CN107091668B

CN107091668B - Hydraulic pressure measuring unit and liquid level meter

Info

Publication number: CN107091668B
Application number: CN201610718599.1A
Authority: CN
Inventors: 山田严
Original assignee: Musasino Co Ltd
Current assignee: Musasino Co Ltd
Priority date: 2016-02-17
Filing date: 2016-08-24
Publication date: 2020-03-24
Anticipated expiration: 2036-08-24
Also published as: JP5918458B1; CN107091668A; JP2017146206A; KR20170096927A; KR102128518B1

Abstract

The invention provides a hydraulic measurement unit and a liquid level meter using the same, which can avoid liquid aeration as a measurement object, prevent the proliferation of bacteria, and do not suffer from the influence of a dead space and do not need to be corrected. A hydraulic pressure measurement unit includes: a diaphragm (5) having one surface side serving as a pressure receiving surface for receiving the pressure of the liquid to be measured and the other surface side serving as an air pressure receiving surface for the air chamber (6); a compressed air supply port having an opening in the air chamber (6); and an exhaust port, an open end (71) of which is opposed to the diaphragm (5) in the air chamber (6). The open end (71) of the exhaust port and the diaphragm (5) are lip-shaped structures, and the open end (71) is closed by the diaphragm (5) by balancing the pressure applied to the pressure receiving surface and the air pressure applied to the air chamber (6).

Description

Hydraulic pressure measuring unit and liquid level meter

Technical Field

The present invention relates to a hydraulic pressure measurement unit and a liquid level meter using the same.

Background

In a container ship or a tanker ship, a level gauge for detecting the level of stored liquid is provided for each ballast tank or cargo tank. Bubble type level gauges, one of the measurement modes of such level gauges. The bubble type liquid level gauge has a structure in which an air supply pipe is provided in a liquid chamber from the bottom to the upper end thereof, compressed air is supplied to the air supply pipe submerged in liquid stored in the chamber, and bubbles are discharged from the lower end of the air supply pipe. The principle of the bubble type liquid level meter is as follows: since the air pressure in the intake pipe changes depending on the liquid level of the liquid stored in the chamber, that is, the height of the liquid surface can be measured by measuring the air pressure.

Since the bubble type liquid level meter generally used at present must continuously supply compressed air from a compressed air supply source to an air supply pipe, there are problems that the consumption of compressed air is large, and the display value is not stable enough due to pulsation caused by discharging compressed air. Therefore, the present applicant has proposed a bubble level gauge comprising: a 1 st electromagnetic valve for opening and closing a compressed air supply path from a compressed air supply source to an air supply pipe; and a 2 nd solenoid valve that opens and closes an air pressure detection path from the air supply pipe to the pressure sensor. The bubble type liquid level meters described in patent documents 1 and 2 are the technical means.

According to the bubble type liquid level meters described in patent documents 1 and 2, the opening and closing of the 1 st electromagnetic valve and the 2 nd electromagnetic valve are controlled to intermittently supply compressed air and detect the air pressure in the air supply pipe. Therefore, according to the bubble type liquid level meters described in patent documents 1 and 2, the consumption amount of compressed air can be reduced, pulsation of compressed air and fluctuation of display values can be eliminated, and stable display can be realized.

Documents of the prior art

Patent document

Patent document 1: japanese patent No. 3992153

Patent document 2: japanese patent No. 3983240

When the bubble type level meter is used for detecting the level of ballast water in a ship, oxygen is supplied to the water in the cabin by the bubbles, and bacteria are easily activated in the water to grow. In particular, in a ship which travels across countries, if bacteria mixed in air adopted in a certain country are proliferated in ballast water, and the ballast water in which the bacteria are proliferated is discharged to another country, bacteria may be proliferated in another country.

In order to prevent the generation of bacteria by the bubble type level gauge, it is preferable that the liquid in the chamber as the measurement object of the liquid level is not brought into contact with the compressed air for measurement. Therefore, a level gauge using a measurement unit having the following structure is known: in this measurement cell, a diaphragm is used, one surface of the diaphragm serves as a pressure receiving surface for receiving the pressure of a liquid to be measured, and the other surface of the diaphragm faces an airtight chamber. Specifically, the structure is as follows: air is continuously supplied to the airtight chamber at a constant flow rate, and the air pressure in the airtight chamber is measured. This is an air-purged level gauge. Since the air pressure in the airtight chamber changes with the difference in the pressure applied to the pressure receiving surface of the diaphragm, the liquid pressure can be obtained by measuring the air pressure in the airtight chamber, and the liquid level of the liquid can be obtained.

Disclosure of Invention

Problems to be solved by the invention

According to the hydraulic pressure measurement unit using the diaphragm, since the pressure receiving surface receiving the pressure of the liquid to be measured and the compressed air are separated by the diaphragm, it is possible to avoid the problem of the increase of bacteria due to the aeration of the ballast water or the like. But there is a dead space at the diaphragm, i.e. the area where hysteresis occurs. The "hysteresis" is a phenomenon in which an output value corresponding to an input value differs depending on the direction of increase and the direction of decrease of the input value. Therefore, the conventional hydraulic pressure measuring unit using the diaphragm has a problem that the measured value is uneven. Therefore, when the diaphragm is used in a hydraulic pressure measurement unit, it is necessary to prevent the measurement value from being affected by the dead space.

In order to prevent the liquid in the chamber from being increased by bacteria by the liquid level meter, it is conceivable to use an electric pressure gauge. However, the use of an electric manometer requires periodic calibration, which causes a problem of troublesome calibration for the user.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a hydraulic measurement unit and a level gauge using the same, which are capable of preventing the aeration of liquid as a measurement target to prevent the proliferation of bacteria, and which are not affected by dead space and do not require calibration.

Means for solving the problems

The invention provides a hydraulic pressure measuring unit, characterized in that,

the hydraulic pressure measurement unit includes:

a diaphragm having one surface side serving as a pressure receiving surface for receiving a pressure of a liquid to be measured and the other surface side serving as an air pressure receiving surface facing the air chamber,

a compressed air supply port having an opening in the air chamber, an

An exhaust port having an open end opposite the diaphragm within the air chamber,

the open end of the exhaust port and the diaphragm are lip-shaped structures, and the open end is closed by the diaphragm by equalizing pressure applied to the pressure-receiving surface and air pressure applied to the air chamber.

The invention relates to a liquid level meter, which is characterized in that,

the liquid level meter has:

the hydraulic pressure measuring unit is provided with a hydraulic pressure measuring unit,

a compressed air supply source that supplies compressed air having a pressure higher than a pressure applied to a pressure receiving surface of the hydraulic pressure measuring unit that receives the hydraulic pressure to an air chamber of the hydraulic pressure measuring unit via a compressed air supply port of the hydraulic pressure measuring unit,

an air pressure measuring instrument connected to the compressed air supply port for measuring air pressure in the air chamber, an

A switching valve selectively connecting the compressed air supply port to the compressed air supply source and the air pressure gauge,

the switching valve is connected to the air pressure gauge after temporarily connecting the compressed air supply port to the compressed air supply source,

and the air pressure measuring instrument calculates the liquid level according to the measured value after the measured value is reduced and stabilized.

ADVANTAGEOUS EFFECTS OF INVENTION

Compressed air is temporarily supplied from a compressed air supply port to an air chamber so that the air pressure applied to an air pressure receiving surface of a diaphragm is greater than the hydraulic pressure applied to the pressure receiving surface of the diaphragm which receives the pressure of a liquid to be measured. During the period when the air pressure is greater than the hydraulic pressure, air is discharged from the air discharge port, and the air pressure applied to the air pressure receiving surface gradually decreases. When the air pressure becomes equal to or slightly less than the hydraulic pressure, the diaphragm closes the exhaust port, the air pressure stops decreasing, and the measured value of the air pressure becomes stable. The air pressure when reaching the stability is the hydraulic pressure. Therefore, since the measurement is performed only when the diaphragm moves in one direction, the hydraulic pressure can be measured with high accuracy without being affected by the dead space specific to the diaphragm.

The pressure receiving surface for receiving the pressure of the liquid to be measured and the air pressure receiving surface are isolated by the diaphragm, and the liquid is not aerated. Therefore, even when the hydraulic pressure measurement unit and the level gauge of the present invention are mounted on a ballast tank of a ship or the like, the increase of bacteria or the like in the ballast water can be suppressed.

Drawings

Fig. 1 is a sectional view showing an embodiment of a hydraulic pressure measuring unit of the present invention along line a-a in fig. 2.

Fig. 2 is a front view of the embodiment.

Fig. 3 is a side view showing a use state of the embodiment.

Fig. 4 is a front view showing a use state of the embodiment.

Fig. 5 is a sectional view showing the flange in the embodiment.

Fig. 6 is a sectional view showing the pressing flange combined with the flange.

Fig. 7 is a front view showing the buffer plate in the embodiment.

Fig. 8 is a longitudinal sectional view showing the gasket in the embodiment.

Fig. 9 is a schematic side view showing a state in which the hydraulic pressure measurement unit is mounted in the chamber.

Fig. 10 is a diagram showing an example of a change in pressure of an air chamber displayed on a liquid level meter using the hydraulic pressure measurement unit.

Description of the symbols

1 Flange

2 pressing flange

3O-shaped ring

4O-shaped ring

5 diaphragm

6 air chamber

7 liner

8 joint

9 pressure detection end

10 joint

11O-shaped ring groove

12 exhaust port

14 compressed air supply port

15 protruding dike

16 projection

17 plane

18 combining hole

21O-shaped ring groove

22 step part

24 groove

25 discharge hole

26 step part

27 step part

28 center hole

29 combining hole

30 buffer board

32 buffer plate

35 fastening part

36 joint

40 cabin

42 liquid

44 valve

50 piping

54 discharge pipe

71 open end

81 compressed air channel

Detailed Description

Next, an embodiment of the hydraulic pressure measuring unit and the liquid level gauge according to the present invention will be described with reference to the drawings.

[ examples ] A method for producing a compound

[ examples of Hydraulic pressure measuring Unit ]

In fig. 1 and 2, the hydraulic pressure measuring unit includes a flange 1 and a pressing flange 2 overlapping the flange 1. The flange 1 and the pressing flange 2 are both made of a high-strength material such as metal and have a circular outer peripheral shape. The flange 1 has a disc shape, and the pressing flange 2 has an annular shape with a center hole 28.

As shown in fig. 5, the flange 1 has an O-ring groove 11 formed along the entire circumference along a circle concentric with the circular shape of the outer circumference. A circular bank 15 is formed along the entire circumference of the flange 1 and concentrically with the outer periphery of the O-ring groove 11 as viewed in a planar direction. At the center of the bank 15 in the width direction, a protrusion 16 is formed along the entire circumference of the flange 1 and along a concentric circle. A flat surface 17 slightly lower than the height of the bank 15 is formed on the inner peripheral side of the flange 1 closer to the inner peripheral side than the O-ring groove 11.

In the center portion of the flange 1, an exhaust port 12 is formed through the flange 1 in the thickness direction of the flange 1. In the flange 1, a compressed air supply port 14 is formed through the flange 1 in the thickness direction of the flange 1 at a radially inner side of the O-ring groove 11 and a radially outer side of the exhaust port 12. In fig. 5, the exhaust port 12 and the compressed air supply port 14 are each opened on the bottom surface side and the flat surface 17 side of the flange 1.

In the exhaust port 12, the bottom surface side of the flange 1 has a large diameter, a female screw is formed in the large diameter portion, the flat surface 17 side has a small diameter portion 122, and a stepped portion 121 is formed between the small diameter portion 122 and the large diameter portion. Similarly, in the compressed air supply port 14, the bottom surface side of the flange 1 has a large diameter, a female screw is formed in the large diameter portion, and the flat surface 17 side has a small diameter portion 171. As described later in detail, the open end of the small-diameter portion 171 on the flat surface 17 is the pressure detection end 9.

As shown in fig. 1, a gasket 7 and a joint 8 are attached to an exhaust port 12 of the flange 1. Fig. 8 shows a longitudinal section of the gasket 7. The gasket 7 has: a base portion 72 which is circular in plan view; a small-caliber portion 73 formed upright from the base portion 72; and an exhaust passage 74 that extends vertically through the base portion 72 and the small-caliber portion 73. The upper end of the exhaust passage 74 is an open end 71. The gasket 7 is inserted into the exhaust port 12 of the flange 1, the small-diameter portion 73 is fitted into the small-diameter portion 122, and the base portion 72 abuts against the step portion 121.

In a state where the gasket 7 is inserted into the exhaust port 12 of the flange 1, the nipple 8 is screwed into the female screw of the exhaust port 12, and the nipple 8 is attached to the flange 1. The joint 8 has an exhaust passage 81 communicating with the exhaust passage 74 of the gasket 7. The adapter 8 presses the base portion 72 of the gasket 7 against the step portion 121 of the exhaust port 12, and firmly fixes the gasket 7 to the exhaust port 12 of the flange 1.

Thus, the air passage of the exhaust port 12 is substantially restricted by the exhaust passage 74 of the gasket 7, and the open end 71 at the upper end of the gasket 7 becomes the open end of the exhaust port 12. Hereinafter, the open end 71 at the upper end of the gasket 7 will be also described as the open end of the exhaust port 12. The open end of the gasket 7 fixed to the flange 1, that is, the open end 71 of the exhaust port 12, is slightly projected from the flat surface 17 of the flange 1 to be located in the air chamber 6. The open end 71 is opposed to the diaphragm 5 with a gap in the air chamber 6.

The connector 10 is screwed into the compressed air supply port 14 of the flange 1, and the connector 10 is attached to the flange 1. The adapter 10 is formed with a central hole. The connector 10 is connected to a compressed air supply source through a switching valve, and is also connected to an air pressure detection path leading to a pressure sensor through the switching valve. The switching valve selectively connects the compressed air supply port 14 to the compressed air supply source and the air pressure gauge. More specifically, the switching valve is connected to the air pressure gauge after temporarily connecting the compressed air supply port 14 to the compressed air supply source. The compressed air supplied from the compressed air supply source is preferably dry air that is dry air to prevent a failure such as a short circuit from occurring in the sensor circuit due to dew condensation of moisture or the like.

Fig. 6 shows the press flange 2 in overlapping engagement with the flange 1. In fig. 6, the pressing flange 2 is shown vertically turned upside down, and the pressing flange 2 shown in fig. 6 is vertically turned upside down and superposed on the flange 1. As shown in fig. 6, the pressing flange 2 has an O-ring groove 21 at a position overlapping with the O-ring groove 11 of the flange 1, and a step portion 22 overlapping with the bank 15 of the flange 1 is provided extending from the outer peripheral edge of the O-ring groove 21. The step portion 22 is formed with a groove 24 overlapping the projection 16 of the flange 1. The O-ring groove 21, the step portion 22, and the groove 24 are formed along concentric circles over the entire circumference of the pressing flange 2.

In the center hole 28 of the pressing flange 2,

large diameter portions

261 and 271 are formed by slightly enlarging the diameter from both sides in the thickness direction of the pressing flange 2, and a stepped portion 26 extending from the large diameter portion 261 and a stepped portion 27 extending from the large diameter portion 271 are formed. The large-diameter portion 271 is located on the side opposite to the flange 1, and the depth, which is the size of the large-diameter portion 271 in the thickness direction of the pressing flange 2, is larger than the depth, which is the size of the other large-diameter portion 261 in the thickness direction of the pressing flange 2. On the side of the pressing flange 2 facing the flange 1, a flat surface 23 is formed between the O-ring groove 21 and the large-diameter portion 271 at a position lower than the surface of the stepped portion 22.

The cushion plate 30 is fitted into the large-diameter portion 261 of the pressing flange 2, and the cushion plate 30 is fixed to the step portion 26. The buffer plate 30 may be formed of a so-called punched metal as shown in fig. 7, and may be formed in a disc-shaped metal plate with a plurality of holes. The cushion plate 32 is fitted into the large-diameter portion 271 of the pressing flange 2, and the cushion plate 32 is fixed to the step portion 27. The buffer plate 32 may also be made of punched metal having the same shape as the buffer plate 30. However, the thickness of the buffer plate 30 and the buffer plate 32 corresponds to the depth of the large-

diameter portions

261 and 271, and the buffer plate 30 is thin and the buffer plate 32 is thick. The outer surface of the cushion plate 32 is substantially flush with the flat surface 23.

The pressing flange 2 has a discharge hole 25 formed from one position of the outer peripheral surface toward the inside in the radial direction. The discharge hole 25 opens on the flat surface 23 radially inward of the pressing flange 2. An internal thread 251 is formed in an opening of the discharge hole 25 on the outer peripheral side of the pressing flange 2.

Fig. 1 shows a configuration of a hydraulic pressure measuring unit in which a pressing flange 2 is overlapped and coupled to a flange 1. Before the pressing flange 2 is overlapped with the flange 1, the O-ring 3 is fitted into the O-ring groove 11 of the flange 1. The diaphragm 5 is overlapped on the O-ring 3 in such a manner as to cover the whole of the O-ring 3 from above. Further, another O-ring 4 is overlapped on the diaphragm 5. The O-ring 4 has the same diameter and thickness as the O-ring 3, and the O-ring 4 is overlapped with the O-ring 3.

Then, the pressing flange 2 is overlapped with the flange 1 in position alignment. The protrusion 16 of the flange 1 is fitted into the groove 24 of the pressing flange 2, and the relative position of the flange 1 and the pressing flange 2 is determined. The surface of the bank 15 of the flange 1 overlaps the surface of the step portion 22 of the pressing flange 2, and the O-ring 4 is fitted in the O-ring groove 21 of the pressing flange 2.

Then, the flange 1 and the pressing flange 2 are fastened by the coupling holes 18 provided near the outer periphery of the flange 1, the coupling holes 29 provided near the outer periphery of the pressing flange 2, bolts 35 (see fig. 2 and 4), and the like. By fastening the flange 1 and the pressing flange 2, the O-ring 3 on the flange 1 side and the O-ring 4 on the pressing flange 2 side are compressed, and the O-

rings

3 and 4 sandwich the outer peripheral edge of the diaphragm 5.

Thus, the diaphragm 5 separates the space on the flange 1 side on one surface side from the space on the pressing flange 2 side on the other surface side. An air chamber 6 defined by one surface of the diaphragm 5, the flat surface 17 of the flange 1, and the O-ring 3 is formed on the one surface of the diaphragm 5 facing the flange 1. The open end of the exhaust port, i.e., the open end 71 of the gasket 7, is located in the air chamber 6, with the open end 71 being opposite the diaphragm 5. The open end 71 and the diaphragm 5 are formed in a lip-shaped structure that closes the open end 71 by substantially equalizing pressures applied to one surface side and the other surface side of the diaphragm 5.

The open end 71 of the lip structure and the air discharge passage 74 communicating therewith preferably have a small cross-sectional area because a large amount of air does not have to be discharged and only a small amount of air needs to be discharged gradually.

Next, a usage of the hydraulic pressure measuring unit having the above-described configuration and a liquid level meter using the hydraulic pressure measuring unit will be described. Fig. 9 is a schematic view showing a state in which the hydraulic pressure measuring unit is attached to the chamber 40, and reference numeral 42 denotes a liquid stored in the chamber 40.

As shown in fig. 1, the pressing flange 2 is provided toward the chamber 40, and the pressure of the liquid contained in the chamber 40 is applied to the diaphragm 5 through the

buffer plates

30 and 32. Therefore, the surface of the diaphragm 5 on the pressing flange 2 side serves as a pressure receiving surface for receiving the pressure of the liquid to be measured. The O-

rings

3 and 4 isolate the liquid pressure receiving surface of the diaphragm 5 from the surface on the air chamber 6 side.

As shown in fig. 3, 4, and 9, the joint 8 connects the compressed air supply port 14 to a compressed air supply source and a pressure measuring instrument via a pipe 50. A switching valve including an electromagnetic valve and the like is provided between the pipe 50, the compressed air supply source, and the air pressure measuring instrument. The switching valve selectively connects the compressed air supply port 14 to the compressed air supply source and the air pressure gauge. More specifically, the switching valve switches the pressure sensor connected to the air pressure gauge after temporarily connecting the compressed air supply port 14 to the compressed air supply source.

When the switching valve is switched to the air pressure measuring instrument side, the air pressure of the air chamber 6 is applied to the pressure sensor of the air pressure measuring instrument from the pressure detecting end 9 via the joint 10, and the air pressure of the air chamber 6 is detected by the pressure sensor. The discharge hole 25 is connected to the discharge recovery portion via a joint 36. As shown in fig. 3 and 9, a valve 44 may be provided between the hydraulic pressure measuring unit and the chamber.

The hydraulic pressure measuring unit is used as a hydraulic pressure measuring device or a liquid level meter, as described below. Before use as a hydraulic pressure measuring device or a liquid level meter, the switching valve is switched to the zero-point adjustment mode, and the air chamber 6 is opened to the atmospheric pressure to perform zero-point adjustment. By performing the zero point adjustment, the hydraulic pressure measurement with high accuracy can be performed.

After the zero point adjustment is completed, the switching valve is switched to the compressed air supply source side, and compressed air is temporarily and intermittently supplied from the compressed air supply source side to the air chamber 6 via the pipe 50 (see fig. 4 and 9) and the compressed air supply port 14. The pressure of the compressed air is set to be higher than the pressure applied to the diaphragm 5 when the liquid level of the liquid as the measurement target is at a maximum.

After the compressed air is temporarily supplied, the compressed air supply port 14 is switched to the barometer side by the switching valve. Immediately after switching the compressed air supply port 14 to the barometer side, the air pressure applied to the air chamber 6 side of the diaphragm 5 is greater than the hydraulic pressure applied to the pressure receiving surface side of the diaphragm 5 that receives the hydraulic pressure, and a gap is formed between the diaphragm 5 and the opening end 71 of the exhaust port 12. Therefore, the air in the air chamber 6 is discharged through the open end 71, and the air pressure in the air chamber 6 gradually decreases. When the air pressure applied to the air chamber 6 side and the pressure applied to the pressure receiving surface side receiving the liquid pressure reach equilibrium, the open end 71 is closed by the diaphragm 5, and the air discharge is stopped. The air pressure of the air chamber 6 tends to be stable. The pressure at which this stabilization is achieved is measured by the pressure sensor.

Since the compressed air is temporarily and intermittently supplied to the air chamber 6 from the compressed air supply source, as shown in fig. 10, the air pressure of the air chamber 6 repeatedly changes with the above intermittent period. Since the air pressure level of the air chamber 6, which tends to be stable, changes in accordance with the liquid level when the liquid level in the chamber changes, the changed liquid level can be calculated by detecting the air pressure when the air pressure reaches a stable level.

The operation control of the switching valve for temporarily and intermittently supplying compressed air from the compressed air supply source, the operation control of the switching valve for detecting the air pressure of the air chamber 6, and the like are not essential to the present invention, and therefore, the description thereof will be omitted. Further, the techniques described in patent documents 1 and 2 can be applied to control of a switching valve for intermittently supplying compressed air.

The hydraulic pressure measuring unit of the present invention may be attached to the bottom plate of the chamber containing the liquid, or may be attached to a side wall near the bottom plate.

When the liquid in the chamber is discharged, the valve connected to the discharge hole 25 is opened to discharge the residue remaining on the liquid pressure receiving surface side of the diaphragm 5.

[ effects of examples ]

According to the embodiments of the hydraulic pressure measuring unit and the liquid level gauge of the present invention described above, the following effects can be obtained.

Since the pressure of the liquid to be measured is measured only when the diaphragm 5 moves in one direction, the hydraulic pressure can be measured with high accuracy while avoiding the influence of the dead space specific to the diaphragm.

Since the diaphragm 5 isolates the pressure receiving surface that receives the pressure of the liquid as the measurement target from the air pressure receiving surface, the liquid is not aerated. Therefore, even when the hydraulic pressure measurement unit and the level gauge in the embodiment are mounted on a ballast tank of a ship or the like, the increase of bacteria or the like in the ballast water can be suppressed.

Even if the volume of the air chamber 6 formed on the air pressure receiving surface side of the diaphragm 5 is extremely small, the pressure of the liquid to be measured can be measured by measuring the air pressure applied to the air chamber 6. Further, the compressed air to be supplied to the air chamber 6 only needs to be supplied temporarily and intermittently, and a small compressed air supply source can satisfy the demand.

Since only the air pressure at the time when the air pressure of the air chamber 6 stops decreasing and becomes stable needs to be measured, the air pressure measurement can measure the hydraulic pressure with high accuracy without causing fluctuation or pulsation of the air pressure.

Claims

1. A hydraulic pressure measuring unit is characterized in that,

the hydraulic pressure measurement unit includes:

a diaphragm having one surface side serving as a pressure receiving surface for receiving a pressure of a measurement object, the measurement object being a liquid, and the other surface side serving as an air pressure receiving surface facing an air chamber,

a compressed air supply port having an opening in the air chamber, an

the open end of the exhaust port and the diaphragm are formed into a lip structure that closes the open end by the diaphragm by equalizing pressure applied to the pressure-receiving surface and air pressure applied to the air chamber.

2. The hydraulic pressure determination unit according to claim 1,

the compressed air supply port doubles as an air pressure detection port.

3. The hydraulic pressure determination unit according to claim 1,

compressed air is supplied to the air chamber intermittently through the compressed air supply port in such a manner that air pressure greater than the pressure of liquid applied to the pressure receiving surface of the diaphragm is applied to the air chamber.

4. The hydraulic pressure determination unit according to claim 1,

the compressed air supplied to the air chamber is dry air.

5. The hydraulic pressure determination unit according to claim 1,

the compressed air supplied to the air chamber is discharged from the discharge port, and the air pressure applied to the air chamber when the air pressure applied to the air chamber stops decreasing is used as a measured value.

6. The hydraulic pressure determination unit according to claim 1,

the diaphragm is held by the flange and the pressing flange sandwiching the peripheral edge portion of the diaphragm.

7. The hydraulic pressure determination unit of claim 6,

the diaphragm is held by the flange-side O-ring and the pressing flange-side O-ring sandwiching the peripheral edge portion of the diaphragm.

8. The hydraulic pressure determination unit of claim 6,

the compressed air supply port and the exhaust port are provided on the flange side, and the pressing flange side is a pressure receiving surface side that receives a pressure of the liquid.

9. The hydraulic pressure determination unit of claim 6,

a cushion plate is fixedly attached to the pressing flange on a pressure receiving surface side of the diaphragm that receives the pressure of the liquid.

10. A liquid level meter is characterized in that a liquid level meter is arranged in the tank,

the liquid level meter has:

the hydraulic pressure determination unit according to any one of claims 1 to 9,

a compressed air supply source that supplies compressed air, whose pressure is higher than a pressure applied to a pressure receiving surface of the hydraulic pressure measuring unit that receives the hydraulic pressure, to an air chamber of the hydraulic pressure measuring unit via a compressed air supply port of the hydraulic pressure measuring unit,