JP2000146865A - Sludge densitometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance measuring sensitivity, and facilitate maintenance and inspection. SOLUTION: One end part of a probe main body 11 comprising a wave guide is closed and its other end part is opened. The opened end part of the probe main body 11 is attached to a window part 13 formed in a sludge transportation pipe 12. The first plate-shaped conductor part 15 having a high-frequency window 14 comprising a ceramic member in its central part is provided in the window part 13. The second plate-shaped conductor part 17 having a through hole 16 in its central part is provided in a sludge face side of the first conductor part 15 with a colse contact condition. An antenna 18 is provided inside the probe main body 11, and the antenna 18 is connected to a microwave transmitter/receiver with a coaxial cable. A hollow part 20 is formed inside the probe main body 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a sludge used in a process for measuring the concentration of solids and suspended solids in sludge in the treatment process of a sewage treatment plant, a wastewater treatment plant, a water purification plant, and a sludge treatment plant thereof. It relates to a densitometer.

[0002]

2. Description of the Related Art Sewage treatment plants, wastewater treatment plants,
In a water treatment plant and those sludge treatment plants, it is very important for plant operation management to constantly monitor and grasp the amount of solid matter generated from each process or transported from one process to another process. It is. The amount of sludge solids can be calculated from two values, sludge flow rate and sludge concentration.

[0003] For measuring the sludge flow rate, an electromagnetic flow meter, an ultrasonic Doppler type flow meter or the like is used. These flowmeters have achieved relatively reliable measurements. On the other hand, the measurement of the sludge concentration is based on the principle of attenuation of ultrasonic waves and the principle of detection based on the amount of transmitted light or reflected light. At present, these sludge densitometers are inferior in reliability and the like as compared with electromagnetic flowmeters due to factors disturbing measurement and complicated maintenance work.

In recent years, microwave-type sludge densitometers using a transmission phase difference detecting method utilizing microwaves have been developed and used to solve the disadvantages of conventional sludge densitometers. This microwave-based sludge densitometer using microwave
It is configured as shown in FIG. In FIG.
Reference numeral 61 denotes a sludge transport pipe which forms a window facing a predetermined portion of the sludge transport pipe 61. A densitometer detection probe 62 on the microwave transmission side and a densitometer detection probe 63 on the microwave reception side are provided in this window. 64 is both probes 62 and 63
Is a sludge concentration meter converter for obtaining sludge concentration from a phase difference between microwave transmission and reception waves.

In the microwave type sludge densitometer shown in FIG. 10, the difference (phase difference) between the phase delay of the microwave transmission wave in the fresh water (concentration 0%) and the phase delay of the microwave transmission wave in the sludge is shown. As shown in FIG. 11, the concentration is measured using the fact that the concentration is linear with the sludge concentration. Note that the concentration is obtained from a constant × Δθ.

FIG. 12 is a schematic diagram showing a reflection type microwave type sludge densitometer having a different detecting means from the above-mentioned microwave type sludge densitometer. In FIG. 12, reference numeral 81 denotes a sludge transport pipe. A window is formed in a predetermined portion of the sludge transport pipe 81, and a microwave reflection intensity detection type probe 82 is formed in the window.
Is provided. The probe 82 transmits microwaves from a set of probes, receives the microwaves reflected on the interface between the probe and the sludge mixture (total sludge including solids) with the same probe, and receives the microwaves with respect to the transmission intensity. Detect the intensity ratio. Reference numeral 83 denotes a microwave transceiver, which detects the ratio of the transmission and reception intensities detected by the microwave transceiver 83 and inputs the detection signal to a sludge concentration converter 84, where the solid matter concentration in the sludge is measured. Measure. The probe 82
, Devices such as coaxial cables and waveguides are used.

[0007]

(1) The two microwave-type sludge densitometers shown in FIG. 10 are based on the principle of measuring the sludge concentration by detecting the phase difference of the microwave transmission component in the sludge. Therefore, for example, a large-diameter transport pipe (for example, φ35
(0 or more), the following problem occurs.

[0008] This is because, if the transport pipe is made large, the attenuation of the microwaves reaching the receiving side becomes large and the phase difference cannot be detected. Therefore, the diameter is reduced or the pipe is partially branched with a small-diameter pipe. (Bypassing) or measures to increase the microwave transmission output are required.

[0009] Although it is possible to some extent, there is a limit, and the condition is that the allowable pressure loss in the plant design should not be exceeded.

[0010] Regarding the above, a large installation space is required, and if there is not enough space, installation may not be possible.

[0011] When the frequency exceeds a certain limit, it is a violation of the Radio Law. In addition to an increase in the price due to an increase in transmission power, a cost for complying with the Radio Law is required, and the total price becomes high.

(2) The microwave type sludge densitometer shown in FIG. 10 is based on the principle of measuring the sludge concentration by detecting the phase difference of the microwave transmission component in the sludge. Are arranged to face each other with the sludge transport pipe interposed therebetween.

For this reason, it is necessary to provide a bypass pipe so that the transmitter and the receiver can be removed together with the sludge transport pipe in consideration of a response when a failure occurs in the densitometer. Therefore, a large installation space is required, and if there is not enough space, installation may not be possible.
Further, in addition to the main body of the concentration meter, construction costs for the bypass pipe are required, so that the total price rises.

(3) In the microwave type sludge concentration meter shown in FIG. 10, the transmitter and the receiver cannot be easily removed because the transmitter and the receiver are arranged opposite to each other in the sludge transport pipe.
Therefore, comparison and calibration between the chemical analysis value of the sludge concentration and the output of the concentration meter must be performed in the sludge pipe. For example, for zero-point calibration in fresh water, calibration can be performed relatively accurately by flowing sludge to the bypass side, closing valves at both ends of the densitometer, and replacing sludge in the densitometer with water for measurement. it can. However, when calibrating the span measurement at a certain sludge concentration, the sludge settles by the same method as the zero point, so that accurate calibration cannot be performed. Therefore, for calibration on the span side, sludge flows through the densitometer as in normal measurement,
At a certain point, the sludge collected at a certain point must be chemically analyzed, and a method of comparing and calibrating the sludge output at that time must be adopted. However, since the sludge concentration in the normal sludge transport pipe changes every moment, the sludge collected and the output of the densitometer are
It is difficult to match them as they are. Therefore, the output calibration of the densitometer cannot be accurately performed in many cases, and there is a problem that the measurement accuracy is greatly affected.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a sludge concentration meter that improves measurement sensitivity and matching with sludge and facilitates maintenance and inspection.

[0016]

According to the present invention, in order to achieve the above-mentioned object, a first invention is to provide a reflection intensity detecting type microwave detecting probe main body in a sludge transport pipe, and to transmit microwaves from the probe main body. Dispatch to the sludge transport pipe,
The same probe body receives the microwave reflected at the boundary of the sludge mixture liquid to be measured in the sludge transport pipe, detects the ratio of the reception intensity to the transmission intensity, and detects the solid concentration and suspended solids in the sludge mixture. In a sludge concentration meter that measures the concentration,
A conductor is provided at an open end of the probe main body, the conductor having a portion for guiding a sample to be measured.

According to a second aspect of the present invention, the probe body is constituted by a waveguide having one end closed and the other end opened.

According to a third aspect of the present invention, the probe main body is formed of a waveguide, and a first conductor having a window of a member through which microwaves can pass is provided in an opening at the other end of the waveguide. A second conductor portion is provided in close contact with the first conductor portion, the second conductor portion being provided with a portion for guiding the sample to be measured in the window.

A fourth invention is characterized in that a dielectric resonator is filled in the probe main body composed of a waveguide.

According to a fifth aspect of the present invention, a protective film is provided between the first conductor and the second conductor.

According to a sixth aspect of the present invention, in the member through which the microwave can be transmitted, the thickness, width, and length of the shape of the member include values obtained from the following equations (a), (b), and (c). , And a value within 10% of the value obtained from these equations.

Thickness = light speed / (square root of ε _s of member × resonance frequency × 4) (a) Width = light speed / (square root of ε _s of member × cutoff frequency × 2) (b) Vertical width = width / 2 (c) where ε _s is the relative dielectric constant.

According to a seventh aspect of the present invention, the shape of the portion for guiding the sample to be measured is such that the width of the portion is determined by the following equation (e), and the value within 10% of the value obtained from this equation is defined as: In addition, the vertical width of the part satisfies the following expression (d).

The vertical width of the groove> the vertical width of a member through which microwaves can pass .... (d) The width of the part = light speed / (square root of sewage ε × resonance frequency of member × 2) (e) where ε Is the dielectric constant.

According to an eighth aspect of the present invention, the depth of the portion for guiding the sample to be measured has a value satisfying the following expression (f).

Depth of the part> = width of the part / 4 (f) The ninth invention is characterized in that the thickness of the protective film has a value satisfying the following expression (g).

Film thickness <= 0.01 × light speed / (square root of ε _s of protective film × resonance frequency of high frequency window) (g)

[0028]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an enlarged sectional view of a main part of a probe portion of a reflection type microwave detecting type sludge densitometer according to a first embodiment of the present invention. In FIG. 1, reference numeral 11 denotes a probe main body composed of a waveguide. One end of the probe main body 11 is closed, and the other end is open. The open end of the probe body 11 is
2 is attached to the window 13 formed. Window 13
Is provided with a plate-shaped first conductor portion 15 having a high-frequency window 14 made of a ceramic member at the center. On the sludge surface side of the first conductor portion 15, a plate-shaped second conductor portion 17 having a through hole 16 at the center is provided in close contact. An antenna 18 is provided in the probe main body 11.
Reference numeral 8 is connected to a microwave transmitter / receiver 83 shown in FIG. 12 via a coaxial cable. Reference numeral 19 denotes a coaxial cable connector. Reference numeral 20 denotes a hollow portion of the probe main body 11.

Next, the operation of the first embodiment will be described. A transmission signal from the microwave transmitter / receiver 83 shown in FIG. 12 is supplied to the antenna 18 via a coaxial cable (not shown).
When the microwave transmitted from the antenna 18 is radiated from the ceramic member high-frequency window 14 into the sludge transport pipe 12, the microwave is reflected on the boundary surface of the sludge mixed liquid, and is again returned to the high-frequency window 1.
4 and is received by the antenna 18 of the probe body 11. The received signal is received by the microwave transmitter / receiver 83, converted into a sludge concentration by the sludge concentration meter converter 84, and displayed on the sludge concentration meter.

As described above, the microwave transmitted from the antenna 18 in the probe body 11 is
-Frequency window 14 made of ceramic member formed in
Since the high-frequency window 14 is made of a ceramic member, the high-frequency window 14 prevents the wastewater (sludge mixture), which is a measurement sample, from entering the probe main body 11 and has an area necessary for measurement. It has a function of stably transmitting microwaves of the frequency.

The second conductor portion 17 allows microwaves to pass through the through-hole 16 to the sewage, and the portion without the through-hole 16 (conductor portion) defines the electromagnetic boundary between the ceramic member and the sewage. Has a role to stabilize. For this reason,
It can be expected that microwave matching between the ceramic member and the sewage is improved.

Here, the waveguide and the waveguide resonator constituting the probe body 11 will be described. A waveguide is a hollow tube made of an electric conductor, and is used for transmitting microwaves having a high frequency of several thousand MHz or more. Microwaves are transmitted inside the tube. The size inside the tube is designed according to the frequency of the microwave used. A rectangular cross section is called a rectangular waveguide, and a circular cross section is called a circular waveguide. In addition, a type in which a dielectric is packed inside the tube instead of air is called a dielectric waveguide.
When the same frequency is used, the size of the dielectric waveguide decreases in inverse proportion to the square root of the dielectric constant of the dielectric.

A waveguide resonator is a microwave circuit device in which both ends or one end of a waveguide are closed so that a standing wave can be generated inside the tube. To reduce the size, a dielectric is often used instead of air.

FIG. 2 is a cross-sectional view showing a second embodiment of the present invention. In the second embodiment, a protective film 21 is provided between the first conductor 15 and the second conductor 17 in the first embodiment. It is a thing. When this protective film 21 is provided, the high frequency window 14 made of a ceramic member is protected from sewage, and the matching frequency can be finely adjusted by changing the thickness of the protective film 21. Note that the protective film 21 includes
Teflon, which has good corrosion resistance and low microwave absorption, is optimal.

FIG. 3 is an enlarged cross-sectional view of a main part of a probe portion of a reflection intensity detecting type microwave type sludge concentration meter according to a third embodiment of the present invention. Will be explained. In the third embodiment, a dielectric resonator 31 is provided in a hollow portion 20 of a probe body 11, the probe body 11 is inserted into a window 13, and the probe main body 11 is It is fixed to. Then, the second conductor portion 17 having the through hole 16 is provided in close contact with the dielectric resonator 31. By providing the dielectric resonator 31 in the hollow portion 20 of the probe body 11 as in the third embodiment, the probe structure can be reduced in size and the configuration can be simplified.

FIG. 4 is a characteristic diagram of an experimental result showing a measurement result of the reflection intensity according to each embodiment of the present invention and a measurement result of the conventional reflection intensity. From the result, the characteristic diagram of each embodiment of the present invention is shown. As described above, since the second conductor portion having the through hole is provided at the contact portion with the sewage, the electromagnetic boundary between the ceramic member and the sewage can be stabilized, and therefore, according to each embodiment of the present invention. In this case, improvement in measurement sensitivity can be expected as compared with the conventional one.

As shown in FIG. 4, the measurement range is 2 to 2.6 GH.
In z, when there is no second conductor 17, the reflection is -3.
In contrast, when there is the second conductor portion 17 having the through hole 16, the matching is improved to about −17 dB. This indicates that the reflectance is significantly improved from a reflectance of 70% or more to a reflectance of 14% in percentage.

Since the first to third embodiments are reflection-detection-detection-type microwave-type sludge densitometers, the necessary conditions among the performances required of the detection probe of the densitometer are as follows. There are the following two.

Requirement 1: Sufficient matching with load sludge is required. Condition 2: Microwave reflection characteristics change in accordance with change in properties of load sludge.

In order to satisfy the above two requirements, the first to third embodiments have been devised. When the concept of the first to third embodiments is compared with an electronic circuit, the high-frequency window has a function of a transmission circuit of a resonance frequency, and the second conductor portion and the sludge in the through hole have a function of a filter circuit for the resonance frequency. The characteristics as a filter change according to the concentration and conductivity of the sludge, and the matching state changes. By measuring this variation from the amount of reflected microwaves, it is possible to obtain information on the concentration and conductivity of the wastewater.

FIG. 5 is also an enlarged sectional view of a main part of a probe part of a reflection-intensity detection type microwave type sludge densitometer showing a fourth embodiment of the present invention having the above-mentioned necessary conditions. The description is given with the same reference numerals. The fourth shown in FIG.
Although the configuration is the same as that of the second embodiment, the configuration in which the protective film 21 is provided between the first conductor 15 and the second conductor 17 is the same.
In the fourth embodiment, the high-frequency window 14 shown in FIG. 6 and the through-hole 16 formed in the second conductor 17 shown in FIG.
The following describes that various differences occur when the configuration is described later. In addition, 22 is a flange of the sludge transport pipe, and 23 is a waterproof shield.

First, the high frequency window 14 shown in FIG.
The specific shape of the through hole 16 shown in FIG.
First, the shape of the high-frequency window 14 will be described.

The high frequency window 14 resonates when the thickness is 1/4 wavelength. Using the following equation (1), the thickness of the high-frequency window 14 with respect to the resonance frequency to be used can be calculated.

Thickness = light speed / (square root of ε _s of high frequency window × resonant frequency × 4) (1) where ε _s is a relative permittivity.

Here, as specific values, the high-frequency window 14
When alumina having a relative dielectric constant of "9" is used, the resonance frequency is 2.2 GHz and the thickness is 11.4 mm.

Next, as for the width, it is desirable to make the characteristics of the microwave equal to the coaxial-waveguide converter to be connected. This is possible by matching the cutoff frequencies. The width of the high frequency window 14 is calculated using the following equation (2).

Width = light speed / (square root of ε _s of high frequency window × cutoff frequency × 2) (2) where ε _s is a relative permittivity.

Here, as a temporary value, alumina having a relative dielectric constant of “9” is used for the high-frequency window 14 and a cutoff frequency is 1.37 GH.
When the coaxial-waveguide converter of z is used, the width becomes about 36.5 mm.

Since it is customary to set the vertical width to a value that is half of the horizontal width, the vertical width can be obtained from the following equation (3).

Vertical width = Horizontal width / 2 (3) Using this equation (3), in the case of the above example, it is about 18.2 mm.

However, the values obtained by the equations (1), (2) and (3) do not significantly affect the characteristics if they are within about 10% of the values.

Next, the shape of the rectangular through hole 16 will be described.
There is no problem if the vertical width is longer than the vertical width of the high frequency window 14. However, when the length is short, unnecessary capacitance affects the operation, which is not preferable. Therefore, the through hole 1
6 and the vertical width of the high-frequency window 14 satisfy the following equation (4).

The vertical width of the through-hole> the vertical width of the high frequency window (4) Next, the horizontal width of the through-hole 16 will be described. Through hole 1
6 is formed so that the cut-off frequency of the hole and the resonance frequency of the high-frequency window 14 match. At this time, the hole resonates,
The way of resonance is an open end on both the outlet side and the inlet side. The width of the through hole 16 is given by the following equation.

Width of hole = light speed / (square root of ε of wastewater × resonant frequency of high frequency window × 2) (5) where ε is a dielectric constant.

As a tentative value, when the resonance frequency is 2.2 GHz and the dielectric constant of water is 76 in the equation (5), the lateral width of the hole is 7.8 mm.
become.

Regarding the depth of the hole, if it is too shallow, the microwave will always pass through, and it will not be a filter. Theoretically, the width of the hole divided by 3.14 is a measure to push back the microwave. However, sludge is highly viscous,
Since the holes may be clogged, the depth of the holes should be as shallow as possible. Therefore, the lower limit of the design is set to 1/4 of the width. In the above example, the depth of the hole is 1.9 mm.

The depth of the hole> = the width of the hole / 4/4 (6) However, the value obtained by the equation (5) does not significantly affect the characteristics if it is within about 10% of the value.

Finally, the protective film 21 will be described. The purpose of the protective film 21 is to protect the high-frequency window 14, improve the coefficient of sliding friction of the through-hole 16, prevent water leakage, and the like. A material that transmits microwaves and is sufficiently thin with respect to the wavelength is used. The thickness is desirably about 0.01 wavelength or less. The calculation formula is given by the following formula (7).

Film thickness <= 0.01 × light speed / (square root of ε _s of protective film × resonance frequency of high frequency window) (7) where ε _s is a relative permittivity.

If Teflon is used at a frequency of 2.2 GHz, since the relative dielectric constant is about 2, the thickness of the protective film is 1 m.
m or less.

Even when the through holes 16 are formed as in the fourth embodiment, the characteristics are as shown in FIG. 4, and the matching with the sludge is greatly improved.

FIG. 8 is a characteristic diagram of an experimental result showing the measurement result of the microwave reflection intensity when the solid concentration changes in the fourth embodiment of the present invention. This indicates that the sensitivity differs. In this case, the peak intensity increases as the concentration increases.

FIG. 9 is a characteristic diagram of an experimental result showing a measurement result of the microwave reflection intensity when the conductivity changes in the fourth embodiment of the present invention. This indicates that there is sensitivity. In this case, the peak frequency increases as the conductivity increases.

With such a configuration as in the fourth embodiment, it becomes possible to obtain information on the solid concentration and the electrical conductivity from the reflection characteristics.

[0065]

As described above, according to the present invention,
In addition to the advantages of easy maintenance such as maintenance and inspection, and low cost, it can be improved without major structural changes in the reflection intensity detection type microwave type sludge densitometer. The second with a through hole
Since the conductor is provided at the contact portion with the sewage, there is an advantage that the electromagnetic boundary between the first conductor and the window can be stabilized and the measurement sensitivity can be improved. In addition, the use of a dielectric resonator can greatly reduce the size and simplify the configuration. Further, since the resolution of the sludge concentration can be controlled by the size of the through-hole, the resolution can be changed according to the use and the use environment. In addition to this, it is possible to improve the matching with the sludge, and it is possible to obtain information on the concentration and the conductivity from the change in the reflection characteristic due to the change in the property of the sludge.

[Brief description of the drawings]

FIG. 1 is an enlarged sectional view of a main part of a probe part according to a first embodiment of the present invention.

FIG. 2 is an enlarged sectional view of a main part showing a second embodiment of the present invention.

FIG. 3 is an enlarged sectional view of a main part of a probe part according to a third embodiment of the present invention.

FIG. 4 is a characteristic diagram of an experimental result showing a measurement result of the reflection intensity according to each embodiment and a measurement result of the conventional reflection intensity.

FIG. 5 is an enlarged sectional view of a main part of a probe part according to a fourth embodiment of the present invention.

FIG. 6 is a front view of the high-frequency window.

FIG. 7 is a front view of a second conductor.

FIG. 8 is a graph showing the reflection characteristics of microwaves when the solid concentration changes.

FIG. 9 is a graph showing the reflection characteristics of microwaves when the conductivity changes.

FIG. 10 is a schematic configuration explanatory view of a conventional transmission-type phase difference detection type microwave-type sludge concentration meter.

FIG. 11 is a phase difference-concentration characteristic diagram of a conventional transmission type phase difference detection type microwave type sludge densitometer.

FIG. 12 is a schematic diagram illustrating the configuration of a conventional reflection-type microwave-type sludge densitometer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Probe main body 12 ... Sludge transport pipe 13 ... Window part 14 ... High frequency window of a ceramic member 15 ... First conductor part 16 ... Through-hole 17 ... Second conductor part 18 ... Antenna 19 ... Connector 20 ... Hollow part 21 ... Protective film 31 ... Dielectric resonator

Claims (9)

[Claims] 1. A sludge transport pipe is provided with a reflection intensity detection type microwave detection probe main body, and a microwave is transmitted from the probe main body into the sludge transport pipe, and a sludge mixture liquid as a sample to be measured in the sludge transport pipe is provided. In the sludge concentration meter which receives the microwave reflected on the boundary surface by the same probe main body, detects the ratio of the reception intensity to the transmission intensity, and measures the solid concentration or the suspended solid concentration in the sludge mixed solution, A sludge densitometer characterized in that a conductor portion provided with a portion for guiding a sample to be measured is formed at an open end. 2. The probe body has one end closed,
2. The sludge densitometer according to claim 1, wherein the sludge densitometer comprises a waveguide having an open end. 3. A sludge transport pipe is provided with a reflection intensity detection type microwave detection probe main body, and a microwave is transmitted from the probe main body into the sludge transport pipe, and a sludge mixture liquid as a sample to be measured in the sludge transport pipe is provided. In the sludge concentration meter which receives the microwave reflected on the boundary surface with the same probe main body, detects the ratio of the reception intensity to the transmission intensity, and measures the solid concentration or the suspended solid concentration in the sludge mixture, the probe main body is A first conductor portion having a window which is formed of a waveguide whose one end is closed and whose other end is opened, and which has a window through which microwaves can pass, is provided in the opening at the other end of the waveguide. A sludge densitometer, wherein a second conductor portion provided with a portion for guiding a sample to be measured is provided in the window in close contact with the first conductor portion. 4. The sludge densitometer according to claim 2, wherein a dielectric resonator is filled in the probe main body composed of a waveguide. 5. The sludge densitometer according to claim 3, wherein a protective film is provided between the first conductor and the second conductor. 6. The member through which the microwave can be transmitted has the following thicknesses, widths, and heights of the member:
6. The sludge concentration meter according to claim 1, wherein the sludge concentration meter comprises values obtained from the expressions (b) and (c) and includes values within 10% of the values obtained from these expressions. Thickness = light speed / (square root of ε _s of member × resonance frequency × 4) (a) Width = light speed / (square root of ε _s of member × cutoff frequency × 2) (b) Vertical width = width / 2… ( c) where ε _s is the relative dielectric constant. 7. The shape of the part for guiding the sample to be measured is such that the width of the part is determined by the following equation (e) and includes a value within 10% of the value obtained from this equation. 6. The sludge concentration meter according to claim 1, wherein the vertical width of the portion satisfies the following expression (d). The vertical width of the groove> the vertical width of the member through which microwaves can be transmitted ... (d) The horizontal width of the portion = light speed / (square root of sewage ε × resonance frequency of member × 2) (e) where ε is the dielectric constant It is. 8. The sludge concentration meter according to claim 1, wherein the depth of the portion for guiding the sample to be measured has a value satisfying the following expression (f). Depth of site> = width of site / 4/4 (f) 9. The protective film has the following thickness (g):
The sludge concentration meter according to claim 5, wherein the sludge concentration meter comprises a value satisfying the expression. Film thickness <= 0.01 × light speed / (square root of ε _s of protective film × resonance frequency of high frequency window) (g)