Patent Publication Number: US-2013233707-A1

Title: Sensor and sensor system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2012-078757 filed on Mar. 30, 2012, Japanese Patent Application No. 2012-052495 filed on Mar. 9, 2012, and Japanese Patent Application No. 2012-082173 filed on Mar. 30, 2012, the contents of which are hereby incorporated by reference into the present application. 
     TECHNICAL FIELD 
     The specification discloses a technique for appropriately detecting a property of liquid. 
     DESCRIPTION OF RELATED ART 
     A technique is known which utilizes a change in a capacitance of a pair of electrodes to detect a property of liquid. For example, Japanese Patent Application Publication No. 2005-351688 discloses a liquid level and quality sensor with a pairs of electrode disposed on each of an upper surface and a lower surface of a substrate. The capacitance of the pair of electrodes disposed on the upper surface of the substrate changes correlatively to a level of engine oil. The capacitance of the pair of electrodes disposed on the lower surface of the substrate changes correlatively to quality of the engine oil but not to the level of the engine oil. The liquid level and quality sensor in Japanese Patent Application Publication No. 2005-351688 detects the level of the engine oil based on a change in the capacitance of the pair of electrodes disposed on the upper surface of the substrate, and detects the quality of the engine oil based on a change in the capacitance of the pair of electrodes disposed on the lower surface of the substrate. 
     SUMMARY 
     The liquid level and quality sensor in Japanese Patent Application Publication No. 2005-351688 may fail to appropriately detect a property of a detection target liquid depending on a detection target liquid. For example, when the capacitance changes insignificantly in response to a change in the quality of the detection target liquid, the sensor may fail to appropriately detect the quality. Furthermore, when the capacitance changes insignificantly in response to a change in the level of the detection target liquid, the sensor may fail to appropriately detect the level. The specification provides a technique for appropriately detecting the property of the liquid using the pair of electrodes. 
     The technique disclosed herein is a sensor for detecting a property of liquid. A first sensor disclosed herein includes a substrate, a first pair of electrodes, and a second pair of electrodes. The first pair of electrodes is disposed on an upper surface of the substrate. The second pair of electrodes is disposed on a lower surface of the substrate. A capacitance of the first pair of electrodes and a capacitance of the second pair of electrodes mutually change correlatively to a same property of the liquid. 
     Compared to the conventional configuration in which a pair of electrodes having a capacitance changing correlatively to one property of liquid is disposed only on an upper surface of a substrate, the above-described first sensor can increase an amount of change in a capacitance of the sensor (that is, the first and second pairs of electrodes) with respect to an amount of change in the property of the liquid. That is, a sensitivity of the sensor to the change in the property of the liquid can be increased. As a result, compared to the conventional configuration, the first sensor can be used to appropriately detect the property of the liquid. 
     A second sensor disclosed herein includes a substrate, a sensor unit including a substrate, a first pair of electrodes with a first electrode and a second electrode disposed on an upper surface of the substrate apart from each other, and a second pair of electrodes with a third electrode and a fourth electrode disposed on a lower surface of the substrate apart from each other, and a connection unit configured to connect a power source and the sensor unit. In the sensor, the first electrode opposes at least a part of the third electrode via the substrate, and the second electrode opposes at least a part of the fourth electrode via the substrate. The connection unit can connect the electrodes in a state in which both the first electrode and the fourth electrode are at a first electric potential and both the second electrode and the third electrode are at a second electric potential different from the first electric potential. 
     In the second sensor, the connection unit connects the power source to the first electrode, the second electrode, the third electrode, and the fourth electrode in the state in which both the first electrode and the fourth electrode are at the first electric potential and both the second electrode and the third electrode are at the second electric potential different from the first electric potential. Thus, a potential difference occurs between the opposite electrodes (between the first electrode and the second electrode, and between the third electrode and the fourth electrode, respectively) on the same surface (respectively on the upper surface and the lower surface) of the substrate. A potential difference also occurs between the electrodes (between the first electrode and the third electrode and between the second electrode and the fourth electrode) at least partly opposite to each other with the substrate in between. As a result, a capacitive component (a capacitance of the capacitive component is denoted by Cz) between the electrodes at least partly opposite to each other with the substrate in between is connected in parallel with a capacitive component (a capacitance of the capacitive component is denoted by Cy) between the opposite electrodes on the surface of the substrate, and a capacitance C of the sensor unit is such that C=Cy+Cz. A dielectric loss tan δ of the sensor unit is expressed by Expression (1) shown below, using an angular frequency ω of the power source, a parasitic resistance R 2  of the sensor unit (which is affected by an electric conductivity of the liquid), and the capacitance C. Expression (1) indicates that when the capacitance C increases by Cz, the dielectric loss tan δ decreases, enabling an adverse effect of the electric conductivity of the liquid to be reduced. 
       tan δ=1 /ωCR 2  (1)
 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a configuration of a sensor system according to a first embodiment. 
         FIG. 2  shows a configuration of a sensor system according to a second embodiment. 
         FIG. 3  shows a configuration of a sensor system according to a third embodiment. 
         FIG. 4  schematically shows a sensor system according to a fourth embodiment. 
         FIG. 5  is a cross-sectional view of the sensor system in  FIG. 4  taken along line V-V in  FIG. 4 . 
         FIG. 6  schematically shows a sensor system according to a variation. 
         FIG. 7  schematically shows a sensor system according to a fifth embodiment. 
         FIG. 8  schematically shows a sensor system according to a variation. 
         FIG. 9  schematically shows a sensor system according to a sixth embodiment. 
         FIG. 10  schematically shows a sensor system according to a variation. 
         FIG. 11  shows a relation between a first output and an ethanol concentration of a mixed fuel. 
         FIG. 12  shows a relation between a second output and an electric conductivity of the mixed fuel. 
         FIG. 13  shows a relation between a second output and the ethanol concentration of the mixed fuel. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT 
     In a first sensor disclosed herein, a first pair of electrodes may include a first signal electrode to which a signal is input from outside and a first reference electrode opposing the first signal electrode with a space in between. A second pair of electrodes may include a second signal electrode to which a signal is input from outside and a second reference electrode opposing the second signal electrode with a space in between. The first signal electrode and the second signal electrode may have a same shape. The first reference electrode and the second reference electrode may have a same shape. A distance between the first signal electrode and the first reference electrode may be equal to a distance between the second signal electrode and the second reference electrode. This configuration can make a capacitance of the first pair of electrodes equal to a capacitance of the second pair of electrodes. 
     In the first sensor, the capacitance of the first pair of electrodes and the capacitance of the second pair of electrodes may mutually change correlatively to a concentration of a specific material included in liquid. This configuration can appropriately detect the concentration of the specific material included in the liquid. 
     The specification further discloses a sensor system including the first sensor. The sensor system may include the first sensor, a signal supplying device, and a comparing device. The signal supplying device may supply a signal to the first pair of electrodes and the second pair of electrodes. The comparing device may compare a first correlation value correlated to the capacitance of the first pair of electrodes during when the signal supplying device is supplying the signal to the first pair of electrodes and a second correlation value correlated to the capacitance of the second pair of electrodes during when the signal supplying device is supplying the signal to the second pair of electrodes, and to output a comparison result. 
     The first correlation value and the second correlation value are correlated to a same property. Thus, the first correlation value and the second correlation value are positively correlated to each other. However, if one of the first pair of electrodes and the second pair of electrodes is damaged or a foreign substance is attached to one of the first pair of electrodes and the second pair of electrodes, the capacitance of the one of the pairs of electrodes may be changed by a factor different from the property of the liquid. In this case, the correlation between the first correlation value and the second correlation value is disrupted, reducing a correlation coefficient for the correlation values. This relation enables determination of whether one of the first correlation value and the second correlation value fails to be correlated to the property of the liquid, using the comparison result output by the comparing device. Thus, the property of the liquid can be prevented from being detected using the correlation value not correlated to the property of the liquid. 
     In the sensor system, the first pair of electrodes may be connectable to the second pair of electrodes in parallel. This configuration enables the capacitance of the sensor to be increased compared to a configuration in which the first pair of electrodes and the second pair of electrodes are connected in series. 
     The specification further discloses another sensor system including the first sensor. The sensor system may include the first sensor and a specifying device connected to the first sensor and configured to specify a property of liquid based on a signal output from the first sensor. The specifying device may include a signal supplying unit configured to supply a signal to the first sensor, a reference equivalent circuit including one or more reference elements corresponding to an equivalent circuit of the first sensor, and an arithmetic unit configured to correct the signal output from the first sensor based on a signal output from the reference equivalent circuit when the signal is fed from the signal supplying unit to the reference equivalent circuit. This configuration corrects the signal output from the first sensor using the reference elements, allowing a variation in detection accuracy among sensor systems to be suppressed. 
     The specifying device may further include a power voltage measurement unit configured to measure a voltage of power supplied to the first sensor and the signal supplying unit, and a temperature measurement unit configured to measure a temperature of the specifying device. The arithmetic unit may further be configured to correct the signal output from the first sensor based on the voltage of power measured by the power voltage measurement unit and the temperature measured by the temperature measurement unit. This configuration can restrain the signal output from the first sensor from being changed by a change in the temperature of the specifying device or in the voltage of power supplied to the first sensor. 
     In a second sensor disclosed herein, the first electrode may oppose the third electrode via the substrate, and the second electrode may oppose the fourth electrode via the substrate. 
     In the second sensor, the connection unit may further include a switching device configured to switch between a state in which both the first electrode and the fourth electrode are at the first electric potential and both the second electrode and the third electrode are at the second electric potential, and a state in which both the first electrode and the third electrode are at the first electric potential and both the second electrode and the fourth electrode are at the second electric potential. 
     In the second sensor, the connection unit may include a switching device configured to switch between a state in which the first pair of electrodes and the second pair of electrodes are connected in parallel and a state in which the first pair of electrodes and the second pair of electrodes are connected in series. 
     The specification provides a method for measuring a property of liquid using a sensor including a substrate, a first pair of electrodes including a first electrode and a second electrode disposed on an upper surface of the substrate apart from each other, and a second pair of electrodes including a third electrode and a fourth electrode disposed on a lower surface of the substrate apart from each other, wherein the first electrode opposes at least a part of the third electrode via the substrate, and the second electrode opposes at least a part of the fourth electrode via the substrate. The method supplies power to the sensor in such a manner that both the first electrode and the fourth electrode are at a first electric potential and both the second electrode and the third electrode are at a second electric potential different from the first electric potential, and measures a capacitance of the liquid using a first output from the sensor. 
     The method for measuring the property of the liquid may further include supplying power to the sensor in such a manner that both the first electrode and the third electrode are at the first electric potential and both the second electrode and the fourth electrode are at the second electric potential, and calculating an electric conductivity of the liquid based on a second output and the first output from the sensor. 
     According to the method for measuring the property of the liquid, the capacitance and electric conductivity of the liquid may be measured using an output from the sensor obtained when power is supplied to the sensor in a state in which the first pair of electrodes and the second pair of electrodes are connected in parallel and an output from the sensor obtained when power is supplied to the sensor in a state in which the first pair of electrodes and the second pair of electrodes are connected in series. 
     Representative, non-limiting examples of the present invention will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed below may be utilized separately or in conjunction with other features and teachings to provide improved sensors and sensor systems, as well as methods for using the same. 
     Moreover, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described and below-described representative examples, as well as the various independent and dependent claims, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. 
     All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter. 
     First Embodiment 
     A sensor system  2  is mounted in an automobile using a mixed fuel of gasoline and ethanol as a fuel. The sensor system  2  is used to detect a concentration of ethanol in the mixed fuel. As shown in  FIG. 1  the sensor system  2  includes a sensor  10  and a specifying device  50 . 
     The sensor  10  includes a substrate  11  and two pairs of electrodes  21  and  31 .  FIG. 1  shows an upper surface  20  and a lower surface  30  of the substrate  11  juxtaposed with each other. In actuality, the sensor  10  includes the single substrate  11 . 
     The pair of electrodes  21  is disposed on the upper surface  20  of the substrate  11 . The pair of electrodes  21  includes a signal electrode  22  and a reference electrode  24 . The signal electrode  22  includes a plurality of (in  FIG. 1 , eight) horizontal electrode portions  22   a  (in  FIG. 1 , only one of the horizontal electrode portions  22   a  is denoted by the reference numeral) and a vertical electrode portion  22   b . The vertical electrode portion  22   b  extends linearly in a longitudinal direction of the substrate  11  (a depth direction of a fuel tank). An upper end of the vertical electrode portion  22   b  is positioned at an upper end of the substrate  11 . The vertical electrode portion  22   b  is connected to ends on one side (left ends in  FIG. 1 ) of the plurality of horizontal electrode portions  22   a . Thus, the plurality of horizontal electrode portions  22   a  is electrically connected to the vertical electrode portion  22   b . The plurality of horizontal electrode portions  22   a  is disposed such that the horizontal electrode portions  22   a  are parallel to one another and also disposed perpendicularly to the vertical electrode portion  22   b . The plurality of horizontal electrode portions  22   a  is arranged such that the horizontal electrode portions  22   a  are disposed at regular intervals in the longitudinal direction of the substrate  11 . 
     On the right of the signal electrode  22 , the reference electrode  24  is disposed in association with the signal electrode  22 . The reference electrode  24  is grounded. The reference electrode  24  includes a plurality of (in  FIG. 1 , eight) horizontal electrode portions  24   a  (in  FIG. 1 , only one of the horizontal electrode portions  24   a  is denoted by the reference numeral) and a vertical electrode portion  24   b . The vertical electrode portion  24   b  extends linearly in the longitudinal direction of the substrate  11 . An upper end of the vertical electrode portion  24   b  is positioned at the upper end of the substrate  11 . The vertical electrode portion  24   b  is connected to the plurality of horizontal electrode portions  24   a . Thus, the plurality of horizontal electrode portions  24   a  is electrically connected to the vertical electrode portion  24   b . The plurality of horizontal electrode portions  24   a  is disposed such that the horizontal electrode portions  24   a  are parallel to one another and also disposed perpendicularly to the vertical electrode portion  24   b . The plurality of horizontal electrode portions  24   a  is arranged such that the horizontal electrode portions  24   a  are disposed at regular intervals in the longitudinal direction of the substrate  11 . The horizontal electrode portions  22   a  and the horizontal electrode portions  24   a  are alternately disposed as viewed along a side of the substrate  11  from the upper end to a lower end of the substrate  11 . 
     The pair of electrodes  31  is disposed on the lower surface  30  of the substrate  11 . The pair of electrodes  31  includes a signal electrode  32  and a reference electrode  34 . The signal electrode  32  includes a plurality of (in  FIG. 1 , eight) horizontal electrode portions  32   a  (in  FIG. 1 , only one of the horizontal electrode portions  32   a  is denoted by the reference numeral) and a vertical electrode portion  32   b . The signal electrode  32  has a same shape as a shape of the signal electrode  22 . Furthermore, the signal electrode  32  is at a same position as a position of the signal electrode  22  on the other side of the substrate  11 . The reference electrode  34  is disposed in association with the signal electrode  32 . The reference electrode  34  is grounded. The reference electrode  34  includes a plurality of (in  FIG. 1 , eight) horizontal electrode portions  34   a  (in  FIG. 1 , only one of the horizontal electrode portions  34   a  is denoted by the reference numeral) and a vertical electrode portion  34   b . The reference electrode  34  has a same shape as a shape of the reference electrode  24 . Furthermore, the reference electrode  34  is at a same position as a position of the reference electrode  24  on the other side of the substrate  11 . In this configuration, a distance between the signal electrode  32  and the reference electrode  34  is equal to a distance between the signal electrode  22  and the reference electrode  24 . More specifically, a distance between the horizontal electrode portions  32   a  and the horizontal electrode portions  34   a  which are adjacent to each other is equal to a distance between the horizontal electrode portions  22   a  and the horizontal electrode portions  24   a  which ae adjacent to each other. Thus, if the pair of electrodes  21  and the pair of electrodes  31  are disposed in a same environment (for example, in a fuel tank), a capacitance of the pair of electrodes  21  is normally equal to a capacitance of the pair of electrodes  31 . An uppermost horizontal electrode portions  22   a  is positioned above an uppermost horizontal electrode portions  24   a . An uppermost horizontal electrode portions  32   a  is positioned above an uppermost horizontal electrode portions  34   a.    
     The specifying device  50  includes an oscillation circuit  52 , two resistors R 1  and R 2 , two switches S 1  and S 2 , a rectifying unit  56 , an amplifying unit  58 , an arithmetic unit  60 , and a temperature detecting unit  62 . The oscillation circuit  52  generates a signal (alternating voltage) with a predetermined period (for example, 10 Hz to 3 MHz). 
     The oscillation circuit  52  is connected to each of the switches S 1  and S 2  via the resistor R 1 . The switches S 1  and S 2  are turned on and off by a control unit (not shown in the drawings). If the switch S 1  is on, the signal electrode  22  and the oscillation circuit  52  are connected together. If the switch S 1  is off, the signal electrode  22  and the oscillation circuit  52  are not connected together. That is, while the switch S 1  is on, the oscillation circuit  52  supplies a signal to the signal electrode  22 . If the switch S 2  is on, the signal electrode  32  and the oscillation circuit  52  are connected together. If the switch S 2  is of, the signal electrode  32  and the oscillation circuit  52  are not connected together. That is, while the switch S 2  is on, the oscillation circuit  52  supplies a signal to the signal electrode  32 . 
     The rectifying unit  56  is connected between the resistor R 1  and both the signal electrodes  22  and  32 . Same signals as the signals supplied to the signal electrodes  22  and  32  is input to the rectifying unit  56 . The rectifying unit  56  rectifies the input signal and outputs the rectified signal to the amplifying unit  58 . The amplifying unit  58  amplifies the input signal and outputs the amplified signal to the arithmetic unit  60  (MCU). 
     The control unit controls the switches S 1  and S 2  to switch the sensor system  2  among a first state to a fourth state. In the first state, the switch S 1  is on, and the switch S 2  is off. In the first state, the oscillation circuit  52  inputs a signal to the signal electrode  22 . As a result, charge is accumulated in the pair of electrodes  21 . The capacitance of the pair of electrodes  21  changes correlatively to the concentration of ethanol in the mixed fuel. The resistor R 1  has a constant resistance value, and thus, an amplitude of the signal supplied to the signal electrode  22  changes correlatively to the concentration of ethanol. The same signal as the signal supplied to the signal electrode  22  is input to the rectifying unit  56 . As a result, a signal correlated to the capacitance of the pair of electrodes  21  is input to the arithmetic unit  60 . 
     In the second state, the switch S 1  is off, and the switch S 2  is on. In the second state, the oscillation circuit  52  inputs a signal to the signal electrode  32 . As a result, charge is accumulated in the pair of electrodes  31 . The capacitance of the pair of electrodes  31  changes correlatively to the concentration of ethanol in the mixed fuel. The resistor R 1  has a constant resistance value, and thus, an amplitude of the signal supplied to the signal electrode  32  changes correlatively to the concentration of ethanol. The same signal as the signal supplied to the signal electrode  32  is input to the rectifying unit  56 . As a result, a signal correlated to the capacitance of the pair of electrodes  31  is input to the arithmetic unit  60 . 
     In the third state, both the switches S 1  and S 2  are on. In the third state, the pair of electrodes  21  and the pair of electrodes  31  are connected together in parallel. In the third state, the oscillation circuit  52  inputs a signal to the signal electrodes  22  and  32 . As a result, charge is accumulated in the pairs of electrodes  21  and  31 . With the resistor R 1  exhibiting a predetermined resistance, the capacitances of the pairs of electrodes  21  and  31  change correlatively to the concentration of ethanol in the mixed fuel. Thus, amplitudes of the signals supplied to the signal electrodes  22  and  32  change correlatively to the concentration of ethanol. The signals supplied to the signal electrodes  22  and  32  are input to the rectifying unit  56 . As a result, a signal correlated to a sum of the capacitance of the pair of electrodes  21  and the capacitance of the pair of electrodes  31  is input to the arithmetic unit  60 . 
     In the fourth state, both the switches S 1  and S 2  are off. In the fourth state, the rectifying unit  56  is connected in series with the resistor R 1 . In this case, no signal is supplied to the pairs of electrodes  21  and  31 . As a result, a signal that does not change depending on the concentration of ethanol in the mixed fuel is input to the arithmetic unit  60  via the rectifying unit  56 . 
     The control unit consecutively switches from the first state to the fourth state. The control unit also repeats the consecutive switching from the first state to the fourth state. The control unit transmits a current state of the sensor system  2  to the arithmetic unit  60 . 
     The temperature detecting unit  62  is connected via the resistor R 2  to a DC power source  54  configured to output, for example, a DC voltage of 5V. The temperature detecting unit  62  includes a resistance temperature detector that has a resistance value changing depending on temperature. The temperature detecting unit  62  uses the resistance temperature detector to detect a temperature of the mixed fuel, and inputs the detected temperature of the mixed fuel to the arithmetic unit  60 . 
     In the third state, the arithmetic unit  60  uses the signal received from the amplifying unit  58  and the temperature of the mixed fuel received from the temperature detecting unit  62  to determine the concentration of ethanol in the mixed fuel. For example, the arithmetic unit  60  uses a database indicative of relations between the capacitances and the temperature of the mixed fuel and the concentration of ethanol which is stored in the arithmetic unit  60 , to determine the concentration of ethanol in the mixed fuel. In the third state, the pair of electrodes  21  and the pair of electrodes  31  are connected together in parallel. This configuration can determine the concentration of ethanol using the sum of the capacitances of the two pairs of electrodes  21  and  31 . Thus, compared to a configuration that determines the concentration of ethanol using the capacitance of one of the pair of electrodes  21  and the pair of electrodes  31 , the above-described configuration can increase a change in the capacitance associated with a change in the concentration of ethanol, that is, the above-described configuration can increase a sensitivity of the sensor  10 . Consequently, the arithmetic unit  60  can appropriately detect the concentration of ethanol. 
     Furthermore, the capacitances of the pairs of electrodes  21  and  31  change depending on the temperature of the mixed fuel. The arithmetic unit  60  uses the temperature of the mixed fuel in detecting the concentration of ethanol. According to this configuration, the arithmetic unit  60  can correct a change in the capacitances of the pair of electrodes  21  and  31  depending on the temperature of the mixed fuel. Thus, the arithmetic unit  60  can more appropriately detect the concentration of ethanol. The concentration of ethanol detected by the arithmetic unit  60  is output to an engine control unit (ECU)  70 . The ECU  70  uses the input concentration of ethanol to control an injector and the like. That is, based on the appropriate concentration of ethanol detected by the arithmetic unit  60 , the ECU  70  can control the injector and the like. Hence, the mixed fuel can be appropriately supplied to an engine. 
     In the first state, the signal input to the arithmetic unit  60  (hereinafter referred to as a “first correlation value”) is correlated to the capacitance of the pair of electrodes  21 . In the second state, the signal input to the arithmetic unit  60  (hereinafter referred to as a “second correlation value”) is correlated to the capacitance of the pair of electrodes  31 . The signal input to the electrodes  21  in the first state is the same as the signal input to the electrodes  31  in the second state. The sensor system  2  is consecutively switched from the first state to the second state. That is, the first correlation value and the second correlation value are consecutively input to the arithmetic unit  60 . While the sensor system  2  is operating normally, the consecutively input first and second correlation values are substantially equal. 
     When the first correlation value and the second correlation value are consecutively input to the arithmetic unit  60 , the arithmetic unit  60  compares the input first and second correlation values with each other. If the first correlation value is different from the second correlation value, at least one of the first correlation value and the second correlation value is not correlated to the concentration of ethanol in the mixed fuel. In this case, the correlation between the first correlation value and the second correlation value is disrupted. Then, the arithmetic unit  60  determines that at least one of the pair of electrodes  21  and the pair of electrodes  31  is suffering from a defect such as damage or adhesion of a foreign substance. In this case, the arithmetic unit  60  outputs a signal indicating that a defect is occurring in at least one of the pair of electrodes  21  and the pair of electrodes  31 , to the ECU  70 . This configuration can determine that a defect is occurring in the sensor  10 . As a result, the ECU  70  can be prevented from controlling the units of the sensor based on an incorrect concentration of ethanol. 
     The arithmetic unit  60  stores a voltage value of the signal input to the arithmetic unit  60  in the fourth state in which the sensor system  2  is operating normally. The arithmetic unit  60  compares the voltage value of the signal input when the sensor system  2  is in the fourth state with a voltage value stored in the arithmetic unit  60 . If the voltage value of the input signal is different from the voltage value stored in the arithmetic unit  60 , the arithmetic unit  60  determines that a defect (damage to any conducting wire, a failure in any of the units  52 ,  56 , and  58 , or the like) is occurring in the sensor system  2 . In this case, the arithmetic unit  60  outputs a signal indicating that a defect is occurring in the sensor system  2 , to the ECU  70 . This configuration can determine that a defect is occurring in the sensor system  2  except for the sensor  10 . As a result, the ECU  70  can be prevented from controlling the units of the sensor based on an incorrect concentration of ethanol. 
     Second Embodiment 
     Differences from the first embodiment will mainly be described. A sensor system  102  shown in  FIG. 2  includes units  52 ,  56 ,  58 ,  60 , S 1 , and S 2  similar to the units  52 ,  56 ,  58 ,  60 , S 1 , and S 2  of the sensor system  2  and lacks the resistors R 1  and R 2 , temperature detecting unit  62 , and the sensor  10 . The sensor system  102  further includes a switch S 3 , a sensor  110 , and a current-voltage conversion unit  155 . 
     The sensor  110  includes a substrate  11  and two pairs of electrodes  21  and  131 . Like  FIG. 1 ,  FIG. 2  shows an upper surface  20  and a lower surface  30  of the substrate  11  juxtaposed with each other. In actuality, the sensor  110  includes the single substrate  11 . A reference electrode  24  of the pair of electrodes  21  is connected to a rectifying unit  56  via the current-voltage conversion unit  155 . 
     The pair of electrodes  131  is disposed on the lower surface  30  of the substrate  11 . The pair of electrodes  131  includes a signal electrode  132  and a reference electrode  134 . A shape of the signal electrode  132  and a position of the signal electrode  132  on the substrate  11  are the same as the shape of the reference electrode  34  in  FIG. 1  and the position of the reference electrode  34  on the substrate  11 . A shape of the reference electrode  134  and a position of the reference electrode  134  on the substrate  11  are the same as the shape of the signal electrode  32  in  FIG. 1  and the position of the signal electrode  32  on the substrate  11 . In this configuration, a distance between the signal electrode  132  and the reference electrode  134  is equal to a distance between a signal electrode  22  and a reference electrode  24 . The reference electrode  134  is connected to the rectifying unit  56  via the current-voltage conversion unit  155 . 
     The oscillation circuit  52  is connected to each of the switches S 1  to S 3 . The switches S 1  to S 3  are turned on and off by a control unit (not shown in the drawings). If the switch S 1  is on, the signal electrode  22  and the oscillation circuit  52  are connected together. If the switch S 1  is off, the signal electrode  22  and the oscillation circuit  52  are not connected together. That is, while the switch S 1  is on, the oscillation circuit  52  supplies a signal to the signal electrode  22 . If the switch S 2  is on, the signal electrode  132  and the oscillation circuit  52  are connected together. If the switch S 2  is off, the signal electrode  132  and the oscillation circuit  52  are not connected together. That is, while the switch S 2  is on, the oscillation circuit  52  supplies a signal to the signal electrode  132 . If the switch S 3  is on, the current-voltage conversion unit  155  and the oscillation circuit  52  are connected directly together (without intervention of the pair of electrodes  21  or  131 ). If the switch S 3  is off, the current-voltage conversion unit  155  and the oscillation circuit  52  are not connected directly together. That is, while the switch S 3  is on, the oscillation circuit  52  supplies a signal to the current-voltage conversion unit  155  without passing the signal through the pair of electrodes  21  or  131 . 
     The control unit controls the switches S 1  to S 3  to switch the sensor system  2  among a first state to a fourth state. In the first state, the switch S 1  is on, the switch S 2  is off, and the switch  3  is off. In the first state, the pair of electrodes  21  and the current-voltage conversion unit  155  are connected together in series. When the oscillation circuit  52  supplies a signal to the signal electrode  22 , charge is accumulated in the pair of electrodes  21 . The capacitance of the pair of electrodes  21  changes correlatively to a concentration of ethanol in a mixed fuel. A current value of the current-voltage conversion unit  155  changes correlatively to the capacitance of the pair of electrodes  21 , that is, the concentration of ethanol in the mixed fuel. The current-voltage conversion unit  155  converts the current value of the current-voltage conversion unit  155  into a signal (voltage value) and outputs the signal to the rectifying unit  56 . Thus, the voltage value of the signal input to the rectifying unit  56  is changed by the capacitance of the pair of electrodes  21 . As a result, a signal correlated to the capacitance of the pair of electrodes  21  is input to the arithmetic unit  60 . 
     In the second state, the switch S 1  is off, the switch S 2  is on, and the switch S 3  is off. In the second state, the pair of electrodes  131  and the current-voltage conversion unit  155  are connected together in series. When the oscillation circuit  52  supplies a signal to the signal electrode  132 , charge is accumulated in the pair of electrodes  131 . A capacitance of the pair of electrodes  131  changes correlatively to the concentration of ethanol in the mixed fuel. The current value of the current-voltage conversion unit  155  changes correlatively to the capacitance of the pair of electrodes  131 , that is, the concentration of ethanol in the mixed fuel. The current-voltage conversion unit  155  converts the current value of the current-voltage conversion unit  155  into a signal (voltage value) and outputs the signal to the rectifying unit  56 . Thus, the voltage value of the signal input to the rectifying unit  56  is changed by the capacitance of the pair of electrodes  131 . As a result, a signal correlated to the capacitance of the pair of electrodes  131  is input to the arithmetic unit  60 . 
     In the third state, both the switch S 1  and the switch S 2  are on, and the switch S 3  is off. In the third state, the pair of electrodes  21  and the pair of electrodes  131  are connected together in parallel and connected in series with the current-voltage conversion unit  155 . When the oscillation circuit  52  supplies a signal to the signal electrodes  22  and  132 , charge is accumulated in the pairs of electrodes  21  and  131 . A sum of the capacitances of the pairs of electrodes  21  and  131  changes correlatively to the concentration of ethanol in the mixed fuel. The current value of the current-voltage conversion unit  155  changes correlatively to the capacitances of the pairs of electrodes  21  and  131 , that is, the concentration of ethanol in the mixed fuel. Thus, the current-voltage conversion unit  155  converts the current value of the current-voltage conversion unit  155  into a signal (voltage value) and outputs the signal to the rectifying unit  56 . The voltage value of the signal input to the rectifying unit  56  is correlated to the sum of the capacitances of the pairs of electrodes  21  and  131 , that is, the concentration of ethanol. As a result, a signal correlated to the concentration of ethanol is input to the arithmetic unit  60 . 
     In the third state, the arithmetic unit  60  uses the signal received from the amplifying unit  58  to determine the concentration of ethanol in the mixed fuel. Like the sensor system  2 , this configuration can increase a change in the capacitance associated with a change in the concentration of ethanol (that is, the configuration can increase a sensitivity of the sensor  110 ). Consequently, the arithmetic unit  60  can appropriately detect the concentration of ethanol. 
     In the fourth state, both the switches S 1  and S 2  are off, and the switch S 3  is on. In the fourth state, the current-voltage conversion unit  155  is connected directly to the oscillation circuit  52  (without intervention of the pair of electrodes  21  or  131 ), with no signal input to the pairs of electrodes  21  and  131 . As a result, a signal that does not change depending on the concentration of ethanol in the mixed fuel is input to the arithmetic unit  60  via the current-voltage conversion unit  155  and the rectifying unit  56 . 
     The control unit consecutively switches from the first state through the fourth state. The control unit also repeats the consecutive switching from the first state through the fourth state. The control unit transmits a current state of the sensor system  2  to the arithmetic unit  60 . 
     In the first state, the signal input to the arithmetic unit  60  (hereinafter referred to as a “first correlation value”) is correlated to the capacitance of the pair of electrodes  21 . In the second state, the signal input to the arithmetic unit  60  (hereinafter referred to as a “second correlation value”) is correlated to the capacitance of the pair of electrodes  131 . The signal input to the electrodes  21  in the first state is the same as the signal input to the electrodes  131  in the second state. The sensor system  102  is consecutively switched from the first state to the second state. That is, the first correlation value and the second correlation value are consecutively input to the arithmetic unit  60 . While the sensor system  102  is operating normally, the consecutively input first and second correlation values are substantially equal. 
     When the first correlation value and the second correlation value are consecutively input to the arithmetic unit  60 , the arithmetic unit  60  compares the input first and second correlation values with each other. If the first correlation value is different from the second correlation value, at least one of the first correlation value and the second correlation value is not correlated to the concentration of ethanol in the mixed fuel. In this case, the arithmetic unit  60  determines that at least one of the pair of electrodes  21  and the pair of electrodes  131  is suffering from a defect such as damage or adhesion of a foreign substance. Like the sensor system  2 , this configuration can prevent the ECU  70  from controlling the units of the sensor based on an incorrect concentration of ethanol. 
     Furthermore, the arithmetic unit  60  compares the voltage value of the signal input when the sensor system  102  is in the fourth state with a voltage value stored in the arithmetic unit  60 . If the voltage value of the input signal is different from the voltage value stored in the arithmetic unit  60 , the arithmetic unit  60  determines that a defect (damage to any conducting wire, a failure in any of the units  52 ,  56 , and  58 , or the like) is occurring in the sensor system  102 . In this case, the arithmetic unit  60  outputs a signal indicating that a defect is occurring in the sensor system  102 , to the ECU  70 . This configuration can determine that a defect is occurring in the sensor system  102  except for the sensor  110 . As a result, the ECU  70  can be prevented from controlling the units of the sensor based on an incorrect concentration of ethanol. 
     Third embodiment 
     A sensor system according to a third embodiment will be described with reference to  FIG. 3 . As shown in  FIG. 3 , the sensor system according to the third embodiment includes a liquid quality sensor  80   a , a liquid level sensor  80   b , and a specifying device  88  connected to the sensors  80   a  and  80   b.    
     The liquid quality sensor  80   a  has substantially the same configuration as the configuration of the sensor  11  according to the first embodiment except that the liquid quality sensor  80   a  further includes a liquid temperature measurement unit  86   a  configured to measure the temperature of the mixed fuel. That is, the liquid quality sensor  80   a  includes a first pair of electrodes  82   a  configured to measure quality of the mixed fuel (that is, an ethanol concentration of the mixed fuel), a second pair of electrodes  84   a  configured to measure the quality of the mixed fuel (that is, the ethanol concentration of the mixed fuel), and the liquid temperature measurement unit  86   a  configured to measure the temperature of the mixed fuel. The first pair of electrodes  82   a  and the second pair of electrodes  84   a  have a same configuration as the configuration of the pairs of electrodes  21  and  31  of the sensor  11  according to the first embodiment. The liquid temperature measurement unit  86   a  is a resistance heat-sensitive element such as a thermistor. 
     The liquid quality sensor  80   a  is disposed on an upper surface of a fuel tank configured to store the mixed fuel. The liquid quality sensor  80   a  is contained in a case to which surplus fuel from a pressure regulator is supplied. That is, the fuel tank includes a fuel pump disposed therein and configured to supply the mixed fuel in the fuel tank to an engine, and the pressure regulator disposed in the fuel tank and configured to adjust a pressure of the mixed fuel discharged from the fuel pump. The pressure regulator is connected via piping to the case containing the liquid quality sensor  80   a  so that surplus fuel from the pressure regulator is fed into the case through the piping. The surplus fuel fed into the case is returned to the fuel tank through a discharge port formed in the case. Consequently, the liquid quality sensor  80   a  comes into contact with the surplus fuel fed into the case to measure quality and a temperature of the surplus fuel. When the fuel pump stops operation, the feeding of the surplus fuel into the case containing the liquid quality sensor  80   a  is stopped. Thus, the mixed fuel in the case is discharged to outside of the case through the discharge port. As a result, the liquid quality sensor  80   a  is exposed to atmosphere. 
     The liquid level sensor  80   b  has a configuration similar to the configuration of the liquid quality sensor  80   a  to measure the level and temperature of the mixed fuel in the fuel tank. That is, the liquid level sensor  80   b  includes a first pair of electrodes  82   b  configured to measure the level of the mixed fuel, a second pair of electrodes  84   b  configured to measure the level of the mixed fuel, and a liquid temperature measurement unit  86   b  configured to measure the temperature of the mixed fuel. The liquid level sensor  80   b  is disposed in the fuel tank to measure the level of the mixed fuel in the fuel tank. Furthermore, the first pair of electrodes  82   b  and the second pair of electrodes  84   b  are disposed so as to extend from a position close to a lower surface of the fuel tank to a position close to an upper surface of the fuel tank. 
     The specifying device  88  is connected to the liquid quality sensor  80   a  and the liquid level sensor  80   b . Signals output by the sensors  80   a  and  80   b  are input to the specifying device  88 . The specifying device  88  is also connected to an external power source (not shown in the drawings) and an engine ECU. Power from the external power source is supplied to the specifying device  88 . The power supplied to the specifying device  88  is fed to the liquid temperature measurement unit  86   a  of the liquid quality sensor  80   a  and to the liquid temperature measurement unit  86   b  of the liquid level sensor  80   b . The specifying device  88  includes an oscillation circuit  98 , a sensor output selecting circuit  97 , a sensor input selecting circuit  91 , a signal amplifying unit  90 , an input selecting unit  92 , an AD converter  94 , an arithmetic unit  96 , a reference equivalent circuit  100 , and a temperature compensating circuit  102 . 
     The oscillation circuit  98  generates a signal (AC voltage) using power supplied by the external power source. The sensor output selecting circuit  97  selects a target to which the signal generated by the oscillation circuit  98  is output. That is, the sensor output selecting circuit  97  outputs the signal (AC voltage) from the oscillation circuit  98  to either the pairs of electrodes  82   a  and  84   a  of the liquid quality sensor  80   a  or the pairs of electrodes  82   b  and  84   b  of the liquid level sensor  80   b  or the reference equivalent circuit  100 . The target to which the signal is output is selected by the sensor output selecting circuit  97  based on an instruction from the arithmetic unit  96 . The sensor input selecting circuit  91  selects either signals output from the pairs of electrodes  82   a  and  84   a  of the liquid quality sensor  80   a  or signals output from the pairs of electrodes  82   b  and  84   b  of the liquid level sensor  80   b  or a signal output from the reference equivalent circuit  100 . The sensor input selecting circuit  91  then inputs the selected signals to the signal amplifying unit  90 . The signal amplifying unit  90  amplifies the signals selected by the sensor input selecting circuit  91 . The input selecting unit  92  selects either a signal received from the signal amplifying unit  90 , a signal received from the liquid temperature measurement unit  86   a  of the liquid quality sensor  80   a  or a signal received from the liquid temperature measurement unit  86   b  of the liquid level sensor  80   b . The input selecting unit  92  then outputs the selected signal to the AD converter  94 . The AD converter  94  converts the signal (analog signal) output from the input selecting unit  92  into a digital signal. The arithmetic unit  96  processes the signal received from the AD converter  94  to determine the quality and level of the mixed fuel, and outputs the determined quality and level to the engine ECU. Furthermore, as described below, the arithmetic unit  96  uses outputs from the reference equivalent circuit  100  and the temperature compensating circuit  102  to carry out a process of correcting (calibrating) the signal output from the liquid quality sensor  80   a  and the signal output from the liquid level sensor  80   b.    
     First, the reference equivalent circuit  100  and the temperature compensating circuit  102  will be described. The reference equivalent circuit  100  includes a reference capacitance  100   a  and a reference resistance  100   b . The reference capacitance  100   a  and the reference resistance  100   b  form a reference equivalent circuit (RC circuit) corresponding to a circuit (that is, an RC circuit) including the pairs of electrodes  82   a  and  84   a  of the liquid quality sensor  80   a  and the pairs of electrodes  82   b  and  84   b  of the liquid level sensor  80   b . The signal from the oscillation circuit  98  is input to the reference equivalent circuit  100 . A signal output from the reference equivalent circuit  100  is input to the arithmetic unit  96  via the input selecting unit  92  and the AD converter  94 . 
     The temperature compensating circuit  102  includes a circuit temperature measurement unit  102   a  and a power voltage measurement section  102   b . The circuit temperature measurement unit  102   a  is a resistance element such as a thermistor which measures a temperature of the specifying device  88 . The power voltage measurement section  102   b  measures a voltage of power supplied to the liquid temperature measurement unit  86   a  of the liquid quality sensor  80   a  and the liquid temperature measurement unit  86   b  of the liquid level sensor  80   b . A signal (the temperature of the specifying device  88 ) output from the circuit temperature measurement unit  102   a  and a signal output from the power voltage measurement section  102   b  are input to the arithmetic unit  96  via the input selecting unit  92  and the AD converter  94 . 
     Now, a sensor output correcting process carried out by the arithmetic unit  96  will be described. The arithmetic unit  96  corrects a signal output from the liquid quality sensor  80   a  and a signal output from the liquid level sensor  80   b  based on the signal (that is, the temperature of the specifying device  88 ) output from the circuit temperature measurement unit  102   a  and the signal (that is, the power voltage) output from the power voltage measurement section  102   b . That is, a change in the temperature of the specifying device  88  changes a frequency of the signal output from the oscillation circuit  98 . A change in frequencies of the signals input to the liquid quality sensor  80   a  and the liquid level sensor  80   b  changes the signals output from the pairs of electrodes  82   a  and  84   a  of the liquid quality sensor  80   a  and the pairs of electrodes  82   b  and  84   b  of the liquid level sensor  80   b . Thus, the arithmetic unit  96  corrects the signals output from the pairs of electrodes  82   a  and  84   a  as well as  82   b  and  84   b  based on the temperature of the specifying device  88 . For example, the arithmetic unit  96  prestores a correction factor associated with the temperature of the specifying device and multiplied by the signals output from the pairs of electrodes  82   a  and  84   a  as well as  82   b  and  84   b.    
     Furthermore, a change in the power voltage supplied to the liquid temperature measurement units  86   a  and  86   b  of the liquid quality sensor  80   a  and the liquid level sensor  80   b  changes voltages of the signals of the liquid temperature measurement units  86   a  and  86   b . That is, the liquid temperature measurement unit  86   a  and  86   b  are resistors, and thus, a change in the voltage of the input power changes the voltage of the output signal. Hence, the arithmetic unit  96  corrects the voltages of the signals of the liquid temperature measurement units  86   a  and  86   b  based on the power voltage. For example, the arithmetic unit  96  multiplies the signals output from the liquid temperature measurement units  86   a  and  86   b  by the correction factor associated with the power voltage. The arithmetic unit  96  determines that at least one of the liquid temperature measurement units  86   a  and  86   b  is at fault if the signal output from the liquid temperature measurement unit  86   a  is significantly different from the signal output from the liquid temperature measurement unit  86   b . A signal indicating that the liquid temperature measurement unit  86   a  or  86   b  is at fault is output from the arithmetic unit  96  to the outside (engine ECU or the like). 
     Moreover, the arithmetic unit  96  corrects the signals output from the pair of electrodes  82   a  and  84   a  of the liquid quality sensor  80   a  and the pair of electrodes  82   b  and  84   b  of the liquid level sensor  80   b  based on the signal output from the reference equivalent circuit  100 . That is, a secular change or the like in the specifying device  88  may change the signal output from the oscillation circuit  98 . A change in the signal output from the oscillation circuit  98  changes the signal output from the reference equivalent circuit  100  and the signals output from the pairs of electrodes  82   a  and  84   a  as well as  82   b  and  84   b . Here, the reference capacitance  100   a  and reference resistance  100   b  of the reference equivalent circuit  100  are known, allowing a frequency or the like of the signal input to the reference equivalent circuit  100  to be determined. Thus, the arithmetic unit  96  determines the frequency or the like of the signal input to the reference equivalent circuit  100  to correct the signals output from the pairs of electrodes  82   a  and  84   a  as well as  82   b  and  84   b  based on the determined frequency or the like. The arithmetic unit  96  determines that the pair of electrodes  82   a ,  84   a ,  82   b  or  84   b  is at fault if the signal output from the pair of electrodes  82   a  is significantly different from the signal output from the pair of electrodes  84   a  or if the signal output from the pair of electrodes  82   b  is significantly different from the signal output from the pair of electrodes  84   b . A signal indicating that the pair of electrodes  82   a  and  84   a  or  82   b  and  84   b  is at fault is output to the outside (engine ECU or the like) by the arithmetic unit  96 . 
     Moreover, while the fuel pump is at rest, the arithmetic unit  96  corrects the signal output from the pair of electrodes  82   a  and  84   a  of the liquid quality sensor  80   a  based on the signal output from the pair of electrodes  82   a  and  84   a  of the liquid quality sensor  80   a . That is, while the fuel pump is at rest, the mixed fuel is discharged from inside the case containing the liquid quality sensor  80   a , with the pair of electrodes  82   a  and  84   a  of the liquid quality sensor  80   a  exposed to the atmosphere. A dielectric constant of the atmosphere (air) is known, and thus, a capacitance of the pair of electrodes  82   a  and  84   a  can be estimated. On the other hand, an individual difference among liquid quality sensors  80   a  or a secular change in the liquid quality sensor  80   a  may cause a capacitance of the liquid quality sensor  80   a  to deviate from a design value. In such a case, the estimated capacitance of the pair of electrodes  82   a  and  84   a  is different from the capacitance of the pair of electrodes  82   a  and  84   a  actually measured while the fuel pump is at rest. Thus, the arithmetic unit  96  determines a correction factor for correcting the signal output from the pair of electrodes  82   a  and  84   a  of the liquid quality sensor  80   a  based on the signal output from the pair of electrodes  82   a  and  84   a  of the liquid quality sensor  80   a  while the fuel pump is at rest. This eliminates a variation in measured value caused by the individual difference among liquid quality sensors  80   a  or the secular change in the liquid quality sensor  80   a.    
     Moreover, the arithmetic unit  96  corrects the signal output from the pair of electrodes  82   b  and  84   b  of the liquid level sensor  80   b  based on the signal output from the pair of electrodes  82   b  and  84   b  of the liquid level sensor  80   b  while the fuel tank is full of fuel. That is, while the fuel tank is full of fuel, all of the pair of electrodes  82   b  and  84   b  of the liquid level sensor  80   b  is immersed in the fuel. On the other hand, a dielectric constant of the fuel can be determined based on the output from the liquid quality sensor  80   a , allowing a capacitance of the pair of electrodes  82   b  and  84   b  to be estimated. On the other hand, an individual difference among liquid level sensors  80   b  or a secular change in the liquid level sensor  80   b  may cause a capacitance of the liquid level sensor  80   b  to deviate from a design value. In such a case, the estimated capacitance of the pair of electrodes  82   b  and  84   b  is different from the capacitance of the pair of electrodes  82   b  and  84   b  actually measured while the fuel tank is full of fuel. Thus, the arithmetic unit  96  determines a correction factor for correcting the signal output from the pair of electrodes  82   b  and  84   b  of the liquid level sensor  80   b  based on the signal output from the pair of electrodes  82   b  and  84   b  of the liquid level sensor  80   b  while the fuel tank is full of fuel. This eliminates a variation in measured value caused by the individual difference among liquid level sensors  80   b  or the secular change in the liquid level sensor  80   b.    
     As described above, the sensor system according to the third embodiment corrects the signals output from the sensors  80   a  and  80   b , and can thus accurately measure the quality and level of the fuel in the fuel tank. The sensor system according to the first embodiment can also self-diagnose a possible failure in the sensors  80   a  and  80   b  and a possible failure in the specifying device  88 . 
     Variation 
     (1) According to the above-described embodiments, if the signal from the oscillation circuit  52  is supplied to the two pairs of electrodes  21  and  31  or the like provided on the upper and lower surfaces, respectively, of the substrate  11 , charge with the same capacitance is accumulated in the pairs of electrodes  21  and  31 . According to the first embodiment, each of the electrodes  22  and  24  of the pair of electrodes  21  has the same shape as the shape of each of the electrodes  32  and  34  of the pair of electrodes  31 . The distance between the two electrodes  22  and  24  is equal to the distance between the two electrodes  32  and  34 . However, the capacitance of the pair of electrodes on the upper surface of the substrate need not be the same as the capacitance of the pair of electrodes on the lower surface of the substrate. In this case, the two pairs of electrodes may be provided such that the capacitance of the pair of electrodes on the upper surface of the substrate is correlated (positively correlated) to the capacitance of the pair of electrodes on the lower surface of the substrate. For example, the two pairs of electrodes may be provided such that the capacitance of the pair of electrodes on the upper surface of the substrate is double the capacitance of the pair of electrodes on the lower surface of the substrate. According to this configuration, if the first correlation value (a value correlated to the capacitance of the pair of electrodes on the upper surface of the substrate) is not substantially double the second correlation value (a value correlated to the capacitance of the pair of electrodes on the lower surface of the substrate), the arithmetic unit  60  may determine that at least one of the two pairs of electrodes is suffering from a defect such as damage or adhesion of a foreign substance. 
     (2) According to the above-described embodiments, the capacitances of the pair of electrodes  21  or the like change correlatively to the concentration of ethanol in the mixed fuel. However, the pair of electrodes  21  or the like may be provided such that the capacitance of the pair of electrode  21  or the like changes correlatively to the level of the fuel. 
     (3) According to the above-described embodiments, the arithmetic unit  60  compares the first correlation value with the second correlation value to determine that at least one of the two pairs of electrodes  21  and  31  or the like is suffering from a defect. The arithmetic unit  60  then outputs, to the ECU  70 , the signal indicating that at least one of the two pairs of electrodes  21  and  31  or the like is suffering from a defect. However, the arithmetic unit  60  may compare the first correlation value with the second correlation value to output a comparison result. For example, the arithmetic unit  60  may output “0” if the two correlation values are equal and output “1” if the two correlation values are different. In this case, the ECU  70  may determine whether or not at least one of the two pairs of electrodes  21  and  31  or the like is suffering from a defect, depending on the value received from the arithmetic unit  60 . Alternatively, the ECU  70  may light a lamp indicating a defect only if the ECU  70  receives the value “1” from the arithmetic unit  60 . 
     Fourth Embodiment 
     (Configuration of the Sensor System  1200 ) 
     As shown in  FIG. 4 , a sensor system  1200  includes a sensor  1010  and a specifying device  1500 . The sensor  1010  is disposed inside a fuel tank (not shown in the drawings), and the specifying device  1500  is disposed outside the fuel tank. The sensor system  1200  is used to determine the ethanol concentration of a mixed fuel containing the ethanol. 
     (Configuration of the Specifying Device  1500 ) 
     The specifying device  1500  includes an oscillation circuit  52 , a resistance unit  54 , a rectifying unit  56 , a signal amplifying unit  58 , an arithmetic unit (MCU)  60 , and a connection unit  1600 . 
     The oscillation circuit  52  generates a signal (voltage) with a specific frequency. The oscillation circuit  52  is connected to the connection unit  1600  via the resistance unit  54 . The connection unit  1600  is connected to the sensor unit  1010  and the rectifying unit  56 . The sensor unit  1010  is connected to the connection unit  1600  at connecting units  1014   c ,  1014   d ,  1012   c , and  1012   d . A specific configuration of the sensor unit  1010  will be described below. 
     The connection unit  1600  includes switches  1610 ,  1620 , and  1630  as a switching device configured to switch a connection state. The switch  1610  switches between a state in which the oscillation circuit  52  and the rectifying unit  56  are connected to a terminal T 10  and a state in which the oscillation circuit  52  and the rectifying unit  56  are connected to a terminal T 11 . The switch  1620  switches between a state in which the connecting unit  1012   d  is connected to a terminal T 20  and a state in which the connecting unit  1012   d  is connected to a terminal T 21 . The switch  1630  switches between a state in which the connecting unit  1012   c  is connected to a terminal T 30  and a state in which the connecting unit  1012   c  is connected to a terminal T 31 . 
     The rectifying unit  56  is connected to the switch  1610 . A signal (voltage) at the switch  1610  is input to the rectifying unit  56 . The rectifying unit  56  rectifies the input signal (voltage). The signal rectified by the rectifying unit  56  is input to the amplifying unit  58 . The amplifying unit  58  amplifies the input signal (voltage). The signal (voltage) at the switch  1610  is input to the arithmetic unit  60  via the rectifying unit  56  and the amplifying unit  58 . The resistance unit  54  has a constant resistance value, and thus, an output value of a voltage signal output to the arithmetic unit  60  changes depending on a capacitance C of the sensor unit  1010 . The arithmetic unit  60  can calculate the capacitance C of the sensor unit  1010  from the output value of the voltage signal output to the arithmetic unit  60 . The capacitance C of the sensor unit  1010  changes depending on the concentration of ethanol contained in the mixed fuel in which the sensor unit  1010  is immersed. The arithmetic unit  60  stores databases that allow the concentration of ethanol to be calculated using the capacitance C obtained. The databases are pre-created based on tests or the like and stored in the arithmetic unit  60 . 
     (Configuration of the Sensor Unit  1010 ) 
     The sensor unit  1010  includes a first pair of electrodes  1014  formed on an upper surface  1021  of a substrate  1020  and a second pair of electrodes  1012  formed on a lower surface  1022  of the substrate  1020 . The substrate  1020  is formed using a dielectric as a material and formed of polyimide according to the present embodiment. However, the substrate  1020  may be formed of another dielectric material such as poly phenylene sulfide resin (PPS). The substrate  1020  is a rectangular plate. The upper surface  1021  and the lower surface  1022  are opposite surfaces of the substrate  1020 . The first pair of electrodes  1014  has a comb-like first electrode  1014   a  and a comb-like second electrode  1014   b  disposed on the upper surface  1021  apart from each other. The second pair of electrodes  1012  has a comb-like third electrode  1012   a  and a comb-like fourth electrode  1012   b  disposed on the lower surface  1022  apart from each other. The first electrode  1014   a , the second electrode  1014   b , the third electrode  1012   a , and the fourth electrode  1012   b  are connected to the connection unit  1600  at the connecting units  1014   c ,  1014   d ,  1012   c , and  1012   d , respectively. The connecting unit  1014   c  is grounded. The connecting unit  1014   d  is connected between the resistance unit  54  and the switch  1610 . The connecting unit  1012   c  is connected to the switch  1630 . The connecting unit  1012   d  is connected to the switch  1620 . 
     As shown in  FIG. 4 , the first electrode  1014   a  includes a handle unit extending, in a negative direction of a Y axis, from the connecting unit  1014   c  along an end of the upper surface  1021  located in a positive direction of an X axis, and a comb tooth unit extending from the handle unit in a negative direction of the X axis. The second electrode  1014   b  includes a handle unit extending, in the negative direction of the Y axis, from the connecting unit  1014   d  along an end of the upper surface  1021  located in the negative direction of the X axis, and a comb tooth unit extending from the handle unit in the positive direction of the X axis. The comb tooth unit of the first electrode  1014   a  and the comb tooth unit of the second electrode  1014   b  are spaced at intervals in the Y direction and are partly opposite to each other. The third electrode  1012   a  includes a handle unit extending, in the negative direction of the Y axis, from the connecting unit  1012   c  along an end of the lower surface  1022  located in the positive direction of the X axis, and a comb tooth unit extending from the handle unit in the negative direction of the X axis. The fourth electrode  1012   b  includes a handle unit extending, in the negative direction of the Y axis, from the connecting unit  1012   d  along an end of the lower surface  1022  located in the negative direction of the X axis, and a comb tooth unit extending from the handle unit in the positive direction of the X axis. The comb tooth unit of the third electrode  1012   a  and the comb tooth unit of the fourth electrode  1012   b  are spaced at intervals in the Y direction and are partly opposite to each other. 
     As shown in  FIG. 4  and  FIG. 5 , the first electrode  1014   a  and the third electrode  1012   a  are disposed so as to pass through a center of the substrate  1020  in a direction of a Z axis and positioned symmetrically with respect to a surface perpendicular to the Z axis (the surface shown by a segment A-B in  FIG. 5 ). That is, the third electrode  1012   a  is formed at a position opposite to the first electrode  1014   a  via the substrate  1020 . Furthermore, the second electrode  1014   b  and the fourth electrode  1012   b  are disposed so as to pass through the center of the substrate  1020  in the direction of the Z axis and positioned symmetrically with respect to the surface perpendicular to the Z axis. That is, the fourth electrode  1012   b  is formed at a position opposite to the second electrode  1014   b  via the substrate  1020 . 
     (Method for Measuring the Capacitance) 
     The specifying device  1500  has a control unit (not shown in the drawings) configured to supply power to the sensor unit  1010  and to measure a capacitance C of the sensor unit  1010 . Specifically, the control unit of the specifying device  1500  switches the switch  1610  and the like of the connection unit  1600  to manipulate connections among the first electrode  1014   a , second electrode  1014   b , third electrode  1012   a , and fourth electrode  1012   b  of the sensor unit  1010  to measure the capacitance C of the sensor unit  1010 . 
     First, as shown in  FIG. 1 , the specifying device  1500  connects the switch  1610  to the terminal T 11 , the switch  1620  to the terminal T 21 , and the switch  1630  to the terminal T 30  to obtain a first output. The arithmetic unit  60  then uses the first output to calculate the capacitance C of the sensor unit  1010 . In this case, the first electrode  1014   a  and the fourth electrode  1012   b  are grounded to provide a first potential V 1  (0 V). Furthermore, the oscillation circuit  52  applies a voltage to the second electrode  1014   b  and the third electrode  1012   a , which provide a second potential V 2  (V 2 ≠V 1 =0). The first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel. As a result, a potential difference with a magnitude V 1  occurs between the opposite electrodes on the upper surface  1021  or lower surface  1022  of the substrate  1020  (between the first electrode  1014   a  and the second electrode  1014   b  and between the third electrode  1012   a  and the fourth electrode  1012   b ). Moreover, a potential difference with the magnitude V 1  also occurs between the electrodes opposite to each other with the substrate  1020  in between (between the first electrode  1014   a  and the third electrode  1012   a  and between the second electrode  1014   b  and the fourth electrode  1012   b ). A capacitive component between the opposite electrodes on the upper surface  1021  or lower surface  1022  of the substrate  1020  (a capacitance of this capacitive component is denoted by Cy) is connected in parallel with a capacitive component between the electrodes opposite to each other with the substrate  1020  in between (a capacitance of this capacitive component is denoted by Cz). Thus, the capacitance C of the sensor unit  1010  is obtained by C( 1 )−Cy+Cz. A voltage signal corresponding to the capacitance C is output to the arithmetic unit  60  as the first output. 
     Then, on the connection unit  1600 , the specifying device  1500  connects the switch  1610  to the terminal T 10 , the switch  1620  to the terminal T 20 , and the switch  1630  to the terminal T 31  to obtain a second output. The arithmetic unit  60  then uses the second output to calculate the capacitance C of the sensor unit  1010 . In this case, the first electrode  1014   a  and the third electrode  1012   a  are grounded to provide the first potential. The oscillation circuit  52  applies a voltage to the second electrode  1014   b  and the fourth electrode  1012   b , which provide the second potential V 2 . The first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel. Thus, a potential difference with the magnitude V 1  occurs between the opposite electrodes on the upper surface  1021  or lower surface  1022  of the substrate  1020 . On the other hand, the electrodes opposite to each other with the substrate  1020  in between are at the same potential. As a result, the capacitance Cz of the capacitive component between the electrodes opposite to each other with the substrate  1020  in between is such that Cz≅0. Only the capacitance Cy of the capacitive component between the opposite electrodes on the upper surface  1021  or lower surface  1022  of the substrate  1020  is detected as the capacitance C( 2 ) of the sensor unit  1010  (C( 2 )=Cy). A voltage signal corresponding to the capacitance C is output to the arithmetic unit  60  as the second output. 
     The specifying device  1500  calculates the concentration of ethanol contained in the mixed fuel and an electric conductivity of the mixed fuel based on the capacitance C( 1 ) obtained from the first output and the capacitance C( 2 ) obtained from the second output. An example of a method for calculating the electric conductivity will be described with reference to  FIG. 11  to  FIG. 13 .  FIG. 11  is a diagram showing a relation between the first output provided to the arithmetic unit  60  in association with the capacitance C( 1 ) and the concentration of ethanol contained in the mixed fuel. An impact of the electric conductivity of the measured liquid on the first output is sufficiently small, and thus, an ethanol concentration E( 1 ) of the mixed fuel can be determined based on the relation shown in  FIG. 11 .  FIG. 12  is a diagram showing a relation at respective ethanol concentrations between the second output provided to the arithmetic unit in association with the capacitance C( 2 ) and the electric conductivity of the mixed fuel. As shown in  FIG. 12 , the second output changes under an effect of the electric conductivity of the mixed fuel. At the same ethanol concentration, the second output increases and decreases consistently with the electric conductivity. If the ethanol concentration is E( 1 ), the second output changes by x1 within a range of the electric conductivity shown in  FIG. 12 . Similarly, at ethanol concentrations E 100  and E 0 , the second output changes by x100 and by x0, respectively.  FIG. 13  is a diagram showing a relation between the second output provided to the arithmetic unit  60  in association with the capacitance C( 2 ) and the concentration of ethanol contained in the mixed fuel. A straight line shown at reference numeral  1  shows a relation between the second output and the ethanol concentration obtained if the electric conductivity is maximum within the range of electric conductivity shown in  FIG. 12 . A straight line shown at reference numeral  2  shows a relation between the second output and the ethanol concentration obtained if the electric conductivity is σ( 2 ). A straight line shown at reference numeral  3  shows a relation between the second output and the ethanol concentration obtained if the electric conductivity is minimum within the range of electric conductivity shown in  FIG. 12 . When the first output and the second output are measured for the mixed fuel, then first, the ethanol concentration E( 1 ) of the mixed fuel can be determined based on the relation shown in  FIG. 11 . Then, based on the relation, shown in  FIG. 12 , between the electric conductivity and the second output at the ethanol concentration E( 1 ) obtained, the electric conductivity σ( 2 ) can be determined. Relations between the second output and the electric conductivity at ethanol concentrations which are not shown in  FIG. 12  can be determined by compensation using relations between the second output and the electric conductivity at a plurality of ethanol concentrations (for example, ethanol concentrations E 1  and E 100 ). The relations between the output and the electric conductivity at the respective ethanol concentrations as shown in  FIG. 12  can be pre-checked by experiments or the like and are stored in the specifying device  1500  as data. Using the relations shown in  FIG. 12 , the specifying device  1500  can calculate the electric conductivity of the mixed fuel based on the first output and the second output. 
     As described above, the connection unit  1600  of the sensor  1200  includes the switches  1610 ,  1620 , and  1630  as a switching device configured to be able to switch between the state in which the first electrode  1014   a  and the fourth electrode  1012   b  are at the first potential V 1  and in which the second electrode  1014   b  and the third electrode  1012   a  are at the second potential V 2  (V 2 ≠V 1 ) and the state in which the first electrode  1014   a  and the third electrode  1012   a  are at the first potential V 1  and in which the second electrode  1014   b  and the fourth electrode  1012   b  are at the second potential V 2 . The specifying device  1500  switches the connection unit  1600  between the two states to supply power to the sensor unit  1010  and measures the capacitance C in the respective states. In the state in which the first electrode  1014   a  and the fourth electrode  1012   b  are at the first potential V 1  and in which the second electrode  1014   b  and the third electrode  1012   a  are at the second potential V 2 , the capacitance C( 1 ) of the sensor unit  1010  increases by the capacitance Cz obtained between the electrodes partly opposite to each other with the substrate in between. A dielectric loss tan δ of the sensor unit  1010  is in inverse proportion to a product of the capacitance C and a parasitic resistance R 2  of the sensor unit  1010 . Thus, an increase in capacitance C reduces the dielectric loss tan δ and an adverse effect of the parasitic resistance R 2  (that is, an adverse effect of the electric conductivity of the measured liquid). The capacitance C can be measured with the dielectric loss tan δ reduced, thus improving measurement accuracy for the capacitance. 
     Furthermore, the connection unit  1600  switches the switches  1610 ,  1620 , and  1630  to allow the capacitance C( 2 ) to be measured in the state in which the first electrode  1014   a  and the third electrode  1012   a  are at the first potential V 1  and in which the second electrode  1014   b  and the fourth electrode  1012   b  are at the second potential V 2 . The capacitance C( 2 ) decreases with increasing parasitic resistance R 2 . A comparison between the capacitance C( 1 ) and the capacitance C( 2 ) enables the electric conductivity of the liquid to be calculated. The electric conductivity obtained allows the measurement accuracy for the capacitance C of the sensor unit  1010  to be improved. 
     Variation 
     The first embodiment carries out voltage detection. However, as shown in  FIG. 6 , a sensor system  1201  may carry out current detection. The sensor system  1201  is different from the sensor system  1200  in a configuration of a specifying device  1501 . In the specifying device  1501 , the oscillation circuit  52  is connected to a connection unit  1601  without the resistance unit  54  interposed between the oscillation circuit  52  and the connection unit  1601 . Furthermore, a connection unit  1301  is connected to the rectifying unit  56  via a current-voltage conversion unit (I-V conversion unit)  55 . 
     The connection unit  1601  includes switches  1640 ,  1650 ,  1660 , and  1670  as a switching device configured to be able to switch a connection state. The switch  1640  switches between connection and disconnection between the oscillation circuit  52  and a terminal T 40 . The switch  1650  switches between connection and disconnection between the oscillation circuit  52  and a terminal T 50 . The switch  1660  switches between connection and disconnection between the connecting unit  1014   d  and a terminal T 60 . The switch  1670  switches between connection and disconnection between the connecting unit  1014   d  and a terminal  770 . 
     The I-V conversion unit  55  is connected to the switch  1670 . A signal (voltage) at the switch  1670  is input to the I-V conversion unit  55 . The voltage signal input to the I-V conversion unit  55  is converted into a current signal, which is then output to the rectifying unit  56 . A remaining part of the configuration is similar to the corresponding part of configuration of the sensor system  1200  according to the fourth embodiment and will thus not be described. 
     As shown in  FIG. 6 , the specifying device  1501  connects the switch  1640  to the terminal T 40 , disconnects the switches  1650  and  1660 , and connects the switch  1670  to the terminal  170 . Thus, as is the case with the fourth embodiment, the capacitance C of the sensor unit  1010  can be measured as the capacitance C( 1 ), which is obtained if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. 
     Furthermore, the specifying device  1501  disconnects the switches  1640  and  1670 , connects the switch  1650  to the terminal T 50 , and connects the switch  1660  to the terminal T 60 . Thus, as is the case with the first embodiment, the capacitance C of the sensor unit  1010  can be measured as the capacitance C( 2 ), which is obtained if no potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. As is the case with the first embodiment, switching the switches  1640 ,  1650 ,  1660 , and  1670  of the connection unit  1601  allows measurement of the capacitance C( 1 ), which is obtained if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between, and the capacitance C( 2 ), which is obtained if no potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. 
     Fifth Embodiment 
     As shown in  FIG. 7 , a sensor system  1202  is different from the sensor system  1200  in a configuration of a connection unit  1602  in a specifying device  1502 . The connection unit  1602  is configured to be able to switch between a state in which a first pair of electrodes  1014  and a second pair of electrodes  1012  are connected together in parallel and a state in which the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series. 
     The connection unit  1602  includes switches  1680 ,  1690 ,  1700 ,  1712 , and  1714  as a switching device configured to be able to switch a connection state. The switch  1680  switches between a state in which an oscillation circuit  52  and a rectifying unit  56  are connected to a terminal T 80  and a state in which the oscillation circuit  52  and the rectifying unit  56  are connected to a terminal T 81 . The switch  1690  switches between a state in which a connecting unit  1014   c  is connected to a terminal T 90  and a state in which the connecting unit  1014   c  is connected to a terminal T 91 . The switch  1700  switches between a state in which a connecting unit  1012   d  is connected to a terminal T 100  and a state in which the connecting unit  1012   d  is connected to a terminal T 101 . The switch  1712  switches connection and disconnection between the switch  1680  and a terminal T 102 . The switch  1714  switches between connection and disconnection between a connecting unit  1014   d  and a terminal T 104 . The connecting unit  1014   c  of a sensor unit  1010  is connected to the switch  1690 . A connecting unit  1014   d  is connected to the switch  1714  and the terminal T 80 . A connecting unit  1012   c  is connected to the terminal T 102  and a rectifying unit  56 . A connecting unit  1012   d  is connected to the switch  1700 . A remaining pan of the configuration is similar to the corresponding part of configuration of the sensor system  1200  according to the first embodiment and will thus not be described. 
     The specifying device  1502  switches between the state in which the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel and the state in which the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series, to supply power to the sensor unit  1010 . The specifying device  1502  then measures the capacitance C of the sensor unit  1010 . 
     As shown in  FIG. 4 , the specifying device  1502  connects the switch  1680  to the terminal T 81 , connects the switch  1690  to the terminal  191 , connects the switch  1700  to the terminal T 101 , connects the switch  1712  to the terminal T 102 , and disconnects the switch  1714 . Thus, a first electrode  1014   a  and a fourth electrode  1012   b  are grounded to provide a first potential V 1  (0V). Furthermore, the oscillation circuit  52  applies a voltage to a second electrode  1014   b  and a third electrode  1012   a , which provide a second potential V 2  (V 2 ≠V 1 =0). The first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel. A capacitance Czp of a capacitive component between the electrodes opposite to each other with a substrate  1020  in between is such that Czp≠0. A capacitance C of the sensor unit  1010  is measured as C( 1 )=Cyp+Czp. A suffix p in Cyp and Czp represents a measured value obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel. 
     Furthermore, the specifying device  1502  connects the switch  1680  to the terminal T 80 , connects the switch  1690  to the terminal T 90 , connects the switch  1700  to the terminal T 100 , disconnects the switch  1712 , and connects the switch  1714  to the terminal T 104 . Thus, the first electrode  1014   a  is connected to the oscillation circuit  52 , the second electrode  1014   b  is connected to the fourth electrode  1012   b , the third electrode  1012   a  is connected to the rectifying unit  56 , and the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series. A capacitance Czs of a capacitive component between the electrodes opposite to each other with the substrate  1020  in between is such that Czs≠0. The capacitance C of the sensor unit  1010  is measured as a capacitance C( 3 ) that can be calculated by an expression 1/C( 3 )=1/Cys+1/Czs. A suffix s in Cys and Czs represents a measured value obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series. The specifying device  1502  calculates a concentration of ethanol contained in a mixed fuel and an electric conductivity of the mixed fuel based on the capacitance C( 1 ) and the capacitance C( 3 ). A method for calculating the electric conductivity may be similar to the calculation method described in the first embodiment. 
     As described above, the sensor  1202  can measure the capacitance C obtained if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between, as is the case with the first embodiment. The capacitance C can be measured with a dielectric loss tan δ reduced, thus improving measurement accuracy for the capacitance. Moreover, the connection unit  1602  in the sensor  1202  is configured to be able to switch the state in which the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel and the state in which the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series. Thus, the electric conductivity of the liquid can be calculated by comparing the capacitance C( 1 ), measured during the parallel connection, with the capacitance C( 3 ), measured during the series connection. The electric conductivity obtained allows the measurement accuracy for the capacitance C of the sensor unit  1010  to be improved. 
     Variation 
     The second embodiment carries out voltage detection. However, as shown in  FIG. 8 , a sensor system  1203  may carry out current detection. The sensor system  1203  is different from the sensor system  1201  according to the variation of the first embodiment in a configuration of a connection unit  1603  in a specifying device  1503 . The connection unit  1603  includes switches  1710 ,  1720 , and  1730  as a switching device configured to be able to switch a connection state. The switch  1710  switches between connection and disconnection between the oscillation circuit  52  and a terminal T 110 . The switch  1720  switches between connection and disconnection between the connecting unit  1014   d  of the sensor unit  1010  and a terminal T 120 . The switch  1730  switches between connection and disconnection between the connecting unit  1014   d  and a terminal T 130 . The oscillation circuit  52  is connected to the connecting unit  1014   c  and the switch  1710 . The connecting unit  1012   c  is connected to the terminal T 130  and an I-V conversion unit  55 . A remaining part of the configuration is similar to the corresponding part of configuration of the sensor system  1201  and will thus not be described. 
     As shown in  FIG. 8 , the specifying device  1503  connects the switches  1710  and  1730  to the terminals T 110  and T 130 , respectively, and disconnects the switch  1720 . Thus, as is the case with the fifth embodiment, the capacitance C of the sensor unit  1010  can be measured as the capacitance C( 1 ), which is obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel and if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. 
     Furthermore, the specifying device  1503  disconnects the switches  1710  and  1730  and connects the switch  1720  to the terminal T 120 . Thus, as is the case with the second embodiment, the capacitance C of the sensor unit  1010  can be measured as the capacitance C( 3 ), which is obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series and if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. As is the case with the second embodiment, switching the switches  1710 ,  1720 , and  1730  of the connection unit  1603  allows measurement of the capacitance C( 1 ), which is obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel and if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between, and the capacitance C( 3 ), which is obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series and if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. 
     Sixth Embodiment 
     As shown in  FIG. 9 , a sensor system  1204  is different from the sensor system  1200  in a configuration of a connection unit  1604  in a specifying device  1504 . The connection unit  1604  is configured to be able to switch between a state in which a first pair of electrodes  1014  and a second pair of electrodes  1012  are connected together in parallel and a state in which the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series. The connection unit  1604  is configured to be able to switch between a state in which a potential difference occurs between electrodes opposite to each other with a substrate  1020  in between and a state in which no potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between, when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel. 
     The connection unit  1604  includes switches  1740 ,  1750 ,  1760 , and  1770  in addition to the switches  1610 ,  1620 , and  1630  according to the sensor system  1200 , as a switching device configured to be able to switch a connection state. The switch  1740  switches between a state in which an oscillation circuit  52  is connected to a terminal T 140  and a state in which the oscillation circuit is connected to the terminal T 141 . The switch  1750  switches between a state in which a connecting unit  1014   c  is connected to a terminal T 150  and a state in which the connecting unit  1014   c  is connected to a terminal T 151 . The switch  1760  switches connection and disconnection between the switch  1740  and a terminal T 160 . The switch  1770  switches connection and disconnection between a terminal T 170  and a connecting unit  1014   d  and also a terminal T 104 . The switch  1610  is connected to the terminal T 160 . The terminal T 170  is connected to a terminal T 10  and a terminal T 20 . A remaining part of the configuration is similar to the corresponding part of configuration of the sensor system  1200  according to the first embodiment and will thus not be described. 
     The specifying device  1504  switches between the state in which the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel and the state in which the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series. The specifying device  1504  also switches between the state in which a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between and the state in which no potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between, when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel. The specifying device  1504  thus supplies power to a sensor unit  1010 , and measures a capacitance C of the sensor unit  1010 . 
     As shown in  FIG. 9 , the specifying device  1504  connects the switch  1610  to a terminal T 11 , connects the switch  1620  to a terminal T 21 , connects the switch  1630  to a terminal T 30 , connects the switch  1740  to the terminal T 141 , connects the switch  1750  to the terminal T 151 , connects the switch  1760  to the terminal T 160 , and disconnects the switch  1770 . Thus, as is the case with the fourth embodiment, a first electrode  1014   a  and a fourth electrode  1012   b  are grounded to provide a first potential V 1  (0V). Furthermore, the oscillation circuit  52  applies a voltage to a second electrode  1014   b  and a third electrode  1012   a , which provide a second potential V 2  (V 2 ≠V 1 =0). The first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel. As is the case with the fourth embodiment and the fifth embodiment, the capacitance C of the sensor unit  1010  can be measured as a capacitance C( 1 ) obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel and if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. 
     Furthermore, the specifying device  1504  connects the switch  1610  to the terminal T 10 , connects the switch  1620  to the terminal T 20 , connects the switch  1630  to a terminal T 31 , connects the switch  1740  to the terminal T 141 , connects the switch  1750  to the terminal T 151 , connects the switch  1760  to a terminal T 160 , and disconnects the switch  1770 . Thus, as is the first embodiment, a first electrode  1014   a  and a third electrode  1012   a  are grounded to provide a first potential V 1  (0V). Furthermore, the oscillation circuit  52  applies a voltage to a second electrode  1014   b  and a fourth electrode  1012   b , which provide a second potential V 2 . The first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel. As is the case with the fourth embodiment, the capacitance C of the sensor unit  1010  can be measured as a capacitance C( 2 ) obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel and if no potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. 
     Additionally, the specifying device  1504  connects the switch  1610  to the terminal T 11 , connects the switch  1620  to the terminal T 20 , connects the switch  1630  to the terminal T 30 , connects the switch  1740  to the terminal T 140 , connects the switch  1750  to the terminal T 150 , disconnects the switch  1760 , and connects the switch  1770  to the terminal T 170 . Thus, as is the fifth embodiment, the first electrode  1014   a  is connected to the oscillation circuit  52 , the second electrode  1014   b  is connected to the fourth electrode  1012   b , the third electrode  1012   a  is connected to a rectifying unit  56 , and the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series. As is the case with the fifth embodiment, the capacitance C of the sensor unit  1010  can be measured as the capacitance C( 3 ) obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series and if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. The specifying device  1504  calculates a concentration of ethanol contained in a mixed fuel and an electric conductivity of the mixed fuel based on the capacitances C( 1 ), C( 2 ), and C( 3 ). A method for calculating the electric conductivity may be similar to the calculation method described in the fourth embodiment and the fifth embodiment. A measured value for the electric conductivity may be an average value for the electric conductivity calculated based on the capacitances C( 1 ) and C( 2 ) as is the case with the fourth embodiment and the electric conductivity calculated based on the capacitances C( 1 ) and C( 3 ) as is the case with the fifth embodiment. This allows an accuracy of electric-conductivity measurement to be improved. 
     As described above, the connection unit  1604  in the sensor  1204  is configured to be able to switch between the state in which the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel and the state in which the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series, as is the case with the fifth embodiment. The connection unit  1604  is further configured to be able to switch between the state in which a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between and a state in which no potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between, when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel. The single sensor  1204  can carry out measurements similar to the measurements performed by the sensors  1200  and  1202 , to obtain the capacitances C( 1 ), C( 2 ), and C( 3 ). A comparison between the capacitances C( 1 ) and C( 2 ) and C( 3 ) allows an impact of the electric conductivity of the liquid to be accurately calculated. Therefore, measurement accuracy for the capacitance is further improved. 
     Variation 
     The sixth embodiment carries out voltage detection. However, as shown in  FIG. 10 , a sensor system  1205  may carry out current detection. The sensor system  1205  is different from the sensor system  1201  according to the fourth embodiment in a configuration of a connection unit  1605  in a specifying device  1505 . The connection unit  1605  includes a switch  1708  in addition to the switches  1640 ,  1650 ,  1660 , and  1670  according to the sensor system  1201 , as a switching device configured to be able to switch a connection state. The switch  1780  switches between connection and disconnection between a connecting unit  1014   c  and a terminal T 180 . The terminal T 180  is connected to an I-V conversion unit  55  via the switch  1670 . A remaining part of the configuration is similar to the corresponding part of configuration of the sensor system  1201  and will thus not be described. 
     As shown in  FIG. 7 , the specifying device  1505  connects the switches  1640 ,  1670 , and  1780  to terminals T 40  and  170  and the terminal T 180 , respectively, and disconnects the switches  1650  and  1660 . Thus, as is the case with the sixth embodiment, the capacitance C of the sensor unit  1010  can be measured as the capacitance C( 1 ), which is obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel and if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. 
     Furthermore, the specifying device  1505  connects the switches  1650 ,  1660 , and  1780  to terminals T 50  and T 60  and the terminal T 180 , respectively, and disconnects the switches  1640  and  1670 . Thus, as is the case with the sixth embodiment, the capacitance C of the sensor unit  1010  can be measured as the capacitance C( 2 ), which is obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in parallel and if no potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. 
     Additionally, the specifying device  1505  connects the switches  1660  and  1670  to the terminals T 60  and T 70 , respectively, and disconnects the switches  1640 ,  1650 , and  1780 . Thus, as is the case with the sixth embodiment, the capacitance C of the sensor unit  1010  can be measured as the capacitance C( 3 ), which is obtained when the first pair of electrodes  1014  and the second pair of electrodes  1012  are connected together in series and if a potential difference occurs between the electrodes opposite to each other with the substrate  1020  in between. 
     In the above-described embodiments and variations, the case is illustrated in which the concentration of ethanol contained in the mixed fuel is measured using the capacitance of the sensor unit. However, the embodiments and variations are not limited to this case. Another physical quantity such as a liquid level in the mixed fuel may be measured. Whichever of the first potential V 1  and the second potential V 2  may be higher than the other provided that the first potential V 1  and the second potential V 2  are different from each other. Furthermore, even if the first electrode  1014   a  and the third electrode  1012   a  or the second electrode  1014   b  and the fourth electrode  1012   b  are partly opposite to each other with the substrate  1020  in between, a capacitance can be obtained by applying a potential difference to between the electrodes.