JP4311145B2 - Concentration detector - Google Patents

Description

The present invention relates to concentration detector.

Patent Document 1 discloses a NOx sensor that detects the concentration of nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine. The NOx sensor described in Patent Document 1 includes a pair of electrodes (in Patent Document 1, reference numerals 8a and 8b are attached, respectively). When a voltage is applied between these electrodes, NOx around one electrode is decomposed into nitrogen (N ₂ ) and oxygen (O ₂ ), and is proportional to the O ₂ concentration around the one electrode. A current flows between the electrodes. In Patent Document 1, the NOx concentration is calculated (detected) from the value of the current flowing between the electrodes at this time.

  By the way, in the NOx sensor described in Patent Document 1, the value of the current flowing between the electrodes when the NOx concentration in the exhaust gas is zero is obtained in advance as a reference current value, and the NOx concentration in the exhaust gas is being detected. The NOx concentration in the exhaust gas is detected based on the value obtained by subtracting the reference current value from the value of the current flowing between the electrodes. However, the characteristics of the electrodes and other components of the NOx sensor may change over time. In this case, the current value flowing between the electrodes when the reference current value obtained in advance is zero in the NOx concentration. May not match.

  Therefore, in Patent Document 1, while the NOx concentration in the exhaust gas is zero (for example, while fuel is not injected into the combustion chamber of the internal combustion engine and therefore combustion is not performed in the combustion chamber), The value of the current flowing between the electrodes is set as a new reference current value.

JP-A-11-148910 JP-A-11-101154 JP 2002-202285 A

  However, even if the value of the current flowing between the electrodes is detected on the assumption that the NOx concentration in the exhaust gas is zero, the NOx concentration in the exhaust gas is often not zero. The current value adopted as the current value does not match the current value flowing between the electrodes when the NOx concentration is zero. In this case, the NOx concentration detection accuracy of the NOx sensor is not high.

In general, this applies also to an apparatus that detects the concentration of an oxygen compound in a gas using a method similar to the NOx sensor described in Patent Document 1. An object of the present invention is to maintain a high concentration detection accuracy of the concentration detecting device.

In order to solve the above-mentioned problem, in the first invention, in the apparatus for detecting the concentration of the detection target made of nitrogen oxide, sulfur oxide or ammonia in the gas, the first solid electrolyte layer having oxygen ion conductivity And a second solid electrolyte layer, a gas chamber formed by the first solid electrolyte layer and the second solid electrolyte layer, into which gas is introduced, and separated from the gas chamber by the second solid electrolyte layer and communicated with the atmosphere. An air chamber, a pair of pump electrodes, one on the gas chamber side of the first solid electrolyte layer and the other on the back side, and oxygen in the gas chamber to the outside via the first solid electrolyte layer Oxygen pumping means for pumping out oxygen pumping means for controlling the voltage between the pump electrodes so that the oxygen concentration in the gas in the gas chamber becomes substantially zero, and one of the oxygen pumping means in the second solid electrolyte layer A pair of sensor electrodes and the other disposed on the air chamber side to the scan chamber side, when one of the detection target decompose voltage above start generating values of oxygen around the sensor electrode is applied between the sensor electrodes First current detecting means for detecting a current value flowing between the sensor electrodes in accordance with the amount of oxygen around the one sensor electrode, and decomposing the detection target around the one sensor electrode to start generating oxygen A second current detecting means for detecting a current value flowing between the sensor electrodes in accordance with the amount of oxygen around the one sensor electrode when a voltage equal to or lower than the value is applied between the sensor electrodes; Detected by the second current detection means during the oxygen pumping by the oxygen pumping means from the current value detected by the first current detection means during the oxygen pumping by the pumping means. Detecting the concentration of the detection target in the gas based on a value obtained by subtracting the current values.
In the second invention, in the apparatus for detecting the concentration of the detection target comprising nitrogen oxide, sulfur oxide or ammonia in the gas, the first solid electrolyte layer and the second solid electrolyte layer having oxygen ion conductivity A gas chamber formed by the first solid electrolyte layer and the second solid electrolyte layer and into which gas is introduced; an air chamber isolated from the gas chamber by the second solid electrolyte layer and communicating with the atmosphere; A pair of pump electrodes, one on the gas chamber side of the first solid electrolyte layer and the other on the back side thereof, and oxygen pumping means for pumping oxygen in the gas chamber to the outside through the first solid electrolyte layer And oxygen pumping means for controlling the voltage between the pump electrodes so that the oxygen concentration in the gas in the gas chamber becomes substantially zero, one side being the gas chamber side of the second solid electrolyte layer and the other being the atmosphere. A pair of sensor electrodes disposed on the side, the oxygen near one of the sensor electrodes above when applied between one of the sensor electrodes around the detection target decomposed above value oxygen begins to produce the voltage to the sensor electrode A first current detecting means for detecting a current value flowing between the sensor electrodes in accordance with the amount of the sensor electrode, and a voltage equal to or lower than a value at which the detection object around the one sensor electrode is decomposed to start generating oxygen A second current detecting means for detecting a current value flowing between the sensor electrodes in accordance with the amount of oxygen around the one sensor electrode when applied in between, and detection of the current value by the second current detecting means. Second current storage means for detecting the current value and storing the detected current value when it is determined that the current value should be detected. The first current detection Detecting the concentration of the detection target in the gas based on a value obtained by subtracting the current value stored by the second current memory means from the detected current value by the step.

According to the first or second invention, the current detected by the second current detecting means is generated due to oxygen generated by being decomposed from a detection target made of nitrogen oxide, sulfur oxide, or ammonia . Current is not included. That is, the current value detected by the second current detection means represents only the concentration of oxygen originally present in the gas. In other words, the current value detected by the second current detection means matches the value when the concentration of the detection target in the gas is zero. According to the first or second invention, the current value detected by the second current detection means or the current value detected by the second current detection means stored in advance is detected by the first current detection means. Current value (this includes the current generated due to the oxygen originally present in the gas and the current generated due to the oxygen generated by decomposition from the detection target in the gas) The concentration of the detection target in the gas is detected based on the value subtracted from. Therefore, according to the present invention, the concentration detection accuracy is maintained high.
In addition, according to the present invention, the concentration of the detection target in the gas can be easily (and therefore many) reduced by setting the voltage applied between the electrodes to be equal to or less than a value at which the detection target is decomposed to start generating oxygen. The current value when it is zero can be detected. Therefore, according to the present invention, the concentration detection accuracy is maintained high.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view of a sensor portion of a NOx sensor that detects the concentration of NOx in gas (for example, exhaust gas discharged from a combustion chamber of an internal combustion engine). For example, when detecting the NOx concentration in the exhaust gas discharged from the fuel chamber of the internal combustion engine, the NOx sensor is attached to an exhaust pipe connected to the fuel chamber.

The sensor part of the NOx sensor is composed of five oxygen ion conductive solid electrolyte layers such as zirconia oxide laminated on each other. These five solid electrolyte layers are hereinafter referred to as a first layer L ₁ and a second layer in order from the top. These are referred to as L ₂ , third layer L ₃ , fourth layer L ₄ , and fifth layer L ₅ .

In the second layer L ₂ between the first layer L ₁ and the third layer L ₃ , a chamber (hereinafter referred to as “gas chamber”) 1 into which a gas is introduced is formed. Further, in the fourth layer L ₄ between the third layer L ₃ and the fifth layer L ₅ , a chamber 2 (hereinafter referred to as “atmosphere chamber”) 2 communicating with the atmosphere is formed.

The first layer L ₁ is provided with an opening 3 for introducing gas into the gas chamber 1. Further, the first layer L ₁ of the outer wall surface, so as to cover the opening 3, for example, made of a porous material (or, pores are formed) diffusion-controlling member 4 is arranged. Accordingly, the gas flows into the gas chamber 1 from the opening 3 via the diffusion rate controlling member 4, and thus the gas chamber 1 is filled with the gas.

Further, the electrode 5 is disposed on the outer wall surface of the first layer L ₁ (the wall surface opposite to the wall surface of the first layer L ₁ facing the gas chamber 1). On the other hand, on the inner wall surface of the first layer L ₁ (first layer L ₁ of the wall surface of the side facing the gas chamber 1), the electrode 6 is disposed. As will be described later, these electrodes 5 and 6 serve to pump (pump) oxygen from the gas in the gas chamber 1 to the outside. The electrode 5 which is disposed on the outer wall surface of the first layer L _1, the voltage source is connected to (hereinafter "pump voltage source" hereinafter) anode 7, hereinafter referred to as anode-side pump electrode. On the other hand, the electrode 6 arranged on the inner wall surface of the first layer L ₁ is connected to the cathode of the pump voltage source 7, hereinafter referred to as the cathode side pump electrode. The cathode-side pump electrode 6 is made of a material having a low reducing property with respect to NOx (for example, an alloy of gold (Au) and platinum (Pt)).

_On the wall surface of the third layer L3 facing the gas chamber 1, an electrode 8 is arranged. On the other hand, the electrode 9 is also disposed on the wall surface of the third layer L ₃ on the side facing the atmosphere chamber 2. These electrodes 8 and 9 function to detect (sense) the NOx concentration in the gas in the gas chamber 1, as will be described later. The electrode 8 disposed on the wall surface of the third layer L3 facing the gas chamber 1 is connected to the cathode of a voltage source (hereinafter referred to as “sensor voltage source”) 10. This is referred to as a side sensor electrode. On the other hand, the electrode 9 disposed on the wall surface of the third layer L ₃ on the side facing the air chamber 2 is connected to the anode of the sensor voltage source 10, hereinafter referred to as anode-side sensor electrode. The cathode side sensor electrode 8 is made of a material (for example, rhodium (Rh) or platinum (Pt)) having a strong reducing property with respect to NOx.

When a voltage is applied between the pump electrodes 5 and 6 by the pump voltage source 7, oxygen in the gas in the gas chamber 1 comes into contact with the cathode-side pump electrode 6 and becomes oxygen ions. The oxygen ion flows toward the first layer L ₁ to the anode side pump electrode 5. Thus, the oxygen in the gas in the gas chamber 1 is pumped to the outside by moving the first layer L _1. At this time, the amount of oxygen pumped to the outside increases as the voltage of the pump voltage source 7 increases. Therefore, if the voltage of the pump voltage source 7 is appropriately controlled, a gas containing almost no oxygen can be supplied to the cathode side sensor electrode 8. In the present embodiment, the voltage of the pump voltage source 7 is controlled so that the oxygen concentration in the gas becomes substantially zero.

As described above, the cathode-side pump electrode 6 is made of a material having a low reducing property with respect to NOx, but when a voltage is applied between the pump electrodes 5 and 6 by the pump voltage source 7, There is a function of reducing NO ₂ in the gas in the gas chamber 1 to NO. Therefore, most of the NOx in the gas arriving around the cathode side sensor electrode 8 is in the form of NO.

When a voltage is applied between the sensor electrodes 8 and 9 by the sensor voltage source 10, NO in the gas in the gas chamber 1 comes into contact with the cathode side sensor electrode 8 and is decomposed into N ₂ and O _2. The O ₂ decomposed and generated here is further converted to oxygen ions. The oxygen ions, the third layer L ₃ moves toward the anode side sensor electrodes 9. At this time, a current indicated by reference numeral 11 (hereinafter referred to as “sensor current”) is proportional to the amount of oxygen ions moving between the sensor electrodes 8 and 9 in the third layer L ₃ toward the anode-side sensor electrode 9. ) Is flows.

  As described above, in the cathode side pump electrode 6, most of the oxygen is exhausted from the gas and NOx is hardly reduced (or, even if it is reduced, it is only reduced to NO). Will come in a gas that contains almost no oxygen and only NOx. Therefore, the sensor current Is is proportional to the NOx concentration contained in the gas, and the NOx concentration in the gas can be known by using this sensor current Is.

Incidentally, the fifth layer L ₅ represents between sixth layer L _6, the electric heater 12 for heating the sensor portion of the NOx sensor is disposed. The electric heater 12 heats the sensor portion of the NOx sensor from 700 ° C. to 800 ° C.

Incidentally, as described above, in the present embodiment, all oxygen is discharged from the gas flowing into the gas chamber 1 by the pump electrodes 5 and 6. However, in practice, when the pump electrodes 5 and 6 deteriorate with time, oxygen remains in the gas, or even if the pump electrodes 5 and 6 are not deteriorated, a small amount of oxygen It will remain in the gas. In this case, oxygen remaining in the gas (hereinafter referred to as “residual oxygen”) is converted into oxygen ions by the sensor electrodes 8 and 9 and moves through the third layer L ₃ toward the anode-side sensor electrode 9. For this reason, the sensor current Is indicated by reference numeral 11 flowing at this time includes a current due to the residual oxygen, and the sensor current Is is assumed on the assumption that the amount of residual oxygen is zero. The obtained NOx concentration is not a true NOx concentration.

  If the residual oxygen amount is always constant, the residual oxygen amount is obtained in advance by experiments or the like, and the NOx concentration calculated from the sensor current Is based on the preliminarily obtained residual oxygen amount becomes the true NOx concentration. If a correction value for correcting the sensor current Is (or a correction value for correcting the NOx concentration obtained from the sensor current Is) is obtained in advance, and the NOx concentration is obtained using these correction values, The determined NOx concentration should be a true NOx concentration. However, as described above, the residual oxygen amount varies depending on the deterioration of the pump electrodes 5 and 6 with time.

  In any case, in order to accurately detect the NOx concentration, the amount of oxygen remaining in the gas arriving at the sensor electrodes 8 and 9 is known, and the sensor current Is (or the sensor current Is according to the amount of remaining oxygen). It is necessary to correct the NOx concentration obtained from (1). Therefore, in this embodiment, the NOx concentration is obtained as follows.

  When the oxygen concentration and the NOx concentration in the gas arriving at the sensor electrodes 8 and 9 are respectively a certain concentration, the relationship between the voltage Vs of the sensor voltage source 10 and the sensor current Is is as shown in FIG. . That is, while the voltage Vs is lower than a certain value V1, the sensor current Is is zero. However, when the voltage Vs exceeds the value V1, the sensor increases as the voltage Vs increases until the voltage Vs exceeds the value V2. The current Is also increases. Until the voltage Vs exceeds the value V2 and exceeds the value V3 larger than the value V2, the sensor current Is is substantially constant regardless of the voltage Vs. Further, until the voltage Vs exceeds the value V3 and exceeds the value V4 larger than the value V3, the sensor current Is increases as the voltage Vs increases. Further, until the voltage Vs exceeds the value V4 and exceeds the value V5 larger than the value V4, the sensor current Is is substantially constant regardless of the voltage Vs. When the voltage Vs exceeds the value V5, the sensor current Is increases as the voltage Vs increases.

  The reason why the sensor current Is changes in this way by the voltage Vs is because the following phenomenon occurs in the cathode-side sensor electrode 8. That is, when the voltage Vs is lower than the value V1, no oxygen ions are generated from the residual oxygen in the gas or from NOx, so the sensor current Is is zero. When the voltage Vs is between the value V1 and the value V3, oxygen ions are generated from the residual oxygen in the gas, so that the sensor current Is becomes larger than zero. However, at this time, oxygen ions are generated only from residual oxygen in the gas, and oxygen ions are not generated from NOx in the gas. Therefore, the sensor current Is has a relatively small value. Further, when the voltage Vs becomes higher than the value V3, oxygen ions are generated from both residual oxygen and NOx in the gas, so that the sensor current Is has a relatively large value.

  As described above, when the voltage Vs is between the value V1 and the value V3, oxygen ions are generated only from the remaining oxygen, so the voltage Vs is kept at a constant value between the value V1 and the value V3. If so, the sensor current Is at this time is a current generated due to residual oxygen. On the other hand, when the voltage Vs is higher than the value V3, oxygen ions are generated from the residual oxygen and NOx. Therefore, if the voltage Vs is kept at a constant value larger than the value V3, The sensor current Is is a current generated due to residual oxygen and NOx.

  Therefore, a voltage having a value between the value V1 and the value V3 is applied to the sensor electrodes 8 and 9 from the sensor current Is detected when a voltage higher than the value V3 is applied to the sensor electrodes 8 and 9. The value obtained by subtracting the sometimes detected sensor current Is is a current value caused only by NOx, and the NOx concentration obtained from this current value is a true NOx concentration.

  Therefore, in the present embodiment, a voltage range in which oxygen ions are generated from residual oxygen but oxygen ions are not generated from NOx at a predetermined time interval (in the example shown in FIG. 2, values V1 and V3 From the viewpoint of accuracy, preferably, the voltage Vs of the sensor voltage source 10 is kept at a predetermined constant value (between the value V2 and the value V3), and the sensor current Is at this time is changed to the remaining oxygen Is stored as the sensor current Iso caused by the above. When the NOx concentration is to be detected, the voltage range in which oxygen ions are generated from both residual oxygen and NOx (in the example shown in FIG. 2, the value is V3 or more, preferably from the viewpoint of accuracy, the value V3 The voltage Vs of the sensor voltage source 10 is kept at a predetermined constant value between the value V5 and more preferably from the viewpoint of accuracy (between the value V4 and the value V5). A value obtained by subtracting the sensor current Iso resulting from the residual oxygen from Is is referred to as a sensor current Isnox caused only by NOx. Then, the NOx concentration is detected from the sensor current Isnox caused only by this NOx.

  According to this, it is detected when the NOx concentration is to be detected (that is, when a voltage having a value equal to or higher than the value V3 (preferably between the values V4 and V5) is applied to the sensor electrodes 8 and 9). The sensor current Is to be corrected is corrected by a correction value that is periodically updated (that is, the sensor current Iso that is caused only by the residual oxygen that is updated at a predetermined time interval). A high NOx concentration is detected.

FIG. 3 is a flowchart showing an example of a routine for detecting the NOx concentration according to the above-described embodiment. In the example shown in FIG. 3, first, in step 10, the counter C (this indicates the time that has elapsed since the current value Iso due to residual oxygen was calculated and updated in step 13 last time. It is determined whether or not “is” is equal to or greater than a predetermined value Cth (C ≧ Cth). Here, when it is determined that C <Cth (that is, when a certain time has not elapsed since the current value Iso due to residual oxygen has been updated last time), the routine proceeds to step 16. Thus, the NOx concentration Cnox is calculated according to the following equation (1).
Cnox = (Is−Iso) × K (1)
Here, Is is the value of the current flowing between the sensor electrodes 8 and 9, Iso is the value of the current caused by the residual oxygen, and K is the value of the current flowing between the sensor electrodes 8 and 9. This is a coefficient for conversion to density.
Next, at step 17, the counter C is incremented (C ← C + 1).

  On the other hand, when it is determined in step 10 that C ≧ Cth (that is, when a predetermined time has elapsed since the current value Iso due to residual oxygen has been updated last time), the routine proceeds to step 11. The voltage Vs applied between the sensor electrodes 8 and 9 is a value VL that generates oxygen ions only from residual oxygen (a value between the value V1 and the value V3 in the example of FIG. 2, and preferably , A value between V2 and V3) is determined (Vs = VL). If Vs = VL is not satisfied, the routine proceeds to step 19 where the voltage Vs is set to the value VL.

  On the other hand, if Vs = VL in Step 11, the routine proceeds to Step 12, and the time T (this is the time elapsed since the voltage Vs was set to the value VL in Step 11). It is determined whether or not a predetermined time Tth or longer (T ≧ Tth). That is, in step 11, since it takes a certain time until the voltage Vs becomes the value VL after the voltage Vs is set to the value VL, step 12 is provided in this routine. .

  When it is determined in step 12 that T <Tth (that is, when it is determined that the voltage Vs is not the value VL), the routine proceeds to step 18 and the time T is incremented ( T ← T + 1). On the other hand, when it is determined in step 12 that T ≧ Tth, the routine proceeds to step 13, where the sensor current Is flowing between the sensor electrodes 8 and 9 is converted into a current value Iso resulting from only residual oxygen. As (ie, update). Thereafter, the newly updated value Iso is used for the calculation of the NOx concentration according to the above equation (1) in step 16.

  Next, in step 14, the voltage Vs applied between the sensor electrodes 8 and 9 is a value VH that generates oxygen ions from residual oxygen and NOx (a value equal to or higher than the value V3 in the example of FIG. Is a value between the value V3 and the value V5, and more preferably a value between the value V4 and the value V5). Next, at step 15, the counter C is reset (C ← 0).

  In the above description, the present invention has been described by taking the case of detecting the NOx concentration in the gas as an example. However, the present invention also applies to the case of detecting the concentration of an oxygen compound such as sulfur oxide (SOx) in the gas. Applicable.

The present invention is also applicable when detecting the ammonia concentration in the gas. That is, ammonia (NH ₃ ) contained in the gas is decomposed into NO and H ₂ O (4NH ₃ + 5O ₂ → 4NO + 6H ₂ O) at the cathode-side pump electrode 6, and this decomposed NO is It arrives at the cathode side sensor electrode 8. This NO is decomposed into N ₂ and O ₂ on the cathode side sensor electrode 8, and the decomposed and generated O ₂ becomes oxygen ions toward the anode side sensor electrode 9 in the third layer L ₃ . Moving. Also at this time, the sensor current Is is proportional to the NH ₃ concentration contained in the gas. Figure 4 shows the relationship between NOx concentration and NH ₃ concentration sensor current Is and gas, from FIG 4, the sensor current Is to be proportional to the NOx concentration and the NH ₃ concentration in the gas I understand. Therefore, the NH ₃ concentration in the gas can also be detected using the sensor current Is.

  Further, as described in connection with Patent Document 1, when the value of the current flowing between the electrodes while the NOx concentration in the exhaust gas is zero is set as a new reference current value, If there is little chance that the NOx concentration in the exhaust gas becomes zero, the reference current value is updated less frequently. Therefore, a reference current value that does not match the current value flowing between the electrodes when the NOx concentration is zero is used. Will be longer. In this case, it cannot be said that the NOx concentration detection accuracy of the NOx sensor is high. However, according to the present invention, it can be said that the NOx concentration detection accuracy of the NOx sensor is high because the sensor current (corresponding to the reference current value) caused only by residual oxygen is periodically updated.

It is sectional drawing of the sensor part of a NOx sensor. It is a figure which shows the relationship between the voltage Vs applied between sensor electrodes, and the sensor electric current Is which flows between sensor electrodes. 3 is a flowchart illustrating an example of a control routine for detecting NOx concentration according to an embodiment of the present invention. Is a diagram showing a relationship between NOx concentration and NH ₃ concentration and sensor current Is.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Gas chamber 2 ... Atmosphere chambers 5, 6 ... Pump electrode 7 ... Pump voltage source 8, 9 ... Sensor electrode 10 ... Sensor voltage source

Claims (2)

In an apparatus for detecting a concentration of a detection target made of nitrogen oxide, sulfur oxide, or ammonia in a gas, a first solid electrolyte layer and a second solid electrolyte layer having oxygen ion conductivity, and the first solid electrolyte layer And a second solid electrolyte layer formed of a gas chamber into which gas is introduced, an atmosphere chamber separated from the gas chamber by the second solid electrolyte layer and communicating with the atmosphere, and one of the gas chamber and the second solid electrolyte layer. A pair of pump electrodes arranged on the back side of the gas chamber and oxygen pumping means for pumping oxygen in the gas chamber to the outside through the first solid electrolyte layer, the gas in the gas chamber An oxygen pumping means for controlling the voltage between the pump electrodes so that the oxygen concentration in the inside becomes substantially zero, and a pair of sensors arranged one on the gas chamber side of the second solid electrolyte layer and the other on the atmosphere chamber side. The sensor according to the electrode and the amount of oxygen near one of the sensor electrodes above when applied between one of the sensor electrodes around the detection target decomposed above value oxygen begins to produce the voltage to the sensor electrode A first current detecting means for detecting a current value flowing between the electrodes, and a voltage equal to or lower than a value at which the detection target around the one sensor electrode is decomposed to start generating oxygen is applied between the sensor electrodes; Second current detecting means for detecting a current value flowing between the sensor electrodes in accordance with the amount of oxygen around one sensor electrode, and the first current detecting means during the pumping of oxygen by the oxygen pumping means. from the detected current value, said analyzing in the gas based on a value obtained by subtracting the detected current value by the second current detection means during oxygen pumping by the oxygen pumping means Apparatus for detecting the concentration of a target. In an apparatus for detecting a concentration of a detection target made of nitrogen oxide, sulfur oxide, or ammonia in a gas, a first solid electrolyte layer and a second solid electrolyte layer having oxygen ion conductivity, and the first solid electrolyte layer And a second solid electrolyte layer formed of a gas chamber into which gas is introduced, an atmosphere chamber separated from the gas chamber by the second solid electrolyte layer and communicating with the atmosphere, and one of the gas chamber and the second solid electrolyte layer. A pair of pump electrodes arranged on the back side of the gas chamber and oxygen pumping means for pumping oxygen in the gas chamber to the outside through the first solid electrolyte layer, the gas in the gas chamber An oxygen pumping means for controlling the voltage between the pump electrodes so that the oxygen concentration in the inside becomes substantially zero, and a pair of sensors arranged one on the gas chamber side of the second solid electrolyte layer and the other on the atmosphere chamber side. The sensor according to the electrode and the amount of oxygen near one of the sensor electrodes above when applied between one of the sensor electrodes around the detection target decomposed above value oxygen begins to produce the voltage to the sensor electrode A first current detecting means for detecting a current value flowing between the electrodes, and a voltage equal to or lower than a value at which the detection target around the one sensor electrode is decomposed to start generating oxygen is applied between the sensor electrodes; A second current detecting means for detecting a current value flowing between the sensor electrodes according to the amount of oxygen around one sensor electrode, and determining whether or not the current value should be detected by the second current detecting means; A second current storage means for detecting the current value and storing the detected current value when it is determined that it is time to detect the current value, the first current detection means; Current detected by Apparatus for detecting the concentration of the detection target of the second current memory means based on the value obtained by subtracting the stored current value by the gas from.

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