JPH10206255A - Hydraulic sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sensor capable of monitoring hydraulic pressure without providing measuring data transmitting devices on individual sensors. SOLUTION: This sensor is provided with a bellows 5 expanded and contracted according to hydraulic pressure and a movable plate 21 interlocked with the bellows 5. Also, the halfway part of an optical fiber 4 is fixed to a fiber fixture 13 and further held by the movable plate 21. When the bellows 5 is expanded and contracted according to hydraulic variation and the movable plate 21 is interlocked with the bellows 5, partial tension is applied to the optical fiber 4 between the fiber fixture 13 and the movable plate. Thus, the hydraulic pressure can be detected by measuring the strain of the optical fiber 4 accompanying the application of the tension.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor for measuring water pressure, and more particularly to a sensor for monitoring water pressure in soil such as in a river embankment and a method for detecting water pressure.

[0002]

2. Description of the Related Art It is considered that the embankment break of a river largely depends on the degree of water infiltration into the embankment, and the degree of infiltration is to measure the water pressure (water level in the soil) in the embankment. Can be monitored by Conventionally, an electric pressure gauge has been known as a technique for measuring this type of water pressure.

However, this type of conventional sensor needs to be provided with a transmission device such as a wireless device in order to collect measurement data obtained at a plurality of installation locations. In this case, a power supply for the transmission device is also required. On the other hand, when there is no transmission device, the measurement data of each sensor must be collected directly, which is time and labor intensive.

Accordingly, an object of the present invention is to provide a sensor capable of monitoring water pressure without installing a transmission device in each sensor and a method for detecting water pressure.

[0005]

The sensor of the present invention has been made to achieve the above-mentioned object, and converts the movement of a bellows which expands and contracts with a change in water pressure into a change in the tension of an optical fiber. The water pressure is detected from a change in the generated distortion.

That is, an optical fiber, a bellows which expands and contracts with the water pressure, and a tension applying portion provided in the middle of the optical fiber and having a movable end and a fixed end, wherein the movable end of the tension applying portion is provided. It is characterized by being linked to the expansion and contraction of the bellows.

The specific configuration of the tension applying section is as follows. (1) The optical fiber includes a bent portion bent in the middle of the optical fiber, a fixed end for holding the bent portion, and a movable end for holding the optical fiber at a position away from the fixed end. Further, a constant pulley and a weight are provided, and the movable end and the weight are connected by a wire hooked on the constant pulley.

[0008] (2) The optical fiber is wound around both hooking portions, with the movable end being a moving hooking portion interlocking with the expansion and contraction of the bellows, and the fixed end being a fixed hooking portion not interlocking with the expansion and contraction of the bellows. As a specific configuration of each engaging portion, a disk-shaped reel or the like can be given. In this case, it is preferable that the interval between the movable engagement portion and the fixed engagement portion is configured to be adjustable. Thereby, the tension of the optical fiber is adjusted, and it is possible to prevent an excessive tension from being applied to the optical fiber at the time of initial setting, or to prevent the optical fiber from being loosened to generate a dead zone for pressure detection.

In any of the above arrangements, it is preferable to provide a case for accommodating the tension applying section, and to provide a ventilation hole in this case. The inside of the case can be kept equal to the atmospheric pressure by the ventilation hole.

It is desirable to provide a case for accommodating the tension applying section, and to provide a moisture-proof mechanism in the middle of the process until the optical fiber is accommodated in the case. If outside air is introduced into the case and dew condensation is repeated due to a temperature change, water may accumulate in the case. Therefore, if a moisture proof mechanism is provided, it is possible to prevent inactivity caused by accumulation of water in the case.

Further, in the water pressure detecting method according to the present invention, the movement of the bellows which expands and contracts in response to a change in the water pressure and the movement of a tension applying section for applying tension to the optical fiber are linked, and the optical fiber associated with the tension is applied. Water pressure is detected by monitoring distortion.

The detection of the water pressure may be performed by detecting a strain that changes with the tension applied to the optical fiber. Therefore, when monitoring the water pressure, a strain measuring device is connected to the end of the optical fiber. For example, BOTDR
Is connected, the strain distribution in the longitudinal direction of the optical fiber is measured, and the water pressure is detected from the increase in strain.

If a plurality of the above-mentioned sensors are formed in the middle of a series of optical fibers and the strain of the optical fibers is measured, measurement data at a plurality of locations can be easily aggregated without installing a transmission device in each sensor. it can.

It is desirable to provide a protective cover around the bellows to prevent intrusion of earth and sand. For example, a part of the wall surface of the protective cover is formed of a net so that water can enter the protective cover but earth and sand cannot enter. For forming this protective cover, various materials having water permeability but preventing permeation of earth and sand can be used in addition to nets and cloths.

In this case, it is more preferable to provide an air vent hole in the protective cover. Accordingly, when the sensor of the present invention is buried in the ground, air bubbles remaining in the protective cover can be discharged to smoothly expand and contract the bellows. It is preferable to reduce the size of the hole or to provide a net so that the sand does not enter the air vent hole.

[0016]

FIG. 1 is a block diagram showing an embodiment of the present invention.
It will be described based on. FIG. 3 is an explanatory view showing the principle of the sensor of the present invention, and shows a state in which the sensor is installed in the embankment. The case 30 of the water pressure sensor of the present invention is buried in the embankment body 31, and the ground and the inside of the case communicate with each other through the vent hole 32 so that the pressure in the case is equal to the atmospheric pressure. The case 30 has a water passage hole 33 on the lower surface, and a bellows 34 is fixed to the inner wall, and the water passage hole 33 is provided.
From 33, water can be introduced into the space surrounded by the bellows outer circumference and the case inner wall. At the lower end of the bellows 34, a pressure receiving plate 35 for receiving water pressure is integrated.
A pressure transmission rod 36 extends upward from 35. On the other hand, an optical fiber 37 is attached in this case. The lower end of the optical fiber 37 is fixed by a holder 38, and the upper end is integrated with the upper end of the pressure transmitting rod 36 via a connecting member 39. Therefore, when the water pressure (water level in the soil) in the embankment rises, the pressure receiving plate 35 of the bellows is pressed and the pressure transmitting rod 36
, And the tension is applied to the optical fiber 37 accordingly.

Here, the water level in the soil is h from the position of the pressure receiving plate.
(M), the force P (kg) applied to the pressure receiving plate is represented by P = Sh / 10, where S (cm ² ) is the effective area of the pressure receiving plate.

On the other hand, since the relationship between the tension applied to the optical fiber and the strain is proportional, if the strain is ε (μ) and the proportional constant is k (kg / μ), it can be obtained by P = kε. . Therefore, BO
If you connect a strain measurement device such as TDR,
From the equation, the soil water level h can be determined from the measured strain by the following equation. h = (10k / S) ε

[0019]

An embodiment will be described with reference to FIG. FIG. 3 is a front view showing the internal structure of the sensor of the present invention. The sensor of the present invention includes an upper protection tube 1, a lower protection tube 2, and a protection cover 3, and a tension applying portion including an optical fiber 4 and the like is stored in the protection tubes 1 and 2, and a bellows 5 is stored in the protection cover 3. ing. Note that the two protection cylinders 1 and 2 correspond to a case.

The upper protection cylinder 1 and the lower protection cylinder 2 are cylindrical containers connected via a partition plate 6. The upper protective cylinder 1 has a lid 8 into which an optical cable 7 is introduced, and a bellows 5 is integrated below a lower protective cylinder 2 via a mounting portion 9. The space in the longitudinal direction from the upper protection cylinder 1 to the bellows 5 is sealed.

The optical fiber 4 is exposed from the optical cable 7 introduced from the lid 8. The exposed optical fiber 4 penetrates through the partition plate 6 and is introduced into the lower protective tube 2, returns, passes through the partition plate 6 again, and returns to the optical cable 7. Here, the optical fiber 4 was built in the stainless steel tube 10 except for the portion immediately after being exposed from the optical cable 7.
A filler is injected between the optical fiber 4 and the stainless steel tube 10, and the optical fiber 4 and the stainless steel tube 10 are integrated. Therefore, if tension is applied to the stainless tube 10, tension is applied to the optical fiber 4. In this embodiment, the stainless steel tube 10 is used, but such a protective tube may be used as needed to protect the optical fiber 4. The material and the necessity of the protective tube are not particularly limited. In addition, an appropriate material such as resin or rubber may be selected as the filler.

A support bar 11 is fixed to the lower surface of the partition plate 6. A fixing plate 12 is attached to a lower end of the support rod 11, and a fiber fixture 13 (fixed end) for stopping the optical fiber 4 stored in the stainless steel tube 10 is attached in the middle of the support rod 11. Since the fixing plate 12 and the fiber fixing device 13 are connected to the partition plate 6 via the support rod 11, the optical fiber 4 is bent in a U-shape (bent portion) in the lower protective tube 2 and its bending diameter is reduced. It is kept constant.

On the other hand, a hollow cylinder 14 is fixed inside the bellows 5, and a pressure transmitting rod 20
Is growing. The pressure transmission rod 20 includes the fixed plate 12 and the partition plate 6.
And reaches the inside of the upper protection cylinder 1. The outer circumference of the bellows 5 is covered with a cylindrical protective cover 3.
The bottom surface of the protective cover 3 is constituted by a net 15, and a plurality of air vent holes 16 are formed on the upper surface. By forming the bottom portion with the net 15, water can enter the protective cover 3, but can prevent earth and sand from entering the protective cover 3, thereby suppressing damage to the bellows 5. The air vent hole 16 is a hole for discharging air remaining in the protective cover 3 when the present sensor is buried in the underground of a bank or the like. Air vent holes for easy removal of remaining air bubbles
It is desirable that the surface on which the 16 is formed is inclined.

Further, a pulley 17 is fixed to the lower surface of the lid 8, and one end of a wire 18 hung on the pulley 17 is a weight.
At 19, the other end is connected to a pressure transmission rod 20 extending from the hollow cylinder 14. The weight 19 is a counterweight for the expansion and contraction of the bellows 5. And the upper protection cylinder 1
A movable plate 21 (movable end) is fixed to the upper end of the transmission rod 20 in the inside, and the optical fiber 4 containing a stainless steel tube is fixed to the movable plate 21. That is, the transmission rod 20
Is moved up and down, the movable plate 21 is also interlocked, and accordingly, the optical fiber 4 is pulled between the fiber fixture 13 and the movable plate 21 to be tensioned (tension applying part).

When measuring the water pressure, a plurality of the above sensors are formed in the middle of a series of optical fibers, and each sensor is arranged at a predetermined measurement place, for example, in the ground of a dike. Then, a strain measuring device (not shown) is connected to the end of the optical fiber. For example, BOTDR (Brillouin Optical Time Domain)
Reflectometer) device may be used. In this method, a light pulse is incident on an optical fiber, and the generation wavelength of Brillouin scattered light in the backscattered light is measured to detect distortion. Then, the position where the Brillouin scattered light of a certain wavelength occurs is specified by the time from when the light pulse is incident to when the backscattered light returns to the incident end. Therefore, data of Brillouin scattered light along the longitudinal direction of the optical fiber can be obtained, and a change in strain due to a change in tension of the optical fiber can be detected.

The sensor buried underground has a protective cover 3
Water penetrates through the net or air vent hole, and the space between the protective cover 3 and the bellows 5 is filled with water. When the water pressure rises, the bellows 5 is pressed and contracts upward. At this time, the transmission rod 20 is also pushed up, and the movable plate 21 is pushed up at the same time. As the movable plate 21 moves, the optical fiber 4 is pulled between the fiber fixture 13 and the movable plate 21 to be tensioned. Therefore, the water pressure can be detected by detecting the distortion of the optical fiber 4 caused by the application of the tension by the distortion measuring device. Since the measurement results can be collectively monitored at the installation location of the strain measurement device, there is no need to provide a measurement data transmission device or a power supply for each sensor.

Next, an embodiment having a configuration different from that of FIG. 2 will be described with reference to FIGS. FIG. 3 is a front view showing the internal structure of the embodiment, and FIG. 4 is a partial cross-sectional view showing the configuration of the pressure applying section viewed from the side. In the present embodiment, a moving reel 42 (moving engaging portion) and a fixed reel 43 (fixing engaging portion) are used as tension applying portions for applying tension to the optical fiber 41 in conjunction with the movement of the bellows 40.

The case 44 is a metal cylinder having corrosion resistance (see FIG. 3). A lid with a vent hole 45 at its upper end
46 (made of corrosion-resistant metal) is screwed in a watertight manner, a bottom plate 47 is fitted in a watertight manner on the lower end, and a protective cover 48 is fitted in the outside.
It is fixed by screws 49. The ventilation hole 45 communicates with the ground when the sensor is buried, and keeps the inside of the case almost equal to the atmospheric pressure. The protective cover 48 is for preventing foreign substances such as earth and sand from entering the bellows and damaging them. A single water passage hole 50 is formed in the bottom plate 47 of the case, and a plurality of small holes 51 that allow water to pass therethrough but prevent foreign substances from entering are formed in the bottom of the protective cover 48.

A bellows 40 is fixed to the upper surface of the case bottom plate 47 in a watertight manner. A pressure receiving plate 52 is attached to the upper surface of the bellows 40 in a watertight manner. The water that has entered through the small holes 51 and the water holes 50 is introduced into the bellows to expand and contract the bellows 40 and move the pressure receiving plate 52 up and down.

At approximately the center of the pressure receiving plate 52, a pressure transmitting rod 53 (pressure transmitting portion) extending upward is fixed. The connection between the pressure receiving plate 52 and the rod 53 may be performed by welding or screwing. The pressure transmission rod 53 is a metal rod with an external thread formed at the upper end, and moves the pressure receiving plate 52 up and down.
Communicate to 42.

The movable reel 42 is held at the upper part in the case via a spring 54 (elastic material) (see FIG. 4). That is, the support plate 55 is fixed to the inner wall of the case, and the spring 54 is installed on the support plate. Then, a spring receiver 56 was arranged above the spring 54, and the movable reel 42 was fixed to the spring receiver 56. On the other hand, the fixed reel 43 is fixed to a support plate 57 fixed to the inner wall of the case between the moving reel 42 and the bellows 40. Each of the reels 42, 43, the support plate 55, and the spring receiver 56 has a coaxial through-hole, and the through-hole and the spring 54 pass through the pressure transmitting rod 53. Therefore, the moving reel
42 can move in the axial direction of the pressure transmitting rod 53 by the elastic force of the spring 54. A spiral groove 58 for winding an optical fiber was formed on the outer peripheral surface of each of the reels 42 and 43. Accordingly, it is possible to prevent the optical fibers from being overlapped and wound and causing local bending strain to be applied.

Here, as shown in FIG. 3, a nut 59 (stop) is screwed onto the upper end of the pressure transmitting rod 53 to regulate the distance that the movable reel 42 can move. Since the position of the nut 59 can be changed according to the degree of screwing into the pressure transmission rod 53, the moving distance of the moving reel 42 can be easily changed.

When performing such initial setting of the water pressure sensor, first, the nut 54 is screwed to compress the spring 54 (for example, about 30 to 50 mm), and the movable reel 42 and the fixed reel 4 are compressed.
Keep the distance to 3 narrow. In this state, the optical fiber 41 is wound along the spiral groove 58 of each of the reels 42, 43, and the optical fiber 41 is wound around the reels 42, 43. When the nut 59 is loosened, the moving distance of the moving reel 42 increases by the distance the nut has moved, and the pressing force of the spring 54 causes the optical fiber 41 to have an appropriate elongation distortion (for example, 0.1 to 0.2).
%). At this time, the optical fiber
The position of the nut 59 is adjusted so that excessive tension is not applied to 41, and conversely, it does not loosen to generate a dead zone for detection.

The optical fiber 41 may be incorporated in a stainless steel tube or the like, or wound around both the reels 42 and 43 in a state of being combined with a metal wire. Has little effect on measurement results. Although not shown, the end of the optical fiber 41 may be guided to the ground through a ventilation hole 45 and connected to a BOTDR device. At this time, it is desirable that the optical fiber passing through the ventilation hole 45 be incorporated in the waterproof pipe.

With the above-mentioned initial settings, the present sensor is embedded in a monitoring target such as a dike. When the soil water level of the embankment rises with the rise of the river water level, the water that has entered through the small holes 51 and the water holes 50 causes the bellows 40 to extend upward. Accordingly, the pressure receiving plate 52 and the pressure transmitting rod 53 (nut 59) are pushed up. Accordingly, the range in which the movable reel 42 can move upward is widened, and the movable reel 42 is pressed upward by the elastic force of the spring 54, so that the distance between the movable reel 42 and the fixed reel 43 is widened, and the tension according to the water pressure (water level in the soil) is increased. Is given to Therefore, if this elongation strain is detected by BOTDR, monitoring of water pressure can be performed.

An example of calculation of the monitoring accuracy of the sensor of the present invention is as follows. Generally, the breaking load of optical fiber is about 6k
g, elongation is about 6%. Therefore, assuming that a tension of up to 3 kg is applied to one optical fiber and the measured water level is 10 m, the water pressure becomes 1 kg / cm ² , and to generate a tension of 6 kg for two optical fibers, 6 cm ² A pressure receiving plate area is required. In addition, since the accuracy of BOTDR is about 0.02%, the detection accuracy of the soil water level at this time is about 7 cm (3% distortion at a water level of 10 m).

In the embodiment shown in FIGS.
The tension of the optical fiber can be adjusted by adjusting the distance between the reel and the fixed reel 43.
When the weight of 53 is light, the moving reel 42 may be simply fixed to the pressure transmitting rod 53 so that the moving reel 42 moves up and down together with the pressure transmitting rod 53. In that case, the spring 54 and the spring receiver 56 may not be provided.

Further, the moistureproof mechanism provided for each of the above sensors will be described. First, an example in which a moisture-proof mechanism is provided in the sensor of FIG. 2 will be described. FIG. 5 is an explanatory view showing a state in which the sensor of FIG. 2 is provided with a moisture-proof mechanism and is embedded, and FIG. 6 is a detailed explanatory view of the inside of the closure of FIG.

A trunk optical cable 60 is embedded in the embankment,
The closure 61 formed in the middle is stored in the manhole 62 (FIG. 5). The branch optical fiber 63 is extended from the closure 61, and a sensor is provided at the tip of the branch optical fiber 63.
Connect 64.

As shown in FIG. 6, the internal structure of the closure 61 is such that an optical fiber core 66 is pulled out of the trunk optical cable 60 via a fusion splicing part 65 and this core 66 is introduced into a telescopic container 67 from one end. I do. The telescopic container 67 can expand and contract like a bellows, and has dry air sealed therein. The introduced optical fiber core wire 66 is pulled out from the other end of the telescopic container 67 as a branch optical cable 63. The inlet of the optical fiber core wire 66 and the outlet of the branch optical cable 63 in the telescopic container 67 have a water-and-honey structure. And this branch optical cable 63
Is pulled out of the closure 61 and guided to the sensor 64.

By terminating the branch optical cable 63 in the telescopic container 67 in which dry air is sealed, moisture can be prevented from being introduced into the sensor. For this reason, dew condensation is repeated in the sensor in accordance with the temperature change, and water accumulates. As a result, erroneous operation or malfunction can be suppressed.

Next, a case where a moisture proof mechanism is provided in the sensor shown in FIGS. FIG. 7 is an explanatory diagram showing a state in which the sensor of FIGS. In this case, as in the example of FIG. 5, the branch optical cable 73 is pulled out from the closure 72 of the trunk optical cable 71 in the manhole 70, and the branch optical cable 73 is introduced into the sensor 74. In this example, the middle of the branch optical cable 73 is drawn into another manhole 75, and further introduced into a container 76 in this manhole 75. A cable protection tube 77 is connected to the bottom of the container 76, and the protection tube 77 is connected to a vent of the sensor 74. The branch optical cable 73 introduced into the container 76 is guided to the sensor 74 through the inside of the protective tube. The inlet of the branch optical cable 76 in the container 76 has a honey structure.

Here, a desiccant such as silica gel is put in the container 76. Although outside air is introduced into the sensor 74 through the protective tube 77, the outside air is taken in through the container 76 containing the moisture absorbent, so that introduction of moisture-containing air into the sensor 74 can be suppressed. By opening the cover of the manhole 75, replacement and inspection of the moisture absorbent can be performed.

As described above with reference to FIGS.
By suppressing the introduction of moisture-containing air into the case of the sensor, it is possible to prevent the sensor from malfunctioning or not operating due to the accumulation of water in the case due to condensation.

[0045]

As described above, according to the sensor and the method of the present invention, it is possible to easily measure the water pressure at a plurality of locations by using a series of optical fibers. In this case, there is no need to provide a measurement data transmission device for each sensor. Further, since the sensor of the present invention does not apply local strain to the optical fiber but performs detection based on elongational strain, damage to the optical fiber can be reduced. Further, by providing the moisture proof mechanism, it is possible to prevent the sensor from malfunctioning or not operating due to the accumulation of water in the case due to dew condensation.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing the basic principle of the sensor of the present invention.

FIG. 2 is a front view showing the internal structure of the sensor of the present invention.

FIG. 3 is a front view showing the internal structure of the sensor of the present invention having a configuration different from that of FIG. 2;

FIG. 4 is a sectional view showing a tension applying section of the sensor of FIG. 3;

FIG. 5 is an explanatory diagram showing a state in which the sensor of FIG. 2 is provided with a moisture-proof mechanism and is embedded.

FIG. 6 is a detailed explanatory view of the inside of the closure of FIG. 5;

FIG. 7 is an explanatory view showing a state in which the sensors of FIGS.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Upper protective cylinder 2 Lower protective cylinder 3,48 Protective cover 4,37,41 Optical fiber 5,34,40 Bellows 6 Partition plate 7 Optical cable 8,46 Lid 9 Mounting part 10 Stainless steel tube 11 Support rod 12 Fixing plate 13 Fiber Fixture 14 Hollow cylinder 15 Net 16 Vent hole 17 Pulley 18 Wire 19 Weight 20,36,53 Pressure transmission rod
21 Movable plate 30,44 Case 31 Embankment 32,45 Vent 33,50
Water holes 35,52 Pressure receiving plate 38 Holder 39 Connector 42 Moving reel 43 Fixed reel 47 Bottom plate 49 Screw 51 Small hole 54 Spring 55,5
7 Support plate 56 Spring receiver 58 Spiral groove 59 Nut 60,71 Main optical cable 61,72 Closure 62,70 Manhole 63,73 Branch optical cable 64,74 Sensor 65 Connection 66 Optical fiber core 67
Telescopic container 75 Manhole 76 Container 77 Cable protection tube

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Yuji Nakura 1-3-1 Shimaya, Konohana-ku, Osaka-shi Sumitomo Electric Industries, Ltd. Osaka Works

Claims (10)

[Claims] An optical fiber, a bellows that expands and contracts with water pressure, and a tension applying portion provided in the middle of the optical fiber and having a movable end and a fixed end, wherein the movable end of the tension applying portion is a bellows. A water pressure sensor characterized by being linked to the expansion and contraction of the water. 2. A tension applying section comprising: a bent portion bent in the middle of an optical fiber; a fixed end for holding the bent portion; a movable end for holding the optical fiber at a position away from the fixed end; The water pressure sensor according to claim 1, comprising a weight, a wire hung on a constant pulley, one end connected to the movable end, and the other end connected to the weight. 3. An optical fiber is wound around both hooking portions, with the fixed hooking portion not linked to the expansion and contraction of the bellows as a fixed end, and the movable hooking portion linked to the expansion and contraction of the bellows as a movable end. The water pressure sensor according to claim 1, wherein: 4. A pressure transmitting portion which is linked with the expansion and contraction of the bellows, an elastic member which presses the moving engaging portion in a direction away from the fixed engaging portion, and a stopper which can be adjusted in position with respect to the pressure transmitting portion. The engaging portion is configured to be movable along the pressure transmitting portion,
The range in which the movable engaging portion moves is restricted by the position of the stop, and the tension of the optical fiber is made adjustable by changing the position of the stop to change the distance between the movable reel and the fixed reel. Item 4. A water pressure sensor according to item 3. 5. The water pressure sensor according to claim 1, further comprising a case for accommodating the tension applying section, wherein the case is provided with a vent hole. 6. The water pressure sensor according to claim 1, further comprising a case for accommodating the tension applying section, and a moisture proof mechanism provided midway before introducing the optical fiber into the case. 7. The water pressure sensor according to claim 1, wherein a strain measuring device is connected to an end of the optical fiber. 8. The water pressure sensor according to claim 1, wherein a protective cover is provided around the bellows to prevent earth and sand from entering. 9. The water pressure sensor according to claim 7, wherein an air vent hole is provided in the protective cover. 10. The movement of the bellows, which expands and contracts in response to a change in water pressure, and the movement of a tension applying section that applies tension to the optical fiber are interlocked with each other, and the distortion of the optical fiber caused by the application of the tension is monitored. A water pressure detection method, characterized by detecting a water pressure.