Gas sensor and concentration measuring method using gas sensor

Abstract

The sensor element is provided with first to fourth cavities communicating in this order from the gas introduction port, and the adjustment pump unit sucks oxygen from the gas to be measured introduced into the first cavity so that NOx, H2O, and CO2 in the gas to be measured are not decomposed. The first measurement pump unit sucks oxygen from the second cavity such that all NOx in the gas to be measured introduced into the second cavity is reduced, the second measurement pump unit sucks oxygen from the third cavity such that all H2O and CO2 in the gas to be measured introduced into the third cavity are reduced, and the third measurement pump unit sucks oxygen into the fourth cavity. And a second measurement pump unit that selectively oxidizes H2 generated by reduction, determines NOx concentration from the suction current of the first measurement pump unit, determines H2O concentration from the suction current of the third measurement pump unit, and determines CO2 concentration on the basis of the determined H2O concentration and the suction current of the second measurement pump unit.

Description

Technical Field

[0001] The present invention relates to a multi-gas sensor capable of monitoring multiple types of monitored gas components and measuring their concentrations. Background Art

[0002] Regarding the measurement for managing the emission amount of automotive exhaust gas, techniques for measuring the concentrations of water vapor (H2O) and carbon dioxide (CO2) are known (see, for example, Patent Document 1 and Patent Document 2). In the gas sensors disclosed in Patent Document 1 and Patent Document 2, it is possible to simultaneously measure the water vapor (H2O) component and the carbon dioxide (CO2) component.

[0003] In addition, there is also known a gas sensor (NOx sensor) having a sensor element with a configuration similar to that of the gas sensors disclosed in Patent Document 1 and Patent Document 2, and capable of measuring NOx by performing pump unit control different from that of the gas sensors disclosed in Patent Document 1 and Patent Document 2 (see, for example, Patent Document 3).

[0004] In the gas sensor having a three-chamber configuration disclosed in Patent Document 1, first, the main pump unit of the pump unit for the first internal cavity operates to suck out O2 contained in the gas to be measured introduced into the first internal cavity, and at the same time, H2O and CO2 also contained in the gas to be measured are all temporarily reduced to generate H2 and CO. The gas to be measured containing H2 and CO is introduced into the second and third internal cavities. Next, by using the first measurement pump unit of the pump unit for the second internal cavity to suck in O2, H2 is selectively oxidized to generate H2O, and further, by using the second measurement pump unit of the pump unit for the third internal cavity to suck in O2, CO is oxidized to generate CO2. And based on the magnitudes of the pump currents flowing through the first measurement pump unit and the second measurement pump unit when oxidizing the above-mentioned H2 and CO, the concentrations of H2O and CO2 in the gas to be measured are measured.

[0005] In this gas sensor, in order to reduce H2O and CO2 in the first internal cavity, it is necessary to set a relatively high applied voltage in the pump unit for the first internal cavity. At the same time, it is also necessary to increase the temperature of the cavity internal pump electrode constituting the main pump unit, that is, the main inner pump electrode. However, maintaining the above-mentioned high applied voltage and the high temperature of the pump electrode may cause cracking of the sensor element mainly composed of an oxygen ion conductive solid electrolyte ceramic, blackening of the solid electrolyte ceramic due to reduction, etc.

[0006] In addition, there are times when it is desired to measure NOx while measuring H2O and CO contained in the exhaust gas. In particular, from the viewpoints of ensuring ease of installation space and cost savings, there is a need to measure NOx simultaneously in the same gas sensor in addition to H2O and CO.

[0007] Prior art documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent No. 5918177

[0010] Patent Document 2: Japanese Patent No. 6469464

[0011] Patent Document 3: Japanese Patent No. 3798412 Summary of the invention

[0012] The present invention has been made in view of the above problems, and an object thereof is to provide a multi-gas sensor that can simultaneously measure water vapor (H2O) components, carbon dioxide (CO2) components, and NOx, suppress the occurrence of cracking and blackening in the sensor element, and is less likely to change in sensitivity even after long-term use, and has excellent long-term reliability compared to the prior art.

[0013] In order to solve the above-mentioned problems, the first scheme of the present invention is a gas sensor capable of measuring the concentrations of multiple monitored gas components, characterized in that it comprises: a sensor element having a structure composed of a solid electrolyte with oxygen ion conductivity; and a controller, which controls the operation of the gas sensor, the sensor element comprising: a gas inlet port for introducing a measured gas; a first cavity, a second cavity, a third cavity and a fourth cavity, which are connected in sequence from the gas inlet port via different diffusion rate control units; an adjustment pump unit, which is composed of an inner electrode formed facing the first cavity, an outer cavity pump electrode provided at a position other than the first cavity, the second cavity, the third cavity and the fourth cavity, and the solid electrolyte present between the inner electrode and the outer cavity pump electrode; A first measuring pump cell, the first measuring pump cell comprising a first measuring electrode formed facing the second cavity, the cavity outer pump electrode, and the solid electrolyte present between the first measuring electrode and the cavity outer pump electrode; a second measuring pump cell, the second measuring pump cell comprising a second measuring electrode formed facing the third cavity, the cavity outer pump electrode, and the solid electrolyte present between the second measuring electrode and the cavity outer pump electrode; a third measuring pump cell, the third measuring pump cell comprising a third measuring electrode formed facing the fourth cavity, the cavity outer pump electrode, and the solid electrolyte present between the third measuring electrode and the cavity outer pump electrode; and a heater, the heater heating the sensor element, the inner electrode comprising a Pt-Au alloy as a metal component and the Au concentration in the Pt-Au alloy being 0.A cermet electrode of 5 wt% or more, the first measurement electrode being another cermet electrode containing a Pt-Rh alloy as a metal component. The adjustment pump unit sucks oxygen from the measured gas introduced into the first cavity from the gas inlet in such a way that NOx, water vapor, and carbon dioxide contained in the measured gas are not decomposed. The first measurement pump unit sucks oxygen from the second cavity in such a way that substantially all of the NOx contained in the measured gas introduced from the first cavity into the second cavity is reduced. The second measurement pump unit sucks oxygen from the third cavity in such a way that substantially all of the water vapor and carbon dioxide contained in the measured gas introduced from the second cavity into the third cavity are reduced. The third measurement pump unit selectively oxidizes hydrogen generated by the reduction of water vapor contained in the measured gas introduced from the third cavity into the fourth cavity in the fourth cavity by sucking oxygen into the fourth cavity. The controller includes: a NOx concentration determination mechanism that determines the concentration of NOx contained in the measured gas based on the magnitude of the oxygen pump current, i.e., the NOx detection current, flowing between the first measurement electrode and the outer cavity pump electrode when sucking oxygen from the second cavity using the first measurement pump unit; a water vapor concentration determination mechanism that determines the concentration of water vapor contained in the measured gas based on the value of the oxygen pump current, i.e., the water vapor equivalent current, flowing between the second measurement electrode and the outer cavity pump electrode when hydrogen is oxidized by the oxygen sucked into the third cavity by the second measurement pump unit; and a carbon dioxide concentration determination mechanism that determines the concentration of carbon dioxide contained in the measured gas based on the value of the water vapor equivalent current and the value of the oxygen pump current, i.e., the full reduction current, flowing between the first measurement electrode and the outer cavity pump electrode when the water vapor and carbon dioxide are reduced by sucking oxygen from the second cavity by the first measurement pump unit.

[0014] Based on the gas sensor according to the first aspect, the second aspect of the present invention is characterized in that the controller further stores: Ip1-NOx data that pre-determines the relationship between the NOx detection current and the concentration of NOx. The NOx concentration determination mechanism determines the concentration of NOx contained in the measured gas based on the NOx detection current when NOx contained in the measured gas is reduced and the Ip1-NOx data.

[0015] Based on the gas sensor involved in the second aspect, the third aspect of the present invention is characterized in that the controller stores: Ip2-H2O data that pre-determines the relationship between the oxygen pump current flowing through the second measurement pump unit and the water vapor concentration when the measured gas contains water vapor but no carbon dioxide, Ip2-CO2 data that pre-determines the relationship between the oxygen pump current flowing through the second measurement pump unit and the water vapor concentration when the measured gas contains carbon dioxide but no water vapor, and Ip3-H2O data that pre-determines the relationship between the oxygen pump current flowing through the third measurement pump unit and the water vapor concentration when the measured gas contains water vapor but no carbon dioxide. The water vapor concentration determination mechanism determines the water vapor concentration corresponding to the value of the water vapor equivalent current in the Ip3-H2O data as the water vapor concentration contained in the measured gas. After the carbon dioxide concentration determination mechanism determines the contribution part of the total reduction current brought by the reduction of water vapor based on the water vapor concentration contained in the measured gas determined by the water vapor concentration determination mechanism and the Ip2-H2O data, it determines the carbon dioxide concentration corresponding to the difference obtained by subtracting the contribution part from the total reduction current in the Ip2-CO2 data as the carbon dioxide concentration contained in the measured gas.

[0016] Based on the gas sensor involved in the second aspect, the fourth aspect of the present invention is characterized in that the controller stores: Ip2-CO2 data that pre-determines the relationship between the oxygen pump current flowing through the second measurement pump unit and the water vapor concentration when the measured gas contains carbon dioxide but no water vapor, Ip3-H2O data that pre-determines the relationship between the oxygen pump current flowing through the third measurement pump unit and the water vapor concentration when the measured gas contains water vapor but no carbon dioxide, and H2O characteristic data that pre-determines the relationship between the water vapor equivalent current and the oxygen pump current corresponding to the contribution part of water vapor in the total reduction current. The water vapor concentration determination mechanism determines the water vapor concentration corresponding to the value of the water vapor equivalent current in the Ip3-H2O data as the water vapor concentration contained in the measured gas. After the carbon dioxide concentration determination mechanism determines the contribution part of the total reduction current brought by the reduction of water vapor based on the water vapor equivalent current and the H2O characteristic data, it determines the carbon dioxide concentration corresponding to the difference obtained by subtracting the contribution part from the total reduction current in the Ip2-CO2 data as the carbon dioxide concentration contained in the measured gas.

[0017] Based on the gas sensor according to any one of the first to fourth aspects of the present invention, a fifth aspect of the present invention is characterized in that the controller further includes an oxygen concentration determination mechanism that determines the concentration of oxygen contained in the gas to be measured based on the magnitude of the current flowing between the inner electrode and the outer cavity pump electrode when oxygen is sucked out of the first cavity by the adjustment pump unit.

[0018] Based on the gas sensor according to any one of the first to fifth aspects of the present invention, a sixth aspect of the present invention is characterized in that the third measurement electrode is a cermet electrode containing a Pt-Au alloy as a metal component, and the Au concentration in the Pt-Au alloy is 1 wt% or more and 50 wt% or less.

[0019] The seventh scheme of the present invention is a method for measuring the concentration of multiple monitored gas components using a gas sensor, characterized in that the gas sensor includes a sensor element, the sensor element has a long plate-shaped structure composed of an oxygen ion conductive solid electrolyte, the sensor element includes: a gas inlet port for introducing a measured gas; a first cavity, a second cavity, a third cavity and a fourth cavity, which are connected in sequence from the gas inlet port via different diffusion rate control parts; an adjustment pump unit, the adjustment pump unit is composed of an inner electrode formed facing the first cavity, an outer cavity pump electrode provided at a position other than the first cavity, the second cavity, the third cavity and the fourth cavity, and the solid electrolyte present between the inner electrode and the outer cavity pump electrode; a first measurement pump unit, the first measurement pump unit A fixed pump unit is composed of a first measuring electrode formed facing the second cavity, the cavity outer pump electrode, and the solid electrolyte present between the first measuring electrode and the cavity outer pump electrode; a second measuring pump unit is composed of a second measuring electrode formed facing the third cavity, the cavity outer pump electrode, and the solid electrolyte present between the second measuring electrode and the cavity outer pump electrode; a third measuring pump unit is composed of a third measuring electrode formed facing the fourth cavity, the cavity outer pump electrode, and the solid electrolyte present between the third measuring electrode and the cavity outer pump electrode; and a heater, which heats the sensor element, the inner electrode is a Pt-Au alloy containing a metal component and the Au concentration in the Pt-Au alloy is 0.A cermet electrode of 5 wt% or more, the first measurement electrode being another cermet electrode containing a Pt-Rh alloy as a metal component. The method includes the following steps: a) Using the adjustment pump unit to suck oxygen from the measured gas introduced into the first cavity from the gas inlet in such a way that NOx, water vapor, and carbon dioxide contained in the measured gas are not decomposed; b) Using the first measurement pump unit to suck oxygen from the second cavity in such a way that substantially all of the NOx contained in the measured gas introduced into the second cavity from the first cavity is reduced; c) Using the second measurement pump unit to suck oxygen from the third cavity in such a way that substantially all of the water vapor and carbon dioxide contained in the measured gas introduced into the third cavity from the second cavity are reduced; d) Selectively oxidizing hydrogen generated by the reduction of water vapor contained in the measured gas introduced into the fourth cavity from the third cavity in the fourth cavity by sucking oxygen into the fourth cavity using the third measurement pump unit; e) Determining the concentration of NOx contained in the measured gas based on the magnitude of the oxygen pump current flowing between the first measurement electrode and the external cavity pump electrode when NOx is reduced by sucking oxygen from the second cavity by the first measurement pump unit, i.e., the NOx detection current; f) Determining the concentration of water vapor contained in the measured gas based on the value of the oxygen pump current flowing between the third measurement electrode and the external cavity pump electrode when hydrogen is oxidized by the oxygen sucked into the fourth cavity by the third measurement pump unit, i.e., the water vapor equivalent current; and g) Determining the concentration of carbon dioxide contained in the measured gas based on the value of the water vapor equivalent current and the value of the oxygen pump current flowing between the second measurement electrode and the external cavity pump electrode when water vapor and carbon dioxide are reduced by sucking oxygen from the third cavity by the second measurement pump unit, i.e., the full reduction current value.

[0020] The eighth aspect of the present invention is based on the concentration measurement method using a gas sensor according to the seventh aspect, and is characterized by including the following step: h) Before steps a) to g), Ip1-NOx data representing the relationship between the NOx detection current and the concentration of NOx is determined in advance. In step e), the concentration of NOx contained in the measured gas is determined based on the NOx detection current when NOx contained in the measured gas is reduced and the Ip1-NOx data.

[0021] Based on the concentration measurement method using a gas sensor according to the eighth aspect of the present invention, the ninth aspect of the present invention is characterized in that in the step h), the following data are further determined: Ip2-H2O data representing the relationship between the oxygen pump current flowing through the second measurement pump unit and the concentration of water vapor when the measured gas contains water vapor but no carbon dioxide, Ip2-CO2 data representing the relationship between the oxygen pump current flowing through the second measurement pump unit and the concentration of water vapor when the measured gas contains carbon dioxide but no water vapor, and Ip3-H2O data representing the relationship between the oxygen pump current flowing through the third measurement pump unit and the concentration of water vapor when the measured gas contains water vapor but no carbon dioxide. In the step f), the concentration of water vapor corresponding to the value of the water vapor equivalent current in the Ip3-H2O data is determined as the concentration of water vapor contained in the measured gas. In the step g), based on the concentration of water vapor contained in the measured gas determined in the step f) and the Ip2-H2O data, the contribution part of the total reduction current due to the reduction of water vapor is determined, and then the carbon dioxide concentration corresponding to the difference obtained by subtracting the contribution part from the total reduction current in the Ip2-CO2 data is determined as the concentration of carbon dioxide contained in the measured gas.

[0022] Based on the concentration measurement method using a gas sensor according to the eighth aspect of the present invention, the tenth aspect of the present invention is characterized in that in the step h), the following data are further determined: Ip2-CO2 data representing the relationship between the oxygen pump current flowing through the second measurement pump unit and the concentration of water vapor when the measured gas contains carbon dioxide but no water vapor, Ip3-H2O data representing the relationship between the oxygen pump current flowing through the third measurement pump unit and the concentration of water vapor when the measured gas contains water vapor but no carbon dioxide, and H2O characteristic data representing the relationship between the water vapor equivalent current and the oxygen pump current corresponding to the contribution part of water vapor in the total reduction current. In the step f), the concentration of water vapor corresponding to the value of the water vapor equivalent current in the Ip3-H2O data is determined as the concentration of water vapor contained in the measured gas. In the step g), based on the water vapor equivalent current and the H2O characteristic data, the contribution part of the total reduction current due to the reduction of water vapor is determined, and then the carbon dioxide concentration corresponding to the difference obtained by subtracting the contribution part from the total reduction current in the Ip2-CO2 data is determined as the concentration of carbon dioxide contained in the measured gas.

[0023] Based on the concentration measurement method using a gas sensor according to any one of the seventh to tenth aspects of the present invention, the eleventh aspect of the present invention is characterized by further comprising the following steps: i) determining the concentration of oxygen contained in the gas to be measured based on the magnitude of the current flowing between the inner electrode and the outer cavity pump electrode when oxygen is sucked out of the first cavity by the adjustment pump unit.

[0024] Based on the concentration measurement method using a gas sensor according to any one of the seventh to eleventh aspects of the present invention, the twelfth aspect of the present invention is characterized in that the third measurement electrode is a cermet electrode containing a Pt-Au alloy as a metal component, and the Au concentration in the Pt-Au alloy is 1 wt% or more and 50 wt% or less.

[0025] According to the first to twelfth aspects of the present invention, a multi-gas sensor with excellent long-term reliability compared to the prior art and capable of simultaneously measuring a large number of gas types can be realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 FIG. schematically shows an example of the configuration of the gas sensor 100.

[0027] Figure 2 FIG. is a block diagram showing the functional components implemented in the controller 110.

[0028] Figure 3 FIG. is a schematic diagram showing the gas inflow and outflow in the four cavities of the sensor element 101 of the gas sensor 100.

[0029] Figure 4 FIG. is a graph showing the relationship between the target value of the electromotive force V0 in the sensor unit 80 for the first cavity and the oxygen pump current Ip0 flowing through the adjustment pump unit 21 when different three model gases flow.

[0030] Figure 5 FIG. shows the dependence of the oxygen pump current Ip2 on the concentration of the gas component to be monitored.

[0031] Figure 6 FIG. shows the dependence of the oxygen pump current Ip3 on the concentration of the gas component to be monitored.

[0032] Figure 7 FIG. illustrates the H2O characteristic data. DETAILED DESCRIPTION OF THE INVENTION

[0033] <Gas Sensor Configuration>

[0034] Figure 1FIG. 0 schematically shows an example of the configuration of the gas sensor 100 according to the present embodiment. The gas sensor 100 is a multi-gas sensor that monitors multiple gas components through the sensor element 101 and measures their concentrations. In the present embodiment, at least water vapor (H2O), carbon dioxide (CO2), and nitrogen oxides (NOx) are the main monitored gas components in the gas sensor 100. The gas sensor 100 is used, for example, by being installed in the exhaust path of an internal combustion engine such as an automobile engine and setting the exhaust gas flowing through the exhaust path as the gas to be measured. Figure 1 A vertical cross-sectional view along the longitudinal direction including the sensor element 101.

[0035] The sensor element 101 has: a long strip-shaped structure body (base portion) 14 formed of an oxygen ion-conductive solid electrolyte; a first diffusion rate control portion 11 formed at one end (the left end in the drawing) of the structure body 14 and also serving as a gas introduction port 10 for introducing the gas to be measured; and a buffer space 12, a first cavity 20, a second cavity 40, a third cavity 61, and a fourth cavity 63, which are formed in the structure body 14 and communicate with each other in sequence starting from the gas introduction port 10 (the first diffusion rate control portion 11). The buffer space 12 communicates with the gas introduction port 10 (the first diffusion rate control portion 11). The first cavity 20 communicates with the buffer space 12 via a second diffusion rate control portion 13. The second cavity 40 communicates with the first cavity 20 via a third diffusion rate control portion 30. The third cavity 61 communicates with the second cavity 40 via a fourth diffusion rate control portion 60. The fourth cavity 63 communicates with the third cavity 61 via a fifth diffusion rate control portion 62.

[0036] The structure body 14 is formed by laminating multiple substrates formed of, for example, ceramics. Specifically, the structure body 14 has a configuration in which six layers including a first substrate 1, a second substrate 2, a third substrate 3, a first solid electrolyte layer 4, an isolation layer 5, and a second solid electrolyte layer 6 are laminated in this order from the lower side. Each layer is formed of an oxygen ion-conductive solid electrolyte such as zirconia (ZrO2).

[0037] The first diffusion rate control portion 11 that also serves as the gas introduction port 10, the buffer space 12, the second diffusion rate control portion 13, the first cavity 20, the third diffusion rate control portion 30, the second cavity 40, the fourth diffusion rate control portion 60, the third cavity 61, the fifth diffusion rate control portion 62, and the fourth cavity 63 are formed in sequence on one end side of the structure body 14 and are formed between the lower surface 6b of the second solid electrolyte layer 6 and the upper surface 4a of the first solid electrolyte layer 4. The portion from the gas introduction port 10 to the fourth cavity 63 is also referred to as the gas flow portion.

[0038] The buffer space 12, the first cavity 20, the second cavity 40, the third cavity 61, and the fourth cavity 63 are formed to penetrate the isolation layer 5 in the thickness direction. In the above cavities and the like, in the upper part of the drawing thereof, the lower surface 6b of the second solid electrolyte layer 6 is exposed, and in the lower part of the drawing thereof, the upper surface 4a of the first solid electrolyte layer 4 is exposed. The side portions of the above cavities and the like are delimited by the isolation layer 5 or any diffusion rate control portion. The lengths (dimensions in the element length direction) of the first cavity 20, the second cavity 40, the third cavity 61, and the fourth cavity 63 are, for example, 0.3 mm to 1.0 mm, the widths (dimensions in the element width direction) are, for example, 0.5 mm to 30 mm, and the heights (dimensions in the element thickness direction) are, for example, 50 μm to 200 μm. However, the sizes of the respective cavities do not need to be the same and may be different.

[0039] It should be noted that, similarly, the gas inlet 10 may also be formed in a manner that penetrates the isolation layer 5 differently from the first diffusion rate control portion 11. In this case, the first diffusion rate control portion 11 is formed adjacent to a position inside the gas inlet 10.

[0040] The first diffusion rate control portion 11, the second diffusion rate control portion 13, the third diffusion rate control portion 30, the fourth diffusion rate control portion 60, and the fifth diffusion rate control portion 62 each have two horizontally long slits. That is, in the upper and lower parts of the drawing, there are openings that extend relatively long in a direction perpendicular to the drawing. The lengths (dimensions in the element length direction) of the slits are, for example, 0.2 mm to 1.0 mm, the widths (dimensions in the element width direction) of the openings are, for example, 0.5 mm to 30 mm, and the heights (dimensions in the element thickness direction) of the openings are, for example, 5 μm to 30 μm.

[0041] In addition, a reference gas introduction space 43 is provided at the other end (the right end in the drawing) of the sensor element 101 opposite to the end where the gas inlet 10 is provided. The reference gas introduction space 43 is formed between the upper surface 3a of the third substrate 3 and the lower surface 5b of the isolation layer 5. In addition, the side portion of the reference gas introduction space 43 is delimited by the side surface of the first solid electrolyte layer 4. For example, oxygen (O2), air is introduced into the reference gas introduction space 43 as a reference gas.

[0042] Regarding the gas inlet 10, the gas inlet 10 (the first diffusion rate control portion 11) is a portion that is open to the external space, and the gas to be measured is introduced into the sensor element 101 from the external space through the gas inlet 10.

[0043] The first diffusion rate control portion 11 is a portion that imparts a predetermined diffusion resistance to the introduced gas to be measured.

[0044] The purpose of providing the buffer space 12 is to eliminate the concentration variation of the gas to be measured caused by the pressure variation of the gas to be measured in the external space. Examples of such pressure variations of the gas to be measured include pulsations in the exhaust pressure of automotive exhaust gas.

[0045] The second diffusion rate control unit 13 is a part that imparts a prescribed diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first cavity 20.

[0046] The first cavity 20 is provided as a space for sucking out oxygen from the gas to be measured introduced through the second diffusion rate control unit 13. The sucking out of this oxygen is achieved by the operation of the adjustment pump unit 21.

[0047] The adjustment pump unit 21 is an electrochemical pump unit composed of an inner pump electrode (adjustment electrode) 22, an outer pump electrode (outer cavity pump electrode) 23, and a solid electrolyte existing in a portion sandwiched between these two electrodes in the structure 14.

[0048] In the adjustment pump unit 21, a voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23 using a variable power source 24 provided outside the sensor element 101, thereby generating an oxygen pump current (oxygen ion current) Ip0. Thereby, the oxygen in the first cavity 20 can be sucked out to the external space. It should be noted that in the present embodiment, the direction of the oxygen pump current Ip0 when sucking out oxygen from the first cavity 20 is set as the positive direction of the oxygen pump current Ip0.

[0049] The inner pump electrode 22 is provided as a top electrode portion 22a and a bottom electrode portion 22b on substantially the entire lower surface 6b of the second solid electrolyte layer 6 that demarcates the first cavity 20 and on substantially the entire upper surface 4a of the first solid electrolyte layer 4. The top electrode portion 22a and the bottom electrode portion 22b are connected by a conduction portion (not shown).

[0050] The inner pump electrode 22 is provided as a porous cermet electrode that is rectangular in plan view and contains, as a metal component, an alloy of platinum (Pt) and gold (Au) that is inactive with respect to NOx, for example, a Pt - Au alloy and zirconia. Considering that NOx, H2O, and CO2 are not reduced and only the oxygen contained in the gas to be measured is reliably sucked out, it is preferable that the Pt - Au alloy contains Au at a concentration of 0.5 wt% or more.

[0051] The outer pump electrode 23 is provided as a porous cermet electrode that is rectangular in plan view and contains, as a metal component, platinum or a Pt - Au alloy, for example, platinum or a Pt - Au alloy and zirconia.

[0052] In addition, in the sensor element 101, the first cavity sensor unit 80 is constituted by the inner pump electrode 22, the reference electrode 42, and the solid electrolyte present in the portion of the structure 14 sandwiched between these two electrodes. The first cavity sensor unit 80 is an electrochemical sensor unit for grasping the oxygen partial pressure in the atmosphere in the first cavity 20.

[0053] The reference electrode 42 is an electrode formed between the first solid electrolyte layer 4 and the third substrate 3, and is provided, for example, as a porous cermet electrode that includes platinum and zirconia and is rectangular in plan view.

[0054] A reference gas introduction layer 48 formed of porous alumina and connected to the reference gas introduction space 43 is provided around the reference electrode 42. The reference gas in the reference gas introduction space 43 is introduced to the surface of the reference electrode 42 via the reference gas introduction layer 48. That is, the reference electrode 42 is always in contact with the reference gas.

[0055] In the first cavity sensor unit 80, an electromotive force (Nernst electromotive force) V0 is generated between the inner pump electrode 22 and the reference electrode 42. The electromotive force V0 is a value corresponding to the difference between the oxygen concentration (oxygen partial pressure) in the first cavity 20 and the oxygen concentration (oxygen partial pressure) of the reference gas. Among them, the oxygen concentration (oxygen partial pressure) of the reference gas is substantially constant. Therefore, the electromotive force V0 is a value corresponding to the oxygen concentration (oxygen partial pressure) in the first cavity 20.

[0056] The third diffusion rate control unit 30 is a part that imparts a predetermined diffusion resistance to the measurement gas introduced from the first cavity 20 to the second cavity 40.

[0057] The second cavity 40 is set as a space in which NOx contained as a monitoring target gas component in the measurement gas introduced through the third diffusion rate control unit 30 is reduced (decomposed), and the oxygen generated thereby is sucked out so that the measurement gas contains H2O and CO2 but substantially does not contain NOx. The suction of this oxygen is achieved by the operation of the first measurement pump unit 50.

[0058] The first measurement pump unit 50 is an electrochemical pump unit constituted by the first measurement electrode 51, the outer pump electrode 23, and the solid electrolyte present in the portion of the structure 14 sandwiched between these two electrodes.

[0059] In the first measurement pump unit 50, a voltage Vp1 is applied between the first measurement electrode 51 and the outer pump electrode 23 by using a variable power source 52 provided outside the sensor element 101, thereby generating an oxygen pump current (oxygen ion current) Ip1. Accordingly, oxygen in the second cavity 40 can be sucked out to the external space. It should be noted that in the present embodiment, the direction of the oxygen pump current Ip1 when oxygen is sucked out from the second cavity 40 is set as the positive direction of the oxygen pump current Ip1.

[0060] The first measurement electrode 51 is provided as a top electrode portion 51a and a bottom electrode portion 51b on substantially the entire lower surface 6b of the second solid electrolyte layer 6 that demarcates the second cavity 40 and on substantially the entire upper surface 4a of the first solid electrolyte layer 4. The top electrode portion 51a and the bottom electrode portion 51b are connected by a conduction portion (not shown).

[0061] The first measurement electrode 51 is set as a porous cermet electrode that is rectangular in plan view and contains, for example, a Pt-Rh alloy and zirconia, with the metal component being an alloy of platinum and rhodium (Pt-Rh alloy). The Rh concentration in the Pt-Rh alloy is preferably 30 wt% or more.

[0062] In addition, in the sensor element 101, the first measurement electrode 51, the reference electrode 42, and the solid electrolyte existing in the portion sandwiched by these two electrodes in the structure 14 constitute a second cavity sensor unit 81. The second cavity sensor unit 81 is an electrochemical sensor unit for grasping the oxygen partial pressure in the atmosphere in the second cavity 40.

[0063] In the second cavity sensor unit 81, an electromotive force (Nernst electromotive force) V1 is generated between the first measurement electrode 51 and the reference electrode 42. The electromotive force V1 is a value corresponding to the difference between the oxygen concentration (oxygen partial pressure) in the second cavity 40 and the oxygen concentration (oxygen partial pressure) of the reference gas. Among them, since the oxygen concentration (oxygen partial pressure) of the reference gas is substantially constant, the electromotive force V1 is a value corresponding to the oxygen concentration (oxygen partial pressure) in the second cavity 40.

[0064] The fourth diffusion rate control unit 60 is a part that imparts a predetermined diffusion resistance to the measured gas containing H2O and CO2 but substantially free of NOx and oxygen introduced from the second cavity 40 into the third cavity 61.

[0065] The third cavity 61 is set as a space in which H2O and CO2 contained as monitored target gas components in the measured gas introduced through the fourth diffusion rate control unit 60 are reduced (decomposed) to generate hydrogen (H2) and carbon monoxide (CO), so that the measured gas contains not only no NOx and oxygen but also is substantially free of H2O and CO2. The above reduction (decomposition) of H2O and CO2 is achieved by the operation of the second measurement pump unit 41.

[0066] The second measurement pump unit 41 is an electrochemical pump unit composed of a second measurement electrode 44, an outer pump electrode 23, and a solid electrolyte existing in a portion of the structure 14 sandwiched between these two electrodes.

[0067] In the second measurement pump unit 41, a voltage Vp2 is applied between the second measurement electrode 44 and the outer pump electrode 23 by a variable power supply 46 provided outside the sensor element 101, thereby generating an oxygen pump current (oxygen ion current) Ip2. Thus, oxygen generated in the third cavity 61 by the reduction of H2O and CO2 can be sucked out to the external space. It should be noted that in the present embodiment, the direction of the oxygen pump current Ip2 when sucking oxygen from the third cavity 61 is set as the positive direction of the oxygen pump current Ip2.

[0068] The second measurement electrode 44 is provided as a top electrode portion 44a and a bottom electrode portion 44b on substantially the entire lower surface 6b of the second solid electrolyte layer 6 that demarcates the third cavity 61 and on substantially the entire upper surface 4a of the first solid electrolyte layer 4. The top electrode portion 44a and the bottom electrode portion 44b are connected by a conduction portion (not shown).

[0069] The second measurement electrode 44 is provided as a porous cermet electrode that is rectangular in plan view with Pt as the metal component.

[0070] In addition, in the sensor element 101, a third cavity sensor unit 82 is composed of a second measurement electrode 44, a reference electrode 42, and a solid electrolyte existing in a portion of the structure 14 sandwiched between these two electrodes. The third cavity sensor unit 82 is an electrochemical sensor unit for grasping the oxygen partial pressure in the atmosphere in the third cavity 61.

[0071] In the third cavity sensor unit 82, an electromotive force (Nernst electromotive force) V2 is generated between the second measurement electrode 44 and the reference electrode 42. The electromotive force V2 is a value corresponding to the difference between the oxygen concentration (oxygen partial pressure) in the third cavity 61 and the oxygen concentration (oxygen partial pressure) of the reference gas. Among them, the oxygen concentration (oxygen partial pressure) of the reference gas is substantially constant. Therefore, the electromotive force V2 is a value corresponding to the oxygen concentration (oxygen partial pressure) in the third cavity 61.

[0072] The fifth diffusion rate control unit 62 is a part that imparts a prescribed diffusion resistance to the measurement gas containing H2 and CO but substantially free of H2O, CO2, NOx, and oxygen introduced from the third cavity 61 to the fourth cavity 63.

[0073] The fourth cavity 63 is set as the following space: the H2 in the measured gas introduced through the fifth diffusion rate control unit 62 is selectively and completely oxidized, and H2O is regenerated again. The oxidation of this H2 to generate H2O is achieved by the operation of the third measurement pump unit 66.

[0074] The third measurement pump unit 66 is an electrochemical pump unit composed of a third measurement electrode 64, an outer pump electrode 23, and a solid electrolyte existing in the part sandwiched by these two electrodes in the structure 14.

[0075] In the third measurement pump unit 66, a voltage Vp3 is applied between the third measurement electrode 64 and the outer pump electrode 23 by a variable power supply 68 provided outside the sensor element 101, thereby generating an oxygen pump current (oxygen ion current) Ip3. Thus, oxygen can be inhaled from the external space into the fourth cavity 63. It should be noted that in this embodiment, the direction of the oxygen pump current Ip3 when oxygen is sucked out from the fourth cavity 63 is set as the positive direction of the oxygen pump current Ip3.

[0076] The third measurement electrode 64 is provided on substantially the entire upper surface 4a of the first solid electrolyte layer 4 that demarcates the fourth cavity 63.

[0077] The third measurement electrode 64 is set as a porous cermet electrode that includes a Pt-Au alloy as a metal component, for example, includes the Pt-Au alloy and zirconia and is rectangular in plan view. The Au concentration in the Pt-Au alloy is preferably 1 wt% or more and 50 wt% or less, more preferably 10 wt% or more and 30 wt% or less. In this case, the selective oxidizability of H2 at the third measurement electrode 64 is better exhibited, that is, when H2 and CO coexist in the fourth cavity 63, only H2 is selectively oxidized by the oxygen inhaled by the third measurement pump unit 66, but CO is not oxidized.

[0078] In addition, in the sensor element 101, a fourth cavity sensor unit 83 is composed of the third measurement electrode 64, a reference electrode 42, and a solid electrolyte existing in the part sandwiched by these two electrodes in the structure 14. The fourth cavity sensor unit 83 is an electrochemical sensor unit for grasping the oxygen partial pressure in the atmosphere in the fourth cavity 63.

[0079] In the fourth cavity sensor unit 83, an electromotive force (Nernst electromotive force) V3 is generated between the third measurement electrode 64 and the reference electrode 42. The electromotive force V3 is a value corresponding to the difference between the oxygen concentration (oxygen partial pressure) in the fourth cavity 63 and the oxygen concentration (oxygen partial pressure) of the reference gas. Among them, the oxygen concentration (oxygen partial pressure) of the reference gas is basically constant. Therefore, the electromotive force V3 is a value corresponding to the oxygen concentration (oxygen partial pressure) in the fourth cavity 63.

[0080] In addition, the sensor element 101 further includes an electrochemical sensor unit 84 composed of an outer pump electrode 23, a reference electrode 42, and a solid electrolyte existing in a portion sandwiched between these two electrodes in the structure 14. The electromotive force Vref generated between the outer pump electrode 23 and the reference electrode 42 in the sensor unit 84 is a value corresponding to the oxygen partial pressure of the gas to be measured existing outside the sensor element 101.

[0081] In addition to the above components, the sensor element 101 further includes a heater unit 70 that undertakes the temperature adjustment function of heating and maintaining the temperature of the sensor element 101, so as to improve the oxygen ion conductivity of the solid electrolyte constituting the structure 14.

[0082] The heater unit 70 mainly includes: a heater electrode 71, a heater member 72, a heater lead 72a, a through hole 73, a heater insulating layer 74, and Figure 1 a heater resistance detection lead (not shown in the figure) is omitted hereinafter. Hereinafter, the heater member 72 will also be simply referred to as the heater 72.

[0083] The heater 72 is arranged in such a way that it is sandwiched between the second substrate 2 and the third substrate 3 from above and below, and generates heat by being powered from the outside through the heater electrode 71, the through hole 73, and the heater lead 72a provided on the lower surface 1b of the first substrate 1. The heater 72 is implanted in the entire area of the range from the buffer space 12 to the fourth cavity 63, and can heat the sensor element 101 to a specified temperature and maintain the temperature.

[0084] The heater 72 is arranged such that, during heating, the vicinity of the first cavity 20 (the vicinity of the inner pump electrode 22) becomes the highest temperature, and in the element length direction, the temperature decreases as the distance from the first cavity 20 increases. In the present embodiment, the temperature in the range from one end of the sensor element 101 where the gas inlet 10 is located to the fourth cavity 63 when the gas sensor 100 is in use (when the sensor element 101 is driven) is referred to as the element driving temperature. The heater 72 is heated in such a way that the element driving temperature reaches within the range of 750°C to 950°C.

[0085] Above and below the heater 72, a heater insulating layer 74 made of alumina or the like is formed for the purpose of obtaining electrical insulation from the second substrate 2 and the third substrate 3. In addition, the heater unit 70 is provided with a pressure release hole 75. The pressure release hole 75 is a portion provided to penetrate the third substrate 3 and communicate with the reference gas introduction space 43. The purpose of providing the pressure release hole 75 is to relieve the increase in internal pressure accompanying the temperature rise in the heater insulating layer 74.

[0086] In addition, the gas sensor 100 further includes a controller 110 that controls the operation of the sensor element 101 and performs the following processing, that is, determines the concentration of the monitored gas component based on the current flowing through the sensor element 101.

[0087] Figure 2 FIG. is a block diagram showing the functional components implemented in the controller 110. The controller 110 is composed of one or more electronic circuits having, for example, one or more CPUs (Central Processing Units) and storage devices. The electronic circuit is also a software functional unit that implements the specified functional components by the CPU executing a specified program stored in the storage device, for example. Of course, it may also be composed of an integrated circuit such as an FPGA (Field-Programmable Gate Array) obtained by connecting multiple electronic circuits according to functions.

[0088] It should be noted that when the gas sensor 100 is installed in the exhaust path of an automotive engine and the exhaust gas flowing through the exhaust path is used as the gas to be measured, part or all of the functions of the controller 110 can be implemented by the ECU (Electronic Control Unit) of the vehicle.

[0089] In the controller 110, as functional components implemented by executing a specified program in the CPU, there are provided: an element operation control unit 120 that controls the operations of respective parts of the above-mentioned sensor element 101; and a concentration determination unit 130 that performs processing to determine the concentration of the monitored gas component contained in the gas to be measured.

[0090] The element operation control unit 120 mainly includes: an adjustment pump unit control unit 121 that controls the operation of the adjustment pump unit 21; a first measurement pump unit control unit 122a that controls the operation of the first measurement pump unit 50; a second measurement pump unit control unit 122b that controls the operation of the second measurement pump unit 41; a third measurement pump unit control unit 122c that controls the operation of the third measurement pump unit 66; and a heater control unit 123 that controls the heating operation of the heater 72.

[0091] On the other hand, the concentration determination unit 130 mainly includes: a NOx concentration determination unit 130N, a water vapor concentration determination unit 130H, and a carbon dioxide concentration determination unit 130C that respectively determine the concentrations of the main monitored gas components in the gas sensor 100, namely NOx, H2O, and CO2.

[0092] The NOx concentration determination unit 130N determines the concentration of NOx contained in the gas to be measured based on the value of the oxygen pump current Ip1 flowing through the first measurement pump unit 50 obtained by the first measurement pump unit control unit 122a.

[0093] The water vapor concentration determination unit 130H determines the concentration of H2O contained in the gas to be measured based on the value of the oxygen pump current Ip3 flowing through the third measurement pump unit 66 obtained by the third measurement pump unit control unit 122c.

[0094] The carbon dioxide concentration determination unit 130C determines the concentration of CO2 contained in the gas to be measured based on the concentration of H2O determined in the water vapor concentration determination unit 130H (the value of the oxygen pump current Ip3 serving as the basis for the determination) and the value of the oxygen pump current Ip2 flowing through the second measurement pump unit 41 obtained by the second measurement pump unit control unit 122b.

[0095] The concentration determination unit 130 further includes an oxygen concentration determination unit 130A that determines the concentration of oxygen contained in the gas to be measured. The oxygen concentration determination unit 130A determines the concentration of oxygen contained in the gas to be measured based on the value of the oxygen pump current Ip0 flowing through the adjustment pump unit 21 obtained by the adjustment pump unit control unit 121. That is, in the gas sensor 100 according to the present embodiment, in addition to NOx, H2O, and CO2, which are the main monitored gas components, oxygen is also monitored as an incidental monitored gas component.

[0096] <Multi-gas monitoring and concentration determination>

[0097] Next, a method for monitoring multiple gas types (multi-gas monitoring) and determining the concentration of the monitored gas implemented in the gas sensor 100 having the above-described configuration will be described. Hereinafter, the gas to be measured is exhaust gas containing oxygen, NOx, H2O, and CO2.

[0098] Figure 3 It is a schematic diagram showing the gas inflow and outflow states in the four cavities (internal cavities) of the sensor element 101 of the gas sensor 100.

[0099] First, in the sensor element 101 included in the gas sensor 100 according to the present embodiment, as described above, the gas to be measured is introduced into the first cavity 20 through the gas inlet 10 (first diffusion rate control unit 11), the buffer space 12, and the second diffusion rate control unit 13. In the first cavity 20, the adjustment pump unit 21 operates to suck out oxygen from the introduced gas to be measured.

[0100] The suction of oxygen described above is carried out as follows. That is, the adjustment pump unit control section 121 of the controller 110 sets the target value (control voltage) of the electromotive force V0 in the first cavity sensor unit 80 to a value within the range of 300 mV to 500 mV (preferably 350 mV). According to the difference between the actual value of the electromotive force V0 and the target value, feedback control is performed on the voltage Vp0 applied by the variable power supply 24 to the adjustment pump unit 21 so that the electromotive force V0 is maintained at this target value. For example, when the measured gas containing a large amount of oxygen reaches the first cavity 20, the value of the electromotive force V0 changes significantly relative to the target value. Therefore, the adjustment pump unit control section 121 controls the pump voltage Vp0 applied by the variable power supply 24 to the adjustment pump unit 21 to reduce this change.

[0101] By sucking oxygen from the first cavity 20 in this way using the adjustment pump unit 21, the oxygen partial pressure (concentration) in the first cavity 20 is maintained at a sufficiently low value within the range where the reduction of H2O and CO2 contained in the measured gas does not occur. For example, by operating the adjustment pump unit 21, the oxygen concentration in the first cavity 20 is made to reach about 0.1 ppm to about 0.00001 ppm.

[0102] Figure 4 It is a diagram for explaining the reason for sucking oxygen within the range where the reduction of H2O and CO2 does not occur by setting the target value of the electromotive force V0 to a value within the range of 300 mV to 500 mV. Specifically, Figure 4 It is a graph showing the relationship between the target value (control voltage) of the electromotive force V0 in the first cavity sensor unit 80 when different three types of model gases flow and the oxygen pump current Ip0 flowing through the adjustment pump unit 21. Regarding the three types of model gases, specifically, the first gas containing 10% oxygen, the second gas containing 10% oxygen and 10% CO2, and the third gas containing 10% oxygen and 10% H2O. In these gases, the rest is nitrogen (N2). It should be noted that the element driving temperature is 800 °C or higher, and the temperature of the model gas is 150 °C.

[0103] By Figure 4 It was confirmed that in the case of the first gas, within the range where the control voltage is 0.3 V or higher, the oxygen pump current Ip0 is substantially constant. In contrast, in the case of the second gas and the third gas, within the range where the control voltage is 0.7 V or lower, it shows a curve substantially the same as that of the first gas. However, if the control voltage exceeds 0.7 V, the oxygen pump current Ip0 increases again. This increase is caused by the superposition of the reduction current of H2O or CO2 generated by the reduction (decomposition) of H2O or CO2 contained in the measured gas and the oxygen flowing through.

[0104] Based on this, in the present embodiment, the target value of the electromotive force V0 is set to a value within the range of 300 mV to 500 mV.

[0105] It should be noted that regarding NOx, similar to H2O and CO2, it is not reduced as the pump unit 21 sucks in oxygen for adjustment. However, this does not depend on the setting method of the target value of the electromotive force V0 because, as described above, the inner pump electrode 22 contains Au that is inactive with respect to NOx.

[0106] In this way, in the gas sensor 100 according to the present embodiment, different from the conventional gas sensors, in the first cavity 20 where the highest temperature is reached in the sensor element 101 during operation, only oxygen is sucked in without reducing H2O and CO2, and H2O and CO2 are not reduced. In addition, NOx is not reduced either.

[0107] The target value of the electromotive force V0 in the sensor unit 80 for the first cavity set for the above-mentioned oxygen suction is 300 mV to 500 mV, which is sufficiently small compared to the target value of 1000 mV to 1500 mV set when reducing H2O and CO2. Accordingly, an increase in the pump voltage Vp0 is suppressed compared to the voltage applied to the corresponding pump unit of the conventional gas sensor accompanied by the reduction of H2O and CO2. Thus, in the gas sensor 100 according to the present embodiment, the occurrence of cracking and blackening caused by applying a high voltage while the inner pump electrode 22 is maintained at a high temperature is well suppressed.

[0108] In addition, since the inner pump electrode 22 contains Au that is inactive with respect to NOx, even when the gas to be measured contains NOx, NOx is not reduced as the pump unit 21 sucks in oxygen for adjustment.

[0109] The gas to be measured in which only oxygen is sucked in without reducing NOx, H2O, and CO2 in the first cavity 20 is introduced into the second cavity 40. And in this second cavity 40, the NOx contained in the gas to be measured is reduced. The first measurement electrode 51 provided in the second cavity 40 and constituting the first measurement pump unit 50 has a Pt - Rh alloy as the metal component and does not contain Au that is inactive with respect to NOx. Therefore, NOx is reduced at the first measurement electrode 51. That is, by operating the first measurement pump unit 50, after sucking in oxygen in the first cavity 20, an oxygen suction operation is further performed on the gas to be measured introduced into the second cavity 40, thereby performing a reduction (decomposition) reaction of NOx (e.g., 2NO → 2N2 + O2) contained in the gas to be measured, and NO is substantially completely decomposed into nitrogen and oxygen.

[0110] The reduction (decomposition) of the above-mentioned NOx and the suction of the resulting oxygen are carried out as follows: The first measurement pump unit control section 122a of the controller 110 sets the target value (control voltage) of the electromotive force V1 in the second cavity sensor unit 81 to a value within the range of 350 mV to 700 mV (preferably 400 mV), and feedback-controls the voltage Vp1 applied by the variable power supply 52 to the first measurement pump unit 50 according to the difference between the actual value of the electromotive force V1 and the target value, so as to keep the electromotive force V1 at this target value.

[0111] By operating the first measurement pump unit 50 in this way, the oxygen partial pressure in the second cavity 40 is kept at a value equal to or slightly lower than that in the first cavity 20. For example, when V2 = 400 mV, it is about 10 －8 atm. Thus, the gas to be measured contains H2O and CO2 (and N2), but substantially does not contain NOx and oxygen.

[0112] Moreover, in the gas sensor 100 according to the present embodiment, the concentration of NOx in the gas to be measured is determined based on the oxygen pump current Ip1 flowing through the first measurement pump unit 50 when sucking out oxygen including the reduction of NOx.

[0113] The oxygen pump current Ip1 flowing through the first measurement pump unit 50 (hereinafter also referred to as the NOx detection current Ip1) flows as oxygen generated by the decomposition of NOx contained in the gas to be measured is sucked out. Therefore, the magnitude of the NOx detection current Ip1 is approximately proportional to the concentration of NOx contained in the gas to be measured introduced from the gas inlet 10. That is, a linear relationship holds between the NOx detection current Ip1 and the concentration of NOx in the gas to be measured. Regarding the data representing this linear relationship (Ip1-NOx data), it is determined in advance using a model gas with a known NOx concentration and stored in the controller 110.

[0114] When the gas sensor 100 actually performs measurement, the NOx concentration determination section 130N obtains the value of the NOx detection current Ip1 from the first measurement pump unit control section 122a. And, referring to the Ip1-NOx data, it determines the value of the oxygen concentration corresponding to the obtained NOx detection current Ip1. Thus, the concentration of NOx in the gas to be measured is determined.

[0115] The measured gas in which NOx has been reduced within a range where H2O and CO2 are not reduced in the second cavity 40 is introduced into the third cavity 61. In the third cavity 61, the reduction of H2O and CO2 contained in the measured gas is carried out. That is, by operating the second measurement pump unit 41, the oxygen suction operation is further performed on the measured gas in which oxygen has been sucked out in the first cavity 20 and NOx has been reduced in the second cavity 40, thereby carrying out the reduction (decomposition) reaction of H2O and CO2 contained in the measured gas (2H2O → 2H2 + O2, 2CO2 → 2CO + O2), and substantially all of H2O and CO2 are decomposed into hydrogen (H2), carbon monoxide (CO), and oxygen.

[0116] The reduction (decomposition) of the above H2O and CO2 and the suction of the generated oxygen are carried out as follows: The second measurement pump unit control section 122b of the controller 110 sets the target value (control voltage) of the electromotive force V2 in the third cavity sensor unit 82 to a value within the range of 1000 mV to 1500 mV (preferably 1000 mV), and feedback-controls the voltage Vp2 applied by the variable power supply 46 to the second measurement pump unit 41 according to the difference between the actual value of the electromotive force V2 and the target value, so as to keep the electromotive force V2 at the above target value. It should be noted that Figure 4 The shown graph also implies that it is more ideal to set the target value of the electromotive force V2 to a value within the range of 1000 mV to 1500 mV.

[0117] By operating the second measurement pump unit 41 in the above manner, the oxygen partial pressure in the third cavity 61 is maintained at a value lower than the oxygen partial pressures in the first cavity 20 and the second cavity 40. For example, when V2 = 1000 mV, it is about 10 －20 atm. Thus, the measured gas contains H2 and CO (and N2), but substantially does not contain NOx, H2O, CO2, and oxygen.

[0118] The measured gas containing H2 and CO, but substantially not containing NOx, H2O, CO2, and oxygen is introduced into the fourth cavity 63.

[0119] In the fourth cavity 63, oxygen is sucked in by operating the third measurement pump unit 66, and only H2 contained in the introduced measured gas is selectively oxidized.

[0120] The inhalation of oxygen is carried out as follows: The third measurement pump unit control section 122c of the controller 110 sets the target value (control voltage) of the electromotive force V3 in the fourth cavity sensor unit 83 to a value within the range of 250 mV to 450 mV (preferably 350 mV), and feedback-controls the voltage Vp3 applied by the variable power supply 68 to the third measurement pump unit 66 according to the difference between the actual value of the electromotive force V3 and the target value, so as to keep the electromotive force V3 at this target value.

[0121] By operating the third measurement pump unit 66 in this way, in the fourth cavity 63, the oxidation (combustion) reaction of 2H2 + O2 → 2H2O is promoted, and an amount of H2O that has a correlation with the amount of H2O introduced from the gas inlet 10 is regenerated. It should be noted that in this embodiment, the so-called "having a correlation with the amount of H2O" means that the amount of H2O introduced from the gas inlet 10 and the amount of H2O regenerated by the oxidation of the H2 decomposed therefrom are the same, or within a certain error range allowed in consideration of the measurement accuracy.

[0122] By setting the target value of the electromotive force V3 to a value within the range of 250 mV to 450 mV, the oxygen partial pressure in the fourth cavity 63 is maintained at a value in the range where H2 is almost completely oxidized but CO is not oxidized. For example, when V3 = 350 mV, it is about 10 －7 atm.

[0123] In addition, as described above, setting the third measurement electrode 64 as a cermet electrode containing a Pt - Au alloy with an Au concentration of 1 wt% or more and 50 wt% or less as a metal component also helps to improve the selective oxidizability of H2.

[0124] In addition, measures such as further suppressing the temperature rise of the third measurement electrode 64 by making efforts on the shape (width, thickness), arrangement (density), etc. of the heater 72 can also be taken.

[0125] In the gas sensor 100 according to this embodiment that operates in the above manner, the concentrations of H2O and CO2 in the gas to be measured are determined based on the oxygen pump current Ip2 flowing through the second measurement pump unit 41 during the oxygen extraction including the reduction of H2O and CO2, and the oxygen pump current Ip3 flowing through the third measurement pump unit 66 during the oxygen inhalation for the oxidation of H2.

[0126] Figure 5 and Figure 6It is a graph showing the dependence of the oxygen pump current Ip2 and the oxygen pump current Ip3 on the concentration of the monitored gas component in the case where only one of H2O and CO2, which are the main monitored gas components, is separately included in the measured gas and in the case where H2O and CO2 at equal concentrations are included in the measured gas.

[0127] Figure 5 and Figure 6 In, the curve showing the case where only H2O is included as the monitored gas component is marked with circles, the curve showing the case where only CO2 is included as the monitored gas component is marked with triangles, and the curve showing the case where H2O and CO2 at equal concentrations are included as the monitored gas component (recorded as "H2O + CO2" in the figure) is marked with quadrilaterals. It should be noted that these curves are obtained by operating the gas sensor 100 in an atmosphere of a model gas with the concentration of the monitored gas component known and the remainder being oxygen and nitrogen. The element driving temperature is 800 °C or higher, and the temperature of the model gas is 200 °C.

[0128] It can be seen from Figure 5 that: in the case where only H2O is included as the monitored gas component and in the case where only CO2 is included as the monitored gas component, the curves both increase monotonically and are approximately linear.

[0129] In addition, the value of the oxygen pump current Ip2 in the case where H2O and CO2 at equal concentrations are included as the monitored gas component is the sum of the oxygen pump currents Ip2 in the cases where H2O and CO2 are separately included. Also, although not shown in the figure, it was confirmed that: the value of the oxygen pump current Ip2 when the ratio of H2O and CO2 is different is also the sum of the oxygen pump currents Ip2 in the cases where H2O and CO2 at concentrations corresponding to their respective ratios are separately included.

[0130] On the other hand, as Figure 6 shown, the curve of the oxygen pump current Ip3 in the case where only H2O is included as the monitored gas component decreases monotonically (the absolute value increases monotonically) and is approximately linear. It should be noted that the oxygen pump current Ip3 is negative because: in the third measurement pump unit 66, as described above, the direction of sucking oxygen is set as the positive direction of the oxygen pump current, and on the other hand, in order to re-oxidize the H2 generated by the reduction in the third cavity 61, the oxygen pump current Ip3 flows in the direction of sucking oxygen.

[0131] In contrast, the value of the oxygen pump current Ip3 in the case where only CO2 is included as the monitored gas component remains approximately zero. This shows that: the CO generated by the reduction in the third cavity 61 is not re-oxidized by the operation of the third measurement pump unit 66.

[0132] In addition, the curve of the oxygen pump current Ip3 in the case where H2O and CO2 at the same concentration are included as the monitored gas components is substantially the same as the curve of the oxygen pump current Ip3 in the case where only H2O is included. This indicates that: the oxygen pump current Ip3 in the case where only CO2 is included as the monitored gas component is substantially zero. It should be noted that although not shown in the figure, it was confirmed that: the values of the oxygen pump current Ip3 when the ratio of H2O and CO2 is different are also substantially the same as the curves of the oxygen pump current Ip3 in the cases where H2O and CO2 are included separately. This means that: the oxygen pump current Ip3 actually depends only on the concentration of H2O. Therefore, if the oxygen pump current Ip3 is known, the concentration of H2O can be determined.

[0133] In this embodiment, the concentrations of H2O and CO2 in the gas to be measured are measured by using the properties of the oxygen pump current Ip2 and the oxygen pump current Ip3 as described above. Hereinafter, the oxygen pump current Ip2 and the oxygen pump current Ip3 during actual measurement using the gas sensor 100 are also referred to as the full reduction current Ip2 and the water vapor equivalent current Ip3, respectively.

[0134] Specifically, before using the gas sensor 100, a model gas with a known concentration is used in advance to obtain Figure 5 characteristic data showing the relationship between the oxygen pump current Ip2 and the concentration of each gas in the case where only one of H2O and CO2 is included in the gas to be measured but the other is not (hereinafter, referred to as Ip2-H2O data and Ip2-CO2 data, respectively), and Figure 6 characteristic data showing the relationship between the oxygen pump current Ip3 and the concentration of H2O in the case where H2O is included in the gas to be measured but CO2 is not (hereinafter, referred to as Ip3-H2O data), and are stored in the controller 110. It should be noted that the Ip2-H2O data and the Ip2-CO2 data are values representing the contribution part of H2O and the contribution part of CO2 in the full reduction current Ip2, respectively.

[0135] It should be noted that the oxygen pump current Ip2 is a value corresponding to the diffusion resistance imparted to the gas to be measured from the gas inlet 10 of the sensor element 101 to the third cavity 61, and the oxygen pump current Ip3 is a value corresponding to the diffusion resistance imparted to the gas to be measured from the gas inlet 10 of the sensor element 101 to the fourth cavity 63. Therefore, strictly speaking, the Ip2-H2O data, the Ip2-CO2 data, and the Ip3-H2O data are different among the individual sensor elements 101 constituting each gas sensor 100. Therefore, it is preferable to determine these characteristic data for each gas sensor 100. Among them, for the gas sensors 100 manufactured under the same conditions and in the same batch, the following scheme may be adopted: when it is confirmed that the error is within the allowable range, the characteristic data obtained for a specific gas sensor 100 is applied to other gas sensors 100 in the same batch.

[0136] Moreover, when the gas sensor 100 actually performs measurement, the gas to be measured is introduced into the sensor element 101 heated to the element driving temperature, and the pump unit 21, the first measurement pump unit 50, the second measurement pump unit 41, and the third measurement pump unit 66 are adjusted to operate in the above-described manner. Then, the water vapor concentration determination unit 130H obtains the water vapor equivalent current Ip3 from the third measurement pump unit control unit 122c, and determines the H2O concentration corresponding to the obtained value based on the Ip3-H2O data.

[0137] After determining the H2O concentration, next, the carbon dioxide concentration determination unit 130C obtains the value of the full reduction current Ip2 from the second measurement pump unit control unit 122b, and based on the Ip2-H2O data, determines the contribution part of the full reduction current Ip2 of the H2O at the determined concentration, that is, the current amount obtained by the reduction of H2O in the full reduction current Ip2. The contribution part of CO2 in the full reduction current Ip2 is determined by subtracting the obtained value from the value of the full reduction current Ip2. Finally, based on the Ip2-CO2 data, the CO2 concentration corresponding to the contribution part of CO2 is determined.

[0138] In the gas sensor 100 according to the present embodiment, the H2O concentration and the CO2 concentration in the gas to be measured are measured by the above operations.

[0139] Alternatively, the relationship between the water vapor equivalent current Ip3 and the oxygen pump current Ip2 corresponding to the contribution part of H2O in the full reduction current Ip2 is determined in advance, and the characteristic data representing this relationship (hereinafter, referred to as H2O characteristic data) is stored in the controller 110. The carbon dioxide concentration determination unit 130C can directly determine the contribution part of H2O in the full reduction current Ip2 according to the water vapor equivalent current Ip3 by using the H2O characteristic data.

[0140] Figure 7This is a diagram illustrating the characteristic data of H2O. It should be noted that Figure 7 in this figure, the absolute value of the water vapor equivalent current Ip3 is set as the x-axis, and the value of the oxygen pump current Ip2 corresponding to the contribution part of H2O in the total reduction current Ip2 is set as the y-axis and shown. As Figure 7 shown, a linear relationship holds between the water vapor equivalent current Ip3 and the contribution part of H2O in the total reduction current Ip2. Therefore, it is only necessary to determine the relational expression representing this linear relationship as the H2O characteristic data.

[0141] Alternatively, the value of the y-intercept in this relational expression is theoretically zero, and in the case of the normally operating gas sensor 100, it is actually also a value small enough to be regarded as zero. Therefore, only the slope of the expression representing the above linear relationship is stored in the controller 110 as the H2O characteristic data in advance. The carbon dioxide concentration determination unit 130C can use the product of the value of this slope and the water vapor equivalent current Ip3 as the contribution part of H2O in the total reduction current Ip2.

[0142] It should be noted that the slope in the H2O characteristic data is equivalent to the ratio of the diffusion resistance imparted to the measured gas from the gas inlet 10 to the fourth cavity 63 to the diffusion resistance imparted to the measured gas from the gas inlet 10 to the third cavity 61.

[0143] In addition, while determining the NOx concentration, H2O concentration, and CO2 concentration, the oxygen concentration is also determined by using the oxygen pump current Ip0 flowing through the adjustment pump unit 21.

[0144] In the gas sensor 100 according to this embodiment, as described above, by operating the adjustment pump unit 21, oxygen is sucked out from the measured gas introduced from the gas inlet 10 in the first cavity 20. The suction of the above oxygen is performed in such a way that the reduction of NOx, H2O, and CO2 does not occur. However, at this time, the oxygen pump current Ip0 (hereinafter, also referred to as the oxygen detection current Ip0) flowing through is approximately proportional to the concentration of oxygen contained in the measured gas introduced from the gas inlet 10. That is, a linear relationship holds between the oxygen detection current Ip0 and the oxygen concentration in the measured gas. Regarding the data (Ip0 - O2 data) representing this linear relationship, it is determined in advance using a model gas with a known oxygen concentration and stored in the controller 110.

[0145] When the gas sensor 100 actually performs measurement, the oxygen concentration determination unit 130A obtains the value of the oxygen detection current Ip0 from the adjustment pump unit control unit 121. Then, referring to the Ip0 - O2 data, the value of the oxygen concentration corresponding to the obtained oxygen detection current Ip0 is determined. Thus, the oxygen concentration in the measured gas is determined.

[0146] As described above, in the gas sensor according to the present embodiment, as in the case of conventional gas sensors, when the gas to be measured contains both H2O and CO2, the concentrations of both can be measured. In addition to this, the concentration of NOx can also be measured simultaneously. Further, the oxygen concentration can be accurately determined.

[0147] In addition, in the case of the gas sensor according to the present embodiment, different from the gas sensors of the prior art, reduction of H2O and CO2 does not occur in the first cavity that reaches the highest temperature during operation. Therefore, the voltage applied to the adjustment pump unit that sucks oxygen from the first cavity is suppressed to be lower than that of the gas sensors of the prior art. Thus, cracking and blackening of the sensor element are well suppressed.

[0148] That is, according to the present embodiment, a multi-gas sensor can be realized that has excellent long-term reliability compared to the prior art and can simultaneously measure a relatively large number of gas species.

Claims

1. A gas sensor capable of measuring the concentration of multiple monitoring target gas components. The gas sensor is characterized in that it has: A sensor element having a structure composed of an oxygen ion conductive solid electrolyte; and a controller, the controller controlling the action of the gas sensor, The sensor element comprises: A gas inlet port, the gas inlet port is used to introduce the measured gas; A first cavity, a second cavity, a third cavity and a fourth cavity, wherein the cavities are sequentially connected from the gas inlet via different diffusion rate control parts; an adjustment pump unit, the adjustment pump unit comprising an inner electrode formed facing the first cavity, an outer cavity pump electrode provided at a location other than the first cavity, the second cavity, the third cavity, and the fourth cavity, and the solid electrolyte existing between the inner electrode and the outer cavity pump electrode; a first measuring pump cell, the first measuring pump cell comprising a first measuring electrode formed facing the second cavity, the cavity outer pump electrode, and the solid electrolyte existing between the first measuring electrode and the cavity outer pump electrode; a second measuring pump cell, the second measuring pump cell comprising a second measuring electrode formed facing the third cavity, the cavity outer pump electrode, and the solid electrolyte existing between the second measuring electrode and the cavity outer pump electrode; a third measuring pump cell, the third measuring pump cell comprising a third measuring electrode formed facing the fourth cavity, the cavity-external pump electrode, and the solid electrolyte existing between the third measuring electrode and the cavity-external pump electrode; and a heater for heating the sensor element, The inner electrode is a metal ceramic electrode containing a Pt-Au alloy as a metal component and the Au concentration in the Pt-Au alloy is 0.5 wt % or more. The first measuring electrode is another metal ceramic electrode containing a Pt-Rh alloy as a metal component, The adjustment pump unit absorbs oxygen from the gas to be measured introduced into the first cavity from the gas introduction port so that NOx, water vapor, and carbon dioxide contained in the gas to be measured are not decomposed. The first measuring pump unit sucks oxygen from the second cavity in such a manner that substantially all of the NOx contained in the gas to be measured introduced from the first cavity into the second cavity is reduced. The second measurement pump unit sucks oxygen from the third cavity in such a manner that substantially all of the water vapor and carbon dioxide contained in the measured gas introduced from the second cavity into the third cavity are reduced. The third measurement pump unit selectively oxidizes hydrogen generated by reduction of water vapor contained in the measured gas introduced from the third cavity into the fourth cavity in the fourth cavity by sucking oxygen into the fourth cavity. The controller includes: a NOx concentration determination mechanism that determines the concentration of NOx contained in the measured gas based on the magnitude of the oxygen pump current, i.e., the NOx detection current, flowing between the first measurement electrode and the external cavity pump electrode when sucking oxygen from the second cavity using the first measurement pump unit; a water vapor concentration determination mechanism that determines the concentration of water vapor contained in the measured gas based on the value of the oxygen pump current, i.e., the water vapor equivalent current, flowing between the second measurement electrode and the external cavity pump electrode when hydrogen is oxidized by oxygen sucked into the third cavity by the second measurement pump unit; and a carbon dioxide concentration determination mechanism that determines the concentration of carbon dioxide contained in the measured gas based on the value of the water vapor equivalent current and the value of the oxygen pump current, i.e., the full reduction current, flowing between the first measurement electrode and the external cavity pump electrode when sucking oxygen from the second cavity using the first measurement pump unit and reducing water vapor and carbon dioxide.

2. The gas sensor according to claim 1, wherein: The controller also stores: pre-determined Ip1-NOx data representing the relationship between the NOx detection current and the concentration of NOx. The NOx concentration determination mechanism determines the concentration of NOx contained in the measured gas based on the NOx detection current when the NOx contained in the measured gas is reduced and the Ip1-NOx data.

3. The gas sensor according to claim 2, wherein: The controller stores: Pre-determined Ip2-H2O data representing the relationship between the oxygen pump current flowing through the second measurement pump unit and the concentration of water vapor when the measured gas contains water vapor but does not contain carbon dioxide, Pre-determined Ip2-CO2 data representing the relationship between the oxygen pump current flowing through the second measurement pump unit and the concentration of water vapor when the measured gas contains carbon dioxide but does not contain water vapor, and Pre-determined Ip3-H2O data representing the relationship between the oxygen pump current flowing through the third measurement pump unit and the concentration of water vapor when the measured gas contains water vapor but does not contain carbon dioxide. The water vapor concentration determination mechanism determines the concentration of water vapor corresponding to the value of the water vapor equivalent current in the Ip3-H2O data as the concentration of water vapor contained in the measured gas. The carbon dioxide concentration determination mechanism determines the contribution part of the total reduction current brought by the reduction of water vapor based on the concentration of water vapor contained in the measured gas determined by the water vapor concentration determination mechanism and the Ip2-H2O data, and then determines the carbon dioxide concentration corresponding to the difference obtained by subtracting the contribution part from the total reduction current in the Ip2-CO2 data as the concentration of carbon dioxide contained in the measured gas.

4. The gas sensor according to claim 2, wherein: The controller stores: Pre-determined Ip2-CO2 data representing the relationship between the oxygen pump current flowing through the second measurement pump unit and the concentration of water vapor when the measured gas contains carbon dioxide but does not contain water vapor, Pre-determined Ip3-H2O data representing the relationship between the oxygen pump current flowing through the third measurement pump unit and the concentration of water vapor when the measured gas contains water vapor but does not contain carbon dioxide, and Pre-determined H2O characteristic data representing the relationship between the water vapor equivalent current and the oxygen pump current corresponding to the contribution part of water vapor in the total reduction current. The water vapor concentration determination mechanism determines the water vapor concentration corresponding to the value of the water vapor equivalent current in the Ip3-H2O data as the concentration of water vapor contained in the measured gas. The carbon dioxide concentration determination mechanism determines the contribution part of the total reduction current brought by the reduction of water vapor based on the water vapor equivalent current and the H2O characteristic data, and then determines the carbon dioxide concentration corresponding to the difference obtained by subtracting the contribution part from the total reduction current in the Ip2-CO2 data as the concentration of carbon dioxide contained in the measured gas.

5. The gas sensor according to any one of claims 1 to 4, characterized in that: The controller also includes an oxygen concentration determination mechanism. The oxygen concentration determination mechanism determines the concentration of oxygen contained in the gas to be measured based on the magnitude of the current flowing between the inner electrode and the outer cavity pump electrode when oxygen is sucked out of the first cavity using the adjustment pump unit.

6. The gas sensor according to any one of claims 1 to 4, characterized in that: The third measurement electrode is a cermet electrode containing a Pt-Au alloy as a metal component, and the Au concentration in the Pt-Au alloy is 1 wt% or more and 50 wt% or less.

7. A method for measuring concentration using a gas sensor, which is a method for measuring the concentration of multiple monitoring target gas components using a gas sensor, The concentration determination method is characterized in that The gas sensor includes a sensor element having a long plate-shaped structure composed of an oxygen ion conductive solid electrolyte. The sensor element comprises: A gas inlet port, the gas inlet port is used to introduce the measured gas; A first cavity, a second cavity, a third cavity and a fourth cavity, wherein the cavities are sequentially connected from the gas inlet via different diffusion rate control parts; an adjustment pump unit, the adjustment pump unit comprising an inner electrode formed facing the first cavity, an outer cavity pump electrode provided at a location other than the first cavity, the second cavity, the third cavity, and the fourth cavity, and the solid electrolyte existing between the inner electrode and the outer cavity pump electrode; a first measuring pump cell, the first measuring pump cell comprising a first measuring electrode formed facing the second cavity, the cavity outer pump electrode, and the solid electrolyte existing between the first measuring electrode and the cavity outer pump electrode; a second measuring pump cell, the second measuring pump cell comprising a second measuring electrode formed facing the third cavity, the cavity outer pump electrode, and the solid electrolyte existing between the second measuring electrode and the cavity outer pump electrode; a third measuring pump cell, the third measuring pump cell comprising a third measuring electrode formed facing the fourth cavity, the cavity-external pump electrode, and the solid electrolyte existing between the third measuring electrode and the cavity-external pump electrode; and a heater for heating the sensor element, The inner electrode is a metal ceramic electrode containing a Pt-Au alloy as a metal component and the Au concentration in the Pt-Au alloy is 0.5 wt % or more. The first measurement electrode is another cermet electrode containing a Pt-Rh alloy as a metal component, The method includes the following steps: a) Using the adjustment pump unit to suck out oxygen from the measured gas introduced into the first cavity from the gas inlet in such a way that NOx, water vapor, and carbon dioxide contained in the measured gas are not decomposed; b) Using the first measurement pump unit to suck out oxygen from the second cavity in such a way that substantially all of the NOx contained in the gas to be measured introduced from the first cavity into the second cavity is reduced; c) Using the second measurement pump unit to suck out oxygen from the third cavity in such a way that substantially all of the water vapor and carbon dioxide contained in the gas to be measured introduced from the second cavity into the third cavity are reduced; d) By using the third measurement pump unit to suck in oxygen into the fourth cavity, selectively oxidizing the hydrogen generated by the reduction of water vapor contained in the gas to be measured introduced from the third cavity into the fourth cavity in the fourth cavity; e) Based on the oxygen pump current flowing between the first measurement electrode and the outer cavity pump electrode when the NOx is reduced by sucking out oxygen from the second cavity by the first measurement pump unit, that is, the magnitude of the NOx detection current, to determine the concentration of NOx contained in the gas to be measured; f) Based on the oxygen pump current flowing between the third measurement electrode and the outer cavity pump electrode when the hydrogen is oxidized by the oxygen sucked into the fourth cavity by the third measurement pump unit, that is, the value of the water vapor equivalent current, to determine the concentration of water vapor contained in the gas to be measured; and g) Based on the value of the water vapor equivalent current and the oxygen pump current flowing between the second measurement electrode and the outer cavity pump electrode when the water vapor and carbon dioxide are reduced by sucking out oxygen from the third cavity by the second measurement pump unit, that is, the value of the full reduction current, to determine the concentration of carbon dioxide contained in the gas to be measured.

8. The concentration measurement method using a gas sensor according to claim 7, characterized in that, The following steps are included: h) Before steps a) to g), Ip1-NOx data representing the relationship between the NOx detection current and the concentration of NOx is determined in advance. In step e), based on the NOx detection current when the NOx contained in the gas to be measured is reduced and the Ip1-NOx data, the concentration of NOx contained in the gas to be measured is determined.

9. The concentration measurement method using a gas sensor according to claim 8, characterized in that, In step h), it is also determined: Ip2-H2O data representing the relationship between the oxygen pump current flowing through the second measurement pump unit and the concentration of water vapor when the gas to be measured contains water vapor but does not contain carbon dioxide, Ip2-CO2 data representing the relationship between the oxygen pump current flowing through the second measurement pump unit and the concentration of water vapor when the gas to be measured contains carbon dioxide but does not contain water vapor, and Ip3-H2O data representing the relationship between the oxygen pump current flowing through the third measurement pump unit and the concentration of water vapor when the measured gas contains water vapor but no carbon dioxide, In the step f), the concentration of water vapor corresponding to the value of the current equivalent to water vapor in the Ip3-H2O data is determined as the concentration of water vapor contained in the measured gas. In the step g), based on the concentration of water vapor contained in the measured gas determined in the step f) and the Ip2-H2O data, the contribution part of the full reduction current due to the reduction of water vapor is determined. Then, the carbon dioxide concentration corresponding to the difference obtained by subtracting the contribution part from the full reduction current in the Ip2-CO2 data is determined as the concentration of carbon dioxide contained in the measured gas.

10. The concentration measurement method using a gas sensor according to claim 8, characterized in that, In the step h), it is also determined that: Ip2-CO2 data representing the relationship between the oxygen pump current flowing through the second measurement pump unit and the concentration of water vapor when the measured gas contains carbon dioxide but no water vapor, Ip3-H2O data representing the relationship between the oxygen pump current flowing through the third measurement pump unit and the concentration of water vapor when the measured gas contains water vapor but no carbon dioxide, and H2O characteristic data representing the relationship between the current equivalent to water vapor and the oxygen pump current equivalent to the contribution part of water vapor in the full reduction current. In the step f), the concentration of water vapor corresponding to the value of the current equivalent to water vapor in the Ip3-H2O data is determined as the concentration of water vapor contained in the measured gas. In the step g), based on the current equivalent to water vapor and the H2O characteristic data, after determining the contribution part of the full reduction current due to the reduction of water vapor, the carbon dioxide concentration corresponding to the difference obtained by subtracting the contribution part from the full reduction current in the Ip2-CO2 data is determined as the concentration of carbon dioxide contained in the measured gas.

11. The concentration measurement method using a gas sensor according to any one of claims 7 to 10, characterized in that, There is also a step as follows: i) Based on the magnitude of the current flowing between the inner electrode and the outer cavity pump electrode when oxygen is sucked out of the first cavity by the adjustment pump unit, the concentration of oxygen contained in the measured gas is determined.

12. The concentration measurement method using a gas sensor according to any one of claims 7 to 10, characterized in that The third measurement electrode is a cermet electrode containing Pt-Au alloy as a metal component, and the Au concentration in the Pt-Au alloy is 1 wt% or more and 50 wt% or less.

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