CN112198085B - Liquid density measuring device based on side hole optical fiber and measuring method thereof - Google Patents

Liquid density measuring device based on side hole optical fiber and measuring method thereof Download PDF

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CN112198085B
CN112198085B CN202011072898.5A CN202011072898A CN112198085B CN 112198085 B CN112198085 B CN 112198085B CN 202011072898 A CN202011072898 A CN 202011072898A CN 112198085 B CN112198085 B CN 112198085B
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liquid
optical fiber
hole
density
side hole
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田野
马占宇
柴全
杨晓煜
廉宇航
于浪
谭滔
段超
张建中
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Harbin Engineering University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N9/00—Investigating density or specific gravity of materials; Analysing materials by determining density or specific gravity
    • G01N9/24—Investigating density or specific gravity of materials; Analysing materials by determining density or specific gravity by observing the transmission of wave or particle radiation through the material
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects
    • G01N13/02—Investigating surface tension of liquids

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Abstract

The invention provides a liquid density measuring device based on a side hole optical fiber and a measuring method thereof, belonging to the field of liquid density measurement.

Description

Liquid density measuring device based on side hole optical fiber and measuring method thereof
Technical Field
The invention relates to a liquid density measuring device based on a side hole optical fiber and a measuring method thereof, belonging to the field of liquid density measurement.
Background
Due to the difference of intrinsic properties of different liquids or different densities of the same liquid in different states, the measurement of the liquid density has important significance in basic theory research and engineering application, and is widely applied to the processes of oil exploitation and transportation, medical detection of blood, detection of electrolyte performance of a storage battery, detection of aerospace liquid fuel and the like.
Aiming at the current development of liquid density measurement technology, the density measurement method can be roughly divided into the following points: resonant, radial, ultrasonic, capacitive, fiber sensing, and the like. The resonant liquid density measuring device mainly realizes measurement by the inherent frequency of a liquid influence measuring system, the performance of the resonant liquid density measuring device is influenced by an elastic element, the volume is relatively large, the measurement of the density of trace liquid cannot be realized, and various mechanical vibrations outside a frequency band need to be restrained or filtered; the ray type has great harm to the health of operators; the ultrasonic liquid density measuring method is widely applied, is commonly used as a sensing device, particularly a surface acoustic wave sensor developed on the basis of an ultrasonic measuring technology, and becomes the current hotspot research direction, particularly a lamb wave sensor, the volume of the lamb wave sensor is relatively smaller, the sensitivity is higher, but the manufacturing process of an ultrasonic probe is relatively stricter and more complex, and the ultrasonic measuring method is greatly influenced by the temperature and the liquid purity; the capacitive liquid density measurement method mainly utilizes the buoyancy force borne by a floater in liquid to convert the buoyancy force into the distance between capacitor plates to realize density measurement, and the device has higher requirement on a circuit and poorer anti-interference capability; the liquid density measuring device using Optical fiber sensing comprises fiber gratings [ Idris S K, Haroon H, Razak H A, et al. investment on fiber Optical sensor using FBG for variable temperature and liquid density [ J ]. Journal of Physics Conference Series,2020,1502:012009 ] and fiber rings [ Bai D, Hu S, Xu T, et al. A novel density sensor based high-double sampling fiber laser [ J ]. Proceedings of SPIE-The International Society for Optical Engineering,2013,8914:89140G ].
Disclosure of Invention
The invention aims to provide a novel method for measuring liquid density based on an optical fiber light path device for the first time aiming at the limitation of the existing liquid density measuring method.
The purpose of the invention is realized as follows: a liquid density measuring device based on a side hole optical fiber is provided, wherein two optical fiber microstructures are arranged in a liquid to be measured, the two optical fiber microstructures are simultaneously connected with an optical fiber coupler, the optical fiber coupler is respectively connected with a light source and a spectrometer, and the optical fiber microstructures comprise a single mode optical fiber and a side hole optical fiber which are welded together from top to bottom.
The invention also includes such structural features:
1. the air holes of the two side-hole fibers are different in size.
2. The diameter and the fiber core of the side hole optical fiber are the same as those of the single mode optical fiber.
3. A liquid density measuring method based on a side hole optical fiber comprises a liquid density measuring device based on the side hole optical fiber, and comprises the following steps:
the method comprises the following steps: calibrating an initial state, turning on a light source, and measuring initial reflection spectrums of two long gratings in the optical fiber microstructure by using a spectrometer;
step two: placing liquid to be measured in a cylindrical container, and measuring and obtaining the bottom area and the liquid height of the cylindrical container;
step three: inserting one end of the edge hole optical fiber of the two optical fiber microstructures into liquid, and measuring and recording the reflection spectrums of the two long gratings at the moment after the liquid level is stable;
step four: and (4) according to the reflection spectrum comparison in the first step and the third step, obtaining the liquid level height of the liquid in the air hole of the optical fiber microstructure, further comparing the stress analysis of the liquid in the two optical fiber microstructures, and finally calculating the density of the liquid.
4. After the optical fiber is inserted into the liquid to be measured, the trace liquid entering the optical fiber through the air hole is a main stress balance analysis object and comprises external atmospheric pressure, air pressure in the hole, liquid level pressure of the liquid outside the optical fiber and surface tension of the liquid in the hole; and comparing and analyzing the two microstructure devices to calculate the density of the liquid and the surface tension coefficient of the liquid.
Compared with the prior art, the invention has the beneficial effects that: the invention provides a liquid density measurement method based on side hole optical fibers for the first time, which mainly utilizes a microstructure device in which the side hole optical fibers written in a grating are welded with a single-mode optical fiber, and two side hole optical fibers with air holes of different sizes are inserted into liquid to compare, calculate and analyze the stress of the liquid entering the optical fibers, thereby realizing the density measurement of trace liquid and simultaneously obtaining the surface tension coefficient of the liquid to be measured. The invention also provides a method for measuring the density of the liquid by welding two optical fiber microstructures of the single-mode optical fiber by using the side hole optical fiber, and simultaneously the surface tension coefficient of the liquid can be measured; writing a long grating in the side-hole optical fiber, and fusing the liquid level height measuring technology into a liquid density measuring method for the first time; based on the miniaturization advantage of the optical fiber microstructure device, the amount of liquid required by measurement is small, and clinical blood density detection is facilitated.
Drawings
FIG. 1 is a schematic view of a fiber optic circuit apparatus;
FIG. 2 is a schematic diagram showing the process of measuring the density of liquid and the parameter labeling of the device.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
As the fiber drawing technology is gradually mature, especially the drawing process of the side hole fiber with various parameters is improved, the uniformity of the axial distribution of the side hole fiber is closer to the ideal parameter state, so that the side hole fiber (such as 2 and 4 in fig. 1) required to be used in the device can be regarded as the side hole fiber with the ideal air hole, such as 23 in fig. 1, and the cross-sectional diameter size is known, and the cross-sectional area is S respectively3And S4As shown in fig. 2. The outermost diameter of the cladding of the edge-holed fiber (21 in FIG. 1) is equal to that of the single-mode fiber (1 in FIG. 1), and has a cross-sectional area S2The two are the same rulerCun be convenient for butt fusion and optical signal's transmission, avoid the collapse that leads to side opening optic fibre air hole simultaneously to a certain extent in the butt fusion process. When the optical fiber fusion splicer is used for fusing the side hole optical fiber and the single-mode optical fiber together, the small current is particularly used for discharging, the collapse of air holes is avoided, and the length L of the side hole optical fiber is not changed before and after fusion splicing.
The essence of grating writing Technology is to form periodic Refractive Index modulation on the Fiber core or cladding of the Fiber, which has been developed for more than thirty years, the Technology is relatively mature, and liquid level sensing by writing grating on the Side-Hole Fiber was realized in 2019 [ Tian Y, Xu B, Chen Y, et al, liquid Surface testing and reflective Index Based on a Side-Hole Fiber, IEEE Photonics Technology Letters,2019,31(12):947-950 ], especially using grating reflection spectrum to measure liquid level change. The invention relates to a method for writing a grating on a fiber core of a single-hole side-hole fiber, and welding the grating with a single-mode fiber to form a fiber microstructure. By measuring the reflection spectrums of the two gratings in real time and comparing the change of the reflection spectrums of the gratings before and after the liquid is inserted, the accurate liquid level height can be obtained, namely the liquid amount of the liquid to be measured entering the side hole optical fiber air hole.
The measuring device of the invention is realized based on a side-hole optical fiber with cylindrical air holes 23 distributed on one side of the fiber core, and the cladding diameter 21 and the fiber core 22 of the side-hole optical fiber are respectively the same as the single-mode optical fibers 1 and 3 in size. Long gratings are written in fiber cores of the side-hole optical fibers respectively, and the central wavelengths of the gratings are different, so that the detection of a spectrometer is facilitated. Then use the fiber cutter, the grating area and a section of single mode fiber of intercepting side hole optic fibre, use optical fiber splicer with its butt fusion, especially need use weak current discharge butt fusion, avoid the air hole of side hole optic fibre to sink, realize the preparation of optic fibre microstructure device.
After the optical fiber is inserted into the liquid to be measured, the trace liquid entering the optical fiber through the air hole is a main stress balance analysis object and comprises external atmospheric pressure, air pressure in the hole, liquid level pressure of the liquid outside the optical fiber and surface tension of the liquid in the hole. The density of the liquid and the surface tension coefficient of the liquid can be calculated by comparative analysis of the two microstructure devices.
Placing the liquid to be measured in a cylindrical container with bottom area (S in FIG. 2)1) And liquid level height (as h in FIG. 2)0) The method can measure the liquid level height H in the container after the optical fiber microstructure device is inserted into the liquid to be stable based on the volume conservation principle. The liquid height in the air hole of the side hole optical fiber is h1And h2And the density of the liquid can be obtained by comparing and analyzing the stress balance of the liquid in the air holes of the two side holes of the optical fiber. The liquid stress in the air hole of the side hole optical fiber comprises internal gas pressure (downward), liquid level pressure of liquid outside the optical fiber (upward), liquid surface tension in the hole (upward) and external atmospheric pressure (upward). Since the air holes of the side-holed fiber are small, the amount of liquid entering therein is also small, and therefore, its own weight is ignored here.
A liquid density measurement method based on a side hole optical fiber is characterized by sequentially comprising the following steps:
step one, a liquid density measuring device is built, a long grating is written in two side hole optical fibers 2 and 4 of air holes 23 with different sizes, the length of a grid area is L, an optical fiber cutter is used for cutting the grid area of the side hole optical fibers, a fusion splicer or a microscope is used for magnifying and observing whether two end faces of the side hole optical fibers are cut neatly, a section of single mode optical fiber 1 and a section of single mode optical fiber 3 are cut out in the same way, an optical fiber microstructure formed by fusion splicing the side hole optical fibers and the single mode optical fibers is prepared by the optical fiber fusion splicer, then the optical fiber microstructure is connected into an optical fiber light path through an optical fiber coupler 5, and the light path further comprises a light source 6 and a spectrometer 7, as shown in figure 1.
Because the structural particularity of the side hole optical fiber easily causes the collapse of the air hole in the welding process, and the preparation of a subsequent device is not facilitated, the weak current discharge point welding is particularly needed, the trial test of the single-mode optical fiber and the single-mode optical fiber welding can be performed, the discharge time and the discharge current of the welding machine are adjusted, and the welding of the side hole optical fiber and the single-mode optical fiber is performed after the optimal state is reached. And a plurality of optical fiber microstructure devices in which the same side hole optical fiber is welded with the single-mode optical fiber can be prepared at the same time, and the subsequent liquid density measurement experiment is carried out by selecting the optical fiber microstructure device with the smallest air hole collapse.
And step two, calibrating the initial state of the whole device, turning on the light source (6), and measuring the initial reflection spectrums of the two long gratings in the optical fiber micro-structure device by using the spectrometer (7).
Step three, placing the liquid in the miniature cylindrical container, measuring and obtaining the bottom area and the liquid height of the cylindrical container, and passing through the bottom area of the miniature cylindrical container (such as S in figure 2)1) And liquid level height (as h in FIG. 2)0) Obtaining the volume V of the liquid0Namely:
V0＝S1×h0 (1)
due to the subsequent need to insert fiber optic microstructure devices, the micro-cylindrical containers cannot be replaced with bulky devices that would otherwise affect the accuracy of the liquid density measurement.
And step four, vertically inserting one end of the side hole optical fiber of the two optical fiber microstructure devices into liquid, which can cause the liquid level in the miniature cylindrical container to rise, simultaneously forcing the liquid to enter the air hole of the side hole optical fiber by the external atmospheric pressure, the liquid level pressure around the side hole optical fiber and the liquid surface tension, wherein the air hole originally belongs to a semi-closed state, and after the liquid enters, the inner liquid level and the outer liquid level of the optical fiber are stable, and at the moment, the reflection spectra of the two long gratings are recorded.
Step five, according to the reflection spectrum comparison in the step two and the step four, the liquid level height of the liquid in the air hole of the optical fiber microstructure device is obtained, then the stress analysis of the liquid in the two optical fiber microstructure devices is compared, and finally the density of the liquid is calculated, specifically:
after the optical fiber was inserted, the liquid level in the micro cylindrical vessel was set to be H as shown in FIG. 2. At this time, the volume V of the liquid outside the optical fiber1Comprises the following steps:
V1＝(S1-2×S2)×H (2)
according to the reflection spectrum comparison of the second step and the fourth step, the liquid level heights of the liquid entering the two air holes are obtained respectivelyIs h1And h2Then the volume V of liquid in the two air holes2Comprises the following steps:
V2＝S3×h1+S4×h2 (3)
according to the results of the second step and the third step, based on the volume constant principle, namely:
V0＝V1+V2 (4)
equations (1), (2), (3) and (4) are combined to determine the height of the liquid level in the cylindrical container after the optical fiber microstructure device is inserted. Accordingly, the liquid in the hole is subjected to stress analysis, namely, the liquid level pressure P of the liquid outside the optical fiberH(upward in direction), this force is the same for two different sizes of air holes:
PH＝ρgH (5)
where ρ is the liquid density and g is the acceleration of gravity.
Then surface tension is Pα1And Pα2(direction up):
Pα1＝2παr1/S3
Pα2＝2παr2/S4 (6)
where α is the surface tension coefficient, an intrinsic parameter of the liquid, r1And r2Respectively, the known radius of the edge hole fiber air hole, and S3＝πr1 2，S4＝πr2 2。
Further comprises the air pressure in the air hole and the external atmospheric pressure, and the air pressure P to the external atmospheric pressure0The value is known, the air pressure inside the air hole of the side-hole optical fiber can be obtained by an ideal gas state equation, namely:
PV＝nRT (7)
where P represents pressure, V represents volume of gas object analyzed, n is amount of substance, R is gas constant, and T is absolute temperature, because before and after insertion of the side-hole fiber into the liquid, the void isSince the gas amount, the gas constant and the temperature in the air hole are constant, it is assumed that the volumes of the entire air hole are V3And V4Namely:
V3＝S3×L
V4＝S4×L (8)
after the optical fiber with the side hole is inserted into the liquid, the volumes of the air in the air hole are respectively V3' and V4', i.e.:
V3’＝S3×(L-h1)
V4’＝S4×(L-h2) (9)
therefore, a constant relationship between the side-hole fiber before and after insertion into the liquid can be obtained according to equation (7):
P0V3＝PP1V3’＝nRT
P0V4＝PP2V4’＝nRT (10)
accordingly, the pressure P of the gas in the air hole of the side-hole optical fiber can be obtained by substituting equations (8) and (9) into equation (10)P1And PP2(downwards direction), i.e.:
PP1＝LP0/(L-h1)
PP2＝LP0/(L-h2) (11)
in conclusion, the liquid in the air holes of the side-hole optical fiber is stressed in a balanced manner, and the following steps are obtained:
P0+Pα1+PH＝PP1
P0+Pα2+PH＝PP2 (12)
by the simultaneous connection of the two equations in the equation (12), the liquid density ρ and the surface tension coefficient α of the liquid can be obtained.
In summary, the invention belongs to the field of liquid density measurement, and particularly relates to a liquid density measurement method based on a side-hole optical fiber. The liquid density measuring device is mainly based on two optical fiber microstructures of a side hole optical fiber and a single mode optical fiber in fusion joint, particularly, a grating needs to be written on the side hole optical fiber, the two side hole optical fibers of air holes with different sizes are inserted into liquid to be measured, the amount of liquid entering the side hole optical fiber air holes is measured according to the reflection spectrum of the optical fiber microstructures, the stress condition of the part of liquid is analyzed, and the liquid density value and the surface tension coefficient can be calculated by comparing the stress analysis of the liquid in the two optical fiber microstructure devices.

Claims (5)

1. The utility model provides a liquid density measuring device based on side hole optic fibre which characterized in that: two optical fiber microstructures arranged in a liquid to be measured, wherein the two optical fiber microstructures are simultaneously connected with an optical fiber coupler, the optical fiber coupler is respectively connected with a light source and a spectrometer, the optical fiber microstructures comprise a single-mode optical fiber and a side hole optical fiber which are welded together up and down, a grating is engraved on the side hole optical fiber, a microstructure device which is welded by the side hole optical fiber and the single-mode optical fiber and is written with the grating is utilized, two side hole optical fibers with air holes of different sizes are inserted into the liquid, the stress of the liquid entering the optical fiber is contrastingly calculated and analyzed, four stresses including the external atmospheric pressure, the liquid surface tension in the side holes, the liquid surface pressure of the liquid outside the optical fiber and the gas pressure in the side holes are balanced up and down, the four stresses of the liquid in the side hole optical fibers of the two air holes of different sizes are balanced up and down, two stress balance equations in the up and down directions are established, and two unknown quantities of the liquid density and the surface tension coefficient of the liquid to be measured are obtained by solving the equations, the density measurement of trace liquid is realized, and the surface tension coefficient of the liquid to be measured can be obtained at the same time.
2. The liquid density measuring device based on the side-hole optical fiber according to claim 1, wherein: the air holes of the two side-hole fibers are different in size.
3. A side-hole optical fiber based liquid density measuring apparatus according to claim 1 or 2, wherein: the diameter and the fiber core of the side hole optical fiber are the same as those of the single mode optical fiber.
4. A liquid density measuring method based on a side hole optical fiber comprises a liquid density measuring device based on a side hole optical fiber, and is characterized in that: the method comprises the following steps:
the method comprises the following steps: calibrating an initial state, turning on a light source, and measuring initial reflection spectrums of two long gratings in the optical fiber microstructure by using a spectrometer;
step two: placing liquid to be measured in a cylindrical container, and measuring and obtaining the bottom area and the liquid height of the cylindrical container;
step three: inserting one end of the edge hole optical fiber of the two optical fiber microstructures into liquid, and measuring and recording the reflection spectrums of the two long gratings at the moment after the liquid level is stable;
step four: and (4) according to the reflection spectrum comparison in the first step and the third step, obtaining the liquid level height of the liquid in the air hole of the optical fiber microstructure, further comparing the stress analysis of the liquid in the two optical fiber microstructures, and finally calculating the density of the liquid.
5. The method for measuring the density of the liquid based on the side-hole optical fiber as claimed in claim 4, wherein: after the optical fiber is inserted into the liquid to be measured, the micro liquid entering the optical fiber through the air hole is a stress balance analysis object, including the external atmospheric pressure P0Pressure P of gas in the side holePLiquid level pressure P of liquid outside optical fiberHAnd surface tension P of the liquid in the wellαThe four stresses are balanced in the vertical direction; the comparative analysis of the two microstructure devices establishes two stress balance equations in the up-down direction, namely:
P0+Pα1+PH＝PP1
P0+Pα2+PH＝PP2
in the formula: p alpha is 2 pi alpha r/S, alpha is the liquid surface tension coefficient, r is the radius of the air hole of the side-hole fiber, and S is the area S of the air hole of the side-hole fiber pi r2；PHρ gH, and ρ is the liquid density, g is the acceleration of gravity, H is the liquid level; pP＝LP0(L-h), wherein L is the length of the side hole optical fiber, h represents the height of the liquid level in the side hole optical fiber, and the lower corner mark represents two different side hole optical fibers; solving an equation set to obtain two unknown quantities of the liquid density and the surface tension coefficient of the liquid to be detected, and calculating the density of the liquid and the surface tension coefficient of the liquid.
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