Gas sensors and sensor elements

Abstract

This suppresses the decrease in measurement accuracy of specific gas concentrations in the gas being measured. [Solution] The gas sensor 100 comprises a sensor element 101 and a control device 95. The sensor element 101 has an element body 102 in which first to third internal cavities 14, 24, and 34 are provided inside, which are not in communication with each other and to which the gas to be measured can reach from outside the sensor element 101. The control device 95 measures at least two of the first to fourth concentrations as specific gas concentrations by performing at least two processes selected in combination from a first concentration measurement process that measures a first concentration based on pump currents Ip1 and Ip2, a second concentration measurement process that measures a second concentration based on pump currents Ip2 and Ip3, a third concentration measurement process that measures a third concentration based on pump current Ip3, and a fourth concentration measurement process that measures a fourth concentration based on pump currents Ip1 and Ip3, in a way that utilizes all of the pump currents Ip1 to Ip3.

Description

Technical Field

[0004]

[0001] The present invention relates to a gas sensor and a sensor element.

Background Art

[0002] Conventionally, gas sensors for measuring the concentration of carbon dioxide in a measured gas such as automotive exhaust gas have been known. For example, Patent Document 1 describes a gas sensor including a sensor element composed of an oxygen ion conductive solid electrolyte layer, which specifies the concentrations of water vapor components and carbon dioxide components in the measured gas. In this gas sensor, the oxygen partial pressure in the first internal cavity of the sensor element is adjusted so that substantially all of the water vapor components and carbon dioxide components in the measured gas are decomposed. Then, oxygen is supplied to the second internal cavity by the first measurement electrochemical pumping cell so that hydrogen generated by the decomposition of the water vapor component selectively burns in the second internal cavity communicating with the first internal cavity, and the concentration of the water vapor component present in the measured gas is specified based on the magnitude of the current flowing at this time. Further, in this gas sensor, oxygen is supplied to the surface of the second measurement inner electrode by the second measurement electrochemical pumping cell so that carbon monoxide generated by the decomposition of the carbon dioxide component selectively burns in the third internal cavity communicating with the second internal cavity, and the concentration of the carbon dioxide component present in the measured gas is specified based on the magnitude of the current flowing at this time.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the gas sensor described in Patent Document 1, as mentioned above, the first, second, and third internal cavities within the sensor element are in communication with each other. In this case, water (water vapor) produced by the combustion of hydrogen in the second internal cavity and carbon dioxide produced by the combustion of carbon monoxide in the third internal cavity may reach the first internal cavity by back diffusion (backflow). Then, the water and carbon dioxide that reach the first internal cavity by back diffusion may be decomposed (reduced) again to produce hydrogen and carbon monoxide, which may reach the second and third internal cavities, respectively, and burn again. It has been found that this can reduce the accuracy of measuring the water concentration (concentration of water vapor component) and carbon dioxide concentration in the gas being measured. Therefore, it has been desirable to suppress the decrease in the accuracy of measuring specific gas concentrations in the gas being measured that is caused by such back diffusion.

[0005] This invention was made to solve these problems, and its main objective is to suppress the decrease in measurement accuracy of specific gas concentrations in the gas being measured. [Means for solving the problem]

[0006] To achieve the main objectives described above, the present invention employs the following means.

[0007] [1] The first gas sensor of the present invention is A gas sensor comprising a sensor element and a control device for measuring the concentration of a specific gas in a gas to be measured, The aforementioned sensor element is The element body has an oxygen ion conductive solid electrolyte layer and is provided with a first chamber, a second chamber, and a third chamber inside, which are not in communication with each other and to which the gas to be measured can reach from outside the sensor element, A first pump cell comprising a first inner electrode disposed in the first chamber and a first outer electrode disposed on the outer surface of the element body, A second pump cell comprising a second inner electrode disposed in the second chamber and a second outer electrode disposed on the outer surface of the element body, A third pump cell comprising a third inner electrode disposed in the third chamber and a third outer electrode disposed on the outer surface of the element body, It has, The control device is A first pump cell control process is performed to control the first pump cell so as to pump oxygen from around the first inner electrode to around the first outer electrode, thereby reducing the target gas, which is two or more oxide gases in the gas to be measured in the first chamber. A second pump cell control process controls the second pump cell to pump oxygen from around the second inner electrode to around the second outer electrode, while suppressing the reduction of a certain type of oxide gas, which is a first type gas, among the gases to be reduced in the gas to be measured in the second chamber, compared to the first pump cell control process. A third pump cell control process controls the third pump cell to pump oxygen from around the third inner electrode to around the third outer electrode, while suppressing the reduction of the second type gas, which is one or more oxide gases other than the first type gas among the gases to be reduced in the gas to be measured in the third chamber, compared to the second pump cell control process, and Perform The control device is A first concentration measurement process that measures a first concentration, which is the concentration of the first type of gas in the gas to be measured, based on a first pump current flowing through the first pump cell by the first pump cell control process and a second pump current flowing through the second pump cell by the second pump cell control process. A second concentration measurement process that measures the second concentration, which is the concentration of the second type of gas in the gas to be measured, based on the second pump current and the third pump current flowing to the third pump cell by the third pump cell control process, A third concentration measurement process that measures a third concentration, which is the total concentration of the gas to be reduced other than the first and second gases and oxygen in the gas to be measured, based on the third pump current, A fourth concentration measurement process that measures a fourth concentration, which is the sum of the first type gas and the second type gas in the gas to be measured, based on the first pump current and the third pump current, By performing at least two processes selected in a combination that utilizes all of the first to third pump currents, at least two of the first to fourth concentrations are measured as the specific gas concentration. It is.

[0008] In this first gas sensor, the control device performs at least two processes selected from the first to fourth concentration measurement processes in a combination that utilizes all of the first to third pump currents, thereby measuring at least two of the first to fourth concentrations based on the first to third pump currents. Here, the first pump current flowing to the first pump cell by the first pump cell control process correlates with the total concentration of the target gas to be reduced and oxygen in the gas being measured. The second pump current flowing to the second pump cell by the second pump cell control process correlates with the total concentration of the target gas to be reduced other than the first gas and oxygen in the gas being measured. The third pump current flowing to the third pump cell by the third pump cell control process correlates with the total concentration of the target gas to be reduced other than the first gas and the second gas, and oxygen in the gas being measured. Therefore, based on the first pump current and the second pump current, the first concentration, which is the concentration of the first gas in the gas being measured, can be measured. Also, based on the second pump current and the third pump current, the second concentration, which is the concentration of the second gas in the gas being measured, can be measured. Based on the third pump current, a third concentration, which is the sum of the reduction target gases other than the first and second gases and oxygen in the gas being measured, can be measured. Based on the first and third pump currents, a fourth concentration, which is the sum of the first and second gases in the gas being measured, can be measured. Moreover, the first, second, and third chambers are not connected to each other, and the gas being measured reaches the first, second, and third chambers from outside the sensor element via independent paths. Therefore, the influence of each gas between the first, second, and third chambers can be suppressed, and the decrease in measurement accuracy due to the aforementioned back diffusion is less likely to occur. Consequently, this gas sensor can suppress the decrease in measurement accuracy of specific gas concentrations in the gas being measured.

[0009] Furthermore, if the first type gas is two or more oxide gases among the gases to be reduced, the first concentration will be the total concentration of those two or more oxide gases. Also, since the second type gas is one or more oxide gases other than the first type gas among the gases to be reduced, there may be a configuration in which the second type gas is all types of oxide gases other than the first type gas among the gases to be reduced. In this configuration, since there are no "the gases to be reduced other than the first type gas and the second type gas in the gas being measured", the oxygen concentration in the gas being measured will be the third concentration. The first type gas may be one or more oxide gases selected in order from the least reduced oxide gas among the gases to be reduced. The second type gas may be one or more oxide gases selected in order from the least reduced oxide gas among the gases to be reduced excluding the first type gas.

[0010] [2] In the first gas sensor described above (the gas sensor described in [1] above), the gas to be reduced is water and carbon dioxide, the first gas is carbon dioxide, and the second gas is water.

[0011] [3] In the first gas sensor described above (the gas sensor described in [2] above), the second inner electrode may contain a first type precious metal having catalytic activity and a second type precious metal that suppresses the reduction of carbon dioxide. By containing a second type precious metal in addition to the first type precious metal in the second inner electrode, the second pump current becomes less susceptible to the influence of the carbon dioxide concentration in the gas being measured. As a result, the measurement accuracy of the first concentration measurement process, i.e., the measurement accuracy of the carbon dioxide concentration based on the first pump current and the second pump current, is improved. In this case, the first inner electrode may contain the first type precious metal.

[0012] [4] In the first gas sensor described above (the gas sensor described in [3] above), the first precious metal is at least one of Pt, Rh, Ir, Ru, and Pd, and the second precious metal may be Au.

[0013] [5] In the first gas sensor described above (the gas sensor described in [3] or [4] above), the ratio R2 of the second inner electrode calculated by the following formula (1) may be 2% or more. By having a ratio R2 of 2% or more of the second inner electrode, the reduction capacity of the second inner electrode for carbon dioxide can be more reliably weakened.

[0014] R2 = S2 / (S1 + S2) × 100 (1) however, S1: Mass percentage of the aforementioned Class 1 precious metal [wt%] S2: Mass percentage of the aforementioned second type of precious metal [wt%]

[0015] [6] In the first gas sensor described above (the gas sensor described in any of [1] to [5] above), the sensor element has a reference electrode disposed inside the element body so as to be in contact with a reference gas, and the control device may, in the first pump cell control process, control the first pump cell so that the first voltage, which is the voltage between the reference electrode and the first inner electrode, becomes a first voltage target value, in the second pump cell control process, control the second pump cell so that the second voltage, which is the voltage between the reference electrode and the second inner electrode, becomes a second voltage target value which is smaller in absolute value than the first voltage target value, and in the third pump cell control process, control the third pump cell so that the third voltage, which is the voltage between the reference electrode and the third inner electrode, becomes a third voltage target value which is smaller in absolute value than the second voltage target value.

[0016] [7] In the first gas sensor described above (the gas sensor described in any of [1] to [6] above), the first concentration measurement process may be a process of measuring the first concentration based on the difference between the first pump current and the second pump current, or a process of measuring the first concentration based on the difference between the total concentration of the gas to be reduced and oxygen in the gas to be measured, derived based on the first pump current, and the total concentration of the gas to be reduced other than the first type gas and oxygen in the gas to be measured, derived based on the second pump current. The second concentration measurement process may be a process of measuring the second concentration based on the difference between the second pump current and the third pump current, or a process of measuring the second concentration based on the difference between the total concentration of the gas to be reduced other than the first type gas and oxygen in the gas to be measured, derived based on the second pump current, and the total concentration of the gas to be reduced other than the first type gas and the second type gas and oxygen in the gas to be measured, derived based on the third pump current. The fourth concentration measurement process may be a process of measuring the fourth concentration based on the difference between the first pump current and the third pump current, or a process of measuring the fourth concentration based on the difference between the total concentration of the gas to be reduced and oxygen in the gas to be measured, derived based on the first pump current, and the total concentration of the gas to be reduced other than the first and second type gases and oxygen in the gas to be measured, derived based on the third pump current. Note that "a process of measuring the first concentration based on the difference between the first pump current and the second pump current" also includes "a process of correcting at least one of the first pump current and the second pump current before deriving the difference and measuring the first concentration based on that difference." "a process of measuring the second concentration based on the difference between the second pump current and the third pump current" also includes "a process of correcting at least one of the second pump current and the third pump current before deriving the difference and measuring the second concentration based on that difference." The "process of measuring the fourth concentration based on the difference between the first pump current and the third pump current" also includes the "process of correcting at least one of the first pump current and the third pump current, deriving the difference, and measuring the fourth concentration based on that difference."

[0017] [8] In the above-described first gas sensor (the gas sensor according to any one of [1] to [7]), the element body has a rectangular parallelepiped shape having first to sixth surfaces as the outer surfaces, and the element body has a first inlet that is an inlet of the gas to be measured from the outside to the first chamber, a second inlet that is an inlet of the gas to be measured from the outside to the second chamber, and a third inlet that is an inlet of the gas to be measured from the outside to the third chamber, and the first inlet, the second inlet, and the third inlet may open on different surfaces among the first to sixth surfaces. By doing so, it is possible to further suppress the mutual influence of the respective gases between the first chamber, the second chamber, and the third chamber.

[0018] [9] In the above-described first gas sensor (the gas sensor according to any one of [1] to [7]), the element body has a rectangular parallelepiped shape having first to sixth surfaces as the outer surfaces, and the element body has a first inlet that is an inlet of the gas to be measured from the outside to the first chamber, a second inlet that is an inlet of the gas to be measured from the outside to the second chamber, and a third inlet that is an inlet of the gas to be measured from the outside to the third chamber, and the first inlet, the second inlet, and the third inlet may open on the same surface among the first to sixth surfaces. By doing so, even when the concentration of a specific gas in the gas to be measured fluctuates in a short time, the gas to be measured reaching each of the first to third chambers is likely to have the same specific gas concentration. Therefore, the measurement accuracy of the concentrations (first, second, and fourth concentrations) measured based on two of the first to third pump currents is improved.

[0019]

[10] The second gas sensor of the present invention is a gas sensor including a sensor element and a control device and measuring the concentration of a specific gas in the gas to be measured, wherein the sensor element has an oxygen ion-conductive solid electrolyte layer, and an element body provided therein with a first chamber and a second chamber that do not communicate with each other and to which the gas to be measured can respectively reach from the outside of the sensor element, a first pump cell including a first inner electrode disposed in the first chamber and a first outer electrode disposed on the outer surface of the element body. A second pump cell comprising a second inner electrode disposed in the second chamber and a second outer electrode disposed on the outer surface of the element body, It has, The control device is A first pump cell control process is performed to control the first pump cell so as to pump oxygen from around the first inner electrode to around the first outer electrode, thereby reducing the target gas containing at least water among the water and carbon dioxide in the gas to be measured in the first chamber, A second pump cell control process controls the second pump cell to pump oxygen from around the second inner electrode to around the second outer electrode, while suppressing the reduction of one or more gases, which are the gases to be reduced, in the gas to be measured in the second chamber, compared to the first pump cell control process. A concentration measurement process that measures a first concentration, which is the concentration of the first type of gas in the gas to be measured, as the specific gas concentration, based on the first pump current flowing through the first pump cell by the first pump cell control process and the second pump current flowing through the second pump cell by the second pump cell control process. Perform The first type of gas is water if the gas to be reduced is water, and if the gas to be reduced is water and carbon dioxide, it is one or more gases from among the gases to be reduced that include at least carbon dioxide. It is.

[0020] In this second gas sensor, the control device measures the first concentration, which is the concentration of the first type gas in the gas being measured, based on the first pump current and the second pump current. Here, the first pump current flowing to the first pump cell by the first pump cell control process correlates with the total concentration of the gas to be reduced and oxygen in the gas being measured. The second pump current flowing to the second pump cell by the second pump cell control process correlates with the total concentration of the gases to be reduced other than the first type gas and oxygen in the gas being measured. Therefore, based on the first pump current and the second pump current, the first concentration, which is the concentration of the first type gas in the gas being measured, can be measured as a specific gas concentration. Moreover, the first and second chambers are not connected to each other, and the gas being measured reaches the first and second chambers from outside the sensor element via independent paths. Therefore, it is possible to suppress the influence of each gas between the first and second chambers, and the decrease in measurement accuracy caused by the aforementioned back diffusion is less likely to occur. Thus, this gas sensor can suppress the decrease in measurement accuracy of the specific gas concentration in the gas being measured. The gas to be reduced contains at least water among water and carbon dioxide. That is, the gas to be reduced is either water or water and carbon dioxide. When the gas to be reduced is water, the first gas is water. When the gas to be reduced is water and carbon dioxide, the first gas is one or more gases from the gases to be reduced that contain at least carbon dioxide. That is, when the gas to be reduced is water and carbon dioxide, the first gas is either carbon dioxide or water and carbon dioxide. When the first gas is water and carbon dioxide, the first concentration is the total concentration of water and carbon dioxide in the gas being measured. The second gas sensor may adopt the same configuration as the various configurations of the first gas sensor described above, or it may add the same configuration as the first gas sensor described above.

[0021]

[11] The sensor element of the present invention is A sensor element for measuring the concentration of a specific gas in a gas to be measured, The element body has an oxygen ion conductive solid electrolyte layer and is provided with a first chamber, a second chamber, and a third chamber inside, which are not connected to each other but to which the gas to be measured can reach from the outside, respectively. A first pump cell comprising a first inner electrode disposed in the first chamber and a first outer electrode disposed on the outer surface of the element body, A second pump cell comprising a second inner electrode disposed in the second chamber and a second outer electrode disposed on the outer surface of the element body, A third pump cell comprising a third inner electrode disposed in the third chamber and a third outer electrode disposed on the outer surface of the element body, A sensor element having [a certain characteristic].

[0022] Similar to the sensor element of the first gas sensor described above, this sensor element has a first chamber, a second chamber, and a third chamber that are not connected to each other, and the gas to be measured reaches the first chamber, the second chamber, and the third chamber from outside the sensor element via independent paths. Therefore, it is possible to suppress the mutual influence of each gas between the first chamber, the second chamber, and the third chamber, and the reduction in measurement accuracy caused by the back diffusion described above is less likely to occur. For this reason, the sensor element of the present invention is suitable for use in the sensor element of the first gas sensor described above. In addition, the sensor element of the present invention may adopt various embodiments similar to those of the sensor element in the first gas sensor described above, or similar configurations may be added. [Brief explanation of the drawing]

[0023] [Figure 1] A schematic cross-sectional diagram illustrating an example of the configuration of the gas sensor 100. [Figure 2] Partial cross-sectional view of spacer layer 5 in Figure 1. [Figure 3] A block diagram showing the electrical connection relationships between the control device 95 and each cell and heater 72. [Figure 4] An explanatory diagram showing an example of the VI characteristics of the pump cells (1st to 3rd measurement pump cells 15, 25, 35). [Figure 5] This diagram illustrates an example of VI characteristics when the inner electrodes (1st to 3rd measuring electrodes 16, 26, 36) contain a Class II precious metal. [Figure 6] A partial cross-sectional view of the element body 102 of a modified example. [Figure 7]A schematic cross-sectional view of the element body 102 of a modified example. [Modes for carrying out the invention]

[0024] Next, embodiments of the present invention will be described with reference to the drawings. Figure 1 is a schematic cross-sectional view showing an example of the configuration of a gas sensor 100, which is one embodiment of the present invention. Figure 2 is a partial cross-sectional view of the spacer layer 5 in Figure 1. Figure 3 is a block diagram showing the electrical connection relationship between the control device 95 and each cell and heater 72. Figure 2 is a partial cross-sectional view from above of the area around the first to third internal cavities 14, 24, and 34 in the cross-section of the spacer layer 5 along the front, back, left, and right directions. Also, in Figure 2, the first to third diffusion rate-limiting sections 13, 23, and 33 are shown with dotted lines for reference. This gas sensor 100 is attached to piping such as the exhaust gas pipe of an internal combustion engine. The gas sensor 100 uses the exhaust gas of an internal combustion engine as the gas to be measured and detects the specific gas concentration, which is the concentration of a specific gas in the gas to be measured. In this embodiment, the gas sensor 100 measures carbon dioxide concentration, water concentration, oxygen concentration, and the total concentration of carbon dioxide and water as the specific gas concentrations.

[0025] The gas sensor 100 comprises a sensor element 101 having a long rectangular parallelepiped-shaped element body 102, cells 15, 25, 35, 18, 28, and 38 provided within the sensor element 101, a heater section 70 provided inside the sensor element 101, and a control device 95 that controls the entire gas sensor 100 and has variable power supplies 17, 27, and 37 and a heater power supply 76. The longitudinal direction of the sensor element 101 (left-right direction in Figure 1) is defined as the front-back direction, the thickness direction of the sensor element 101 (up-down direction in Figure 1) is defined as the up-down direction, and the width direction of the sensor element 101 (perpendicular to the front-back and up-down directions, and the up-down direction in Figure 2) is defined as the left-right direction. Since the element body 102 is a rectangular parallelepiped, as shown in Figures 1 and 2, the element body 102 has six outer surfaces: the first surface 102a (top surface), the second surface 102b (bottom surface), the third surface 102c (left side surface), the fourth surface 102d (right side surface), the fifth surface 102e (front end surface), and the sixth surface 102f (rear end surface).

[0026] The element body 102 is a laminate in which six layers are stacked in this order from the bottom in the drawing: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each consisting of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO2). The solid electrolytes forming these six layers are dense and airtight. The element body 102 is manufactured, for example, by performing predetermined processing and printing circuit patterns on ceramic green sheets corresponding to each layer, stacking them, and then firing them to integrate them.

[0027] On the front end side of the sensor element 101 (element body 102), a first gas inlet 11, a first buffer space 12, a first diffusion rate-limiting section 13, and a first internal cavity 14 are formed adjacent to each other between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, in a manner that they communicate in this order from front to back. Similarly, a second gas inlet 21, a second buffer space 22, a second diffusion rate-limiting section 23, and a second internal cavity 24 are formed adjacent to each other between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, in a manner that they communicate in this order from left to right. Furthermore, a third gas inlet 31, a third buffer space 32, a third diffusion rate-limiting section 33, and a third internal cavity 34 are formed adjacent to each other between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, in a manner that they communicate in this order from right to left.

[0028] The first gas inlet 11 is the inlet for the gas to be measured from outside the sensor element 101 to the first internal cavity 14, and in this embodiment it opens on the fifth surface 102e. The second gas inlet 21 is the inlet for the gas to be measured from outside the sensor element 101 to the second internal cavity 24, and in this embodiment it opens on the third surface 102c. The third gas inlet 31 is the inlet for the gas to be measured from outside the sensor element 101 to the third internal cavity 34, and in this embodiment it opens on the fourth surface 102d. Therefore, in this embodiment, the first gas inlet 11, the second gas inlet 21, and the third gas inlet 31 open on different surfaces from the first to sixth surfaces 102a to 102f.

[0029] The first to third gas inlets 11, 21, 31, the first to third buffer spaces 12, 22, 32, and the first to third internal cavities 14, 24, 34 are spaces inside the sensor element 101, provided in a manner in which the spacer layer 5 has been hollowed out, with the upper part partitioned by the lower surface of the second solid electrolyte layer 6, the lower part partitioned by the upper surface of the first solid electrolyte layer 4, and the sides partitioned by the side surfaces of the spacer layer 5.

[0030] The first diffusion-limiting section 13 is provided as two horizontally elongated slits (with their longitudinal openings perpendicular to the drawing in Figure 1). As shown in Figure 1, these two slits of the first diffusion-limiting section 13 are provided as the gap between the lower surface of the second solid electrolyte layer 6 and the upper surface of the spacer layer 5, and the gap between the upper surface of the first solid electrolyte layer 4 and the lower surface of the spacer layer 5. The second diffusion-limiting section 23 and the third diffusion-limiting section 33 are also provided as two horizontally elongated slits (with their longitudinal openings perpendicular to the drawing in Figure 1), although they are not shown in the illustration.

[0031] The first buffer space 12 is a space provided to guide the gas to be measured, introduced from the first gas inlet 11, to the first diffusion rate-limiting section 13. In this embodiment, the first buffer space 12 opens to the fifth surface 102e, and this opening is the first gas inlet 11. The first diffusion rate-limiting section 13 is a section that imparts a predetermined diffusion resistance to the gas to be measured introduced from the first buffer space 12 into the first internal cavity 14. The second buffer space 22 is a space provided to guide the gas to be measured, introduced from the second gas inlet 21, to the second diffusion rate-limiting section 23. In this embodiment, the second buffer space 22 opens to the third surface 102c, and this opening is the second gas inlet 21. The second diffusion rate-limiting section 23 is a section that imparts a predetermined diffusion resistance to the gas to be measured introduced from the second buffer space 22 into the second internal cavity 24. The third buffer space 32 is a space provided to guide the gas to be measured, introduced from the third gas inlet 31, to the third diffusion rate-limiting section 23. In this embodiment, the second buffer space 22 opens to the fourth surface 102d, and this opening is the third gas inlet 31. The third diffusion rate-limiting section 33 is a part that imparts a predetermined diffusion resistance to the gas to be measured introduced from the third buffer space 32 into the second internal cavity 34.

[0032] When the gas to be measured is introduced from outside the sensor element 101 into the first internal cavity 14, the gas to be measured, which is rapidly drawn into the sensor element 101 from the first gas inlet 11 due to pressure fluctuations of the gas to be measured in the external space (pulsations of exhaust pressure if the gas to be measured is automobile exhaust gas), is not directly introduced into the first internal cavity 14. Instead, the pressure fluctuations of the gas to be measured are canceled out through the first buffer space 12 and the first diffusion rate-limiting section 13 before it is introduced into the first internal cavity 14. As a result, the pressure fluctuations of the gas to be measured introduced into the first internal cavity 14 become almost negligible. Similarly, the gas to be measured introduced into the sensor element 101 from the second gas inlet 21 is introduced into the second internal cavity 24 after the pressure fluctuations are canceled out through the second buffer space 22 and the second diffusion rate-limiting section 23. The gas to be measured, introduced into the sensor element 101 from the third gas inlet 31, is introduced into the third internal cavity 34 after pressure fluctuations are canceled out through the third buffer space 32 and the third diffusion rate-limiting section 33.

[0033] The area from the outside of the sensor element 101 to the first internal cavity 14 (here, the first gas inlet 11, the first buffer space 12, and the first diffusion rate-limiting section 13) is also referred to as the first gas to be measured flow section. The area from the outside of the sensor element 101 to the second internal cavity 24 (here, the second gas inlet 21, the second buffer space 22, and the second diffusion rate-limiting section 23) is also referred to as the second gas to be measured flow section. The area from the outside of the sensor element 101 to the third internal cavity 34 (here, the third gas inlet 31, the third buffer space 32, and the third diffusion rate-limiting section 33) is also referred to as the third gas to be measured flow section. The gas to be measured can reach the first internal cavity 14 from the outside of the sensor element 101 via this first gas to be measured flow section. The gas to be measured can reach the second internal cavity 24 from the outside of the sensor element 101 via this second gas to be measured flow section. The gas to be measured can reach the third internal cavity 34 from outside the sensor element 101 via this third gas flow section. As shown in Figures 1 and 2, the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34 are not connected to each other and are independently provided inside the element body 102. More specifically, the first gas flow section, the second gas flow section, and the third gas flow section are not connected to each other, and there are no gas flow paths inside the element body 102 that allow the flow of the gas to be measured between at least two of the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34. Therefore, the gas to be measured reaches the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34 from outside the sensor element 101 via independent paths (first to third gas flow sections).

[0034] The sensor element 101 (element body 102) is equipped with a reference gas introduction section 49 that allows a reference gas to flow from outside the sensor element 101 to the reference electrode 42 when measuring the concentration of a specific gas. The reference gas introduction section 49 has a reference gas introduction space 43 and a reference gas introduction layer 48. The reference gas introduction space 43 is a space provided inward from the sixth surface 102f of the sensor element 101. The reference gas introduction space 43 is located between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and is provided at a position where its sides are partitioned by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 opens to the sixth surface 102f of the sensor element 101, and this opening functions as the inlet 49a of the reference gas introduction section 49. The reference gas is introduced into the reference gas introduction space 43 from this inlet 49a. The reference gas introduction section 49 introduces the reference gas introduced from the inlet 49a to the reference electrode 42 while imparting a predetermined diffusion resistance to it. In this embodiment, the reference gas was the atmosphere.

[0035] The reference gas introduction layer 48 is provided between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. The reference gas introduction layer 48 is a porous material made of ceramics such as alumina. A portion of the upper surface of the reference gas introduction layer 48 is exposed within the reference gas introduction space 43. The reference gas introduction layer 48 is formed to cover the reference electrode 42. The reference gas introduction layer 48 allows the reference gas to flow from the reference gas introduction space 43 to the reference electrode 42.

[0036] The reference electrode 42 is an electrode formed in such a manner that it is sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, a reference gas introduction layer 48 connected to the reference gas introduction space 43 is provided around it. Furthermore, as will be described later, it is possible to measure the oxygen concentration (partial pressure of oxygen) in the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34 using the reference electrode 42.

[0037] The first internal cavity 14 is provided as a space for adjusting the partial pressure of oxygen in the gas to be measured, which is introduced through the first diffusion rate-limiting unit 13. This partial pressure of oxygen is adjusted by the operation of the first measuring pump cell 15. The second internal cavity 24 is provided as a space for adjusting the partial pressure of oxygen in the gas to be measured, which is introduced through the second diffusion rate-limiting unit 23. This partial pressure of oxygen is adjusted by the operation of the second measuring pump cell 25. The third internal cavity 34 is provided as a space for adjusting the partial pressure of oxygen in the gas to be measured, which is introduced through the third diffusion rate-limiting unit 33. This partial pressure of oxygen is adjusted by the operation of the third measuring pump cell 35.

[0038] The first measuring pump cell 15 is an electrochemical pump cell composed of a first measuring electrode 16, an outer pump electrode 40, and a second solid electrolyte layer 6, a spacer layer 5, and a first solid electrolyte layer 4 that serve as current paths between these electrodes. The first measuring electrode 16 is disposed in the first internal cavity 14 and is positioned to cover most of the region of the upper surface of the first solid electrolyte layer 4 that faces the first internal cavity 14 (i.e., the region that constitutes the bottom surface of the first internal cavity 14). The outer pump electrode 40 is an electrode disposed on the first surface 102a of the outer surface of the element body 102. The outer pump electrode 40 is disposed in a manner that exposes it to the outside of the sensor element 101, but it may be covered with a protective layer that is a porous material through which the gas to be measured can pass.

[0039] In the first measuring pump cell 15, by applying a desired voltage Vp1 between the first measuring electrode 16 and the outer pump electrode 40, and flowing a pump current Ip1 in the positive or negative direction between the first measuring electrode 16 and the outer pump electrode 40, it is possible to pump oxygen from the first internal cavity 14 to the external space, or pump oxygen from the external space into the first internal cavity 14.

[0040] Furthermore, in order to detect the oxygen concentration (partial pressure of oxygen) in the atmosphere of the first internal cavity 14, an electrochemical sensor cell, i.e., a first sensor cell 18, is constructed from a first measuring electrode 16, a first solid electrolyte layer 4, a third substrate layer 3, and a reference electrode 42.

[0041] The oxygen concentration (partial pressure of oxygen) in the first internal cavity 14 can be determined by measuring the electromotive force (voltage V1) between the first measuring electrode 16 and the reference electrode 42 in the first sensor cell 18. Furthermore, the pump current Ip1 is controlled by feedback control of the voltage Vp1 of the variable power supply 17 so that the voltage V1 becomes a target value. This adjusts the oxygen concentration in the first internal cavity 14.

[0042] The second measuring pump cell 25 is an electrochemical pump cell composed of a second measuring electrode 26, an outer pump electrode 40, and a second solid electrolyte layer 6, a spacer layer 5, and a first solid electrolyte layer 4, which form the current path between these electrodes. The second measuring electrode 26 is disposed in the second internal cavity 24 and is positioned to cover most of the upper surface of the first solid electrolyte layer 4 that faces the second internal cavity 24 (i.e., the area that constitutes the bottom surface of the second internal cavity 24).

[0043] In the second measuring pump cell 25, by applying a desired voltage Vp2 between the second measuring electrode 26 and the outer pump electrode 40, and flowing a pump current Ip2 in the positive or negative direction between the second measuring electrode 26 and the outer pump electrode 40, it is possible to pump oxygen from the second internal cavity 24 to the outside space, or pump oxygen from the outside space into the second internal cavity 24.

[0044] Furthermore, in order to detect the oxygen concentration (partial pressure of oxygen) in the atmosphere of the second internal cavity 24, an electrochemical sensor cell, i.e., a second sensor cell 28, is constructed from a second measuring electrode 26, a first solid electrolyte layer 4, a third substrate layer 3, and a reference electrode 42.

[0045] The oxygen concentration (partial pressure of oxygen) in the second internal cavity 24 can be determined by measuring the electromotive force (voltage V2) between the second measuring electrode 26 and the reference electrode 42 in the second sensor cell 28. Furthermore, the pump current Ip2 is controlled by feedback control of the voltage Vp2 of the variable power supply 27 so that the voltage V2 becomes a target value. This adjusts the oxygen concentration in the second internal cavity 24.

[0046] The third measuring pump cell 35 is an electrochemical pump cell composed of a third measuring electrode 36, an outer pump electrode 40, and a second solid electrolyte layer 6, a spacer layer 5, and a first solid electrolyte layer 4, which form the current path between these electrodes. The third measuring electrode 36 is disposed in the third internal cavity 34 and is positioned to cover most of the upper surface of the first solid electrolyte layer 4 that faces the third internal cavity 34 (i.e., the area that constitutes the bottom surface of the third internal cavity 34).

[0047] In the third measuring pump cell 35, by applying a desired voltage Vp3 between the third measuring electrode 36 and the outer pump electrode 40, and flowing a pump current Ip3 in the positive or negative direction between the third measuring electrode 36 and the outer pump electrode 40, it is possible to pump oxygen from the third internal cavity 34 to the outside space, or pump oxygen from the outside space into the third internal cavity 34.

[0048] Furthermore, in order to detect the oxygen concentration (partial pressure of oxygen) in the atmosphere of the third internal cavity 34, an electrochemical sensor cell, i.e., a third sensor cell 38, is constructed from a third measuring electrode 36, a first solid electrolyte layer 4, a third substrate layer 3, and a reference electrode 42.

[0049] The oxygen concentration (partial pressure of oxygen) in the third internal cavity 34 can be determined by measuring the electromotive force (voltage V3) between the third measuring electrode 36 and the reference electrode 42 in the third sensor cell 38. Furthermore, the pump current Ip3 is controlled by feedback control of the voltage Vp3 of the variable power supply 37 so that the voltage V3 becomes a target value. This adjusts the oxygen concentration in the third internal cavity 34.

[0050] Here, each electrode 16, 26, 36, 40, and 42 will be described. The first measuring electrode 16, the second measuring electrode 26, and the third measuring electrode 36 each contain a first-class precious metal having catalytic activity. Examples of first-class precious metals include at least one of Pt, Rh, Ir, Ru, and Pd. The outer pump electrode 40 and the reference electrode 42 also contain a first-class precious metal. The second measuring electrode 26 preferably contains a second-class precious metal in addition to the first-class precious metal to suppress the catalytic activity of the first-class precious metal toward carbon dioxide. By containing a second-class precious metal in the second measuring electrode 26, the reduction ability of the second measuring electrode 26 toward carbon dioxide can be weakened. An example of a second-class precious metal is Au. Each electrode 16, 26, 36, 40, and 42 is preferably a cermet containing a precious metal and an oxide having oxygen ion conductivity (e.g., ZrO2). Each electrode 16, 26, 36, 40, and 42 is preferably a porous material. In this embodiment, each electrode 16, 26, 36, 40, and 42 is a porous cermet electrode made of Pt and ZrO2 that does not contain a second-class precious metal.

[0051] If the second measuring electrode 26 contains a Class II precious metal, it is preferable that the proportion R2 of the second measuring electrode 26, calculated by the following formula (1), is 2% or more. If the proportion R2 of the second measuring electrode 26 is 2% or more, the reduction capacity of the second measuring electrode 26 to carbon dioxide can be more reliably weakened. The proportion R2 of the second measuring electrode 26 may be 5% or more. The proportion R2 of the second measuring electrode 26 may be 10% or less, or 5% or less. Similar to the proportion R2, the proportion of Class II precious metal in the first measuring electrode 16 is denoted as proportion R1, and the proportion of Class II precious metal in the third measuring electrode 36 is denoted as proportion R3. The proportions R1 and R3 are calculated in the same way as the following formula (1). The proportions R1 to R3 are values ​​measured using an electron probe microanalyzer (EPMA).

[0052] R2 = S2 / (S1 + S2) × 100 (1) however, S1: Mass percentage of the aforementioned Class 1 precious metal [wt%] S2: Mass percentage of the aforementioned second type of precious metal [wt%]

[0053] The sensor element 101 is equipped with a heater section 70 that plays a role in temperature control by heating and maintaining the sensor element 101 to enhance the oxygen ion conductivity of the solid electrolyte. The heater section 70 comprises a heater connector electrode 71, a heater 72, a through-hole 73, a heater insulating layer 74, and a pressure relief hole 75.

[0054] The heater connector electrode 71 is an electrode formed in such a manner that it is in contact with the lower surface of the first substrate layer 1. By connecting the heater connector electrode 71 to the heater power supply 76 (see Figure 3), power can be supplied from the heater power supply 76 to the heater unit 70.

[0055] The heater 72 is an electrical resistor formed sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to the heater connector electrode 71 via a through-hole 73, and generates heat when power is supplied from the heater power supply 76 through the heater connector electrode 71, thereby heating and maintaining the temperature of the solid electrolyte forming the sensor element 101.

[0056] Furthermore, the heater 72 is embedded throughout the entire region where the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34 exist, making it possible to adjust the entire sensor element 101 to a temperature at which the solid electrolyte is activated.

[0057] The heater insulating layer 74 is an insulating layer formed on the upper and lower surfaces of the heater 72 using an insulator such as alumina. The heater insulating layer 74 is formed to provide electrical insulation between the second substrate layer 2 and the heater 72, and between the third substrate layer 3 and the heater 72.

[0058] The pressure relief holes 75 penetrate the third substrate layer 3 and the reference gas introduction layer 48 and are provided to communicate with the reference gas introduction space 43, and are formed for the purpose of mitigating the rise in internal pressure due to the rise in temperature within the heater insulating layer 74.

[0059] As shown in Figure 3, the control device 95 comprises the variable power supplies 17, 27, and 37 described above, the heater power supply 76 described above, and the control unit 96. The control unit 96 is a microprocessor equipped with a CPU 97 and a memory unit 98. The memory unit 98 is a non-volatile memory that can be rewritten and can store various programs and various data, for example. The control unit 96 receives the voltage V1 of the first sensor cell 18, the voltage V2 of the second sensor cell 28, the voltage V3 of the third sensor cell 38, the pump current Ip1 flowing through the first measuring pump cell 15, the pump current Ip2 flowing through the second measuring pump cell 25, and the pump current Ip3 flowing through the third measuring pump cell 35 as input. Furthermore, the control unit 96 controls the voltages Vp1, Vp2, and Vp3 output by the variable power supplies 17, 27, and 37 by outputting control signals to them, thereby controlling the first measuring pump cell 15, the second measuring pump cell 25, and the third measuring pump cell 35. The control unit 96 controls the power supplied by the heater power supply 76 to the heater 72 by outputting control signals to the heater power supply 76. The memory unit 98 also stores target values ​​V1*, V2*, V3*, which will be described later. The CPU 97 of the control unit 96 controls each of the pump cells 15, 25, and 35 by referring to these target values ​​V1*, V2*, V3*.

[0060] The control unit 96 performs a first measurement pump control process to control the first measurement pump cell 15 so that it pumps oxygen from around the first measurement electrode 16 to around the outer pump electrode 40. Specifically, the control unit 96 controls the first measurement pump cell 15 by feedback control of the voltage Vp1 of the variable power supply 17 so that the voltage V1 becomes a target value V1*. The target value V1* is defined as a value such that the oxygen concentration in the first internal cavity 14 becomes a predetermined low concentration that is sufficiently low to substantially reduce all of the target gases, which are two or more oxide gases in the gas to be measured. In this embodiment, the target gases are water and carbon dioxide. When this first measurement pump control process is performed, in the first internal cavity 14, water in the gas to be measured is reduced to generate hydrogen and oxygen, and carbon dioxide in the gas to be measured is reduced to generate carbon monoxide and oxygen. Then, the oxygen generated by these reductions, along with the oxygen that was present in the gas being measured before reduction, is pumped from around the first measuring electrode 16 to around the outer pump electrode 40 by the pump current Ip1 flowing through the first measuring pump cell 15. Therefore, the pump current Ip1 flowing through the first measuring pump cell 15 by the first measuring pump control process is correlated with the total concentration of the gas to be reduced and oxygen in the gas being measured.

[0061] The control unit 96 performs a second measurement pump control process to control the second measurement pump cell 25 so that it pumps oxygen from around the second measurement electrode 26 to around the outer pump electrode 40. Specifically, the control unit 96 controls the second measurement pump cell 25 by feedback control of the voltage Vp2 of the variable power supply 27 so that the voltage V2 becomes a target value V2*. The target value V2* is set to a value such that the oxygen concentration in the second internal cavity 24 becomes a predetermined low concentration that suppresses the reduction of a certain type of oxide gas among the gases to be reduced in the gas to be measured, namely the first type gas (in this case, carbon dioxide), compared to the first measurement pump control process. When this second measurement pump control process is performed, in the second internal cavity 24, the gases to be reduced other than the first type gas in the gas to be measured (in this case, water) are reduced to generate hydrogen and oxygen, while the reduction of the first type gas (in this case, carbon dioxide) in the gas to be measured is suppressed. Then, the oxygen generated by the reduction of water and the oxygen that was present in the gas being measured before reduction are pumped from around the second measuring electrode 26 to around the outer pump electrode 40 by the pump current Ip2 flowing through the second measuring pump cell 25. Therefore, the pump current Ip2 flowing through the second measuring pump cell 25 by the second measuring pump control process is correlated with the total concentration of water (i.e., gases to be reduced other than the first type gas) and oxygen in the gas being measured.

[0062] The control unit 96 performs a third measuring pump control process to control the third measuring pump cell 35 so that it pumps oxygen from around the third measuring electrode 36 to around the outer pump electrode 40. Specifically, the control unit 96 controls the third measuring pump cell 35 by feedback control of the voltage Vp3 of the variable power supply 37 so that the voltage V3 becomes a target value V3*. The target value V3* is set to a value such that the oxygen concentration in the third internal cavity 34 becomes a predetermined low concentration that suppresses the reduction of one or more oxide gases other than the first gas among the gases to be reduced in the gas to be measured, namely the second gas (in this case, water), compared to the second measuring pump control process. When this third measuring pump control process is performed, the reduction of the first gas (in this case, carbon dioxide) and the second gas (in this case, water) among the gases to be reduced in the gas to be measured is suppressed in the third internal cavity 34, and if oxide gases other than the first gas and the second gas are present among the gases to be reduced, those oxide gases are reduced and oxygen is generated. Then, the oxygen generated by this reduction, along with the oxygen that was present in the gas being measured before the reduction, is pumped from around the third measuring electrode 36 to around the outer pump electrode 40 by the pump current Ip3 flowing through the third measuring pump cell 35. Therefore, the pump current Ip3 flowing through the third measuring pump cell 35 due to the third measuring pump control process correlates with the total concentration of the gas to be reduced other than the first and second gases, and oxygen in the gas being measured. In this embodiment, as described above, the gases to be reduced are water (second gas) and carbon dioxide (first gas), so there are no "gases to be reduced other than the first and second gases in the gas being measured." Therefore, in this embodiment, the pump current Ip3 flowing through the third measuring pump cell 35 due to the third measuring pump control process correlates with the oxygen concentration in the gas being measured.

[0063] Furthermore, the control unit 96 performs a first concentration measurement process to measure the first concentration (here, carbon dioxide concentration), which is the concentration of the first type gas (here, carbon dioxide) in the gas to be measured, based on the pump current Ip1 flowing through the first measuring pump cell 15 by the first measuring pump control process and the pump current Ip2 flowing through the second measuring pump cell 25 by the second measuring pump control process. As described above, the pump current Ip1 flowing through the first measuring pump cell 15 by the first measuring pump control process is correlated with the total concentration of the gas to be reduced and oxygen in the gas to be measured. Also, the pump current Ip2 flowing through the second measuring pump cell 25 by the second measuring pump control process is correlated with the total concentration of water (i.e., the gas to be reduced other than the first type gas) and oxygen in the gas to be measured. Therefore, based on these pump currents Ip1 and Ip2, the first concentration (here, carbon dioxide concentration), which is the concentration of the first type gas (here, carbon dioxide) in the gas to be measured, can be measured. For example, the control unit 96 may measure the carbon dioxide concentration based on the difference between pump current Ip1 and pump current Ip2. In this case, a first correspondence between the difference between pump current Ip1 and pump current Ip2 and the carbon dioxide concentration may be stored in the storage unit 98 beforehand. The first correspondence can be, for example, a relational expression such as a linear function or a map. This first correspondence can be determined in advance by experimentation or analysis. The control unit 96 can then derive the difference between pump current Ip1 and pump current Ip2 and derive (measure) the carbon dioxide concentration based on the derived value and the first correspondence stored in the storage unit 98.

[0064] The control unit 96 performs a second concentration measurement process to measure the second concentration (in this case, water concentration), which is the concentration of the second type gas (in this case, water) in the gas to be measured, based on the pump current Ip2 flowing through the second measuring pump cell 25 by the second measuring pump control process and the pump current Ip3 flowing through the third measuring pump cell 35 by the third measuring pump control process. As described above, the pump current Ip2 flowing through the second measuring pump cell 25 by the second measuring pump control process correlates with the total concentration of water (i.e., gases to be reduced other than the first type gas) and oxygen in the gas to be measured. Also, the pump current Ip3 flowing through the third measuring pump cell 35 by the third measuring pump control process correlates with the total concentration of the gas to be reduced other than the first and second type gases and oxygen in the gas to be measured (in this case, the oxygen concentration in the gas to be measured). Therefore, based on these pump currents Ip2 and Ip3, the second concentration (in this case, water concentration), which is the concentration of the second type gas (in this case, water) in the gas to be measured, can be measured. For example, the control unit 96 may measure the water concentration based on the difference between pump current Ip2 and pump current Ip3. In this case, a second correspondence between the difference between pump current Ip2 and pump current Ip3 and the water concentration may be stored in the storage unit 98 beforehand. The second correspondence can be, for example, a relational expression such as a linear function or a map. This second correspondence can be determined in advance by experimentation or analysis. The control unit 96 can then derive the difference between pump current Ip2 and pump current Ip3 and derive (measure) the water concentration based on the derived value and the second correspondence stored in the storage unit 98.

[0065] The control unit 96 performs a third concentration measurement process to measure the third concentration, which is the sum of the reduction target gases other than the first and second gases and oxygen in the gas to be measured, based on the pump current Ip3 flowing to the third measurement pump cell 35 by the third measurement pump control process. As described above, in this embodiment, the oxide gases contained in the reduction target gas are the first gas (carbon dioxide) and the second gas (water), and no other oxide gases exist, so the oxygen concentration in the gas to be measured becomes the third concentration. Also, as described above, the pump current Ip3 flowing to the third measurement pump cell 35 by the third measurement pump control process is correlated with the oxygen concentration in the gas to be measured. Therefore, the third concentration (in this case, the oxygen concentration) in the gas to be measured can be measured based on this pump current Ip3. In this case, the third correspondence relationship between the pump current Ip3 and the oxygen concentration may be stored in advance in the storage unit 98. The third correspondence relationship can be, for example, a relational expression such as a linear function or a map. This third correspondence relationship can be determined in advance by experimentation or analysis. The control unit 96 can then derive (measure) the oxygen concentration based on the pump current Ip3 and the third correspondence stored in the memory unit 98.

[0066] The control unit 96 performs a fourth concentration measurement process to measure the fourth concentration (here, the total concentration of carbon dioxide and water), which is the total concentration of the first type gas and the second type gas in the gas to be measured, based on the pump current Ip1 flowing through the first measuring pump cell 15 by the first measuring pump control process and the pump current Ip3 flowing through the third measuring pump cell 35 by the third measuring pump control process. As described above, the pump current Ip1 flowing through the first measuring pump cell 15 by the first measuring pump control process correlates with the total concentration of the gas to be reduced and oxygen in the gas to be measured. Also, the pump current Ip3 flowing through the third measuring pump cell 35 by the third measuring pump control process correlates with the total concentration of the gas to be reduced other than the first type gas and the second type gas, and oxygen (here, the oxygen concentration in the gas to be measured). Therefore, based on these pump currents Ip1 and Ip3, the fourth concentration, which is the total concentration of the first type gas (here, carbon dioxide) and the second type gas (here, water) in the gas to be measured, can be measured. For example, the control unit 96 may measure a fourth concentration based on the difference between pump current Ip1 and pump current Ip3. In this case, a fourth correspondence between the difference between pump current Ip1 and pump current Ip3 and the fourth concentration may be stored in the storage unit 98 beforehand. The fourth correspondence can be, for example, a relational expression such as a linear function or a map. This fourth correspondence can be determined in advance by experimentation or analysis. The control unit 96 can then derive the difference between pump current Ip1 and pump current Ip3 and derive (measure) the water concentration based on the derived value and the fourth correspondence stored in the storage unit 98.

[0067] The control unit 96 outputs a control signal to the heater power supply 76 to perform heater control processing so that the temperature of the heater 72 reaches a target temperature (for example, 800°C). Here, the temperature of the heater 72 can be expressed as a linear function of the resistance value of the heater 72. Therefore, in the heater control processing, the control unit 96 calculates the resistance value of the heater 72 as a value that can be considered as the temperature of the heater 72 (a value that can be converted to temperature), and feedback controls the heater power supply 76 so that the calculated resistance value becomes the target resistance value (the resistance value corresponding to the target temperature). The control unit 96 can, for example, acquire the voltage of the heater 72 and the current flowing through the heater 72, and calculate the resistance value of the heater 72 based on the acquired voltage and current. The control unit 96 may also calculate the resistance value of the heater 72 using, for example, the three-terminal method or the four-terminal method. When energizing the heater 72, the heater power supply 76 adjusts the power supplied to the heater 72 by changing the value of the voltage applied to the heater 72 based on, for example, the control signal from the control unit 96.

[0068] Furthermore, the control device 95, including the variable power supplies 17, 27, and 37 and the heater power supply 76 shown in Figure 2, is actually connected to each electrode inside the sensor element 101 via lead wires (not shown) formed inside the sensor element 101 and connector electrodes (not shown) formed on the rear end side of the sensor element 101 (only the heater connector electrode 71 is shown in Figure 1).

[0069] Here, we will explain an example of the target values ​​V1*, V2*, V3* mentioned above. Figure 4 is an explanatory diagram showing an example of the relationship (VI characteristic) between the target voltage values ​​(V1*, V2*, V3*) and the pump current (Ip1, Ip2, Ip3) in the pump cells (1st to 3rd measuring pump cells 15, 25, 35). The thick solid line graph A in Figure 4 was obtained by investigating the relationship between the target voltage value and the pump current in the pump cell when the base gas is nitrogen and the model gas to be measured contains oxygen, water, and carbon dioxide. The thick dotted line graph B was obtained by investigating the relationship between the target voltage value and the pump current in the pump cell when the base gas is nitrogen and the model gas to be measured contains oxygen but does not contain water or carbon dioxide. The thin solid line graph C was obtained by investigating the relationship between the target voltage value and the pump current in the pump cell when the base gas is nitrogen and the model gas to be measured contains water but does not contain oxygen or carbon dioxide. The fine dotted graph D shows the relationship between the target voltage in the pump cell and the pump current when the model gas to be measured is nitrogen as the base gas, contains carbon dioxide, and does not contain oxygen or water. Figure 4 shows a graph for a gas sensor 100 in which the VI characteristics of the first to third measuring pump cells 15, 25, and 35 are made the same, for example, by making the diffusion resistance of each of the first to third gas flow sections to be measured the same, and by making the first to third measuring electrodes 16, 26, and 36 electrodes of the same material. Therefore, in the explanation of Figure 4 and Figure 5 described later, the first to third measuring pump cells 15, 25, and 35 are not distinguished and are simply referred to as "pump cells," the target values ​​V1*, V2*, and V3* are not distinguished and are simply referred to as "voltage target values," the pump currents Ip1, Ip2, and Ip3 are not distinguished and are simply referred to as "pump currents," the first to third internal cavities 14, 24, and 34 are not distinguished and are simply referred to as "internal cavities," the first to third measuring electrodes 16, 26, and 36 are not distinguished and are simply referred to as "inner electrodes," and the ratios R1, R2, and R3 are not distinguished and are simply referred to as "ratio R." Also, Figure 4 shows the VI characteristics when the inner electrodes do not contain a Class II precious metal.

[0070] As shown in Figure 4, in all graphs A to D, a tendency was observed where the pump current increased as the absolute value of the target voltage increased. Furthermore, the larger the absolute value of the target voltage, the more the pump cell is controlled to lower the target oxygen concentration in the internal cavity relative to the oxygen concentration of the reference gas surrounding the reference electrode 42 (i.e., to pump out more oxygen from the internal cavity).

[0071] In Graph D of Figure 4, the pump current is nearly constant, close to zero, in the region where the target voltage is between 200mV and 700mV. In the region where the target voltage is above 700mV, the pump current increases as the target voltage rises. Then, in the region between 1250mV and 1400mV, the pump current remains nearly constant. In other words, the pump current is at the limit current. This region is called the plateau region. From Graph D, it can be seen that in the region where the target voltage is above 700mV, carbon dioxide is reduced, and the oxygen generated by the reduction is pumped out by the pump current. Furthermore, for carbon dioxide, the region where the target voltage is between 1250mV and 1400mV is a plateau region, and in this region, the pump current is correlated with the carbon dioxide concentration in the internal cavity. Similarly, from Graph C, it can be seen that for water, the region where the target voltage is between 1000mV and 1400mV is a plateau region, and in this region, the pump current is correlated with the water concentration in the internal cavity. Furthermore, from graph B, it can be seen that for oxygen, the region where the target voltage is between 200mV and 1400mV is a plateau region, and in this region, the pump current is correlated with the oxygen concentration in the internal cavity. From these graphs B to D, it can be seen that carbon dioxide, water, and oxygen have different lower limits for the voltage target values ​​at which the plateau region appears. The lower limit for the voltage target value at which the plateau region appears is lowest for oxygen (200mV), followed by water (1000mV), and highest for carbon dioxide (1250mV). And, since the lower limit for the voltage target value at which the plateau region appears is higher for carbon dioxide than for water, it can be seen that carbon dioxide is less easily reduced than water.

[0072] Furthermore, in Graph A of Figure 4, which shows the VI characteristics obtained using a gas under test containing oxygen, water, and carbon dioxide, it can be seen that three plateau regions appear, corresponding to the respective plateau regions in Graphs B to D. In this embodiment, the target voltage values ​​(V1*, V2*, V3*) are set to be different from each other, and the relationship of their absolute values ​​is set to |V3*|<|V2*|<|V1*|, so that the target values ​​V1*, V2*, and V3* correspond to the three plateau regions in Graph A. In this way, the pump currents Ip1 to Ip3 can be made to correspond to different gas concentrations. Specifically, the target value V1* is set to a predetermined value of 1250mV to 1400mV, corresponding to the region with the highest target voltage among the three plateau regions in Graph A. In this region, as can be seen from Graphs B to D, carbon dioxide, water, and oxygen are all plateau regions, and the pumping of oxygen and the reduction of water and carbon dioxide from the gas under test occur. Therefore, when the first measurement pump control process is performed based on the target value V1* set in this way, the pump current Ip1 becomes a relatively large value, and the pump current Ip1 becomes a value correlated with the total concentration of water, carbon dioxide, and oxygen in the gas being measured. The target value V2* is set as a predetermined value between 1000mV and 1200mV, corresponding to the second highest voltage target value among the three plateau regions in graph A. As can be seen from graphs B to D, this region is a plateau region for water and oxygen, while it is outside the carbon dioxide plateau region (a region with a lower voltage target value than the carbon dioxide plateau region). In this region, oxygen is pumped out and water is reduced from the gas being measured, while the reduction of carbon dioxide is suppressed. Therefore, when the second measurement pump control process is performed based on the target value V2* set in this way, the pump current Ip2 becomes a smaller value than the pump current Ip1 due to the suppression of carbon dioxide reduction, and the pump current Ip2 becomes a value correlated with the total concentration of water and oxygen in the gas being measured. The target value V3* is set as a predetermined value between 200mV and 700mV, corresponding to the region with the lowest voltage target value among the three plateau regions of graph A.As can be seen from graphs B to D, this region is an oxygen plateau region, while it is outside the plateau regions for water and carbon dioxide (a region where the voltage target value is lower than that of the water and carbon dioxide plateau regions). In this region, oxygen is pumped out of the gas being measured, while the reduction of water and carbon dioxide is suppressed. Therefore, when the third measurement pump control process is performed based on the target value V3* set in this way, the pump current Ip3 becomes smaller than the pump current Ip2 due to the suppression of the reduction of water and carbon dioxide, and the pump current Ip3 becomes a value correlated with the oxygen concentration in the gas being measured.

[0073] As described above, even when the gas to be measured contains oxygen, water, and carbon dioxide (Graph A in Figure 4), the reduction of some or all of the target gases (in this case, water and carbon dioxide) can be selectively suppressed by changing the voltage target value. More specifically, as the absolute value of the voltage target value is reduced, the reduction of the target gases (in this case, water and carbon dioxide) is suppressed in order from the gas that is less likely to be reduced (in this case, the reduction of carbon dioxide is suppressed first, and then the reduction of water is suppressed). This can be used to adjust the relationship between the pump current and the gas concentration according to the voltage target value. In this embodiment, by setting the target values ​​V1*, V2*, and V3* to the values ​​described above, the pump current Ip1 corresponds to the total concentration of water, carbon dioxide, and oxygen, the pump current Ip2 corresponds to the total concentration of water and oxygen, and the pump current Ip3 corresponds to the oxygen concentration, thereby enabling the measurement of the first to fourth concentrations described above.

[0074] Next, we will explain the VI characteristics when the inner electrode contains a Class II precious metal. Figure 5 shows the VI characteristics of a pump cell when the inner electrode contains a Class II precious metal such that the percentage R is between 2% and 10%. The measured gases in graphs A to D in Figure 5 are the same as the measured gases in graphs A to D in Figure 4. Comparing graph D in Figure 4 and graph D in Figure 5, both show that when the voltage target value exceeds 700mV, the pump current increases as the voltage target value increases (the pump current rises). However, in graph D in Figure 5, compared to graph D in Figure 4, the pump current does not rise much in the region where the voltage target value is between 900mV and 1100mV, and the pump current begins to rise when the voltage target value exceeds 1100mV. Furthermore, in graph D of Figure 5, similar to graph D of Figure 4, the region where the voltage target value is between 1250mV and 1400mV is a plateau region, but the increase in pump current before reaching the plateau region was smaller than in graph D of Figure 4. These results confirm that including a second-class precious metal in the inner electrode suppresses the reduction of carbon dioxide around the inner electrode. In contrast, graphs B and C of Figure 5 showed almost no change from graphs B and C of Figure 4. That is, even when a second-class precious metal is included in the inner electrode, the reduction of water around the inner electrode is hardly suppressed, and the oxygen pumping by the pump cell is hardly affected. From the above comparison results of Figures 4 and 5, it was confirmed that including a second-class precious metal in the inner electrode selectively suppresses the reduction of carbon dioxide.

[0075] Furthermore, a comparison of Graph A in Figure 4 and Graph A in Figure 5 confirmed a trend stemming from the difference between Graph D in Figure 4 and Graph D in Figure 5, due to the selective suppression of carbon dioxide reduction as described above. Specifically, the pump current in Graph A in Figure 4 is slightly upward sloping in the region where the voltage target value is between 1000mV and 1200mV, because it is affected not only by the plateau region in Graph C in Figure 4 but also by the rise in the pump current in Graph D in Figure 4 (i.e., carbon dioxide reduction). In contrast, in the region where the voltage target value is between 1000mV and 1100mV, the rise in the pump current in Graph D in Figure 5 is suppressed as described above, so the shape of the plateau region in Graphs B and C in Figure 5 is more strongly expressed, and the pump current is flatter compared to Graph A in Figure 4. In other words, in the region where the voltage target value is between 1000mV and 1100mV, the influence of the pump current originating from carbon dioxide reduction is suppressed more in Graph A in Figure 5 than in Graph A in Figure 4. Therefore, if the second measuring electrode 26 contains a Class II precious metal, the target value V2* should be set as a predetermined value between 1000mV and 1100mV, corresponding to the second highest voltage target value among the three plateau regions in graph A of Figure 5. In this way, the pump current Ip2 is less affected by the carbon dioxide concentration in the gas being measured, and the pump current Ip2 corresponds more accurately to the total concentration of water and oxygen. As a result, when measuring the carbon dioxide concentration based on the difference between pump current Ip1 and pump current Ip2, this difference corresponds more accurately to the carbon dioxide concentration. Thus, by including a Class II precious metal in the second measuring electrode 26, the measurement accuracy of the first concentration measurement process, i.e., the measurement accuracy of the carbon dioxide concentration based on pump current Ip1 and pump current Ip2, is improved. For these reasons, it is preferable for the second measuring electrode 26 to contain a Class II precious metal.

[0076] Regarding the first measuring electrode 16, as mentioned above, it is sufficient that it contains a Class 1 precious metal, but it may also contain a Class 2 precious metal, or it may not contain a Class 2 precious metal. When the first measuring electrode 16 contains a Class 2 precious metal, as can be seen from the comparison between graph D in Figure 4 and graph D in Figure 5, in the region where the voltage target value is between 1250 mmV and 1400 mV, the pump current value derived from the reduction of carbon dioxide tends to decrease, but a carbon dioxide plateau region appears. Therefore, if the voltage target value V1* is set to a predetermined value between 1250 mV and 1400 mV, the pump current Ip1 will be a value correlated with the total concentration of water, carbon dioxide, and oxygen in the gas being measured. Thus, even if the first measuring electrode 16 contains a Class 2 precious metal, the carbon dioxide concentration can be measured based on the pump current Ip1 and pump current Ip2. Furthermore, if the first measuring electrode 16 contains a second-class precious metal, the proportion R1 of the second-class precious metal in the first measuring electrode 16 may be 2% or more, or 5% or more. The proportion R1 may be 10% or less, or 5% or less. The proportion R1 may be less than or equal to the proportion R2, or less than the proportion R2.

[0077] The third measuring electrode 36 is used at a voltage target value (V3*) where carbon dioxide is hardly reduced. Therefore, as mentioned above, it is sufficient for it to contain a Class I precious metal, but it may also contain a Class II precious metal, or it may not contain a Class II precious metal. Even if the third measuring electrode 36 contains a Class II precious metal, the target value V3* should be set to a predetermined value of 200mV to 700mV, similar to Figure 4, as the value corresponding to the region with the lowest voltage target value among the three plateau regions in graph A of Figure 5. When the third measuring electrode 36 contains a Class II precious metal, the proportion R3 of the third measuring electrode 36 may be 2% or more, or 5% or more. The proportion R3 may be 10% or less, or 5% or less. The proportion R3 may be a value less than or equal to the proportion R2, or a value smaller than the proportion R2.

[0078] The VI characteristics and target values ​​V1*, V2*, and V3* in Figures 4 and 5 are examples only. For example, if the diffusion resistance values ​​of each of the first to third gas flow sections change, the numerical range of the voltage target values ​​in which the three plateau regions appear in graph A of Figure 4 may also change. In this case as well, the target values ​​V1*, V2*, and V3* should be determined based on the VI characteristics of each of the first to third measuring pump cells 15, 25, and 35. However, it is preferable that the diffusion resistance values ​​of each of the first to third gas flow sections are close to or the same as each other. For example, in Figure 2, the first buffer space 12 has a shorter length along the gas flow direction and a larger length (i.e., width) perpendicular to the gas flow direction compared to the second buffer space 22 and the third buffer space 32. Therefore, the first buffer space 12 has a smaller diffusion resistance than the second buffer space 22 and the third buffer space 32. Even in such cases, by, for example, making the diffusion resistance of the first diffusion rate-limiting section 13 higher than that of the second diffusion rate-limiting section 23 and the third diffusion rate-limiting section 33 (for example, by reducing the cross-sectional area of ​​the slit in the first diffusion rate-limiting section 13), the diffusion resistance values ​​of each of the first to third gas flow sections to be measured can be made close to or the same.

[0079] As shown in Figure 2, in this embodiment, the top-view areas of the first to third measuring electrodes 16, 26, and 36 are all different, with the first measuring electrode 16 having the largest area, the second measuring electrode 26 having the next largest area, and the third measuring electrode 36 having the smallest area. This corresponds to the fact that, as described above, the magnitude of the pump current flowing through the first to third measuring pump control processes is Ip1 > Ip2 > Ip3. Since a larger area of ​​the inner electrode increases the oxygen pumping capacity of the pump cell, it is preferable to increase the area of ​​the inner electrode for pump cells through which a larger pump current flows.

[0080] An example of using the gas sensor 100 configured in this way is described below. With the gas sensor 100 attached to a pipe through which the gas to be measured flows, the control unit 96 first performs the heater control process described above to control the temperature of the heater 72 to the target temperature. When the temperature of the heater 72 reaches the target temperature (or near the target temperature), the control unit 96 starts the first to third measurement pump control processes described above. While the first to third measurement pump control processes are continuously performed, the control unit 96 acquires (measures) the pump currents Ip1 to Ip3, and based on the acquired values, performs the first to fourth concentration measurement processes to measure the specific gas concentrations in the gas to be measured (in this case, the first to fourth concentrations, i.e., carbon dioxide concentration, water concentration, oxygen concentration, and the total concentration of carbon dioxide and water).

[0081] Here, the correspondence between the components of this embodiment and the components of the present invention will be clarified. The sensor element 101 of this embodiment corresponds to the sensor element of the present invention, the element body 102 corresponds to the element body, the first internal cavity 14 corresponds to the first chamber, the second internal cavity 24 corresponds to the second chamber, the third internal cavity 34 corresponds to the third chamber, the first measuring electrode 16 corresponds to the first inner electrode, the first measuring pump cell 15 corresponds to the first pump cell, the second measuring electrode 26 corresponds to the second inner electrode, the second measuring pump cell 25 corresponds to the second pump cell, the third measuring electrode 36 corresponds to the third inner electrode, and the third measuring pump cell 35 corresponds to the third pump cell, the outer pump electrode 40 corresponds to the first outer electrode, the second outer electrode, and the third outer electrode, the control device 95 corresponds to the control device, the first measuring pump control process corresponds to the first pump cell control process, the second measuring pump control process corresponds to the second pump cell control process, the third measuring pump control process corresponds to the third pump cell control process, the pump current Ip1 corresponds to the first pump current, the pump current Ip2 corresponds to the second pump current, and the pump current Ip3 corresponds to the third pump current. Also, voltage V1 corresponds to the first voltage, target value V1* corresponds to the first voltage target value, voltage V2 corresponds to the second voltage, target value V2* corresponds to the second voltage target value, voltage V3 corresponds to the third voltage, and target value V3* corresponds to the third voltage target value. The first gas inlet 11 corresponds to the first inlet, the second gas inlet 21 corresponds to the second inlet, and the third gas inlet 31 corresponds to the third inlet.

[0082] As described in detail above, the gas sensor 100 of this embodiment allows the control device 95 to measure the first to fourth concentrations as specific gas concentrations based on the pump currents Ip1 to Ip3. Here, the pump current Ip1 flowing to the first measuring pump cell 15 by the first measuring pump control process correlates with the total concentration of the gas to be reduced and oxygen in the gas to be measured. The pump current Ip2 flowing to the second measuring pump cell 25 by the second measuring pump control process correlates with the total concentration of water (i.e., the gas to be reduced other than the first type gas) and oxygen in the gas to be measured. The pump current Ip3 flowing to the third measuring pump cell 35 by the third measuring pump control process correlates with the oxygen concentration in the gas to be measured. Therefore, based on the pump currents Ip1 and Ip2, the first concentration (here, the carbon dioxide concentration), which is the concentration of the first type gas (here, carbon dioxide) in the gas to be measured, can be measured. Furthermore, based on pump currents Ip2 and Ip3, the second concentration (water concentration in this case), which is the concentration of the second type gas (water in this case) in the gas being measured, can be measured. Based on pump current Ip3, the third concentration (oxygen concentration in this case) in the gas being measured can be measured. Based on pump currents Ip1 and Ip3, the fourth concentration, which is the total concentration of the first type gas (carbon dioxide in this case) and the second type gas (water in this case) in the gas being measured, can be measured. Moreover, in the gas sensor 100, the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34 are not connected to each other, and the gas being measured reaches the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34 from outside the sensor element 101 via independent paths. Therefore, it is possible to suppress the mutual influence of each gas between the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34, and the reduction in measurement accuracy due to the aforementioned back diffusion is less likely to occur. For example, if the first internal cavity 14 and the second internal cavity 24 are in communication with each other inside the sensor element 101, hydrogen and carbon monoxide produced by the reduction of water and carbon dioxide in the first internal cavity 14 may reach the second internal cavity 24, which may reduce the measurement accuracy of the first and / or second concentrations based on the pump current Ip2. However, this is unlikely to occur in the gas sensor 100 of this embodiment.Therefore, this gas sensor 100 can suppress a decrease in the measurement accuracy of the specific gas concentration in the gas being measured.

[0083] Furthermore, by including a Class II precious metal in addition to the Class I precious metal in the second measuring electrode 26, the pump current Ip2 becomes less susceptible to the influence of the carbon dioxide concentration in the gas being measured. As a result, the measurement accuracy of the first concentration measurement process, i.e., the measurement accuracy of the carbon dioxide concentration based on the pump currents Ip1 and Ip2, is improved. In addition, by having a ratio R2 of 2% or more of the second measuring electrode 26, the reduction capacity of the second measuring electrode 26 for carbon dioxide can be more reliably weakened.

[0084] Furthermore, since the first gas inlet 11, the second gas inlet 21, and the third gas inlet 31 open to different faces among the first to sixth faces 102a to 102f, the mutual influence of each gas between the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34 can be further suppressed.

[0085] It goes without saying that the present invention is not limited in any way to the embodiments described above, and can be implemented in various forms as long as they fall within the technical scope of the present invention.

[0086] For example, in the embodiment described above, the control device 95 performed the first to fourth concentration measurement processes to measure the first to fourth concentrations, but it is sufficient to perform at least two of the first to fourth concentration measurement processes. In this case, at least two of the first to fourth concentration measurement processes may be performed in a combination that utilizes all of the pump currents Ip1 to Ip3. For example, a combination that performs only the second and third concentration measurement processes is undesirable because it does not utilize the pump current Ip1. Similarly, a combination that performs only the third and fourth concentration measurement processes is undesirable because it does not utilize the pump current Ip2. Furthermore, the control device 95 may omit the fourth concentration measurement process. That is, the control device 95 may perform the first to third concentration measurement processes to measure the first to third concentrations.

[0087] In the embodiment described above, the control device 95 measured the first concentration based on the difference between pump current Ip1 and pump current Ip2 in the first concentration measurement process. However, it is not limited to this, and the first concentration may be measured based on pump current Ip1 and pump current Ip2. For example, the control device 95 may derive the total concentration of the gas to be reduced and oxygen in the gas to be measured (also referred to as the first total concentration) based on pump current Ip1, and the total concentration of the gas to be reduced other than the first gas and oxygen in the gas to be measured (also referred to as the second total concentration) based on pump current Ip2, and then derive (measure) the difference between the first total concentration and the second total concentration as the first concentration. In this case, the correspondence between pump current Ip1 and the first total concentration, and the correspondence between pump current Ip2 and the second total concentration are stored in the storage unit 98 in advance, and the control device 95 may measure the first total concentration, the second total concentration, and the first concentration using pump current Ip1, pump current Ip2, and their correspondences. The same approach can be taken for measuring the second and fourth concentrations. For example, in the second concentration measurement process, the control device 95 may derive the second total concentration based on the pump current Ip2, and then derive the total concentration of the gas to be reduced other than the first and second gases and oxygen in the gas to be measured (also referred to as the third total concentration) based on the pump current Ip3, and then derive (measure) the difference between the second total concentration and the third total concentration as the second concentration. In the fourth concentration measurement process, the control device 95 may derive the first total concentration based on the pump current Ip1, and then derive the third total concentration based on the pump current Ip3, and then derive (measure) the difference between the first total concentration and the third total concentration as the fourth concentration.

[0088] In the embodiment described above, the correspondence between the difference between pump current Ip1 and pump current Ip2 and the carbon dioxide concentration is stored in the storage unit 98 as the first correspondence. However, the correspondence between pump current Ip1, pump current Ip2, and the first concentration may also be stored in the storage unit 98 as the first correspondence. In this case, the control device 95 may derive the first concentration based on the pump currents Ip1 and Ip2 and the first correspondence without deriving the difference between pump current Ip1 and Ip2. The same considerations can be applied to the measurement of the second and fourth concentrations.

[0089] In the embodiments described above, the control device 95 measured the first concentration based on the difference between pump current Ip1 and pump current Ip2 in the first concentration measurement process. However, the control device 95 may also correct at least one of pump current Ip1 and pump current Ip2 before deriving the difference and measuring the first concentration based on that difference. That is, in the first concentration measurement process, the control device 95 may measure the first concentration based on the difference between pump current Ip1' (which is the corrected pump current Ip1) and pump current Ip2, or it may measure the first concentration based on the difference between pump current Ip1 and pump current Ip2' (which is the corrected pump current Ip2), or it may measure the first concentration based on the difference between pump current Ip1' and pump current Ip2'. These embodiments are also included in the embodiments of "measuring the first concentration based on the difference between pump current Ip1 and pump current Ip2". It is preferable that the correction of at least one of pump current Ip1 and pump current Ip2 is performed in such a way that the difference between the sensitivity of pump current Ip1 to the gas and the sensitivity of pump current Ip2 to the gas is reduced. Basically, correction can be applied to either the pump current Ip1 or the pump current Ip2, but as mentioned above, correction may be applied to both. Here, for example, if the proportion R1 of the second type of precious metal in the first measuring electrode 16 and the proportion R2 of the second type of precious metal in the second measuring electrode 26 are different, the VI characteristics of the first measuring pump cell 15 and the second measuring pump cell 25 may differ, as shown in Figures 4 and 5. Also, as mentioned above, if the diffusion resistance values ​​are different in the first gas flow section and the second gas flow section, the VI characteristics of the first measuring pump cell 15 and the second measuring pump cell 25 may differ. And when the VI characteristics of the first measuring pump cell 15 and the second measuring pump cell 25 are different, there may be a difference in the sensitivity of pump current Ip1 to the gas and the sensitivity of pump current Ip2 to the gas. For example, if the composition of the gas to be measured is the same in the first internal cavity 14 and the second internal cavity 24, and the carbon dioxide concentration is 0%, then if the sensitivity of the first measuring pump cell 15 and the second measuring pump cell 25 to the gas is the same, then the pump current Ip1 flowing through the first measuring pump control process and the pump current Ip2 flowing through the second measuring pump control process will be basically the same value.In contrast, if there is a difference in the gas sensitivity between the first measuring pump cell 15 and the second measuring pump cell 25, even if the composition of the gas to be measured is the same in the first internal cavity 14 and the second internal cavity 24 and the carbon dioxide concentration is 0%, a discrepancy may occur between the value of the pump current Ip1 flowing due to the first measuring pump control process and the value of the pump current Ip2 flowing due to the second measuring pump control process. In this case, if there is a difference in the gas sensitivity between the first measuring pump cell 15 and the second measuring pump cell 25, the difference between the pump current Ip1 and the pump current Ip2 includes not only the carbon dioxide concentration but also the aforementioned discrepancy. Therefore, by correcting at least one of the pump current Ip1 and the pump current Ip2 to reduce this discrepancy, the corrected difference will correspond to the carbon dioxide concentration with greater accuracy. The correction of the pump current Ip1 may be performed, for example, by multiplying the pump current Ip1 by a predetermined correction coefficient. For example, if the pump current Ip2 is 0.9 times the value of pump current Ip1 when the composition of the gas to be measured is the same in the first internal cavity 14 and the second internal cavity 24 and the carbon dioxide concentration is 0%, then the corrected pump current Ip1' may be obtained by multiplying pump current Ip1 by a correction factor of 0.9. The correction of pump current Ip1 may also be performed by deriving pump current Ip1' using the correspondence between pump current Ip1 and the corrected pump current Ip1'. Such correction factors and correspondences can be determined in advance by experiment or analysis and stored in the memory unit 98. The correction of pump current Ip2 can be performed in the same manner. The measurement of the second and fourth concentrations can also be considered in the same way. For example, in the second concentration measurement process, the control device 95 may correct at least one of pump current Ip2 and pump current Ip3, derive the difference, and measure the second concentration based on that difference. This embodiment is also included in the embodiment of "measuring the second concentration based on the difference between pump current Ip2 and pump current Ip3". In the fourth concentration measurement process, the control device 95 may correct at least one of the pump currents Ip1 and Ip3, derive the difference, and measure the fourth concentration based on that difference. This embodiment is also included in the embodiment of "measuring a fourth concentration based on the difference between pump current Ip1 and pump current Ip3".

[0090] In the embodiments described above, the gases to be reduced were water and carbon dioxide, the first gas was carbon dioxide, and the second gas was water, but the invention is not limited to these. The gases to be reduced are not limited to water and carbon dioxide, but can be two or more oxide gases in the gas being measured. Some of the oxide gases among the gases to be reduced can be designated as the first gas. One or more oxide gases other than the first gas among the gases to be reduced can be designated as the second gas. The first gas is one or more oxide gases selected from the two or more oxide gases contained in the gases to be reduced, in order from the least easily reduced oxide gas. The second gas is one or more oxide gases selected from the oxide gases other than the first gas in the gases to be reduced, in order from the least easily reduced oxide gas. For example, consider a case where the gases to be reduced are four types: gas a, gas b, gas c, and gas d, and these are oxide gases that are less easily reduced in that order. In this case, the first gas is selected from the oxide gases among the gases to be reduced, in order from the least easily reduced oxide gas, for example, gas a, or gas a and gas b. Furthermore, if the first type gas is gas a, the second type gas is selected from among the oxide gases other than the first type gas in the gas to be reduced, in order from the oxide gases that are difficult to reduce, such as gas b, or gas b and gas c. In the embodiment described above, since the gases to be reduced are water and carbon dioxide, carbon dioxide, which is difficult to reduce, is designated as the first type gas, and water, which is the other oxide gas, is designated as the second type gas. Two or more gases including carbon dioxide may be designated as the first type gas. Two or more gases including water may be designated as the second type gas. Note that even if an oxide gas is contained in the gas to be measured, an oxide gas that is not reduced by any of the processes performed by the control device 95 (at least two processes performed in a combination that utilizes all of the pump currents Ip1 to Ip3 from the first to fourth pump cell control processes) is not a gas to be reduced.

[0091] In the embodiments described above, the first gas inlet 11, the second gas inlet 21, and the third gas inlet 31 opened to different faces among the first to sixth faces 102a to 102f, but this is not limited to this. Two of the first gas inlet 11, the second gas inlet 21, and the third gas inlet 31 may open to the same face among the first to sixth faces 102a to 102f. Also, as in the modified element body 102 shown in Figure 6, the first gas inlet 11, the second gas inlet 21, and the third gas inlet 31 may open to the same face among the first to sixth faces 102a to 102f. In the modified example in Figure 6, the first gas inlet 11, the second gas inlet 21, and the third gas inlet 31 all open to the third face 102c. As a result, even if the concentration of a specific gas in the gas being measured fluctuates rapidly, the gas being measured is more likely to reach the same specific gas concentration in each of the first internal cavities 14, 24, and 34. Therefore, the measurement accuracy of the concentrations (first, second, and fourth concentrations) measured based on two of the first to third pump currents is improved.

[0092] In the embodiments described above, the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34 are all spaces formed by hollowing out the spacer layer 5, but the invention is not limited to this. Two or more of the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34 may be formed in different layers among the multiple layers provided by the element body 102. For example, as in the modified element body 102 shown in Figure 7, the first internal cavity 14, the second internal cavity 24, and the third internal cavity 34 may be formed in different layers. In the modified example in Figure 7, the element body 102 includes a third solid electrolyte layer 7 and a fourth solid electrolyte layer 8 in addition to the layers 1 to 6 of the embodiments described above. The first internal cavity 14 is formed by hollowing out the spacer layer 5, the second measuring pump cell 25 is formed by hollowing out the second solid electrolyte layer 6, and the third measuring pump cell 35 is formed by hollowing out the third solid electrolyte layer 7. Furthermore, the outer pump electrode 40 is positioned on the upper surface of the fourth solid electrolyte layer 8, which is the first surface 102a of the element body 102.

[0093] In the embodiment described above, one of the first internal cavity 14, second internal cavity 24, and third internal cavity 34 provided in the element body 102 of the sensor element 101 may be omitted. In this case, the two internal cavities that were not omitted from the first internal cavity 14, second internal cavity 24, and third internal cavity 34 correspond to the first and second chambers of the second gas sensor of the present invention. Even in this case, the control device 95 can perform processing corresponding to the two internal cavities that were not omitted from the first to third measurement pump control processing of the embodiment described above, and flow two pump currents from pump currents Ip1 to Ip3, and measure the specific gas concentration based on those two pump currents. For example, if the third internal cavity 34 is omitted, the control unit 96 may measure the first type gas (e.g., carbon dioxide concentration) in the gas to be measured as the specific gas concentration based on the pump current Ip1 flowing to the first measurement pump cell 15 by the first measurement pump control processing and the pump current Ip2 flowing to the second measurement pump cell 25 by the second measurement pump control processing. The same considerations can be applied to cases where the first internal void 14 is omitted or where the second internal void 24 is omitted.

[0094] In the embodiment described above, the outer pump electrode 40 served as the first outer electrode paired with the first measuring electrode 16 in the first measuring pump cell 15, the second outer electrode paired with the second measuring electrode 26 in the second measuring pump cell 25, and the third outer electrode paired with the third measuring electrode 36 in the third measuring pump cell 35. In other words, the first to third outer electrodes were configured as a common outer pump electrode 40. However, this is not the only configuration. For example, two of the first to third outer electrodes may be arranged on the outer surface of the element body 102 as common electrodes and the remaining one as an electrode independent of the outer pump electrode 40. Alternatively, the first to third outer electrodes may each be provided as independent electrodes on the outer surface of the element body 102. [Industrial applicability]

[0095] This invention can be used in gas sensors that detect specific gas concentrations, such as carbon dioxide concentration, water concentration, and oxygen concentration, in a gas to be measured, such as automobile exhaust gas. [Explanation of Symbols]

[0096] 1 First substrate layer, 2 Second substrate layer, 3 Third substrate layer, 4 First solid electrolyte layer, 5 Spacer layer, 6 Second solid electrolyte layer, 7 Third solid electrolyte layer, 8 Fourth solid electrolyte layer, 11, 21, 31 First to third gas inlets, 12, 22, 32 First to third buffer spaces, 13, 23, 33 First to third diffusion rate-limiting sections, 14, 24, 34 First to third internal cavities, 15, 25, 35 First to third measurement pump cells, 16, 26, 36 First to third measurement electrodes, 17, 27, 37 Variable power supply, 18, 28, 38 First to third sensor cells, 40 Outer pump electrode, 42 Reference electrode, 43 Reference gas introduction space, 48 Reference gas introduction layer, 49 Reference gas introduction section, 49a Inlet section, 70 Heater section, 71 Heater connector electrode, 72 Heater, 73 Through-hole, 74 Heater insulating layer, 75 Pressure relief hole, 76 Heater power supply, 95 Control device, 96 Control unit, 97 CPU, 98 Memory unit, 100 Gas sensor, 101 Sensor element, 102 Element body, 102a~102f First to sixth surfaces.

Claims

1. A gas sensor comprising a sensor element and a control device, which measures the concentration of a specific gas in a gas to be measured, The aforementioned sensor element is The element body has an oxygen ion conductive solid electrolyte layer and is provided with a first chamber, a second chamber, and a third chamber inside, which are not connected to each other and to which the gas to be measured can reach from outside the sensor element, A first pump cell comprising a first inner electrode disposed in the first chamber and a first outer electrode disposed on the outer surface of the element body, A second pump cell comprising a second inner electrode disposed in the second chamber and a second outer electrode disposed on the outer surface of the element body, A third pump cell comprising a third inner electrode disposed in the third chamber and a third outer electrode disposed on the outer surface of the element body, It has, The control device is A first pump cell control process is performed to control the first pump cell so as to pump oxygen from around the first inner electrode to around the first outer electrode, thereby reducing the target gas, which is two or more oxide gases in the gas to be measured in the first chamber. A second pump cell control process controls the second pump cell to pump oxygen from around the second inner electrode to around the second outer electrode, while suppressing the reduction of a certain type of oxide gas, which is a first type gas, among the gases to be reduced in the gas to be measured in the second chamber, compared to the first pump cell control process, A third pump cell control process controls the third pump cell to pump oxygen from around the third inner electrode to around the third outer electrode, while suppressing the reduction of the second type gas, which is one or more oxide gases other than the first type gas among the gases to be reduced in the gas to be measured in the third chamber, compared to the second pump cell control process, Perform The control device is A first concentration measurement process that measures a first concentration, which is the concentration of the first type of gas in the gas to be measured, based on a first pump current flowing through the first pump cell by the first pump cell control process and a second pump current flowing through the second pump cell by the second pump cell control process. A second concentration measurement process that measures the second concentration, which is the concentration of the second type of gas in the gas to be measured, based on the second pump current and the third pump current flowing to the third pump cell by the third pump cell control process, A third concentration measurement process that measures a third concentration, which is the total concentration of the gas to be reduced other than the first gas and the second gas in the gas to be measured, and oxygen, based on the third pump current, A fourth concentration measurement process that measures a fourth concentration, which is the sum of the first type gas and the second type gas in the gas to be measured, based on the first pump current and the third pump current, By performing at least two processes selected in a combination that utilizes all of the first to third pump currents, at least two of the first to fourth concentrations are measured as the specific gas concentration. Gas sensor.

2. A gas sensor according to claim 1, The gases to be reduced are water and carbon dioxide. The aforementioned first type of gas is carbon dioxide, The aforementioned second type of gas is water. Gas sensor.

3. A gas sensor according to claim 2, The second inner electrode contains a first type of precious metal having catalytic activity and a second type of precious metal that suppresses the reduction of carbon dioxide. Gas sensor.

4. A gas sensor according to claim 3, The aforementioned first type of precious metal is at least one of Pt, Rh, Ir, Ru, and Pd. The aforementioned second type of precious metal is Au. Gas sensor.

5. A gas sensor according to claim 3 or 4, The second inner electrode has a ratio R2 of 2% or more, calculated by the following formula (1). R2=S2 / (S1+S2)×100 (1) however, S1: Mass percentage of the first type of precious metal [wt%] S2: Mass percentage of the second type of precious metal [wt%] Gas sensor.

6. A gas sensor according to any one of claims 1 to 4, The sensor element has a reference electrode disposed inside the element body so as to be in contact with a reference gas. The control device is In the first pump cell control process, the first pump cell is controlled so that the first voltage, which is the voltage between the reference electrode and the first inner electrode, becomes a first voltage target value. In the second pump cell control process, the second pump cell is controlled so that the second voltage, which is the voltage between the reference electrode and the second inner electrode, is a second voltage target value that has an absolute value smaller than the first voltage target value. In the third pump cell control process, the third pump cell is controlled so that the third voltage, which is the voltage between the reference electrode and the third inner electrode, is a third voltage target value that has an absolute value smaller than the second voltage target value. Gas sensor.

7. A gas sensor according to any one of claims 1 to 4, The first concentration measurement process is a process of measuring the first concentration based on the difference between the first pump current and the second pump current, or a process of measuring the first concentration based on the difference between the total concentration of the gas to be reduced and oxygen in the gas to be measured, derived based on the first pump current, and the total concentration of the gas to be reduced other than the first type gas and oxygen in the gas to be measured, derived based on the second pump current. The second concentration measurement process is a process of measuring the second concentration based on the difference between the second pump current and the third pump current, or a process of measuring the second concentration based on the difference between the total concentration of the gas to be reduced other than the first gas and oxygen in the gas to be measured, derived based on the second pump current, and the total concentration of the gas to be reduced other than the first gas and the second gas and oxygen in the gas to be measured, derived based on the third pump current. The fourth concentration measurement process is a process of measuring the fourth concentration based on the difference between the first pump current and the third pump current, or a process of measuring the fourth concentration based on the difference between the total concentration of the gas to be reduced and oxygen in the gas to be measured, derived based on the first pump current, and the total concentration of the gas to be reduced other than the first gas and the second gas and oxygen in the gas to be measured, derived based on the third pump current. Gas sensor.

8. A gas sensor according to any one of claims 1 to 4, The element body has a rectangular parallelepiped shape with the first to sixth faces as its outer surfaces. The element body is The first inlet is the entrance for the gas to be measured from the outside to the first chamber, The second inlet is the entrance for the gas to be measured from the outside to the second chamber, The third inlet is the entrance from the outside to the third chamber for the gas to be measured, It has, The first inlet, the second inlet, and the third inlet open to different faces among the first to sixth faces. Gas sensor.

9. A gas sensor according to any one of claims 1 to 4, The element body has a rectangular parallelepiped shape with the first to sixth faces as its outer surfaces. The element body is The first inlet is the entrance for the gas to be measured from the outside to the first chamber, The second inlet is the entrance for the gas to be measured from the outside to the second chamber, The third inlet is the entrance from the outside to the third chamber for the gas to be measured, It has, The first, second, and third inlets open to the same surface among the first to sixth surfaces. Gas sensor.

10. A gas sensor comprising a sensor element and a control device, which measures the concentration of a specific gas in a gas to be measured, The aforementioned sensor element is The element body has an oxygen ion conductive solid electrolyte layer and is provided with a first chamber and a second chamber inside that are not connected to each other but to which the gas to be measured can reach from outside the sensor element, A first pump cell comprising a first inner electrode disposed in the first chamber and a first outer electrode disposed on the outer surface of the element body, A second pump cell comprising a second inner electrode disposed in the second chamber and a second outer electrode disposed on the outer surface of the element body, It has, The control device is A first pump cell control process is performed to control the first pump cell so as to pump oxygen from around the first inner electrode to around the first outer electrode, thereby reducing the target gas containing at least water among the water and carbon dioxide in the gas to be measured in the first chamber, A second pump cell control process controls the second pump cell to pump oxygen from around the second inner electrode to around the second outer electrode, while suppressing the reduction of one or more gases, which are the gases to be reduced, in the gas to be measured in the second chamber, compared to the first pump cell control process. A concentration measurement process that measures a first concentration, which is the concentration of the first type of gas in the gas to be measured, as the specific gas concentration, based on the first pump current flowing through the first pump cell by the first pump cell control process and the second pump current flowing through the second pump cell by the second pump cell control process. Perform The first type of gas is water if the gas to be reduced is water, and if the gas to be reduced is water and carbon dioxide, it is one or more gases from the gases to be reduced that include at least carbon dioxide. Gas sensor.

11. A sensor element for measuring the concentration of a specific gas in a gas to be measured, The element body has an oxygen ion conductive solid electrolyte layer and is provided with a first chamber, a second chamber, and a third chamber inside, which are not connected to each other but to which the gas to be measured can reach from the outside, respectively. A first pump cell comprising a first inner electrode disposed in the first chamber and a first outer electrode disposed on the outer surface of the element body, A second pump cell comprising a second inner electrode disposed in the second chamber and a second outer electrode disposed on the outer surface of the element body, A third pump cell comprising a third inner electrode disposed in the third chamber and a third outer electrode disposed on the outer surface of the element body, A sensor element having [a certain characteristic].

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