JPH0758279B2 - Humidity sensor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a limiting current type humidity sensor for measuring the concentration of water vapor in the atmosphere or a measurement gas.

(Prior Art) FIG. 4 is a schematic configuration diagram of a conventional humidity sensor. In the figure, a humidity sensor 100 includes a solid electrolyte portion (detection portion) 10 made of an oxygen ion conductor such as zirconia in which an ionic current flows between a positive electrode 1 and a negative electrode 2, and another solid electrolyte in which a heater 5 is embedded. Section (heater section) 11, small hole 4 for limiting diffusion of measurement gas, space section 3 connected to this small hole 4, and lead section for connecting between electrodes 1 and 2 and terminals 9A and 9D, respectively. 6,
7 and through holes 8B, 8C, 8D and heel terminals 9B, 9C.

The negative electrode 2 of the detection unit 10 is provided on the wall surface of the space 3, and oxygen gas in the measurement gas existing in the space is converted into oxygen ions, and water vapor is electrolyzed into oxygen ions. It has the function of feeding into the electrolyte. Further, the positive electrode 1 has a function of converting oxygen ions, which are ionized by the negative electrode 2 and carried in the solid electrolyte of the detection unit 10, into oxygen molecules and diffused to the outside. For this reason, it is provided on the outer surface of the detection unit 10.

(Problems to be solved by the invention) However, in the humidity sensor 100 configured as described above, a current value much larger than the magnitude of the oxygen ion current estimated from the water vapor concentration in the gas to be measured is observed. There was a problem of being done. This current value is shown by the dotted line b in FIG. 2, and some electrochemical reaction occurs on the surface of the positive electrode 1 which is in direct contact with the measurement gas, which is a factor that causes a current higher than the deposition current. It will be deposited. For this reason, the inability to accurately measure the amount of water vapor has been an important issue to be solved.

The present invention has been made in view of the above points, and it is an object of the present invention to effectively solve the problems of the prior art, to eliminate the cause of measurement error, and to provide a humidity sensor in which the measurement accuracy is significantly improved.

(Means for Solving the Problems) In order to achieve such an object, the present invention provides a positive electrode formed in a porous part of a detection part in a solid electrolyte part without directly contacting the atmosphere or a measurement gas. It is characterized in that it is configured to be embedded.

(Function) As a result of repeated years of research and experiment by the inventors, since the positive electrode is originally formed in a porous state, it is possible to sufficiently dissipate the generated oxygen gas even if the positive electrode is formed in the embedded type. As a result, the factor of measurement error is reduced, and the measurement accuracy is significantly improved.

(Example) Next, the Example of this invention is described in detail based on drawing.

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention, (A)
Is a front view thereof, and (B) is a sectional view taken along line AA of (A).
In the figure, parts having the same functions as those in FIG. 4 are designated by the same reference numerals. The humidity sensor 200 includes an enzyme ion conductor, for example, a green sheet made of a solid electrolyte such as zirconia. It is made by stacking green sheets, pressing and joining them with a hot press to integrate them, and then firing. The detection unit 10 is formed on one wall surface side of the space 3. That is, the negative electrode 2 is provided in contact with one wall surface of the space 3, and the positive electrode 1 is provided so as to face the negative electrode 2 with the zirconia layer 10A having an appropriate thickness, for example, about 100 μm interposed therebetween. . In addition, space 3
Are connected to the outside air through the small holes 4 that limit the oxygen diffusion amount. The heater portion 11 is formed in close contact with the detection portion 10, and has a heater 5 composed of a platinum rhodium thick film resistor on the outermost side and an appropriate thickness on the inner side thereof, for example, about 100 μm.
It consists of a zirconia layer 11A to a degree. Furthermore, both electrodes 1, 2
Are connected to terminals 9A and 9D by lead portions 6 and 7 and through holes 8A and 8D, respectively.

In this way, the positive electrode 1 is embedded between the zirconia layers 10A and 11A, and is naturally porous even if it is not directly contacted with air or the measurement gas. Is sufficiently dissipated, and the measurement error factor due to the positive electrode 1 in the prior art directly contacting the measurement gas is reduced.

Next, FIG. 2 shows a characteristic diagram of inter-electrode current and inter-electrode voltage of the humidity sensor. In the figure, a solid line a is a current / voltage characteristic diagram of the humidity sensor 200 according to the present invention, and a dotted line b is a current / voltage characteristic diagram of the conventional humidity sensor 100. Due to the diffusion limiting small holes 4, the amount of oxygen or the amount of water vapor supplied by diffusion cannot increase beyond a certain value determined by the partial pressure of oxygen or water vapor in the atmosphere in which the humidity sensor 200 is placed. Therefore, a plateau region having a constant current value is generated in the parts A and B, and the amount of oxygen and the amount of water vapor in the measurement gas can be measured by this current value. That is, part A is a plateau region due to the ion current of oxygen in the measurement gas, IL is the current value at that time, part B is the plateau region where the ion current generated by electrolysis of water vapor is added to part A, IH is the current value.

Next, the humidity sensor 200 of the present invention will be described in comparison with the prior art and theoretical calculation values, taking the humidity measurement in the atmosphere as an example.

a) Measured value As for the measured value, the current value increase C% due to the water vapor in the atmosphere from the current values IL and IH is defined by the following equation (1).

Since the current values IH and IL can be measured as the output current of the humidity sensor, C% can be obtained as a measured value.

b) Regarding theoretical calculated values As a premise of theoretical calculated values, the atmospheric pressure is constant at 1 atm.

FIG. 3 shows a current / voltage characteristic diagram of the humidity sensor of the present invention during dry air measurement and during atmospheric measurement. In the figure, parts having the same functions as those in FIG. 2 are designated by the same reference numerals. The solid line a1 shows the current-voltage characteristics when measuring dry air, and the dotted line b1 shows the current-voltage characteristics when measuring atmospheric air. Assuming that the amount of oxygen in dry air is 20.6% and the amount of water vapor in the atmosphere is α%, the current value IL is proportional to the oxygen amount, and the current value IH is the oxygen amount and the oxygen (ion) amount generated from the water vapor. Proportional to the sum of. Therefore, the current values IL and IH are as shown in Table 1 when measured with dry air and air.

Here, α% is a value calculated from the saturated water vapor amount and relative humidity at the time of measurement.

In the measurement of the atmosphere, the amount of oxygen is reduced by the amount of water vapor compared to dry air, so the current value IL is 20.6 (100-
It is proportional to α) / 100%.

The reason why the current value IH is proportional to the amount of oxygen + the amount of water vapor / 2 in the measurement of the atmosphere will be described below.

On the negative electrode 2 side, an electrochemical reaction represented by the following equations (2) and (3) occurs in the voltage region of the portion B, and oxygen ions are generated.

_{^{O 2+ 4e - → 20 2- ......}} (2) H 2 O + 2e - → H 2 + O 2- ...... (3) that is, with respect to the oxygen in the atmosphere can two oxygen ions , From the electrolysis of water vapor, one oxygen ion
I can only do it. Since the atmospheric pressure is constant, the amount of oxygen in the atmosphere decreases in proportion to the amount of water vapor in proportion to dry air. As shown in Table 1, the oxygen ions involved in the water vapor are only half of the oxygen ions produced from the oxygen gas reduced by that amount.

Based on the above results, when the current value increase D% due to the water vapor in the atmosphere similar to the equation (2) is obtained by the theoretical calculation equation, the following equation (4) is obtained.

By rearranging this equation (4), c) Regarding the comparison between the actually measured value and the theoretically calculated value, the current values IH and IL of the humidity sensors 100 and 200 of the conventional type and the present invention type are measured to obtain the current increase C%, while the amount of water vapor in the atmosphere at the time of measurement Table 2 shows an example in which the current increase D% was calculated from α% and compared.

Inventive type (humidity sensor 200) as shown in Table 2
Is the conventional type (the humidity sensor 10
Compared to 0), it was able to be much closer to 1. That is, it is considered that the humidity sensor 200 is hardly affected by the outside air in the operating state because the oxygen carried from the positive electrode 2 side to the negative electrode 1 side creates an oxygen atmosphere close to 100%.

(Effects of the Invention) In order to achieve such an object, the present invention originally has a structure in which the positive electrode formed in the porous detection section is embedded in the solid electrolyte layer, and thus the conventional method is adopted. The technical problems are effectively solved, the factor of measurement error is removed, and the measurement accuracy is dramatically improved.

[Brief description of drawings]

FIG. 1 shows a schematic configuration diagram of an embodiment of the present invention. FIG. 1 (A) is a front view thereof, and FIG. 1 (B) is AA of FIG.
Sectional view, FIG. 2 is a characteristic diagram of inter-electrode current and inter-electrode voltage of the humidity sensor, FIG. 3 is current-voltage characteristic diagram of the humidity sensor of the present invention during dry air measurement and atmospheric measurement, and FIG. The figure shows a schematic configuration diagram of a conventional humidity sensor, in which (A) is a front view thereof and (B) is a cross-sectional view taken along line BB of (A). 1: Positive electrode, 2: Negative electrode, 3: Space part, 5: Heater, 10: Detection part, 11: Heater part, 200: Humidity sensor.

Claims (1)

[Claims] 1. A limiting current method for detecting, as a current value, the amount of oxygen ions resulting from the electrolysis of water in the atmosphere or in a measurement gas generated in a solid electrolyte portion made of an oxygen ion conductor and having a detection portion and a heater portion. In the humidity sensor,
A humidity sensor, characterized in that the porous positive electrode of the detection part is embedded in the solid electrolyte part without directly contacting the atmosphere or the measurement gas.