portion  128   a  when loading of the fuel cell  5  becomes lower. 
   In addition, when a purge is conducted to drain water left in the fuel cell system due to an increase of the offgas flowing through the outlet pipes  8  and  9  when stopping operation of the fuel cell  5 , the control device  2  increases the volume of the saturated water vapor of the ambient gas in the gas detection chamber  127  by raising the temperature of the gas detection chamber  127  by increasing power distribution to the tube main portion  126   a  and the base flange portion  128   a . Thereby, condensation in the gas detection chamber  127  is prevented. 
   The control device  2  starts power distribution to the gas detection element  131 , the temperature compensation element  132 , the tube main portion  126   a , and the base flange portion  128   a  before starting offgas flow in the outlet pipe  9  of the oxygen electrode when starting the fuel cell  5 , and stops power distribution to the gas detection element  131 , the temperature compensation element  132 , the tube main portion  126   a , and the base flange portion  128   a  after stopping offgas flow in the outlet pipe  9  of the oxygen electrode when stopping the fuel cell  5 . 
   Functions of the gas sensor  101  of the aforementioned embodiment are described. The dehumidifier  137  provided in the gas detection chamber  127  absorbs and releases water in accordance with the humidity of the ambient gas in the gas detection chamber  127 . For example, relative humidity in the gas detection chamber  127  decreases when power is distributed to the tube main portion  126   a  and the base flange portion  128   a  during operation of the gas sensor  101 , due to the rise of the temperature inside the gas detection chamber  127 . Thereby, the dehumidifier  137  releases water, and thus, more water can be absorbed therein. When the power distribution to the tube main portion  126   a  and the base flange portion  128   a  is stopped after offgas flow in the outlet pipe  9  is stopped when the fuel cell  5  is stopped, the relative humidity in the gas detection chamber  127  increases due to the decline of the temperature inside the gas detection chamber  127 . At this time, the dehumidifier  137  absorbs water including water vapor in the ambient gas in the gas detection chamber  127  in accordance with its capacity. 
   Thereby, conditions in which condensation occurs such as the relative humidity in the gas detection chamber  127  becoming 100% and the temperature of the gas detection chamber  127  becoming equal to or less than the dew-point temperature can be prevented from occurring. In addition, even if condensation occurs when a temperature of a portion of the gas detection chamber  127  becomes equal to or less than the dew-point temperature, the water generated by condensation can be absorbed by the dehumidifier  137 . 
   The dehumidifier  137  provided nearer the gas introduction portion  127   a  than the tube main portion  126   a  formed by the ceramic heater, between the gas detection element  131  and the temperature compensation element  132 , and the gas introduction portion  127   a  can decrease the relative humidity of the ambient gas, to which the gas detection element  131  and the temperature compensation element  132  are exposed, and prevent condensation on the surfaces of the gas detection element  131  and the temperature compensation element  132 . 
   Furthermore, since the inner wall of the gas detection chamber  127  is delimited by the tube main portion  126   a  formed by the ceramic heater, condensation on the inner wall of the gas detection chamber  127  can be prevented compared with the case where the tube portion  126  is formed by a metal or the like which has relatively higher heat conductivity. 
   As described above, the gas sensor  101  of the present embodiment can set a desired temperature and humidity in the gas detection chamber  127 , maintaining uniformity of temperature and humidity distribution of the ambient gas in the gas detection chamber  127  and temperature distribution on the surfaces of the gas detection element  131  and the temperature compensation element  132 , and prevent condensation caused by nonuniformity of temperature and humidity in the gas detection chamber  127 . 
   In addition, since there is no need to provide an additional heater in the gas detection chamber  127 , and the sintered filter  139  and the water-repellent filter  140  are provided in the outer case  138 , the inner space of the gas detection chamber  127  can be made larger while preventing enlargement of the gas sensor  101  itself. This leads to preventing rapid change of the temperature and humidity in the inner space, and rapid decline of the temperatures of the ambient gas, the gas detection element  131 , and the temperature compensation element  132  to below the dew-point temperature. Detection accuracy is also improved because the amount of the observed gas introduced in the gas detection chamber  127  increases. 
   Furthermore, since the dehumidifier  137  which absorbs water including water vapor in the ambient gas in the gas detection chamber  127  contacts the tube main portion  126   a  of the ceramic heater, the temperature of the dehumidifier  137  can be directly controlled by the ceramic heater. Therefore, for example, when offgas of high relative humidity brings water into the gas detection chamber  127 , water adhesion on the surfaces of the gas detection element  131  and the temperature compensation element  132  and condensation in the gas detection chamber  127  can be prevented by rapidly heating and drying the dehumidifier  137  and controlling the absorbing capacity thereof. 
   In the above embodiment, a heat insulating cover  151  made of heat insulating material may cover the outside of the tube portion  126  like a variation shown in  FIGS. 11 to 13 . In the heat insulating cover  151 , apertures  151   a  and  151   b  are provided to communicate with the stay providing portion  129  of the tube portion  126  and the gas introduction portion  127   a.    
   According to the variation, rapid change of the temperature in the gas detection chamber  127  and rapid decline of the temperatures of the ambient gas, the gas detection element  131 , and the temperature compensation element  132  to below the dew-point temperature can be prevented. 
   Although the gas sensor  101  is a hydrogen sensor in the aforementioned embodiment, the gas sensor of the present invention is not limited thereto. The gas sensor may be one which measures other gases such as combustible gas of carbon monoxide and methane or the like. 
   In addition, although the gas sensor  101  is a catalytic combustion sensor in the aforementioned embodiment, the gas sensor of the present invention is not limited thereto. The gas sensor may be of another type such as a semiconductor type. 
   Furthermore, although the bridge circuit to which the gas detection element  131  and the temperature compensation element  132  are connected is provided in the gas sensor  101  of the aforementioned embodiment, the circuit is not limited thereto. The circuit may be of another type such as a series circuit and values of voltage or current between predetermined connecting points may be output to the control device  2  as parameters related to the resistance value R 14  of the gas detection element  131 . 
   While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.