JP2003302277A - Level sensor with specific gravity correction function - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate influence of fluctuation in a liquid temperature and specific gravity resulting in an error of a level measurement value for improving measurement precision in a sensor measuring a level by converting a fluid pressure into deflection of an optical fiber. <P>SOLUTION: This level sensor is provided with a pressure detection part 1 transmitting displacement by a fluid pressure in bellows 11 to a movable shaft 12 so that deflection matching the fluid pressure is generated in the optical fiber 5a having an FBG element between rollers 13 and 14, a specific gravity detection part 2 generating deflection in the optical fiber 5b having the FBG element between rollers 24 and 25 in accordance to a difference of pressures working on bellows 21 and 22 arranged face to face, and a temperature detection part 3 generating deflection matching a temperature in the optical fiber 5b between the rollers 24 and 31. By this level sensor, temperature correction and specific gravity correction of a fluid level measurement value can be carried out. <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 level sensor which replaces a liquid pressure with a strain of an optical fiber and obtains a liquid level from the strain, and a liquid level monitoring system using the same.

[0002]

2. Description of the Related Art The applicant has developed and put into practical use a system that uses an optical fiber water level sensor to remotely monitor and manage the water level of a river or the like in real time.
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.

Although the effect of seawater has been described here, fluctuations in the specific gravity of water that cause variations in water level measurement values may occur due to other causes than seawater contamination.

Therefore, in the case of the monitoring system using the above-mentioned optical fiber water level sensor, it is necessary to know the specific gravity of the liquid at the monitoring point (water level sensor installation point) for correction.

Further, the specific gravity of a liquid including the salinity must be measured online in order to take advantage of the above-mentioned system. Therefore, a new specific gravity measuring method capable of measuring the specific gravity of river water online is required. Specific gravity sensors are now required.

Originally, the above-mentioned monitoring system was not premised on the fluctuation of the specific gravity of the liquid, but if the influence of the fluctuation of the specific gravity can be eliminated, the application and function thereof can be expanded.

The specific gravity of the liquid can be measured by knowing its volume and weight.

As a measuring instrument for measuring the specific gravity in real time, there is a measuring instrument for transmitting the radiation (γ ray) to the liquid in the pipe and measuring 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. The method of measuring the difference in weight in liquid (buoyancy) to obtain the specific gravity of the liquid is used, but the specific gravity of river water etc. can be measured online and the one that can cope with the fluctuation of the specific gravity of the liquid is used. There was no

For this reason, an ultrasonic sensor which is not influenced by the specific gravity has been used for measuring the water level when the fluctuation of the specific gravity is considered. Instead of this, a system using an optical fiber water level (liquid level) sensor is used. In this case, multiple sensors can be connected serially to a single optical fiber that also serves as a signal transmission line, enabling multipoint remote monitoring. There is no need for power supply or signal transmission equipment in the field. low cost. There are advantages such as, and much more advantageous.

Therefore, an object of the present invention is to improve the measurement accuracy by removing the component due to the fluctuation of the specific gravity of the liquid or the temperature change, in the liquid level measured value by the optical fiber liquid level sensor. The task is to

[0012]

In order to solve the above-mentioned problems, in the present invention, a pressure detecting portion for generating a strain in an optical fiber according to the liquid pressure by utilizing the displacement of the first pressure receiving body by the liquid pressure. And a specific gravity detection unit that causes the optical fiber to generate strain in accordance with the difference in pressure applied to both pressure receiving bodies by utilizing the displacement of the second and third pressure receiving bodies that are arranged facing each other with a height difference, A pressure detection unit is provided with a temperature detection unit that causes strain due to thermal expansion and contraction in the optical fiber, and the liquid specific gravity obtained from the strain of the optical fiber of the specific gravity detection unit and the temperature obtained from the strain of the optical fiber of the temperature detection unit are used. With a specific gravity correction function that corrects the measured hydraulic pressure at the sensor installation point determined from the strain of the optical fiber and removes the error component due to the specific gravity fluctuation of the liquid and the temperature change from the liquid level measured value obtained from the measured hydraulic pressure Provide a liquid level sensor .

This sensor comprises a pressure detector, a specific gravity detector,
It is preferable to use an optical fiber having an FBG element as each optical fiber of the temperature detection unit.

It is also preferable to use bellows as the first, second and third pressure receiving bodies, and further to integrate the pressure detecting section, the specific gravity detecting section and the temperature detecting section.

In addition, in the liquid level monitoring system using the liquid level sensor of the present invention, the difference between the specific gravity of the liquid detected by the specific gravity detection part and the theoretical liquid specific gravity at the temperature detected by the temperature detection part is calculated. It is also preferable to have a function of obtaining the weight% of the substance mixed in the monitoring liquid.

[0016]

The sensor of the present invention converts the hydraulic pressure at the sensor installation point acting on the first pressure receiving body into the strain of the optical fiber for measurement,
The liquid level is measured by using the relational expression between the amount of strain of the optical fiber and the liquid level that has been examined in advance.

Further, the difference in pressure acting on the second and third pressure receivers (pressure difference due to the depth difference) is converted into strain of the optical fiber and measured (for the measurement, the second and third pressure receivers are measured). The relationship between the pressure difference between the two and the strain amount of the optical fiber is investigated in advance), and the specific gravity of the liquid is obtained using the relational expression between the pressure difference and the specific gravity. When the pressure difference is ΔP, the pressure receiving areas of the second and third pressure receiving bodies are S, and the height difference between both pressure receiving body installation points is H, the equation ΔP = ρ · S · H holds, and from this equation, the specific gravity ρ of the liquid Can be asked.

Further, the temperature at the sensor installation point is obtained from the strain of the optical fiber of the temperature detecting section.

Thus, the liquid specific gravity under the temperature condition at the time of detection is obtained, the liquid pressure measured by the pressure detection unit is corrected, and the liquid surface level of the liquid as close to the true value as possible, that is, the variation in the specific gravity of the liquid and the temperature. It becomes possible to measure the liquid level from which the influence of the change (error component due to them) is eliminated.

Further, since the specific gravity of the liquid is obtained from the pressure difference of the liquid due to the difference in height, it is possible to measure in real time the liquid which cannot be measured by the conventional specific gravity meter or specific gravity measuring device, for example, river water in which specific gravity changes. The specific gravity can be measured.

The sensor using the FBG element can be downsized, the structure can be simplified, and the pressure measurement can be facilitated by taking advantage of the characteristics of the FBG element.

An FBG (fiber Bragg grating) element 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 can be serially connected to one optical fiber. It has a feature that the detection sensitivity is also high. In the measurement of the pressure (strain) by this FBG element, a wide band light source with a wide wavelength range is used to inject light into an optical fiber, and the wavelength reflected by the FBG is measured with a wavelength meter.

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.

The structure using the bellows as the first, second and third pressure receiving members can further simplify the structure. Regarding the specific gravity detection unit, it is preferable that two bellows are interlocked to extract the pressure difference.
Expansion / contraction strain due to a pressure difference can be reliably applied to the element to reduce the measurement error of the specific gravity.

The integrated pressure detecting section, specific gravity detecting section and temperature detecting section can be further downsized and the structure can be simplified.
The liquid level can be measured more accurately.

[0026]

DETAILED DESCRIPTION OF THE INVENTION An embodiment of a liquid level sensor of the present invention will be described below with reference to FIG.

In FIG. 1, 1 is a pressure detecting section, 2 is a specific gravity detecting section, and 3 is a specific gravity detecting section.
Is a temperature detection unit, which are integrated and co-located as shown in the figure.

In the pressure detecting section 1, a bellows 11 as a first pressure receiving body is attached to the lower portion of the main chamber of the sensor main body 4 which covers the case 4a to ensure the airtightness inside, and the bellows 11 is attached.
The displacement of the pressure receiving portion 11a is transmitted to the movable shaft 12 supported by the body frame 4b of the sensor. Also,
An optical fiber 5a having an FBG element is fixed and stretched between the fixed roller 13 attached to the main body frame 4b and the movable roller 14 attached to the movable shaft 12 so as not to cause slippage. The structure is such that elongation strain is given according to the amount of shaft displacement (that is, hydraulic pressure).

Further, in the specific gravity detecting portion 2, a sub-chamber for covering the sensor body 4 with a case 4c to ensure airtightness inside is provided adjacent to the main chamber, and second and third pressure receiving chambers are provided above and below the sub-chamber. As a body, two bellows 21 and 22 having the same pressure-receiving area S are attached so as to face each other, and pressure-receiving portions 21a of both bellows,
21b are connected by a movable shaft 23. Further, an optical fiber 5b having an FBG element is stretched between the fixed roller 24 attached to the main body frame 4b and the movable roller 25 attached to the movable shaft 23 so as not to slip. In the illustrated specific gravity detection unit 2 thus configured, the bellows 21 contracts due to the pressure difference due to the height difference H,
The bellows 22 grows. Then, due to the displacement of the movable shaft 23 due to this, extension strain is generated in the optical fiber 5b according to the pressure difference.

The specific gravity detecting section 2 may include a dummy optical fiber 5c for balancing the forces, but if the displacement amount due to the pressure difference is small, the slip between the cladding and the buffer layer in the optical fiber strand will not occur. Since it hardly occurs, there is no problem even without the dummy optical fiber.

Reference numeral 26 is a cone for preventing sedimentation of earth and sand mounted on the pressure receiving portion of the bellows 22, and is provided as a preferable element. When the conical body 26 is provided, it is preferable to attach a dummy conical body 27 to the lower bellows 21 to balance the load.

The temperature detecting section 3 is constructed by arranging an optical fiber 5d having an FBG element between the fixed roller 24 and the fixed roller 31 separately attached to the main body frame 4b so as not to slip. The optical fiber 5d expands and contracts due to temperature changes. The amount of expansion / contraction is the amount of thermal expansion / contraction of itself added to the amount of thermal expansion / contraction of the case 4a. Therefore, this is considered to be a component due to the temperature change included in the strain of the optical fiber 5a. By subtracting from the strain, it becomes possible to correct the temperature of the liquid level obtained from the strain of the optical fiber 5a.

When this temperature correction is performed, the error component contained in the liquid level measurement value is only due to the change in the specific gravity of the liquid.
The remaining error component is removed (specific gravity correction) using the data detected by the specific gravity detection unit 2.

The specific gravity detecting section 2 has bellows 21, 22 where Scm ^{2 is} the pressure receiving area of each of the bellows 21, 22, H is the height difference between the bellows 21, 22 and ρ is the specific gravity of the liquid.
The pressure difference between them is given by the formula ρ · S · H (g). If S = 10 cm ² , H = 40 cm, and ρ = 1.0, the pressure difference between the bellows 21 and 22 is 400 gf.
(≈3.923N). When a bellows with a spring constant of 100 gf is used, the strain of the optical fiber is 1% / kgf, so the length of the FBG element in the optical fiber 5b is 10 c.
If m, the relationship between displacement and force is 0.1mm / 100g
Therefore, the FBG element bears and receives most of the load of 100 gf.

On the other hand, since the FBG element has a strain measurement accuracy of 0.0004%, the FBG element having a length of 10 cm can measure the load with an accuracy of 0.4 g when the spring force of the bellows is ignored. 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.

Although the specific gravity is measured here with an accuracy of 1/1000, 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.

The optical fibers 5a, 5b, 5d and 5c provided as necessary are a series of fibers.
Is connected to the optical fiber inside. At one corner of the optical cable 5, a supervisory controller (not shown) and optical fibers 5a, 5
A measuring device for measuring the liquid level by measuring the reflection wavelength from each FBG element in b and 5d and performing temperature correction and specific gravity correction is connected.

As shown in the figure, the pressure detector 1 and the specific gravity detector 2
By integrating the above, the FBG element (optical fiber 5d) for temperature correction can be shared, and the number of liquid level sensors connected to the one-core optical fiber can be increased to construct a system with a large monitoring capability.

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.

In other words, 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 if 10m is ±
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).

In the liquid level sensor of the present invention, even if the liquid temperature or the specific gravity of the liquid changes, the pressure measured by the pressure detecting unit 1 is obtained by obtaining the temperature at that time and the specific gravity including the influence of the temperature. It is possible to correct the liquid level and measure the liquid level with a small error.Therefore, it is possible to measure the water level at a place where the specific gravity changes due to the mixture of seawater and fresh water, and during the daytime and nighttime, as well as in the summer and winter. When used for water level measurement in places where the water temperature changes, the measurement accuracy can be improved.

Further, even when the water level of water mixed with earth and sand due to heavy rain is measured, the influence of earth and sand can be reduced. Since the apparent specific gravity of water mixed with soil and sand increases, there is a possibility that the water level will be overestimated, but if the specific gravity is detected and corrected, the measurement accuracy will not vary greatly.

If the liquid level sensor of the present invention is used,
By subtracting the theoretical liquid specific gravity (density) under the temperature detected by the temperature detecting unit 3 from the liquid specific gravity detected by the specific gravity detecting unit 2, the weight% of the substance mixed in the monitoring liquid is obtained. It also becomes possible.

Therefore, when the change in specific gravity over the change in density due to temperature is large, it is possible to assume that the soil is mixed due to heavy rain, and by knowing that information in addition to the rise in water level, If applied to a place where the water level of a river is measured, the occurrence of a landslide may be detected depending on the measurement place.

When water levels are measured at multiple points along the length of the river, when there is no possibility of soil contamination on the upstream side and sediment contamination on the downstream side is assumed, a landslide may occur between them. Sex can be detected.

If the sensor of the present invention is installed and used at the bottom of the tank for detecting the liquid level of an oil tank or the like, it is possible to detect the mixture of water and the like. When a liquid having a specific gravity larger than that of oil is mixed, the layers are separated and the detection data of the specific gravity detecting unit 2 is changed.

In addition, it can be applied to observation of suspended sediment in rivers that flow into dam reservoirs (use as basic data for dam sediment) and observation of suspended sediment diffused on the lake surface (where to settle) Is.

If the sensor of the present invention is arranged at the bottom of the lake of the reservoir with an appropriate interval, the specific gravity measurement value of the sensor at the position where the suspended sediment settles and accumulates gradually increases. You can do it.

[0054]

As described above, since the liquid level sensor of the present invention has both the temperature correction function and the specific gravity correction function, the liquid level can be measured even when it is used in a place where liquid temperature fluctuation and liquid specific gravity fluctuation are considered. The value does not vary greatly, which improves reliability and eases installation restrictions depending on the installation location.

Further, since the liquid pressure, the liquid specific gravity, and the temperature are all detected by the optical fiber, real-time multipoint remote monitoring can be performed, and a simple and economical liquid level monitoring system can be constructed.

In addition, the one using the FBG element, the one using the bellows as the pressure receiving body, the one integrating the pressure detecting portion, the specific gravity detecting portion, and the temperature detecting portion, simplifies the sensor,
It can also be downsized.

Further, the liquid level monitoring system using the sensor of the present invention can be used not only for liquid level monitoring but also for monitoring the mixing of soil and the like, the estimation of the point of landslide, the accumulation of suspended sand, etc. Can be achieved.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a liquid level sensor of the present invention.

[Explanation of symbols]

1 Pressure detector 2 Specific gravity detector 3 Temperature detector 4 sensor body 4a, 4c case 4b body frame 5 optical cable 5a, 5b, 5d Optical fiber having FBG element 11, 21, 22 Bellows 12, 23 Movable shaft 13, 24, 31 Fixed roller 14, 25 movable rollers 26 cone 27 Dummy cone

   ─────────────────────────────────────────────────── ─── 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. F term (reference) 2F014 AA04 BA10                 2F075 AA02 AA03 EE02

Claims (4)

[Claims] 1. A pressure detecting portion for generating a strain corresponding to a hydraulic pressure in an optical fiber by utilizing a displacement of a first pressure receiving body due to a hydraulic pressure, and second and third pressure receiving portions which are arranged to face each other with a height difference. A specific gravity detection unit that generates strain in the optical fiber according to the difference in pressure applied to both pressure receiving bodies by utilizing the displacement of the body due to hydraulic pressure, and a temperature detection unit that generates strain in the optical fiber due to thermal expansion and contraction are installed together. , The liquid specific gravity obtained from the strain of the optical fiber of the specific gravity detection unit and the temperature obtained from the strain of the optical fiber of the temperature detection unit are used to correct the measured hydraulic pressure at the sensor installation point obtained from the strain of the optical fiber of the pressure detection unit. , A liquid level sensor with a specific gravity correction function that removes error components due to fluctuations in the specific gravity of the liquid and temperature changes from the liquid level measured value obtained from the measured liquid pressure. 2. The liquid level sensor with a specific gravity correction function according to claim 1, wherein each of the optical fibers of the pressure detecting section, the specific gravity detecting section and the temperature detecting section has an FBG element. 3. A liquid level with a specific gravity correction function according to claim 1, wherein bellows are used as the first, second and third pressure receiving bodies, and the pressure detecting section, the specific gravity detecting section and the temperature detecting section are integrated. Sensor. 4. The liquid level sensor according to any one of claims 1 to 3 is adopted, and in addition to the liquid level measuring function, the specific gravity of the liquid detected by the specific gravity detection unit and the temperature detected by the temperature detection unit are controlled. A liquid level monitoring system that has the function of obtaining the weight difference of the substances mixed in the monitoring liquid by obtaining the theoretical difference in the liquid specific gravity.