JP2002090208A - Liquid level detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid level detector capable of detecting the liquid level at a high accuracy and easy to set the mounting position or angle of an ultrasonic probe. SOLUTION: The level detector comprises an ultrasonic probe 31 and an operational indicator 27. The probe 31 is mounted outside a bottom wall 29a of a tank 29 to radiate ultrasonic waves vertically toward a liquid surface 28a and receive ultrasonic waves reflected from this surface 28a. The operational indicator 27 calculates the liquid level, based on the time taken for receiving the ultrasonic wave reflected from the liquid surface 28a after the probe 31 radiates the ultrasonic waves, and the propagation velocity of the ultrasonic wave in a liquid stored in the tank 29.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting the liquid level of a liquid such as liquefied petroleum gas (LPG) stored in a liquid storage device such as a high-pressure gas container or a liquid high-pressure gas storage tank. The present invention relates to a liquid level detection device.

[0002]

2. Description of the Related Art A float type liquid level detecting device shown in FIG. 9 is conventionally known. This liquid level detecting device includes a float 3 fixed to one end of a rod 2 connected to a rotating gear 1. When the float 3 moves up and down as the liquid level 5a of the liquid 5 stored in the storage tank 4 rises and falls, the rotary gear 1 rotates, and this rotation angle is converted into a liquid level L and displayed on the display 6. Is done.

However, in this liquid level detector, the relationship between the liquid level L and the rotation angle of the rotary gear 1 is non-linear, and it is difficult to calculate the liquid level L accurately.
Further, the float 3 may be caught on the inner wall surface of the storage tank 4 and the rod 2 may not be able to rotate. In this case, it is necessary to open the inside of the storage tank 4 for repair. This repair work is very cumbersome and requires large repair costs.

On the other hand, in recent years, a liquid level detecting device using an ultrasonic probe has been proposed. For example,
Japanese Patent Publication No. 3036172 discloses a liquid level detecting device as shown in FIG.

In this liquid level detecting device, a first ultrasonic probe 11 for detecting the liquid level is provided on a side peripheral portion of the high-pressure gas container 10 and a second ultrasonic probe 11 for measuring the propagation speed of the ultrasonic wave in the liquid 5. Two ultrasonic probes 12 are attached.

The first ultrasonic probe 11 has an attachment angle set so as to transmit ultrasonic waves obliquely upward, and the intersection between the liquid surface 5a and the inner peripheral surface of the side peripheral portion of the high-pressure gas container 10. The ultrasonic wave reflected at the position P1 is received. The liquid surface level L is calculated based on the time from when the first ultrasonic probe 11 transmits an ultrasonic wave to when the reflected ultrasonic wave is received and the propagation speed of the ultrasonic wave in the liquid 5.

On the other hand, the second ultrasonic probe 12
The mounting angle is set so as to transmit ultrasonic waves in the horizontal direction. Since the distance from the second ultrasonic probe 12 to the horizontal facing position P2 is known, the distance is set to the second position.
Is divided by the time from the transmission of the ultrasonic wave by the ultrasonic probe 12 to the reception thereof, whereby the propagation speed of the ultrasonic wave in the liquid 5 can be obtained. The propagation speed of the ultrasonic wave in the liquid depends on the temperature of the liquid, but since the propagation speed is actually measured by the second ultrasonic probe 12, the propagation of the ultrasonic wave corresponding to the temperature of the liquid at the time of detecting the liquid level is performed. Speed is obtained.

[0008]

However, the liquid level detecting device shown in FIG. 10 has the following problems. First, the first ultrasonic probe 11 is connected to the high-pressure gas container 10.
Of the high-pressure gas container 1
If the position is near the bottom of 0 and lower than the mounting position of the ultrasonic probe 11, the liquid level cannot be detected. The first ultrasonic probe 11 is mounted obliquely with respect to the side periphery of the high-pressure gas container 10, but the side periphery of the high-pressure gas container or the storage tank for liquid high-pressure gas is generally curved. It is difficult to accurately mount the ultrasonic probe 11 at a desired mounting angle. Further, a second ultrasonic probe 12 for detecting a propagation velocity is provided separately from the first ultrasonic probe 11 for detecting a liquid level, and a total of two ultrasonic probes are required. The structure is complicated.

The present invention has been made to solve the above-mentioned problems in the conventional liquid level detecting device. That is, an object of the present invention is to provide a liquid level detecting device which can detect the liquid level with high accuracy and can easily set the mounting position and the mounting angle of the ultrasonic probe.

[0010]

SUMMARY OF THE INVENTION Accordingly, the present invention relates to a liquid level detecting device for detecting the liquid level of a liquid stored in a liquid reservoir, wherein the liquid level detecting device detects the liquid level of the liquid stored in the liquid reservoir. And an ultrasonic probe that transmits ultrasonic waves in the vertical direction toward the liquid surface and receives the ultrasonic waves reflected by the liquid surface. A liquid level detecting device including a calculator for calculating a liquid level based on a time until the reflected ultrasonic wave is received and a propagation speed of the ultrasonic wave in the liquid stored in the liquid reservoir. Is provided.

In the liquid level detecting device of the present invention, the ultrasonic probe is fixed not to the side of the liquid reservoir but to the outside of the bottom wall, and emits ultrasonic waves in the vertical direction toward the liquid surface. Therefore, even when the liquid level is near the bottom of the liquid reservoir, that is, when the liquid level is low, the liquid level can be reliably detected. Further, it is easy to set the mounting position and the mounting angle of the ultrasonic probe with respect to the liquid reservoir.

[0012] It is preferable that a temperature sensor attached to the outside of the wall portion of the liquid reservoir is provided, and the computing unit corrects the propagation speed of the ultrasonic wave in the liquid based on a measurement value of the temperature sensor. .

Specifically, the arithmetic unit stores the relationship between the temperature of the liquid and the propagation speed of the ultrasonic wave in the liquid, and calculates the temperature of the liquid estimated from the measurement value of the temperature sensor, It is preferable to determine the propagation speed of the ultrasonic wave in the liquid from the relationship with the propagation speed.

In this case, since the arithmetic unit corrects the propagation speed of the ultrasonic wave based on the actual measurement value of the temperature sensor, the propagation speed of the ultrasonic wave can be corrected with high accuracy, and the liquid level can be accurately detected. .

[0015] It is preferable that the ultrasonic probe and the temperature sensor are incorporated in a single mounting element, and the mounting element is mounted on the outside of the bottom wall of the liquid reservoir.

In this case, since the ultrasonic probe and the temperature sensor are one unit, the structure is simplified. Also,
The ultrasonic probe and the temperature sensor can be attached to the liquid reservoir at the same time.

When the liquid stored in the liquid storage is a mixed liquid of a plurality of liquids, the arithmetic unit determines the propagation speed of the ultrasonic wave in the mixed liquid based on the mixing ratio of each liquid. It is preferable to correct.

Specifically, the arithmetic unit may calculate the propagation speed of the ultrasonic wave in the mixed liquid composed of n kinds of liquids stored in the liquid storage based on the following equation. preferable.

[0019]

(Equation 2) (K = 1, 2,..., N−1, n) V _t : Propagation velocity of ultrasonic wave in the mixed liquid V _k : Propagation velocity of ultrasonic wave in each liquid r _k : Ratio of each liquid in the mixed liquid

In this case, even if the liquid stored in the liquid storage is a mixture of a plurality of types of liquids, the liquid level can be detected with high accuracy.

It is preferable that the frequency of the ultrasonic wave transmitted from the ultrasonic probe is set in a range of 0.5 MHz to 2.5 MHz. If the frequency of the ultrasonic wave transmitted by the ultrasonic probe is set in this range, a sufficient liquid level resolution can be obtained, and the energy loss when the ultrasonic wave propagates from the wall of the liquid reservoir to the liquid can be reduced. Can be reduced.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention shown in the drawings will be described in detail. FIG. 1 shows a liquid level detecting device 21 and a centralized management system thereof according to an embodiment of the present invention. A plurality of liquid level detecting devices 21 are connected via a modem 22 and a public line (communication circuit) 23, respectively. Connected to a central management server (management device) 24.

As shown in FIGS. 2 to 4, each liquid level detecting device 21 includes a sensor unit 26 and an operation / display unit (operation unit) 27.

As shown in FIGS. 2 and 4, the sensor unit 26 includes a bracket member 26a and a sensor body (mounting element) 26b.
8 is attached to an outer surface of a wall portion 29a constituting a bottom portion of the storage tank 29 in which the storage tank 8 is enclosed. The bracket member 26a is provided with a female screw hole 26c penetrating in the thickness direction, and is fixed to the outside of the wall portion 29a by an adhesive, welding, or a magnet. On the other hand, the sensor main body 26b incorporates an ultrasonic probe 31 and a temperature sensor 32. A male screw portion 26d is provided on the outer periphery of the sensor main body 26b. Female screw hole 26 of the bracket member 26a
By screwing the male screw portion 26d of the sensor main body 26b to the bracket c, the sensor main body 26b is detachably attached to the bracket member 26a. The ultrasonic probe 31 is arranged at the upper end in the figure of the sensor main body 26b, and is in close contact with the wall 29a.

The mounting position and mounting angle of the sensor unit 26 are set so that the ultrasonic probe 31 emits ultrasonic waves in the vertical direction toward the liquid surface 28a.
That is, the ultrasonic wave transmitted by the ultrasonic probe 31 propagates vertically upward in the LPG 28 via the wall portion 29a, and is reflected by the liquid surface 28a. The ultrasonic wave reflected by the liquid surface 28a propagates vertically downward in the LPG 28 and enters the ultrasonic probe 31 via the wall 29a.

As described above, in the liquid level detecting device 21 of the present embodiment, the ultrasonic probe 31 is fixed not to the side of the storage tank 29 but to the outside of the bottom wall 29a, and
To transmit ultrasonic waves vertically toward the liquid surface 28a,
Even when the liquid level 28a is near the bottom of the storage tank 29, that is, when the liquid level is low, the liquid level L can be reliably detected. Further, the ultrasonic probe 31 transmits the ultrasonic wave not vertically but obliquely to the storage tank 29.
The mounting position and the mounting angle can be easily set. Further, since the ultrasonic probe 31 and the temperature sensor 32 are configured as one sensor unit 26, the mounting structure for the storage tank 29 is simple, and the ultrasonic probe 31 and the temperature sensor 32 can be mounted on the storage tank 29 at the same time. Can be.

As shown in FIG. 2, the operation / display 27
Is attached to the side periphery of the storage tank 29. FIG.
As shown in the figure, the calculation / display 27 includes a transmission / reception unit 27a, a temperature conversion unit 27b, a liquid level calculation unit 27c,
It has a capacity conversion unit 27d and a display unit 27e.

The transmitting / receiving section 27a instructs the ultrasonic probe 31 to transmit an ultrasonic wave. In addition,
The receiving unit 27a measures a time (reflection round-trip time T) from when the ultrasonic probe 31 transmits the ultrasonic wave to when the ultrasonic wave reflected by the liquid surface 28a is received (reflection round-trip time T). Is output to the liquid level calculation unit 27c. FIG. 7 shows an example of the waveform of an ultrasonic wave transmitted and received by the ultrasonic probe 31. In the example of FIG. 7, the ultrasonic probe 31 transmits an ultrasonic wave at a time s1 according to a command from the transmitting / receiving unit 27a, and the liquid level 2 at a time s2.
The ultrasonic probe 31 receives the ultrasonic wave reflected by 8a. The time from time s1 to time s2 is the reflection round trip time T measured by the transmission / reception unit 27a.

The temperature conversion section 27b is connected to the temperature sensor 3
The temperature t of the LPG 28 is estimated based on the detection signal from the control unit 2 and the temperature t is output as a digital signal to the liquid level calculation unit 27c. Since there is no large difference between the actual temperature of the LPG 28 and the temperature detected by the temperature sensor 32, the temperature detected by the temperature sensor 32 may be the temperature t of the LPG 28 itself.

The liquid level calculator 27c is a digital calculator, and is based on the following equation (1) based on the reflection round trip time T input from the transmitter / receiver 27a and the propagation speed V of the ultrasonic wave in the LPG 28. To calculate the liquid level L.

[0031]

L = 1/2 * T * V * 10 ^-3 (1) L: liquid level (m) T: repetition reciprocating time (msec) V: propagation speed of ultrasonic wave in LPG (m / sec) )

The propagation speed V of the ultrasonic wave in the LPG 28 is L
Depends on the temperature of the PG 28, and as the temperature rises, the propagation velocity V
Decrease. LPG28 is not 100% propane, but butane may be mixed with propane.
The propagating speed of the ultrasonic wave differs between propane and butane. Therefore, if the LPG 28 is a mixture of propane and butane, an error occurs if the liquid level L is calculated at a propagation speed assumed to be 100% of propane. Therefore, the liquid level calculation unit 27c outputs the LP input from the temperature conversion unit 27b.
G28 and temperature t of the mixing ratio of propane and butane in LPG28: based on (propane / butane = r _p r _b) to be corrected propagation velocity of the ultrasonic wave. As will be described later, the mixing ratio of propane and butane depends on the central management server 24.
And the setting unit 35 can store it in the liquid level calculation unit 27c.

First, the temperature correction will be described. The liquid level calculator 27c stores the relationship between the temperature and the propagation speed of ultrasonic waves (temperature-speed characteristic) in propane and butane shown in FIG. The liquid level calculation unit 27c calculates the propane velocity V with respect to the temperature t of the LPG 28 from FIG.
Calculate _p and butane speed _Vb .

Next, the mixing ratio correction will be described. The liquid level calculating unit 27c calculates the LPG by the following equation (2) based on the mixing ratio (r _p : r _b ) of propane and butane.
Calculates the ultrasonic wave propagation velocity V _t of 28, and the propagation velocity V _t after the correction by calculating the liquid level L is substituted into the equation (1).

[0035]

Equation 4] _{V t = {r p / (} r p + r b)} * V p + {r b / (r p + r b)} * V b (2) V t: the ultrasonic velocity of propagation in LPG (M / sec) V _p : Propagation speed of ultrasonic wave in propane (m / sec) V _b : Propagation speed of ultrasonic wave in butane (m / sec)

As described above, the liquid level detection device 21 of the present embodiment corrects the ultrasonic wave propagation speed in the liquid based on the temperature of the LPG 28 and the mixing ratio of propane and butane, so that the liquid level is detected with high accuracy. The plane level L can be detected.

In general, when a mixed liquid composed of n kinds of liquids is stored in the storage tank 29, the liquid level detecting device 21 uses the ultrasonic wave propagation velocity V _t based on the following equation.
Is calculated.

[0038]

(Equation 5) (K = 1, 2,..., N−1, n) (3) Vt: velocity of propagation of ultrasonic waves in the mixture (m / sec) V _k : velocity of propagation of ultrasonic waves in each liquid (m / sec) r _k : ratio of each liquid in the mixture

The capacity converting section 27d calculates the capacity of the LPG 28 remaining in the storage tank 29 from the liquid level L calculated by the liquid level calculating section 27c. For more information,
The capacity conversion unit 27d stores a table (tank table) indicating the relationship between the liquid level and the capacity shown in FIG.
The liquid level L is converted into a capacity based on the tank table. Note that the liquid level calculating section 27c may calculate only the liquid level L without providing the capacity converting section 27d.

The display section 27e is composed of, for example, a liquid crystal display device, and displays the hydraulic pressure level L calculated by the liquid level calculation section 27c and the capacity calculated by the capacity conversion section 27d.

The frequency of the ultrasonic wave transmitted from the ultrasonic probe 31 is preferably set in a range of 0.5 MHz to 2.5 MHz. The reason will be described below. Generally, the higher the frequency of the ultrasonic wave, the greater the attenuation in the liquid, and the lower the frequency, the lower the resolution of the liquid level L. For example, the sound speed in LPG at 10 ° C. is 800 m /
sec, and when the frequency is 1 MHz (10 ⁶ HZ), the wavelength is about 0.8 mm (800 (m) / 10 ⁶ = 0.8
× 10 ^-3 (m)). In order to measure the liquid level L with a resolution of 2 mm or less, the frequency needs to be 0.5 MHz or more.

On the other hand, the thickness D (see FIG. 4) of the wall portion 29a is generally about 10 mm to 15 mm. The sound speed in the wall 29a of the storage tank 29 is 5900 m / s in the case of iron.
In the case of aluminum, it is 6260 m /. As shown schematically by two-dot chain lines A1 and A2 in FIG. 4, the frequency of the ultrasonic wave is set such that the thickness D of the wall 29a is １ ／ of the wavelength of the ultrasonic wave.
When the frequency is set to (1 / 2λ), the inner surface of the wall portion 29a (the portion where the wall portion 29a contacts the LPG 28) becomes an antinode of vibration (longitudinal wave), and the propagation efficiency of ultrasonic waves from the wall portion 29a to the LPG 28 Is good. Similarly, when the frequency of the ultrasonic wave is set to an integral multiple of the 1 / 2λ frequency, the wall portion 29a
Inside is the antinode of the longitudinal wave. As shown in Table 1 below, the 1 / 2λ frequency when the wall 29a is made of iron or aluminum is in the range of 0.197 MHz to 0.313 MHz. However, since the ultrasonic probe in this frequency band has a large vibrator size and low efficiency of the vibrator itself, a frequency of 0.4 MHz to 0.6 MHz or more, which is about twice the 1 / 2λ frequency, is appropriate. . Further, since the ultrasonic waves in the LPG 28 are attenuated about 10 times or more per unit meter as compared to underwater, it is not preferable to set the frequency too high. Accordingly, the upper limit of the frequency is 1.
It is about 6 MHz to 2.4 MHz.

[0043]

[Table 1]

As described above, the frequency of the ultrasonic wave transmitted from the ultrasonic probe 31 is in the range of 0.5 MHz to 2.5 MHz, and the material and thickness D of the wall portion 29 a of the storage tank 29 are set to D.
If the resonance frequency is set to the corresponding 1 / 2λ resonance frequency, a sufficient liquid level resolution can be obtained, and the LPG
The energy loss when the ultrasonic wave propagates to the M. 28 can be reduced.

A signal indicating the liquid level L is transmitted from each liquid level detector 21 to the central management server 24 via the public line 23, and the liquid level L of the plurality of storage tanks 29 is transmitted to the central management server 24.
Can be monitored collectively. Further, the central management server 24 can transmit the mixture ratio of propane and butane to the calculation / display 27 via the public line 23 and store the mixture ratio in the liquid level detection device 21. Therefore, the central management server 24 monitors the liquid level L of each storage tank 29 and stores it in the liquid level calculation unit 27c without going to the site where the individual storage tank 29 including the liquid level detection device 21 is installed. The setting and the change of the mixing ratio can be performed.

The portable setting device 35 shown in FIGS. 2 and 4
Indicates that the liquid level detection device 21 is not connected to the public line 23 (including both the case where the modem 22 is not provided and the case where communication with the central management server 24 is stopped). This is for setting and changing the mixture ratio stored in the surface level calculator 27c.

As shown in FIG. 2, the setting unit 35 calculates
A connector 35a for connecting to the display 27, buttons 35b and 35c for setting numerical values, a button 35d for transmitting the set numerical values to the operation / display 27, and a display unit 35e for displaying the numerical values are provided. . By using the setting device 35, it is possible to set and change the mixing ratio at the site where the storage tank 29 is arranged.

As described above, the liquid level detecting device 21 is
However, as shown in FIG. 8, the liquid level device 21 of the present embodiment can be applied to the high-pressure gas container 37 as well.

[0049]

As is clear from the above description, in the liquid level detecting device of the present invention, the ultrasonic probe is fixed not to the side of the liquid reservoir but to the outside of the bottom wall, and the ultrasonic probe is fixed to the liquid surface. Since the ultrasonic waves are emitted vertically toward the container, the liquid level can be reliably detected even when the liquid level is near the bottom of the container, that is, when the liquid level is low. Further, it is easy to set the mounting position and the mounting angle of the ultrasonic probe with respect to the container. Furthermore, if the propagation speed of the ultrasonic wave in the liquid is corrected based on the temperature and the mixing ratio of the liquid, the liquid level can be detected with higher accuracy. Furthermore, if the frequency of the ultrasonic wave transmitted by the ultrasonic probe is set in the range of 0.5 MHz to 2.5 MHz, a sufficient liquid level resolution can be obtained, and the ultrasonic wave from the container wall to the liquid can be obtained. Energy loss when a sound wave propagates can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a liquid level detection device and a management system therefor according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing a liquid level detection device.

FIG. 3 is a block diagram showing a calculation / display unit.

FIG. 4 is a partially enlarged sectional view showing a sensor unit.

FIG. 5 is a diagram illustrating an example of a relationship between a temperature of a liquid and a propagation speed of an ultrasonic wave in the liquid.

FIG. 6 is a diagram showing an example of a relationship between a liquid level and a liquid volume.

FIG. 7 is a waveform diagram illustrating an example of a waveform of an ultrasonic wave transmitted and received by an ultrasonic probe.

FIG. 8 is a schematic view showing an example in which the high-pressure gas container is provided with the liquid level detecting device of the present invention.

FIG. 9 is a schematic view showing a conventional float type liquid level detecting device.

FIG. 10 is a schematic diagram showing a liquid level detecting device using a conventional ultrasonic probe.

[Explanation of symbols]

 Reference Signs List 21 liquid level detecting device 22 modem 23 public line 24 centralized management server 26 sensor unit 26a bracket member 26b sensor main body 26c female screw hole 26d male screw part 27 calculation / display 27a transmission / reception part 27b temperature conversion part 27c liquid level calculation part 27d Volume conversion unit 27e Display unit 28 LPG 28a Liquid surface 29 Storage tank 29a Wall 31 Ultrasonic probe 32 Temperature sensor 35 Setting device L Liquid level

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Koji Okamoto 3-48 Takahata-cho, Nishinomiya City, Hyogo Prefecture Inside Kawatetsu Advantech Co., Ltd. (72) Inventor Takagen Ogata 3-48 Takahata-cho, Nishinomiya City, Hyogo Prefecture Co., Ltd. F-term (reference) 2F014 AA04 AA05 AB01 AC02 FB01

Claims (7)

[Claims] 1. A liquid level detecting device for detecting a liquid level of a liquid stored in a liquid reservoir, the liquid level detecting device being attached to an outer side of a bottom wall of the liquid reservoir and being vertically directed toward the liquid surface. An ultrasonic probe that transmits an ultrasonic wave in the direction and receives the ultrasonic wave reflected by the liquid surface, and an ultrasonic probe that transmits the ultrasonic wave and then receives the ultrasonic wave reflected by the liquid surface. A liquid level detection device comprising: a calculator that calculates a liquid level based on time and a propagation speed of an ultrasonic wave in a liquid stored in a liquid storage. 2. A temperature sensor mounted outside a wall of the liquid reservoir, wherein the computing unit corrects a propagation speed of ultrasonic waves in the liquid based on a measurement value of the temperature sensor. The liquid level detecting device according to claim 1, wherein: 3. The arithmetic unit stores a relationship between a temperature of the liquid and a propagation speed of an ultrasonic wave in the liquid, and calculates a temperature of the liquid estimated from a measurement value of the temperature sensor, 3. The liquid level detecting device according to claim 2, wherein a propagation speed of the ultrasonic wave in the liquid is determined from a relationship with the speed. 4. The apparatus according to claim 1, wherein said ultrasonic probe and said temperature sensor are integrated into a single mounting element, said mounting element being mounted outside the bottom wall of said liquid reservoir. The liquid level detecting device according to claim 2 or 3. 5. When the liquid stored in the liquid storage is a mixed liquid of a plurality of types of liquids, the arithmetic unit transmits the ultrasonic wave in the mixed liquid based on a mixing ratio of each liquid. The liquid level detecting device according to claim 1, wherein the speed is corrected. 6. The computer according to claim 1, wherein the arithmetic unit calculates the propagation speed of the ultrasonic wave in the mixed liquid including the n kinds of liquids stored in the liquid storage based on the following equation. The liquid level detecting device according to claim 5. (Equation 1) (K = 1, 2,..., N−1, n) V _t : Propagation velocity of ultrasonic wave in the mixed liquid V _k : Propagation velocity of ultrasonic wave in each liquid r _k : Ratio of each liquid in the mixed liquid 7. The apparatus according to claim 1, wherein the frequency of the ultrasonic wave transmitted by the ultrasonic probe is set in a range from 0.5 MHz to 2.5 MHz. The liquid level detecting device according to item 1.