JPH08292081A - Capacitance type level sensor - Google Patents

Abstract

PURPOSE: To enable detection of the surface of a storage liquid and an interface between this storage liquid and water with certainty by installing a rectifying-smoothing circuit which converts the output of a buffer circuit into that of a sensor after rectifying and smoothing it, and a high frequency power source. CONSTITUTION: When a detecting element 1 is in a water phase, a dielectric constant of water is very high as 80, and since an interval between electrode plates of this detecting part 1 is approximate to a state of being shorted, voltage in a bridge output terminal 5 is the same phase as the voltage of a high frequency power source 4, therefore its amplitude also becomes electric potential approximate to this phase. Accordingly, a large rectifying output appears in an output terminal 8, and in the case where a synchronous commutation is carried out in a rectifying-smoothing circuit 7, polarity of output voltage in this output terminal 8 becomes negative, by way of example, if voltage from the high frequency power source 4 is negative. Next, when the detecting element 1 is in a gas phase, capacity in this detecting element 1 decreases, and thereby electric potential in the bridge output end 5 comes nearer to that of a high frequency power source 3. In addition, when the circuit 7 is used, whether the detecting element 1 is in the water phase or the gas phase can be judged by a polarity judgement in output potential, so that it is no need to take a dynamic range in a buffer circuit 6 so wide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrostatic capacitance type level sensor for detecting the surface and interface of a liquid from the electrostatic capacitance value of a detecting portion composed of a pair of facing electrode plates.

[0002]

2. Description of the Related Art In a conventional capacitance type level sensor, a capacitance value between a detection electrode which is insulated from the inner wall of a tank and a ground electrode which is connected to the inner wall of the tank is the surface or ratio of the liquid. It is used to detect the surface or interface of two liquids having different dielectric constants, for example, the change at the interface between water and a stored liquid.

[0003]

For example, when a petroleum product is unloaded from an oil tanker, seawater that has entered during voyage is accumulated at the bottom of the tank. The petroleum product must be sent out after the end, and the pump must be stopped when the petroleum product is sent out and the tank is emptied. The level sensor used for pump control described above must be able to detect the interface between water and oil and the oil surface with one unit, and since it handles petroleum products, it must have an intrinsically safe explosion-proof structure. I won't.

It is an object of the present invention to provide a level sensor which can be used safely and can reliably detect the surface of a stored liquid and the interface between the stored liquid and water.

[0005]

Means and embodiments for achieving the above-mentioned object are as follows.

<Means 1> A detection unit composed of a pair of electrode plates insulated from the inner wall of the tank is provided near the bottom plate of the tank for storing a liquid having a relative dielectric constant lower than that of water. In a level sensor for detecting a surface of a liquid and an interface between a stored liquid and water, a high-frequency half bridge which has the detection section as one side and is substantially balanced when the stored liquid exists between electrode plates of the detection section, and this half bridge. Of the output voltage of the sensor circuit, a rectifying / smoothing circuit for rectifying and smoothing the output of the buffer circuit to obtain a sensor output, and a high-frequency power source for forming the half bridge. The stored liquid is distinguished by discriminating whether the liquid existing between the electrode plates of the detection part is the stored liquid or water, or neither the stored liquid nor the water. Configured to detecting the interface surface and storage liquid and water.

<Means 2> A detection unit composed of a pair of electrode plates insulated from the inner wall of the tank is provided near the bottom plate of the tank for storing a liquid having a lower relative dielectric constant than water, and stored from this capacitance value. In a level sensor for detecting a surface of a liquid and an interface between a stored liquid and water, a high frequency full bridge which has the detection section as one side and is substantially balanced when the stored liquid exists between electrode plates of the detection section, and A pair of buffer circuits for impedance-converting the output voltage of the bridge; a rectifying / smoothing circuit for rectifying and smoothing the output of the buffer circuit to obtain a sensor output; and a high-frequency power supply for exciting the full bridge, The stored liquid is discriminated from the sensor output whether there is stored liquid or water between the electrode plates of the detection unit, or whether neither stored liquid nor water exists. A structure for detecting the interface between the surface and the storage liquid and water.

<Embodiment 1> The rectifying / smoothing circuit is a synchronous rectifying / smoothing circuit using a voltage from a high frequency power supply as a reference voltage.

<Embodiment 2> The rectifying / smoothing circuit and the high frequency power source are provided outside the tank, the high frequency bridge and the buffer circuit are provided in the vicinity of the detecting portion, and the rectifying / smoothing circuit, the high frequency power source and the high frequency power are provided via a cable. The bridge and the buffer circuit are electrically connected to each other.

<Embodiment 3> The electrode plate of the detector is provided so as to be substantially horizontal in the tank, the upper electrode plate is connected to the bridge output side, and the lower electrode plate is connected to the high frequency power source side. .

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic construction of a level sensor according to claim 1 of the present invention will be described below with reference to FIG. In the figure, reference numeral 1 is a detection unit composed of a pair of facing electrode plates provided near the bottom surface of the oil tank and insulated from the inner wall of the tank, 2 is a capacitor having a fixed capacitance, and 3 and 4 are stable output voltage amplitudes. The high frequency power supplies are almost equal and have opposite phases.

A half bridge is composed of the detector 1, the capacitor 2, and the high frequency power sources 3 and 4, and the capacitor 2
For the high frequency power source 3, 4, the bridge is selected so that the bridge is almost in equilibrium when the stored liquid oil is present between the electrode plates of the detector 1, that is, when the detector 1 is in the oil phase. For this, a rectangular wave power source having a frequency of several tens of kHz to several hundreds of kHz is used.

A buffer circuit 6 for impedance conversion, for example, a source follower having a field effect transistor as the first stage is connected to the bridge output terminal 5 to generate a voltage equal to the bridge output voltage on a low resistance.

Reference numeral 7 in the figure rectifies the bridge output voltage.
A rectifying / smoothing circuit section that smoothes and outputs to the output terminal 8. The rectifying / smoothing circuit 7 is a high-frequency power source 3 for the reason described below.
It is preferable that the voltage of _No. 4 be the reference voltages P ₁ and P ₂ , respectively.

Next, the operation of this embodiment will be described. First, when the detection unit 1 is in the oil phase, the half bridge is almost balanced, so the output voltage from the output terminal 8 is almost 0v.

When the detector 1 is in the water phase, the relative permittivity of water is as high as 80, and the electrode plates of the detector 1 are close to a short-circuited state.
The voltage has the same phase as the voltage and the amplitude is close to this. Therefore, a large rectified output appears at the output terminal 8, and when synchronous rectification is performed in the rectifying / smoothing circuit 7, the polarity of the output voltage at the output terminal 8 is negative if the voltage from the high frequency power source 4 is negative, for example. become.

Next, when the detector 1 is in the gas phase,
The capacitance of the detection unit 1 decreases, and the potential of the bridge output terminal 5 approaches the potential of the high frequency power supply 3.

To give specific numerical examples of the above-mentioned operation, for example, both the capacitance value in the oil phase of the detector 1 and the capacitance value of the capacitor 2 are 40 pF, the relative permittivity of oil is 2, the high frequency power source 3 and The voltage amplitude of 4 is 5v
_{If p} and -5v _p (v _p indicates the peak voltage value),
The capacitance value of the detector 1 is changed from 40 pF to 40/2 pF,
Therefore, the potential of the rectifying / smoothing circuit 7 is (40-20) / (40 + 20) × 5v _p = 1.7v _p , and its phase is the same as the voltage phase of the high frequency power supply 3. In this case, the output obtained from the output terminal 8 is
If the smoothing circuit 7 performs synchronous rectification, the polarity is positive.

Therefore, a DC output corresponding to 5 v _p is obtained from the output terminal 8 when the detecting section 1 is in the water phase, approximately 0 v when the detecting section 1 is in the oil phase, and 1.7 v _p when the detecting section 1 is in the gas phase. It is possible to easily identify whether the detection unit 1 is in the water phase, the oil phase or the gas phase by comparing the magnitudes of the output values.

Further, if synchronous rectification is performed in the rectifying / smoothing circuit 7, in addition to identifying Ov, the polarity of the output potential from the output terminal 8 may be identified, which makes identification easier. Note that load effects and saturation characteristics are not taken into consideration in the calculation of the above-described numerical example.

Next, the features of the configuration of this embodiment will be described. In addition to a half bridge, a full bridge is generally known as a bridge, but in the full bridge, it is necessary to make one of the excitation side and the output detection side a balanced type and the other an unbalanced type. Use a transformer on either the excitation side or the detection side.

When the sensor of this embodiment is used in a petroleum tank, an intrinsically safe explosion-proof structure is required. However, if a transformer is used, its realization is difficult due to the inductance of the transformer. On the other hand, in this embodiment, since the half bridge is adopted as the high frequency bridge, there is no need to use a transformer, which is advantageous in terms of explosion protection.

Further, in the half bridge, the amplitudes of the two high frequency power supplies must be stable and the phases of them must be in opposite phase to each other. However, if a high frequency power supply with a rectangular wave is used as in this embodiment, A power supply circuit is easier to manufacture than a high frequency power supply of sine wave.

Further, if a synchronous rectifying / smoothing circuit is used for the rectifying / smoothing circuit 7, it can be discriminated by detecting the polarity of the output potential whether the detecting section 1 is in the water phase or in the gas phase. It is not necessary to have a wide dynamic range of 6. Therefore, a power supply voltage for operating the rectifying / smoothing circuit 7 is, for example, a direct current of about 5 V, which is also one of the features that facilitates the realization of an intrinsically safe explosion-proof structure.

The input impedance of the buffer circuit 6 connected to the bridge exerts a load effect on the bridge. Capacitance values of the detection unit 1 and the capacitor 2 are C _X and C, respectively, and an input impedance of the buffer circuit 6 is a capacitance value C _i.
The potential V ₀ of the bridge output terminal 5 is represented by the following equation. V ₀ = (C−C _X ) / (C + C _X + C _i ) × V _S (V _S : voltage of the high frequency power supply 3) Therefore, if C _i is not sufficiently smaller than C or C _X , the sensitivity will decrease. However, if a field effect transistor is used in the first stage of the buffer circuit 6, the field effect transistor has an input capacitance of as small as a few pF or less, so there is no risk of sensitivity deterioration.

Further, if C _i is small, C and C _X can also be made small, so that the distance between the electrode plates of the detection unit 1 can be made sufficiently large, and water or oil remains between the electrode plates due to surface tension. Is prevented and the reliability of the sensor is improved. Furthermore, the field effect transistor has a high input resistance,
It is also suitable from the viewpoint of reducing the load effect on the bridge.

Next, a specific example of the level sensor according to the present invention will be described with reference to FIGS. FIG. 2 is a specific circuit diagram of the level sensor according to the present invention. The same components as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

In the figure, reference numeral 11 is a capacitor as a coupling capacitance for taking out the potential of the bridge output terminal 5, 12 and 13 are bias resistors, 14 is a junction type field effect transistor, 15 is a load resistance of the field effect transistor, 16 is a PNP transistor, 17
Indicates the source resistance.

The field effect transistors 14, 16 and the like have a so-called complementary compound structure and generate the same voltage as the potential change of the bridge output terminal 5 on the source resistor 17. However, when the bridge unbalanced voltage appearing at the bridge output terminal 5 is large, the amplitude of the voltage on the source resistor 17 is limited due to the saturation of the circuit.

The above-mentioned capacitor 11 and bias resistor 1
2, 13, field effect transistor 14, load resistor 15,
The transistor 16 and the source resistance 17 form the buffer circuit 6 in FIG.

In the figure, reference numerals 18 and 19 denote C-MOS type analog switch ICs for synchronous rectification, and the analog switch ICs 18 and 19 are 0.
It is controlled by rectangular wave voltages (reference voltages) of v and 5v having opposite phases, and the switch is closed when the reference voltage is 5v. Reference numerals 20 and 21 denote reception resistors, and reference numerals 22 and 23 denote smoothing capacitors.

The analog switch ICs 18 and 1 described above
9, receiving resistors 20 and 21, and capacitors 22 and 23
The rectifying / smoothing circuit 7 in FIG.

Point 2 on the circuit sandwiching the capacitors 22 and 23
The DC voltage appearing between 4 and 25 is proportional to the amplitude of the rectangular wave voltage generated in the source resistor 17, and the polarity changes depending on the phase.

Since it is convenient to convert the DC voltage appearing between the points 24 and 25 on the circuit into a voltage with the circuit common (COM) as a reference, the operational resistors 26, 27, 28, 29 and the operational amplifier 30 are used. The calculation is performed and the output is given to the output terminal 8. The conversion gain can be easily changed by appropriately selecting the resistance values of the calculation resistors 26 to 29.

Reference numeral 31 in the figure indicates a multivibrator using a general-purpose timer IC, and the oscillation frequency of this multivibrator 31 is several tens kHz to several hundreds kHz.
Set to an appropriate value within the range. 32 is C- for waveform shaping
MOS type inverter ICs 33 and 34 are 5v Zener diode and Zener current regulating resistor for potential shift, respectively, and 35 is a C-MOS type inverter IC whose operating current is shifted 5v to the negative side of the inverter IC 32, respectively. . The outputs of the inverter ICs 32 and 35 are given to the electrostatic capacitance detector 1 and the capacitor 2 which form a half bridge, and their amplitudes are C-MOS type I.
Since the characteristic of C is almost equal to the power supply voltage, the output of the inverter IC is extremely stable if the power supply voltage is stable.

It is also clear that these phases are in the opposite phase to each other, and the characteristics imposed on the high frequency power supply in the present invention are realized with a simple structure. Reference numeral 36 is also an inverter IC for obtaining a voltage for controlling the analog switch IC 19.

The general-purpose timer IC 31, the inverter IC 32, the Zener diode 33, and the Zener current regulating resistor 3 described above.
4, the inverter ICs 35 and 36 constitute the high frequency power supplies 3 and 4 in FIG.

In the circuit shown in FIG. 2, when an electrode plate having a capacitance of 40 pF when the detecting unit 1 is in the oil phase is used as the detecting unit, the output is output when the detecting unit 1 is in the oil phase. While the output from the terminal 8 was 0 v, the output was about −2 v when in the water phase and about 1 v when in the gas phase.

Next, a specific example of mounting the level sensor according to the present invention on the tank of the oil tanker will be described with reference to FIG. In the figure, reference numeral 40 is an outer wall of a tank for storing a liquid such as petroleum, 41 is a well provided on the bottom of the tank, 42 and 43 are electrode plates of the detection unit 1, and 44 is an in-tank housing for holding the electrode plates. , 45 is a supporting conduit, 46 is a level sensor mounting flange, and 47 is a tank outer housing that accommodates an external wiring terminal board.

The electrode plates 42 and 43 of the detection unit 1 are mounted as shown in FIG. 4, and the inner diameter of the lower end portion of the electric wire tube 45a connected below the in-tank housing 44 is widened to form a stepped portion. 50 is provided, and a fluororesin such as PF is provided in this step.
A cylindrical block 51 of A is attached, and a cap nut 52 is tightened so that the inside of the conduit tube 45a is watertight. The block 51 includes two kinds of conductive columns 53, 54 having different lengths.
Attached in pairs, and the short pole 53 is attached to the upper electrode plate 42.
The lower electrode plate 43 is attached to the long pillar 54. The lower end of the long pillar 54 is projected from the hole 42a formed in the upper electrode plate 42 to the lower surface side of the upper electrode plate 42 so as not to contact the upper electrode plate 42. The distance between the electrode plates is such that water or oil does not stay between the electrode plates due to surface tension when the tank is empty.

Both electrode plates are preferably provided so as to be substantially horizontal in order to enhance sensitivity to fluctuations in the water-oil interface and the oil surface. However, when the electrode plates are provided horizontally, water and oil accumulate on the electrode plates. It will be easier. Therefore, a rising frame 42b is provided around the hole 42a formed in the upper electrode plate 42 so that water and oil remaining on the electrode plate 42 do not drip from the hole 42a onto the lower electrode plate 43. .

The upper electrode plate 42 has an area slightly larger than that of the lower electrode plate 43, and water or oil dripping from the outer periphery of the upper electrode plate 42 does not drip on the lower electrode plate 43. In addition, a large number of small holes 42a are formed in the lower electrode plate 43 so that water or oil on the upper surface of the lower electrode plate 43 can be dropped downward from the small holes 43a. .

Due to the structure of the detecting section 1 described above, the capacity of the detecting section is rapidly changed, and the water-oil interface and the oil surface are quickly detected. When connecting the electrode plates to form a bridge, the lower electrode plate 43 is connected to the bridge excitation side and the upper electrode plate 42 is connected to the bridge output side, which is less susceptible to electrical influence from the tank bottom surface. .

When the temperature of the liquid in the tank is room temperature, all of the electric circuit shown in FIG. 2 may be housed in the housing 44 inside the tank, and the housing 47 outside the tank may be a housing for terminals only. However, when the temperature of the liquid is high, it is desirable to minimize the number of electronic parts in the in-tank housing 44 that are kept at a high temperature to ensure the reliability of the circuit.

In this case, the bridge and buffer circuit 6 may be left in the tank inner casing 44, and the rectifying / smoothing circuit 7, high-frequency power sources 3 and 4 may be separately provided in the tank outer casing 47. A circuit for separately providing the bridge and buffer circuits, the rectifying / smoothing circuit, and the high frequency power source will be described with reference to FIG.

In the figure, the same components as those of the circuit shown in FIG. 2 are not shown. In the figure, reference numerals 60 and 61 denote coaxial cables for supplying high-frequency voltage from a power source in the tank outer casing 46 to a bridge in the tank inner casing 43, and 62 denotes a buffer circuit 6 in the tank inner casing 43. The coaxial cables 63 and 64 for guiding the output to the rectification / smoothing circuit 7 in the tank outer casing 46 are 5
v, common (COM) lead wires, 65, 66, and 67 denote terminating resistors for reducing waveform distortion, and broken line A
The circuit enclosed by is provided in the tank inner casing 44, and the circuit enclosed by the broken line B is provided in the tank outer casing 47.

When the distance between the tank inner casing 44 and the tank outer casing 47 is short and the excitation frequency is low, a normal shielded wire or a general insulation covered wire is used instead of the coaxial cables 60 and 61. Alternatively, the coaxial cable 62 may be replaced with a shield wire. Resistance 65, 66
Also, 67 and 67 need not be strictly matched to the characteristic impedance of the coaxial cable, and may be omitted depending on the case.

Next, the basic structure of the level sensor according to claim 2 of the present invention will be described with reference to FIG. In the figure, reference numeral 70 designates a detection unit composed of a pair of electrode plates, 71 to 73.
Shows fixed-capacity capacitors forming a full bridge, and those capacitors having a capacitance value such that the bridge is almost in equilibrium when the detecting section is in an oil phase as a stored liquid are selected.

Buffer circuits 76 and 77 are connected to the bridge output terminals 74 and 75, respectively.
The outputs from 77 are synchronous rectification / smoothing circuits 78 and 7 respectively.
It becomes direct current by 9. This DC voltage is output by the output terminal 81 after being subtracted by the differential amplifier 80. The high frequency power supply 82 for exciting the bridge is also used for the synchronous rectification / smoothing circuits 78 and 79 and the reference voltage P for the synchronous rectification.
give. Comparing this configuration with the configuration shown in FIG.
In this case, the buffer circuit is connected to the galvanic circuit of the bridge and the influence on the bridge side element is relatively small as described above. In the present embodiment shown in FIG. 6, if a transformer is used in a full bridge, a configuration in which a buffer circuit is connected to a galvanic circuit can be adopted as in the embodiment shown in FIG. 1, but it is intrinsically safe. Transformers cannot be used to provide explosion-proof construction. Therefore, in the configuration of the present embodiment shown in FIG. 6, since the buffer circuit is connected in parallel to the side elements 71 and 73, there is a slight disadvantage in terms of the accuracy of the bridge. However, there is an advantage that the excitation power source is simpler than that of the configuration of FIG. 1, and a simple level sensor can be realized, which is suitable for use in an alarm circuit or the like.

[0050]

The capacitance level sensor according to the present invention has the following effects due to the above-mentioned configuration. Since the high-frequency bridge of the level sensor of the present invention does not require an inductance element, it is easy to realize an intrinsically safe explosion-proof structure, and sufficient safety can be ensured especially when used for unloading petroleum products and the like. it can.

Further, by using a synchronous rectifying / smoothing circuit in which the voltage from the high frequency power source is used as a reference voltage for the rectifying / smoothing circuit, it is possible to distinguish between the case where the detector is in the water phase and the case where it is in the gas phase. Since the polarity of the sensor output only needs to be discriminated, the discrimination is easy, and since the buffer circuit does not require a wide dynamic range, the operating voltage of the synchronous rectification / smoothing circuit can be low and the safety is further improved.

Furthermore, by providing the components other than the bridge and the buffer circuit outside the tank, the electronic circuit is not easily affected by the heat from the liquid even when the liquid temperature of the stored liquid is high, and the reliability of the circuit is improved. Can be improved. In addition, the electrode plate of the detection unit is provided so as to be almost horizontal in the tank, the upper electrode plate is connected to the bridge output side, and the lower electrode plate is connected to the high frequency power source side. Is susceptible to electrical influence from the bottom surface of the tank, but the high-frequency power source side connected to the lower electrode plate is less susceptible to electrical influence, and therefore highly accurate detection can be performed.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a basic configuration of a level sensor according to claim 1 of the present invention.

FIG. 2 is a circuit diagram showing an embodiment of a level sensor according to claim 1 of the present invention.

FIG. 3 is a vertical sectional view showing a mounting structure of a level sensor according to the present invention.

FIG. 4 is a partially longitudinal front view showing the structure of the detection unit.

FIG. 5 is a circuit diagram showing a specific example of a case where a part of an electric circuit of the level sensor according to the present invention is separately attached.

FIG. 6 is a configuration diagram showing a basic configuration of a level sensor according to claim 2 of the present invention.

[Explanation of symbols]

 1 Detector 2 Capacitor 3, 4 High Frequency Power Supply 5 Bridge Output Terminal 6 Buffer Circuit 7 Rectification / Smoothing Circuit 8 Output Terminal 11 Capacitor 12, 13 Bias Resistor 14 Field Effect Transistor 15 Load Resistor 16 PNP Transistor 17 Source Resistor 18, 19 C -MOS type analog switch IC 20,21 Reception resistance 22,23 Capacitor 26, ..., 29 Operational resistance 30 Operational amplifier 31 Multivibrator 32 C-MOS type inverter IC 33 Zener diode 34 Zener current regulation resistance 35 C-MOS type Inverter IC 36 Inverter IC 40 Tank outer wall 41 Well 42, 43 Electrode plate 44 Tank inner casing 45 Conduit 46 Mounting flange 47 Tank outer casing 50 Step 51 Cylindrical block 52 Cap nut 53, 54 Electrode plate support 60 , 61, 62 Coaxial cable 63, 64 Common conductor 65, 66, 67 Termination resistor 70 Detector 71, 72, 73 Capacitor 74, 75 Bridge output terminal 76, 77 Buffer circuit 78, 79 Rectification / smoothing circuit 80 Differential amplifier 81 Output terminal 82 High frequency power supply

Claims (2)

[Claims] 1. A detection unit comprising a pair of electrode plates insulated from an inner wall of a tank is provided near a bottom plate of a tank for storing a liquid having a relative dielectric constant lower than that of water. In a level sensor for detecting a surface and an interface between a stored liquid and water, a high frequency half bridge having one side of the detection section and being substantially balanced when the stored liquid exists between electrode plates of the detection section, and an output of the half bridge A buffer circuit for impedance-converting the voltage, a rectifying / smoothing circuit for rectifying and smoothing the output of the buffer circuit to obtain a sensor output, and a high-frequency power source for forming the half bridge are provided, and the detection is performed from the sensor output. The liquid existing between the electrode plates of the parts is either stored liquid or water, or neither stored liquid nor water is present to identify the surface of the stored liquid and the liquid. Capacitive level sensor for detecting the interface of the storage liquid and water. 2. A detection unit comprising a pair of electrode plates insulated from the inner wall of the tank is provided near the bottom plate of the tank that stores a liquid having a lower relative dielectric constant than water, and the capacitance of the stored liquid is determined from this capacitance value. In a level sensor for detecting a surface and an interface between a stored liquid and water, a high-frequency full bridge which has the detection section as one side and is substantially balanced when the stored liquid exists between the electrode plates of the detection section, and a full bridge of this full bridge. The sensor includes a pair of buffer circuits for impedance-converting the output voltage, a rectifying / smoothing circuit for rectifying and smoothing the output of the buffer circuit to obtain a sensor output, and a high-frequency power supply for exciting the full bridge. From the output, it is discriminated whether there is a stored liquid or water between the electrode plates of the detection unit, or neither the stored liquid nor the water is present, and the surface of the stored liquid and Capacitive level sensor for detecting the interface between the built liquid and water.