JPS61274223A - Level detection of liquid reservoir by supersonic wave - Google Patents

Abstract

PURPOSE:To detect a level height of liquid unaffected by dimensions and construction of a liquid reservoir, condition in the reservoir, classification and characteristic of the liquid, by introducing supersonic wave from outside surface of the reservoir wall and detecting the liquid level inside the reservoir by comparing damping rates of the reflected wave. CONSTITUTION:Regarding a sound pressure reflecting rate of a supersonic wave, when the supersonic wave (Longitudinal wave) is projected vertically onto the border surface 5 of reservoir wall S and air A, the sound pressure reflecting rate RA reaches 99.9981%. This represents approximate total reflection on this surface 5. On the other hand, the sound pressure reflecting rate RW for vertical irradiation of the supersonic wave (Longitudinal wave) onto the border surface 6 of reservoir wall S and water W reaches 93.7684%. As the sound pressure is proportional to the height of echo appearing on CRT, when values of RA and RW are compared, the comparison value h becomes 0.559dB. When the echo heights of multiple recting waves are compared, every reflection accumulates the difference of echo heights by 0.559dB and thus, detection of the water level 2 becomes available.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method of detecting a liquid level in a liquid tank from the outside of the liquid tank using ultrasonic waves.

A liquid tank here refers to a container in which liquid (water, oil, fuel, paint, acids, etc.) is stored, such as a fuel tank of an automobile or aircraft, or a transformer in which a fixed amount of insulating oil is poured. , storage tanks for various liquids in chemical plants, surge tanks for hydroelectric power plants, etc., and there are no differences in the type and properties of the liquid, the size and shape of the container, and the thickness of the container wall.

The container may be made of any material that can transmit ultrasonic waves, including metals and non-metals (glass, ceramic, concrete, synthetic resin, etc.).

Moreover, from the outside of the liquid tank means from the outer surface of the tank wall that is in direct contact with the liquid of the container forming the liquid tank.

[Background of the invention]

Measuring the liquid level in a liquid tank involves measuring the distance from a reference horizontal plane to the liquid surface, but for this purpose, detection of the liquid level is a prerequisite. There are many conventional methods for measuring the liquid level, depending on the dimensions and structure of the liquid tank, the pressure and temperature inside the liquid tank, the amount and speed of fluctuation in the liquid level, the measurement accuracy of the liquid level, etc. The measurement method is (1
) Methods based on measuring displacement and length, (2) Methods based on measuring pressure and differential pressure, and (3) Methods based on measuring other physical quantities associated with changes in liquid level (Japan Society of Mechanical Engineers, published in 1968. (Refer to "Mechanical Engineering Handbook" 5th edition 6-31 to 32), but there are also liquid level detection methods corresponding to each of these methods.

In the above method H, first, a conical point called a hook r-ji or a point r-zo, which has a conical shape so that contact with the liquid surface can be clearly recognized, is attached to a metallic prism with a scale or a metallic prism. Lower the tape attached to the tip of the kuiyarode vertically from a fixed point above the liquid level, read the scale when the tip of the 4-inch contacts the liquid level, and directly measure the distance to the liquid level. There is a method. Next, a float is floated on the liquid level, and the liquid level is detected from the value converted into a mechanical or electrical quantity of the amount of vertical displacement of the float due to changes in the liquid level, and the amount of change in the liquid level is detected. In addition, by using the fact that the buoyant force of an object placed in a liquid is equal to the weight of the liquid displaced by that object, the buoyant force of an object placed partially in the liquid was converted into a mechanical quantity or an electrical quantity. There are methods such as detecting the liquid level from the value and determining changes in the liquid level.

These methods of detecting the liquid level have the advantage of a simple detection mechanism, but they do not require the installation of measuring instruments within the liquid tank, such as by bringing a point into contact with the liquid surface, or by placing a float or object on the liquid surface or in the liquid. Therefore, in order to detect the liquid level in a sealed liquid tank, such as a column-mounted transformer, the liquid tank must be disassembled again. If the pressure or temperature of the liquid is particularly high or low, or if the size of the liquid tank is very large, the measurement scale will fluctuate greatly, making measurement difficult and making it impossible to respond in real time. There are some defects. Next, for method (2) above, first, the hydrostatic pressure at each point in the liquid is proportional to the height from that point to the liquid level, and either a normal pressure gauge or a tank is used. There is a method of measuring hydrostatic pressure by mechanically or electrically detecting the deformation caused by the pressure of a diaphragm attached directly to the wall, and detecting the liquid level to determine the liquid level height in the tank. In addition, in high-pressure tanks, etc., where the pressure inside the tank fluctuates, the pressure inside the liquid in the tank also fluctuates, so the pressure in the gas phase is detected and the difference between the pressure at the bottom of the liquid and the pressure at the bottom of the liquid is measured using a differential pressure gauge. In addition, compressed air is ejected from the tip of a thin tube inserted into the liquid, and the flow rate of the air to be ejected and the pressure inside the thin tube are determined by measuring the liquid level. There is a method using air parts that detects the liquid level from the back pressure of a thin tube when it is made almost equal to the hydrostatic pressure, and calculates the liquid level height. Detection of the liquid level in method (2) has the advantage of being easy because it is done by visually observing the scale display of the pressure gauge or differential pressure gauge, but it is difficult to detect the liquid level due to fluctuations in pressure, temperature, liquid level, etc. in the liquid tank. However, accurate tracking is difficult and measurement accuracy is poor, and if the liquid tank is large and has a complicated structure, the liquid level detection device becomes large. Next, in the method (3) above,
First, there is a method using a radiation liquid level meter, which takes advantage of the fact that the absorption of radiation, particularly the r-rays of COR-60, is significantly different between liquids and gases, and there are approximately three types of methods.

In type 1, a radiation source is installed on a float floating on the liquid surface, and a detector is installed at the top of the liquid tank facing the radiation source.
It is a 70-meter type detector that detects changes in the intensity of radiation reaching the detector from changes in the distance between the two as the liquid level changes, and detects the liquid level from the amount of change. Type 2 is a transmission type in which a radiation source and a detector are fixed to the tank wall facing each other, and the liquid level is detected from the amount of change in the intensity of the transmitted radiation between them, which changes depending on the thickness of the liquid. In Type 3, the radiation source and detector are placed on the tank wall so that they can slide up and down, facing each other, and a serge mechanism is installed so that the mutual positional relationship of the radiation source, detector, and liquid level is always the same. The liquid level is detected from the position of the gravimeter. The methods of Types 1 to 3 above are relatively less affected by fluctuations in pressure, temperature, liquid level, etc. in the liquid tank, but they all require a radiation source and a detector, and they must be arranged in advance to face each other. There must be. For this reason, it is not possible to detect the liquid level in real time for the desired liquid tank, and it is difficult to install them facing each other when the liquid tank is large in size or has a complicated tank wall structure. And it's annoying. Another problem is that since radiation is involved, safety concerns that do not exist with other methods remain. Next, a method classified as method (3) above is a capacitive liquid level meter that utilizes the property that the capacitance between opposing electrodes is a function of the dielectric constant of the medium between the electrodes. . For example, the dielectric constants of gas and liquid differ by a factor of several to several tens of times.

If the electrodes that are placed facing each other in the liquid tank are structured so that the amount of liquid interposed between the electrodes differs depending on the change in the height of the liquid due to changes in the liquid level, the height of the liquid level can be changed. It can be converted to the size of capacitance, and the liquid level can be detected from the size of capacitance. If the liquid is conductive, there is a method in which the liquid is used as one electrode and the area of the opposing electrode changes as the liquid level changes. However, Jin's method also requires electrodes to be placed in the liquid tank in advance, and has the same problems as the radiation liquid level meter, as well as the measurement accuracy being affected by the placement, shape, and dimensions of the electrodes. It has a defect that is greatly affected.

On the other hand, there is a method using ultrasonic waves that falls under the method (3) above. This is called an ultrasonic liquid level gauge, and it takes advantage of the sharp directivity of ultrasonic waves and the property of ultrasonic waves being reflected at boundaries between media. Specifically, as part 1, both transmitting and receiving ultrasonic probes are installed on the bottom of the liquid tank, and the ultrasonic waves emitted toward the liquid surface are reflected by the liquid surface, and the reflected waves are generated. There is a method of measuring the propagation time in a liquid until it is received.
There is a method of installing both receiving ultrasonic probes above the liquid level in a liquid tank and measuring the propagation time in the gas until the ultrasonic waves emitted towards the liquid surface are received. . A specific example of this is described on pages 749 to 751 of the newly revised edition of "Ultrasonic Technology Handbook" published by Nikkan Kogyo Shimbun in 1978. That is, the first specific example is an example in which the water level of a surge tank of a hydroelectric power plant is recorded and monitored, where the water level fluctuates greatly due to startup of a generator, sudden load cutoff, and the like. The above-mentioned method for measuring the propagation time of ultrasonic waves in liquid has been reported as an underwater method, and the method for measuring the propagation time in gas has been reported as an aerial method. An underwater type is suitable for this purpose, and an aerial type is considered to be suitable for something that is a little more convenient.

Specific example 2 is an example in which the liquid level of a fuel tank or a chemical tank is measured, and the underwater type is used. Specific example 3 is an example of a liquid level alarm, in which a probe is installed at a predetermined liquid level in order to know whether the fuel in the aircraft fuel tank is at a predetermined liquid level. In this example, the relay operation is determined depending on whether or not the probe is immersed in the fuel, and the liquid level is detected. As described in the above-mentioned literature, the above-mentioned conventional ultrasonic liquid level gauge can measure liquid level fluctuations over a (+) wide range with high precision. (11) Easy to follow rapid liquid level fluctuations. (
II+) Remote indication of liquid level, continuous recording, alarm, decimal display, etc. are relatively easy. It has the following advantages, and although it is somewhat expensive, it is becoming popular. It is introduced as but,
In order to detect the liquid level using the conventional ultrasonic liquid level gauge described above, an ultrasonic probe must be installed in the liquid tank in advance, similar to the other liquid level gauges described above. However, the other liquid level meters have problems that occur because the measuring device is installed inside the liquid tank, such as not being able to detect the liquid level in a liquid tank with a closed structure such as a transformer in real time. In addition, it is greatly affected by the properties of the liquid. In other words, when the concentration of a liquid is high, such as muddy water, paint, dye, etc., the scattering attenuation is large when ultrasonic waves pass through the liquid, and the degree of attenuation varies depending on the concentration. cannot be measured well, and the propagation velocity of ultrasonic waves is
Since it changes significantly depending on temperature, the measured value of the propagation time of the ultrasonic wave in a liquid cannot be used until -1 and must be corrected according to each temperature, so that a decrease in measurement accuracy can be avoided. do not have. There are major problems such as: Furthermore, if the water level in the liquid tank is particularly high, there are cases where reflected waves from the liquid surface cannot be obtained, and in such cases, there are many problems such as the inability to detect the liquid level.

As explained above, none of the conventional methods for detecting the liquid level in a liquid tank can detect the liquid level easily, in an extremely short period of time, with high accuracy, and in real time. A method has been awaited.

[Purpose of the invention]

The present invention solves the problems of the prior art as described above, and detects the liquid level in a liquid tank based on the dimensions and structure of the liquid tank, the conditions inside the liquid tank (pressure, temperature, amount and speed of liquid level fluctuation, etc.). ), type of liquid (water, oil, fuel, paint.

Ultrasonic technology enables highly accurate detection in an extremely short time, easily and in real time, without being affected by the properties (concentration, temperature, density, viscosity, etc.) The purpose of the present invention is to provide a method for detecting a liquid level in a liquid tank.

[Summary of the invention]

The present invention involves injecting ultrasonic waves into the liquid tank from the outer surface of the tank wall of the liquid tank through the tank wall, and detecting the interface between the liquid in the liquid tank and the tank wall, and the interface between the gas and the tank wall. By comparing the degree of attenuation of multiple reflected waves reflected from the interface between the This method allows highly accurate detection to be easily performed in a very short time and in real time, without being affected by the type and properties of the liquid.

[Embodiments of the invention]

Embodiments of the present invention will be described with reference to FIGS. 1 to 3. This example is a method for detecting the liquid level when water at 20° C. is stored in a water tank made of a steel plate.

FIG. 1 is an explanatory diagram showing a method of detecting a liquid level.

1 is an aquarium, made of a steel plate with a thickness of 3.2 mm (material: 5S41-
This is a container consisting of a bottom and a tank wall S made of JIS G 3101).

Water W is stored in a water tank 1, and 2 is the water surface. 3 is the outer surface of the tank wall S, and a vertical probe 4 is in contact with the outer surface 3 so as to be able to scan vertically. 5 is an interface between the tank wall S and air A, and 6 is an interface between the tank wall S and water W. Vertical probe 4
is connected by a high-frequency cable to an f-lux reflection type ultrasonic flaw detector (hereinafter simply referred to as an ultrasonic flaw detector) 7 displayed on the A scope display.

8 is the CRT.

Now looking at the sound pressure reflectance of ultrasonic waves, when an ultrasonic wave (longitudinal wave) is perpendicularly incident on the interface 5 between the tank wall S and the air A, the sound pressure reflectance RA at that time is expressed by the following formula. shown.

Here, zg=ρ8・Cs...
・・・・・・・・・・・・(2) zA=ρe・C,...
・・・・・・・・・・・・(3), z8=acoustic impedance of tank wall S, 4=acoustic impedance of air A,
ρ3=density of tank wall S, ρe=density of air, C,=sound velocity of longitudinal wave in tank wall S, CA=sound velocity of air A. Each of these values is known, ρ3=7, 8 f/Ctll,
CB = 5900 rrV/s, ρA = 0.00
13 f/C1/l, Ca=344 min, formula f2), (zS rzA calculated by substituting into 31)
Substituting into equation (1), RA = 99.9981%
becomes.

As shown in FIG. 2, this indicates that almost total reflection occurs at the boundary surface 5, but only 0.0019% is propagated into the air A as a passing wave 9. On the other hand, when an ultrasonic wave (longitudinal wave) is perpendicularly incident on the interface 6 between the tank wall S and the water W, the sound pressure reflectance R, is expressed as, and the acoustic impedance zw of the water W is 2, = ρ7・Cw・
・・・・・・・・・・・・・・・(5) shows. Here, ρ=density of water W, Cw==sound velocity of water W. Known ρ, = 1. O5F76yf, Cw=14
Substituting the value of 85rn/s into equation (5) to find zW, and further calculating using equation (4), R, == 93.7684
It becomes 1. This indicates that, as shown in FIG. 3, 93.7684% of the light is reflected at the boundary surface 6, and the remaining 6.2316% becomes the passing wave 10 and propagates into the water W. Next, since the sound pressure is proportional to the echo height appearing on the CRT, the sound pressure reflectance R calculated using equations (1) and (4)
When the values of 4 and R are compared, the comparison value h and the difference in echo height expressed in side decipels can be determined by the following formula.

In formula (6), RA=99.9981%, Rw: 93
．． When calculated by substituting 7684%, the difference in echo height expressed in decibels h=0.559 (dB). This means that when the vertical probe 4 is scanned up and down the outer surface 3 of the tank wall S, the difference in echo height of the first reflected waves from the boundary surfaces 5 and 6 is only 0.559 dB. This shows that this is within the error range of normal measurement, and the water surface 2 cannot be detected. However, the above value is only for the first reflected wave, and if we compare the echo heights of multiple reflected waves that appear many echoes of reflected waves from interfaces 5 and 6, we can see that the number of reflections is 1. With each increase, the difference in echo height is accumulated by 0.559 dB, and the difference expands.

Then, in the final n-th echo that appears on CRT a, the difference in echo height is sufficiently expanded, and it is easy to determine whether the echo is the multiple reflected wave from the boundary surface 5 or the echo from the boundary surface 6. The water surface 2 can be detected from the position where the echo heights of the multiple reflected waves from the image boundary surfaces 5 and 6 switch on the CRTa. As can be seen from the above description, the greater the number of echoes of the reflected waves from the boundary surfaces 5 and 6, the greater the difference in echo height, which makes detection of the water surface 2 easier. Another method for detecting the water surface 2 is to make the echoes of the reflected waves from the interfaces 5 and 6 appear on the CRTs, so that the echo height of the reflected waves is 2 for both.
There is a method of scanning the vertical probe 4 up and down. In this case, the water surface 2 is located at the echo height 2 of the image boundary surface.

In the method of the present invention, it is necessary to obtain multiple reflected waves reflected from the interface between the tank wall and the liquid and gas in the liquid tank.
It is desirable to obtain the multiple reflected waves in a state where they are resolved as clearly as possible. For this reason, it is desirable that the tank wall has a certain thickness, for example, a thickness of about 5 mm or more in the case of a steel plate. This is because even a normal commercially available vertical probe can clearly obtain multiple reflected waves, making it easy to detect the liquid level in real time. However, even if the tank wall is made of steel plate, the thickness of the plate may be as thin as 2.3m to 3.2cm, such as in a small transformer. It may be difficult to clearly resolve multiple reflected waves with commercially available vertical probes. However, in such cases, if a vertical probe with high frequency and high resolution is used, multiple reflected waves can be clearly obtained. As in the case, the liquid level can be easily detected.

Next, an example in which the method of the present invention is applied to an actual product will be explained with reference to FIGS. 4 to 7.

The target product is a transformer, which is housed in a case made of steel plate (material: 5S41-JISG 3101) with a thickness of 3.2 m, and has a density of 0.95 t/cr/l and a sound velocity of 1557.9 m.
It is filled with insulating oil having characteristics of m1m and acoustic impedance of 0.148 y/s-1. The liquid level of the sealed insulating oil must be inspected not only at the time of sealing, but also for natural consumption due to evaporation, leakage, etc. The inspection in this example is based on the method shown in Figures 1 to 3 above, in which a high-resolution vertical probe with a frequency of 10 MHz is brought into contact with the outer surface of the transformer case, and the probe is scanned up and down while moving the case. The echoes of multiple reflected waves reflected from the interface between the insulating oil and the air were made to appear on the CRT of an ultrasonic flaw detector, and the echoes were observed. Figure 4 shows the echoes of the multiple reflected waves reflected from the interface between the transformer case and the air displayed on CBr4, and Figure 5 shows the multiple reflected waves reflected from the interface between the case and the insulating oil. This shows the echo of the image displayed on CRTs. In both figures, the horizontal axis is the beam path, which is 100 W at the sound speed of longitudinal waves in steel.
m scale, and the vertical axis is the multi-reflected wave.
The echo height is displayed in cents (4), and the sensitivity is set to 100% of the echo height of the first reflected wave.

In both figures, on a scale of 100 wm, the echoes of up to the 31st reflected wave are resolved and appear, and if the echo heights of these multiple reflected waves are connected with dotted lines, the envelope 11 in Figure 4 can be seen. However, in FIG. 5, envelopes 12 are obtained. Comparing the graphs in Figures 4 and 5, we find that the echo height decreases at the case-insulating oil interface in Figure 5 compared to the case-air interface in Figure 4. It can be clearly observed that the degree of attenuation of multiple reflected waves is rapid. Specifically, when comparing the degree of attenuation of the sixth echo height, it is approximately 45% in FIG. 4, while it rapidly decreases to approximately 25% in FIG. This is due to the above-described difference in sound pressure reflectance, and the difference can be clearly observed by comparing the slopes of the envelopes shown by dotted lines. As can be seen from this observation, the liquid level is located at the position where the echo height of the multiple reflected waves changes from Figure 4 to Figure 5 or vice versa, that is, the multiple reflected waves in both Figures. This is the position where the echo height of the middle waveform is obtained. 6th graph
As shown in the figure. Both the horizontal and vertical axes are the same as in FIGS. 4 and 5. Also, the envelope is 13. C from Figure 4 to Figure 6
It can be seen that the liquid level can be easily detected by comparing the echo height on RTg and the appearance/gutter of the envelope.

Next, FIG. 7 is obtained by summarizing the relationship between the echo height of the multiple reflected waves and the beam path obtained in FIGS. 4 and 5. The horizontal axis of the figure is the beam path length X (unit: W), and the vertical axis is the echo height h (unit: dB) when the first echo height is the reference sensitivity O. In the figure, the mark is a plot of the value at the interface between the transformer case and the air shown in Figure 4, and the O mark is the value at the interface between the transformer case and the insulating oil shown in Figure 5. The values are plotted. Both the * and O marks on the graph indicate that a linear correlation exists between the beam path length
In the case of a mark at the interface between the transformer case and the air,
h shown by solid line = -0,43x + 1.38
・・・・・・・・・・・・・・・・・・・・・(7) In the case of O mark at the interface between the case and the insulating oil, h = −0,73x shown by the dotted line. +2.34...
・・・・・・・・・・・・・・・・・・(8) Comparing equations (7) and (8), we find that equation (8)
It can be seen that the degree of decrease in the echo height h is approximately 1.7 times. Specifically, when comparing the 31st value, the value marked with . is -41 dB, while the value at the interface between the case marked O and the insulating oil is -69 dB, and the difference is 28 dB.
It can be seen that the liquid level can be easily detected with high accuracy. In this example, we explained the case where the liquid level is detected by scanning vertically up and down with a vertical probe that is in contact with the outer surface of the transformer case, but it is necessary to always maintain a predetermined liquid level. Liquid tanks, liquid tanks where it is necessary to know when the liquid level is above or below a predetermined level, liquid tanks where it is necessary to know a wide range of liquid level fluctuations, especially large liquid tanks, etc., where remote indication or continuous recording is required. For liquid tanks, etc., it is possible to accurately and easily measure the liquid level by fixing one or more vertical probes in a predetermined position and contacting the outer surface of the tank wall of the liquid tank. can be detected and responded to.

Since the method of the present invention is a method in which the vertical probe is brought into contact with the outer surface of the tank wall, it also has the advantage that the position of contact can be arbitrarily changed as required. By calculating the regression equation for each type of liquid tank to be tested, it is possible to detect the liquid level very simply and easily, and with high precision, as in the above-described embodiment.

The liquid level detection method explained in this example is a visual detection method by displaying echoes of multiple reflected waves on a CRT. The analog quantity is digitized by commonly used means and stored in a storage device, and the liquid level position is calculated from the correlation between the degree of attenuation of multiple reflected waves and the sound pressure reflectance of air and liquid. It is also possible to display the position numerically.

〔Effect of the invention〕

As explained above, the present invention allows ultrasonic waves to be incident into a liquid tank from the outer surface of the tank wall of a liquid tank through the tank wall, and to detect the interface between the liquid in the liquid tank and the tank wall of the ultrasonic wave, Comparing the degree of attenuation of multiple reflected waves reflected from the interface between the gas in the liquid tank and the tank wall,
Since this is a method of detecting the liquid level in the liquid tank using the comparison value as an evaluation index, it is possible to
It is possible to easily and accurately detect the liquid level in a very short time without being affected by the type and properties of the liquid, and moreover, it has a remarkable practical effect in that it can be carried out in real time.

BRIEF DESCRIPTION OF THE DRAWINGS The figures are all explanatory diagrams of the present invention. Figures 1 to 3 are detailed explanatory diagrams of the present invention, Figure 1 is an explanatory diagram showing the liquid level detection method in this embodiment, and Figure 2 is a diagram showing the interface between the tank wall (made of steel plate) and air. FIG. 3 is an explanatory diagram of the sound pressure reflectance at the interface between the tank wall (made of steel plate) and water. Figures 4 to 7 are explanatory diagrams of examples in which the present invention is applied to an actual product transformer. Figure 4 shows a CRT image of multiple reflected waves at the interface between the transformer case (made of steel plate) and air. Figure 5 is a diagram showing the echo pattern of multiple reflected waves on a CRT at the interface between the transformer case and insulating oil, and Figure 6 is a diagram showing the echo pattern of multiple reflected waves on the CRT at the interface between the transformer case and the insulating oil.
FIG. 7 is an explanatory diagram showing the relationship between the beam path length and the echo height in FIGS. 4 and 5. 1...Aquarium, 2...Water surface, 3...Outer surface of tank wall, 4
... Vertical probe, 5... Interface between tank wall and air, 6.
・・Interface between tank wall and water, 8...CRT, 9.10・
... Passing wave, 11°12, 13... Envelope, A...
Air, W...water, S...tank wall.

Claims (1)

[Claims] 1. Ultrasonic waves are introduced into the liquid tank from the outer surface of the tank wall through the tank wall, and the ultrasonic waves are transmitted to the interface between the liquid in the liquid tank and the tank wall, and the liquid A method of detecting the liquid level in a liquid tank by comparing the degree of attenuation of multiple reflected waves reflected from the interface between the gas in the tank and the tank wall, and using the comparison value as an evaluation index. 2. Claims characterized in that a vertical probe is brought into contact with the outer surface of the tank wall of the liquid tank, and the ultrasonic waves are made to enter the liquid tank so as to be incident almost perpendicularly to the tank wall. 2. The method for detecting a liquid level in a liquid tank according to item 1. 3. The degree of attenuation of the echo of multiple reflected waves is shown in C on the A scope display.
The method for detecting a liquid level in a liquid tank according to claim 1, characterized in that the attenuation degree of the displayed echoes is compared and the comparison value is used as an evaluation index. 4. While scanning the vertical probe up and down along the tank wall,
A method for detecting a liquid level in a liquid tank according to claim 2, characterized in that an ultrasonic wave is made incident into the liquid tank.