A micro soil moisture content measuring device and system

Abstract

The micro soil moisture content measuring device and system provided by the present application include: a protective sleeve, a photothermal conversion device, an ordinary optical fiber, a first fiber grating, a second fiber grating, and a heat insulation seat; one end of the photothermal conversion device is connected to one end of the ordinary optical fiber by fiber fusion splicing; the first fiber grating is arranged on the photothermal conversion device, the second fiber grating is arranged on the ordinary optical fiber, the first fiber grating and the second fiber grating are arranged in the protective sleeve, and a heat insulation seat is arranged between the first fiber grating and the second fiber grating. By organically combining the photothermal conversion characteristics of the optical fiber and the fiber grating temperature measurement technology, using the photothermal conversion device as a heating element, synchronously monitoring the temperature changes of the heating optical fiber and the soil environment by using the fiber grating, obtaining the temperature characteristic value by measuring the temperature change of the heating optical fiber, and obtaining the soil moisture content based on the linear relationship between the temperature characteristic value and the moisture field, it is convenient to miniaturize and simplify the soil moisture content measuring device.

Description

Technical Field

[0001] This application relates to the technical field of soil environment measurement, and particularly to a micro soil moisture content measurement device and system. Background Art

[0002] The moisture content in soil is one of the most basic characterization parameters required for disaster early warning such as landslides, collapses and debris flows, and for the analysis of the growth and health of crops. The measurement of soil moisture content is of great significance in many aspects. For example: for places with natural disaster risks such as landslides where the mountain soil is prone to occur, by measuring the water content of the soil mass, the possibility of disasters can be estimated, playing a role in early warning; detecting the soil moisture content of mines can be used to evaluate the stability of mine geology; in addition, the moisture content of the soil also plays a decisive role in the growth and health of crops. Therefore, monitoring the moisture content of the involved area / site and obtaining the information of soil moisture content in a timely and accurate manner is an important way to predict and prevent possible environmental geological disasters.

[0003] According to different measurement principles, the current methods for measuring soil moisture content include: drying method, resistivity method, time domain reflectometry, frequency domain reflectometry, remote sensing method, etc. However, these measurement methods all have defects; for example: the drying method has great destructiveness to the soil mass; the resistivity method is greatly affected by external conditions and is not suitable for widespread promotion in engineering; the time domain and frequency domain reflectometry methods have high costs and are easily interfered; the measurement accuracy of the remote sensing method is relatively low.

[0004] At the same time, in the existing methods for measuring soil moisture content, the heating basically uses an external power supply to heat a carbon fiber optical cable or a heating resistance wire. Using an external power supply will increase the complexity of the system; in addition, the volume of the sensing probe in the method for measuring soil moisture content is relatively large and the energy consumption is also relatively large, especially not suitable for the case where it is difficult to obtain power in the wild. Summary of the Invention

[0005] The embodiments of this application provide a micro soil moisture content measurement device and system, which realize the miniaturization and simplification of the probe and facilitate the measurement of soil moisture content.

[0006] In a first aspect, a micro soil moisture content measurement device provided by the embodiments of this application includes: a protective sleeve, a photo-thermal conversion device, an ordinary optical fiber, a first fiber grating, a second fiber grating, and a heat insulation seat; wherein:

[0007] One end of the photo-thermal conversion device is connected to one end of the ordinary optical fiber by fiber fusion splicing;

[0008] The first fiber Bragg grating is disposed on the photothermal conversion device, the second fiber Bragg grating is disposed on the ordinary optical fiber, the first fiber Bragg grating and the second fiber Bragg grating are disposed in the protective sleeve, and the heat insulation seat is disposed between the first fiber Bragg grating and the second fiber Bragg grating.

[0009] Optionally, in the micro soil moisture content measuring device provided in the first aspect, the photothermal conversion device includes a doped optical fiber, and the first fiber Bragg grating is written on the doped optical fiber by femtosecond laser.

[0010] Optionally, in the micro soil moisture content measuring device provided in the first aspect, the protective sleeve includes a first sub-protective sleeve and a second sub-protective sleeve, and the first sub-protective sleeve is connected to the second sub-protective sleeve;

[0011] The first fiber Bragg grating is located in the first sub-protective sleeve, the second fiber Bragg grating is located in the second sub-protective sleeve, and the heat insulation seat is located at the connection between the first sub-protective sleeve and the second sub-protective sleeve.

[0012] Optionally, in the micro soil moisture content measuring device provided in the first aspect, the distance between the first fiber Bragg grating and the second fiber Bragg grating is not less than 5 cm.

[0013] Optionally, in the micro soil moisture content measuring device provided in the first aspect, the protective sleeve is a stainless steel tube sleeve provided with an open end, the protective sleeve is filled with silicone grease, and the open end of the protective sleeve is encapsulated with encapsulating glue.

[0014] Optionally, in the micro soil moisture content measuring device provided in the first aspect, a jumper and a jumper connector are disposed on the other end of the ordinary optical fiber.

[0015] Optionally, in the micro soil moisture content measuring device provided in the first aspect, the central wavelength range of the first fiber Bragg grating and the second fiber Bragg grating is 1520 - 1580 nm.

[0016] In the micro soil moisture content measurement device and system provided by the embodiments of the present application, when pump light is injected into the photothermal conversion device, the photothermal conversion device generates heat. Thus, the photothermal conversion device can be used as the heating element of the measurement device. The first fiber Bragg grating disposed on the photothermal conversion device is used to monitor the temperature change on the photothermal conversion device, and the second fiber Bragg grating on the ordinary optical fiber is used to monitor the soil environment temperature. Then, based on the sensing principle of the fiber Bragg grating, the change in the reflection spectrum or transmission spectrum of the fiber Bragg grating is detected to inversely obtain the corresponding temperature change information to detect the temperature change of the heating optical fiber and obtain the temperature characteristic value. Then, according to the linear function relationship between the temperature characteristic value and the soil moisture content, the soil moisture content is calculated. In the micro soil moisture content measurement device and system provided by the embodiments of the present application, the photothermal conversion characteristics of the optical fiber and the fiber Bragg grating temperature measurement technology are organically combined through an optical fiber provided with a photothermal conversion device. The photothermal conversion device is used as the heating element, and the first fiber Bragg grating and the second fiber Bragg grating are used to synchronously monitor the temperature changes of the heating optical fiber and the soil environment. The temperature characteristic value is obtained by measuring the temperature change of the heating optical fiber, and then the soil moisture content is obtained based on the linear relationship between the temperature characteristic value and the moisture field. Therefore, the micro soil moisture content measurement device and system provided by the embodiments of the present application facilitate the miniaturization and simplification of the soil moisture content measurement device and are convenient for measuring the soil moisture content.

[0017] In the second aspect, a soil moisture content measurement system provided by the embodiments of the present application includes a micro soil moisture content measurement device, and the micro soil moisture content measurement device is the micro soil moisture content measurement device provided in the first aspect above.

[0018] The micro soil moisture content measurement system further includes a wavelength division multiplexer, a demodulator, an analysis host, and a pump light source.

[0019] Wherein: the first input end of the wavelength division multiplexer is optically connected to the input end of the demodulator, the second input end of the wavelength division multiplexer is optically connected to the pump light source, the output end of the wavelength division multiplexer is connected to the other end of the ordinary optical fiber, and the output end of the demodulator is connected to the analysis host.

[0020] In the third aspect, a micro soil moisture content measurement device provided by the embodiments of the present application includes: a protective sleeve, a photothermal conversion device, a first ordinary optical fiber, a second ordinary optical fiber, a first fiber Bragg grating, a second fiber Bragg grating, and a heat insulation seat; wherein:

[0021] One end of the photothermal conversion device is connected to one end of the first ordinary optical fiber by optical fiber fusion.

[0022] The second ordinary optical fiber is connected in series with the first fiber grating and the second fiber grating. The photothermal conversion device, the first fiber grating and the second fiber grating are located in the protective sleeve. The first fiber grating is close to the photothermal conversion device, the second fiber grating is far from the photothermal conversion device, and a heat insulation seat is arranged between the first fiber grating and the second fiber grating.

[0023] In a fourth aspect, a micro soil moisture content measurement system provided by an embodiment of the present application includes a micro soil moisture content measurement device, and the micro soil moisture content measurement device is the micro soil moisture content measurement device provided in the third aspect;

[0024] The micro soil moisture content measurement system further includes a demodulator, an analysis host and a pump light source;

[0025] Wherein: the other end of the first ordinary optical fiber is optically connected to the pump light source, the second ordinary optical fiber is optically connected to the input end of the demodulator, and the output end of the demodulator is connected to the analysis host.

[0026] In the micro soil moisture content measurement device and system provided by the embodiment of the present application, when pump light is injected into the photothermal conversion device, the photothermal conversion device generates heat. Furthermore, the photothermal conversion device can be used as a heating element of the measurement device. The first fiber grating arranged near the photothermal conversion device is used to monitor the temperature change on the photothermal conversion device, and the second fiber grating on the second ordinary optical fiber is used to monitor the soil environment temperature. Furthermore, by using the sensing principle of the fiber grating, the change of the reflection spectrum or transmission spectrum of the fiber grating is detected to inversely obtain the corresponding temperature change information to detect the temperature change of the heating optical fiber and obtain the temperature characteristic value; then, according to the linear function relationship between the temperature characteristic value and the soil moisture content, the soil moisture content is calculated. The micro soil moisture content measurement device and system provided by the embodiment of the present application organically combine the photothermal conversion characteristics of the optical fiber and the fiber grating temperature measurement technology, use the photothermal conversion device as a heating element, and use the first fiber grating and the second fiber grating to synchronously monitor the temperature changes of the heating optical fiber and the soil environment. The temperature characteristic value is obtained by measuring the temperature change of the heating optical fiber, and then the soil moisture content is obtained based on the linear relationship between the temperature characteristic value and the moisture field. Therefore, the micro soil moisture content measurement device and system provided by the embodiment of the present application facilitate the miniaturization and simplification of the soil moisture content measurement device and are convenient for measuring the soil moisture content. Description of the Drawings

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0028] Figure 1 Structural diagram of a micro soil moisture content measuring device provided by an embodiment of the present application;

[0029] Figure 2 Structural schematic diagram of a micro soil moisture content measuring system provided by an embodiment of the present application;

[0030] Figure 3 Structural diagram of another micro soil moisture content measuring device provided by an embodiment of the present application;

[0031] Figure 4 Structural schematic diagram of another micro soil moisture content measuring system provided by an embodiment of the present application. Specific implementation manners

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0033] The terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit this application. The singular forms "a" and "the" used in this application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0034] To solve the above technical problems, the inventive concept of the embodiments of the present application is: organically combining the photothermal conversion characteristics of a photothermal conversion device and the optical fiber grating temperature measurement technology, using the photothermal conversion device as a heating element, and synchronously monitoring the temperature change of its surrounding environment by using the optical fiber grating, and obtaining the soil moisture content based on the linear relationship between the temperature characteristic value and the moisture field.

[0035] The following will describe in detail some embodiments of the present application with reference to the drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0036] To solve the above technical problems, an embodiment of the present application provides a micro soil moisture content measuring device. Figure 1 It is a schematic structural diagram of a micro soil moisture content measuring device provided by an embodiment of the present application. As Figure 1 shown, the micro soil moisture content measuring device 1 provided by an embodiment of the present application includes: a protective sleeve 11, a photothermal conversion device 12, a common optical fiber 13, a first fiber grating 14, a second fiber grating 15, and a heat insulation seat 16.

[0037] In an embodiment of the present application, one end of the photothermal conversion device 12 is connected to the common optical fiber 13. Optionally, one end of the photothermal conversion device 12 is connected to one end of the common optical fiber 13 by fiber fusion splicing, and light can be transmitted between the photothermal conversion device 12 and the common optical fiber 13. The photothermal conversion device 12 in the embodiment of the present application is an element that can generate heat through the photothermal conversion principle; optionally, the photothermal conversion device 12 is a doped optical fiber. A doped optical fiber refers to an optical fiber doped with a certain special element (such as cobalt, etc.), which changes some characteristics of the optical fiber. When a pump light source is injected, the doped ions absorb the pump light and transition from the ground state to the excited state, and then through the non-radiative transition (multi-phonon relaxation) process, the laser energy is efficiently converted into heat energy, thereby realizing the all-fiber passive heating function; the photothermal conversion device 12 can also be a metal-coated probe, a fiber taper probe, a misaligned fusion splicing probe, etc. or a combination of several methods. In addition, when the photothermal conversion device 12 uses a doped optical fiber, different doped optical fiber types or different doped concentration optical fiber types can be replaced according to the measurement range, measurement accuracy, etc. requirements of the micro soil moisture content measuring device. The common optical fiber 13 in the embodiment of the present application is a fiber made of glass or plastic and is used to transmit light.

[0038] In an embodiment of the present application, the first fiber grating 14 is arranged on the photothermal conversion device 12, and the second fiber grating 15 is arranged on the common optical fiber 13. Furthermore, the first fiber grating 14 and the second fiber grating 15 are connected in series through a single optical fiber including the photothermal conversion device 12. The first fiber grating 14 and the second fiber grating 15 are diffraction gratings formed by axially periodically modulating the refractive index of the optical fiber core through a certain method. Optionally, the first fiber grating 14 is written on the photothermal conversion device 12 by femtosecond laser. In an embodiment of the present application, the excitation pump light of the optional photothermal conversion device 12 can use light with a wavelength of 1480 nm. Furthermore, the central wavelengths of the first fiber grating 14 and the second fiber grating 15 should avoid 1480 nm; optionally, the central wavelengths of the first fiber grating 14 and the second fiber grating 15 can be selected between 1520 - 1580 nm. Of course, the central wavelengths of the first fiber grating 14 and the second fiber grating 15 need to be selected within the demodulation range of the demodulator.

[0039] The photothermal conversion device 12, the first fiber grating 14, the second fiber grating 15, etc. are arranged in the protective sleeve 11, and the protective sleeve 11 is used to protect the photothermal conversion device 12, the first fiber grating 14, the second fiber grating 15, etc. Optionally, the protective sleeve 11 is selected as a sleeve with good heat conduction performance; such as a stainless steel sleeve, which has good corrosion resistance, heat resistance and heat conduction, and thus the stainless steel sleeve can simplify the structure of the sensor probe, protect the photothermal conversion device 12, the first fiber grating 14, the second fiber grating 15, etc., and can also prevent the influence of external stress changes on the temperature measured by the grating.

[0040] An insulating seat 16 is arranged between the first fiber grating 14 and the second fiber grating 15. When the photothermal conversion device 12 is heated, it prevents the heat generated by the photothermal conversion device 12 from being transferred to the second fiber grating 15, so as to prevent the heat generated by the photothermal conversion device 12 from affecting the measurement result of the second fiber grating 15 for measuring the soil environment temperature. Further, to prevent the heat generated by the photothermal conversion device 12 from affecting the second fiber grating 15, the distance between the first fiber grating 14 and the second fiber grating 15 is not less than 5 cm.

[0041] In the embodiment of the present application, the protective sleeve 11 includes a first sub-protective sleeve 111 and a second sub-protective sleeve 112, and the first sub-protective sleeve 111 is connected to the second sub-protective sleeve 112; the first fiber grating 14 is located in the first sub-protective sleeve 111, the second fiber grating 15 is located in the second sub-protective sleeve 112, and the connection between the first sub-protective sleeve 111 and the second sub-protective sleeve 112 is provided. Further, in the embodiment of the present application, the diameter of the first sub-protective sleeve 111 is smaller than the diameter of the second sub-protective sleeve 112, which is convenient for inserting into the soil to be detected during the use of the micro soil moisture content measuring device. Optionally, the first sub-protective sleeve 111 and the second sub-protective sleeve 112 can be an integrally formed structure.

[0042] Further, silicone grease 17 is injected into the protective sleeve 11. On the one hand, the silicone grease 17 realizes the fixation of the relative positions of the photothermal conversion device 12, the ordinary optical fiber 13, the first fiber grating 14, the second fiber grating 15, etc. in the protective sleeve 11. On the other hand, it can make the first fiber grating 14 and the second fiber grating 15 be heated evenly, avoiding the chirping phenomenon caused by uneven heating of the first fiber grating 14 and the second fiber grating 15, resulting in the loss of grating data and making the demodulation device unable to correctly measure the wavelength value, affecting the accuracy of the soil moisture content measurement result.

[0043] In the embodiment of the present application, the protective sleeve 11 is a protective sleeve with open ends at both ends, which facilitates the arrangement of the photothermal conversion device 12, the ordinary optical fiber 13, the first fiber grating 14, the second fiber grating 15, the heat insulation seat 16, etc. inside the protective sleeve 11. After the photothermal conversion device 12, the ordinary optical fiber 13, the first fiber grating 14, the second fiber grating 15, the heat insulation seat 16, etc. are fixed inside the protective sleeve 11, the open ends of the protective sleeve 11 are encapsulated with the encapsulating adhesive 18. The encapsulating adhesive 18 has the characteristics of high temperature resistance and waterproofness, which is convenient for ensuring the tightness of the internal space of the protective sleeve 11.

[0044] In the implementation of the present application, for the convenience of the use and connection of the micro soil moisture content measuring device, a jumper 19 is sleeved on the other end of the ordinary optical fiber 13 and extends out of the protective sleeve 11, and a jumper connector is provided at the end of the corresponding jumper 19, and other devices for measuring the soil moisture content are connected through the jumper connector.

[0045] The micro soil moisture content measuring device provided by the embodiment of the present application can be heated by a fixed power of the photothermal conversion device without an external power supply, simplifies the structure and has low power consumption; the first fiber grating and the second fiber grating have high sensitivity and good long-term stability, and can timely measure the temperature change of the heating optical fiber and the soil environment.

[0046] Based on the micro soil moisture content measuring device provided by the above embodiment, the embodiment of the present application provides a micro soil moisture content measuring system. Figure 2 The structure diagram of a micro soil moisture content measuring system provided by the embodiment of the present application is as follows. Figure 2 As shown, the micro soil moisture content measuring system provided by the embodiment of the present application further includes a wavelength division multiplexer (WDM) 21, a demodulator 22, an analysis host 23, and a pump source 24 in addition to the micro soil moisture content measuring device 1 provided by the above embodiment.

[0047] The first input end of the wavelength division multiplexer 21 is optically connected to the input end of the demodulator 22, the second input end of the wavelength division multiplexer 21 is optically connected to the pump source 24, the output end of the wavelength division multiplexer is connected to the other end of the ordinary optical fiber 13, and the output end of the demodulator 22 is connected to the analysis host 23. The wavelength division multiplexer 21 is used to multiplex the light output by the pump source 24 and the light output by the demodulator 22 into one optical fiber for transmission. The demodulator 22 is a multi-channel FBG demodulator.

[0048] If the demodulator 22 outputs light with a wavelength of 1550 nm and the pump light source 24 outputs light with a wavelength of 1480 nm, then the wavelength division multiplexer 21 is connected to the pump light source 24 and the demodulator 22. The demodulator 22 is connected to the 1550 nm end of the wavelength division multiplexer 21, the pump light source is connected to the 1480 nm end of the WDM, and the other end of the ordinary optical fiber 13 is connected to the other end of the wavelength division multiplexer 21. Through the wavelength division multiplexer 21, optical signals with different wavelengths on the optical fiber string are multiplexed into one optical fiber for transmission. The 1480 nm light emitted by the pump light source is absorbed by the photo-thermal conversion device 12, and the light energy is converted into heat energy through non-radiative transition, increasing the surface temperature of the first fiber grating 14, resulting in a change in the wavelength of the first fiber grating 14, realizing the functions of fixed-power heating and heating fiber temperature measurement. The second fiber grating 15 connected in series with the first fiber grating 14 changes its grating pitch with the change of the soil temperature, thereby causing a change in the reflected wavelength. The demodulation device derives the external temperature by detecting the change in wavelength. Optionally, the other end of the ordinary optical fiber 13 can be connected to the other end of the wavelength division multiplexer 21 through a jumper connector.

[0049] When the micro soil moisture content measurement system provided by the embodiment of the present application is specifically used, when the micro soil moisture content measurement device 1 is placed in the soil, the photo-thermal conversion device 12 inside the micro soil moisture content measurement device 1 is heated at a fixed power under the action of the pump light source, causing a change in the temperature of the photo-thermal conversion device 12, and then resulting in an offset of the reflection wavelength of the first fiber grating 14 synchronously arranged inside the photo-thermal conversion device 12. The demodulator 22 inversely calculates the wavelength offset into a corresponding temperature value, and finally the host 23 calculates the soil moisture content according to the linear function relationship between the temperature characteristic value and the moisture content. The thermal conductivity of the soil increases with the increase of the moisture content. Therefore, the higher the moisture content of the soil, the better the thermal conductivity of the soil, so the temperature characteristic value shows a decreasing trend with the increase of the soil moisture content. Since the external environment will affect the soil temperature, the second fiber grating 15 is used to monitor the soil temperature in real time to reduce the influence of the external environment on the measurement result.

[0050] In the micro soil moisture content measurement system provided by the embodiment of the present application, the specific calculation is as follows:

[0051] The temperature characteristic value is defined as: after heating for a period of time with power on and the measured temperature tends to be stable, a certain characteristic time interval [t1, t2] is selected, and the arithmetic mean of multiple temperature values measured within this time interval is obtained as the temperature average value T, and then subtracting the initial temperature T0, the temperature characteristic value ΔT is obtained. t 。

[0052] The calculation formula is:

[0053]

[0054] where, ΔT t is the temperature characteristic value, T t is the temperature value measured at equal time intervals within the characteristic time interval [t1, t2], and n is the number of temperature measurements within the characteristic time interval.

[0055] Assume that the soil sample to be measured is homogeneous and isotropic. The heat transfer problem can be regarded as a one-dimensional problem. Select the heat source per unit length as the object of study. According to Ohm's law, the energy generated by the heat source per unit length per unit time is:

[0056]

[0057] where: Q1 is the energy generated by the heat source per unit length per unit time; U is the voltage applied across the heat source per unit length; R is the resistance of the heat source per unit length; I is the current passing through the heat source per unit length. Since the voltage, current, and resistance are all constants, the heat generated by the heat source per unit time is also a constant.

[0058] According to Fourier's law, the heat dissipated by the heat source per unit length per unit time is:

[0059]

[0060] where: Q2 is the energy dissipated by the heat source per unit length per unit time; λ is the thermal conductivity, which is related to the properties of the soil itself; is the temperature gradient. Among them, the temperature gradient is the phenomenon of stepped increase or decrease in air temperature, water temperature, or soil temperature in nature with the change of land height or water depth and soil depth.

[0061] According to the law of conservation of energy, the heat generation of the heat source per unit time is expressed as:

[0062] Q3 = Q1 - Q2 = cm(T - T0) = cmΔT t (4)

[0063] where: Q3 is the energy used for fiber heating; c is the specific heat capacity of the heat source; m is the mass of the heat source; T0 is the initial temperature of the FBG before heating; T is the measured temperature of the FBG after heating; ΔT t is the degree of heat diffusion in the soil after heating, that is, the temperature characteristic value.

[0064] Combining equations (2) - (4) gives:

[0065]

[0066] λ is obtained through equation (5):

[0067]

[0068] When the temperature field is stable, due to the isotropy and uniformity of the soil to be measured, the temperature gradient is a constant. Therefore, Equation (6) can be simplified to:

[0069] λ = k0ΔT t + b0 (7)

[0070] Where: Both k0 and b0 are constants.

[0071] Also, since the soil contains substances in solid, liquid, and gas phases, the thermal conductivity of the gas is very small compared to that of the liquid and solid. Therefore, it is neglected during the derivation. Then the soil thermal conductivity is:

[0072] λ = λ w + λ s (8)

[0073] In the formula: λ w is the thermal conductivity of the liquid substance; λ s is the thermal conductivity of the solid substance.

[0074] λ w is closely related to the water content of the soil sample. That is, the greater the water content in the soil, the stronger the heat conduction ability of the soil sample. When the temperature tends to be stable, the temperatures of the soil particles and water are the same, and the heat transfer between them can be ignored. Therefore, λ w and the water content of the soil sample can be approximately considered to be positively correlated, that is:

[0075] λ w = aw (9)

[0076] In the formula: a is a constant; w is the water content in the soil sample.

[0077] Combining Equations (7)-(9) gives:

[0078]

[0079] Further rearrangement gives:

[0080] w = k1ΔT t + b1 (11)

[0081] In the formula: k1 is the water temperature conversion coefficient, a constant; b1 is the water temperature correction coefficient, b1 = -λ2 / a + b0 / a, a constant.

[0082] The temperature characteristic value ΔT tIt has a linear function relationship with the water content w. The above relationship can be used for quantitative monitoring of the soil water content. Among them, when the soil is heated at a fixed power by a photothermal conversion device, only when the soil conditions are the same, the water temperature conversion coefficient k1 and the water temperature correction coefficient b1 are constants. When the soil conditions are different, k1 and b1 change. Therefore, for soils with different conditions, various parameters of the soil need to be measured on-site.

[0083] In the micro soil moisture content measurement system provided by the embodiment of the present application, when pump light is injected into the photothermal conversion device, the photothermal conversion device generates heat. Furthermore, the photothermal conversion device can be used as the heating element of the measurement device. The first fiber Bragg grating arranged on the photothermal conversion device is used to monitor the temperature change on the photothermal conversion device, and the second fiber Bragg grating on the ordinary optical fiber is used to monitor the soil environment temperature. Then, using the sensing principle of the fiber Bragg grating, the change in the reflection spectrum or transmission spectrum of the fiber Bragg grating is detected to inversely obtain the corresponding temperature change information to detect the temperature change of the heating optical fiber and obtain the temperature characteristic value. Then, according to the linear function relationship between the temperature characteristic value and the soil water content, the soil water content is calculated.

[0084] Therefore, in the micro soil moisture content measurement system provided by the embodiment of the present application, the photothermal conversion characteristics of the optical fiber and the fiber Bragg grating temperature measurement technology are organically combined through an optical fiber provided with a photothermal conversion device. The photothermal conversion device is used as the heating element, and the first fiber Bragg grating and the second fiber Bragg grating are used to synchronously monitor the temperature changes of the heating optical fiber and the soil environment. The temperature characteristic value is obtained by measuring the temperature change of the heating optical fiber, and then the soil water content is obtained based on the linear relationship between the temperature characteristic value and the moisture field. Furthermore, it is convenient to miniaturize and simplify the soil moisture content measurement device and facilitate the measurement of the soil moisture content.

[0085] To solve the above technical problems, the embodiment of the present application also provides a micro soil moisture content measurement device. Figure 3 It is a schematic structural diagram of another micro soil moisture content measurement device provided by the embodiment of the present application. As Figure 3 shown, another micro soil moisture content measurement device 3 provided by the embodiment of the present application includes: a protective sleeve 31, a photothermal conversion device 32, a first ordinary optical fiber 33, a second ordinary optical fiber 34, a first fiber Bragg grating 35, a second fiber Bragg grating 36, and a heat insulation seat 37.

[0086] In the embodiment of the present application, one end of the photothermal conversion device 32 is connected to the first ordinary optical fiber 33. Optionally, one end of the photothermal conversion device 32 is connected to one end of the first ordinary optical fiber 33 by fiber fusion splicing, and light can be transmitted between the photothermal conversion device 32 and the first ordinary optical fiber 33. The photothermal conversion device 32 in the embodiment of the present application is an element that can generate heat through the photothermal conversion principle; optionally, the photothermal conversion device 12 is a doped optical fiber, which refers to an optical fiber doped with a certain special element (such as cobalt, etc.), so that some characteristics of the optical fiber are changed. When a pump light source is injected, the doped ions transition from the ground state to the excited state by absorbing the pump light, and then through the non-radiative transition (multi-phonon relaxation) process, the laser energy is efficiently converted into heat energy, thus realizing the all-fiber passive heating function; the photothermal conversion device 12 can also be a metal-coated probe, a fiber taper probe, a misaligned fusion splicing probe, etc. or a combination of several methods. In addition, when the photothermal conversion device 12 uses a doped optical fiber, different doped optical fiber types or different doped concentration optical fiber types can be replaced according to the measurement range, measurement accuracy, etc. of the micro soil moisture content measuring device. The first ordinary optical fiber 33 in the embodiment of the present application is a fiber made of glass or plastic, and is used to transmit light to the photothermal conversion device 32.

[0087] The first ordinary optical fiber 33 and the second ordinary optical fiber 34 connected with the photothermal conversion device 32 are arranged in parallel in the protective sleeve 31. The second ordinary optical fiber 34 is connected in series with the first fiber grating 35 and the second fiber grating 36. The first fiber grating 35 is close to the photothermal conversion device 32, and the second fiber grating 36 is far from the photothermal conversion device 32. Furthermore, the photothermal conversion device 32, the first fiber grating 35 and the second fiber grating 36, etc. are arranged in the protective sleeve 31. In the embodiment of the present application, optionally, the pump light for exciting the photothermal conversion device 32 can be light with a wavelength of 1480 nm. Furthermore, the central wavelengths of the first fiber grating 35 and the second fiber grating 36 should avoid 1480 nm; optionally, the central wavelengths of the first fiber grating 35 and the second fiber grating 36 can be selected between 1520 - 1580 nm. Of course, the central wavelengths of the first fiber grating 35 and the second fiber grating 36 need to be selected within the demodulation range of the demodulator. In the embodiment of the present application, the second ordinary optical fiber 34 transmits light to the first fiber grating 35 and the second fiber grating 36 and transmits the reflected light of the first fiber grating 35 and the second fiber grating 36.

[0088] The photothermal conversion device 32, the first fiber grating 35, the second fiber grating 36, etc. are arranged in the protective sleeve 31, and the protective sleeve 31 is used to protect the photothermal conversion device 32, the first fiber grating 35, the second fiber grating 36, etc. Optionally, the protective sleeve 31 is selected as a sleeve with good heat conduction performance; such as a stainless steel sleeve, which has good corrosion resistance, heat resistance and heat conduction, so that the stainless steel sleeve can simplify the structure of the sensor probe, protect the photothermal conversion device 32, the first fiber grating 35, the second fiber grating 36, etc., and can also prevent the influence of external stress changes on the temperature measured by the grating.

[0089] An insulating seat 37 is arranged between the first fiber grating 35 and the second fiber grating 36. When the photothermal conversion device 32 is heated, it prevents the heat generated by the photothermal conversion device 32 from being transferred to the second fiber grating 36, so as to prevent the heat generated by the photothermal conversion device 32 from affecting the measurement result of the second fiber grating 36 for measuring the soil environment temperature. Further, to prevent the heat generated by the photothermal conversion device 32 from affecting the second fiber grating 36, the distance between the first fiber grating 35 and the second fiber grating 36 is not less than 5 cm.

[0090] In the embodiment of the present application, the protective sleeve 31 includes a first sub-protective sleeve 311 and a second sub-protective sleeve 312, and the first sub-protective sleeve 311 is connected to the second sub-protective sleeve 312; the first fiber grating 35 is located in the first sub-protective sleeve 311, and the second fiber grating 36 is located in the second sub-protective sleeve 312, and the connection between the first sub-protective sleeve 311 and the second sub-protective sleeve 312 is provided. Further, in the embodiment of the present application, the diameter of the first sub-protective sleeve 311 is smaller than the diameter of the second sub-protective sleeve 312, which is convenient for inserting the micro soil moisture content measuring device into the soil to be detected during use. Optionally, the first sub-protective sleeve 311 and the second sub-protective sleeve 312 can be an integrally formed structure.

[0091] Further, silicone grease 38 is injected into the protective sleeve 31. On the one hand, the silicone grease 38 realizes the fixation of the relative positions of the photothermal conversion device 32, the first ordinary optical fiber 33, the second ordinary optical fiber 34, the first fiber grating 35, the second fiber grating 36, etc. in the protective sleeve 31. On the other hand, it can make the first fiber grating 35 and the second fiber grating 36 be heated evenly, avoiding the chirping phenomenon caused by uneven heating of the first fiber grating 35 and the second fiber grating 36, resulting in the loss of grating data, so that the demodulation device cannot correctly measure the wavelength value and affect the accuracy of the soil moisture content measurement result.

[0092] In the embodiment of the present application, the protective sleeve 31 is a protective sleeve with open ends at both ends, which facilitates the arrangement of the photothermal conversion device 32, the first ordinary optical fiber 33, the second ordinary optical fiber 34, the first fiber grating 35, the second fiber grating 36, the heat insulation seat 37, etc. inside the protective sleeve 31. After the photothermal conversion device 32, the first ordinary optical fiber 33, the second ordinary optical fiber 34, the first fiber grating 35, the second fiber grating 36, etc. are fixed inside the protective sleeve 11, the open ends of the protective sleeve 31 are encapsulated with the encapsulating glue 39. The encapsulating glue 39 has the characteristics of high temperature resistance and waterproofness, which is convenient for ensuring the airtightness of the internal space of the protective sleeve 31.

[0093] In the implementation of the present application, for the convenience of the use and connection of the micro soil moisture content measuring device, a first jumper 331 is sleeved on the other end of the first ordinary optical fiber 33 and a second jumper 341 is sleeved on the other end of the second ordinary optical fiber 34 and extends out of the protective sleeve 31. Corresponding jumper connectors are respectively arranged at the ends of the first jumper 331 and the second jumper 341, and other devices for measuring the soil moisture content are connected through the jumper connectors.

[0094] Based on the micro soil moisture content measuring device 3 provided in the above embodiment, the embodiment of the present application provides a micro soil moisture content measuring system. Figure 4 It is a schematic structural diagram of a micro soil moisture content measuring system provided by the embodiment of the present application. As Figure 4 shown, the micro soil moisture content measuring system provided by the embodiment of the present application, in addition to including the micro soil moisture content measuring device 3 provided in the above embodiment, further includes a demodulator 41, an analysis host 42, and a pump light source 43.

[0095] The other end of the first ordinary optical fiber 33 is optically connected to the pump light source 43, the second ordinary optical fiber 34 is optically connected to the input end of the demodulator 41, and the output end of the demodulator 41 is connected to the analysis host 42. The demodulator 41 is a multi-channel FBG demodulator.

[0096] The light of 1480 nm emitted by the pump light source is absorbed by the photothermal conversion device 32, and the light energy is converted into heat energy through non-radiative transition, increasing the surface temperature of the first fiber grating 35, resulting in a change in the wavelength of the first fiber grating 35, realizing the functions of fixed-power heating and heating fiber temperature measurement. The second fiber grating 36 connected in series with the first fiber grating 35 changes its own grating pitch with the change of the soil temperature, thereby causing a change in the reflected wavelength, and the demodulation device deduces the external temperature by detecting the change in the wavelength. Optionally, the other end of the first ordinary optical fiber 33 can be connected to the pump light source 43 through the connector at the end of the first jumper 331, and the other end of the second ordinary optical fiber 34 can be connected to the demodulator 41 through the connector at the end of the second jumper 341.

[0097] For the specific use of the micro soil moisture content measurement system provided in this embodiment and the calculation of the soil moisture content, reference may be made to the specific use of the micro soil moisture content measurement system and the calculation of the soil moisture content provided in the foregoing embodiment, which will not be elaborated herein.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments or equivalently replace some of the technical features. These modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A micro soil moisture content measuring device, characterized in that, Comprising: A protective sleeve, a photothermal conversion device, a common optical fiber, a first fiber grating, a second fiber grating, and a heat insulation seat; wherein: One end of the photothermal conversion device is connected to one end of the common optical fiber by fiber fusion splicing; The first fiber grating is arranged on the photothermal conversion device, the second fiber grating is arranged on the common optical fiber, the first fiber grating and the second fiber grating are arranged in the protective sleeve, the heat insulation seat is arranged between the first fiber grating and the second fiber grating, and both the first fiber grating and the second fiber grating are diffraction gratings formed by axially periodically modulating the refractive index of the fiber core; The first fiber grating is used to measure the temperature change of the photothermal conversion device, the second fiber grating is used to measure the temperature change of the soil environment, and a temperature characteristic value is obtained according to the temperature change of the photothermal conversion device and the temperature change of the soil environment; The soil moisture content is obtained based on the linear relationship between the temperature characteristic value and the moisture field.

2. The micro soil moisture content measuring device according to claim 1, wherein, The photothermal conversion device includes a doped optical fiber, and the first fiber grating is written on the doped optical fiber by femtosecond laser.

3. The micro soil moisture content measuring device according to claim 1, wherein The protective sleeve includes a first sub-protective sleeve and a second sub-protective sleeve, and the first sub-protective sleeve is connected to the second sub-protective sleeve; The first fiber grating is located in the first sub-protective sleeve, the second fiber grating is located in the second sub-protective sleeve, and the heat insulation seat is located at the connection between the first sub-protective sleeve and the second sub-protective sleeve.

4. The micro soil moisture content measuring device according to claim 1, characterized in that, The distance between the first fiber grating and the second fiber grating is not less than 5 cm.

5. The micro soil moisture content measuring device according to claim 1, characterized in that The protective sleeve is a stainless steel pipe sleeve provided with an open end, the protective sleeve is filled with silicone grease, and the open end of the protective sleeve is encapsulated with encapsulating glue.

6. The micro soil moisture content measuring device according to claim 1, characterized in that, A jumper and a jumper connector are arranged on the other end of the common optical fiber.

7. The micro soil moisture content measuring device according to claim 1, characterized in that, The central wavelength range of the first fiber grating and the second fiber grating is 1520 - 1580 nm.

8. A micro soil moisture content measurement system, characterized in that, Including a micro soil moisture content measuring device, and the micro soil moisture content measuring device is the micro soil moisture content measuring device according to any one of claims 1 - 7; The micro soil moisture content measuring system further includes a wavelength division multiplexer, a demodulator, an analysis host, and a pump light source; Wherein: the first input end of the wavelength division multiplexer is optically connected to the input end of the demodulator, the second input end of the wavelength division multiplexer is optically connected to the pump light source, the output end of the wavelength division multiplexer is connected to the other end of the common optical fiber, and the output end of the demodulator is connected to the analysis host.

9. A micro soil moisture content measuring device, characterized in that, Comprising: A protective sleeve, a photothermal conversion device, a first common optical fiber, a second common optical fiber, a first fiber grating, a second fiber grating, and a heat insulation seat; wherein: One end of the photothermal conversion device is connected to one end of the first common optical fiber by fiber fusion splicing; The second ordinary optical fiber is connected in series with the first fiber Bragg grating and the second fiber Bragg grating. The photothermal conversion device, the first fiber Bragg grating and the second fiber Bragg grating are located in the protective sleeve. The first fiber Bragg grating is close to the photothermal conversion device, the second fiber Bragg grating is far from the photothermal conversion device, and a heat insulation seat is arranged between the first fiber Bragg grating and the second fiber Bragg grating. Both the first fiber Bragg grating and the second fiber Bragg grating are diffraction gratings formed by axially periodically modulating the refractive index of the fiber core; The first fiber Bragg grating is used to measure the temperature change of the photothermal conversion device, and the second fiber Bragg grating is used to measure the temperature change of the soil environment. A temperature characteristic value is obtained according to the temperature change of the photothermal conversion device and the temperature change of the soil environment; The soil moisture content is obtained based on the linear relationship between the temperature characteristic value and the moisture field.

10. A micro soil moisture content measurement system, characterized in that, It includes a micro soil moisture content measuring device, and the micro soil moisture content measuring device is the micro soil moisture content measuring device described in claim 9; The micro soil moisture content measuring system further includes a demodulator, an analysis host and a pump light source; Wherein: the other end of the first ordinary optical fiber is optically connected to the pump light source, the second ordinary optical fiber is optically connected to the input end of the demodulator, and the output end of the demodulator is connected to the analysis host.

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