JPH0820411B2 - Humidity measuring method and humidity measuring apparatus using oxygen sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention provides a pair of electrodes on the surface of a solid electrolyte, and oxygen concentration for measuring oxygen concentration in a gas to be measured by limiting gas diffusion to a cathode. The present invention relates to a humidity measuring method and a humidity measuring device for measuring humidity (moisture concentration) in a gas to be measured using a detection sensor (hereinafter referred to as "oxygen sensor").

[Prior Art] As a humidity measuring method for measuring the oxygen concentration in a gas by using an oxygen sensor, an oxygen partial pressure in a gas to be measured is disclosed as in the invention introduced in JP-A-60-222761. Is measured before and after dehumidification as the limiting current value of the flat portion due to oxygen diffusion limitation, and a method for obtaining the humidity based on each limiting current value, as in the invention introduced in JP-A-62-150151. A method of measuring the limiting current value due to diffusion restriction based on the difference between the oxygen concentration and the water concentration in the gas to be measured is obtained in a two-step flat portion. As disclosed in Japanese Patent Laid-Open No. 150152, at least one of the limiting current values of the two flat portions depending on the oxygen concentration or the water concentration of the measured gas, and the limiting current value of the dried measured gas after dehumidification, How to measure based on the difference of It has been known.

[Problems to be Solved by the Invention] In the humidity measuring method according to each of the above measuring methods, a drying device for dehumidifying and drying the measured gas is required, and the measured gas before dehumidification and the measured gas after dehumidification are used. Since it is necessary to measure the limiting current value for each of the measurement gases,
There is a problem that a large-scale measurement system is required and the response is poor.

In view of the above matters, the inventors of the present application have conducted studies and tests on a simple method for measuring the moisture concentration of the gas to be measured, and as a result, have found that the limiting current value I in the first flat portion F1 that appears depending on the oxygen concentration is I. _It was found that _L1 is influenced not only by the oxygen concentration alone but also by the oxygen concentration because the oxygen partial pressure changes depending on the water concentration contained in the gas to be measured.

That is, the limiting current value I _L1 in the first flat portion F1 according to the oxygen concentration at each moisture concentration in the measured gas is
As shown in FIG. 5, it was found that the limiting current value I _L1 decreases as the water concentration increases, which is different for each water concentration.

Further, the limiting current value I _L2 in the second flat portion F2 due to the decomposition of water increases as the water concentration increases, as shown in the above inventions.

Furthermore, the inventors of the present application have found that when the voltage applied between the electrodes is increased, the limiting current value that changes from the first flat portion F1 to the second flat portion F2 is such that the oxygen concentration in the measured gas is constant. Then, it has been found that at a certain constant voltage value V _M , a constant current value I _M is exhibited irrespective of the water concentration, and it becomes an inflection point P at which the sign of the second derivative changes.

At the same time, the current value I _{M at the} inflection point P is the same as the first flat portion F1 (second flat portion) when water is not contained (humidity 0%).
It was also found that the limiting current value is the same as F2), which is constant for each oxygen sensor regardless of the temperature of the gas to be measured.

The present invention provides a very excellent humidity measuring method and humidity measuring device obtained as a result of the above research.

[Means for Solving the Problems] A first invention of the present invention is to provide a pair of electrodes made of a porous body in close contact with the surface of a solid electrolyte having oxygen ion conductivity, and a gas diffusion control means. By applying a voltage between the electrodes of an oxygen sensor in which gas diffusion to the electrodes is limited, the humidity in the measured gas is adjusted based on the limiting current value between the electrodes that is limited by the gas diffusion control means. In the humidity measuring method for measuring, a change between a first limiting current value limited according to the oxygen concentration in the measured gas and a second limiting current value limited according to the humidity in the measured gas. It is characterized in that the humidity is obtained based on the current value at the inflection point and at least one of the first limiting current value and the second limiting current value.

A second invention of the present invention is an apparatus for carrying out the method of the first invention, wherein a voltage is applied between the electrodes of the oxygen sensor, and the voltage is applied according to the oxygen concentration in the gas to be measured. A current value at an inflection point between a first limit current value to be limited and a second limit current value to be limited according to the humidity in the gas to be measured, the first limit current value, and the second limit current value. The humidity is obtained based on at least one of the limiting current values of.

[Operation] In the present invention, in the oxygen sensor to be used, when diffusion control to the negative electrode is performed and a voltage is applied between the electrodes, the value of the current flowing between the electrodes appears according to the oxygen concentration. In the area, the water content decreases as the water concentration increases, and the current value of the second flat portion that appears according to the water concentration increases as the water concentration increases, and inflection between the first flat portion and the second flat portion. The current value at a point is always constant regardless of the water concentration at the same oxygen concentration, and the water concentration at this point is calculated based on the fact that the current value at this time corresponds to the current value when dried. It was used.

The procedure will be described below. For the oxygen sensor to be used, the voltage value at the inflection point is checked in advance, and the voltage value at the inflection point thus investigated is applied first to obtain the current value at that time. . With this, it is possible to obtain the limiting current value corresponding to the drying of the gas to be measured.

Next, using the same oxygen sensor, a voltage value for the first flat portion or the second flat portion is continuously applied to obtain the current value. Since each current value in the first flat portion and the second flat portion changes according to the water concentration of the gas to be measured, the water concentration in the gas to be measured is compared with the current value corresponding to the above-described drying time. Can be asked.

[Effect of the Invention] In the present invention, the current value corresponding to the dry state of the gas to be measured can be easily checked by previously obtaining the voltage value at the inflection point for the oxygen sensor to be used. Therefore, it is not necessary to dry the gas to be measured, and the humidity can be measured simply by changing the applied voltage using the same oxygen sensor, so that the measurement system such as the humidity measuring device is simplified.

Further, when the oxygen concentration is constant, once the current value at the inflection point is obtained, thereafter, the first flat portion or the second flat portion can be obtained.
The water concentration at that time can be measured simply by applying a voltage to the flat portion of the. Therefore, it is possible to perform a highly responsive measurement.

Further, when the oxygen concentration in the gas to be measured changes, it can be obtained by alternately applying the voltage at the inflection point and the voltage at each flat portion.

[Examples] Next, the humidity measuring method of the present invention will be described based on Examples.

FIG. 2 shows an oxygen sensor 1 used in implementing the present invention.

The oxygen sensor 1 includes a sensor element 10 and a ceramic heater 20.
Consists of

The sensor element 10 includes an oxygen ion conductive plate 11, a positive electrode 12, a negative electrode 13, an alumina porous layer 14, and a glaze layer 15.

The oxygen ion conductive plate 11 is a plate made of stabilized zirconia as a solid electrolyte in which yttrium oxide as a stabilizer is added to zirconium oxide to form a solid solution. In this embodiment,
The oxygen ion conductive plate 11 is 5 × 7 mm square and 0.3 mm thick.

A positive electrode 12 and a negative electrode 13 are formed on one surface of the oxygen ion conductive plate 11 at intervals. Each electrode 12, 13 is
The platinum paste is printed on the oxygen ion conductive plate 11 and is a porous platinum electrode simultaneously fired at 1500 ° C. at the same time as the oxygen ion conductive plate 11, and the positive electrode 12 and the negative electrode 13 are electrode portions 12a and 13a, respectively. It is composed of connection parts 12b and 13b for energization.

On the oxygen ion conductive plate 11 on the negative electrode 13 side, an alumina porous layer formed by applying a paste prepared by mixing glass with alumina powder.
14 is provided so as to cover only a part of the electrode portion 13a of the negative electrode 13 and the connection portion 13b, further, the alumina porous layer 14 so that the measured gas does not touch the electrode portion 13a of the negative electrode 13. Electrode part by the glaze layer 15 coated with glass to
The alumina porous layer 14 and the glaze layer 15 covered with 13a and a part of the connecting portion 13b are baked on the oxygen ion conductive plate 11 at 850 ° C to 900 ° C.

Therefore, as shown in FIG. 3, the electrode portion 13a of the negative electrode 13 is
Is separated from the gas to be measured, and the connecting portion 13b of the negative electrode 13 is exposed from the glaze layer 15, so that the electrodes 1 are connected at the connecting portion 13b between the end 15a of the glaze layer 15 and the oxygen ion conductive plate 11.
When a voltage is applied to 2 and 13, it also serves as a gas diffusion control body for controlling the oxygen diffusion amount and the water vapor diffusion amount.

Here, each electrode 12 and 13 has a thickness t of 20 μm, and each electrode portion 12a and 13a is a square having one side of 2.5 mm.

In addition, at the connecting portion 13b, as shown in FIG.
mm, and the length L covered by the glaze layer 15 was 2 mm.

Here, assuming that the area of the electrode portions 12a and 13a is S and the cross-sectional area given by the product of the width W and the thickness t of the connecting portion 13b is s, the diffusion amount of oxygen into the electrode portion 13a becomes the cross-sectional area s. It is proportional and inversely proportional to the length L.

Based on these values, the range of the ratio R of the area S of the electrode portion 13a of the negative electrode 13 to the connection portion 13b as the gas diffusion electrode, which is particularly effective for obtaining the limiting current value, is calculated. Since L / S = 1 × 10 ⁻⁵ to 8 × 10 ⁻² , and in this embodiment, s = 0.02, L = 2, and S = 6.25, the value of this ratio R is R = 1.6 ×. It was 10 ^-3 .

The sensor element 10 is mounted by baking glass at about 800 ° C. by applying glass to the ceramic heater 20.

As shown in FIG. 4, the ceramic heater 20 is formed by printing a metal paste made of tungsten (W) on the surface of the green sheet 20A of 96% alumina (Al ₂ O ₃ ) so as to form the heater pattern 20a. Green sheet of 20
It is a plate-shaped heater that is coated with B and fired. Both ends of the heater pattern 20a in the ceramic heater 20 have conductor patterns 20
b, 20c, the electrode 2 on the surface 20d of the ceramic heater 20
1 and 22 are connected respectively.

Here, since the gas diffusion is limited by the connecting portion 13b of the negative electrode 13 made of a porous material, the heater pattern 20a of the ceramic heater 20 is configured to locally heat only the electrode portions 12a, 13a of the electrodes 12, 13. The pumping by the connecting portion 13b is prevented.

A vent hole 23 is formed in the central portion of the ceramic heater 20 in order to improve the heating efficiency of the sensor element 10, and in the portion where the sensor element 10 is baked, through holes 24, 25 penetrating the front and back sides, 26 are arranged in multiple rows.

Further, the surface 20d of the ceramic heater 20 is provided with sensor electrodes 27, 28 connected to the respective connecting portions 12b, 13b by a printed pattern of ruthenium oxide for energizing the respective electrodes 12, 13 of the sensor element 10. Has been. The sensor electrodes 27 and 28 are printed with a pattern forming paste,
When the sensor element 10 is printed and mounted, it is baked at the same time.

The oxygen sensor 1 of the present embodiment having the above configuration is
As shown in the figure, it is used as a sensor portion of a humidity measuring device A to which a voltage is applied from a voltage-variable power source E between the sensor electrodes 27 and 28, and the applied voltage and current value are measured respectively. At this time, the ceramic heater 20 is energized,
300 to 700 centered on each electrode part 12a, 13a of the sensor element 10
Keep at ℃.

 Hereinafter, the operation of the oxygen sensor 1 will be described.

When the oxygen sensor 1 is placed in the gas to be measured and a voltage is applied between the positive electrode 12 and the negative electrode 13, the oxygen in the electrode portion 13a covered with the glaze layer 15 is ionized to become oxygen ions, Oxygen in the measurement gas is pumped to the positive electrode 12 according to the voltage applied from the negative electrode 13 to the positive electrode 12.

At this time, in the negative electrode 13, only the electrode portion 13a is locally heated, and the connection portion 13b is not sufficiently heated to exhibit oxygen ion conductivity. Therefore, oxygen is supplied from the connection portion 13b to the glaze layer 15.
Diffuses into the electrode portion 13a covered with.

When the applied voltage is increased, the current value flowing between the electrodes increases according to the applied voltage.

The amount of oxygen diffusion into the electrode portion 13a is controlled by the connection portion 13b of the negative electrode 13 and is limited according to the oxygen concentration in the gas to be measured.Therefore, if the amount of diffusion is limited, the current value is also limited accordingly. The diffusion limited current value I _L1 (first flat portion F1) is shown.

The oxygen partial pressure decreases as the moisture concentration in the measured gas increases, so the diffusion limiting current value I
_As shown in FIG. 5, _L1 becomes lower as the water concentration becomes higher. Note that FIG. 5 shows the case where the temperature of the gas to be measured is 80 ° C.

When the applied voltage becomes higher than the voltage value at which the diffusion limited current value I _L1 is obtained, the moisture (water vapor) in the gas to be measured is decomposed, and the oxygen ionization caused by the decomposition is caused by the positive electrode 12
Since the water is pumped, the moisture also diffuses from the connecting portion 13b of the negative electrode 13 into the electrode portion 13a at this time, and the current value increases according to the amount of diffusion.

When the applied voltage is further increased, the current value further increases in accordance with the water concentration, but when the diffusion amount of water is limited at the connection portion 13b of the negative electrode 13, the current value is also limited accordingly, and the water concentration increases. A diffusion limiting current value I _L2 (second flat portion F2) according to the above is shown.

Here, these diffusion limiting current values I _L1 and I _L2 respectively change according to the humidity (moisture concentration) in the gas to be measured as described above, and when the oxygen concentration is constant, the diffusion limiting current values I _L1 and I _L2 I _L1 decreases as the humidity increases, and conversely the diffusion limiting current value I _L2 increases as the humidity increases.

Further, when the high voltage applied between the electrodes from a voltage value of the diffusion limiting current I _L1 to the voltage value of the diffusion limiting current value I _L2, the current value is changed from the diffusion limited current value I _L1 to diffusion limited current value I _L2 As a result of research and consideration by the inventors of the present invention, the current value I _M of the inflection point P that occurs during the process is determined only by the oxygen concentration of the gas to be measured, and when the oxygen concentration is constant regardless of the water concentration. It was revealed that a constant value was exhibited and that this value corresponded to a dry state in which the water concentration in the measured gas was 0%.

Therefore, when the voltage value V _{M at} which the inflection point P is obtained is obtained in advance for the oxygen sensor 1 to be used, for the gas to be measured, the voltage value V _M at the inflection point P is applied and its current value I _M is measured, and then the current value I _M becomes the diffusion limited current value I _L1 at the first flat portion F1 or the diffusion limited current value I _L2 at the second
The water concentration (humidity) can be obtained by applying the voltage value of the flat portion F2 of, the current value at that time is measured, and the current values are compared.

In this case, the current value I _{M at} the inflection point P can be measured once with respect to the gas to be measured, and then only the current value with respect to one of the flat portions can be obtained. Can be measured.

Further, when there is a possibility that the oxygen concentration of the gas to be measured changes, it is only necessary to obtain the current value I _M at the inflection point P and then obtain the current value for any flat portion.

Further, FIG. 6 shows the characteristics of the current values I _L1 and I _L2 of the flat portions F1 and F2 with respect to changes in humidity when the oxygen concentration in the measured gas (atmosphere) is constant.

In FIG. 6, the solid line indicates the diffusion limiting current value I _L1 in the first flat portion F1, the broken line indicates the diffusion limiting current value I _L2 in the second flat portion F2, and the temperature in the gas to be measured (atmosphere) respectively. 40
It shows about the case of ℃, 60 ℃, 80 ℃. The alternate long and short dash line shows the current value I _M at the inflection point P.

As described above, according to the present invention, unlike the conventional case, it is not necessary to dry the gas to be measured, so that the measurement system of the humidity measuring device is simplified. Further, when the oxygen concentration is constant, it is possible to measure humidity with good responsiveness. Further, even when the oxygen concentration in the gas to be measured changes, it can be easily handled without changing the measurement system.

In the above embodiments, the oxygen sensor 1 shown in FIG. 1 and the like was used. However, as shown in FIG. 7, the oxygen sensor used is the positive electrode 31 and the negative electrode 31 on the opposite surface of the oxygen ion conductive plate 30. Electrodes 32 are respectively provided, the negative electrode 32 is covered with a box 34 having micropores 33, and oxygen diffusion and water vapor diffusion in the cavity 35 are limited by the micropores 33, as shown in FIG. Alternatively, the negative electrode 32 may be covered with a box 36 made of a porous material to limit oxygen diffusion and water vapor diffusion into the voids 35.

[Brief description of drawings]

1 to 4 each show a first embodiment for carrying out the method of the present invention. FIG. 1 is a schematic view of a humidity measuring device, FIG. 2 is a perspective view of an oxygen sensor, and FIG. FIG. 4 is a sectional view of the oxygen sensor, FIG. 4 is a perspective view showing the structure of the ceramic heater of the present embodiment, FIG. 5 is a voltage-current characteristic diagram showing the measurement by the humidity measuring apparatus of the first embodiment, and FIG. FIG. 7 is a characteristic diagram showing a current value characteristic of each flat portion with respect to humidity in one example.
FIG. 1 is a schematic diagram showing a second embodiment for carrying out the present invention,
FIG. 8 is a schematic diagram showing a third embodiment for carrying out the present invention. In the figure, 1 ... Oxygen sensor, 11 ... Oxygen ion conductive plate (solid electrolyte), 12 ... Positive electrode, 13 ... Cathode electrode, 13b ... Connection part (gas diffusion control means), A ... Humidity measurement apparatus.

Claims (2)

[Claims] 1. An oxygen sensor in which a pair of electrodes made of a porous body are provided in close contact with the surface of a solid electrolyte having oxygen ion conductivity, and gas diffusion control means limits the gas diffusion to the electrodes. In the humidity measurement method of applying a voltage between the electrodes, to measure the humidity in the measured gas based on the limiting current value between the electrodes limited by the gas diffusion control means, A current value at an inflection point between a first limiting current value limited according to oxygen concentration and a second limiting current value limited according to humidity in the measured gas, and the first limiting current A humidity measuring method using an oxygen sensor, wherein the humidity is obtained based on a value and at least one of the second limiting current value. 2. An oxygen sensor in which a pair of electrodes made of a porous material are provided in close contact with the surface of a solid electrolyte having oxygen ion conductivity, and gas diffusion control means limits the gas diffusion to the electrodes. In the humidity measuring device for applying a voltage between the electrodes, to measure the humidity in the measured gas based on the limiting current value between the electrodes limited by the gas diffusion control means, A current value at an inflection point between a first limiting current value limited according to oxygen concentration and a second limiting current value limited according to humidity in the measured gas, and the first limiting current A humidity measuring device using an oxygen sensor, wherein the humidity is obtained based on a value and at least one of the second limiting current value.