JP2002286619A - Suspended-state measuring instrument for slurry - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an instrument and method for measuring the suspended state of slurry by which the concentration of particles in a sample can be measured with high accuracy. SOLUTION: The suspended state measuring instrument for slurry includes at least one current (magnetic field) detecting means which is provided in the circumference of a fluid, a means which oscillates the particles in the fluid by impressing a sound field or electric field upon the fluid in a non-contacting state, and a means which detects the current caused by the particles by means of the current (magnetic field) detecting means without coming into contact with the fluid. The instrument also includes a means which relates the signal generated from the current (magnetic field) detecting means to the concentration of the particles in the fluid.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for analyzing fine particles, and more particularly to an apparatus for analyzing fine particles in liquid suitable for analyzing fine particles present in a fluid.

[0002]

2. Description of the Related Art As a conventional apparatus for analyzing fine particles in a fluid, there is known an apparatus for measuring fine particles in a fluid by measuring a zeta potential. Here, the zeta potential generally refers to an electrokinetic phenomenon (electrophoresis, electroosmosis, streaming potential, sedimentation potential, or the like) as indicated by another name of electrokinetic potential. Such a method of measuring the zeta potential is generally called electroacoustic spectroscopy,
Specifically, the ultrasonic vibration potential method (Ultrasound Vibration P
otential: UVP and Electrokinetic Sonic Am
Plitude (ESA) method is known. Next, these methods will be briefly described. The ultrasonic vibration potential method is a method for measuring a potential difference proportional to the zeta potential caused by the difference between the ease of movement of particles and the ease of movement of counter ions when particles are forcibly vibrated by ultrasonic waves. This principle is Deby
First proposed by e, also called the Debye effect or ion oscillating potential. FIG. 7 schematically shows such a conventional apparatus. As a zeta potentiometer that measures using this ultrasonic vibration potential method, for example, Dispersion Technol
ogy ultrasonic method / particle size distribution and zeta potential measurement device D
T-100, DT-200, DT-1200 and PEN-KEM-Sys manufactured by PEN-KEM
There is a tem-7000. These devices generally include a constant temperature water inlet 70, a constant temperature water outlet 72, a receiver input 74, and a pulse input 7.
6, a stirrer 78, a titrator 80, a preamplifier computer interface 82, and a PH electrode 84, and a constant temperature is maintained around a measurement sample using constant temperature water. In the evaluation of the dispersibility of the fine particle dispersion system, even if the target sample 8 has a high concentration, the particle concentration is measured without diluting the sample 8. Therefore, it is possible to avoid the fluctuation of the zeta potential due to the dilution. .

[0003] The electrokinetic acoustic method is a technique in which, when a high-frequency voltage is applied to a colloid solution, ultrasonic waves generated by the movement of particles in the colloid solution are measured by a detector, and the intensity and phase of the ultrasonic waves are measured. This is a method of calculating the zeta potential from the deviation. This method makes use of the fact that particles with a large inertia cannot follow the electric field when the frequency increases. As this frequency, a high frequency of about 1 kHz is used. ESA is the amplitude of the generated ultrasonic wave per unit electric field,
(Omega) = represented as P / E = C · Δρ · φ · G f · μ d (ω). Here, ω is each frequency, P is the amplitude of the ultrasonic wave, E is the supplied electric field, C is the speed of sound in the suspension, Δρ is the density difference between the particles and the liquid, φ is the volume fraction of the particles, G _f is the correction coefficient, μ _d is dynamic mobility. Since the zeta potential is proportional to the easiness of the particles to vibrate due to the high-frequency electric field (dynamic mobility), the zeta potential can be calculated from the ESA by determining G _f using a standard substance. In this method, it is possible to measure with stirring or in a flowing system because the movement of particles does not affect the measurement, and since the inertial force is proportional to the particle diameter,
It is also possible to calculate the particle size distribution together with the zeta potential by measurement under a condition where the frequency is sufficiently high. Devices using this method include ESA8000 manufactured by Matec Applied Sciences.
There is. For details of the zeta potential, refer to “Zeta Potential Zeta Potential: Physical Chemistry of Fine Particle Interface” by Fumio Kitahara (Scientist).

[0004]

However, such a conventional method has the following problems. First, since the number of parameters required for calculating the zeta potential is large, when measuring the zeta potential, it is necessary to obtain the parameters in advance by another method. Further, in the conventional method, it is necessary to insert an electrode into a measurement object at the time of voltage measurement or voltage application, so that measurement accuracy is deteriorated due to contamination of the electrode surface. This is because a DC electric field is applied to the particles in the fluid through the electrodes,
In the electrolyte solvent, electrolysis occurs at the electrode portion, bubbles are generated, and it becomes difficult to measure the zeta potential of particles in the fluid. This is because the electrode section 2 of the voltage measurement section 10 for measuring the zeta potential in FIG. In particular, when such a conventional apparatus is applied to a line in a plant, there is a problem that voltage fluctuation occurs due to contamination of the electrode or consumption of the electrode. In addition, as a non-contact measurement device,
For example, Japanese Patent Application Laid-Open No. 7-318476 discloses an apparatus for measuring fine particles in a fluid using light. However, in this device, since an image of particles in a fluid formed by light is detected, it is difficult to detect the particles when the concentration of the particles in the fluid is high. Thus, there is a need for a highly accurate and highly reliable measuring device that can be controlled in real time or in-situ without depending on the concentration of particles in a fluid in a line in a plant. ing.

An object of the present invention is to provide a measuring device capable of continuously detecting the concentration of particles flowing in a fluid (slurry) in a non-contact manner in view of the above problems. Another object of the present invention is to provide a highly accurate measuring device having stability that enables continuous measurement.

[0006]

SUMMARY OF THE INVENTION The present invention provides an apparatus and a method for measuring the concentration of a component in a fluid by applying ultrasonic waves to the fluid without contacting the fluid. The conceptual diagram is shown in FIG. Compared to the conventional device of FIG.
Using the coil unit 12 around the fluid 8, the zeta potential caused by particles and ions in the fluid is measured by the current or voltage measuring unit 1.
4 indicates that the measurement is performed in a non-contact manner. That is, when the charged particles move by the ultrasonic waves, the electric charges of the particles also move at the same time, so that an alternating current having a frequency equal to the frequency of the ultrasonic waves is generated. Ampere's law of electromagnetism indicates that a magnetic field is generated around a current in proportion to its magnitude. Therefore, applying ultrasonic waves to the suspension,
By detecting the alternating magnetic field generated around it, the current generated by the movement of the particle charge is obtained, and then the particle charge,
The zeta potential can be determined. As a means for detecting a magnetic field, a Rogowski coil, a Hall element, a squid (SQUID, superconducting quantum interference element) and the like can be considered. Specifically, the present invention provides at least one current (magnetic field) detecting means provided at a peripheral portion of a fluid, and applies a sound field or an electric field to the fluid in a non-contact manner to shake particles in the fluid. Moving means, a means for detecting a current generated by the particles in a non-contact manner with the fluid by the current (magnetic field) detecting means, a signal generated in the at least one current (magnetic field) detecting means, Means for associating the concentration of the particles with the concentration of the particles in the fluid. Here, the current (magnetic field) detecting means is arranged at an interval of 1/4 wavelength of a sound field or an electric field applied to the fluid, or the current (magnetic field) detecting means is a Rogowski coil or a squid. An embodiment is preferably mentioned. Further, according to the present invention, there is provided at least one squid provided in a peripheral portion of a fluid, means for detecting a current or a magnetic field due to movement of the particles in a non-contact manner with the fluid by the at least one squid, and There is provided an apparatus for measuring a suspended state of a slurry, comprising: means for associating a detected current or magnetic field with the concentration of the particles in the fluid. Here, a preferred embodiment is one in which the squid is a flux gate or a Hall element. Here, prior to the means for detecting the current or the magnetic field, means for controlling the flow rate or flow rate of the fluid may be included.

The fluid refers to a fluid such as a slurry which is composed of a mixture of a solvent and particles, and includes a sol, a gel and the like. In addition, in the case of being provided in the peripheral part of the fluid here, for example, a case where a part of the fluid is included in a coil in a non-contact manner using a tube having a multiplex structure in the diameter direction, or a large diameter And a case in which the present invention is applied to a smaller diameter branch pipe branched from the main pipe. In addition, the particles referred to herein include not only the particles themselves, but also the components constituting the particles and the bonds between them and other components in the medium (for example, hydrates). It includes ionized ions and the like.

[0008] The magnetic field detecting means used herein is a generic term for electric wires, elements, junctions, or devices including them provided at the periphery of a fluid to be measured and detecting electric fields, magnetic fields, and the like. Specifically, the magnetic field detecting means includes, for example, a metal wire such as a Rogowski coil, at least one copper wire, or a magnetic field, which is provided around the fluid so as to include the fluid to be measured. It includes a Hall element used for measurement, a Josephson junction formed by a superconducting material, and the like. In particular, when the width of the Rogowski coil is not sufficiently small for a quarter of the wavelength of the sound, electric or magnetic field, or when the separation of components in the fluid shows a gradual response to this interval. In some cases, the resolution of one coil may be better than the resolution of a Rogowski coil.

The sound field referred to here is a gas, a liquid,
A wave caused by the density of a medium propagating in a medium such as solid or plasma, and includes, for example, a case where ultrasonic waves propagate through air or water. In addition, the oscillation here means that particles in the fluid are physically moved and vibrated.For example, in a case where a medium or particles in the fluid are physically mixed by a stirrer or the like. included.

Separation of particles in a fluid into components by applying ultrasonic waves, electric or magnetic fields, etc. to the fluid in a non-contact manner, so that the components respond individually electrically or magnetically. And a response based on this component can be detected in a non-contact manner by at least one current (magnetic field) detecting means provided on the periphery of the fluid.
It is possible to provide a highly accurate and highly reliable measuring device that is free from fluid contamination and does not fluctuate due to aging of components of the device. In addition, at least one current (magnetic field)
By correlating the difference between the electrical or magnetic measurement results detected by the detection means and the particle concentration in the fluid, the concentration of the particles in the fluid can be managed in a non-contact manner with high accuracy and high reliability in real time It is possible to do.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As a specific embodiment, a case where a Rogowski coil is used will be described. First, the principle will be briefly described. For example, when an ultrasonic wave is applied to a fluid, as shown in FIG. 1, the responses of the distribution states of the charges of the particles 4 and the ions 6 generated in the fluid are different from each other (in FIG. Indicated by the difference). Here, for simplicity, the description will be made on the assumption that the particles in the fluid have no particle size distribution and are monodisperse. When a particle is negatively charged, it is electrically neutral throughout the solution and therefore has a positive charge equal to that of the particle (eg, H ⁺
A pair of ions appears. In general, the equation of motion of a particle subjected to ultrasonic waves in a liquid is

(Equation 1) It is expressed as Here, m in the above equation (1) is mass, v
Is the velocity of the particle, η is the viscosity coefficient, a is the radius of the particle, and f is the ultrasonic pressure. In the case of ions, since the viscosity is negligible, the speed of the ions is approximately equal to the speed of sound. Therefore,
When the current due to counterion and i _c, a i _c = -e _c v _s. Here, e _c is the charge of the counter ion, and v _s is the speed of sound.
On the other hand, in the case of particles, the viscosity of the particles is very large, so that the acceleration applied to the particles can be almost ignored. If the particle velocity v _p is obtained assuming (dv / dt) ≒ 0 in equation (1), then v _p = f / 6πη _{p ap} . Therefore, if the current due to particles is i _p ,

(Equation 2) Becomes Here, e _p is the charge of the particle, η _p is the viscosity coefficient of the particle, and a _p is the radius of the particle. The total current value i including the particle and the counter ion is

(Equation 3) Becomes Accordingly, by measuring the current generated in the fluid while changing the pressure of the ultrasonic wave, e is calculated from the slope and intercept of the graph of the ultrasonic pressure and the current value at that time.
_p / η _p and e _c are obtained. Furthermore, from the condition e _p = e _c where the total charge becomes zero, the charge of the viscosity coefficient and particle are determined. Further, the particle size when the shape of the particle is approximated by a sphere can be obtained from the viscosity coefficient thus obtained by using Stokes' law.

FIG. 3 is a schematic diagram of an apparatus using a Rogowski coil. The apparatus includes a flow controller 16, an ultrasonic transmitter 18 for introducing ultrasonic waves into a fluid (slurry), an ultrasonic transmitter 20 for generating ultrasonic waves,
At least one Rogowski coil 22 and 24, an ultrasonic sound pressure meter 26, and a lock-in amplifier 28 for capturing signals from the Rogowski coil 22 and 24, respectively.
, 30 and a storage oscilloscope 32.

In this apparatus, an ultrasonic wave is applied to a fluid, and a current is detected near a position corresponding to the node and the antinode. The ultrasonic frequency at this time is 10 kHz to 5
The optimum value is determined in the range of 00 kHz. At this time, it is desirable to use as high a frequency as possible so as to increase the difference in movement between the particles and the ions. However, if the frequency is too high, the wavelength will be too short, smaller than the size of the magnetic field detecting means, and it will be difficult to detect a change in the magnetic field. Since the wavelength of a 500 kHz ultrasonic wave in water is about 2 mm, it is possible to use a higher frequency ultrasonic wave if there is a sufficiently small magnetic field detecting means.

Next, at least one Rogowski coil 22 and 24 is arranged near the position corresponding to the node and the antinode to detect the current at the position of the node and the antinode of the standing wave of the ultrasonic wave. . The distance between the Rogowski coils 22 and 24 is desirably １ ／ wavelength of the ultrasonic wave applied to the fluid. Also, in order to increase the current detection sensitivity (resolution), it is desirable that the diameter of the Rogowski coil is sufficiently smaller than the wavelength of the ultrasonic wave and is about 2 mm or smaller. Although it is desirable that the number of turns of the Rogowski coil be as large as possible, the number of turns may be one in order to emphasize the response speed to the current. Also,
It is desirable to receive and detect with high resistance so as not to be affected by the inductance of the Rogowski coil.

The lock-in amplifiers 28 and 30 connected to the Rogowski coils 22 and 24 receive a reference signal from an ultrasonic transmitter. Lock-in amplifier 2
8 and 30 phase-detect the input signals from the Rogowski coils 22 and 24 and the reference signal, and output them to the storage oscilloscope 32. The storage oscilloscope 32 can detect the current generated by the applied ultrasonic wave by obtaining the difference between the output signals from the lock-in amplifiers 28 and 30B.

Further, the detected current and the ultrasonic sound pressure meter 26
From the value of (1), the charge of the particles in the fluid can be obtained (see Equation (3) above). Thereby, the charge density per unit volume in the fluid can be obtained.

As described above, the charged particles and ions in the fluid slurry vibrate using the ultrasonic waves,
An alternating current having the same frequency as the ultrasonic waves is generated, and the charge of the particles can be evaluated by measuring the value of the alternating current. This takes advantage of the fact that particles and ions have different forces received from ultrasonic waves and viscous resistance in a slurry and have different easiness of movement. Further, by changing the pressure, that is, the power of the ultrasonic wave, and measuring, the signals of the particles and the ions can be easily distinguished.

Further, when the present invention is applied to a line in a plant, since a target fluid has a flow velocity, prior to detection of current or voltage by the apparatus of the present invention,
It is desirable to provide a control unit for controlling the flow velocity and flow rate with a mass flow meter or a flow meter. In such a case,
The fact that the electrical or magnetic change detected by at least one coil differs significantly from the normal control range,
It can be determined that the flow rate of the fluid itself has changed (fluid leakage, malfunction of the flow rate control unit, etc.).

As another specific embodiment, a superconducting quantum interference device (Superconductor) is used as a current (magnetic field) detecting means.
A description will be given of a case in which a squid called an SQUID (in Quantum Interference Device: SQUID) is used. First, the principle will be briefly described. When superconductors with different phases are joined via a barrier layer, a tunnel current flows between the two superconductors, which weakens when the magnetic flux passing through the junction surface is an integral multiple of the flux quantum,
It shows a Fraunhofer-type quantum interference effect of strengthening each other. Such a property is called a Josephson effect, and a device in which one or more Josephson junctions are connected by a superconducting loop is generally called a squid. The squid includes an AC squid having one Josephson junction and a DC squid having two Josephson junctions, and can output a current or a voltage according to a change in magnetic flux.

FIG. 4A schematically shows the structure of a DC squid as an example. The squid includes a voltage measuring unit 34, a junction 36 called a Josephson junction, an opening 38 called a squid hole,
Oxide high-temperature superconductor such as YBa ₂ Cu ₃ O _y or HoBa ₂ Cu ₃ O _7-x or superconducting thin film material portion 40 made of metal such as niobium (Nb)
And a bias current applying unit 42. This element is a very sensitive magnetic sensor capable of measuring a weak magnetic field of about 1 / 50,000,000 to 1 / 100,000,000 of the geomagnetism (about 0.5 Gauss). Furthermore, it is possible to measure the fluid in a non-contact and non-destructive manner. When using a squid, liquid nitrogen (boiling point temperature 77K) or liquid helium (boiling point temperature 4K) is used to bring the superconducting thin film material portion made of a metal such as oxide superconductor or niobium into a superconducting state. And cool. Examples of such a squid include a high-temperature superconducting squid manufactured by Sumitomo Electric Hitex Co., Ltd.
Squid manufactured by Quantum Design is known.

An opening 38 called a squid hole
4B, the voltage of the current flowing through the Josephson junction 36 becomes a magnetic flux quantum Φ ₀ (Φ ₀ = 2.03 × 10 ⁻¹⁵ Wb) as shown in the example of the Φ-V characteristic in FIG. It is modulated in a very small unit. Furthermore, if there is at least one squid in the periphery of the fluid, the concentration of the particles in the fluid can be measured. Therefore, as compared with the case of the Rogowski coil, there is a possibility that the apparatus can be provided relatively inexpensively because the apparatus configuration is simplified.

FIG. 5 shows a specific configuration of the apparatus.
The apparatus includes a flow control unit 44, a squid unit 48 including a squid element 46, and a cooling unit 5 for introducing liquid nitrogen and liquid helium to bring the squid into a superconducting state.
0, a magnetic shield section 52 for eliminating the influence of geomagnetism and the like, a lock-in amplifier 54, a storage oscilloscope 56, and a control section 58 for controlling the flow rate control section, the lock-in amplifier, and the storage oscilloscope. It becomes. First, the particles in the fluid whose flow rate is controlled by the flow control unit 44 are passed beside the squid element 46 in the squid unit 48. At the time of this passage, it is desirable that the flow velocity and the volume of the fluid containing the particles are constant. Therefore, prior to the passage beside the squid element 46, a mechanism that controls the flow velocity or the flow rate of the particles by a flow meter may be used. Good. This flow meter includes an electromagnetic flow meter and an ultrasonic flow meter. In this way, the current or magnetic field due to the movement of the particles in the fluid is detected in the squid portion 48, and the lock-in amplifier 54
And to the control unit 58 via the scope oscilloscope 56.
In the control unit 58 using this computer, the flow velocity and volume of the fluid and the current or magnetic field due to the motion of the particles in the fluid detected by the squid are monitored at regular time intervals in real time, and these are correlated. Therefore, the concentration of the particles in the fluid can be measured without using these data.

In the case where there is no change in the flow velocity or flow rate of the fluid, if a large change appears in the current detected by the squid, for example, as shown in FIG. It is assumed that impurities 60 made of metal are included. Thus, besides measuring the concentration of particles in a fluid, the squid can be used as a device for managing impurities in the fluid. When a squid is used, unlike a case where a coil or the like is used, as shown in FIG. 6, if the squid is installed near the fluid 8, magnetic flux due to magnetization in particles penetrates through the squid. Can be detected. Since the squid is very sensitive, there is a certain degree of freedom in the arrangement of the squid, unlike the case of the coil.

The measurement accuracy of magnetization by the above-mentioned squid is 10
^It can measure with very high sensitivity of about ^-8 emu. Further, upon measurement, the magnetic field dependence at a constant temperature, the temperature dependence at a constant magnetic field, the angle dependence at a constant temperature or a constant magnetic field, and the like can be measured. All of the operations of such a system can be controlled by a computer, and by setting up a measurement program in advance, it is possible to automatically measure the concentration of particles contained in the fluid, impurities, the state of the apparatus, and the like in real time.

The case where a squid is used has been described. However, the present invention is not limited to this. For example, a magnetic flux can be measured using a flux gate or a Hall element instead of the squid. Further, a digital oscilloscope or a sampling oscilloscope may be used instead of the storage oscilloscope used in the case of the Rogowski coil and the squid.

Regarding other effects such as charging of the pipe, the charge in the fluid slurry is in proportion to the charging amount of the pipe. Therefore, the amount of charge can be obtained by measuring the streaming potential or the flowing current. In an actual system, the particle size distribution of the particles exists, but if the particle size distribution is known in advance by another measurement, the resistance received by each particle can be obtained.
In this case, for example, an assumption that the surface charge is proportional to the surface area may be introduced. However, in the case of a dissolved salt, even if the positive ions and the negative ions are balanced, it is desirable to apply the salt aqueous solution after verifying it based on the measurement data.

[0027]

According to the present invention, since it is possible to perform measurement without contacting a fluid, it is possible to solve the problem of the prior art that the measurement accuracy is deteriorated due to contamination of electrodes used for measurement. In addition, since there is no electrode contamination, there is no need to replace parts such as electrodes, and the manufacturing cost and management cost of the apparatus can be reduced. Moreover, in a line in a plant, highly accurate and highly reliable measurement can be performed in real time or in-situ. Further, when the fluid has a flow velocity such as a line in a plant, the electrical or magnetic change detected by at least one current (magnetic field) detecting means is compared with a normal control value, whereby the fluid is controlled. In addition to the concentration of the particles therein, the flow rate of the fluid can also be measured.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of an apparatus for measuring zeta potential in a fluid generated by application of ultrasonic waves by a conventional apparatus.

FIG. 2 is a conceptual diagram of an apparatus for measuring zeta potential in a non-contact manner according to the present invention.

FIG. 3 is a configuration diagram of an apparatus for measuring zeta potential in a non-contact manner using a Rogowski coil of the present invention.

FIG. 4A is a configuration diagram of a general DC squid. (B) is a schematic diagram showing a change in current or magnetic field detected by the squid when a magnetic flux penetrates through the squid hole of (a).

FIG. 5 is a block diagram of an apparatus for measuring a concentration of particles in a fluid by detecting a current or a magnetic field caused by the motion of the particles in the fluid in a non-contact manner using a squid in another embodiment of the present invention. is there.

FIG. 6 is a conceptual diagram of an apparatus for non-contactly measuring magnetization due to impurities contained in a fluid using a squid of the present invention.

FIG. 7 is a configuration diagram of a conventional apparatus for measuring zeta potential using ultrasonic waves.

[Explanation of symbols]

 2 Electrode part 4 Particle 6 Ion 8 Fluid (slurry) 10 Voltage measuring part 12 Coil part 14 Current or voltage measuring part 16, 44 Flow control part 18 Ultrasonic transmitter 20 Ultrasonic transmitter 22, 24 Rogowski coil 26 Ultrasonic Sound pressure meter 28, 30, 54, 56 Lock-in amplifier 32, 56 Storage oscilloscope 34 Voltage measuring unit 36 Josephson junction 38 Squid hole opening 40 High temperature superconducting thin film unit 42 Bias current applying unit 46 Squid element 48 Squid unit 50 Cooling unit 52 Magnetic shield part 58 Control part 60 Impurity

Continued on the front page (72) Inventor Masahiko Nagai 5-717-1, Fukabori-cho, Nagasaki-shi, Nagasaki Sanishi Heavy Industries, Ltd. Nagasaki Research Laboratory (72) Inventor Hideki Tonaka 4-6-kannonshinmachi, Nishi-ku, Hiroshima-shi, Hiroshima No. 22 Mitsubishi Heavy Industries, Ltd. Hiroshima Research Laboratory F-term (reference) 2G053 AA02 AB11 AB14 CA03 CA05 CA10 CA18 CB16 DA01 DB05

Claims (5)

[Claims] At least one of the fluids is provided at a peripheral portion of the fluid.
Magnetic field detecting means, means for applying a sound field or an electric field to the fluid in a non-contact manner to oscillate particles in the fluid, and detecting a magnetic field generated by a current generated by the particles. A slurry suspension state comprising: means for detecting a current by detecting the fluid in a non-contact manner with the fluid; and means for correlating an output of the at least one magnetic field detection means with a concentration of the particles in the fluid. Measuring device. 2. The apparatus according to claim 1, wherein said magnetic field detecting means is arranged at intervals of １ ／ wavelength of a sound field or an electric field applied to said fluid. 3. The apparatus according to claim 1, wherein the magnetic field detecting means is a coil such as a Rogowski coil, a Hall element, or a squid. 4. The apparatus according to claim 4, wherein the squid is a flux gate or a Hall element. 5. The apparatus according to claim 1, further comprising means for controlling a flow rate or a flow rate of the fluid, prior to the means for detecting a magnetic field generated by the current.