KR100211114B1 - Liquid level detector - Google Patents

Abstract

The liquid level detector of the present invention includes a first electrode plate and a second electrode plate for dielectric constant detection, a third electrode plate and a fourth electrode plate for level detection, an operational amplifier, a first electronic switching circuit, and a second electronic switching circuit. The first electrode plate and the second electrode plate for detecting the dielectric constant are located at the bottom of the liquid. The third electrode plate and the fourth electrode plate for level detection are built on the bottom surface of the liquid with a considerable height in the measurement range. The operational amplifier has at least a first input terminal and an output terminal, and generates a voltage for level detection through the output terminal. The first electronic switching circuit is connected between the reference input voltage terminal and the first input terminal of the operational amplifier to generate an electrical equivalent resistance inversely proportional to the capacitance between the third and fourth pole plates for level detection. The second electronic switching circuit is connected between the first input terminal and the output terminal of the operational amplifier, to generate an electrical equivalent resistance inversely proportional to the capacitance between the first electrode plate and the second electrode plate for dielectric constant detection.

Description

Liquid level detector

The present invention relates to a liquid level detector.

The liquid level detector refers to a device for detecting the liquid level of a stored liquid or a flowing liquid. The detected liquid level is proportional to the amount of liquid currently stored or the amount of liquid flowing per unit time. Therefore, the liquid level detector is used in various industrial fields for liquid fuel amount measurement, water level measurement, flow rate measurement, and the like.

As a representative example of a conventional liquid level detector, a liquid level detector using buoyancy is mentioned. The liquid level detector is provided with a member that floats on the liquid, and the resistance value of the variable resistor changes depending on the height of the member. The output voltage changes as the resistance of the variable resistor changes. That is, since the height of the member is equal to the liquid level, the liquid level can be detected by the output voltage.

Another example of a conventional liquid level detector is a liquid level detector using capacitance. This liquid level detector consists of electrode plates for capacitive detection and complex circuits. The electrode plates are erected on the underside of the liquid, and the capacitance therebetween varies with the liquid level. The changing capacitance controls the output frequency of the oscillator, which is converted into a voltage signal through a complex circuit. That is, since the capacitance is proportional to the liquid level, the liquid level can be detected by the voltage signal.

The conventional liquid level detector as described above has the following problems. First, in the case of a detector using buoyancy, the mechanism continues to operate in accordance with the lifting and lowering motion of the member, thereby shortening the lifespan. Second, in the case of a detector using an electrostatic capacitance, its output value changes according to the permittivity of the liquid to be measured. In other words, depending on the permittivity of the liquid, the compatibility and accuracy of detection are inferior.

It is an object of the present invention to provide a liquid level detector which is not affected by the permittivity of the liquid without using a buoyancy mechanism.

1 is a schematic perspective view showing pole plates of a level detector according to an embodiment of the present invention.

2 is a view showing the overall circuit of the liquid level detector according to an embodiment of the present invention.

3A is a basic circuit diagram for explaining a control principle of the circuit of FIG. 2.

3B is a timing diagram of a control pulse signal to be applied to the circuit of FIG. 3A.

4 is an equivalent circuit diagram for describing an operation of the circuit of FIG. 2.

FIG. 5 is an equivalent circuit diagram of the simplified circuit of FIG. 2.

* Explanation of symbols for main parts of the drawings

11, 12: pole plate for detecting dielectric constant 21, 22: pole plate for level detection

L1, L2: Height of pole plates a: Width of pole plates

d: Spacing of pole plates, H ... liquid level

B1: first electronic switching circuit B2: second electronic switching circuit

B1-1: First-1-1 Electronic Switching Circuit

B2-1: 2-1 electronic switching circuit

Vk: input reference voltage Vout: output voltage

The liquid level detector of the present invention for achieving the above object comprises a first electrode plate and a second electrode plate for dielectric constant detection, a third electrode plate and a fourth electrode plate for level detection, an operational amplifier, a first electronic switching circuit and a second electronic switching circuit. Include. The first electrode plate and the second electrode plate for detecting the dielectric constant are located at the bottom of the liquid. The third electrode plate and the fourth electrode plate for level detection are built on the bottom surface of the liquid with a considerable height in the measurement range. The operational amplifier has at least a first input terminal and an output terminal, and generates a voltage for level detection through the output terminal. The first electronic switching circuit is connected between a reference input voltage terminal and a first input terminal of the operational amplifier to generate an electrical equivalent resistance inversely proportional to the capacitance between the third electrode plate and the fourth electrode plate for level detection. . The second electronic switching circuit is connected between the first input terminal and the output terminal of the operational amplifier to generate an electrical equivalent resistance inversely proportional to the capacitance between the first electrode plate and the second electrode plate for detecting the dielectric constant.

Since the output voltage of the liquid level detector of the present invention is proportional to the capacitance between the third electrode plate and the fourth electrode plate for level detection, and inversely proportional to the capacitance between the first electrode plate and the second electrode plate for dielectric constant detection, the dielectric constant of the liquid This offset is unaffected and proportional to the liquid level.

Preferably, the first electronic switching circuit and the second electronic switching circuit are electronic switches, between the third electrode plate and the fourth electrode plate for level detection, and between the first electrode plate and the second electrode plate for dielectric constant detection. In each case, the charging and discharging are performed continuously.

The first electronic switching circuit may further include a first electronic switch having one end connected with the reference input voltage terminal and the other end connected with the third electrode plate; A second electronic switch, one end of which is connected to the third electrode plate and the other end of which is grounded; A third electronic switch, one end of which is connected to the fourth electrode plate and the other end of which is grounded; And a fourth electronic switch, one end of which is connected to the fourth electrode plate and the other end of which is connected to the first input terminal of the operational amplifier.

The second electronic switching circuit may include a fifth electronic switch having one end connected to a first input terminal of the operational amplifier and the other end connected to the first electrode plate for detecting the dielectric constant; A sixth electronic switch having one end connected to the first electrode plate for detecting the dielectric constant and the other end grounded; A seventh electronic switch, one end of which is connected to the second electrode plate for detecting the dielectric constant and the other end of which is grounded; And an eighth electronic switch, one end of which is connected to the second electrode plate for detecting the dielectric constant and the other end of which is connected to an output terminal of the operational amplifier.

Hereinafter, preferred embodiments of the present invention will be described in detail.

1 shows a structure of pole plates of a level detector according to an embodiment of the present invention. Referring to the drawings, the pole plates of the level detector according to the present embodiment include the first pole plate 11 and the second pole plate 12 for detecting the dielectric constant, the third pole plate 21 and the fourth pole plate 22 for the level detection. Include. Dielectric constant detection plates 11 and 12 are located at the bottom of the liquid. The electrode plates 21 and 22 for level detection have a height L2 in the measurement range and are spaced apart on the dielectric plates 11 and 12. The dielectric plates for detecting dielectric constants 11 and 12 have a flat plate structure facing each other. Similarly, the level detecting pole plates 21 and 22 also have a flat plate structure facing each other. The pole plates 11, 12, 21, 22 have a width and a distance d. The height of the dielectric plates for detecting dielectric constants 11 and 12 is L1, which is shorter than the height of the level detecting plates 21 and 22. Reference numeral H denotes a liquid level height.

2 shows an overall circuit of a liquid level detector according to an embodiment of the present invention. Referring to the drawings, the circuit of the level detector according to the present embodiment includes an operational amplifier U ₁ , a first electronic switching circuit B1, a second electronic switching circuit B2, and a 1-1 electronic switching circuit B1-1. 1) and the 2-1 electronic switching circuit B2-1. The operational amplifier U ₁ has a negative input terminal (−), a positive input terminal (+), and an output terminal, and generates a level detection voltage Vout through the output terminal. The first electronic switching circuit B1 is connected between the reference input voltage Vk terminal and the negative input terminal (−) of the operational amplifier U ₁ to connect the third electrode plate 21 of FIG. An electrical equivalent resistance inversely proportional to the capacitance Cd between the four pole plates (22 in Fig. 1) is generated. The second electronic switching circuit B2 is connected between the negative input terminal (-) and the output terminal of the operational amplifier U ₁ , so that the first electrode plate 11 of FIG. 1 and the second electrode plate FIG. Generates an electrical equivalent resistance inversely proportional to the capacitance (Ce).

The first-first electronic switching circuit B1-1 and the second-first electronic switching circuit B2-1 are auxiliary circuits for increasing the accuracy of the level detection voltage Vout from the operational amplifier U ₁ . . The first-first electronic switching circuit B1-1 is connected in parallel with the first electronic switching circuit B1 to generate an electrical equivalent resistance inversely proportional to the capacitance C ₁ of the first capacitor. In addition, the 2-1 electronic switching circuit B2-1 is connected in parallel with the second electronic switching circuit B2 to generate an electrical equivalent resistance inversely proportional to the capacitance C ₂ of the second capacitor.

The first electronic switching circuit B1 and the second electronic switching circuit B2 are electronic switches S ₁ , S ₂ , S ₃₁ , S ₄₁ , S ₄₂ , S ₃₂ , and S ₅ operated by a control pulse signal. , S ₆ , and are charged and discharged between the third electrode plate 21 and the fourth electrode plate 22 for level detection and between the first electrode plate 11 and the second electrode plate 12 for detecting the dielectric constant, respectively. This is done continuously.

[0075] The electronic switching circuit (B1-1) and the second-first electronic switching circuit (B2-1) is an electronic switch operable by a control pulse signal _{(S 7, S 8, S} 33, S 43, S ₄₄ , S ₃₄ , S ₉ , S ₁₀ ) and capacitors, so that charging and discharging are continuously performed in the capacitor of the capacitance C _{1 and} the capacitor of C ₂ , respectively.

The first electronic switching circuit B1 includes: a first electronic switch S ₁ having one end connected to a reference input voltage Vk terminal and the other end connected to the third electrode plate 21; A second electronic switch S ₂ having one end connected to the third electrode plate 21 and the other end grounded; A third electronic switch S ₃₁ having one end connected to the fourth electrode plate 22 and the other end grounded; And a fourth electronic switch S ₄₁ having one end connected to the fourth electrode plate 22 and the other end connected to the negative input terminal (−) of the operational amplifier U ₁ .

The second electronic switching circuit B2 includes a fifth electronic switch having one end connected to the negative input terminal (−) of the operational amplifier U ₁ and the other end connected to the first electrode plate 11 for detecting the dielectric constant. S ₄₂ ); A sixth electronic switch S ₃₂ having one end connected to the first electrode plate 11 for detecting the dielectric constant and having the other end grounded; A seventh electronic switch S ₅ having one end connected to the second electrode plate 12 for detecting the dielectric constant and having the other end grounded; And an eighth electronic switch S ₆ , one end of which is connected to the second electrode plate 12 for detecting the dielectric constant and the other end of which is connected to an output terminal of the operational amplifier U ₁ .

Like the first electronic switching circuit B1 and the second electronic switching circuit B2, the first-first electronic switching circuit B1-1 and the second-first electronic switching circuit B2-1 also use the same capacitors. It is a π-type structure with a center.

FIG. 3A shows a basic circuit for explaining the control principle of the circuit of FIG. 2. 3B shows a control pulse signal to be applied to the circuit of FIG. 3A. The basic circuit of FIG. 3A includes the first electronic switching circuit B1, the second electronic switching circuit B2, the first-first electronic switching circuit B1-1, and the second-first electronic switching circuit B2- of FIG. 2. Applies to all 1). The elements of the basic circuit of FIG. 3A correspond to the respective electronic switching circuits B1, B2, B1-1, and B2-1 of FIG. 2 as follows. That is, the electronic switch S ₁₇ of FIG. 3A is S ₁ , S ₄₂ , S ₇ , S ₄₄ of FIG. 2, and the electronic switch S ₁₈ of FIG. 3A is S ₂ , S ₃₂ , S ₈ , S ₃₄ of FIG. 2, The electronic switch S ₁₉ of FIG. 3A corresponds to S ₃₁ , S ₅ , S ₃₃ , and S ₉ of FIG. 2, and the electronic switch S ₂₀ of FIG. 3A corresponds to S ₄₁ , S ₆ , S ₄₃ , and S ₁₀ of FIG. 2. In addition, the capacitance C of FIG. 3A is a capacitance Ce between the dielectric plates for detecting dielectric constants 11 and 12 of FIG. 1, a third electrode plate for level detection (21 in FIG. 1), and a fourth electrode plate (22 in FIG. 1). ) Corresponds to the capacitance Cd), the capacitance C ₁ of the first capacitor, and the capacitance C ₂ of the second capacitor.

Referring to the drawings of FIGS. 3A and 3B, the control principle for the basic circuit of FIG. 3A will be described. As shown in Fig. 3B, the period of the control pulse signal to be applied is T, and the period T consists of the first control period T1 and the second control period T2. In Fig. 3A, reference numeral V1 denotes an input voltage, I denotes a current, and V2 denotes an output voltage. In FIG. 3B, reference numeral V denotes a voltage and t denotes a time. During the first control period T1, when the electronic switches S17 and S20 are turned on and S18 and S19 are turned off, the moving charge amount Q1 is obtained by the following equation (1).

[Equation 1]

( V1 - V2 )Q1 = C

(V1-V2)

During the second control period T2, when the electronic switches S17 and S20 are turned off and S18 and S19 are turned on, the capacitor C discharges so that its internal charge amount is zero. Therefore, the average amount of moving charges Q passing through the electronic switches S17 and S20 during one period T is obtained by the following equation (2).

[Equation 2]

( V1 - V2 ) = I

TQ = C

(V1-V2) = I

T

Here, if the frequency of the control pulse signal is f, the equivalent resistance R for the basic circuit of Fig. 3A is obtained by the following equation (3).

[Equation 3]

C )R = (V1-V2) / I = T / C = 1 / (f

C)

That is, by operating the electronic switches (S17, ... S20) as a control pulse signal as described above, it can be seen that the basic circuit of Figure 3a performs the function of the equivalent resistor R.

On the other hand, when the electronic switches S17 and S19 are turned on and S18 and S20 are turned off during the first control period T1, the moving charge amount Q1 is obtained by the following equation (4).

[Equation 4]

V1Q1 = C

V1

During the second control period T2, when the electronic switches S17 and S19 are turned off and S18 and S20 are turned on, the mobile charge amount Q2 is obtained by the following expression (5).

[Equation 5]

V2Q2 = C

V2

Therefore, the average amount of moving charges Q passing through the electronic switches S17 and S20 during one period T is obtained by the following equation (6).

[Equation 6]

( V1 - V2 ) = - I

TQ = Q1-Q2 = C

(V1-V2) =-I

T

The reason why the negative sign (-) is attached to the current I in Equation 6 is because the direction of the current is opposite to the direction of I shown in FIG. 3A.

Here, if the frequency of the control pulse signal is f, the equivalent resistance R for the basic circuit of Fig. 3A is obtained by the following equation (7).

[Equation 7]

C )R = (V1-V2) / I = T / C = 1 / (f

C)

That is, by operating the electronic switches (S17, ... S20) as a control pulse signal as described above, it can be seen that the basic circuit of Figure 3a performs the function of the negative equivalent resistance-R.

As described above, by operating the electronic switches (S17, ... S20) as a control pulse signal, it can be seen that the basic circuit of Figure 3a performs the function of the equivalent resistance R or negative equivalent resistance -R. Using this characteristic and operational amplifier, the overall circuit of the liquid level detector of FIG. 2 can be represented as the equivalent circuit of FIG. In other words, an electrical equivalent resistance inversely proportional to the capacitance Cd between the third electrode plate 21 (see FIG. 1) and the fourth electrode plate 22 (FIG. 1) for level detection in the first electronic switching circuit B1. Rd may be generated. In addition, in the second electronic switching circuit B2 of FIG. 2, an electrical equivalent resistance Re inversely proportional to the capacitance Ce between the first electrode plate 11 of FIG. 1 and the second electrode plate 12 of FIG. 1. Can be generated. In addition, the 1-1 st electronic switching circuit (B1-1 of FIG. 2) may generate an electric equivalent resistor R ₁ inversely proportional to the capacitance C ₁ of the first capacitor. Then, the second-first electronic switching circuit (B2-1 Fig. 2), the second is parallel-connected to the electronic switching circuit (B2) electrically equivalent resistance that is inversely proportional to the capacitance (C ₂₎ of the second kaepesiteo -R ₂ Can be generated.

Therefore, the output voltage Vout of the liquid level detector is proportional to the capacitance Cd between the third electrode plate 21 and the fourth electrode plate 22 for level detection, and the first electrode plate 11 and the second electrode plate for dielectric constant detection ( Since it is inversely proportional to the capacitance Ce between 12), the permittivity of the liquid is canceled out and is not affected and proportional to the liquid level (H in FIG. 1) only. On the other hand, an appropriate equivalent resistance can be generated by the 1-1 electronic switching circuit B1-1 and the 2-1 electronic switching circuit B2-1, thereby increasing the accuracy of the output voltage Vout of the liquid level detector. Specific actions in this regard are described below.

은 아래의 수학식 8로써 구해진다.In FIG. 4, when the combined resistance of the equivalent resistors -Rd and R ₁ is R _T1 , and the combined resistance of Re and -R ₂ is R _T2 , the ratio of the output voltage Vout to the reference input voltage Vk

Is obtained by Equation 8 below.

[Equation 8]

= -

=-

In Equation 8, the combined resistance R _T1 of the equivalent resistance -Rd and R ₁ is obtained by Equation 9 below.

[Equation 9]

=

=

이고 Rd는

이다. 여기서 f는 상기 제어 펄스 신호의 주파수를 나타낸다. 이를 상기 수학식 9에 대입하면, 아래의 수학식 10이 성립한다.Referring to Equation 3, the equivalent resistance R ₁ is

And Rd is

to be. Where f represents the frequency of the control pulse signal. Substituting this into Equation 9, Equation 10 below holds.

[Equation 10]

=

=

Similarly, the equivalent resistance Re and the composite resistance R _T2 of -R ₂ are obtained by the following equation (11).

[Equation 11]

=

=

이고 R₂는

이므로, 이를 상기 수학식 11에 대입하면, 아래의 수학식 12가 성립한다.Where equivalent resistance Re is

And R ₂ is

Therefore, by substituting this into Equation 11, Equation 12 below holds.

[Equation 12]

=

=

Substituting Equations 10 and 12 into Equation 8 below Equation 13 holds.

[Equation 13]

= -

=-

=-

=

Here, the capacitance Ce between the dielectric plates for detecting the dielectric plates 11 and 12 of FIG. 1 is obtained by the following equation (14) in the structure shown in the drawing of FIG.

[Equation 14]

L1

/ dCe = a

L1

/ d

L1

₀

_r/ d= a

L1

₀

_r / d

_r = Ke

_r

은 액체의 유전율,

₀는 유전 상수( 약 8.855

10^-12),

_r은 액체의 비유전율(比誘電率), 그리고 Ke는 유전율 검출용 극판들(도 1의 11, 12)의 구조에 따른 상수이다. 따라서, 유전율 검출용 극판들(도 1의 11, 12) 사이의 정전 용량 Ce는 액체의 비유전율

_r에 정비례한다.here,

Permittivity of silver liquid,

₀ is the dielectric constant (approximately 8.855)

10 ^-12 ),

_r is the relative dielectric constant of the liquid, and Ke is a constant according to the structure of the electrode plates for detecting the dielectric constant (11, 12 of FIG. 1). Therefore, the capacitance Ce between the dielectric plates for detecting dielectric constants 11 and 12 of FIG. 1 is the relative dielectric constant of the liquid.

_It is directly proportional to _r .

On the other hand, the capacitance Cd between the level detecting pole plates 21 and 22 of FIG. 1 is the structure of the structure shown in the drawing of FIG. 1, which is the capacitance of the space not filled with the liquid and the capacitance of the filled area. It is sum. Therefore, the capacitance Cd between the level detecting pole plates 21 and 22 is obtained by Equation 15 below.

[Equation 15]

Cd = capacitance of the liquid-filled space + capacitance of the liquid-filled area

(L2 - H)

₀/ d + a

H

₀

_r/ d= a

(L2-H)

₀ / d + a

H

₀

_r / d

₀와 같다. 레벨 검출용 극판들(21, 22)의 높이 L2에 대한 액면 높이 H의 비율(H/L2)을 Hr이라 하면, 액면 높이 H는 L2

Hr 이다. 이를 상기 수학식 16에 대입하면, 아래의 수학식 16이 성립한다.The capacitance of the space not filled with liquid is the capacitance by air. Since the relative dielectric constant of air is 1, the dielectric constant is dielectric constant

Same as ₀ When the ratio H / L2 of the liquid level H to the height L2 of the level plates 21 and 22 is Hr, the liquid level H is L2.

Hr. Substituting this into Equation 16, Equation 16 below holds.

[Equation 16]

(L2 - L2

Hr)

₀/ d + a

L2

Hr

₀

_r/ dCd = a

(L2-L2

Hr)

₀ / d + a

L2

Hr

₀

_r / d

L2

(1 - Hr)

₀/ d + a

L2

Hr

₀

_r/ d= a

L2

(1-Hr)

₀ / d + a

L2

Hr

₀

_r / d

L2

₀) / d를 상수 Kd라 하면, 상기 수학식 16은 아래의 수학식 17과 같다.Where (a

L2

_{When 0} ) / d is a constant Kd, Equation 16 is expressed by Equation 17 below.

[Equation 17]

(1 - Hr) + Kd

Hr

_r Cd = Kd

(1-Hr) + Kd

Hr

_r

Hr + Kd

Hr

_r = Kd-Kd

Hr + Kd

Hr

_r

Hr

_r- Kd

Hr + Kd= Kd

Hr

_r -Kd

Hr + Kd

Hr

(

_r- 1) + Kd= Kd

Hr

(

_r -1) + Kd

Therefore, by substituting Equations 14 and 17 into Equation 13, Equation 18 below holds.

Equation 18

=

=

=

=

In Equation 18, when the capacitance C ₁ of the first capacitor is set equal to the value of Kd and the capacitance C ₂ of the second capacitor is equal to the value of Ke, Equation 19 below is established.

[Equation 19]

=

=

=

Therefore, the output voltage Vout of the liquid level detector is generated according to Equation 20 below.

[Equation 20]

=

=

That is, it can be seen that the output voltage Vout of the liquid level detector is proportional to only the liquid level without being affected by the dielectric constant of the liquid being canceled.

On the other hand, by appropriately adjusting the phase of the control pulse signal to be applied to each of the electronic switching circuits (B1, B2, B1-1, and B2-1 in FIG. 2), the circuit of the liquid level detector of FIG. 2 is simplified as in the circuit of FIG. can do. That is, the electronic switch S ₃ of FIG. 5 may perform the functions of the electronic switches S ₃₁ , S ₃₂ , S ₃₃ and S ₃₄ of FIG. 2, and the electronic switch S ₄ of FIG. 5 is the electronic switches S of FIG. 2. ₄₁ , S ₄₂ , S ₄₃ and S ₄₄ can perform the functions.

As described above, according to the liquid level detector according to the present invention, since it is not influenced by the permittivity of the liquid even without using the buoyancy mechanism, the life thereof is extended, and the compatability and precision of the detection are improved.

The present invention is not limited to the above embodiments, and modifications and improvements are possible at the level of those skilled in the art.

Claims (6)

A first electrode plate and a second electrode plate for detecting permittivity positioned on a bottom surface of the liquid; A third electrode plate and a fourth electrode plate for level detection, which have a height corresponding to the measurement range, and are mounted on the bottom surface of the liquid; An operational amplifier having at least a first input terminal and an output terminal for generating a level detection voltage through the output terminal; A first electronic switching circuit connected between a reference input voltage terminal and a first input terminal of the operational amplifier to generate an electrical equivalent resistance inversely proportional to the capacitance between the third electrode plate and the fourth electrode plate for level detection; And A second electronic switching circuit connected between the first input terminal and the output terminal of the operational amplifier, the second electronic switching circuit generating an electrical equivalent resistance inversely proportional to the capacitance between the first electrode plate and the second electrode plate for detecting the dielectric constant; Liquid level detector. The method of claim 1, wherein the first electronic switching circuit and the second electronic switching circuit, And electronic switches, so that charging and discharging are continuously performed between the third and fourth electrode plates for level detection and between the first and second electrode plates for dielectric constant detection, respectively. Detector. The method of claim 2, wherein the first electronic switching circuit, A first electronic switch having one end connected to the reference input voltage terminal and the other end connected to the third electrode plate; A second electronic switch, one end of which is connected to the third electrode plate and the other end of which is grounded; A third electronic switch, one end of which is connected to the fourth electrode plate and the other end of which is grounded; And And a fourth electronic switch, one end of which is connected to the fourth electrode plate and the other end of which is connected to the first input terminal of the operational amplifier. The method of claim 3, wherein the second electronic switching circuit, A fifth electronic switch, one end of which is connected to the first input terminal of the operational amplifier and the other end of which is connected to the first electrode plate for detecting the dielectric constant; A sixth electronic switch having one end connected to the first electrode plate for detecting the dielectric constant and the other end grounded; A seventh electronic switch, one end of which is connected to the second electrode plate for detecting the dielectric constant and the other end of which is grounded; And And an eighth electronic switch, one end of which is connected to the second electrode plate for detecting permittivity and the other end of which is connected to an output terminal of the operational amplifier. The method of claim 1, wherein the first input terminal of the operational amplifier, A liquid level detector, characterized by being a cathode input terminal. The positive input terminal of the operational amplifier of claim 2, Liquid level detector, characterized in that grounded.

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