JPH079085Y2 - Humidity sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a humidity detector for detecting the ratio of water vapor contained in a gas to be measured.

[Prior Art] Conventionally, an oxygen sensor is used, and a first voltage smaller than a theoretical decomposition voltage of water and a second voltage larger than the theoretical decomposition voltage are alternately switched and applied between electrodes of a solid electrolyte. A humidity sensor that calculates is known (Shokai 62-126766
Issue).

[Problems to be Solved by the Invention] However, the conventional humidity sensor has the following drawbacks.

(A) When the solid electrolyte is aged, the difference in the limiting current is reduced from I _HL to I _H ′ _-L ′ as shown in FIG. 6, and the humidity is measured low.

(A) As shown in FIG. 7, since the limiting current characteristics of solid electrolytes are different from each other, the difference in limiting current is also I _h-1 ,
It varies from product to product, such as I _h ′ _-l ′, so that replacement of the oxygen sensor is not possible.

An object of the present invention is to provide a humidity sensor which can detect accurate humidity for a long time and which is excellent in compatibility of parts constituting an oxygen sensor.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a solid electrolyte having good conductivity of oxygen ions, a first electrode and a solid electrolyte having the end surface in close contact with each other. A second electrode, a gas supply limiting means for limiting diffusion of a measured gas containing water vapor and oxygen to the first electrode, a first voltage lower than a decomposition start voltage at which electrolysis of water starts, and a decomposition start voltage A voltage applying means for alternately applying the above-mentioned second voltage between the electrodes, a ratio of a first limiting current value flowing when the first voltage is applied, and a second limiting current value flowing when the second voltage is applied, Then, a configuration including a calculator for calculating the ratio of water vapor contained in the gas to be measured based on the obtained ratio is adopted.

[Operation and Effect of Invention] (Operation) When applied voltage and humidity are changed, the current draws a characteristic having two flat portions as shown in FIG.

As shown in FIG. 6, the ratio of the first limiting current value flowing when the first voltage is applied to the second limiting current value flowing when the second voltage is applied is initially _IL : _IH , and the solid electrolyte is aged. Then, I _L ′: I _H ′, but from various experiments, it has been found that I _L : I _H = I _L ′: I _H ′, regardless of the amount of water vapor.

In one product, the ratio between the first limiting current value flowing when the first voltage is applied and the second limiting current value flowing when the second voltage is applied is I _l : I _{h as} shown in FIG. Then
Although I _l ′: I _h ′, it has been found from various experiments that I _l ： I _h = I _l ′: I _h ′, regardless of the amount of water vapor.

(Effect) The ratio of the first limiting current value flowing when the first voltage is applied and the second limiting current value flowing when the second voltage is applied is constant even if the solid electrolyte is aged, for any given amount of water vapor. Therefore, accurate humidity can be detected for a long period of time.

The ratio between the first limiting current value flowing when the first voltage is applied and the second limiting current value flowing when the second voltage is applied is constant even if the portion forming the oxygen sensor is replaced, for any given amount of water vapor. For this reason, it is excellent in compatibility with the portion forming the oxygen sensor.

[Embodiment] A first embodiment of the present invention will be described with reference to FIGS.

As shown in FIG. 1, the humidity sensor S comprises a stabilized zirconia plate 1 which is a solid electrolyte, a cathode 2 and an anode 3 which are arranged by closely adhering the end faces 21 and 31 to the plate 1, and a cover for the cathode 2. A gas supply limiting means 4 for limiting the diffusion of the measurement gas, a voltage applying means 5 for applying a DC voltage to the anode 3 and the cathode 2, and a calculator 6 for calculating the water vapor pressure are provided.

The stabilized zirconia plate 1 has a thickness of 0.3 mm and a size of 5 mm × 7 mm, and has good conductivity of oxygen ions. This plate 1 is fixed with sealing glass on a ceramic heater 100 in which a heating pattern is embedded, in which only electrode portions 22 and 32, which will be described later, are locally heated at 300 ° C to 700 ° C.

The cathode 2 and the anode 3 are porous platinum electrodes (thickness 20 μm), and have electrode portions 22 and 32 having a side of 2.5 mm and connecting portions 23 and 33 having a width of 1 mm and a length of 2 mm.

As shown in FIG. 2, the gas supply restricting means 4 is composed of an alumina porous layer 41 and a grace layer 42, which are electrode parts of the cathode 2.
The diffusion of the gas to be measured is restricted with respect to the cathode 2 by covering 22 and isolating it from the gas to be measured and exposing the connecting portion 23.

The voltage applying means 5 is a voltage source such that 1 V, which is lower than the decomposition start voltage (about 1.23 V) at which electrolysis of water starts, and 2 V, which is higher than the decomposition start voltage, are alternately applied between the cathode 2 and the anode 3. The electronic switch 53 is used to switch between 51 and 52.

The computing unit 6 detects the voltage across the current detection resistor 61 and receives the switching information of the voltage switch 53 to input 1V.
The first limiting current value that flows when the voltage is applied and the second limit current that flows when 2V is applied
The ratio with the limiting current value is obtained, and the water vapor pressure contained in the measured gas is calculated based on the ratio and displayed on the display 62.

Next, the operation of the humidity sensor S of the present invention will be described.

When the humidity sensor S is placed in the gas to be measured and a DC voltage is applied between the anode 3 and the cathode 2, the oxygen is ionized in the electrode portion 22 covered with the Grace layer 42 to become oxygen ions, and the measurement is performed. Gas oxygen is pumped from the cathode 2 to the anode 3. At this time, in the cathode 2, the electrode part
Since only 22 is locally heated and the connecting portion 23 is not heated to the extent that it exhibits oxygen ion conductivity, oxygen diffuses from the connecting portion 23 into the electrode portion 22 covered with the grace layer 42.

Fig. 3 shows the relationship between each humidity and current when the measured gas temperature is 80 ° C.

(I) Initially, the current is increased according to the applied voltage.

(II) Applied voltage between cathode 2 and anode 3 is about 0.5V
In the range of 1.2V, the diffusion amount of oxygen into the electrode part 22 is controlled by the connection part 23 of the cathode 2 and is limited according to the oxygen concentration in the gas to be measured. Along with this, the current value is also limited to a flat first limit current value.

(III) When the applied voltage is around the decomposition start voltage of water, the current tends to increase according to the applied voltage. When the decomposition start voltage is exceeded, water vapor in the measurement gas is electrolyzed, and oxygen ions generated by the decomposition are also pumped to the anode 3.

(IV) When the applied voltage is further increased to a voltage of 1.7 V to 2.4 V, the diffusion amount of water is limited at the connecting portion 23 of the cathode 2,
Along with this, the current value is also limited, and becomes a flat second limit current value according to the water vapor pressure.

Next, the effect of the humidity sensor S will be described.

The first and second limiting current values are mainly limited by the connecting portion 23 as the gas supply limiting means, and the stabilized zirconia plate 1
Is determined by the density and thickness of the electrode, the area of the electrode portion 22 of the cathode 2 formed on the surface of the plate 1, and the like, and varies for each humidity sensor S due to variations in manufacturing the oxygen sensor portion. Further, when the stabilized zirconia plate 1 deteriorates due to long-term use, the first and second limiting current values decrease as compared with the initial stage of use.

However, the ratio between the first limit current value and the second limit current value (for example, R1 shown in FIG. 4 = second limit current value / first limit current value)
Indicates a constant value regardless of the degree of variation or deterioration during manufacturing, as long as the water vapor pressure in the measured gas is constant. Therefore, if the relationship between the current value ratio R1 and the water vapor pressure is measured in advance, the water vapor pressure for R1 can be obtained.

That is, since the humidity is obtained from the current value ratio in each flat portion, even if each limiting current value differs due to the dispersion of the stabilized zirconia plate 1, the calculated water vapor pressure is not affected, Further, even if the oxygen sensor portion is replaced, it is not necessary to recalibrate the arithmetic unit 6. further,
Even if each limiting current value changes due to the deterioration of the plate 1, the calculated water vapor pressure is not affected by the deterioration.

FIG. 8 shows a second embodiment of the present invention.

As shown in FIG. 8, the humidity sensor S1 is provided with an anode 3 and a cathode 2 on opposite surfaces of a stabilized zirconia plate 1, respectively. In the present embodiment, the diffusion of the gas to be measured into the void 45 is restricted by the box 44 having the minute holes 46 covering the cathode 2.

FIG. 9 shows a third embodiment of the present invention.

In the humidity sensor S2 of this embodiment, as shown in FIG. 9, the cathode 2 is covered with a box 47 made of a porous material to limit the diffusion of the gas to be measured into the void 45.

Also in the second and third embodiments, the humidity is obtained from the current value ratio in each flat portion, so that the same effect as that of the first embodiment is obtained.

[Brief description of drawings]

1 to 4 show a first embodiment of the present invention. FIG. 1 is a diagram for explaining the principle of the humidity sensor, FIG. 2 is a cross-sectional view of the main part thereof, FIG. 3 is a graph showing the relationship between the voltage applied to the sensor and the current, and FIG. 4 is the current value ratio and water vapor ratio. It is a graph. 5 to 7 are diagrams for explaining the function and effect of the present invention.
It is a graph which shows the relationship between an applied voltage and current. 8 and 9 are sectional views of the humidity sensor in the second and third embodiments of the present invention. In the figure, 1 ... Stabilized zirconia plate (solid electrolyte) 2 ... Cathode (first electrode), 3 ... Anode (second electrode), 4 ... Gas supply limiting means, 5 ... Voltage applying means, 6 ...
Calculator, 21, 31 ... End face

Claims (1)

[Scope of utility model registration request] 1. A solid electrolyte having good conductivity of oxygen ions, a first electrode and a second electrode arranged in close contact with each end surface of the solid electrolyte, and water vapor and oxygen with respect to the first electrode. Gas supply limiting means for limiting the diffusion of the gas to be measured including, a first voltage lower than the decomposition start voltage at which electrolysis of water starts,
Voltage applying means for alternately applying a second voltage equal to or higher than the decomposition start voltage between the electrodes, a first limit current value flowing when the first voltage is applied, and a second limit current value flowing when the second voltage is applied. And a calculator for calculating the ratio of water vapor contained in the gas to be measured based on the ratio.