Optical fiber diaphragm sensor and method for measuring liquid sample parameters

Abstract

The present invention discloses an optical fiber diaphragm sensor, which is used for measuring liquid sample parameters, and comprises a sample chamber and a detection chamber, wherein the sample chamber and the detection chamber are fixedly connected; a water inlet and a water outlet are provided on the peripheral side wall of the sample chamber, which are used for the inflow and outflow of the liquid sample to be measured; a detection port is provided on the side of the sample chamber facing the detection chamber; the detection chamber comprises a diaphragm and a first optical fiber Bragg grating, the detection chamber is connected to the atmosphere, and the diaphragm is attached to the side of the sample chamber where the detection port is provided, so as to seal the detection port; the first optical fiber Bragg grating is fixed to the side of the diaphragm surface away from the detection port; when liquid flows into the sample chamber, the diaphragm can be deformed and drive the first optical fiber Bragg grating to generate strain; the optical fiber diaphragm sensor disclosed by the present invention can measure liquid level, hydraulic pressure and temperature simultaneously, and has a large measuring range, so as to meet the actual needs of engineering.

Description

Technical Field

[0001] The invention relates to the field of optical fiber diaphragm sensors, and in particular to an optical fiber diaphragm sensor and a method for measuring liquid sample parameters. Background Art

[0002] In many fields such as chemical production, metallurgy, and petroleum industry, real-time online and accurate monitoring of liquid level and hydraulic pressure is the key to ensuring the safe and normal operation of storage devices and systems. Compared with traditional capacitive, ultrasonic, and magnetostrictive liquid level sensors, optical fiber liquid level and hydraulic pressure sensors have greater competitive advantages in harsh environments such as flammable and explosive, highly corrosive, and strong electromagnetic interference, due to their advantages of anti-electromagnetic interference, acid and alkali corrosion resistance, electrical insulation, continuous measurement, small geometric dimensions, and long service life.

[0003] At present, the fiber optic liquid level sensors reported at home and abroad mainly include: tip reflection type, light leakage type, light intensity reflection type and fiber Bragg grating type sensors. Among them, fiber Bragg grating liquid level sensors include refractive index type, buoyancy type and pressure diaphragm type. Pressure diaphragm fiber Bragg grating liquid level sensor has simple structure, high sensitivity and high precision. It is the most potential continuous large-range fiber optic liquid level sensor applied to engineering practice.

[0004] Although the pressure diaphragm fiber Bragg grating liquid level sensor has many advantages mentioned above, existing research mainly focuses on improving the sensitivity of the sensor, while the maximum range of liquid level detection is less than 130cm, which is difficult to meet the actual needs of engineering.

[0005] More importantly, there is no literature report on the simultaneous measurement of a wide range of liquid level, hydraulic pressure and temperature using a fiber Bragg grating (FBG). Therefore, it is necessary to develop a fiber Bragg grating sensor that can accurately and simultaneously detect a wide range of liquid level, hydraulic pressure and temperature. Summary of the invention

[0006] The present invention aims to provide a fiber optic diaphragm sensor and a method for measuring liquid sample parameters, so as to solve the technical problem of small detection range of the existing pressure diaphragm fiber grating liquid level sensor.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A fiber optic diaphragm sensor is used for measuring liquid sample parameters, comprising a sample chamber 1 and a detection chamber, wherein the sample chamber 1 is fixedly connected to the detection chamber; a water inlet 2 and a water outlet 3 are provided on the side walls of the sample chamber 1 for the inflow and outflow of the liquid sample to be measured; a detection port 5 is provided on the side of the sample chamber 1 facing the detection chamber; the detection chamber comprises a diaphragm 6 and a first fiber Bragg grating 9, wherein the detection chamber is connected to the atmosphere, and the diaphragm 6 is attached to the side of the sample chamber 1 where the detection port 5 is provided to seal the detection port 5; the first fiber Bragg grating 9 is fixed to the side of the diaphragm 6 away from the detection port 5; when liquid flows into the sample chamber 1, the diaphragm 6 can be deformed and drive the first fiber Bragg grating 9 to generate strain.

[0009] Preferably, the detection chamber further comprises a fixing plate 13 , the fixing plate 13 is fixedly connected to a side of the diaphragm 6 away from the sample chamber 1 , and a second fiber Bragg grating 12 is fixed on the fixing plate 13 .

[0010] Preferably, the first fiber Bragg grating includes a first grating area 20 corresponding to the position of the diaphragm 6, and a first pigtail area 10 extending from the position of the diaphragm 6 toward the detection chamber; the second fiber Bragg grating includes a second grating area 26 corresponding to the position of the fixed plate 13, and a second pigtail area 14 extending from the position of the fixed plate 13 toward the detection chamber.

[0011] Preferably, a constant pressure chamber 16 is fixed on the side of the fixed plate 13 away from the diaphragm 6, and a fiber guide hole 15 is opened on the side of the constant pressure chamber 16 facing the fixed plate 13. A conduit 17 for connecting the inside of the constant pressure chamber 16 with the external atmosphere is installed on the top of the constant pressure chamber 16, and the free end of the first pigtail area 10 and the free end of the second pigtail area 14 pass through the conduit 17.

[0012] Preferably, a first mounting groove 8 is opened on the side of the diaphragm 6 corresponding to the detection port 5 and away from the detection port 5, the first fiber Bragg grating 9 is fixed in the first mounting groove 8, and the first grating area 20 is located in the first mounting groove 8; a second mounting groove 11 is opened on the side of the fixing plate 13 away from the diaphragm 6, the second fiber Bragg grating 12 is fixed in the second mounting groove 11, and the second grating areas 26 and 24 are located in the first mounting groove 8.

[0013] Preferably, a reserved deformation opening 19 is provided at a position on the fixing plate 13 corresponding to the detection opening 5 .

[0014] Preferably, the shape of the detection port 5 of the sample chamber 1 is elliptical, and the major axis of the ellipse is parallel to the installation direction of the first fiber Bragg grating 9 .

[0015] The present invention also discloses a method for measuring liquid sample parameters, using the optical fiber diaphragm sensor as described above, characterized in that it comprises the following steps:

[0016] Step 1, establishing a mathematical model between the sensitivity coefficient of the optical fiber diaphragm sensor and the parameters of the liquid sample to be measured;

[0017] Step 2: Determine the sensitivity coefficient K of the optical fiber diaphragm sensor through experiments L , K T,1 , K T,2 and K P , obtaining a relationship between the Bragg wavelength change value of the first fiber Bragg grating and the second fiber Bragg grating and the parameters of the liquid sample to be measured;

[0018] Step 3, placing the sample chamber in the liquid sample to be tested;

[0019] Step 4, obtaining Bragg wavelength change values ​​of the first fiber Bragg grating and the second fiber Bragg grating;

[0020] Step 5: Substitute the values ​​into the mathematical model in step 1 to obtain the parameter values ​​of the liquid sample to be tested.

[0021] Preferably, in step 1, the mathematical model between the sensitivity coefficient of the optical fiber diaphragm sensor and the parameters of the liquid sample to be measured is:

[0022]

[0023] In the formula, λ B,L Refers to the absolute Bragg wavelength of the first fiber Bragg grating at different liquid levels, λ B,T Refers to the absolute Bragg wavelength of the second fiber Bragg grating at different temperatures, λ B,P refers to the absolute Bragg center wavelength of the first fiber Bragg grating under different hydrostatic pressures, K L Refers to the liquid level sensitivity coefficient, K T,1 Refers to the first fiber Bragg grating temperature sensitivity coefficient, K T,2 Refers to the temperature sensitivity coefficient of the second fiber Bragg grating, K p refers to the pressure sensitivity coefficient, △L refers to the change in liquid level, △T refers to the change in temperature, △P refers to the change in hydrostatic pressure, and λ B,0 refers to the initial Bragg wavelength of the first fiber Bragg grating, λ T,0 Refers to the initial central wavelength of the second fiber Bragg grating.

[0024] Preferably, in step 2, the relationship between the Bragg wavelength change value of the first fiber Bragg grating and the second fiber Bragg grating and the parameters of the liquid sample to be measured is:

[0025]

[0026] In the formula, △L refers to the change in liquid level, △T refers to the change in temperature, △P refers to the change in hydrostatic pressure, and K L Refers to the liquid level sensitivity coefficient, K T,1 Refers to the first fiber Bragg grating temperature sensitivity coefficient, K T,2 Refers to the temperature sensitivity coefficient of the second fiber Bragg grating, K p Refers to the pressure sensitivity coefficient, △λ B,L Refers to the Bragg wavelength drift of the first fiber Bragg grating caused by the liquid level change, △λ B,T Refers to the Bragg wavelength drift of the second fiber Bragg grating due to temperature change, △λ B,P Refers to the Bragg wavelength drift of the first fiber Bragg grating caused by pressure change.

[0027] The present invention has the following beneficial effects:

[0028] 1. The optical fiber diaphragm sensor disclosed in the present invention can measure liquid level, hydraulic pressure and temperature simultaneously, and has a large measuring range, which can meet the actual needs of engineering.

[0029] 2. The present invention adopts two fiber Bragg gratings fixed on the diaphragm and the fixed plate respectively, and the monitoring data of the second fiber Bragg grating is coupled with the monitoring data of the first fiber Bragg grating, which can eliminate the influence of the change of the diaphragm temperature on the measurement result of the first fiber Bragg grating, thereby realizing the accurate measurement of the three parameters of liquid level, hydraulic pressure and temperature.

[0030] 3. The present invention also discloses a theoretical model for the optical fiber diaphragm sensor to simultaneously detect liquid level, hydraulic pressure and temperature, which provides a good theoretical basis for the optical fiber diaphragm sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to make the purpose, technical solution and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0032] Figure 1 It is a schematic diagram of the sensor structure of the present invention.

[0033] Figure 2 For the present invention Figure 1 Schematic diagram of the constant pressure cabin structure, view from the A direction in the middle.

[0034] Figure 3 It is a schematic diagram of the diaphragm structure of the present invention.

[0035] Figure 4 It is a schematic diagram of assembling the fiber Bragg grating of the present invention on the surface of the diaphragm.

[0036] Figure 5 It is a schematic diagram of the fixing plate structure of the present invention.

[0037] Figure 6 It is a schematic diagram of assembling the fiber Bragg grating of the present invention on the surface of a fixed plate.

[0038] Figure 7 It is a schematic diagram of the local structure of the fiber Bragg grating of the present invention.

[0039] Figure 8 Schematic diagram of the temperature response characteristics of the sensor according to an embodiment of the present invention.

[0040] Fig. 9 Schematic diagram of the liquid level and pressure response characteristics before and after the temperature compensation of the first fiber Bragg grating according to an embodiment of the present invention.

[0041] Explanation of the reference numerals: 1. sample chamber; 2. water inlet; 3. water outlet; 4. screw hole; 5. detection port; 6. diaphragm; 7. adhesive; 8. first mounting groove; 9. first fiber Bragg grating; 10. first pigtail area; 11. second mounting groove; 12. second fiber Bragg grating; 13. fixing plate; 14. second pigtail area; 15. fiber guide hole; 16. constant pressure chamber; 17. conduit; 18. screw; 19. reserved deformation opening; 20. first grating area; 21. fiber core; 22. cladding; 23. coating layer; 24. grating area; 25. pigtail area with coating removed; 26. second grating area. DETAILED DESCRIPTION

[0042] To make the purpose, technical solution and advantages of the embodiments of the present invention more clear, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0043] It should be noted that similar numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings. In the description of the present invention, it should be noted that the orientation or position relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inside", "outside", etc. is based on the orientation or position relationship shown in the drawings, or the orientation or position relationship in which the invention product is usually placed when used, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention. In addition, the terms "first", "second", "third", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance. In addition, the terms "horizontal", "vertical", etc. do not mean that the components are absolutely horizontal or suspended, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly tilted. In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0044] The invention solves the technical problem that the existing pressure diaphragm type optical fiber grating liquid level sensor has a small detection range.

[0045] The fiber Bragg grating mentioned in the present invention generally includes a grating area and a pigtail area, wherein the grating area is composed of a row of parallel semi-reflective mirrors with a spacing of Λ, Λ is called the Bragg spacing. The basic characteristic of the fiber Bragg grating is a narrow-band optical filter centered on a resonant wavelength, which is called the Bragg wavelength. The grating acts like a strong reflector.

[0046] Specifically in the present invention, the region of the first fiber Bragg grating with grating function is called the first grating region, and the pigtail region of the first fiber Bragg grating is called the first pigtail region; the region of the second fiber Bragg grating with grating function is called the second grating region, and the pigtail region of the second fiber Bragg grating is called the second pigtail region.

[0047] In the present invention, polymethyl methacrylate is used to make an optical fiber diaphragm sensor with a length, width and height of 1 to 30 cm, which can accurately measure three parameters of liquid working fluid (working fluid is a medium that can realize the mutual conversion of thermal energy and mechanical energy, such as gas, steam, etc.) temperature in the range of 10 to 80°C, liquid level height of 0 to 10m and hydraulic pressure of 0 to 100KPa.

[0048] like Figure 1 and Figure 2 As shown, based on the above technical problems to be solved, the present invention discloses an optical fiber diaphragm sensor, comprising a sample chamber 1 and a detection chamber, wherein the sample chamber 1 and the detection chamber are fixedly connected;

[0049] The sample chamber 1 is provided with a water inlet 2 and a water outlet 3 on its side wall for the inflow and outflow of the liquid sample to be tested; the sample chamber 1 is provided with a detection port 5 on its side facing the detection chamber;

[0050] The detection chamber comprises a diaphragm 6 and a first fiber Bragg grating 9. The detection chamber is connected to the atmosphere. The diaphragm 6 is attached to the side of the sample chamber 1 where the detection port 5 is opened to seal the detection port 5. The first fiber Bragg grating 9 is fixed to the side of the diaphragm 6 away from the detection port 5.

[0051] When liquid flows into the sample chamber 1, the diaphragm 6 is deformed and causes the first fiber Bragg grating 9 to generate strain.

[0052] Specifically, the liquid sample parameters are liquid level, hydraulic pressure and liquid temperature.

[0053] Specifically, the diaphragm 6 is attached to the side of the sample chamber 1 where the detection port 5 is opened, forming a seal for the detection port 5; the diaphragm 6 can completely cover the detection port 5. After the liquid sample to be tested enters the sample chamber 1, the liquid sample to be tested in the sample chamber 1 generates pressure on the diaphragm 6. At the same time, the detection chamber is in a relatively independent environment, and the interior of the detection chamber is air, and the internal air pressure is consistent with the external atmospheric pressure. Therefore, a pressure difference can be formed on both sides of the diaphragm 6, thereby driving the first fiber Bragg grating 9 to generate strain. The resonant center wavelength of the first fiber Bragg grating 9 will drift with the deformation of the diaphragm 6 and the temperature change. Therefore, the first fiber Bragg grating 9 can respond to the liquid level hydraulic pressure and temperature change information at the same time.

[0054] Specifically, the water inlet 2 is opened at the top of the sample chamber 1, and the water outlet 3 is opened at the bottom of the sample chamber 1. The diameter of the water inlet is 0.2-5 cm, and the diameter of the water outlet is 0.2-5 cm.

[0055] As an embodiment of the positions of the water inlet and the water outlet on the surface of the sample chamber, the water inlet 2 and the water outlet 3 can also be arranged at symmetrical positions on the top and bottom of the sample chamber 1, or can be both opened on the sides of the sample chamber, both of which can realize the functions of the optical fiber diaphragm sensor disclosed in the present invention and achieve accurate measurement of the liquid parameters to be measured.

[0056] Specifically, the sample chamber 1 is in the shape of a rectangular parallelepiped structure, and its length, width and height are 1 to 30 cm, 1 to 30 cm and 1 to 30 cm respectively.

[0057] Specifically, the detection port 5 of the sample chamber 1 is circular or elliptical in shape.

[0058] Preferably, the detection port 5 of the sample chamber 1 is elliptical in shape, with a major axis length of 1 to 5 cm and a minor axis length of 0.5 to 4 cm.

[0059] Compared with circular holes or holes of other shapes, elliptical holes can increase the effective deformation diameter of the diaphragm, thereby improving the sensitivity of the sensor; at the same time, elliptical holes can also help reduce the diaphragm breakage rate and increase the service life of the sensor.

[0060] Specifically, the diaphragm 6 is rectangular, and its shape is adapted to the side shape of the sample chamber 1. The length, width and thickness of the diaphragm 6 are 1-30 cm, 1-30 cm and 0.2-1 cm respectively, and the diaphragm 6 is made of rubber.

[0061] Specifically, the reflection center wavelength of the first fiber Bragg grating 9 is 1515-1580 nm, the length is 2-20 mm, and the pigtail length is 0.5-10 m.

[0062] As a preference, Figure 3 and Figure 4 as well as Figure 7 As shown, a first mounting groove 8 is provided on the side of the diaphragm 6 corresponding to the detection port 5 and away from the detection port 5, a first fiber Bragg grating 9 is fixed in the first mounting groove 8, and a first grating area 20 is located in the first mounting groove 8. The first fiber Bragg grating includes a first grating area 20 corresponding to the position of the diaphragm 6, and a first pigtail area 10 extending from the position of the diaphragm 6 toward the detection chamber.

[0063] The steps of installing the first fiber Bragg grating 9 in the first mounting groove 8 of the diaphragm 6 are as follows:

[0064] Step 1.1, remove the pigtail coating 23 at 1 to 5 cm above and below the grating area 24 of the first fiber Bragg grating 9 and 1 to 3 cm in length, and then use a blade to carve a first mounting groove 8 with a depth and width of 125 to 500 μm on the surface of the diaphragm 6; the grating area 24 here is the first grating area 20 of the first fiber Bragg grating 9.

[0065] Step 1.2, placing the first fiber Bragg grating 9 into the first mounting groove 8, and allowing the first fiber Bragg grating 9 to be in a free extension state;

[0066] Step 1.3: Use adhesive 7 to bond the pigtail of the first fiber Bragg grating 9 with the coating layer 23 removed to the diaphragm 6 .

[0067] In step 1.3, the adhesive 7 is UV shadowless glue or epoxy resin AB glue.

[0068] The detection chamber further comprises a fixing plate 13, which is fixedly connected to a side of the diaphragm 6 away from the sample chamber 1, and a second fiber Bragg grating 12 is fixed on the fixing plate 13. The second fiber Bragg grating 12 is used to respond to temperature change information.

[0069] Specifically, the length, width and thickness of the fixing plate 13 are 1-30 cm, 1-30 cm and 0.2-1 cm respectively, the reflection center wavelength of the second fiber Bragg grating 12 is 1515-1580 nm, the length is 2-20 mm, and the pigtail length is 0.5-10 m.

[0070] As a preference, Figure 5 and Figure 6 as well as Figure 7 As shown, a second mounting groove 11 is formed on a side of the fixing plate 13 away from the diaphragm 6 , a second fiber Bragg grating 12 is fixed to the second mounting groove 11 , and the second grating regions 26 , 24 are located in the first mounting groove 8 .

[0071] The second fiber Bragg grating includes a second grating region 26 corresponding to the position of the fixing plate 13 and a second pigtail region 14 extending from the position of the fixing plate 13 toward the detection chamber.

[0072] The second fiber Bragg grating 12 is installed in the second installation groove 11 of the fixing plate 13 by the following steps:

[0073] Step 2.1, remove the pigtail coating 23 at 1 to 5 cm above and below the grating region 24 of the second fiber Bragg grating 12 and 1 to 3 cm in length; the grating region 24 here is the second grating region 26 of the second fiber Bragg grating 12.

[0074] Step 2.2, using a blade to carve a second mounting groove 11 with a depth and width of 125 to 200 μm in the non-hole area on the surface of the fixing plate 13, placing the second fiber Bragg grating 12 in the second mounting groove 11, and allowing the second fiber Bragg grating 12 to be in a free extension state;

[0075] Step 2.3, using adhesive 7 to bond the pigtail of the second fiber Bragg grating 12 with the coating layer 23 removed to the fixing plate 13;

[0076] The adhesive 7 in step 2.3 is UV shadowless glue or epoxy resin AB glue.

[0077] Preferably, a reserved deformation opening 19 is provided at a position corresponding to the detection opening 5 on the fixing plate 13. The reserved deformation opening 19 on the fixing plate 13 is the same size as the detection opening 5 of the sample chamber 1 and has a corresponding position relationship. The reserved deformation opening 19 is used to reserve space for the diaphragm 6 to deform.

[0078] Specifically, the reserved deformation opening 19 is circular or elliptical in shape.

[0079] Preferably, the reserved deformation opening 19 is elliptical in shape, with a major axis of 1 to 5 cm and a minor axis of 0.5 to 4 cm;

[0080] Compared with circular holes or holes of other shapes, elliptical holes can increase the effective deformation diameter of the diaphragm, thereby improving the sensitivity of the sensor; at the same time, elliptical holes can also help reduce the diaphragm breakage rate and increase the service life of the sensor.

[0081] A constant pressure chamber 16 is fixed on the side of the fixing plate 13 away from the diaphragm 6, and a fiber guide hole 15 is opened on the side of the constant pressure chamber 16 facing the fixing plate 13. A conduit 17 for connecting the inside of the constant pressure chamber 16 with the external atmosphere is installed on the top of the constant pressure chamber 16, and the free end of the first pigtail area 10 and the free end of the second pigtail area 14 pass through the conduit 17.

[0082] The conduit 17 connects the interior of the constant pressure cabin 16 with the external atmospheric environment, so that the air pressure in the constant pressure cabin 16 is consistent with the external atmospheric pressure.

[0083] Specifically, the length, width and height of the constant pressure chamber 16 are 1-30 cm, 1-30 cm and 1-30 cm respectively; the fiber guide hole 15 is elliptical in shape, with a major axis of 1-5 cm and a minor axis of 0.5-4 cm; the inner and outer diameters of the catheter 17 are 0.3-100 mm and 1-200 mm respectively.

[0084] The first pigtail area 10 of the first fiber Bragg grating 9 is arranged in the detection cabin as follows: the pigtail of the first fiber Bragg grating 9 first passes through the reserved deformation opening 19, then passes through the fiber guide hole 15 on the constant pressure cabin 16, and then passes through the conduit 17 and is connected to the fiber grating demodulator.

[0085] The second pigtail area 14 of the second fiber Bragg grating 12 is arranged in the detection cabin as follows: the pigtail of the second fiber Bragg grating 12 first passes through the fiber guide hole 15 on the constant pressure cabin 16, then passes through the conduit 17, and is connected to the fiber Bragg grating demodulator.

[0086] The fiber Bragg grating demodulator is used to obtain information about the changes of the resonance center wavelengths of the first fiber Bragg grating 9 and the second fiber Bragg grating 12 with the liquid level, hydraulic pressure and temperature.

[0087] The free end of the first pigtail region 10 of the first fiber Bragg grating 9 and the free end of the second pigtail region 14 of the second fiber Bragg grating 12 are fixed to the inner wall of the conduit 17 by means of adhesive 7. The adhesive 7 is UV shadowless adhesive or epoxy resin AB adhesive.

[0088] The diaphragm 6, the fixed plate 13 and the constant pressure chamber 16 are fixedly connected to form a detection chamber with a relatively independent space to prevent the liquid sample to be tested from entering the detection chamber. The detection chamber is connected to the external air environment through a conduit 17 to ensure that the air pressure in the detection chamber is consistent with that of the external air environment, thereby ensuring that the diaphragm 6 is not affected by the air pressure when it is deformed.

[0089] In specific implementation, the sample chamber 1 and the detection chamber are provided with a plurality of screw holes 4, and the sample chamber 1 and the detection chamber are fixedly connected by screws 18. The number of the screw holes 4 is 4 to 8, and the diameter is 2 to 10 mm. Preferably, a layer of sealing silicone grease is applied between the sample chamber 1 and the diaphragm 6, between the diaphragm 6 and the fixing plate 13 in the detection chamber, and between the fixing plate 13 and the constant pressure chamber 16 to enhance the spatial independence of the sensor, and also to prevent the sample from leaking out, thereby improving the measurement accuracy.

[0090] Preferably, the material of the sensor itself is a material that is resistant to acid and alkali corrosion and high temperature expansion, such as polymethyl methacrylate, Teflon, polytetrafluoroethylene sheet, etc.

[0091] The working principle of the present invention is: injecting a constant temperature liquid sample (temperature control accuracy is ±0.5°C) into the sample chamber 1, causing the sensor diaphragm 6 to deform, thereby causing the first fiber Bragg grating 9 to produce strain, resulting in a drift in the central wavelength; the higher the liquid level, the greater the pressure difference formed on both sides of the diaphragm 6, causing the diaphragm 6 to deform more seriously, causing the first fiber Bragg grating 9 to drift more. At a constant temperature, the liquid level height can be measured by detecting the drift of the central wavelength of the first fiber Bragg grating 9. In addition, the liquid level height and pressure have a one-to-one correspondence. According to this correspondence, the sensor can simultaneously measure the liquid level and hydraulic binary parameters. Because the second fiber Bragg grating 12 is fixed on a fixed plate 13 that is not easy to deform, it only responds to the liquid phase temperature change information; coupling the second fiber Bragg grating 12 and the first fiber Bragg grating 9 can eliminate the influence of temperature on the measurement result of the first fiber Bragg grating 9.

[0092] In summary, the present invention also solves the technical problem that the existing pressure diaphragm type fiber Bragg grating liquid level sensor cannot measure liquid level, hydraulic pressure and temperature at the same time. The fiber optic diaphragm sensor disclosed in the present invention can realize the simultaneous measurement of three parameters of liquid level, hydraulic pressure and temperature, and has high measurement accuracy.

[0093] The present invention also discloses a method for measuring liquid sample parameters, using the optical fiber diaphragm sensor as described above, characterized in that it comprises the following steps:

[0094] Step 1, establishing a mathematical model between the sensitivity coefficient of the optical fiber diaphragm sensor and the parameters of the liquid sample to be measured;

[0095] Step 2, placing the sample chamber in the liquid sample to be tested;

[0096] Step 3, obtaining Bragg wavelength change values ​​of the first fiber Bragg grating and the second fiber Bragg grating;

[0097] Step 4: Substitute the values ​​into the mathematical model in step 1 to obtain the parameter values ​​of the liquid sample to be tested.

[0098] In step 1, the process of establishing the mathematical model between the sensitivity coefficient of the optical fiber diaphragm sensor and the parameters of the liquid sample to be measured is as follows:

[0099] First, the Bragg center wavelength (λ B ) and the effective refractive index of the optical fiber (n eff ) and the grating period (Λ) show a linear relationship, as shown in equation (1):

[0100] λ B =2n eff Λ (1)

[0101] In formula (1), λ B Refers to the Bragg center wavelength of the fiber Bragg grating, n eff refers to the effective refractive index of the optical fiber, and Λ refers to the grating period.

[0102] Due to the optical fiber photoelastic effect (the phenomenon that the presence of stress waves in the medium can change the dielectric constant or refractive index of the medium, thereby affecting the propagation characteristics of light in the medium), strain can make λ B Drift occurs;

[0103] At the same time, due to the thermo-optic effect (thermo-optic effect, which refers to the phenomenon that the optical properties of liquid crystal change with temperature by heating or cooling the liquid crystal to change its molecular arrangement, and its effect on the change of refractive index is opposite to the plasma dispersion effect, the higher the temperature, the greater the refractive index) and thermal expansion, temperature will cause λ B drift;

[0104] In summary, the first fiber Bragg grating is sensitive to both strain and temperature;

[0105] The drift of the Bragg center wavelength (Δλ B ) is defined as:

[0106] Δλ B =λ B [(1-P ε )ε f +(α+ζ)ΔT] (2)

[0107] In formula (2), λ B Refers to the Bragg center wavelength of the fiber Bragg grating, P ε Refers to the effective elastic-optical coefficient of optical fiber, ε f It refers to the stress on the optical fiber, α refers to the thermal expansion coefficient of the optical fiber material, ζ refers to the thermo-optical coefficient, and △T refers to the temperature change.

[0108] From formula (2), it can be concluded that under the same temperature and stress, Δλ B It depends not only on the physical parameters of the optical fiber, but also on the geometric parameters of the diaphragm.

[0109] In the optical fiber diaphragm sensor disclosed in the present invention, the liquid sample in the external environment enters the detection chamber from the water inlet of the detection chamber, causing the diaphragm to deform, thereby causing the first optical fiber Bragg grating to deform, resulting in the Bragg center wavelength drift. The higher the liquid level in the external environment, the greater the pressure on the diaphragm, resulting in greater deformation of the diaphragm, and causing a greater drift of the center wavelength of the first optical fiber Bragg grating.

[0110] However, changes in the temperature of the diaphragm itself will also cause the central wavelength of the first fiber Bragg grating to drift, resulting in the first fiber Bragg grating being affected by multi-parameter cross-influence. At the same time, the second fiber Bragg grating is installed on a fixed plate that is not easily deformed and only responds to temperature changes.

[0111] Therefore, in order to achieve accurate measurement of liquid level, hydraulic pressure and temperature by the optical fiber diaphragm sensor, a second optical fiber Bragg grating as a temperature compensation unit is used to compensate for the temperature interference signal of the first optical fiber Bragg grating.

[0112] First, assume that the liquid level sensitivity coefficient (K L ) is as follows:

[0113] K L =k(1-P ε )ε f (3)

[0114] In formula (3), ε f refers to the stress on the optical fiber, k refers to the correction factor, P ε Refers to the effective elastic-optical coefficient of optical fiber.

[0115] Assuming the temperature sensitivity coefficient (K T,1 ) is as follows:

[0116] K T,1 =(α+ζ)△T (4)

[0117] In formula (4), α refers to the thermal expansion coefficient of the optical fiber material, ζ refers to the thermo-optical coefficient, and △T refers to the temperature change.

[0118] Then, under different liquid levels, the absolute Bragg wavelength (λ B,L ) and the initial Bragg wavelength (λ B,0 ), △T and liquid level change (△L) can be expressed as:

[0119] λ B,L =K L ΔL+K T,1 ΔT+λ B,0 (5)

[0120] In formula (5), λ B,L refers to the absolute Bragg wavelength of the first fiber Bragg grating, K L refers to the liquid level sensitivity coefficient, △L refers to the liquid level change, K T,1 refers to the temperature sensitivity coefficient of the first fiber Bragg grating, △T refers to the temperature change, λ B,0Refers to the initial Bragg wavelength of the first fiber Bragg grating.

[0121] Furthermore, the liquid level height and the hydrostatic pressure (P) have the following functional relationship:

[0122] P=ρgh (6)

[0123] In formula (6), P refers to the hydrostatic pressure, ρ refers to the liquid density, g refers to the gravitational acceleration, and h refers to the liquid level.

[0124] Under different hydrostatic pressures, the absolute Bragg center wavelength (λ B,P ) and the initial Bragg wavelength (λ B,0 ), △T and pressure change (△P) can be expressed as:

[0125] λ B,P =K P ΔP+K T,1 ΔT+λ B,0 (7)

[0126] In formula (7), λ B,P refers to the absolute Bragg center wavelength of the first fiber Bragg grating under different hydrostatic pressures, K p refers to the pressure sensitivity coefficient, △P refers to the change in hydrostatic pressure, K T,1 refers to the temperature sensitivity coefficient of the first fiber Bragg grating, △T refers to the temperature change, λ B,0 Refers to the initial Bragg wavelength of the first fiber Bragg grating.

[0127] In addition, for the second fiber Bragg grating, since it is not affected by the change of liquid level and hydraulic pressure, its absolute Bragg wavelength (λ B,T ) is only affected by temperature and can be expressed as:

[0128] λ B,T =K T,2 ΔT+λ T,0 (8)

[0129] In formula (8), λ B,T Refers to the absolute Bragg wavelength of the second fiber Bragg grating at different temperatures, K T,2 refers to the temperature sensitivity coefficient of the second fiber Bragg grating, λ T,0 refers to the initial central wavelength of the second fiber Bragg grating, and ΔT refers to the temperature change.

[0130] From equations (5), (7) and (8), we can get the following matrix:

[0131]

[0132] In formula (9), λ B,L Refers to the absolute Bragg wavelength of the first fiber Bragg grating at different liquid levels, λ B,T Refers to the absolute Bragg wavelength of the second fiber Bragg grating at different temperatures, λ B,P refers to the absolute Bragg center wavelength of the first fiber Bragg grating under different hydrostatic pressures, K L Refers to the liquid level sensitivity coefficient, K T,1 Refers to the first fiber Bragg grating temperature sensitivity coefficient, K T,2 Refers to the temperature sensitivity coefficient of the second fiber Bragg grating, K p refers to the pressure sensitivity coefficient, △L refers to the change in liquid level, △T refers to the change in temperature, △P refers to the change in hydrostatic pressure, and λ B,0 refers to the initial Bragg wavelength of the first fiber Bragg grating, λ T,0 Refers to the initial central wavelength of the second fiber Bragg grating.

[0133] The sensor sensitivity coefficient K can be determined by experiment L , K T,1 , K T,2 and K P , and then through the inverse operation of the matrix, the liquid level and hydraulic pressure change information can be obtained:

[0134]

[0135] In formula (10), △L refers to the change in liquid level, △T refers to the change in temperature, △P refers to the change in hydrostatic pressure, and K L Refers to the liquid level sensitivity coefficient, K T,1 Refers to the first fiber Bragg grating temperature sensitivity coefficient, K T,2 Refers to the temperature sensitivity coefficient of the second fiber Bragg grating, K p Refers to the pressure sensitivity coefficient, △λ B,L Refers to the Bragg wavelength drift of the first fiber Bragg grating caused by the liquid level change, △λ B,T Refers to the Bragg wavelength drift of the second fiber Bragg grating due to temperature change, △λ B,P Refers to the Bragg wavelength drift of the first fiber Bragg grating caused by pressure change.

[0136] Formula (10) shows that coupling the second fiber Bragg grating with the first fiber Bragg grating can eliminate the influence of the change of the diaphragm temperature on the measurement result of the first fiber Bragg grating. The fiber optic diaphragm sensor disclosed in the present invention can realize the accurate measurement of the three parameters of liquid level, hydraulic pressure and temperature.

[0137] The optical fiber diaphragm sensor and the method for measuring liquid sample parameters disclosed in the present invention have the following technical effects: the optical fiber diaphragm sensor disclosed in the present invention can measure liquid level, hydraulic pressure and temperature simultaneously, and has a large measuring range, which can meet the actual needs of the project; the present invention adopts two optical fiber Bragg gratings fixed on the diaphragm and the fixed plate respectively, and the monitoring data of the second optical fiber Bragg grating and the first optical fiber Bragg grating are coupled, which can eliminate the influence of the change of the diaphragm temperature on the measurement result of the first optical fiber Bragg grating, thereby realizing the accurate measurement of the three parameters of liquid level, hydraulic pressure and temperature; the present invention also discloses a theoretical model of the optical fiber diaphragm sensor for simultaneously detecting liquid level, hydraulic pressure and temperature, which provides a good theoretical basis for the optical fiber diaphragm sensor.

[0138] Example

[0139] This embodiment adopts the above-mentioned optical fiber diaphragm sensor to analyze and verify its response characteristics.

[0140] In this embodiment, the sensor housing is made of polymethyl methacrylate sheet material, the length, width and height of the detection cabin are 4cm, 1.5cm and 4cm respectively, the diameter of the water inlet and outlet is 3mm, the detection port is elliptical, its major axis is 20mm and the minor axis is 15mm. The diaphragm is made of natural rubber, the thickness of the diaphragm is 500μm, the width of the first installation groove is 300μm and the depth is 300μm. The fixing plate is made of polymethyl methacrylate with a thickness of 5mm, the reserved deformation opening is elliptical, the major axis is 20mm and the minor axis is 15mm, the width of the second installation groove is 150μm and the depth is 150μm. The dimensions of the constant pressure cabin are: length, width and height are 4cm, 1.5cm and 4cm respectively, the fiber guide hole is elliptical, its major axis is 20mm and the minor axis is 15mm, the catheter on the constant pressure cabin uses a corrosion-resistant silicone tube with inner and outer diameters of 0.5mm and 2mm respectively, and the catheter length is 3m.

[0141] In this embodiment, the reflection center wavelength of the first fiber Bragg grating and the second fiber Bragg grating is 1550 nm, the length is 5 mm, and the pigtail length is 4 m.

[0142] The coating layer of the pigtail is removed at a distance of 1 cm above and below the grating area of ​​the first optical fiber Bragg grating and about 5 mm in length, and the pigtail area 25 without the coating layer is fixed in the first mounting groove with UV shadowless adhesive to sense the liquid level hydraulic pressure and liquid phase temperature change information caused by the deformation of the diaphragm due to the liquid level change.

[0143] The pigtail coating is removed at a distance of 1 cm above and below the second optical fiber Bragg grating area and about 5 mm in length, and the pigtail area 25 without the coating is fixed in the second mounting groove with UV shadowless adhesive for sensing temperature change information.

[0144] The pigtail of the first fiber Bragg grating and the pigtail of the second fiber Bragg grating are passed out of the catheter, and the free ends of the pigtail of the first fiber Bragg grating and the pigtail of the second fiber Bragg grating are passed out of the catheter to the outside of the sensor, and are connected to the fiber Bragg grating demodulator to obtain information on the changes of the resonant center wavelengths of the first fiber Bragg grating and the second fiber Bragg grating with the liquid level, hydraulic pressure and temperature.

[0145] (a) Sensor response characteristics to temperature.

[0146] In order to study the response characteristics of the sensor to temperature, the sensor was placed in a constant temperature water bath, the water bath temperature range was set at 25-70°C, and the temperature increase step was 5°C. At each step, after the change of the fiber Bragg grating reflection center wavelength stabilized, the fiber Bragg grating demodulator was used to continuously record the stable center wavelength data for 3 minutes, and the average value of this set of data was taken as the reflection wavelength value corresponding to the temperature. The experimental results are as follows Figure 8 shown.

[0147] Figure 8 It shows that the reflection center wavelength of the fiber Bragg grating redshifts during the temperature rise process. The sensitivities of the first fiber Bragg grating and the second fiber Bragg grating are 10.07pm / ℃ and 11.20pm / ℃ respectively, and the linearities are 0.9998 and 0.9997 respectively. The maximum relative error is 0.25%. This shows that the sensor can accurately measure the temperature, and the second fiber Bragg grating can be used to perform temperature compensation on the first fiber Bragg grating, so that the sensor can accurately measure the liquid level and hydraulic pressure.

[0148] (II) Sensor dynamic liquid level and hydraulic response characteristics

[0149] When the liquid level rise rate is 60 cm / min and the liquid level drop rate is 40 cm / min, and the temperature is within the range of 20-60°C, the relationship between the first fiber Bragg grating resonant center wavelength and the liquid level height and hydraulic pressure change is as follows: Fig. 9The temperature information of the first fiber Bragg grating is compensated by the second fiber Bragg grating, and the response characteristics of the first fiber Bragg grating to the liquid level and hydraulic pressure at different temperatures after compensation are obtained, as shown in FIG. Fig. 9 (cd) shown.

[0150] Fig. 9 (ab) shows the liquid level and pressure response characteristics of the first fiber Bragg grating before temperature compensation: in the temperature range of 20-30℃ and the hydraulic pressure range of 0-22KPa, the linear sensitivity of the first fiber Bragg grating reaches 0.985, and the liquid level and hydraulic pressure sensitivities are 34.55pm / cm and 354pm / KPa respectively, which can meet the actual measurement requirements of liquid level and hydraulic pressure in normal temperature environment. However, in the range of 30-60℃, as the temperature increases, the linear sensitivity of the first fiber Bragg grating to liquid level and hydraulic pressure decreases, and at the liquid level of 220cm, the maximum measurement error reaches 34%; the reason is that the high temperature causes the diaphragm to expand thermally, resulting in a decrease in the elastic deformation of the diaphragm to the liquid level strain, which in turn causes the sensor sensitivity to decrease with the increase in temperature. In the liquid level range of 0-220cm and the hydraulic pressure range of 0-22KPa, it is difficult to achieve accurate measurement of liquid level and hydraulic pressure.

[0151] Fig. 9 (cd) shows the liquid level and pressure response characteristics after the first fiber Bragg grating temperature compensation: in the temperature range of 20-60℃, after the first fiber Bragg grating is temperature compensated by the temperature result measured by the second fiber Bragg grating, the average linearity of the sensor output signal reaches R2=0.983, the average liquid level and hydraulic sensitivity are 35.4pm / cm and 351.6pm / KPa respectively, and the maximum relative error is 4.7%, which meets the actual needs of liquid level and hydraulic pressure measurement. It shows that within the range of 0-220cm liquid level and 0-22KPa hydraulic pressure, the first fiber Bragg grating can realize accurate measurement of liquid level and hydraulic binary parameters under the temperature compensation of the second fiber Bragg grating.

[0152] Therefore, in this embodiment, the optical fiber diaphragm sensor can simultaneously achieve accurate measurement of three parameters: liquid level, hydraulic pressure, and temperature.

[0153] The liquid level sensitivity coefficient K obtained in this embodiment is L =35.40pm / cm, the first fiber Bragg grating temperature sensitivity coefficient K T,1 =10.07pm / ℃, the temperature sensitivity coefficient of the second fiber Bragg grating K T,2＝ 11.20pm / ℃, pressure sensitivity coefficient K p=351.6pm / KPa, substituting into formula (10) can obtain the matrix equation of the sensor of this embodiment, as shown in formula (11):

[0154]

[0155] In formula (11), △L refers to the change in liquid level, △T refers to the change in temperature, △P refers to the change in hydrostatic pressure, and △λ B,L Refers to the Bragg wavelength drift of the first fiber Bragg grating caused by the liquid level change, △λ B,T Refers to the Bragg wavelength drift of the second fiber Bragg grating due to temperature change, △λ B,P Refers to the Bragg wavelength drift of the first fiber Bragg grating caused by pressure change.

[0156] It is understood that the present invention is described by some embodiments, and those skilled in the art are aware that various changes or equivalent substitutions may be made to these features and embodiments without departing from the spirit and scope of the present invention. Under the teachings of the present invention, these features and embodiments may be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. The embodiments described in the present invention are part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings herein may be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all other embodiments obtained by ordinary technicians in the art without creative work based on the embodiments in the present invention belong to the scope of protection of the present invention.

Claims

1. An optical fiber diaphragm sensor for measuring liquid sample parameters, characterized in that: It comprises a sample chamber (1) and a detection chamber, wherein the sample chamber (1) and the detection chamber are fixedly connected; The sample chamber (1) is provided with a water inlet (2) and a water outlet (3) on its peripheral side wall for the inflow and outflow of the liquid sample to be tested; the sample chamber (1) is provided with a detection port (5) on its side facing the detection chamber; The detection chamber comprises a diaphragm (6) and a first fiber Bragg grating (9), the detection chamber is connected to the atmosphere, the diaphragm (6) is attached to the side of the sample chamber (1) where a detection port (5) is provided, and the detection port (5) is sealed; the first fiber Bragg grating (9) is fixed to a side of the surface of the diaphragm (6) away from the detection port (5); the detection chamber further comprises a fixing plate (13), the fixing plate (13) is fixedly connected to a side of the diaphragm (6) away from the sample chamber (1), and a second fiber Bragg grating (12) is fixed to the fixing plate (13); When liquid flows into the sample chamber (1), the diaphragm (6) can be deformed and drive the first fiber Bragg grating (9) to generate strain.

2. The optical fiber diaphragm sensor according to claim 1, characterized in that: The first fiber Bragg grating comprises a first grating region (20) corresponding to the position of the diaphragm (6), and a first pigtail region (10) extending from the position of the diaphragm (6) toward the detection chamber; The second fiber Bragg grating includes a second grating region (26) corresponding to the position of the fixing plate (13), and a second pigtail region (14) extending from the position of the fixing plate (13) toward the detection chamber.

3. The optical fiber diaphragm sensor according to claim 2, characterized in that: A constant pressure chamber (16) is fixed to the side of the fixed plate (13) away from the diaphragm (6), and a fiber guide hole (15) is opened on the side of the constant pressure chamber (16) facing the fixed plate (13). A conduit (17) for connecting the interior of the constant pressure chamber (16) with the external atmosphere is installed on the top of the constant pressure chamber (16), and the free end of the first pigtail area (10) and the free end of the second pigtail area (14) pass through the conduit (17).

4. The optical fiber diaphragm sensor according to claim 1, characterized in that: A first mounting groove (8) is provided on the side of the diaphragm (6) corresponding to the detection port (5) and away from the detection port (5), a first fiber Bragg grating (9) is fixed in the first mounting groove (8), and the first grating area (20) is located in the first mounting groove (8); a second mounting groove (11) is provided on the side of the fixing plate (13) away from the diaphragm (6), a second fiber Bragg grating (12) is fixed in the second mounting groove (11), and the second grating area (26) is located in the first mounting groove (8).

5. The optical fiber diaphragm sensor according to claim 1, characterized in that: A reserved deformation opening (19) is provided at a position on the fixing plate (13) corresponding to the detection opening (5).

6. The optical fiber diaphragm sensor according to claim 1, characterized in that: The shape of the detection port (5) of the sample chamber (1) is an ellipse, and the major axis of the ellipse is parallel to the installation direction of the first fiber Bragg grating (9).

7. A method for measuring liquid sample parameters, using the optical fiber diaphragm sensor as claimed in claim 1, characterized in that: The following steps are involved: Step 1: Establish a mathematical model between the sensitivity coefficient of the optical fiber diaphragm sensor and the parameters of the liquid sample to be measured; the mathematical model between the sensitivity coefficient of the optical fiber diaphragm sensor and the parameters of the liquid sample to be measured is: In the formula, λ B,L Refers to the absolute Bragg wavelength of the first fiber Bragg grating at different liquid levels, λ B,T Refers to the absolute Bragg wavelength of the second fiber Bragg grating at different temperatures, λ B,P refers to the absolute Bragg center wavelength of the first fiber Bragg grating under different hydrostatic pressures, K L Refers to the liquid level sensitivity coefficient, K T,1 Refers to the first fiber Bragg grating temperature sensitivity coefficient, K T,2 Refers to the temperature sensitivity coefficient of the second fiber Bragg grating, K p refers to the pressure sensitivity coefficient, △L refers to the change in liquid level, △T refers to the change in temperature, △P refers to the change in hydrostatic pressure, and λ B,0 refers to the initial Bragg wavelength of the first fiber Bragg grating, λ T,0 refers to the initial central wavelength of the second fiber Bragg grating; Step 2: Determine the sensitivity coefficient K of the optical fiber diaphragm sensor through experiments L , K T,1 , K T,2 and K P , obtaining a relationship between the Bragg wavelength change value of the first fiber Bragg grating and the second fiber Bragg grating and the parameters of the liquid sample to be measured; Step 3, placing the sample chamber in the liquid sample to be tested; Step 4, obtaining Bragg wavelength change values ​​of the first fiber Bragg grating and the second fiber Bragg grating; Step 5: Substitute the values ​​into the mathematical model in step 1 to obtain the parameter values ​​of the liquid sample to be tested.

8. The method for measuring liquid sample parameters according to claim 7, characterized in that: In step 2, the relationship between the Bragg wavelength change value of the first fiber Bragg grating and the second fiber Bragg grating and the parameters of the liquid sample to be measured is: In the formula, △L refers to the change in liquid level, △T refers to the change in temperature, △P refers to the change in hydrostatic pressure, and K L Refers to the liquid level sensitivity coefficient, K T,1 Refers to the first fiber Bragg grating temperature sensitivity coefficient, K T,2 Refers to the temperature sensitivity coefficient of the second fiber Bragg grating, K p Refers to the pressure sensitivity coefficient, △λ B,L Refers to the Bragg wavelength drift of the first fiber Bragg grating caused by the liquid level change, △λ B,T Refers to the Bragg wavelength drift of the second fiber Bragg grating due to temperature change, △λ B,P Refers to the Bragg wavelength drift of the first fiber Bragg grating caused by pressure change.

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