Patent Publication Number: US-2007115308-A1

Title: Liquid quantity sensing device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2005-338799, filed Nov. 24, 2005, the entire contents of which are incorporated herein by reference.  
     BACKGROUND  
      1. Field  
      One embodiment of the invention relates to a liquid quantity sensing device which uses multiple electrodes to sense the quantity of liquid stored in a container, and more particularly to an arrangement of the electrodes.  
      2. Description of the Related Art  
      An apparatus such as a fuel cell unit or an inkjet printer includes a container, which stores liquid therein. Sometimes, a liquid quantity sensor which senses the quantity of liquid stored in the container, is disposed in such a container.  
      For example, Japanese Patent Application Publication (KOKAI) No. 2003-291367 and U.S. Pat. No. 7,059,696 disclose a liquid remaining quantity displaying device which senses the remaining quantity of an ink stored in a container. The liquid remaining quantity displaying device has electrode sections, voltage applying means, and liquid sensing means. The electrode sections are placed respectively at multiple positions in the container which stores liquid, and, when in contact with the liquid, are set to a conductible state. The voltage applying means applies a voltage to the electrode sections. The liquid sensing means senses the presence or absence of the liquid at the positions of the electrode sections, based on the conduction states of the electrode sections when the voltage is applied. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      A general architecture that implements the various feature of the invention will now be described with. reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention.  
       FIG. 1  is an exemplary perspective view of a fuel cell unit according to a first embodiment of the invention;  
       FIG. 2  is an exemplary perspective view showing a state where a portable computer is mounted on the fuel cell unit shown in  FIG. 1 ;  
       FIG. 3  is an exemplary perspective view of a DMFC unit according to the first embodiment of the invention;  
       FIG. 4  is an exemplary section view diagrammatically showing the interior of the fuel cell unit shown in  FIG. 1 ;  
       FIG. 5  is an exemplary perspective view of a mixing section shown in  FIG. 3 ;  
       FIG. 6  is an exemplary section view diagrammatically showing the mixing section shown in  FIG. 3 ;  
       FIG. 7  is an exemplary view diagrammatically showing the operation principle of a liquid quantity sensor shown in  FIG. 6 ;  
       FIG. 8  is an exemplary section view showing a state where a liquid drop adheres to an upper wall of a mixing tank shown in  FIG. 6 ;  
       FIG. 9  is an exemplary section view showing a state where a liquid drop is detached from the upper wall of the mixing tank shown in  FIG. 6 ;  
       FIG. 10  is an exemplary section view showing a state where the mixing tank shown in  FIG. 6  is filled;  
       FIG. 11  is an exemplary section view diagrammatically showing a mixing section according to a second embodiment of the invention;  
       FIG. 12  is an exemplary section view diagrammatically showing a mixing section according to another embodiment of the invention;  
       FIG. 13  is an exemplary section view diagrammatically showing a mixing section according to a third embodiment of the invention;  
       FIG. 14  is an exemplary section view diagrammatically showing a mixing section according to a fourth embodiment of the invention; and  
       FIG. 15  is an exemplary section view diagrammatically showing a mixing section according to a further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment of the invention, there is provided a liquid quantity sensing device including: a container that stores electrically conductive liquid, the container including an upper wall; a sensor body that is attached to the upper wall of the container and that extends toward an interior of the container; a first electrode that is disposed on the sensor body; a plurality of second electrodes that are disposed on the sensor body and are separated from each other in a movement direction of a liquid level that changes according to a quantity of the liquid; and a sensing mechanism that senses conduction states between the respective second electrodes and the first electrode. At least one of the first electrode and an uppermost electrode of the second electrodes is separated from the upper wall by a distance larger than a maximum thickness of a liquid drop that adheres to the upper wall.  
      FIGS.  1  to  10  show a fuel cell unit  1  of a first embodiment of the invention.  FIG. 1  discloses an exemplary embodiment of a liquid quantity sensing device, namely the fuel cell unit  1 . For example, the fuel cell unit  1  is a direct methanol fuel cell (DMFC) device in which a methanol aqueous solution is used as a fuel. As shown in  FIG. 2 , the fuel cell unit  1  has a size which allows the unit to be used as a power source of, for example, a portable computer  2 .  
      As shown in  FIG. 1 , the fuel cell unit  1  has a device body  3  and a stand portion  4 . The device body  3  is formed into a slender shape which extends in the width direction of the portable computer  2 . The stand portion  4  horizontally projects from the front end of the device body  3  so that a rear end portion of the portable computer  2  can be placed on the stand portion. A power source connector  5  is placed on the upper face of the stand portion  4 . When the portable computer  2  is placed on the stand portion  4 , the power source connector  5  is electrically connected to the portable computer  2 .  
      As shown in  FIG. 1 , the device body  3  includes a housing  7 . The housing  7  houses a DMFC unit  8  shown in  FIG. 3 , therein. The DMFC unit  8  includes a holder  10 , a fuel cartridge  11 , a mixing section  12 , an air intake section  13 , a DMFC stack  14 , a cooling section  15 , and a controlling section  16 .  
      First, the whole DMFC unit  8  will be described with reference to  FIGS. 3 and 4 .  
      As shown in  FIG. 3 , the fuel cartridge  11  is detachably attached to the holder  10 . High-concentration methanol which is to be used in electricity generation is charged in the fuel cartridge  11 . As shown in  FIG. 4 , the fuel in the fuel cartridge  11  is fed to the mixing section  12  through a fuel supply pipe  21  opened in the holder  10  and a fuel pump  22 .  
      The mixing section  12  dilutes the high-concentration methanol supplied from the fuel cartridge  11  to produce a methanol aqueous solution having a concentration of, for example, several % to several tens % methanol. The methanol aqueous solution produced in the mixing section  12  is fed to the DMFC stack  14  through a liquid supply pipe  23  and a liquid supply pump  24 .  
      As shown in  FIGS. 3 and 4 , the air intake section  13  has an air intake hole  13   a  which provides air to DMFC stack  14 . The air intake section  13  takes external air into the DMFC unit  8  through the air intake hole  13   a . This air is fed to the DMFC stack  14  through an air supply pipe  25  and an air supply pump  26 .  
      The DMFC stack  14  is one example of an electromotive part. The DMFC stack  14  has an anode  14   a , a cathode  14   b , and an electrolyte film  14   c . The DMFC stack  14  causes the methanol aqueous solution to chemically react with oxygen in the air, thus generating electricity. As a result of the electricity generating operation, carbon dioxide and water vapor are produced. The produced carbon dioxide and water vapor and unreacted methanol are fed to the cooling section  15 .  
      The cooling section  15  has a first cooling mechanism  15   a  and a second cooling mechanism  15   b . The first cooling mechanism  15   a  cools the carbon dioxide and unreacted methanol aqueous solution which have passed through the anode  14   a . The second cooling mechanism  15   b  cools the water vapor and air which have passed through the cathode  14   b.    
      Part of the water, which has been cooled to return to the liquid state, and the methanol aqueous solution are recirculated to the mixing section  12  so that they can be used in production of methanol aqueous solution. The produced carbon dioxide is fed, together with the methanol aqueous solution, to the mixing section  12 . Then the carbon dioxide is separated from the methanol aqueous solution in the mixing section  12  so that the carbon dioxide can be discharged to the outside of the DMFC unit  8 .  
      As shown in  FIG. 4 , the controlling section  16  is housed in the stand portion  4 . The controlling section  16  monitors the states of the mixing section  12 , the air intake section  13 , the DMFC stack  14 , and the cooling section  15  and controls the operations of these units  12 ,  13 ,  14 , and  15 . The controlling section  16  supplies the electricity generated in the DMFC stack  14  to the power source connector  5 .  
      Next, the mixing section  12  will be described in detail with reference to FIGS.  5  to  7 .  
      As shown in  FIG. 5 , the mixing section  12  includes a mixing tank  31  and a liquid quantity sensor  32 . The mixing tank  31  is one example of the container. The mixing tank  31  has a tank body  34 , and a cover  35  which covers the upper face of the tank body  34 . The tank body  34  and the cover  35  cooperate with each other to form a box-like shape having an upper wall  31   a , a bottom wall  31   b , and a side wall  31   c.    
      As shown in  FIG. 4 , the high-concentration methanol is supplied to the mixing tank  31  through the fuel supply pipe  21 . Furthermore, water which has been recovered in the cooling section  15  is supplied to the mixing tank  31 . The mixing tank  31  uses both the high-concentration methanol and the water to produce a methanol aqueous solution having a desired concentration and stores the produced methanol aqueous solution. The methanol aqueous solution is one example of the electrically conductive liquid.  
      As diagrammatically shown in  FIG. 6 , the liquid quantity sensor  32  includes a sensor body  41 , a reference electrode E 0 , first to fourth sensing electrodes E 1 , E 2 , E 3 , E 4 , and a sensing mechanism  42 .  
      The sensor body  41  is attached to a middle portion of the upper wall  31   a  of the mixing tank  31 . The sensor body  41  is formed into a plate-like shape and extends from the upper wall  31   a  toward the interior of the mixing tank  31 . As shown in  FIG. 6 , the lower end  41   a  of the sensor body  41  is separated from the bottom wall  31   b  of the mixing tank  31 .  
      As shown in  FIG. 8 , when the methanol aqueous solution is stored in the mixing tank  31 , a phenomenon sometimes occurs in which a liquid drop D of the methanol aqueous solution adheres to the inner face of the upper wall  31   a . When the inner face of the upper wall  31   a  has convex portions, the adhering of the liquid drop D easily occurs in the concave and convex portions. In the mixing tank  31 , therefore, such adhering occurs in an attachment portion between the upper wall  31   a  and the sensor body  41  as shown in  FIG. 8 . Furthermore, the size of the liquid drop D depends on the kind of liquid, particularly the viscosity of the liquid. When the kind of the liquid is identified, the maximum value of the adhering liquid drop D is specified.  
      As indicted by the one-dot chain line in  FIG. 6 , therefore, a region of the sensor body  41  which, when the liquid drop D adheres to the upper wall  31   a , is presumed to be in contact with the liquid drop D is specified as a wetting region  43 . In the case where the liquid drop D is fresh water, for example, the maximum thickness of the liquid drop D is about 3 millimeters (mm). In the specification, “maximum thickness of liquid drop” means the width of the maximum liquid drop D which may adhere to the upper wall  31   a , extending from the upper wall  31   a  to the lower end of the liquid drop D.  
      Furthermore, research by the inventors has shown that the maximum thickness of the liquid drop D of a methanol aqueous solution having a concentration of several percentage (%) to several tens % methanol is smaller than that of the liquid drop D of fresh water. Namely, the maximum thickness of the liquid drop D of a methanol aqueous solution is smaller than 3 mm. In the embodiment, therefore, the distance between the upper wall  31   a  to the lower end  43   a  of the wetting region  43  is smaller than 3 mm.  
      As shown in  FIG. 6 , the reference electrode E 0  is disposed in a left end portion of the sensor body  41  and extends in the same direction as the sensor body  41 . The reference electrode E 0  is one example of the first electrode. In the embodiment, only one reference electrode E 0  is disposed. Alternatively, plural reference electrodes E 0  may be separately disposed so as to correspond to the multiple sensing electrodes E 1 , E 2 , E 3 , E 4 , respectively.  
      As shown in  FIG. 6 , in order to prevent the reference electrode E 0  from being in contact with the liquid drop D, the reference electrode is separated from the upper wall  31   a  by a distance w which is larger than the maximum thickness of the liquid drop D. In other words, the reference electrode E 0  is disposed in a portion outside the wetting region  43 .  
      Moreover, the upper end of the reference electrode E 0  is positioned in the vicinity of the wetting region  43 . Namely, the upper end of the reference electrode E 0  is disposed in an upper end portion of a region which is not in contact with the liquid drop D. For example, the upper end of the reference electrode E 0  is formed at a position which is separated from the upper wall  31   a  by 3 mm in the vertical direction.  
      The reference electrode E 0  is exposed to the interior of the mixing tank  31 . When the methanol aqueous solution is stored in the mixing tank  31 , the reference electrode E 0  is in contact with the methanol aqueous solution. The reference electrode E 0  is electrically connected to the sensing mechanism  42 .  
      As shown in  FIG. 6 , the first to fourth sensing electrodes E 1 , E 2 , E 3  and E 4  are arranged at intervals in the extension direction of the sensor body  41 . In the embodiment, the extension direction of the sensor body  41  means the movement direction of a liquid level S according to a change of the quantity of the methanol aqueous solution. The first to fourth sensing electrodes E 1  to E 4  are one example of the second electrodes.  
      The first to fourth sensing electrodes E 1  to E 4  are placed respectively at plural heights which are set in the sensor body  41 . Namely, one sensing electrode is placed at one liquid level. The term “liquid level” means a height index which is set in the sensor body  41  in order to indicate the height of the liquid level S.  
      The fourth sensing electrode E 4  is placed in the vicinity of the upper wall  31   a , and positioned in the wetting region  43 . Namely, the fourth sensing electrode E 4  is separated from the upper wall  31   a  by a distance which is smaller than the maximum thickness of the liquid drop D. In a manner similar to the reference electrode E 0 , the first to fourth sensing electrodes E 1  to E 4  are exposed to the interior of the mixing tank  31 , and electrically connected to the sensing mechanism  42 .  
      In order to isolate a wiring pattern, which electrically connects the sensing electrodes E 0 , E 1 , E 2 , E 3 , E 4  to the sensing mechanism  42 , from the methanol aqueous solution, the surface of the sensor body  41  is coated except portions where the sensing electrodes E 0 , E 1 , E 2 , E 3 , E 4  are exposed. As the coating material, a material having a methanol resistance, water repellency, and electrical insulation is preferably used. For example, a parylene coating using a polyparaxylylene resin is preferably employed.  
      As diagrammatically shown in  FIG. 7 , the sensing mechanism  42  applies a reference voltage V REF  to the reference electrode E 0 , and measures sensing voltages V 1 , V 2 , V 3 , V 4  of currents passing through the first to fourth sensing electrodes E 1 , E 2 , E 3 , E 4 . Therefore, the sensing mechanism  42  can sense conduction states between the sensing electrodes E 1 , E 2 , E 3 , E 4  and the reference electrode E 0 . In  FIG. 7 , R 1 , R 2 , R 3 , and R 4  diagrammatically indicate the electric resistances between the first to fourth sensing electrodes E 1  to E 4  and the reference electrode E 0 , respectively.  
      When the sensing voltages V 1 , to V 4  exceed a preset threshold, the sensing mechanism  42  determines that the corresponding first to fourth sensing electrodes E 1  to E 4  are positioned in the liquid. In the following description, a sensing voltage which exceeds the threshold is indicated by V=HIGH, and that which is lower than the threshold is indicated by V=LOW. The sensing mechanism  42  is set so that a sensing result of the lower side is preferentially employed unless the liquid levels transit stepwise.  
      As shown in  FIG. 4 , the mixing section  12  further includes: a temperature sensor  38  which senses the temperature of the methanol aqueous solution; and a concentration sensor  39  which senses the concentration of the methanol aqueous solution. Data which are sensed by the liquid quantity sensor  32 , the temperature sensor  38 , and the concentration sensor  39 , and which relate to the liquid quantity are sent to the controlling section  16  and then used in the control of the operation of the fuel cell unit  1 .  
      Next, the function of the fuel cell unit  1  will be described with reference to FIGS.  6  to  10 .  
      For example,  FIG. 6  shows a state where adhering of the liquid drop D does not occur, and the liquid level S is positioned between the second sensing electrode E 2  and the third sensing electrode E 3 . At this time, between the reference electrode E 0  and the third sensing electrode E 3 , and the reference electrode E 0  and the fourth sensing electrode E 4 , a highly conductive material does not exist and the resistances R 3 , R 4  are high. Therefore, a substantially no current flows between the reference electrode E 0  and the third sensing electrode E 3 , and the reference electrode E 0  and the fourth sensing electrode E 4 , and V 3 =LOW and V 4 =LOW are attained.  
      By contrast, a part of the reference electrode E 0  and the first and second sensing electrodes E 1 , E 2  are positioned in the liquid. Since the methanol aqueous solution exists between the reference electrode E 0  and the first sensing electrode E 1 , and the reference electrode E 0  and the. second sensing electrode E 2 , the resistances R 1 , R 2  are considerably lower than resistances in the case where the reference electrodes are in the air. Therefore, a current flows between the reference electrode E 0  and the first sensing electrode E 1 , and the reference electrode E 0  and the second sensing electrode E 2 , and V 1 =HIGH and V 2 =HIGH are attained.  
      As a result, the liquid quantity sensor  32  can determine that the liquid level S is between the second sensing electrode E 2  and the third sensing electrode E 3 . On the same principle, the liquid quantity sensor  32  can sense the liquid quantity in the five steps, or the height of the liquid level S is (i) below the first sensing electrode E 1 , (ii) between the first sensing electrode E 1  and the second sensing electrode E 2 , (iii) between the second sensing electrode E 2  and the third sensing electrode E 3 , (iv) between the third sensing electrode E 3  and the fourth sensing electrode E 4 , or (v) above the fourth sensing electrode E 4 .  
      Next, the case where adhering of the liquid drop D to the upper wall  31   a  occurs will be described.  
      Even if adhering of the liquid drop D to the upper wall  31   a  occurs, when the liquid level S is below the third sensing electrode E 3 , V 3 =LOW is attained, and hence erroneous sensing is suppressed.  
      For example,  FIG. 8  shows a state where adhering of the liquid drop D occurs, and the liquid level S is positioned between the third sensing electrode E 3  and the fourth sensing electrode E 4 . At this time, also the fourth sensing electrode E 4  is in contact with the methanol aqueous solution, but the reference electrode E 0  is not in contact with the liquid drop D. Therefore, the resistance R 4  between the reference electrode E 0  and the fourth sensing electrode E 4  is high. Consequently, V 4 =LOW is attained. As a result, the liquid quantity sensor  32  senses that the liquid level S is between the third sensing electrode E 3  and the fourth sensing electrode E 4 .  
      When the liquid quantity is further increased from the state shown in  FIG. 8 , the liquid level S is contacted with the lower end of the liquid drop D, and the liquid drop D is detached from the upper wall  31   a  so as to join with the other major portion of the methanol aqueous solution. The state where the liquid drop D is detached is shown in  FIG. 9 . In a state where the liquid drop D is detached, such as that shown in  FIG. 9 , the fourth sensing electrode E 4  is exposed in the air, and hence V 4 =LOW is attained. Therefore, the liquid quantity sensor  32  senses that the liquid level S is between the third sensing electrode E 3  and the fourth sensing electrode E 4 .  
      When the fourth sensing electrode E 4  is immersed in the liquid as shown in, for example,  FIG. 10 , the gap between the reference electrode E 0  and the fourth sensing electrode E 4  is filled with the methanol aqueous solution, and the resistance R 4  between the reference electrode E 0  and the fourth sensing electrode E 4  is lowered, whereby V 4 =HIGH is attained. Therefore, the liquid quantity sensor  32  senses that the liquid level S is above the fourth sensing electrode E 4 .  
      In the thus configured fuel cell unit  1 , the accuracy of liquid quantity sensing can be enhanced. Namely, the reference electrode E 0  in the embodiment is disposed at a position which, even when the liquid drop D adheres to the upper wall  31   a , is not in contact with the liquid drop D. According to the configuration, even when the fourth sensing electrode E 4  is in contact with the liquid drop D, erroneous sensing of the liquid quantity can be suppressed. Improvement of the sensing accuracy leads to stability of sensing in the liquid quantity sensor  32  and contributes to stability and safety of the operation control of the fuel cell unit  1 .  
      In the embodiment, the reference electrode E 0  is disposed at the position which is not in contact with the liquid drop D. Alternatively, the first to fourth sensing electrodes E 1  to E 4  may be disposed at positions which are not in contact with the liquid drop D. Also in the alternative, erroneous sensing is suppressed. Alternatively, all of the reference electrode E 0  and the first to fourth sensing electrodes E 1  to E 4  may be disposed at positions which are not in contact with the liquid drop D.  
      In contrast, in the configuration where the fourth sensing electrode E 4  is disposed in the vicinity of the upper wall  31   a , the full state where the level of the methanol aqueous solution is near the upper wall  31   a  can be surely sensed. Namely, the liquid quantity can be sensed until the liquid level is positioned in the wetting region  43 . Even when the reference electrode E 0  is separated from the upper wall  31   a , therefore, a large sensing range of the liquid level can be ensured.  
      Even when, for example, the reference electrode E 0  is disposed in any portion, the above-described effects can be attained as far as the electrode is disposed outside the wetting region  43 . When the upper end of the reference electrode E 0  is placed in an upper end portion of the region which is not in contact with the liquid drop D, however, the distance between the reference electrode E 0  and the fourth sensing electrode E 4  can be reduced.  
      As the distance between the reference electrode E 0  and the fourth sensing electrode E 4  becomes shorter, the resistance R 4  between the reference electrode E 0  and the fourth sensing electrode E 4  when submerged in the liquid is lower. Namely, it is possible to determine more surely whether the reference electrode E 0  and the fourth sensing electrode E 4  are in the liquid or in the air. This contributes to improvement of the sensing accuracy of the liquid quantity sensor  32 .  
      When the sensor body  41  is attached to the upper wall  31   a , it is not required to dispose an opening or the like for attaching the liquid quantity sensor  32  in the bottom wall  31   b , and hence liquid leakage from the bottom wall  31   b  can be prevented. When the lower end  41   a  of the sensor body  41  is separated from the bottom wall  31   b , the methanol aqueous solution hardly stagnates in the mixing tank  31 , and the concentration of the methanol aqueous solution is more uniform.  
      In the configuration where the sensor body  41  is attached to the middle portion of the upper wall  31   a , even when the mixing tank  31  is inclined, the height change of the liquid level S is least. Namely, the liquid quantity sensor  32  is hardly affected by inclination of the liquid level S. Therefore, the disposition of the sensor body  41  in the middle portion of the upper wall  31   a  contributes to improvement of the accuracy of the sensing of the liquid quantity.  
      Next, a fuel cell unit  51  which is a liquid quantity sensing device of a second embodiment of the invention will be described with reference to  FIG. 11 . The components having the same function as those of the fuel cell unit  1  of the first embodiment are denoted by the same reference numerals, and their description is omitted.  
      A liquid quantity sensor  52  of the fuel cell unit  51  includes first to ninth sensing electrodes E 1  to E 9 . The intervals of the first to ninth sensing electrodes E 1  to E 9  in the vertical direction are smaller than those in the liquid quantity sensor  32  of the first embodiment. The liquid quantity sensor  52  can sense a change of. the liquid quantity which is smaller than that in the case of the liquid quantity sensor  32  of the first embodiment.  
      As shown in  FIG. 11 , the first to ninth sensing electrodes E 1  to E 9  are alternately arranged on both sides of the reference electrode E 0  in the horizontal direction that is orthogonal to the movement direction of the liquid level. Specifically, the first, third, fifth, seventh, and ninth sensing electrodes E 1 , E 3 , E 5 , E 7  and E 9  are placed on the left side of the reference electrode E 0 . The second, fourth, sixth, and eighth sensing electrodes E 2 , E 4 , E 6  and E 8  are placed on the right side of the reference electrode E 0 . The sensing electrodes E 1  to E 9  are separately arranged in different levels so as not to overlap with each other in the horizontal direction.  
      Next, the function of the fuel cell unit  51  will be described.  
      The principle of the liquid quantity sensing in the liquid quantity sensor  52  is identical with that in the liquid quantity sensor  32  in the first embodiment. The liquid quantity sensor  52  in the embodiment is characterized in that erroneous sensing can be suppressed when a liquid drop d adheres to the front of the sensor body  41 .  
      As further shown in  FIG. 12 , a liquid quantity sensor  55  in which the first to ninth sensing electrodes E 1  to E 9  are placed on one of the right and left sides of the reference electrode E 0  may be used to sense small change of the liquid quantity,. For this embodiment of the liquid quantity sensor  55 , however, there is a possibility that the liquid quantity is erroneously sensed when the liquid drop d adheres to a portion of the sensor body  41  which is immediately above the liquid level S.  
      In the state shown in  FIG. 12 , for example, the gaps between the fifth sensing electrode E 5  and the reference electrode E 0 , and the sixth sensing electrode E 6  and the reference electrode E 0  are in a conduction state by the liquid drop d. Although the liquid level S is between the fourth sensing electrode E 4  and the fifth sensing electrode ES, therefore, the liquid quantity sensor  55  may erroneously sense that the liquid level S is between the sixth sensing electrode E 6  and the seventh sensing electrode E 7 .  
      In contrast, according to the liquid quantity sensor  52  in the embodiment, erroneous sensing of the liquid quantity can be suppressed even when the liquid drop d adheres to a portion immediately above the liquid level S as shown in  FIG. 11 . Namely, even when the reference electrode E 0  passes current to the sixth sensing electrode E 6 , the reference electrode E 0  does not pass current to the fifth sensing electrode E 5 , and hence the liquid quantity sensor  52  can correctly sense that the liquid level S is between the fourth sensing electrode E 4  and the fifth sensing electrode E 5 .  
      According to the thus configured liquid quantity sensor  52 , even when the interval between adjacent liquid levels is small, adjacent sensing electrodes can be largely separated from each other. According to the configuration, even when the liquid drop d adheres to a certain sensing electrode, the liquid drop d hardly adheres to a sensing electrode which is positioned at the adjacent liquid level. In the liquid quantity sensor  52 , therefore, erroneous sensing can be suppressed even when the liquid drop d adheres to the front of the sensor body  41 .  
      It is a matter of course that, also in the liquid quantity sensor  52  in the embodiment, erroneous sensing due to the liquid drop D adhering to the upper wall  31   a  can be suppressed in the same manner as the liquid quantity sensor  32  in the first embodiment.  
      Next, a fuel cell unit  61  which is a liquid quantity sensing device of a third embodiment of the invention will be described with reference to  FIG. 13 . The components having the same function as those of the fuel cell unit  1  of the first embodiment are denoted by the same reference numerals, and their description is omitted.  
      A liquid quantity sensor  62  of the fuel cell unit  61  includes first to tenth sensing electrodes E 1  to E 10 . As shown in  FIG. 13 , the first to tenth sensing electrodes E 1  to E 10  are placed separately on the right and left sides of the reference electrode E 0  so that pairs of sensing electrodes are placed respectively at plural heights which are set in the sensor body  41 .  
      Namely, the first and second sensing electrodes E 1 , E 2  are placed at the same liquid level. Similarly, the third and fourth sensing electrodes E 3 , E 4 , the fifth and sixth sensing electrodes E 5 , E 6 , the seventh and eighth sensing electrodes E 7 , E 8 , and the ninth and tenth E 9 , E 10  are placed at the respective same liquid levels.  
      Next, the function of the fuel cell unit  61  will be described.  
      The principle of the liquid quantity sensing in the liquid quantity sensor  62  is identical with that of the liquid quantity sensor  32  in the first embodiment. The embodiment is characterized in that, when the fuel cell unit  61  is inclined, the liquid quantity sensor  62  can sense also the inclination.  
      For example,  FIG. 13  shows a state of the mixing section  12  when the fuel cell unit  61  is inclined. When the fuel cell unit  61  is inclined, the sensor body  41  is inclined with respect to the liquid level S. When the sensor body  41  is inclined with respect to the liquid level S, even in a pair of sensing electrodes which are placed at the same height in the sensor body  41 , a state where one of the sensing electrodes is exposed in the air, and the other sensing electrode is submerged in the liquid is produced. In  FIG. 13 , for example, among the third and fourth sensing electrodes E 3 , E 4  which are placed at the same height, the third sensing electrode E 3  is exposed in the air, and the fourth sensing electrode E 4  is submerged in the liquid.  
      According to the configuration, the liquid quantity sensor  62  can determine that the fuel cell unit  61  is inclined. Namely, the liquid quantity sensor  62  senses and considers the inclination of the liquid level S, so that the accuracy of liquid quantity sensing can be further improved. The number of sensing electrodes which are disposed at one liquid level is not restricted to two, and may be three or more.  
      It is a matter of course that, also in the liquid quantity sensor  62  in the embodiment, erroneous sensing due to the liquid drop D adhering to the upper wall  31   a  can be suppressed in the same manner as the liquid quantity sensor  32  in the first embodiment.  
      In the liquid quantity sensor  62 , a plate face on which the sensing electrodes are arranged may be placed along the section A-A in  FIG. 1  for the following reason. The fuel cell unit  61  attached to the portable computer  2  is often used while placed together with the portable computer  2  on the lap of the user. In such a case, the portable computer  2  is often inclined in the anteroposterior direction, and hence the fuel cell unit  61  is inclined along the section A-A in  FIG. 1 .  
      Alternatively, two or more liquid quantity sensor  62  which are disposed respectively along intersecting directions may be used, and the inclinations along the section A-A in  FIG. 1  and a plane intersecting with the section A-A may be sensed. Alternatively, the sensor body  41  may have two plate faces which intersect with each other as viewed from the top, and three or more sensing electrodes may be disposed at each liquid level on the sensor body  41  to sense inclinations in two or more directions.  
      Next, a fuel cell unit  71  which is a liquid quantity sensing device of a fourth embodiment of the invention will be described with reference to  FIG. 14 . The components having the same function as those of the fuel cell unit  1  of the first embodiment are denoted by the same reference numerals, and their description is omitted.  
      The mixing tank  31  of the fuel cell unit  71  includes a partition  72 . The partition  72  is one example an inner wall. The partition  72  is attached to the upper wall  31   a  and extends toward the interior of the mixing tank  31 . The partition  72  is formed into, for example, a cylindrical shape. The partition  72  is disposed in (proximate to) the periphery of the sensor body  41  so as to surround the sensor body  41 . The lower end  72   a  of the partition  72  is separated from the bottom wall  31   b  of the mixing tank  31 , and the liquid can freely move between the outside and inside of the partition  72 .  
      Next, the function of the fuel cell unit  71  will be described.  
      The principle of sensing the liquid quantity is identical with that of the liquid quantity sensor  32  in the first embodiment. The embodiment is characterized in that, even when an external factor such as vibration is applied to the fuel cell unit  71 , lowering of the sensing accuracy of the liquid quantity sensor  32  can be suppressed.  
      In the case where vibration is applied to the fuel cell unit  71 , the liquid level S swings in the mixing tank  31  as shown in  FIG. 14 . When the liquid level S swings, the sensing accuracy of the liquid quantity is lowered. When the partition  72  is disposed as shown in  FIG. 14 , however, the swing of the liquid level S around the liquid quantity sensor  32  is suppressed, whereby lowering of the sensing accuracy of the liquid quantity can be suppressed. The shape of the partition  72  is not restricted to a cylindrical shape and can have any structure as far as the partition surrounds the sensor body  41 .  
      It is a matter of course that erroneous sensing due to the liquid drop D adhering to the upper wall  31   a  can be suppressed in the same manner as the liquid quantity sensor  32  in the first embodiment.  
      In the above, the fuel cell units  1 ,  51 ,  61 ,  71  of the first to fourth embodiments have been described. The invention is not restricted to the embodiments. As shown in  FIG. 15 , for example, openings  81  which pierce through the sensor body  41  may be disposed in portions of the sensor body  41  where the electrodes E 0  to E 4  are not disposed. Since the sensor body  41  has the openings  81 , stagnation of the methanol aqueous solution in the mixing tank  31  can be further suppressed.  
      The components of the embodiments may be adequately combined with each other in a liquid quantity sensing device to which the invention is applied. The electrically conductive liquid is not restricted to methanol aqueous solution, and may be another liquid fuel such as alcohols, or an ink-like material. The range to which the invention can be applied is not restricted to a fuel cell unit, and may be applied to, for example, an ink container for an inkjet printer.