JP2014041054A - Moisture sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a practical moisture sensor capable of correctly measuring a moisture amount.SOLUTION: A swelling material holding part 22 has a cross-sectional recess housing part 23, a large number of through-holes 24 are formed on a bottom part of the housing part 23 or (and) on a peripheral wall, a swelling material 25 is filled into the housing part 23, an opening of the housing part 23 is closed by a membrane 26, and an opening of a casing 21 housing a pressure sensor 27 therein is closed by the swelling material 25 and the swelling material holding part 22 with the membrane 26.

Description

  The present invention relates to a moisture sensor that detects and measures the amount of moisture in soil, for example, and in particular, moisture using a swelling material (hereinafter abbreviated as a swelling material) that swells and shrinks according to the amount of moisture in the sensitive part. It relates to sensors.

  It is very important to understand the groundwater flow field in the safety evaluation of geological disposal of radioactive waste and the construction of underground cavities. By observing the soil moisture in the unsaturated zone of the surface ground, the groundwater recharge at that point can be obtained directly. It is also important to detect and measure moisture intrusion into the buffer material layer made of clay (bentonite) around the waste.

  Conventionally, an electric sensor using a dielectric constant such as ADR or FDR has been used as a method for detecting and measuring the amount of moisture in the soil, but has the following problems.

(A) Since it is vulnerable to electrical noise, noise countermeasures are necessary near, for example, substations, power transmission lines, or special factories, and monitoring may not be possible in some cases.
(B) Since current flows even if it is weak, it cannot be used in a place or environment where explosion protection is required.
(C) The reliability of data becomes a problem when the insulation of the transmission cable is lowered after long-term use.
(D) Since the transmission cable is relatively thick, in the case of multipoint measurement, it is necessary to route a large amount of the transmission cable, the arrangement of the sensor is complicated, and in some cases, a weak part is formed in the structure body. Become.
(E) Lightning strikes can easily damage the sensor and measurement system, and in the worst case, they can be completely destroyed.

  In contrast, a measurement system using an optical fiber eliminates the drawbacks of the electrical methods described above, has the following features, and is promising as a next-generation monitoring system.

(A) There is no influence of electrical noise.
(B) Since it is completely explosion-proof in nature, it is not limited to the use environment.
(C) No electrical insulation failure occurs.
(D) The outer diameter of the optical fiber including the buffer layer is usually 0.25 mm, which is thinner than that of the electric wire, so that the cable routing does not become a problem.
(E) Since it is essentially an insulator, it is not affected by lightning.

Conventionally, as a method using an optical fiber in observing soil moisture and the like, for example, the following non-patent documents 3 and 4 can be cited.
Non-Patent Document 3: Alessi, RS, L. Prunty: Soil-water Determination Using Fiber Optics, Soil Sci, Soc. Am, Vol. 50, pp. 860-863, 1986.
In the research described in Non-Patent Document 3, the water content is measured for glass beads and sandy soil using an optical fiber sensor. Specifically, after filling the sample in the container and adjusting the water content by increasing the air pressure, the results obtained can be correlated in a linear relationship from the relationship between the obtained voltage, pressure and volumetric water content. Yes.

Non-Patent Document 4: Texier, S., S. Pamukcu, J. Toulouse: Advances in subsurface water content measurement with a distributed Brillouin scattering fiber-optic sensor, Proceedings of SPIE, Vol.5855, pp.555-558, 2005.
In the research described in Non-Patent Document 4, the moisture of a low water permeable material is absorbed using a distributed type Burrian scattering optical fiber sensor, the tensile strain of the optical fiber due to swelling of the polymer, and the optical fiber compression due to drying of the polymer. The distortion is measured as the Burrian scattering shift, and the shift amount is measured.
In addition, regarding the moisture sensor in soil, for example, the following patent documents 1 and 2 and non-patent documents 1 and 2 can be cited.

JP 2005-351663 A JP 2005-127744 A

October 15-17, 2009 Abstracts of the Autumn Lecture Meeting of the Japan Association of Groundwater Hydrology “15. Study on soil moisture measurement using fiber optic pressure gauge” November 11-13, 2010 Abstracts of the Autumn Lecture Meeting of the Japan Association of Groundwater Hydrology “15. Examination of water-swelling materials for development of optical fiber soil moisture meter” Alessi, R.S., L. Prunty: Soil-water Determination Using Fiber Optics, Soil Sci, Soc. Am, Vol. 50, pp. 860-863, 1986. Texier, S., S. Pamukcu, J. Toulouse: Advances in subsurface water content measurement with a distributed Brillouin scattering fiber-optic sensor, Proceedings of SPIE, Vol.5855, pp.555-558, 2005.

  The above-mentioned Patent Documents 1 and 2 and Non-Patent Documents 1 to 4 do not describe a specific structure of a moisture sensor according to the present invention described later.

In addition, the conventional technique of Non-Patent Document 3 has a problem as a technique for measuring the amount of water in an optical fiber sensor having a diameter of about 1 mm, and there is a possibility that a local value may be obtained transiently. Have been described.
Furthermore, in the prior art of the said nonpatent literature 4, it is described that the water content which can be measured is in the range of 0 to 30%, and it is difficult to measure the water content close to saturation.
An object of the present invention is to provide a practical moisture sensor that eliminates the drawbacks of the prior art and can accurately measure the amount of moisture.

In order to achieve the above object, the first means of the present invention is as follows:
A swelling material that swells and shrinks depending on water absorption or dryness;
A swelling material holding portion for filling and holding the swelling material;
A membrane that is elastically deformed by a swelling pressure due to water absorption of the swelling material filled in the swelling material holding portion;
A pressure sensor for detecting the amount of distortion of the membrane is provided.

According to a second means of the present invention, in the first means,
The pressure sensor has an optical fiber provided with a sensor unit for detecting the amount of strain of the membrane;
The strain generated by elastic deformation of the membrane is configured to act in the optical axis direction of the optical fiber.

According to a third means of the present invention, in the second means,
The swelling material holding portion has a concave-shaped storage section, and a large number of through holes are formed in the bottom or (and) the peripheral wall of the storage section,
The storage portion is filled with the swelling material, and the opening of the storage portion is closed by the membrane,
The opening of the casing that houses the pressure sensor is closed by the swelling material and the swelling material holding portion with the membrane.

According to a fourth means of the present invention, in the second means,
The swelling material holding part is composed of a cylindrical body and a porous body having continuous pores that block one opening of the cylindrical body,
The cylindrical body is filled with the swelling material, the other opening of the cylindrical body is closed by the membrane,
The opening of the casing that houses the pressure sensor is closed by the swelling material and the swelling material holding portion with the membrane.

According to a fifth means of the present invention, in the second means,
The swelling material holding part has a cylindrical shape having openings at both ends,
One opening of the swelling material holding part is closed by the membrane,
The opening of the casing housing the pressure sensor is closed by the swelling material and the swelling material holding portion with the membrane,
The swelling material is filled in a space portion having a concave cross-sectional shape formed by the peripheral wall of the swelling material holding portion and the membrane,
The other opening of the swelling material holding portion is brought into contact with the surface of the object to be measured containing moisture, and a part of the swelling material is directly in contact with the surface of the object to be measured. It is.

According to a sixth means of the present invention, in any one of the third to fifth means,
The casing has a cup shape, and an opening of the casing is closed by the swelling material and the swelling material holding portion with the membrane,
The pressure sensor is
A displacement-strain conversion member having a substantially U-shaped side surface, one end of the displacement-strain conversion member attached to the membrane, and the other end of the displacement-strain conversion member attached to the bottom of the casing; ,
The sensor part of the optical fiber is affixed to the side surface of the bridge-like part protruding to one side at the intermediate part of the displacement-strain converting member.

According to a seventh means of the present invention, in any one of the third to fifth means,
The casing has a cup shape, and an opening of the casing is closed by the swelling material and the swelling material holding portion with the membrane,
The pressure sensor is
A displacement transmission member attached from the membrane toward the bottom of the casing;
A fiber support member attached from the bottom of the casing toward the displacement transmission member;
It is comprised from the optical fiber fixed on the said fiber support member from the said displacement transmission member so that the said sensor part may be arrange | positioned in the clearance gap formed between the said displacement transmission member and the said fiber support member. It is characterized by.

According to an eighth means of the present invention, in any one of the second to seventh means,
The sensor unit is characterized by having a configuration in which diffraction gratings having different refractive indexes are arranged in parallel at regular intervals on the optical axis of the core of the optical fiber.

  The ninth means of the present invention is characterized in that a plurality of sensor sections of the eighth means can be attached to one optical fiber at an arbitrary interval.

According to a tenth means of the present invention, in any one of the second to seventh means,
The sensor unit includes a first semi-transmissive mirror and a second semi-transmissive mirror that are opposed to each other with a gap on the optical axis of the core of the optical fiber, and reflected by the first semi-transmissive mirror. The first reflected light and the second reflected light reflected by the second transflective mirror are separated for each wavelength and interfere with each other.

The eleventh means of the present invention is any one of the first to tenth means,
The swelling material is a urethane-based swelling rubber material.

  The present invention is configured as described above, and can provide a practical moisture sensor that can accurately measure the amount of moisture.

It is sectional drawing of the moisture sensor which concerns on 1st Example of this invention. It is a characteristic view which shows the result of having performed the swelling / shrinkage test of the moisture sensor which concerns on the 1st Example. It is sectional drawing of the moisture sensor which concerns on 2nd Example of this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating the principle of an optical fiber type pressure sensor in the FBG system, where (a) is a side view of an optical fiber including a plurality of FBG sensor units, and (b) is an enlarged view showing an internal state of the FBG sensor unit. It is a block diagram of a FBG sensor part. It is a wave form diagram of the transmitted light which permeate | transmits the input light which injects into an FBG sensor part, the reflected light reflected by an FBG sensor part, and the FBG sensor part. It is a block diagram of a FPI sensor part. Sample No. of each swelling material And a table summarizing material names and material shapes. It is the table | surface which put together the result examined about the swelling property, shrinkage | contraction property, durability, workability, and water quality dependence of each swelling material. It is the table | surface which put together the result of having performed the swelling and shrinkage | contraction characteristic test about each swelling material. Sample No. It is the table | surface which put together the result of having repeated the swelling and shrinkage | contraction test about the swelling material of 9,11,13. It is sectional drawing of the moisture sensor which shows the 1st specific example of the pressure sensor in this invention. It is sectional drawing of the moisture sensor which shows the 2nd specific example of the pressure sensor in this invention. It is sectional drawing of the moisture sensor which shows the 3rd specific example of the pressure sensor in this invention. It is a schematic block diagram which shows the water | moisture-content sensor group which can carry out the multi-continuous measurement which embeds several water | moisture-content sensor parts of the FBG type | system | group based on the Example of this invention in soil, and measures a water content. It is a schematic block diagram which shows the moisture sensor group which embeds the several moisture sensor part of the FBG system or FPI system which concerns on the Example of this invention, and measures a moisture content. It is a partial cross section figure of the moisture sensor which measures the moisture content in concrete concerning the 3rd example of the present invention.

(Principle of optical fiber type pressure sensor)
Considering the accuracy and size of the measurement system when considering a moisture sensor using an optical fiber, the FBG method or the FPI method is compared with other methods such as the OTDR method, the B-OTDR method, and the R-OTDR method. Are particularly suitable.
The FBG is an abbreviation of Fiber Bragg Grating. By arranging portions of different refractive indexes called diffraction gratings in a lattice pattern at regular intervals in a part of the core of an optical fiber, only light of a specific wavelength proportional to the interval is arranged. Is reflected. This part is used as an FBG sensor part. The reflection wavelength λ ₀ corresponding to the speed of light in vacuum is expressed by the following equation (1).
λ ₀ = 2nD (1)
In the equation, λ ₀ is a reflection wavelength, n is a refractive index, and D is a distance between diffraction gratings.

The relationship between wavelength, pressure, and temperature is expressed by the following equation (2).
(∂λ / λ) = Aε + BdT (2)
In the equation, λ is a wavelength, A and B are coefficients, ε is strain, and T is temperature.

With such reflection wavelength lambda ₀ is elastic spacing D of the grating equation (1), the reflection wavelength lambda ₀ is changed accordingly. Therefore, the FBG distortion ε can be found by measuring the reflection wavelength λ ₀ . The magnitude of the pressure can be known based on the strain ε. The refractive index n varies with the temperature T.

Next, embodiments of the present invention will be described with reference to the drawings.
4A and 4B are explanatory views of the principle of an optical fiber type pressure sensor in the FBG system. FIG. 4A is a side view of an optical fiber having a plurality of FBG sensor units, and FIG. 4B is an internal state of the FBG sensor unit. FIG. FIG. 5 is a block diagram showing an example of the FBG pressure / temperature sensor. FIG. 6 shows the input light incident on the FBG sensor, the reflected light reflected by the diffraction grating of the FBG sensor, and the diffraction grating of the FBG sensor. It is a wave form diagram of transmitted light.

  As shown in FIG. 4A, a plurality of (for example, 6 to 7) FBG sensor units 2 can be provided at arbitrary intervals along the optical axis direction of one optical fiber 1. The optical fiber 1 used in this example has an outer diameter of 0.25 mm including the buffer layer 3b (see FIG. 4B).

  As shown in FIG. 4B, diffraction gratings 4 having different refractive indexes are arranged in parallel in the optical axis direction at regular intervals in the FBG sensor unit 2 as shown in FIG. The FBG sensor unit 2 is formed by specially processing the tip of the optical fiber 1 or an arbitrary part in the middle of the optical fiber 1.

  The input light 5 incident on the FBG sensor unit 2 has a wavelength waveform as shown in FIG. 6A, and a part of the input light 5 is reflected by the diffraction grating 4 of the FBG sensor unit 2 to be reflected light. Return as 6. The reflected light 6 has a wavelength waveform as shown in FIG.

  As shown in FIG. 6C, the transmitted light 7 transmitted through the diffraction grating 4 of the FBG sensor unit 2 is partially reflected from the input light 5 incident on the FBG sensor unit 2 by the FBG sensor unit 2. Thus, the wavelength waveform has a shape that is dropped out, and is then incident on the adjacent FBG sensor unit 2 having a different wavelength as the input light 5 as it is.

  For this reason, the FBG type pressure sensor uses broadband light having a wide frequency band instead of a single wavelength such as laser light. Multiple continuous measurement is possible, which is a major feature of the FBG method.

  As shown in FIG. 5, the FBG sensor unit 2 has a specific configuration in which the optical fiber 1 is housed in a protective tube 8 such as a stainless steel tube, and one opening end of the protective tube 8 is a thin elastically deformable membrane 9. It is blocked by fixing. The tip of the optical fiber 1 is fixed 10 to the inner surface of the membrane 9 with an adhesive or the like, and the portion other than the FBG sensor portion 2 of the optical fiber 1 is fixed 10 to the inner peripheral surface of the protective tube 8 with an adhesive or the like. . The protective tube 8 is made of an electrically insulating tube such as synthetic resin or synthetic rubber.

  A tensile stress is applied to the FBG sensor unit 2 in advance along the optical axis direction. When a pressure 11 is applied to the membrane 9 as shown in FIG. 5, the central portion of the membrane 9 is pressed and deformed to the FBG sensor unit 2 side. Depending on the degree, the FBG sensor unit 2 is distorted, and the wavelength of the reflected light 6 (see FIG. 4B) generated by the FBG sensor unit 2 changes.

  The FPI is an abbreviation for Fabry-Perot Interferometer, and FIG. 7 is a configuration diagram of the FPI sensor unit.

  As shown in FIG. 7, two first semi-transmissive mirrors 12 a and second semi-transmissive mirrors 12 b are opposed to each other with a gap 13 on the optical axis of the optical fiber 1. The first transflective mirror 12a is supported by the propagation optical fiber 1a, and the second transflective mirror 12b is supported by the reflective optical fiber 1b. This FPI sensor part is formed by specially processing the tip of the optical fiber 1.

  White light is incident as input light 5, and the first reflected light 6a reflected by the first semi-transmissive mirror 12a and the second reflected light 6b reflected by the second semi-transmissive mirror 12b are separated for each wavelength. This is a method of separating and interfering.

  The gap 13 (0 to several tens of μm) becomes the cavity length (I), and the propagation optical fiber 1a and the reflection optical fiber 1b are fixed to the protective tube 8 or the membrane 9 with a gauge length (Lg) of 3 to 10 mm inside the sensor. 10 The pressure accompanying the water pressure 11 is measured as the ratio (I / Lg) of the cavity length (I) to the gauge length (Lg). With simple interference, the output appears as a striped pattern, so the pressure can be measured by counting the number of stripes on the striped pattern.

  The optical fiber type pressure sensor in this FPI system can measure with high accuracy of ± 0.1 μm, has excellent decomposition performance and temperature characteristics, and can measure dynamic changes.

  The measurement principle of the moisture sensor using the optical fiber according to the present invention is to measure by expanding or contracting the FBG type or FPI type pressure sensor using the optical fiber described above according to the change in the moisture content in the soil. Therefore, a swelling material is used to effectively expand and contract the sensor part of the moisture sensor.

(Selection of swelling material)
Generally, swelling materials are classified into four types: synthetic rubber, sponge-like rubber, highly water-absorbing and highly hygroscopic fibers (nonwoven fabric), and polymer polymers. FIG. 8 shows the sample No. of each swelling material. FIG. 9 is a table summarizing the results of examining the swelling, shrinkage, durability, workability and water quality dependence of each swelling material.

  As shown in FIG. Regarding the synthetic rubbers 1 to 9, the material shapes are solid (sample Nos. 1 and 2), pastes (samples No. 3 and 4), and liquids (samples No. 5 to 9) made of a urethane stock solution. is there. The polymer shown in FIG. 9 is a material used for paper diapers and the like, and its swelling property is superior to that of other materials. It was inferior to the liquid synthetic rubber of 5-9, and there was a fault that the swelling characteristic strongly depends on the water quality, and it was judged that it was unsuitable for a moisture sensor. Therefore, no polymer is shown in FIG.

  Next, sample No. shown in FIG. The swelling / shrinkage property test was conducted for 1 to 13, and the results are summarized in FIG. In this swelling / shrinkage characteristic test, specimens made of each material were swollen by immersing them in distilled water in an open (unconstrained) state, then dried and shrunk, and the volume change at that time was measured. The volume change was determined by measuring the amount of change with a vernier caliper based on the marker attached to the specimen, and calculating the swelling volume magnification.

  As a result of this swelling / shrinkage property test, sample No. 1 (solid synthetic rubber), 2 (solid synthetic rubber), 3 (paste synthetic rubber), 4 (paste synthetic rubber), 10 (sponge rubber), 12 (sponge rubber) It took 4-8 days or more, and 3-5 days or more in the contraction process. As a sensor for measuring the amount of moisture in the soil, reaction time is a very important factor. Therefore, these materials were judged to be unsuitable as swelling materials for moisture sensors.

  Sample No. 5 (liquid synthetic rubber) and 6 (liquid synthetic rubber) have a swelling volume ratio of 1.02 to 1.05 times smaller than other swelling materials. Sample No. 7 (liquid synthetic rubber) was excluded from the swollen material because the material did not cure. Sample No. 8 (liquid synthetic rubber) had a relatively large swelling and shrinkage time of 1 day and a swelling volume ratio of 3.4 times, but was excluded from the swelling material because the material shrunk from the initial stage in the shrinking process. did.

  Sample No. 9 (liquid synthetic rubber) and 13 (nonwoven fabric) swelled within a few seconds to one day during the swelling process, and contracted within one day during the shrinking process, resulting in a short reaction time. Sample No. 11 (sponge-like rubber) swells in one day and takes two days in the contraction process, but the swelling volume magnification is at least two times higher and gives pressure to the sensitive part of the moisture sensor As suitable. These determination results are described in the determination column of FIG.

  Next, sample no. For the three types of swelling materials 9, 11, and 13, repeated swelling / shrinkage tests were performed, and the results are shown in FIG. In this test method, the measurement time in the swelling / shrinking process was set to 1 day, and swelling / shrinking was repeated 5 times without restraining the specimen, and the change with time in volume change was measured.

  As is apparent from this figure, the sample No. Nos. 11 and 13 showed a tendency for shape reproducibility to be obtained and weight to decrease. As a cause of this, it is considered that the superabsorbent resin in the material is eluted by repeating the swelling and shrinking. In contrast, sample no. No. 9 has a shape reproducibility and was confirmed to be a stable material as a swelling material for the moisture sensor sensing part. Sample No. used in this example. 9 is a urethane-based swellable rubber material such as Adeka Co., Ltd., trade name A-50N. This trade name A-50N is a urethane-based liquid rubber material containing a urethane prepolymer, a plasticizer and toluene diisocyanate.

  The moisture sensor using the optical fiber of the present invention is a moisture sensor that applies the principle of the optical fiber pressure sensor described in FIG. 5 or FIG. For example, there are various effects in terms of soil pressure and durability, such as a moisture sensor in the soil, so that the structure is designed for installation in the soil.

(First embodiment)
FIG. 1 is a cross-sectional view of a moisture sensor according to a first embodiment of the present invention.
As shown in FIG. 1, the lower end opening of a casing 21 made of a cup-shaped metal such as stainless steel or hard synthetic resin has a lid-shaped holding block made of a rigid material such as stainless steel or hard synthetic resin. 22 is occluded. Specifically, the holding block 22 is screwed into the lower end opening of the casing 21. A storage portion 23 having a circular planar shape and having a container shape (concave section) is integrally provided at a substantially central portion of the holding block 22, and the storage portion 23 slightly protrudes from the lower end portion of the casing 21 and has a convex structure. It has become.

A large number of through holes 24 having a hole diameter of, for example, about 1 to 2 mm are formed in the bottom portion and the peripheral wall of the storage portion 23.
On the inner side of the storage portion 23, the sample No. 2 made of urethane-based swelling rubber material is used. Nine swelling materials 25 are filled. The upper opening of the storage section 23 having a concave cross section is closed by a thin and elastically deformable metal membrane 26. The peripheral part of the membrane 26 is fixed to the opening of the storage part 23.

  A pressure deformation of the membrane 26 described later is detected by a pressure sensor 27 housed in the casing 21. A specific configuration of the pressure sensor 27 applied in the present invention will be described later.

  When this moisture sensor is embedded in the soil, moisture 28 in the soil permeates into the inside of the bottom portion of the storage portion 23 and the through hole 24 of the peripheral wall, and the swelling material 25 absorbs it to swell, and the swelling pressure 29 As a result, the center part of the membrane 26 is pressed and deformed so as to rise.

  Then, the magnitude of the pushing force 30 (pressure) of the membrane 26 is measured by the pressure sensor 27, whereby the amount of moisture in the soil can be detected.

  When the amount of moisture in the soil is large, the swelling material 25 absorbs a large amount of moisture and swells greatly. Due to the swelling pressure 29, the pushing force 30 of the membrane 26 is large. On the other hand, when the amount of moisture in the soil is small, the swelling pressure 29 accompanying the water absorption of the swelling material 25 is low, and the pushing force 30 is small.

  FIG. 2 is a characteristic diagram showing the result of a swelling / shrinkage test of a moisture sensor having a pressure generating portion in which a housing portion 23 having an outer diameter of 6 mm and a height of 4.5 mm is filled with a swelling material 25. In this test, the temperature in the temperature-controlled room was kept constant at 23 ° C. and the humidity was 50%, and a moisture sensor was immersed in a container containing distilled water, and the change in swelling pressure with time was observed.

  As a result of this swelling / shrinkage test, it was found that the swelling time was about 10 hours, the swelling pressure was 4.5 MPa, it swelled in a short time, and the swelling pressure was sufficient, so it was suitable as a soil moisture sensor. . Note that the swelling pressure can be adjusted by adjusting the filling amount of the swelling material 25 in the pressure generating portion.

  In the present embodiment, a storage portion 23 is provided at a substantially central portion of the holding block 22, and the storage portion 23 has a convex structure protruding from the bottom of the holding block 22. It is also possible to form a large number of through holes 24 on the bottom of the container-like holding block 22 or on the peripheral wall as necessary.

(Second embodiment)
FIG. 3 is a cross-sectional view of a moisture sensor according to a second embodiment of the present invention. This embodiment differs from the first embodiment in the configuration of the pressure generating unit.

  The holding block 22 attached to the lower end opening of the casing 21 includes a cylindrical body 31 made of a metal such as stainless steel, and a disk-shaped hard porous body 32 that closes the lower end opening of the cylindrical body 31. ing. The porous body 32 is made of porous metal obtained by sintering metal particles of a predetermined size to each other, has continuous pores having a diameter of about 1 to 2 mm, and is fixed to the cylindrical body 31 by a plurality of screws 33. Has been.

  In the space formed by the upper surface of the porous body 32 and the inner peripheral surface of the cylindrical body 31, the sample No. 1 made of a urethane-based swellable rubber material is used. Nine swelling materials 25 are filled. The upper end opening of the holding block 22 (cylinder 31) is closed by a thin and elastically deformable membrane 26, and the periphery of the membrane 26 is fixed to the upper end opening of the holding block 22 (cylinder 31). Yes.

  When this moisture sensor is embedded in the soil, moisture 28 in the soil permeates into the inside of the porous body 32 through the pores, and the swelling material 25 absorbs it and swells. The membrane is pressed and deformed so that the center portion of the membrane 26 is raised.

  The magnitude of the pushing force 30 of the membrane 26 is measured by the pressure sensor 27, whereby the amount of moisture in the soil can be detected.

In this embodiment, a porous body made of a sintered metal is used, but a water-permeable porous body made of a sintered body of inorganic particles can also be used.
In this embodiment, the disk-shaped porous body 32 is used. However, for example, a container-shaped porous body in which a bottom portion and a peripheral wall portion are integrally formed may be used.

FIG. 12 is a cross-sectional view of a moisture sensor showing a first specific example of the pressure sensor 27. In this specific example, an FBG type pressure sensor is used.
As shown in FIG. 12, a displacement-strain converting member 33 made of a metal plate having a substantially U-shaped side surface is arranged in the casing 21, and one end 34 a of the displacement-strain converting member 33 is the center of the membrane 26. The other end 34 b of the displacement-strain converting member 33 is joined to the bottom 35 of the casing 21.

  The FBG pressure sensor unit 2 provided at the tip of the optical fiber 1 is attached in a tensioned state to the side surface of the bridge-like portion 36 protruding to one side at the middle portion of the displacement-strain converting member 33. Yes.

As described above, when the swelling material 25 absorbs the moisture 28 in the soil and swells and the swelling pressure 29 causes the central portion of the membrane 26 to rise, the pushing force 30 is applied to the displacement-strain conversion member 33. By acting, the bridge-like portion 36 of the displacement-strain converting member 33 is curved outward. Since the FBG pressure sensor portion 2 of the optical fiber 1 is attached to the bridge portion 36, the amount of strain can be detected from the degree of curvature of the bridge portion 36.
Note that the relationship between the amount of moisture in the soil and the amount of strain is known in advance, and the amount of moisture in the soil can be known by detecting the amount of strain.

FIG. 13 is a sectional view of a moisture sensor showing a second specific example of the pressure sensor 27. In this specific example, an FBG type pressure sensor is also used.
In the case of this specific example, as shown in FIG. 13, a displacement transmission member 37 made of a metal plate is erected from the center of the membrane 26 toward the bottom 35 of the casing 21. A fiber support member 38 made of a metal plate is suspended from the bottom 35 of the casing 21 toward the displacement transmission member 37, and a gap 39 is formed between the displacement transmission member 37 and the fiber support member 38. ing.

  The tip of the optical fiber 1 is attached to the fiber support member 38 from the displacement transmission member 37, and the FBG pressure sensor 2 provided on the optical fiber 1 is applied with tension at the position of the gap 39. It is erected.

  In the case of this specific example, the central portion of the membrane 26 is swelled by the swelling pressure 29 of the swelling material 25 due to the absorption of moisture 28 in the soil, and the pushing force 30 (pressure) is the FBG pressure of the optical fiber 1 via the displacement transmission member 37. The distortion amount of the membrane 26 can be detected by being transmitted to the sensor unit 2.

  The displacement-strain conversion member 33 and the displacement transmission member 37 in the first and second specific examples have a function of transmitting the strain of the membrane 26 in the optical axis direction of the FBG pressure sensor unit 2.

FIG. 14 is a cross-sectional view of a moisture sensor showing a third specific example of the pressure sensor 27. In this specific example, a strain gauge type pressure sensor is used.
In the case of this specific example, as shown in FIG. 14, a strain gauge 40 made of a resistance bridge is attached to the upper surface of the membrane 26, and the amount of strain of the membrane 26 that changes due to the push-up force 30 can be detected as a voltage change. .

  Since the amount of strain may vary depending on the position of the membrane 26, the strain gauges 40 are attached to a plurality of locations (four locations in the present specific example) to suppress the deviation of the detected values. In the figure, 41 is a relay board, and 42 is a sensor cable.

  In the specific example, the case where an FBG type pressure sensor or an electric pressure sensor is used has been described. However, the present invention uses another optical sensor such as an FPI type, an OTDR type, or a B-OTDR type, or The present invention is also applicable when using a semiconductor pressure sensor having a silicon chip on which a silicon gauge formed by impurity diffusion is formed.

  FIG. 15 shows the measurement of the amount of moisture at each depth by continuously embedding a plurality of FBG-type (seven in the present embodiment) moisture sensor portions 47 a to 47 g in the depth direction of the soil 45. The moisture sensor group which can carry out multiple continuous measurement is shown. In the figure, reference numeral 25 denotes a swelling material loaded in the sensor, and 26 denotes a membrane.

  As shown in FIG. 15, by embedding each of the moisture sensor units 47a to 47g in various directions, it is possible to measure the amount of moisture in all directions of the soil 45.

  Data from each of the moisture sensor units 47a to 47g is input to the measuring device 43 to measure the moisture content in the soil 45, and the obtained measurement data is recorded in the recording device 44 to monitor the moisture content in the soil. Is called.

  If a plurality of moisture sensor portions 47a to 47g are embedded at predetermined intervals in the depth direction of the soil 45 as in the present embodiment, the infiltration state of the precipitation 46 in the soil 45 can be directly grasped. can do.

  FIG. 16 shows a moisture sensor group in which a plurality of moisture sensor units 48a to 48c of the FBG method or FPI method are individually embedded in the soil 45 and the moisture content in the soil 45 is measured. In this case, although the depth of each moisture sensor part 48a-48c is changed and embed | buried in the soil 45, it is also possible to change an embedment position by the same depth, respectively.

  Also in this case, the data of each of the moisture sensors 48a to 48c is input to the measuring device 43 to measure the moisture content in the soil 45, and the obtained measurement data is recorded in the recording device 44 to monitor the moisture content in the soil. Is done.

Each of the moisture sensor units 47 and 48 has the structure shown in FIG.
In addition, each of the moisture sensor units 47 and 48 can form a hole in the soil of the monitoring target region and embed the moisture sensor unit in the hole. In addition, when providing a cushioning material layer around the waste body, when forming the cushioning material layer, the moisture sensor unit is placed in each position and orientation together with clay (bentonite), and the cushioning material with the moisture sensor is provided. Layers can also be formed.

  In the above-described embodiment, the case where the moisture content in the soil is measured has been described. It can also be applied to the detection and measurement of moisture in objects.

FIG. 17 is a partial cross-sectional view of a moisture sensor for measuring the moisture content in concrete according to the third embodiment of the present invention.
A cylindrical holding block 22 having openings at both ends is attached to the lower end opening of the cup-shaped casing 21, and the inside of the intermediate portion in the height direction of the holding block 22 is closed by a membrane 26. Is fixed to the intermediate portion of the holding block 22.

  One of the pressure sensors 27 described in the specific example is mounted inside the casing 21. In the space having a concave cross section formed by the inner peripheral surface of the substantially lower half of the holding block 22 and the lower surface of the membrane 26, the sample No. 1 made of a urethane-based swelling rubber material is used. Nine swelling materials 25 are filled.

  The lower end opening of the holding block 22 filled with the swelling material 25 is brought into contact with the surface 50 of the concrete 49 whose moisture content is to be measured, and the lower end opening is fixed to the concrete surface 50 with an adhesive 51. For this reason, a part of the swelling material 25 is in direct contact with the concrete surface 50.

  The moisture 28 in the concrete 49 is absorbed by the swelling material 25 from the lower end opening of the holding block 22, and the swelling pressure 29 generated accordingly acts on the membrane 26, and the strain amount of the membrane 26 is measured by the pressure sensor 27. The amount of water in the concrete 49 can be detected.

  It should be noted that moisture 28 accumulates in the through holes 24 of the storage portion 23 shown in FIG. 1 or in the pores in the porous body 32 shown in FIG. 3. In this embodiment, the swelling material 25 is applied to the concrete surface 50. Since it is in direct contact, it has the feature of being directly absorbed by the swelling material 25.

1: optical fiber,
2: FBG sensor part,
4: Diffraction grating,
5: Input light,
6: reflected light,
7: transmitted light,
9: membrane,
11: pressure,
12a: first transflective mirror,
12b: second transflective mirror,
13: void,
21: casing,
22: holding block,
23: storage section,
24: through hole,
25: swelling material,
26: Memblem,
27: Pressure sensor,
28: moisture,
29: swelling pressure,
30: Pushing force,
31: cylinder,
32: porous body,
45: In the ground,
47, 48: Moisture sensor unit.

Claims (11)

A swelling material that swells and shrinks depending on water absorption or dryness;
A swelling material holding portion for filling and holding the swelling material;
A membrane that is elastically deformed by a swelling pressure due to water absorption of the swelling material filled in the swelling material holding portion;
A moisture sensor comprising a pressure sensor for detecting a distortion amount of the membrane. The moisture sensor according to claim 1,
The pressure sensor has an optical fiber provided with a sensor unit for detecting the amount of strain of the membrane;
The moisture sensor is configured so that strain caused by elastic deformation of the membrane acts in an optical axis direction of the optical fiber. The moisture sensor according to claim 2,
The swelling material holding portion has a concave-shaped storage section, and a large number of through holes are formed in the bottom or (and) the peripheral wall of the storage section,
The storage portion is filled with the swelling material, and the opening of the storage portion is closed by the membrane,
A moisture sensor, wherein an opening of a casing housing the pressure sensor is closed by the swelling material and the swelling material holding portion with the membrane. The moisture sensor according to claim 2,
The swelling material holding part is composed of a cylindrical body and a porous body having continuous pores that block one opening of the cylindrical body,
The cylindrical body is filled with the swelling material, the other opening of the cylindrical body is closed by the membrane,
A moisture sensor, wherein an opening of a casing housing the pressure sensor is closed by the swelling material and the swelling material holding portion with the membrane. The moisture sensor according to claim 2,
The swelling material holding part has a cylindrical shape having openings at both ends,
One opening of the swelling material holding part is closed by the membrane,
The opening of the casing housing the pressure sensor is closed by the swelling material and the swelling material holding portion with the membrane,
The swelling material is filled in a space portion having a concave cross-sectional shape formed by the peripheral wall of the swelling material holding portion and the membrane,
Moisture characterized in that a part of the swelling material is brought into direct contact with the surface of the object to be measured by bringing the other opening of the swelling material holding part into contact with the surface of the object to be measured containing water Sensor. The moisture sensor according to any one of claims 3 to 5,
The casing has a cup shape, and an opening of the casing is closed by the swelling material and the swelling material holding portion with the membrane,
The pressure sensor is
A displacement-strain conversion member having a substantially U-shaped side surface, one end of the displacement-strain conversion member attached to the membrane, and the other end of the displacement-strain conversion member attached to the bottom of the casing; ,
The moisture sensor, wherein the sensor part of the optical fiber is affixed to a side surface of a bridge-like part protruding to one side at an intermediate part of the displacement-strain converting member. The moisture sensor according to any one of claims 3 to 5,
The casing has a cup shape, and an opening of the casing is closed by the swelling material and the swelling material holding portion with the membrane,
The pressure sensor is
A displacement transmission member attached from the membrane toward the bottom of the casing;
A fiber support member attached from the bottom of the casing toward the displacement transmission member;
It is comprised from the optical fiber fixed on the said fiber support member from the said displacement transmission member so that the said sensor part may be arrange | positioned in the clearance gap formed between the said displacement transmission member and the said fiber support member. Moisture sensor characterized by The moisture sensor according to any one of claims 2 to 7,
The moisture sensor according to claim 1, wherein the sensor unit has a configuration in which diffraction gratings having different refractive indexes are arranged in parallel at regular intervals on the optical axis of the core of the optical fiber.   A moisture sensor, wherein a plurality of sensor units according to claim 8 can be attached to an optical fiber at an arbitrary interval. The moisture sensor according to any one of claims 2 to 7,
The sensor unit includes a first semi-transmissive mirror and a second semi-transmissive mirror that are opposed to each other with a gap on the optical axis of the core of the optical fiber, and reflected by the first semi-transmissive mirror. A moisture sensor, wherein the reflected light of 1 and the second reflected light reflected by the second semi-transmissive mirror are separated for each wavelength and interfere with each other. The moisture sensor according to any one of claims 1 to 10,
A moisture sensor, wherein the swelling material is a urethane-based swelling rubber material.

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