CN110057426B

CN110057426B - Dark pool liquid level measurement system and method based on strain layer around Bragg grating

Info

Publication number: CN110057426B
Application number: CN201910371117.3A
Authority: CN
Inventors: 梁磊; 李美格; 徐刚; 王慧; 南秋明
Original assignee: Wuhan University of Technology WUT
Current assignee: Wuhan University of Technology WUT
Priority date: 2019-05-06
Filing date: 2019-05-06
Publication date: 2021-03-16
Anticipated expiration: 2039-05-06
Also published as: CN110057426A

Abstract

The invention discloses a dark pool liquid level measurement system and method based on a strain layer around Bragg gratings. The measurement system includes a fiber grating liquid level sensing component, a fiber grating demodulator, a junction box and a data processing system. The fiber grating liquid level sensing component is buried in the dark pool, and its pigtail is connected to the fiber grating demodulator through the junction box. The fiber grating demodulator transmits the detected center wavelength data to the data processing system, which corresponds to the corresponding bra The center wavelength of the grating is compared, and the Bragg grating with the largest wavelength change is located in the sensing area closest to the liquid surface immersed in the liquid, and the liquid level of the liquid in the dark pool is determined according to the position between the sensing areas. The system of the invention is used to measure the liquid level of the dark pool of the sponge city complex, and has the advantages of anti-electromagnetic interference, long measurement distance and good signal stability, and is suitable for long-term work in a harsh working environment. have important value.

Description

Dark pool liquid level measurement system and method based on strain layer around Bragg grating

Technical Field

The invention belongs to the technical field of liquid level measurement, and particularly relates to a dark pool liquid level measurement system and method based on a Bragg grating peripheral strain layer.

Background

The sponge city is a brand new product of the development of the Internet of things technology and the smart city. At present, sponge city on-line monitoring based on thing networking has become the focus, and it fuses the theory in the sponge city into the wisdom city, through information technologies such as thing networking, sensor, cloud computing, big data, implements in coordination various concentrated or distributed water resource, green facility and sponge city construction to make the construction and the management in sponge city more high-efficient, wisdom. Of these technologies, sensing technology is the most fundamental and important key technology. Because if the sensing signal is unstable, the collected mass data information is unreliable, and the functions of the online monitoring system can be played.

Many techniques for measuring or tracking liquid levels exist, examples of which are capacitive sensors, diode-based sensors and differential pressure sensors, each adapted to specific operating conditions. As a novel urban rainfall flood management system, the sponge city has the defects of weak anti-interference performance, poor signal stability, difficult remote transmission, inconvenient large-scale networking and the like in the traditional electromagnetic sensing technology, so that the functions of the online monitoring system cannot be effectively exerted, and the liquid level detection is not enough.

Disclosure of Invention

The invention aims to solve the technical problem of providing a dark pool liquid level measurement system and a method based on a Bragg grating surrounding strain layer, wherein the system is used for measuring the liquid level of a sponge city complex dark pool, has the advantages of electromagnetic interference resistance, long measurement distance and good signal stability, is suitable for long-term work in a severe working environment, and has important value in mastering the real-time condition of rainwater in a sponge city.

The technical scheme adopted by the invention for solving the technical problems is as follows: the fiber bragg grating liquid level sensing assembly is arranged in the dark pool and used for sensing the liquid level in the dark pool, and a fiber pigtail of the fiber bragg grating liquid level sensing assembly is connected with an optical cable and is connected with the fiber bragg grating demodulator through the junction box; the fiber grating demodulator is used for recording the central wavelength of the Bragg grating of the fiber grating liquid level sensing assembly at each junction box and transmitting data to the data processing system; and the data processing system receives the central wavelength of each channel grating sent by the fiber grating demodulator, analyzes and compares the central wavelength with the stored corresponding original wavelength data, determines a sensing area contacted with the liquid, and generates and outputs the display.

According to the technical scheme, one optical fiber in the fiber bragg grating liquid level sensing assembly is axially fixed in a capillary, the tail fiber of the fiber bragg grating liquid level sensing assembly is connected with an armored optical cable and is connected to a fiber bragg grating demodulator through a junction box, the optical fiber comprises a plurality of sensing areas spaced along the length of the optical fiber, and each sensing area comprises a bragg grating generating a reflection spectrum responding to incident light and a strain layer around the bragg grating. Wherein each strain layer is configured to induce strain on the optical fiber at its respective corresponding bragg grating based on the temperature of the strain layer, thereby causing a change in the center wavelength of the reflectance spectrum of the bragg grating, the bragg grating having the greatest change in wavelength being in a sensing region of the liquid immersed in the liquid that is closest to the liquid level, and the liquid level of the liquid in the cell being determined based on the position between the sensing regions.

According to the technical scheme, the fiber bragg grating liquid level sensing assembly further comprises an optical fiber, the two optical fibers are axially fixed in the same capillary, the second optical fiber comprises a second bragg grating group and a strain layer around the second bragg grating group, the first optical fiber and the second optical fiber are connected into different channels of the fiber bragg grating demodulator, and the bragg gratings on the two optical fibers are offset along the length direction of the optical fibers.

According to the technical scheme, 10 Bragg gratings with the length of 5mm are engraved on each optical fiber at intervals of 1.5cm, the Bragg gratings keep the prestretching amount of 3nm, the wavelengths are sequentially increased by 3nm from bottom to top, the wavelengths of the two optical fibers can be the same or different, the wavelength range is 1534 nm-1558 nm, the minimum wavelength is not more than 1532nm, and the maximum wavelength is not more than 1557 nm.

According to the technical scheme, the fiber bragg grating liquid level sensing assembly further comprises a heating device, the heating device heats every 4 hours (the heating device can be set to be opened at six points, ten points, fourteen points, eighteen points and twenty-two points every day at regular time), the heating device is turned off after heating for 15-25 minutes, a heat source in the heating device is connected with a heating coil to heat the capillary tube so as to increase the heat difference between the part, higher than the liquid level, of the optical fiber and the other parts, lower than the liquid level, of the optical fiber, and the heating coil surrounds the capillary tube. To facilitate heating of the capillary tube.

According to the technical scheme, the demodulation range of the fiber grating demodulator is 1530 nm-1560 nm. The ASE light source emits light signals, the power of the emitted light signals is attenuated to a power range acceptable by the filter through the optical attenuator, the light signals are filtered by the filter, are averagely divided into 8 paths through the optical splitter, enter the optical fiber through different channels and are transmitted along the optical fiber; the optical detector detects the reflection spectrum of the optical fiber and converts the optical signal into an electric signal, and the processor receives the detection signal from the optical detector, converts the detection signal into a central wavelength and outputs the central wavelength through the interface.

According to the technical scheme, the data processing system comprises a memory for storing calibration data, and the processor compares the initial center wavelength of the specific Bragg grating with the measurement center wavelength of the specific Bragg grating; liquid is immersed from the lower part of the capillary tube, the temperature of the liquid is lower than that of air, the wavelength is reduced, the sensing area where the Bragg grating with the largest wavelength change is located is the sensing area which is immersed in the liquid and is closest to the liquid level, and the liquid level of the liquid in the dark pool is determined according to the position between the sensing areas.

According to the technical scheme, the thermal expansion coefficient of the strain layer is different from that of the optical fiber, and the strain layer comprises a polymer layer; the optical fiber comprises a plurality of spaced regions, the spaced regions of the plurality of spaced regions are arranged between each pair of adjacent sensing regions, and the spaced regions do not comprise a strain layer; the capillary tube is made of stainless steel or other corrosion-resistant metal materials, so that the fiber grating sensor has the characteristics of electromagnetic resistance, corrosion resistance, high sensitivity and high measurement precision, the outer layer of the heating coil is wrapped with the heat-insulating layer, the inner part and the outer part of the heat-insulating layer are made of stainless steel, the middle part of the heat-insulating layer is vacuumized to delay heat dissipation, and the outer layer is coated with a waterproof and corrosion-resistant material. The sealing performance is good, and the heat preservation performance is good.

The invention also provides a dark pool liquid level measurement method of the strain layer around the Bragg grating, which comprises the following steps of storing the reference center wavelength corresponding to the Bragg grating in the fiber bragg grating liquid level sensing assembly in a data processing system before installation, and determining the relative distance between the sensing areas; the method comprises the following steps that firstly, a fiber bragg grating liquid level sensing assembly is configured in a dark pool, a tail fiber of an optical fiber is connected with an armored optical cable, and the tail fiber is connected with a demodulator through a junction box; recording the central wavelength of the Bragg grating of the fiber grating liquid level sensing assembly at each junction box by a fiber grating demodulator, and transmitting data to a data processing system; thirdly, the data processing system receives the central wavelength of each channel grating sent by the demodulator, analyzes the detected central wavelength and identifies the central wavelength corresponding to each Bragg grating; comparing the initial central wavelength of the specific Bragg grating with the measured central wavelength of the specific Bragg grating to determine the Bragg grating with the central wavelength which changes maximally; determining the liquid level of liquid in the dark pool according to the sensing areas where the determined Bragg gratings with the maximum change of the central wavelength are located and the positions among the sensing areas; and step six, generating an output representing the liquid level.

According to the technical scheme, when the second optical fiber exists in the optical fiber of the optical fiber grating liquid level sensing assembly, the data processing system further more accurately determines the liquid level according to the sensing area where the Bragg grating with the maximum change of the central wavelength on the two optical fibers is located and the position between the sensing areas.

The invention has the following beneficial effects: the defects of poor interference resistance, unstable signals and the like of the traditional liquid level measurement technology are overcome, data such as soil temperature, water level, flow and water pressure are integrated, and the requirements of stable transmission, strong interference resistance, high precision and the like are met. The data transmission that utilizes optic fibre and relevant network technology to gather to the database of system, all-round liquid level to the sponge city monitors and manages, trails the monitoring in real time, accumulates actual operation data, can monitor the long-term operation effect of facility, in time discovers operation risk and problem and carries out effectual processing and disposes, improves the operation guarantee rate of facility, provides the essential data for the continuous improvement of sponge city construction level.

According to the monitoring method provided by the invention, the optical fiber is axially fixed in the capillary, the capillary is heated to increase the thermal difference between the part of the optical fiber, which is higher than the liquid level, and the other parts of the optical fiber, which are lower than the liquid level, and the liquid level in the capillary rises faster after heating, so that the measurement result is more real-time and more accurate.

Drawings

The invention will be further described with reference to the accompanying drawings and examples, in which:

FIG. 1 is a block diagram of a dark pool liquid level measurement system of a sponge city complex based on a strain layer around a Bragg grating according to an embodiment of the invention;

FIG. 2 is a schematic structural diagram of a fiber grating liquid level sensing assembly in an embodiment of the invention;

FIG. 3 is a particular embodiment of a liquid level sensing assembly;

FIG. 4 is a second embodiment of a liquid level sensing assembly;

FIG. 5 is a flow chart of an example of a method of measuring a liquid level.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The embodiment of the invention discloses a dark pool liquid level measuring system based on strain layers around Bragg gratings, which comprises a fiber bragg grating liquid level sensing assembly for reflecting light along an optical fiber by utilizing the Bragg gratings, wherein each Bragg grating is provided with one strain layer around, and the strain of the Bragg gratings is triggered by the strain layers so as to change the central wavelength of the reflection spectrum of the Bragg gratings. The liquid level sensing assembly is calibrated before being installed, calibration data are stored in the data processing system, when the temperature is changed, the strain layer generates strain on the Bragg grating, and the central wavelength of the Bragg grating is changed. This is related to the difference in the coefficient of thermal expansion of the strained layer and the optical fiber, the strained layer comprising a polymer. In addition, the optical fiber liquid level sensing assembly can avoid potential short circuit of any electrical element due to lack of electrical elements, and can be free from electromagnetic interference, so that the optical fiber liquid level sensing assembly can be used in various environments.

Fig. 1 illustrates a specific embodiment of a liquid level measurement system 100, the liquid level measurement system 100 comprising a fiber grating demodulator 101, a data processing system 102 and a dark pool 111. The liquid level sensing assembly 115 is disposed within the dark basin and fused to a respective closure disposed on the fiber optic cable and connected to the fiber grating demodulator 101, the assembly being disposed within the dark basin 115. The level sensing assembly 115 includes an optical fiber 114 and a heating device 119. The optical fiber 114 includes a plurality of sensing regions 116 and a plurality of spacer regions 117 between the sensing regions 116. Each sensing region 116 comprises a bragg grating 112 and a strain layer 113 surrounding the bragg grating 112; the optical fiber 114 is axially fixed in a capillary 120 of a heating device 119. The heating device 119 includes a capillary tube 120 and a heating coil 123 surrounding the capillary tube, an insulating layer 121, and a heat source 122.

The cuvette 111 contains a liquid 124 and the level sensing assembly 115 may include a plurality of optical fibers, such as a second optical fiber 118. When another fiber is present, another grating is also present. For example, the second optical fiber 118 includes a second grating (e.g., similar to the bragg grating 112) and a second strained layer (similar to the strained layer 113) separated by a spacer region (similar to the spacer region 117).

A heat source 122 in heating device 119 is coupled to a heating coil 123 surrounding capillary 120 to heat the capillary to increase the thermal differential between the portion of the fiber above the level of liquid 124 (e.g., the first sensing region) and the other portion of the fiber below the level of liquid 124 (e.g., the second sensing region). The capillary 120 is made of stainless steel or other corrosion-resistant metal. The heating coil 123 is wrapped by an insulating layer 121. The inside and outside of the insulating layer 121 are stainless steel, the middle is vacuumized to delay heat dissipation, and the outer layer is coated with a waterproof and anticorrosive material.

The fiber grating demodulator 101 measures the center wavelength 103 of the bragg grating 112 and provides it to the data processing system 102. The demodulation range of the fiber grating demodulator is 1530 nm-1560 nm, the ASE light source emits light signals, the emitted light signals pass through the optical attenuator, the power of the emitted light signals is attenuated to the acceptable power range of the filter, the emitted light signals are filtered by the filter, the emitted light signals are evenly divided into 8 paths by the optical splitter, enter the optical fiber through different channels and are transmitted along the optical fiber; the optical detector detects the reflection spectrum of the optical fiber and converts the optical signal into an electric signal, and the processor receives the detection signal from the optical detector, converts the detection signal into a central wavelength and outputs the central wavelength through the interface.

The data processing system 102 comprises a memory 104 for storing calibration data 106, a processor 105 and an interface 108, the processor 105 comparing the initial center wavelength of a particular bragg grating stored in the memory 104, i.e. the calibration data 106, with the measured center wavelength 103 of the particular bragg grating. And the sensing area where the Bragg grating with the largest wavelength change along the axial direction of the optical fiber is located is the sensing area which is immersed in the liquid and is closest to the liquid level, and the liquid level of the liquid in the dark pool is determined according to the position between the sensing areas. The data processing system 102 compares the bragg gratings on the first and second fibers that have the greatest change in center wavelength to further accurately determine the level 107 and output a level signal 109 via interface 108.

FIG. 2 is a schematic diagram of a fiber grating liquid level sensing assembly 200, an example of the liquid level sensing assembly 125 shown in FIG. 1. The fluid level sensing assembly 200 includes an optical fiber 201 having a plurality of sensing regions 202, a capillary tube 120, an insulation layer 121, and a heating coil 123. The sensing region 202 is located along the length of the optical fiber 201. Each sensing region 202 includes a bragg grating in the fiber 201 and a strained layer surrounding the bragg grating, such as the bragg grating 112 and the strained layer 113 in fig. 1. Thus, fiber 201 is similar to fiber 117 shown in FIG. 1.

As shown in fig. 2, each sensing region 202 includes a bragg grating (e.g., bragg grating 112) having a different center wavelength. The fiber grating demodulator 101 will detect the center wavelength of each bragg grating and output it to the data processing system 102.

FIG. 3 illustrates a particular embodiment of a fluid level sensing assembly 300, which may be an example of the fluid level sensing assembly 115 shown in FIG. 1. The liquid level sensing assembly 301 includes an optical fiber 301 having a plurality of sensing regions, such as sensing regions 302, 304, 306, and 308. Optical fiber 301, similar to optical fiber 201 shown in fig. 2, is an example of optical fiber 114 shown in fig. 1. Although four sensing regions are shown in FIG. 3, the fluid level sensing assembly 300 may include more or less than four sensing regions depending on the particular configuration. The sensing regions 302, 304, 306, 308 are arranged along the length of the optical fiber 301. Each sensing region comprises a bragg grating surrounded by a strained layer. The first sensing region 302 includes a bragg grating 313 disposed in the optical fiber 301 and a strain layer 309 surrounding the optical fiber 301 and the bragg grating 313. The second sensing region 304 comprises a bragg grating 314 in the optical fiber 301 and a strain layer 310 around the optical fiber 301 and the bragg grating 314. The third sensing region 306 comprises a bragg grating 315 within the optical fiber 301 and a strained layer 311 around the optical fiber 301 and the bragg grating 315. The fourth sensing region 308 includes a bragg grating 316 within the optical fiber 301 and a strained layer 312 around the optical fiber 301 and the bragg grating 316. The initial center wavelengths of the

bragg gratings

313, 314, 315 and 316 are 1541nm, 1538nm, 1535nm and 1532nm, respectively. The sensing region 202 shown in FIG. 2 or the sensing region 116 shown in FIG. 1 can be configured in accordance with the sensing regions 302, 304, 360, and 308. Similarly, the bragg grating 112 in fig. 1 may also be configured in accordance with

bragg gratings

313, 314, 315 and 316.

In the particular embodiment shown in FIG. 3, the sensing regions 302, 304, 306, 308 are separated by a spacing region. The first sensing region 302 and the second sensing region 304 are separated by a first spacer region 303. The second sensor region 304 and the third sensor region 306 are separated by a second spacer region 305, and the third sensor region 306 and the fourth sensor region 308 are separated by a third spacer region 307. The spacer region 117 shown in fig. 1 may be configured in accordance with the spacers 303, 305, 307. In the embodiment shown in fig. 3, the spacers 303, 305, 307 do not comprise a strained layer or bragg grating. In other embodiments the spacer regions 303, 305, 307 may comprise a portion of the strained layer but not the bragg grating. Fig. 3 also illustrates that liquid 124 is separated from gas 318 by liquid level 317.

The data processing system 102 compares the received wavelength 103 with corresponding calibration data 106 in the memory 104 to obtain the bragg grating with the largest change in wavelength. The Bragg grating is located in a sensing region immersed in the liquid and closest to the liquid level, and the liquid level in the cell is determined based on the position between the sensing regions. As shown in fig. 3, the bragg grating 315 is immersed in the liquid 124 and the bragg grating 314 is not immersed in the liquid 124, so that the bragg grating 315 having the largest wavelength variation analyzed by the data processing system 102 is the bragg grating, and thus it can be determined that the level of the liquid level 317 of the liquid 124 is between the second sensing region 304 and the third sensing region 306. Based on the relative distances between the sensing regions, the calibration data can accurately determine the level of the liquid surface 124 within the darkwell. In a particular embodiment, depending on certain characteristics of the fiber 301 and the light source, about 200 bragg gratings are formed in the fiber 301, which is more accurate than measuring the level of the liquid 124 in the cuvette with a fiber having fewer bragg gratings. Furthermore, although the sensing regions 302, 304, 306, 308 are shown in FIG. 3 as being approximately equally spaced along the optical fiber 301. In other embodiments, however, the fibers 301 may comprise sensing regions that are not equally spaced. For example, the sensing regions may be spaced closer together (i.e., more densely) in certain portions of the optical fiber 301 than in other portions of the optical fiber 301. The denser sensing area can provide higher liquid level sensing resolution in the area within the dark pool.

FIG. 4 illustrates another embodiment of a fluid level sensing assembly 400 similar to the fluid level sensing assembly 115 of FIG. 1 and the fluid level sensing assembly 300 of FIG. 3. Except that the fluid level sensing assembly 400 includes two optical fibers. Specifically, the fluid level sensing assembly 400 of FIG. 4 includes the optical fiber 301 of FIG. 3 and an additional optical fiber 405. The fiber 405 has substantially the same characteristics as the fiber 301, such as having a plurality of sensing regions, each comprising a bragg grating and a strain layer disposed along the length of the fiber 405. The sensing region of the optical fiber 405 is offset from the sensing region of the optical fiber 301 but is partially aligned with the spaced region of the optical fiber 301. The fiber 405 comprises a first sensing region formed by a bragg grating 410 and a corresponding strained layer 406, a second sensing region formed by a second bragg grating 411 and a corresponding strained layer 407, a third sensing region formed by a third bragg grating 412 and a corresponding strained layer 408, and a fourth sensing region formed by a bragg grating 413 and a corresponding strained layer 409. The bragg grating 112 shown in fig. 1 may also be configured in accordance with the

bragg gratings

410, 411, 412, 413.

As shown in fig. 4, the first sensing region 401 of the optical fiber 405 is offset from the first sensing region 302 of the optical fiber 301 along the length of the optical fiber. Likewise, the second sensing region 402 of the optical fiber 405 is offset from the second sensing region 304 of the optical fiber 301. The remaining sensing regions along the length of fiber 405 are offset from the remaining sensing regions along the length of fiber 301 by a small amount. The fiber 301 and the fiber 405 are connected to different channels of the fiber grating demodulator. The center wavelength of the

bragg gratings

410, 411, 412, 413 may be the same as the

bragg gratings

313, 314, 315, 316, 1541nm, 1538nm, 1535nm, 1532nm, respectively. It may also be distributed according to another arithmetic progression, such as 1543nm, 1540nm, 1537nm, 1534 nm. The embodiment shown in FIG. 4 may have a higher sensitivity of the fluid level sensing assembly 400 than the fluid level sensing assembly 300 of FIG. 3.

Although the sensing regions of

optical fibers

301 and 405 are spaced approximately the same distance in fig. 4, in other embodiments the sensing regions may be spaced differently. Such as uniform spacing of the sensing regions of optical fiber 301 and non-uniform spacing of the sensing regions of optical fiber 405. In other embodiments, the sensing area of fiber 301 and the sensing area of fiber 405 may be non-uniform, but the denser sensing area is approximately the same area of the dark pool shown in FIG. 1. In other embodiments, the sensing area of fiber 301 and the sensing area of fiber 405 may be unevenly spaced, with the denser sensing area being distributed in different areas of the dark pool.

Further, as described above with respect to FIG. 3, upon analysis by data processing system 102, sensing assembly 300 of FIG. 3 detects that level 317 of liquid 124 is between second sensing region 304 and third sensing region 306 of optical fiber 301. By offsetting the sensing area of the optical fiber 405 from the sensing area of the optical fiber 301, it may be used to further refine the measurements of the level sensing assembly 300.

In fig. 4, the data processing system 102 compares the received wavelength 103 with the corresponding calibration data 106 in the memory 104 to obtain the bragg grating region with the largest wavelength variation, and determines the position of the liquid level. As shown in fig. 4, the bragg grating 412 is immersed in the liquid 124, and the bragg grating 411 is not immersed in the liquid 124, so that the bragg grating with the largest wavelength variation analyzed by the data processing system 102 is the bragg grating 412, and thus it can be determined that the level of the liquid level 317 of the liquid 124 is between the second sensing region 402 and the third sensing region 403. Thus, it can be more accurately determined that the liquid level 317 is between the third sensing region 403 of the optical fiber 405 and the second sensing region 306 of the optical fiber 301. Thus, the resolution of the measurements for the scheme shown in FIG. 4 is more accurate than the measurements for the scheme shown in FIG. 3. However, in the event of an operational transient associated with the optical fiber 301, the additional optical fiber may appear redundant.

The different components of fig. 1-4 correspond to each other. For example, the block diagram of fig. 1 may correspond to any of the embodiments shown in fig. 2-4, with fiber 114 of fig. 1 corresponding to fiber 201 of fig. 2, fiber 301 of fig. 3, or fiber 405 of fig. 4. Similarly, sensing region 116 of FIG. 1 corresponds to sensing region 202 of FIG. 2 or sensing regions 302, 304, 306, and 308 of FIG. 3.

FIG. 5 illustrates a method 500 of measuring a fluid level. The method 500 may be performed by the system 100 of FIG. 1, the system 200 of FIG. 2, and the fluid

level sensing assemblies

300 and 400 of FIGS. 3 and 4, or a combination of both.

The method 500 comprises, before installation, storing reference center wavelengths corresponding to bragg gratings in the fiber grating liquid level sensing assembly in a data processing system, and determining the relative distances 501 between sensing regions; the fiber bragg grating liquid level sensing assembly is configured in a hidden pool, the tail fiber of the optical fiber is connected with an armored optical cable and is connected to a demodulator 502 through a junction box; the fiber grating demodulator is used for recording the central wavelength of the Bragg grating of the fiber grating liquid level sensing assembly at each junction box and transmitting data to the data processing system 503; the data processing system receives the central wavelength 504 of each channel grating sent by the demodulator, analyzes the detected central wavelength, and identifies the central wavelength 505 corresponding to each bragg grating; comparing the initial center wavelength of a particular bragg grating with the measured center wavelength of the particular bragg grating to determine the bragg grating 506 having the center wavelength with the greatest change; determining the liquid level 5076 of the liquid in the dark pool according to the sensing areas where the determined Bragg gratings with the maximum change of the central wavelength are positioned and the positions among the sensing areas; an output 508 is generated indicative of the fluid level.

It will be understood that modifications and variations can be made by persons skilled in the art in light of the above teachings and all such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims.

Claims

1. A dark pool liquid level measurement system based on a strain layer around a Bragg grating is characterized by comprising a fiber bragg grating liquid level sensing assembly, a fiber bragg grating demodulator, a junction box and a data processing system, wherein the fiber bragg grating liquid level sensing assembly is arranged in a dark pool and used for sensing the liquid level in the dark pool, and a fiber pigtail of the fiber bragg grating liquid level sensing assembly is connected with an optical cable and is connected with the fiber bragg grating demodulator through the junction box; the fiber grating demodulator is used for recording the central wavelength of the Bragg grating of the fiber grating liquid level sensing assembly at each junction box and transmitting data to the data processing system; a data processing system for receiving the central wavelength of each channel grating transmitted by the fiber grating demodulator, analyzing and comparing the central wavelength with the stored original wavelength data, determining the sensing region in contact with the liquid, and generating an output display, wherein an optical fiber is axially fixed in a capillary tube in the fiber grating liquid level sensing assembly, the pigtail of which is connected with an optical cable and is connected to the fiber grating demodulator through a junction box, the optical fiber comprises a plurality of sensing regions spaced along the length of the optical fiber, each sensing region comprises a Bragg grating generating a reflection spectrum in response to incident light and a strain layer around the Bragg grating, each strain layer is configured to induce strain on the optical fiber on the corresponding Bragg grating according to the temperature of the strain layer, so that the central wavelength of the reflection spectrum of the Bragg grating is changed, and the fiber grating liquid level sensing assembly further comprises an optical fiber, two optic fibre are fixed in same capillary along the axial, and the second optic fibre contains second Bragg grating group and the strain layer around it, and wherein first optic fibre and second optic fibre access fiber grating demodulator's different passageways, and the Bragg grating on two optic fibre all shifts along the length direction of optic fibre.

2. A dark pool level measurement system based on a strain layer around a bragg grating as claimed in claim 1, wherein the bragg grating is engraved on each fiber at a distance of 1.5cm, the bragg grating has a pre-stretching amount of 3nm, the wavelengths are sequentially increased by 3nm from bottom to top, the wavelengths of the two fibers can be the same or different, the wavelength range is 1534nm to 1558nm, the minimum wavelength is not more than 1532nm, and the maximum wavelength is not more than 1557 nm.

3. The dark pool liquid level measurement system based on the strain layer around the Bragg grating as claimed in claim 1 or 2, wherein the fiber Bragg grating liquid level sensing assembly further comprises a heating device, the heating device is heated every 4 hours and is turned off after heating for 15-25 minutes, a heat source in the heating device is connected with a heating coil, the capillary is heated, so that the thermal difference between the part of the optical fiber above the liquid level and the other part of the optical fiber below the liquid level is increased, and the heating coil surrounds the capillary.

4. The dark pool level measurement system based on the Bragg grating peripheral strain layer as claimed in claim 1 or 2, wherein the demodulation range of the fiber grating demodulator is 1530 nm-1560 nm.

5. A dark pool level measurement system based on a strain layer around a bragg grating according to claim 1 or 2, wherein the data processing system comprises a memory for storing calibration data, the processor comparing the initial center wavelength of a specific bragg grating with the measured center wavelength of the specific bragg grating; liquid is immersed from the lower part of the capillary tube, the temperature of the liquid is lower than that of air, the wavelength is reduced, the sensing area where the Bragg grating with the largest wavelength change is located is the sensing area which is immersed in the liquid and is closest to the liquid level, and the liquid level of the liquid in the dark pool is determined according to the position between the sensing areas.

6. A dark pool liquid level measurement system based on a strained layer around a bragg grating according to claim 1 or 2, characterized in that the coefficient of thermal expansion of the strained layer is different from the coefficient of thermal expansion of the optical fiber, the strained layer comprising a polymer layer; the optical fiber comprises a plurality of spaced regions, the spaced regions of the plurality of spaced regions are arranged between each pair of adjacent sensing regions, and the spaced regions do not comprise a strain layer; the capillary tube is made of stainless steel or other corrosion-resistant metal materials, the outer layer of the heating coil is wrapped with the heat-insulating layer, the inner part and the outer part of the heat-insulating layer are made of stainless steel, the middle of the heat-insulating layer is vacuumized, and the outer layer of the heat-insulating layer is coated with a waterproof and corrosion-resistant material.

7. A dark pool liquid level measurement method of a strain layer around Bragg grating based on the system of claim 1 or 2, characterized in that the method comprises the following steps, firstly, a fiber bragg grating liquid level sensing assembly is arranged in a dark pool, the tail fiber of the optical fiber is connected with an armored optical cable, and the demodulation instrument is accessed through a junction box; recording the central wavelength of the Bragg grating of the fiber grating liquid level sensing assembly at each junction box by a fiber grating demodulator, and transmitting data to a data processing system; thirdly, the data processing system receives the central wavelength of each channel grating sent by the demodulator, analyzes the detected central wavelength and identifies the central wavelength corresponding to each Bragg grating; comparing the initial central wavelength of the specific Bragg grating with the measured central wavelength of the specific Bragg grating to determine the Bragg grating with the central wavelength which changes maximally; determining the liquid level of liquid in the dark pool according to the sensing areas where the determined Bragg gratings with the maximum change of the central wavelength are located and the positions among the sensing areas; and step six, generating an output representing the liquid level.

8. The method of claim 7 wherein the data processing system further determines the liquid level more accurately based on the sensing zone of the Bragg grating having the greatest change in center wavelength on both fibers and the position between the sensing zones when the second fiber is present in the fiber of the fiber grating level sensing assembly.