JP2003302326A - Measuring method and sensor for specific gravity of liquid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To online measure the specific gravity of river water or the like mixed with seawater. <P>SOLUTION: Two bellows 2, 3 used to receive pressures in positions of different depths are arranged so as to be faced, the pressure receiving parts of the bellows 2, 3 are connected by a movable shaft 4, a strain according to a displacement is generated in an optical fiber having an FBG element by using the displacements of the bellows due to a pressure difference, and a reflected wavelength of the FBG element is measured. Thereby, a difference between pressures applied to the bellows 2, 3 is found, and the specific gravity of a liquid is measured by using a relational expression of the pressure difference and the specific gravity. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid specific gravity measuring method and a liquid specific gravity sensor for measuring the specific gravity of liquid using an optical fiber.

[0002]

2. Description of the Related Art The specific gravity of a liquid can be measured by knowing its volume and weight.

As a measuring instrument for measuring the specific gravity in real time, there is one that transmits radiation (γ-rays) through the liquid in the pipe and measures the change in the radiation intensity according to the density of the liquid to obtain the specific gravity. Other than that, basically take out a part of the target object and put it on the measuring instrument, or read the scale on the target liquid by floating it on the target liquid, sink the weight in the liquid, and measure the weight in the air. A method of measuring the difference in weight in liquid (buoyancy) to determine the specific gravity of the liquid is used.

[0004]

In the method using radiation, the measurement target is limited to the liquid in the conduit.

Further, a float type liquid hydrometer, in which a float whose center of gravity is stabilized is floated in a liquid whose specific gravity is known in advance, and a liquid level position of the float is graduated, is mixed with liquids having different specific gravities. When they are separated from each other, the specific gravity of the liquid in the upper layer can be measured, but the specific gravity of the liquid in the lower layer cannot be measured.

Furthermore, the method of calculating the specific gravity by comparing the weight in air and the weight in water of the object to be measured requires the extraction of a sample, and cannot perform online measurement.

On-line measurement cannot be easily performed by the method of putting a liquid in a constant volume container and calculating the specific gravity from the weight.

The present applicant has developed and put into practical use a system for remote monitoring and management of the water level of a river or the like in real time by using an optical fiber water level sensor. In that system, a strain corresponding to the water pressure is generated in the optical fiber by using the water pressure change accompanying the water level change, and the strain is measured and converted into a water level. When measuring, the salt concentration of river water (brackish water) may change due to the ebb and flow of the tide, and the salt concentration may affect the measured value.

At this time, if the salt concentration at the point where the water level sensor is installed is known, correction can be performed to improve the accuracy of water level measurement.

The salt concentration must be measured online in order to take advantage of the above-mentioned system. Therefore, a new specific gravity measuring method and a specific gravity sensor capable of measuring the specific gravity of river water etc. online are required. It became so.

An object of the present invention is to meet such a demand.

[0012]

In order to solve the above problems, in the present invention, two pressure receiving bodies are arranged in the liquid at different installation depth positions, and the pressure difference applied to each pressure receiving body is different. ΔP is measured by measuring the strain of the optical fiber due to the pressure difference, and the pressure-receiving area of each pressure-receiving body is S, and the height difference of the pressure-receiving body installation point is H, and the specific gravity of the liquid ρ from the formula of ΔP = ρ · S · H A method for measuring the specific gravity of a liquid is provided.

Further, as sensors used for carrying out the method, two bellows, which have the same pressure receiving area and are arranged to face each other, which directly or indirectly receive and expand the pressure at different depth positions of the liquid, respectively. Provided is a liquid specific gravity sensor including an FBG element that produces strain according to the amount of expansion or contraction of a bellows or a difference in expansion or contraction amount.

Preferred forms of such sensors are listed below. The pressure receiving parts of the two bellows are connected by a movable shaft. Two sets of FBG elements are provided, and one FBG element is given a strain according to the difference in expansion / contraction amount of the two bellows, and the other FBG element is given a strain according to the thermal expansion / contraction amount of the sensor. Two sets of FBG elements having the same length are provided, and one FBG element is given an extension strain according to the difference in expansion / contraction amount of two bellows, and the other FBG element is given a contraction. The reflection wavelengths of the above two sets of FBG elements are made different, the difference in pressure applied to the two bellows is measured from the difference value of the reflection wavelength peak values from both FBG elements, and the specific gravity of the liquid is calculated based on a predetermined arithmetic expression. A measurement device for calculating A stopper that stops the expansion of the FBG element at the position where the elongation strain reaches a set value is included. The FBG element is distorted by the amplified displacement, including the amplification mechanism that mechanically increases the displacement of the movable shaft. Mount the cone on the pressure receiving surface of the bellows that is to be placed upward in the liquid.

The pressure of the liquid may be directly applied to the two bellows, or two pressure receiving members having a bellows structure may be added and the pressure receiving members may be arranged at different depth positions of the liquid to receive the pressures. The pressure received by the body may be applied to the two bellows via a pressure medium enclosed in a pressure guiding tube.

[0016]

According to the present invention, the pressure difference (pressure difference due to the depth difference) at the pressure receiving body installation point is converted into the strain of the optical fiber for measurement (for the measurement, the relationship between the pressure difference and the strain amount of the optical fiber is measured). The specific gravity of the liquid is calculated using the relational expression between the pressure difference and the specific gravity. According to this method, specific gravity can be measured even for river water or liquid in a liquid tank that cannot be measured by a device that uses radiation. Further, since the pressure receiving body can be arranged at any position, the liquid specific gravity of the lower layer portion, which cannot be measured by the float type hydrometer, can be measured, and online multi-point measurement using one optical fiber can be performed.

Further, in the specific gravity sensor of the present invention, the pressure at the pressure receiving point is directly or indirectly applied to the two bellows arranged opposite to each other, and the strain corresponding to the expansion or contraction amount or the difference in expansion or contraction amount of each bellows is FB.
Since it is applied to the G element, the pressure difference measurement can be facilitated, the structure can be simplified, and the cost can be reduced.

The FBG (fiber Bragg grating) element used in this sensor is an element in which a Bragg diffraction grating that reflects only a specific wavelength is formed in a part of an optical fiber, and a plurality of elements are provided in one optical fiber. It has the characteristics that it can be connected serially and the detection sensitivity is high.
In the measurement of pressure (strain) by this FBG element, light is injected into an optical fiber by using a broadband light source with a wide wavelength range,
The wavelength reflected by G is measured with a wavemeter.

The wavelength reflected by the FBG changes due to the change in the pitch of the grating due to the addition of strain, and therefore the strain (that is, pressure) generated in the optical fiber from the measured value of the reflected wavelength.
Can be asked.

It should be noted that the one adopting the above-mentioned structure is
Since the bellows work together to extract the pressure difference, FB
Expansion and contraction strain due to the pressure difference can be reliably applied to the G element to reduce the measurement error.

Further, in the case of adopting the configuration of (2), the error component due to the temperature contained in the measured value of the pressure difference can be removed by using the distortion due to the temperature of the other FBG element as a correction value.

With the configuration of (2), the difference between the pressures applied to the two bellows is measured from the difference value of the reflection wavelength peak values from the two FBG elements using the measuring device of (2). When this method is adopted, temperature correction (removal of an error component caused by temperature) is automatically performed, and it is not necessary to measure the temperature itself (strain due to temperature).

With the stopper of (1), application of excessive stress to the FBG element is blocked, and the FBG element can be prevented from being destroyed even in an environment where the sensor vibrates due to turbulence or the like.

Further, the one having the amplification mechanism of is F
It is possible to place the BG element at a position apart from the space between the bellows to increase the degree of freedom in the layout of the element and the degree of attachment, and to facilitate the manufacture of the sensor. Further, since the pressure difference is amplified, the pressure difference between the two measurement points may be small,
The sensor length (interval between bellows) can be shortened and the specific gravity can be measured even in a shallow place.

The bellows with the cone attached is
Sediment is prevented from depositing on the bellows, and fluctuations in measurement accuracy due to deposits do not occur.

In addition, in the case of indirectly applying the pressure at the measuring point to the bellows via the pressure guiding tube, it is not necessary to immerse the main body of the sensor in the liquid. Further, the pressure receiving areas of the pressure receiving body and the bellows may be different to amplify and transmit the pressure.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to FIG.
It will be described with reference to FIG.

FIG. 1 shows the basic structure of the liquid specific gravity sensor of the present invention. As shown in the drawing, bellows 2 and 3, which are expanded and contracted by receiving hydraulic pressure, are provided at the upper and lower ends of the sensor body 1 so as to face each other. Pressure receiving parts 2a, 3a of each bellows
(The pressure receiving areas of both are equal) are connected via a movable shaft 4, and when pressure is applied, the bellows 2,
It is displaced to a position corresponding to the pressure difference applied to 3.

A fixed roller 5B is attached to the frame 1a of the sensor body 1, and an optical fiber 6a having an FBG element applies a predetermined tension between the fixed roller 5B and the movable roller 5A attached to the movable shaft 4, It is wound around and fixed on both rollers. In addition, the fixed roller 5B and another movable roller 5C attached to the movable shaft 4
Dummy optical fiber 6 for balancing forces between and
b is wound around the roller and fixed, and is connected to the roller. The optical fibers 6a and 6b are a series of fibers and are connected to the optical fibers in the optical cable 6.

The dummy optical fiber 6b is the optical fiber 6a.
May be provided in parallel between the rollers 5A and 5B.
Also, if the displacement amount due to the pressure difference is small, the slippage between the cladding and the buffer layer in the strand of the optical fiber hardly occurs, so the dummy optical fiber 6b may be omitted.

One end of each of the bellows 2 and 3 is fixed to the frame 1a of the main body 1, and a case 1b is covered between the two bellows to ensure airtightness inside.

In the liquid specific gravity sensor of FIG. 1 configured as above, when the pressure receiving area of each of the bellows ² and 3 is Scm ² , the height difference between the bellows ² and 3 is H, and the specific gravity of the liquid is ρ,
The pressure difference between the bellows 2 and 3 is given by the formula ρ · S · H (g). Suppose now that S = 10 cm ² , H = 40 cm, ρ
= 1.0, the pressure difference generated between the bellows 2 and 3 is 400 gf (≈3.923 N). When using a bellows with a spring constant of 100 gf, the strain of the optical fiber is 1%.
Therefore, if the length of the FBG element in the optical fiber 6a is 10 cm, the relationship between the displacement and the force is 0.1 mm.
/ 100gf, and most of the 100gf load is FB
The G element bears the burden.

On the other hand, since the FBG element has a strain measurement accuracy of 0.0004%, the FBG element with a length of 10 cm can measure the load with an accuracy of 0.4 g, considering the spring force of the bellows. Become. Therefore, if we consider measuring the specific gravity of seawater, if the salt concentration is 3%, the specific gravity will be 1.03, so we will measure the concentration% to one digit below (1.03 × 400) -400 = 12
Since g can be decomposed with 0.4 g, 3% can be decomposed to 1/30, that is, 0.1%.

The fact that it is possible to measure up to 0.1% in terms of% concentration means that the specific gravity can be measured with an accuracy of 1/1000.

As described above, in the case of the monitoring system in which the optical fiber is distorted by water pressure to measure the water level near the river mouth,
If the salt concentration is known, the measurement error due to salt can be corrected to reduce the influence of seawater. The expression to reduce the effect is that accurate correction can be performed if the river water at the measurement point is sufficiently mixed with seawater, but in reality there is a difference in concentration between the river bottom and near the water surface. ,
This is because correction in a strict sense is difficult with just one sensor.

Here, the specific gravity is measured with an accuracy of 1/1000 as an example, but the pressure receiving area S and the height difference H
By appropriately changing the dimension of, the amount of strain applied to the FBG element can be changed and the resolution can be further enhanced.

Further, the height difference H can be made small and the pressure receiving area S can be made large so that the specific gravity can be measured at a shallow water depth while maintaining accuracy.

FIG. 2 is a view in which a cone 7 for preventing the accumulation of earth and sand is mounted on the pressure receiving portion of the bellows 2 arranged in the upward direction of the sensor of FIG. To balance the load,
As shown in the figure, it is preferable to attach the dummy cone 8 to the pressure receiving portion of the lower bellows 3 as well.

In FIG. 3, a sub-chamber 1c is provided on the side of the sensor body 1, bellows 2 and 3 are attached to the upper and lower sides of the sub-chamber, and the pressure receiving portions of both bellows are connected by a movable shaft 4 to form a shaft 4
The displacement due to the pressure difference between the
The optical fiber 6a having the element is transmitted to the optical fiber 6a.
A liquid specific gravity sensor adapted to give a strain corresponding to a pressure difference between the bellows 2 and 3 is shown in a.

Fixed rollers 5B are provided on the upper and lower ends of the frame 1a,
5D is provided, and the optical fiber 6a is stretched between the movable roller 5A and the fixed roller 5B attached to the lever 9 so as not to slip between the roller and the movable roller 5A and the fixed roller 5D. An optical fiber 6c having an FBG element is stretched in the same manner as 6a. A series of optical fibers 6
The a and 6c and the FBG element provided in each optical fiber have the same length. Further, these elements have different reflection wavelengths so that the wavelengths of the two do not overlap even when the wavelength shift amount due to the pressure change becomes maximum.

In this way, for example, the bellows 3 contracts due to the pressure difference, and when the bellows 2 expands, expansion strain is given to the optical fiber 6a and the optical fiber 6c contracts. When the specific gravity of the liquid is measured by obtaining the pressure difference applied to the bellows 2 and 3 from the difference between the peak values of the reflection wavelengths from the two FBG elements at this time, the F of the optical fiber 6a side is measured by the temperature change.
If the BG element is extended by 0.1 mm, for example, the FBG element on the optical fiber 6c side is also extended by 0.1 mm, the extension (strain) due to the temperature change is canceled, and the temperature is automatically corrected.

Since the FBG element is sensitive to temperature, temperature correction is indispensable. For the temperature correction, the fixed rollers 5E and 5F are provided in parallel with the rollers 5A and 5D of FIG. 3 while being supported by the body frame (the rollers 5E and 5F are 5A,
Since the position overlaps with 5D, the drawing is shown as being taken out from the case 1b), and an optical fiber 6d having an FBG element having the same specifications as the optical fiber 6a between the rollers 5E and 5F.
(This is connected to the optical fiber 6a) is stretched and the optical fiber 6d is used to distort the sensor due to temperature change (roller 5
It is also possible to detect (strain between A and 5B) and subtract the component due to temperature from the measured value of the pressure difference by the optical fiber 6a, but if the structure for automatic correction is used, the optical fiber 6d for temperature correction can be used. Since it is not necessary to provide the sensor, it is possible to simplify the sensor and increase the number of sensors connected to one optical fiber line.

FIG. 4 shows another embodiment of the liquid gravity sensor and a water level monitoring system using the same for correction. The liquid specific gravity sensor shown in FIG.
It is arranged in the case 1b. Pressure receiving part 2 of both bellows
a and 3a are connected by a movable shaft 4, and an FBG is provided between a movable roller 5A attached to the back surface of the pressure receiving portion of the bellows 2 and a fixed roller 5B attached to a frame (not shown) of the main body.
Optical fiber 6a having elements and dummy optical fiber 6b
Further, an optical fiber 6c having an FBG element and a dummy optical fiber 6b are respectively stretched between a movable roller 5C attached to the back surface of the pressure receiving portion of the bellows 3 and a fixed roller 5D attached to the frame of the main body. .

Further, the frame of the main body is provided with a stopper 10 for restricting the expansion and contraction amount of each bellows to protect the FBG element.

Further, two pressure receiving bodies 11 and 12 having a bellows structure arranged at different depth positions in water are provided, and the pressure applied to the pressure receiving portions of both pressure receiving bodies 11 and 12 is controlled by the pressure guiding tube 13,
The pressure medium 15 such as water sealed in 14 is transmitted to the pressure receiving portions of the bellows 2 and 3, respectively.

The water level monitoring system of FIG. 4 is equipped with an optical fiber water level sensor 16 for measuring water level by replacing water pressure with strain of the optical fiber. The water level sensor 16 and the liquid gravity sensor are the optical cable 6 and the connection box 17. It is connected to the monitoring control device 18 and the measuring device 19 via.

The liquid specific gravity sensor shown in FIG.
When the optical fiber 6a is stretched due to the difference in pressure applied to 2, 6c
Shrinks. Therefore, the specific gravity of water at the point where the pressure receiving body is installed can be obtained from the difference value of the reflection wavelength peak values of the optical fibers 6a and 6c from the FBG elements. Further, also in this specific gravity sensor, an error due to temperature is automatically corrected as in the sensor of FIG.

The water level monitoring system of FIG. 4 can measure the water level using the sensor 16 and correct the error due to the fluctuation of the specific gravity of water using the specific gravity sensor.

The change in the specific gravity of the liquid depending on the temperature will be briefly described below. Taking water as an example, the density of water (specific gravity)
Changes with temperature as follows. For example, the maximum density at 4 ° C is 0.99997. On the other hand, 4
The density at 0 ° C. is 0.99221, and the change is 0.776%.

Therefore, considering the case where the water depth is about 10 m, 4
C. is 999.97 cm, 40.degree. C. is 992.97 cm.
It becomes 21 cm, and an error of slightly more than 7.7 cm occurs.

That is, if a measuring instrument with a measurement accuracy of 0.1% has an error of ± 1 cm when measuring a water depth of 10 m, there will be an error of 7.7 cm only due to the temperature change. It can be seen that temperature correction that excludes the influence of water temperature is also important.

Here, I talked about the extreme, but ± 10m
In the case of actually measuring the water depth using a water level sensor that can measure with an accuracy of 1 cm, it is possible that the water temperature may change in the range of 4 ° C to 24 ° C. , 1-0.99729 / 0.9
997 = 0.0268, which is 2.7c at 10m.
An error of 3 m is accompanied by an error of 0.9 cm (about 1 cm).

The water level monitoring system shown in FIG. 4 uses both a water level sensor having a temperature correction function and a liquid specific gravity sensor for temperature correction, so that the measurement accuracy in water level measurement near the river mouth can be improved.

The specific gravity sensor of the present invention can be used for detecting the mixing of water with petroleum in addition to the above-mentioned applications. The density of petroleum (kerosene) at room temperature is 0.80-0.
Since the specific gravity is 83 when water (density about 1) is mixed and water and oil are separated into layers, it can be seen that water is mixed.

In addition, the liquid specific gravity sensor of the present invention can be used for controlling the concentration of the aqueous solution.

[0056]

As described above, according to the method and sensor of the present invention, the pressure difference between two pressure receiving body installation points is converted into the strain of the optical fiber to determine the specific gravity of the liquid. Real-time online measurement of river water and aqueous solution in aquarium that could not be done with a meter can be performed, and correction applications such as liquid level measurement values that include error components due to fluctuations in the specific gravity of liquids,
It can be used to detect the mixture of different-gravity liquids that are separated into layers and to control the concentration of aqueous solutions.

The sensor that detects the pressure difference by the FBG element has high sensitivity and can measure the salt concentration of river water near the river mouth.

Further, in the case where two sets of FBG elements are provided for temperature correction, an error component due to a temperature change can be removed and the measurement accuracy is improved.

Two FBs, one of which expands and the other contracts due to a pressure difference
The one that calculates the difference value of the reflection wavelength peak value of the G element and converts it into the specific gravity does not require temperature measurement for temperature correction,
Simplification of sensors and measuring instruments and simplification of measurement can be achieved.

Further, the one provided with the stopper for restricting the extension of the optical fiber can protect the FBG element from excessive stress and impact.

Further, the one having the mechanism for amplifying the displacement due to the pressure difference can measure the specific gravity even if the pressure difference between the measurement points is small, and the manufacture of the sensor can be facilitated.

In addition, in the case where the bellows is provided with the cone, the sediment can be prevented from being deposited on the pressure receiving portion, and the measurement accuracy is not deteriorated due to the sediment.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of a liquid specific gravity sensor of the present invention.

FIG. 2 is a cross-sectional view of another embodiment.

FIG. 3 is a sectional view of a sensor having a displacement amplification mechanism.

FIG. 4 is a diagram showing an outline of still another embodiment of a sensor and a water level monitoring system using the same.

[Explanation of symbols]

1 sensor body 1a frame 1b case A few bellows 2a, 3a Pressure receiving part 4 Movable shaft 5A, 5C movable roller 5B, 5D, 5E, 5F Fixed roller 6 optical cable Optical fiber having 6a, 6c and 6d FBG elements 6b Dummy optical fiber 7 cone 8 dummy cone 9 leverage 10 stopper 11, 12 Pressure receiver 13, 14 impulse pipe 15 Pressure medium 16 Optical fiber water level sensor 18 Monitoring and control equipment 19 Measuring device

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Takashi Fujieda             6-17-19 Shimbashi, Minato-ku, Tokyo Inn Co., Ltd.             Inside Hula Info Systems (72) Inventor Hidehiko Kikutani             6-17-19 Shimbashi, Minato-ku, Tokyo Japan Construction Con             Sultant Co., Ltd.

Claims (10)

[Claims] 1. Two pressure receiving bodies are arranged in the liquid at different installation depth positions, and the pressure difference ΔP applied to each pressure receiving body is measured by measuring the strain of the optical fiber due to the pressure difference. A liquid specific gravity measuring method for obtaining the specific gravity ρ of the liquid from the equation ΔP = ρ · S · H, where S is the pressure receiving area of each pressure receiving body and H is the height difference of the pressure receiving body installation points. 2. The two bellows, which have the same pressure receiving area and are arranged to face each other to directly or indirectly receive and expand the pressure at different depth positions of the liquid, and the expansion or contraction amount or the expansion or contraction amount difference between the bellows. A liquid gravity sensor comprising an FBG element that produces a corresponding strain. 3. The liquid specific gravity sensor according to claim 1, wherein the pressure receiving portions of the two bellows are connected by a movable shaft. 4. Two sets of FBG elements are provided and one FBG element is provided.
The liquid specific gravity sensor according to claim 3, wherein the element is given a strain corresponding to a difference in expansion / contraction amount of the two bellows, and the other FBG element is given a strain corresponding to a thermal expansion / contraction amount of the sensor. 5. A pair of FBG elements having the same length are provided, and one FBG element is provided with elongation strain corresponding to a difference in expansion and contraction amount of two bellows, and the other FBG element is contracted. 3. The liquid specific gravity sensor described in 3. 6. The difference between the reflection wavelengths of the two sets of FBG elements is measured, the difference in pressure applied to the two bellows is measured from the difference value of the reflection wavelength peak values from both FBG elements, and based on a predetermined arithmetic expression. The liquid specific gravity sensor according to claim 4, further comprising a measuring device for calculating the specific gravity of the liquid. 7. The FBG element including a stopper for stopping the extension of the element at a position where the extension strain reaches a set value.
7. The liquid specific gravity sensor according to any one of 1 to 6. 8. The liquid specific gravity sensor according to claim 3, wherein the FBG element is distorted by the amplified displacement, including an amplifying mechanism that mechanically increases the displacement of the movable shaft. 9. The liquid specific gravity sensor according to claim 2, wherein a conical body is attached to a pressure receiving surface of a bellows arranged upward in the liquid. 10. A pressure receiving body having two bellows structures is added, the pressure receiving bodies are arranged at different depth positions of a liquid, and the pressure received by each pressure receiving body is passed through a pressure medium enclosed in a pressure guiding tube. 10. The method according to claim 2, wherein the bellows are added to the two bellows.
The liquid specific gravity sensor according to any one of 1.