JPH06160338A - Limiting current type gas sensor - Google Patents

Abstract

PURPOSE:To replace a sensor unit at a low cost by connecting resistors electrically in series and in parallel to electrodes of the sensor unit, respectively. CONSTITUTION:A sensor unit S wherein resistors Ra and Rb are connected in series and in parallel to a cathode electrode 2 and an anode electrode 3 respectively is disposed in a gas to be measured and a voltage is impressed between sensor electrodes 23 and 33. Oxygen in the gas is pumped from the electrode 2 to the electrode 3 in accordance with the voltage V and diffused from a connecting part 22 into an electrode part 21. When the voltage impressed between the electrode 23 and the resistor Ra is set herein at a certain value, the amount of diffusion is limited in accordance with the concentration of oxygen and a current I between the electrodes 23 and 33 is also limited and becomes a diffusion control current value IL1. When the voltage V is increased further, water vapor in the gas is also diffused and limited and a diffusion control current value IL2 corresponding the concentration of the water vapor is obtained. While the concentrations of the oxygen and the water vapor are detected from these current values IL1 and IL2, however non-uniformity occurs for each unit S due to a minute difference in a gas diffusion limiting means 4. Therefore, the nonuniformity is eliminated by setting resistance values of the resistors Ra and Rb and thus the interchangeability of the unit S is established.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a limiting current type gas sensor.

[0002]

2. Description of the Related Art A limiting current type gas sensor usually has a solid electrolyte plate having good oxygen ion conductivity, a pair of porous electrodes arranged in close contact with the solid electrolyte plate, and diffusion of a gas to be measured is limited. DC voltage is applied between the sensor unit having the gas supply limiting means and the electrode, and the oxygen concentration and the amount of water vapor in the measured gas are detected based on the magnitude of the limiting current at which the output current value becomes flat. And a detection circuit.
The following measures are taken against deterioration and damage of the sensor unit. (A) Replace the sensor unit with a new one, and replace the detection circuit with one that is compatible with the sensor unit. (A) In advance, multiple sensor units with similar characteristics,
Select it and replace the defective sensor unit with this new one.

[0003]

However, the conventional techniques have the following drawbacks. In the case of (a), it is necessary to replace the detection circuit or readjust the sensor unit every time the sensor unit is deteriorated or damaged, which is costly. Further, in the above (a), it is very difficult to manufacture a sensor unit having similar characteristics, and it takes time and cost.

An object of the present invention is to provide a limiting current type gas sensor in which the sensor unit can be replaced at low cost.

[0005]

[Means for Solving the Problems] In order to solve the above problems,
The present invention is a solid electrolyte plate having good conductivity of oxygen ions,
A sensor unit having a pair of porous electrodes arranged in close contact with the solid electrolyte plate and gas diffusion limiting means for limiting gas diffusion to the electrodes, has uniform characteristics in series and in parallel with the electrodes. The resistances (Ra, Rb) for electrical conversion are electrically connected, and the terminal voltage of the resistance (Ra) reaches a ceiling with respect to the rise of the DC voltage applied between one end of the electrode and the other end of the resistance (Ra). A configuration is adopted in which the gas concentration in the measured atmosphere is measured based on the current value.

[0006]

[Operation] When the sensor unit is placed in the atmosphere to be measured,
Oxygen ion conductivity of the solid electrolyte plate located at the electrode is enhanced. The gas in the atmosphere to be measured diffuses into one electrode, is ionized into oxygen ions, and is pumped to the other electrode according to the voltage applied between the electrodes. When the DC voltage applied between one end of the electrode and the other end of the resistance (Ra) is set to a certain value, the amount of oxygen diffusion into the electrode on the one hand depends on the gas diffusion limiting means, and the amount of oxygen diffusion into the measured atmosphere (for example, A value determined according to (water vapor concentration) reaches a peak, and the terminal voltage of the resistor (Ra) does not rise even if the voltage is increased. The gas concentration in the atmosphere to be measured is determined based on the limiting current value (terminal voltage value) at which the rise in the terminal voltage of the resistance (Ra) reaches a peak. By setting the respective resistance values of the resistors (Ra, Rb), it is caused by the difference in the gas diffusion limiting means, etc.
It is possible to eliminate variations in gas concentration-terminal voltage characteristics for each sensor unit.

[0007]

[Effects of the Invention] By setting the resistance values of the resistors (Ra, Rb), it is possible to eliminate the variations in the gas concentration-terminal voltage characteristics for each sensor unit, and to set the reference gas concentration-terminal voltage characteristics. it can. For this reason, for example, if you prepare the required number of resistors (Ra, Rb) whose resistance values are set so that the standard gas concentration-terminal voltage characteristics are attached to the sensor unit side, Exchange can be done at low cost.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIGS. As shown in FIGS. 1 to 4, the limiting current type gas sensor includes a stabilized zirconia plate 1, a porous negative electrode 2 and a positive electrode 3, gas diffusion limiting means 4 provided to the negative electrode 2, and a ceramic heater. Sensor unit S with 5
And resistors Ra and Rb electrically connected in series and in parallel to the negative electrode 2 and the positive electrode 3, respectively, and in this embodiment, the indoor humidity is detected. The resistors Ra and Rb are arranged inside a box (not shown) into which the sensor unit S is inserted, and the box is a 5P socket (not shown) of the detection circuit with the sensor unit S inserted. Be attached to.

The stabilized zirconia plate 1 is a solid electrolyte having good conductivity of oxygen ions in which zirconium oxide is added with yttrium oxide as a stabilizer to form a solid solution. In this embodiment, the length is 5 mm, the width is 7 mm, and the thickness is 0 mm. The one with a thickness of 0.3 mm is used.

The negative electrode 2 and the positive electrode 3, which are arranged in close contact with the upper surface of the stabilized zirconia plate 1, respectively, have an electrode portion 21 and an electrode portion 21, respectively.
31 and connection parts 22 and 32 for energization. In this embodiment, the negative electrode 2 and the positive electrode 3 are squares having a thickness of 20 μm and a side of 2.5 mm.

The gas diffusion limiting means 4 shown in FIG. 2 comprises an alumina porous layer 41 covering a part of the connecting portion 22 of the negative electrode 2 and the electrode portion 21, and alumina so that the gas to be measured does not directly enter the electrode portion 21. The glaze layer 42 covers the periphery of the electrode portion 21 including the porous layer 41, and the exposed portion of the connection portion 22 not covered by the glaze layer 42.

The ceramic heater 5 is formed by burying tungsten in a ceramic mainly composed of alumina, and the electrode portions 21 and 31 are heated to 300 ° C. to 700 ° C.
Locally heat to. Incidentally, 51 is an exposed portion for energization, and 52 is a vent for increasing the heating efficiency of the stabilized zirconia plate 1.

Here, a method of manufacturing the sensor unit S will be described. A platinum paste, which becomes the negative electrode 2 and the positive electrode 3 after firing, is printed on the green sheet that becomes the stabilized zirconia plate 1 after firing, and integrally fired at 1500 ° C. Negative electrode 2,
On the stabilized zirconia plate 1 on which the positive electrode 3 is formed, a paste (which becomes the alumina porous layer 41 after firing) made by mixing alumina powder with glass is used as a part of the connecting portion 22 of the negative electrode 2 and the electrode portion 21. Is coated so as to cover the glass substrate, glass powder (which becomes the glaze layer 42 after firing) is further coated thereon, and baked at 850 ° C. to 900 ° C. to manufacture a sensor body chip. A heater pattern 530 is printed with a tungsten paste on the upper surface of a green sheet 500 of 96% by weight of alumina in which a window 520 to be the ventilation hole 52 is formed after firing, and a conductive paste 511 to be the exposed portion 51 for energization after firing is printed thereon. Then, a similar green sheet 501 printed with a ruthenium oxide paste that will become the sensor electrodes 23 and 33 after firing is covered, and this is fired and integrated to manufacture the ceramic heater 5 (see FIG. 3). The sensor chip and the ceramic heater 24 are sealed with sealing glass at about 800 ° C.
Join with. The sensor electrodes 23 and 33 and the connecting portions 22 and 3
Braze 2 and.

Next, the operation of the sensor unit S alone will be described with reference to FIG. The sensor unit S is placed in the gas to be measured, the ceramic heater 5 is energized, and the sensor electrode 2
A voltage is applied between 3 and 33. Oxygen inside the electrode portion 21 of the negative electrode 2 is ionized into oxygen ions, and oxygen in the gas to be measured is pumped from the negative electrode 2 to the positive electrode 3 according to the applied voltage V. At this time, only the stabilized zirconia plate 1 of the electrode portion 21 is locally heated, and the stabilized zirconia plate 1 of the connection portion 22 is not sufficiently heated to exhibit oxygen ion conductivity. Therefore, oxygen is not covered by the glaze layer 42. The electrode part 21 covered with the glaze layer 42 from the exposed part of the part 22
Diffuse in. Here, sensor electrode 23-sensor electrode 3
The current I flowing between 3 changes as shown in FIG. When the applied voltage V is a voltage value V _{1 to} V ₂ , the oxygen diffusion amount into the electrode portion 21 is controlled by the gas diffusion limiting means 4 of the negative electrode 2 and is limited according to the oxygen concentration in the gas to be measured. Because
The diffusion amount is limited, and accordingly, the current I is also limited to the diffusion limited current value I _L1 and the first flat portion F1 is obtained. When the applied voltage V becomes higher than the voltage value V _{2 at} which the diffusion limiting current value I _L1 is obtained, the water vapor (water content) in the measurement gas is decomposed, and the oxygen ions generated by the decomposition are pumped to the positive electrode 3. Therefore, the water vapor also diffuses from the exposed portion of the connection portion 22 which is not covered by the glaze layer 42 into the electrode portion 21 which is covered by the glaze layer 42, and the current value increases according to the amount of diffusion. When the applied voltage V is further increased to the voltage values V _{3 to} V ₄ , the current I further increases according to the water vapor concentration, but the gas diffusion limiting means 4 of the negative electrode 2 limits the diffusion amount of the water vapor, and accordingly. The current value is also limited and becomes the diffusion limited current value I _L2 according to the water vapor concentration, which indicates the second flat portion F2.
Here, V ₁ to V are provided between the sensor electrode 23 and the sensor electrode 33.
_When the voltage of ₂ is applied and the diffusion limited current value I _L1 is measured, the oxygen concentration can be detected from the magnitude of the diffusion limited current value I _L1 .
If the diffusion limiting current value I _L2 is measured by applying a voltage of V _{3 to} V _4, the water vapor concentration can be detected from the magnitude of the diffusion limiting current value I _L2 .

As shown in FIG. 6, the oxygen gas concentration and water vapor concentration-diffusion limiting current values I _L1 and I _L2 characteristics are different for each sensor unit S due to slight differences (size and shape) of the gas diffusion limiting means 4. Vary. Therefore, the water vapor concentration-diffusion limited current value I _L2 characteristic of a certain sensor unit S is now expressed by the following formula. I _L2 = a × C + b, where C is the water vapor concentration, a and b are constants, and the sensor unit S represented by the formula is connected in series to the sensor electrodes 23 and 33. Further, the resistors Ra and Rb are electrically connected in parallel, and the DC voltage E is applied between the terminals 61 and 62 so that the magnitude of the output current can be converted into the terminal voltage of the resistor Ra and detected.

Further, the gas concentration of all the sensor units S-
An equation for converting the diffusion limiting current value characteristic (for example, water vapor concentration-diffusion limiting current value I _L2 characteristic) is shown below. I ′ _L2 = A × C + B, where A and B are constants, and I ′ _L2 is a current flowing through a resistor Ra ′ having a certain resistance value. In other words, the resistors Ra and Ra 'are set so that the potential difference between the resistors Ra and Ra' are the same.
If the resistance value of Rb is determined, the water vapor concentration expressed by the formula-
The diffusion limiting current value I _L2 characteristic can be converted into the same formula, and the water vapor concentration-diffusion limiting current value I _L2 characteristic becomes one, and compatibility is established in all the sensor units S.

Next, how to set the resistance values of the resistors Ra and Rb
The method will be explained. The potential difference between the resistors Ra is V_out,
The potential difference between anti-Ra 'is V_out’, V _out, V
_out′ Can be expressed by the following equation. V_out= Ra (aC + b + i) V_out′ = Ra ′ (AC + B) where i is the current value flowing through the resistor Rb and V_out= V_out′ Is satisfied, Ra
(AC + b + i) = Ra '(AC + B)
Holds at an arbitrary water vapor concentration C. Therefore, Ra = (A * Ra ') / a ... Also, i = (a * B) / Ab is obtained, and the resistance Ra
When the potential difference is sufficiently smaller than the applied voltage E, the resistance R
For insertion of a, between the sensor electrode 23 and the sensor electrode 33
The applied voltage changes according to the diffusion limiting current value I_L2Is obtained
Since it is in the flat area, it can be ignored.
You can set it as follows. Rb ≒ E / i …………

The limiting current type gas sensor of this embodiment has the following advantages. (A) For each of all the sensor units S to be manufactured, the resistors Ra and Rb having the resistance value obtained from the above formula are
If all the sensor units S are arranged in each box into which each sensor unit S is inserted and are electrically connected in series and in parallel to the sensor electrode 23-sensor electrode 33.
It is possible to make the water vapor concentration-diffusion limited current value I _L2 characteristic uniform. When replacing the sensor, the sensor unit S
At the same time, the box having the resistors Ra and Rb is replaced with a new one. That is, it is not necessary to precisely manufacture the sensor units S so that the characteristics of the sensor units S are uniform, and the detection circuit for detecting the water vapor concentration based on the diffusion limiting current value I _L2 is replaced when the units are replaced. No need. Therefore, the sensor can be replaced at low cost and the detection accuracy can be maintained. (Ii) Without directly arranging the resistors Ra and Rb on the sensor unit S side, the resistor Ra,
Since Rb is arranged, it is not necessary to change the manufacturing method of the sensor unit S.

The present invention includes the following embodiments in addition to the above embodiments. a. The sensor unit S of the limiting current type gas sensor is shown in FIG.
As shown in FIG.
It is also possible to provide a housing 45 having a void portion 44 in which a (gas diffusion limiting means) is formed on the negative electrode 2 side, and as shown in FIG. 8, a void portion for limiting the diffusion of the gas to be measured. A porous housing 46 having 44 (gas diffusion limiting means) may be provided on the negative electrode 2 side. b. The limiting current type gas sensor may measure oxygen concentration, CO ₂ concentration, NO ₂ concentration, etc. in addition to the amount of water vapor. c. The limiting current type gas sensor may be used as an oxygen sensor for detecting the oxygen gas concentration. In this case, the oxygen concentration-diffusion limiting current value I _L1 characteristic of the sensor unit S is set to the resistance R
It may be made uniform by setting the resistance values of a and Rb. d. The resistors Ra and Rb may be directly attached to the sensor unit S. e. The resistors Ra and Rb are variable resistors, and the sensor unit S
Side or the detection circuit side. f. The negative electrode and the positive electrode may be embedded in the solid electrolyte plate.

[Brief description of drawings]

FIG. 1 is an assembly diagram of a sensor unit according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a main part of the sensor unit.

FIG. 3 is an explanatory diagram for explaining a method of manufacturing a ceramic heater of the sensor unit.

FIG. 4 is an electric circuit diagram showing electric connection between the sensor unit and a resistor.

FIG. 5 is a graph showing applied voltage-current characteristics of the sensor unit.

FIG. 6 is a graph showing gas concentration-current characteristics of the sensor unit.

FIG. 7 is a sectional view of a sensor unit according to another embodiment of the present invention.

FIG. 8 is a sectional view of a sensor unit according to another embodiment of the present invention.

[Explanation of symbols]

 1 stabilized zirconia plate (solid electrolyte plate) 2 negative electrode (electrode) 3 positive electrode (electrode) 4 gas diffusion limiting means Ra resistance Rb resistance S sensor unit

Claims (1)

[Claims] 1. A sensor comprising a solid electrolyte plate having good oxygen ion conductivity, a pair of porous electrodes arranged in close contact with the solid electrolyte plate, and gas diffusion limiting means for limiting gas diffusion to the electrodes. Resistors (Ra, Rb) for equalizing the characteristics of the unit are electrically connected in series and in parallel to the electrodes, and the DC voltage applied between one end of the electrodes and the other end of the resistance (Ra). A limiting current type gas sensor for measuring a gas concentration in an atmosphere to be measured based on a limiting current value at which a terminal voltage of a resistance (Ra) reaches a peak with respect to an increase.