FI99054C - Capacitive humidity sensor - Google Patents

Description

99054

Capacitive humidity sensor

The invention relates to a humidity sensor using an organic polymer as a moisture sensitive material.

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A conventional moisture sensor of the above type uses an organic polymer, e.g., cellulose acetate butyrate (hereinafter simply referred to as "SAB"), as a moisture-sensitive material, and utilizes a change in electrical capacitance in a moisture-sensitive film formed of such a moisture-sensitive material. for detecting moisture, as disclosed in Japanese Public Patent Application No. 62-88951.

SAB with 17% butyryl base, typically used as a moisture-sensitive material in a conventional moisture-sensitive element of the type mentioned above, has an absorption rate of about 70 to 90 mg / g at equilibrium at 30 ° C and 90 ° RH relative humidity. The relationship between the amount of water absorption and the relative humidity is explained in detail below.

The moisture-sensitive element having a moisture-sensitive film made of SAB has a 2-4% RH hysteresis (difference in moisture-sensitive property between moisture absorption and desorption) at about 25 ° C and at 5 minute intervals. Thus, it is not possible to reduce hysteresis below 1% RH, which is considered ideal. The hysteresis is also temperature-dependent and increases by 2-3% RH at 40 ° C compared to the value obtained at 25 ° C if the other conditions are the same.

In addition, if a moisture-sensitive element 35 of the above type is left in the same atmosphere for a long time, the effect of moisture to which the element has previously been exposed causes a change in hysteresis and moisture-sensitive property.

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In particular, if the element is left, e.g., 11% RH at relative humidity and room temperature for an extended period of time, the hysteresis increases from an initial value of 2% RH to 4-6% RH. If the same element is left at 84% RH relative humidity and 5 room temperature for a long time, the hysteresis decreases from the initial value, i. 2% RH to 1% RH. However, relative to the moisture sensitivity characteristic curve, the output increases by 5-7% RH at relative humidity in the range of 10-90% RH, i. there is a creep at the start. In addition, the ratio of the value of the electrical capacitance 10 at a relative humidity of 90% RH to the value at a relative humidity of 10% RH (C90 / C10) varies when the element is used for a long time.

In addition, at high temperature and high humidity environment, condensation of moisture, dripping of water, the period of temperature and humidity and so on also causes the moisture sensitivity characteristic of the element to creep in a largely positive direction.

20 These problems are caused by the fact that the water absorption rate of the moisture-sensitive material used in the above-mentioned moisture-sensitive element is too large to be an ideal capacitance or impedance-type moisture-sensitive element. Thus, the interaction of the water absorbed in the moisture-sensitive material causes a change in the response of the element and the equilibrium of the amount of water absorbed in the measured humidity atmosphere, thus causing a creep at the exit of the element.

OBJECTS AND SUMMARY OF THE INVENTION 30

In light of the above problems, the present invention aims to provide a moisture-sensitive element having low hysteresis at various operating temperatures from high to low temperature.

Another object of the present invention is to provide a moisture-sensitive element capable of developing

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35 3 99054 stable output under various operating conditions, such as high humidity, high temperature, intermittently varying humidity, low humidity, moisture condensation, water dripping and so on.

To achieve these objects, the capacitive humidity sensor according to the present invention is characterized by the features set forth in the characterizing part of claim 1.

These and other objects, features and advantages of the present invention will become apparent from the following detailed description of the best embodiments with reference to the accompanying drawings. The preferred embodiments are described in the dependent claims.

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Fig. 1 is a perspective view showing an embodiment of a moisture-sensitive element according to the invention.

Figure 2 is a top plan view of the moisture sensitive element 20 shown in Figure 1.

Figure 3 is a graph showing the relationship between the amount of water absorption and the hysteresis of the moisture-sensitive element of the invention.

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Fig. 4 is a graph showing the relationship between the amount of water absorption and the capacitance ratio of the moisture-sensitive element of the invention.

Fig. 5 is a graph showing the relationship between the amount of water absorption and the relative humidity of a conventional moisture-sensitive element (SAB containing 17% butyryl base); and Fig. 6 is a graph showing a moisture sensitivity characteristic of a conventional moisture-sensitive element (SAB with 17% butyryl base).

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An embodiment of a moisture-sensitive element according to the present invention will now be described with reference to Figures 1 and 2.

These drawings show an insulating substrate 1, 5 made of e.g. alumina, glass, thermally oxidized silicon or the like, and having a lower electrode 2 made of e.g. platinum on its upper surface. The moisture-sensitive film 3 is stacked on and across the lower electrode 2. This film is made of a polymeric moisture-sensitive material 10, the main component of which is either polyether sulfone (hereinafter simply referred to as "PES") or polysulfone.

The upper electrode 4, which is made of e.g. gold, is formed on a moisture-sensitive film 3. That is, the moisture-sensitive film 3 is sandwiched between the lower 2 and upper 4 electrodes. The conductors 2a, 4a are connected to the lower and upper electrodes 2, 4, respectively, for detecting a change in the electrical capacitance in response to the relative humidity of the moisture-sensitive film 3.

The moisture-sensitive film 3 may be made of a polymeric moisture-sensitive material comprising polysulfone, polyether sulfone, polyetherimide, polybenzimidal, polyether, polyimide, polyamideimide, polyphenylene oxide, polyphenylene oxide, polycarbonate, polyallylate, polymethacrylate, polymethacrylate , polyetheretherketone, polyacetal or the like.

Accordingly, such polymeric materials are coated on a quartz vibrator to form moisture-sensitive films of about 10 μm or less in thickness. The moisture-sensitive films are then placed in a constant humidity atmosphere to measure the corresponding amount of water absorption at about 30 ° C based on the change in oscillation frequency of the quartz vibrator.

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Each of the above polymeric materials is also coated on a lower electrode 2 placed on an insulating substrate, about 10 μm or less, to form a moisture-sensitive film 3 on which the upper electrode 4 is formed as mentioned above. With this structure, the ratio between the electrical capacitance ratio and the relative humidity is measured at about 30 ° C. The same measurements are also made on a moisture-sensitive film 10 3 made of a conventionally used cellulose acetate butyrate. as shown in Figure 6.

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From the results of the above measurement, it was found that the cellulose acetate butyrate film (containing 17% butyryl base) has a water absorption amount of about 70-90 mg, a hysteresis of 2-4% RH and an electrical capacitance ratio (C90 / C10) of 1.21 to 1.42, while cellulose the acetate butyrate film (containing 50% butyryl base) has a water absorption rate of 40-60 mg, a hysteresis of 1-2% RH and an electrical capacitance ratio (C90 / C10) of 1.10 to 1.20. The polymethacrylic acid methyl film gave a satisfactory result with a water absorption of 10 to 30 mg, a hysteresis of 0.5 to 1.0% RH and C90 / C10 1.07 to 1.15.

• Good results were also obtained with polyethersulfone film and polysulfone film. The former has a water absorption of 10-30 mg, a hysteresis of 0.5 to 1.5% RH and C90 / C10 1.12 to 1.17 and the latter has a water absorption of 10 to 15 mg, a hysteresis of 0.2 to 1.0% RH and C90 / C10 1.05 to 1.08.

: The measurement results show that the number of water molecules in the polymer material causes a large change in the moisture-sensitivity characteristic.

In particular, in the range of relative humidity below 60% RH, the water absorption rate is 35 to 40 mg / g and consequently the hysteresis is 0.3 to 0.4% RH, while the linearity of the moisture sensitivity characteristic curve is quite satisfactory. However, in the range of relative humidity above 60%, the amount of RH water absorption exceeds 40 mg / g, with the interaction occurring with water molecule-5 channels and therefore the hysteresis gradually increases. For example, when measurements are made at 10% RH, 70% RH, and 10% RH relative humidity, respectively, the maximum water absorption rate was 40-45 mg / g and the hysteresis was 1.2-1.5% RH. While measurements were performed at 10% RH, 80% RH, and 10% RH, respectively, the maximum water absorption rate was 50 to 55 mg / g and the hysteresis was 1.5 to 2.1% RH. From these results, it can be seen that the hysteresis increases as the amount of water absorption increases. Such a large hysteresis causes the linearity of the sensor output to deteriorate.

The above results show that there is a close correlation between the water absorption rate and the moisture sensitivity characteristic of the polymeric material. If the hysteresis is expected to be e.g. less than 1% RH, the amount of water absorbed should not exceed 40 mg / g. It is to be understood that the information shown herein is typical, and the hysteresis will vary depending on the manufacturing method, manufacturing conditions, measurement method, measurement conditions, humidity to which the element has previously been exposed, and the like. Thus: the characteristic curves shown in Figures 3 and 4 may have a certain width. Incidentally, when the moisture-sensitive element shown in Figs. 1 and 2 is made of a polymeric material having an water absorption rate of 10 to 40 mg / g 30 at 90% RH relative humidity, its capacitance ratio (C90 / C10) is 1.05 to 1.2.

In the embodiment described above, the moisture-sensitive element 35 is used at a temperature in the range of -30 to 100 ° C, so that there are no problems with the polymeric materials forming the moisture-sensitive film 3, e.g., destruction of the upper electrode 4.

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7 99054 or greater changes in sensor characteristics will not occur. With respect to the dielectric breakdown voltage, it has been found that a polymer film having a thickness of 0.5 to 10 μm can withstand voltages of 1.5 to 5 V (+/-) applied to it. In addition, when 5 elements were measured in connection with the oscillation circuit, no problem arose with respect to the resistance included in the volume.

As described above, the invention can provide an improved moisture-sensitive element that achieves less than 1% hysteresis and elimination of temperature dependence. The moisture-sensitive element according to the invention can also maintain a stable output and electrical capacitance ratio, even if it is left in the same atmosphere for a long time. In addition, the creep of the moisture sensitivity characteristic curve is quite small, even if it is left in an atmosphere of high temperature and high humidity, condensation of moisture, dripping of water, temperature and humidity cycle, etc ..

Since many modifications to the above structure may be made and many apparently wide variety of embodiments of the present invention may be made without departing from its scope, it is intended that all matters contained in the above description or shown in the accompanying drawings be construed as illustrative and not restrictive.

Claims (5)

A capacitive moisture sensor comprising a moisture-sensitive membrane (3) made of polymeric material, the thickness of the moisture-sensitive membrane (3) being in the range 0.5 µm-10 µπι; and two electrodes (2, 4) between which the moisture-sensitive membrane (3) is sandwiched; characterized in that the water absorption amount of the moisture sensitive membrane is in the range of 10-40 mg / g at an equilibrium position at a temperature of 30 ° C and a relative humidity of 90% RH, and that the capacitive humidity sensor comprises in addition, a substrate (1) on which the two electrodes (2, 4) and the moisture-sensitive membrane (3) are arranged. 1 5 Humidity sensor according to claim 1, characterized in that the moisture-sensitive membrane (3) is made of polymeric material whose thermal deflection temperature under the load is above 120 ° C or whose heat resistance temperature is above 100 ° C. Humidity sensor according to claim 1, characterized in that the moisture-sensitive membrane (3) is made of polymeric material whose glazing temperature is 100 ° C or whose melting point is above 150 ° C. Humidity sensor according to claim 1, characterized in that said moisture sensitive membranes (3) are made of polymeric material, whose volume resistance is above 1014 Q / cm 3. Humidity sensor according to claim 1, characterized in that the moisture-sensitive membrane (3) is made of polymeric material whose dielectric breakdown voltage is over 14 KV / mm.