JP2000180409A - Method for beaking leak current in gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively eliminate noise component such as a leak current that flows into a gas sensor element by providing a conductive shield between the sensor element and a heater. SOLUTION: A conductive shield 4c (such as a metal plate and a conductive covering) is provided between a sensor element and a heater 4. Then, when a gas sensor element is heated by the heater 4 in the atmosphere of gas to be detected and the sensor element is operated, electromotive force is generated between a detection electrode 3 and a reference electrode 2. The electromotive force is amplified by an amplifier 6 and is outputted, thus detecting gas concentration. During the detection, a leak current that is generated from the heater 4 is shielded by the shield 4c provided between alumina insulators 4a and 4b of the heater 4, thus preventing the leak current from affecting the electromotive force of the sensor element and accurately detecting the gas concentration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention is primarily measured CO ₂ concentration in the gas phase, other O _2, NOx and SOx
The present invention relates to a small-sized solid electrolyte gas sensor capable of detecting gas concentration with high accuracy, and more specifically to a gas sensor capable of effectively removing noise components such as leak current flowing into a gas sensor element. And a method of interrupting the leakage current in the above.

[0002]

2. Description of the Related Art In recent years, in order to deal with atmospheric environmental pollution, the development of gas sensors capable of measuring mainly the CO ₂ concentration in a gas phase or other NOx and SOx has been actively carried out. Have been done. As one of such gas sensors, there is a so-called solid electrolyte type gas sensor in which a detection electrode and a reference electrode are provided on the surface of a solid electrolyte (JP-A-9-264873, etc.). The sensor element of a solid electrolyte gas sensor (hereinafter referred to as a gas sensor element) detects and uses an electromotive force generated when an internal chemical substance reacts with a substance to be detected. Such a gas sensor element is generally used as a concentration cell. It is called a mold, and the gas sensor element itself works as a battery. If a current is taken out of the gas sensor element when the electromotive force is taken out because it is a battery, the gas sensor element deteriorates so that the battery is consumed. Therefore, an amplifier circuit that receives an electromotive force generally has an extremely high input impedance and is designed to flow only a very small current (1 × 10 ⁻¹⁰ to 1 × 10 ⁻¹² A). Further, such a gas sensor element has a high internal impedance.

FIG. 6 shows the basic structure of a conventional carbon dioxide sensor.
More specifically, referring to FIG.
01 is a solid electrolyte composed of an alkali metal ion conductor,
102 is a detection electrode made of an alkali metal carbonate, 103 is a reference electrode made of an alkali metal composite oxide, 104 is an electrode, 105 is a heater for heating, and 106 is an amplifier. The heating heater 105 is for heating the gas sensor element to an operable temperature of 400 ° C. to 500 ° C.
The gas sensor generates an electromotive force between the detection electrode 102 and the reference electrode 103 when the gas sensor element is heated by the heater 105 in the state shown in the atmosphere of the gas to be measured. , 104, and amplify and output and detect the gas concentration.

[0004]

In the case of a carbon dioxide sensor having the above structure, the internal impedance is about 1 × 10 ⁵ to 1 × 10 ⁶ Ω. Therefore, when the gas sensor element is actually operated for such a reason, the following problems occur. (1) Fluctuation of electromotive force due to leakage current from heating heater A generally used heating heater uses ceramics such as alumina (alumina insulator) or magnesia as an insulator 105a as shown in FIG. Body 105a
In many cases, a platinum resistor 105b is used as a heater (heating element). However, such a heating heater 105
Is used for heating the gas sensor element, as shown in FIG. 9, leakage from the heating element 105b side shown in FIG. 9 occurs due to the inherent insulation resistance of the insulator 105a separating the heating element 105b and the sensor element. Electric current flows to the sensor element. Further, even when the surface of the insulator is contaminated with a conductive substance (for example, sodium contained in human sweat), a leak current similarly flows. The electromotive force from the gas sensor element changes due to the leak current, resulting in a large measurement error.

This state will be described in detail with reference to FIG. 10 which is an equivalent circuit of FIG.
Is a heater power supply (generally a voltage of several volts to several tens of volts), Rr
Is the insulation resistance of the insulator shown in the figure, Ri is the internal impedance of the sensor element (generally 10 ⁵ to 10 ⁶ Ω), EM
F is the true electromotive force of the sensor element (generally several hundred mV),
Vo is the electromotive force of the sensor element actually observed.
As is clear from the figure, the heater voltage V is divided by the insulation resistance Rr and the internal impedance Ri of the sensor element.
H is added to the true electromotive force EMF of the sensor element, which is measured as Vo. The divided heater voltage VH directly becomes an error. Further, when the temperature of the heating heater is controlled, the error is not constant because VH constantly changes.

FIG. 8 shows the result of measuring the leak current in the thickness direction of the alumina insulator 105a of the heater 105 shown in FIG. The size of the alumina substrate (insulator) used in this measurement was a square of 8 mm × 8 mm, the plate thickness was 0.65 mm, and the resistance value of the platinum resistance at room temperature was 8Ω. An electrode for measurement was attached to the opposite side (the side on which the sensor element is usually mounted), and a voltage was applied as shown in FIG. 7 to measure a leak current Ir flowing through the alumina insulator. In addition, in order to make it the same as an actual operation state, the whole sample was heated to 500 ° C. and measured. When the resistance of the alumina insulator is calculated from the leak current value shown in FIG. 7, it becomes about 6.8 × 10 ⁸ Ω. Table 1 shows the results obtained by applying the above results to the equivalent circuit of FIG. 10 and calculating the effect of the leak current on the electromotive force of the sensor element. In FIG. 9, the internal impedance Ri of the sensor element is 1 × 10 ⁶ Ω, and the true electromotive force EMF of the sensor element is shown.
Was calculated as 200 mV.

[0007] As is apparent from the above results, at present, the leak current from the heater for heating affects the electromotive force of the sensor element, causing an error of several percent. This effect becomes a serious problem when trying to improve the accuracy of the sensor.

[0008]

Accordingly, the present invention provides a conductive shield (metal) between a sensor element and a heater for effectively removing noise components such as leak current flowing into the gas sensor element. Plate, a conductive film, etc.) to provide a method for shutting off a leak current in a gas sensor capable of blocking a leak current from a heater for heating. It is intended to provide a leak current blocking method in a gas sensor capable of blocking a leak current from a heater for heating while giving a degree of freedom to the polarity and structure of the element, and further in a gas sensor capable of sharing the shield with an electrode of a sensor element. It is intended to provide a method for interrupting leakage current, and to solve the above problems by using these methods. To.

[0009]

For this reason, the technical solution adopted by the present invention is to provide a gas sensor having a sensor element and a heater for heating, wherein a conductive shield is provided between the sensor element and the heater for heating. (A metal plate, a conductive film or the like) is provided, and a leakage current from a heating heater is shielded.

[0010]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a structural view of a solid electrolyte type CO ₂ gas sensor according to a first embodiment of the present invention. In the following description, the gas sensor element is an element composed of a solid electrolyte, a detection electrode, a reference electrode, and an electrode, and the heating heater is for heating the gas sensor element, and these are combined. A solid electrolyte sensor is configured. In FIG. 1, 1 is a solid electrolyte made of an alkali metal ion conductor, 2 is a detection electrode made of an alkali metal carbonate, 3 is a reference electrode made of an alkali metal composite oxide, and 4 is a temperature 400 at which the gas sensor element can operate. ° C to 500
In order to heat to ° C, a heater for heating, 5 is an electrode, and 6 is an amplifier. The heater 4 for heating is an alumina insulator 4a,
4b and a heating element 4d composed of a platinum resistor or the like, and the insulator is a first or second insulator 4a, 4b and a metal plate or a conductive film (for example, gold, platinum, etc.) disposed therebetween. ) 4c. The metal plate or the conductive film (hereinafter referred to as a shield) 4c is formed, for example, by sputtering, vapor deposition,
The shield 4c is formed on the insulator 4a or 4b by a method such as plating, and the shield 4c is connected to a potential that cancels a leak current flowing into the gas sensor element.

This solid electrolyte type gas sensor is similar to a conventional gas sensor. When the gas sensor element is heated by a heater 4 in a state shown in the atmosphere of a gas to be measured in the state shown in FIG. An electromotive force is generated between the reference electrode 3 and the electromotive force. The electromotive force is taken out through the electrode 5, amplified, and output to detect the gas concentration. During this detection, as is apparent from FIG. 1, the leakage current generated from the heater 4 is caused by the shield 4 provided between the alumina insulators 4a and 4b of the heater.
c can be shielded from affecting the electromotive force of the sensor element, and highly accurate density detection can be performed.

To further explain using the equivalent circuit shown in FIG. 2, the sensor element includes resistors Rr ₁ and Rr ₂ of the alumina insulators 4b and 4a and a ground disposed between the resistors Rr ₁ and Rr _2. The shield 4c and the heating element (heater 4d) are connected. As a result, the sensor output voltage Vo becomes the internal impedance R of the sensor element.
i, will be divided by the resistor Rr ₂ of the second insulator. However, the resistance value of the alumina constituting the insulator of the heating heater 4 (resistances Rr ₁ and Rr ₂ of the insulators 4 a and 4 b) is about 6.8 × 10 ⁸ Ω, while the actual solid electrolyte Since the internal impedance of the gas sensor element using the element can be controlled to about 1 × 10 ⁶ Ω, the effect on the output voltage is about 0.15%, which is negligible compared to the influence of the leak current shown in Table 1. . As described above, the leak current generated from the heating heater 4 is shielded by the shield 4c provided between the first insulator 4a and the second insulator 4b of the heating heater 4, so that the influence of the leak current is determined. This makes it possible to detect the gas concentration with a small size and high accuracy.

Next, the second embodiment, the third embodiment, and the fourth embodiment
Solid electrolyte type CO according to the embodiment _TwoGas sensor element
Explain the structure of these, they prevent the effect of leakage current
Is connected to an arbitrary potential.
Use the most suitable method for the intended use depending on the form.
Can be. The second embodiment differs from the first embodiment in the basic configuration.
The same, but with the heater power and
The feature is that the shield 4c is connected to make the common
There is. Leakage current from heating heater 4 is shielded
Electromotive of sensor element to flow to ground through 4c
Does not affect power.

Although the third embodiment has the same basic configuration as the first embodiment, the output voltage of the sensor element is impedance-converted by an operational amplifier 7 as shown in FIG. 4, and the output is connected to a shield. The advantage of this method is shield 4
The potential difference between c and the sensor element is always equal to the output voltage of the sensor element, and a stable leakage current shielding effect can be obtained regardless of the polarity of the electrode of the sensor element mounted on the alumina insulator 4a. Further, the ground of the power supply for the heater and the sensor element can be separated. Since the output of the operational amplifier 7 is a voltage source having an extremely low impedance, there is no effect even if a leak current flows. Further, in this embodiment, the reference electrode side is grounded, but it is also possible to connect the detection electrode side to ground and measure with a negative voltage. Alternatively, the structure may be reversed and the detection electrode side may be joined to the heater for heating.

The fourth embodiment has a configuration in which the second insulator 4a in the first embodiment is omitted, and a gas sensor element is directly disposed on the shield 4c. As shown in FIG. Since the low potential side (ground potential side) and the shield are shared, the structure of the gas sensor can be simplified.
However, in this method, the detection electrode side cannot be grounded because measurement at a negative potential cannot be performed later. As described above, according to the present invention, by providing a conductive shield (a metal plate, a conductive coating, or the like) between the sensor element and the heater for heating, a leak current from the heater for heating can be shielded.

[0016]

As described in detail above, according to the present invention,
By providing a conductive shield between the sensor element and the heater for heating, the leak current from the heater for heating is shielded, and noise components such as leak current flowing into the gas sensor element are effectively removed to achieve high accuracy. An excellent effect of enabling gas detection can be achieved.

[Brief description of the drawings]

FIG. 1 is a structural diagram of a gas sensor-element according to a first embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of the gas sensor element according to the first embodiment.

FIG. 3 is a structural view of a gas sensor-element according to a second embodiment of the present invention.

FIG. 4 is a structural diagram of a gas sensor-element according to a third embodiment of the present invention.

FIG. 5 is a structural view of a gas sensor-element according to a fourth embodiment of the present invention.

FIG. 6 is a basic structural diagram of a conventional gas sensor element.

FIG. 7 is a structural explanatory view of a heater.

FIG. 8 is a diagram of a leak current of a heater.

FIG. 9 is a basic structural view showing a heater section of a conventional gas sensor element in detail.

FIG. 10 is an equivalent circuit diagram of the gas sensor element shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Solid electrolyte 2 Detection electrode 3 Reference electrode 4 Heater 4a 1st insulator 4b 2nd insulator 4c Shield 4d Heating element

Claims (3)

[Claims] 1. A gas sensor comprising a sensor element and a heater for heating, wherein a conductive shield is provided between the sensor element and the heater for heating to shield a leak current from the heater for heating. Method of interrupting leakage current in a gas sensor. 2. The method according to claim 1, wherein said shield is connected to an arbitrary potential. 3. The method according to claim 1, wherein the shield is shared with an electrode of a sensor element.