Fiber Bragg grating sensor for moisture monitoring

Abstract

The invention discloses a fiber bragg grating sensor for moisture monitoring, and relates to the field of moisture monitoring, and the fiber bragg grating sensor comprises a sensor body and a processor. The sensor body comprises a shell, a heating wire, at least one optical fiber and at least one fiber bragg grating; the heating wire and the optical fiber are inserted into the shell in parallel; the fiber bragg grating is arranged on the optical fiber; the processor is connected with the fiber bragg grating; when moisture of to-be-measured soil is measured, the sensor body is vertically inserted into a heating line in the to-be-measured soil along the direction of the heating line to heat the to-be-measured soil under a set heating condition, the fiber bragg grating measures the deviation of optical wavelength in the optical fiber in the heating process, and the processor determines temperature distribution of the to-be-measured soil according to the deviation. According to the soil moisture monitoring method and the soil moisture monitoring device, the moisture monitoring of the large-area soil can be accurately realized, and the cost is low.

Description

Technical Field

[0001] The present application relates to the field of moisture monitoring, and in particular to a fiber Bragg grating sensor for moisture monitoring. Background Art

[0002] Due to the limitations of traditional monitoring technology, the internal changes in soil moisture content are difficult to investigate. Among the internal factors, the "basin effect" of the impermeable ground layer is significant, which strongly changes the heat and moisture transfer in the subsoil. In order to more clearly understand the "basin effect" inside the soil, the temperature and water content profiles should be accurately obtained.

[0003] At present, some technologies for measuring soil moisture have been developed, and the most widely used soil moisture measurement methods include: drying weighing method, neutron method, soil moisture sensor method, dielectric property method, nuclear magnetic resonance method, separation tracer method, remote sensing method, etc. All of the above methods have certain limitations and cannot achieve accurate measurement of soil moisture over a large area at low cost. Summary of the invention

[0004] The purpose of this application is to provide a fiber Bragg grating sensor for moisture monitoring, which can accurately monitor the moisture of a large area of ​​soil at a low cost.

[0005] To achieve the above objectives, this application provides the following solutions:

[0006] A fiber Bragg grating sensor for moisture monitoring, comprising: a sensor body and a processor;

[0007] The sensor body comprises: a housing, a heating wire, at least one optical fiber and at least one fiber Bragg grating; the heating wire and the optical fiber are inserted into the housing in parallel; the fiber Bragg grating is arranged on the optical fiber; the processor is connected to the fiber Bragg grating;

[0008] When measuring the moisture content of the soil to be tested, the sensor body is vertically inserted into the soil to be tested along the direction of the heating wire; the heating wire is used to heat the soil to be tested under set heating conditions; the fiber Bragg grating is used to measure the offset of the light wavelength in the optical fiber during the heating process; the processor is used to determine the temperature distribution of the soil to be tested according to the offset, calculate the thermal conductivity of the soil to be tested according to the temperature distribution, and determine the moisture content of the soil to be tested according to the thermal conductivity of the soil to be tested and the thermal conductivity-water content fitting curve.

[0009] Optionally, the heating wire is a resistance wire.

[0010] Optionally, there are two optical fibers; and the number of the fiber Bragg gratings is not less than two;

[0011] The heating line is located on the center line of the housing; the two optical fibers are symmetrically arranged on both sides of the heating line; and the plurality of fiber Bragg gratings are symmetrically arranged on the two optical fibers;

[0012] In terms of determining the temperature distribution of the soil to be measured according to the offset, the processor is specifically configured to determine the temperature distribution of the soil to be measured according to an average value of the offsets measured by fiber Bragg gratings on the two optical fibers at the same soil depth.

[0013] Optionally, the processor includes:

[0014] A temperature distribution determination module, used to determine the temperature distribution of the soil to be tested according to the offset;

[0015] A thermal conductivity determination module, used to determine a temperature change value according to the temperature distribution, and to determine the thermal conductivity of the soil to be tested according to the relationship between the temperature change value and the heating time;

[0016] The fitting curve determination module is used to perform curve fitting using the least square method based on experimental data obtained from multiple groups of experimental tests to obtain a thermal conductivity-water content fitting curve; the experimental data includes: soil thermal conductivity and corresponding real water content obtained by measuring soils with different water contents using a fiber Bragg grating sensor;

[0017] The moisture calculation module is used to determine the moisture content of the soil to be tested according to the thermal conductivity of the soil to be tested and the thermal conductivity-water content fitting curve.

[0018] Optionally, the sensor body further comprises: a tip portion; the tip portion is located at the bottom of the shell; the tip portion is used to drive the shell to be inserted into the soil to be tested.

[0019] Optionally, the sensor body further comprises: a fixing portion; the fixing portion is located at the top of the housing; the fixing portion is used to fix the heating wire and the optical fiber.

[0020] Optionally, a plurality of through holes are provided inside the shell; the number of the through holes is equal to the sum of the number of the heating wires and the optical fibers; one of the through holes is used to place the heating wire, and the optical fibers are placed in each of the remaining through holes in a one-to-one correspondence.

[0021] Optionally, the gap between the heating wire and the inner wall of the through hole is filled with hot glue; the gap between the optical fiber and the inner wall of the through hole is filled with hot glue.

[0022] Optionally, the shell is an alumina ceramic tube.

[0023] Optionally, the diameter of the shell is 6 mm; the diameter of the through hole is 1 mm; and the length of the shell is 1000 mm.

[0024] According to the specific embodiments provided in this application, this application has the following technical effects:

[0025] The present application provides a fiber Bragg grating sensor for moisture monitoring, which uses a heating wire to actively heat the soil to be tested, and uses a fiber Bragg grating (FBG) to measure the offset of the light wavelength in the optical fiber during the heating process, so as to determine the temperature distribution of the soil to be tested, and determine the moisture of the soil to be tested in combination with the thermal conductivity-water content fitting curve according to the temperature distribution. The present application is suitable for large-area soil moisture monitoring of various soil types, and realizes moisture monitoring based on the thermal conductivity-water content fitting curve, thereby improving the accuracy of moisture monitoring. Compared with monitoring equipment such as time domain reflectometers, the present application can realize measurement by using heating wires and fiber Bragg gratings, and has low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0027] Figure 1 A schematic diagram of the structure of a fiber Bragg grating sensor for moisture monitoring provided in one embodiment of the present application;

[0028] Figure 2 A top view of the top surface of the housing provided in one embodiment of the present application;

[0029] Figure 3 A schematic diagram of a laboratory test provided in accordance with an embodiment of the present application;

[0030] Figure 4 A schematic diagram of a field simulation arrangement provided for an embodiment of the present application.

[0031] Reference numerals: housing—1, heating wire—2, optical fiber—3, fiber Bragg grating—4, tip portion—5, fixing portion—6, soil sample—7. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0033] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0034] Among the relevant soil moisture monitoring methods, the drying and weighing method is the simplest and most accurate method, but it requires stratified sampling, the process is complicated, it damages the soil, it takes a long time, and it cannot measure the soil moisture content of a large area; the sensor measures soil moisture quickly, is inexpensive, and can be measured for a long time, but the sensor itself has a greater impact on the measurement results; the neutron method can meet the requirements for continuous measurement of soil moisture content in a certain area and depth, but there is a radiation hazard; the time domain reflectometer in the dielectric property method is convenient and fast, and the accuracy is much higher than that of the neutron instrument, but the equipment is expensive and the operation is complicated. The use of detection radar can measure large areas over a large range, but it is not suitable for all soil types; the nuclear magnetic resonance method is easily affected by electromagnetic noise; the separation tracer method can measure the horizontal and vertical distribution of soil moisture content over a large range, but the price increases with the measurement area; the remote sensing method is more affected by factors of the soil itself. For example, in order to reduce errors, the corresponding cost needs to be increased.

[0035] Some limitations of the above methods have led to the rapid development of active heating optical fiber methods due to their advantages such as high precision, long distance, distributed monitoring, and good durability. They can fill this gap and thus obtain temperature and soil moisture changes at multiple points along the optical fiber with high precision.

[0036] In an exemplary embodiment, Figure 1 As shown, a fiber Bragg grating sensor for moisture monitoring is provided, comprising: a sensor body and a processor. The sensor body comprises: a housing 1, a heating wire 2, at least one optical fiber 3 and at least one fiber Bragg grating 4; the heating wire 2 and the optical fiber 3 are inserted into the housing 1 in parallel; the fiber Bragg grating 4 is arranged on the optical fiber 3; the processor is connected to the fiber Bragg grating 4. The fiber Bragg grating 4 can be a common structure on the market.

[0037] When measuring the moisture content of the soil to be tested, the sensor body is vertically inserted into the soil to be tested along the direction of the heating wire 2; the heating wire 2 is used to heat the soil to be tested under set heating conditions; the fiber Bragg grating 4 is used to measure the offset of the light wavelength in the optical fiber 3 during the heating process; the processor is used to determine the temperature distribution of the soil to be tested according to the offset, calculate the thermal conductivity of the soil to be tested according to the temperature distribution, and determine the moisture content of the soil to be tested according to the thermal conductivity of the soil to be tested and the thermal conductivity-water content fitting curve. Setting the heating conditions includes setting the heating time and setting the heating power.

[0038] The actively heated fiber Bragg grating sensor in this embodiment is an actively heated fiber method based on high-precision fiber Bragg grating temperature measurement. Electric heating is applied to the outer shell 1 protecting the optical fiber 3 to generate a heat pulse, which applies a certain amount of heat to the surrounding soil. Due to the different thermal conductivity coefficients of water, soil and air, the diffusion speed of energy in different soils is different, thereby measuring the soil temperature. This method is extended from point measurement to quasi-distributed measurement, and can achieve large-area, high-precision monitoring.

[0039] In another exemplary embodiment of the present application, the heating wire 2 is a resistance wire, and the resistance of the resistance wire may be 28Ω / m.

[0040] In another exemplary embodiment of the present application, please refer to Figure 1 , there are two optical fibers 3; the number of the fiber Bragg gratings 4 is not less than two. The heating wire 2 is located on the center line of the housing 1; the two optical fibers 3 are symmetrically arranged on both sides of the heating wire 2; a plurality of fiber Bragg gratings 4 are symmetrically arranged on the two optical fibers 3; the fiber Bragg gratings 4 on each optical fiber 3 are connected in series.

[0041] In terms of determining the temperature distribution of the soil to be measured according to the offset, the processor is specifically used to determine the temperature distribution of the soil to be measured according to an average value of the offsets measured by the fiber Bragg gratings 4 on the two optical fibers 3 at the same soil depth.

[0042] When the number of the fiber Bragg gratings 4 is greater than two, the fiber Bragg gratings 4 are distributed at different soil depths, and the processor can determine the temperature distribution at different soil depths, thereby realizing the monitoring of soil temperature and moisture at different depths. This embodiment is applicable to the monitoring of soil temperature and moisture at different depths of the slope.

[0043] In order to obtain the temperature and moisture profiles at different depths, this embodiment measures temperature by using a fiber Bragg grating 4, and then uses the relationship between thermal conductivity and water content to obtain the soil moisture profile. The accuracy of the curve fitting is verified by field testing, and the temperature and moisture distribution at different depths are measured.

[0044] In another exemplary embodiment of the present application, the processor includes: a temperature distribution determination module, a thermal conductivity determination module, a fitting curve determination module and a moisture calculation module.

[0045] A temperature distribution determination module is used to determine the temperature distribution of the soil to be tested according to the offset.

[0046] The thermal conductivity determination module is used to determine the temperature change value according to the temperature distribution, and determine the thermal conductivity of the soil to be tested according to the relationship between the temperature change value and the heating time.

[0047] The fitting curve determination module is used to perform curve fitting using the least square method based on the experimental data obtained from multiple sets of experimental tests to obtain a thermal conductivity-water content fitting curve; the experimental data includes: the soil thermal conductivity and the corresponding real water content obtained when the fiber Bragg grating sensor is used to measure soils with different water contents. When performing experimental tests, the resistance wire is still heated under the set heating conditions.

[0048] Actively heated fiber Bragg grating sensors cannot directly measure moisture content. The relationship between thermal conductivity and moisture content is fitted using the least squares method, and then in-situ tests are performed on the slope site to measure the temperature and moisture profiles at different depths of the soil. The least squares method is a mathematical optimization technique that finds the best function match for the data by minimizing the sum of squares of the error to fit the curve, so that the data points are not far above or below the curve, which can reflect the overall distribution of the data without causing large local fluctuations. It can also reflect the characteristics of the approximated data, so that the deviation between the obtained curve and the original data is minimized in a certain way. The least squares method is widely used because of its simple principle, fast convergence speed, and easy understanding and implementation.

[0049] The moisture calculation module is used to determine the moisture content of the soil to be tested according to the thermal conductivity of the soil to be tested and the thermal conductivity-water content fitting curve.

[0050] In another exemplary embodiment of the present application, please refer to Figure 1 , the sensor body further includes: a tip portion 5 and a fixing portion 6. The tip portion 5 is located at the bottom of the housing 1; the tip portion 5 is used to drive the housing 1 to be inserted into the soil to be measured. The fixing portion 6 is located at the top of the housing 1; the fixing portion 6 is used to fix the heating wire 2 and the optical fiber 3; the fixing portion 6 can be a flange. The fixing portion 6, the housing 1 and the tip portion 5 are fixed together by epoxy resin, and the portion of the housing 1 that contacts the tip portion 5 is provided with insulating glue to separate the heating wire 2 in the housing 1 from the end face of the tip portion 5.

[0051] In another exemplary embodiment of the present application, a plurality of through holes are opened inside the shell 1; the number of the through holes is equal to the sum of the number of the heating wires 2 and the optical fibers 3; one of the through holes is used to place the heating wire 2, and the remaining through holes are each used to place the optical fiber 3 in a one-to-one correspondence.

[0052] The gap between the heating wire 2 and the inner wall of the through hole is filled with hot glue; the gap between the optical fiber 3 and the inner wall of the through hole is filled with hot glue. In practical applications, the length of the housing 1 is 1000 mm, see Figure 2 , the diameter of the shell 1 is 6 mm; the diameter of the through hole is 1 mm.

[0053] In another exemplary embodiment of the present application, the housing 1 is an alumina ceramic tube. The alumina ceramic tube is a ceramic material with aluminum oxide as the main body, which has good chemical corrosion resistance, high temperature resistance, is not easy to explode, and has high mechanical strength. It is used as the housing 1 of the fiber Bragg grating sensor, so that the sensor has very good thermal conductivity, can protect the optical fiber 3 from the influence of extreme temperature, abrasive substances and other harsh environments, and realize accurate temperature measurement.

[0054] The present application utilizes internal resistance wires for heating. As the heating time changes, the energy distribution of the surrounding soil changes, thereby obtaining the soil temperature distribution, converting the temperature into apparent thermal conductivity, and then using the least squares method for curve fitting to establish a mathematical relationship between thermal conductivity and water content. The moisture content can be measured using a fiber Bragg grating sensor. In addition, on-site in-situ tests are performed on the slope to verify the accuracy of the curve fitting, indicating that the actively heated fiber Bragg grating sensor can be used to measure the temperature and moisture distribution of soil at different depths in an actual environment.

[0055] The fiber Bragg grating sensor for moisture monitoring is further introduced below by taking a heating wire 2 and two optical fibers 3 as an example.

[0056] A fiber Bragg grating sensor for moisture monitoring is prepared to measure the temperature and moisture content of the soil. The fiber Bragg grating sensor includes a sensor body and a processor. The sensor body includes an alumina ceramic tube, a resistance wire, a tip 5 and a plurality of fiber Bragg gratings 4 for temperature measurement. The diameter of the alumina ceramic tube is 6 mm, and there are three holes with a diameter of 1 mm. The central hole is used to install the resistance wire, and the other two are used to install the optical fiber 3. The resistance of the resistance wire is selected to be 28Ω / m. After the resistance wire and the optical fiber 3 are installed in the corresponding holes, the gap between the resistance wire and the optical fiber 3 and the inner surface of the hole is filled with hot melt adhesive. The tip 5 is fixed at the bottom to facilitate the insertion of the sensor body into the soil. The tip 5 and the resistance wire are separated by insulating glue. The flange is installed on the top of the alumina ceramic tube to protect and fix the optical fiber 3 and the resistance wire. The flange, the alumina ceramic tube and the tip 5 are fixed together by epoxy resin. The processor is connected to each fiber Bragg grating 4.

[0057] When the processor uses the least squares method to perform curve fitting, it uses laboratory measured data (i.e. experimental data), such as Figure 3 As shown, the sensor body with a length of 60 mm (sensor body for experiment) is completely inserted into the center of the soil sample 7 prepared by the square container. The container is a cube with a side length of 70 mm. Eight groups of different soil samples 7 are prepared. The water content of each group of soil samples 7 is different. The other conditions are the same. The curve fitting is performed according to the measured data. According to the semi-theoretical and semi-empirical Johansen model, R 2 The value of R is 0.65 and the value of RMSE is 0.063. This model (i.e., thermal conductivity-water content fitting curve) is selected to evaluate the water content. 2 Indicates the proportion of the variability in the dependent variable that can be explained by the independent variable in the regression model.

[0058] The active heating fiber Bragg grating sensor for soil temperature and moisture monitoring in this embodiment uses the sensor body to obtain the temperature profile inside the soil that changes with heating time. The processor establishes a mathematical equation between the apparent thermal conductivity and the soil water content through the Johansen model fitted by the least squares method to obtain the change in moisture content, and verifies the accuracy of the curve fitting through on-site in-situ testing, thereby providing an effective method for measuring temperature and moisture profiles using an active heating fiber Bragg grating sensor.

[0059] The following takes 16 fiber Bragg gratings 4 as an example to further introduce the use of the fiber Bragg grating sensor to monitor the temperature and moisture of soil at different depths of the slope.

[0060] See also Figure 4, the length of each sensor body is increased from 60mm used in the laboratory to 1000mm, and 16 fiber Bragg gratings 4 are connected in series at depths of 150mm, 250mm, 350mm, 450mm, 550mm, 650mm, 750mm and 850mm respectively. The sensor body is placed in the excavation hole and fixed in the center, and then the excavated soil is backfilled and compacted in layers. After the backfilling is completed, it takes one month to ensure that the soil reaches a state of equilibrium in terms of soil moisture, temperature and deformation, which is convenient for subsequent measurements. The heating power and heating time should be exactly the same as those used in the laboratory test, and the soil temperature data is obtained, and then the moisture content can be calculated using the Johansen model. Before each heating, the soil temperature is recorded for 3 minutes as the initial soil temperature value, and the temperature change value is calculated by subtracting the initial soil temperature value from the temperature determined by the offset of the light wavelength measured by the fiber Bragg grating 4, so as to determine the temperature change value and obtain the thermal conductivity. Finally, the fiber Bragg grating sensor based on active heating realizes the measurement of slope temperature and moisture distribution.

[0061] The following is a specific example to analyze the impact of different soil covers on the internal temperature and moisture content of the slope.

[0062] Actively heated fiber Bragg grating sensors were used to simulate the monitoring of temperature and moisture distribution at different depths. Figure 4, the field test was conducted on the soil samples collected for laboratory tests. The fiber Bragg grating sensor was placed on the slope through the excavation hole. The diameter of each excavation hole was 20mm and the depth was 1100mm. Six fiber Bragg grating sensors were placed along the slope, and the interval between each fiber Bragg grating sensor was 10m. After the hole was formed, the fiber Bragg grating sensor was fixed in the center position, and then the excavated soil was backfilled and compacted layer by layer, and each layer was about 100mm. After the backfilling was completed, the grass around the first and second fiber Bragg grating sensors was cleared, and the surface was exposed. The original slope grass was around the third and fourth fiber Bragg grating sensors. A layer of plastic film was covered on the fifth and sixth fiber Bragg grating sensors, and the edge of the plastic film was buried in the soil to prevent it from being blown away by the wind. The field test of the fiber Bragg grating sensor was carried out from June 1 to August 1, and the measurement time was 9:00 am and 17:00 pm. The fiber Bragg grating sensor was heated for 10 minutes, and the heating current was set to 0.5A during the heating process. During the measurement, the water content at a depth of 0-150 mm is measured by a soil moisture detection sensor. During the heating process, the data collected by the fiber Bragg grating 4 in the fiber Bragg grating sensor is recorded, and the soil temperature distribution is calculated. Then, the thermal conductivity is obtained by using the relationship between the temperature change value and the heating time, and the thermal conductivity is converted into water content using a simulation curve. The temperature and moisture profiles at different depths of the slope can be measured. By processing the soil cover around the fiber Bragg grating sensor, the effects of different soil covers on the temperature and water content inside the slope can be analyzed.

[0063] In recent years, fiber optic sensing technology has developed rapidly. Fiber Bragg grating sensor is a rapidly developing method for measuring temperature and moisture. This application is based on the active heating fiber method, using a sensor with internal heating function to measure temperature, and establishing a model between temperature change information and moisture content to achieve soil moisture content measurement. This application uses fiber Bragg grating sensor to measure moisture content, which is of great significance for actual monitoring.

[0064] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0065] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A fiber Bragg grating sensor for moisture monitoring, characterized in that: The fiber Bragg grating sensor for moisture monitoring comprises: a sensor body and a processor; The sensor body comprises: a housing, a heating wire, at least one optical fiber and at least one fiber Bragg grating; the heating wire and the optical fiber are inserted into the housing in parallel; the fiber Bragg grating is arranged on the optical fiber; the processor is connected to the fiber Bragg grating; When measuring the moisture content of the soil to be tested, the sensor body is vertically inserted into the soil to be tested along the direction of the heating wire; the heating wire is used to heat the soil to be tested under set heating conditions; the fiber Bragg grating is used to measure the offset of the light wavelength in the optical fiber during the heating process; the processor is used to determine the temperature distribution of the soil to be tested according to the offset, calculate the thermal conductivity of the soil to be tested according to the temperature distribution, and determine the moisture content of the soil to be tested according to the thermal conductivity of the soil to be tested and the thermal conductivity-water content fitting curve.

2. The fiber Bragg grating sensor for moisture monitoring according to claim 1, characterized in that: The heating wire is a resistance wire.

3. The fiber Bragg grating sensor for moisture monitoring according to claim 1, characterized in that: There are two optical fibers; the number of the fiber Bragg gratings is not less than two; The heating line is located on the center line of the housing; the two optical fibers are symmetrically arranged on both sides of the heating line; a plurality of fiber Bragg gratings are symmetrically arranged on the two optical fibers; the fiber Bragg gratings on each optical fiber are connected in series; In terms of determining the temperature distribution of the soil to be measured according to the offset, the processor is specifically configured to determine the temperature distribution of the soil to be measured according to an average value of the offsets measured by fiber Bragg gratings on the two optical fibers at the same soil depth.

4. The fiber Bragg grating sensor for moisture monitoring according to claim 1, characterized in that: The processor comprises: A temperature distribution determination module, used to determine the temperature distribution of the soil to be tested according to the offset; A thermal conductivity determination module, used to determine a temperature change value according to the temperature distribution, and to determine the thermal conductivity of the soil to be tested according to the relationship between the temperature change value and the heating time; The fitting curve determination module is used to perform curve fitting using the least square method based on experimental data obtained from multiple groups of experimental tests to obtain a thermal conductivity-water content fitting curve; the experimental data includes: soil thermal conductivity and corresponding real water content obtained by measuring soils with different water contents using a fiber Bragg grating sensor; The moisture calculation module is used to determine the moisture content of the soil to be tested according to the thermal conductivity of the soil to be tested and the thermal conductivity-water content fitting curve.

5. The fiber Bragg grating sensor for moisture monitoring according to claim 1, characterized in that: The sensor body further comprises: a tip portion; the tip portion is located at the bottom of the shell; the tip portion is used to drive the shell to be inserted into the soil to be tested.

6. The fiber Bragg grating sensor for moisture monitoring according to claim 1, characterized in that: The sensor body further comprises: a fixing portion; the fixing portion is located at the top of the housing; the fixing portion is used to fix the heating wire and the optical fiber.

7. The fiber Bragg grating sensor for moisture monitoring according to claim 1, characterized in that: A plurality of through holes are provided inside the shell; the number of the through holes is equal to the sum of the number of the heating wires and the optical fibers; one of the through holes is used to place the heating wire, and the optical fibers are placed in each of the remaining through holes in a one-to-one correspondence.

8. The fiber Bragg grating sensor for moisture monitoring according to claim 7, characterized in that: The gap between the heating wire and the inner wall of the through hole is filled with hot glue; the gap between the optical fiber and the inner wall of the through hole is filled with hot glue.

9. The fiber Bragg grating sensor for moisture monitoring according to claim 1, characterized in that: The shell is an alumina ceramic tube.

10. The fiber Bragg grating sensor for moisture monitoring according to claim 7, characterized in that: The diameter of the shell is 6 mm; the diameter of the through hole is 1 mm; and the length of the shell is 1000 mm.

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