JPH0713628B2 - Liquid boiling detector - Google Patents

Description

The present invention relates to a boiling detection device for water and other liquids.

(Prior Art) Conventionally, as a method for detecting boiling of water or other liquid,
A method of detecting boiling with a temperature sensor has been proposed.
For example, in an electric pot or the like, by utilizing the fact that the generation of steam becomes remarkable when water completely boils in the container, the steam generated in the inner container of the electric pot main body is covered with a hinge inside the lid. A temperature sensor that detects the temperature of the steam is installed on the way to the air pump, and a discharge path is provided to let the steam escape to the outside on the way from the inner container to the bellows of the air pump. The boiling is being detected at.

(Problems to be solved by the invention) However, in the boiling detection device adopting this type of boiling detection method, the output of the temperature sensor provided on the lid is transmitted to the control circuit on the electric pot body side, and therefore, the lead of the temperature sensor is used. There is a drawback that the structure and assembly are complicated because it is necessary to wire the wires through the hinge part between the electric pot main body and the lid, and this is a big problem in the recent trend toward compact electric pots. ing. Moreover, it is extremely difficult for the conventional boiling detection device to detect boiling of the liquid in the container without the lid.

(Means for Solving Problems) As a means for solving the above problem, the present invention is basically a vibration for converting a vibration of a liquid or a container caused by a temperature rise of the liquid into an electric signal. A sensor, boiling reference voltage setting means for setting a value between the boiling point output voltage of the vibration sensor and the equilibrium level output voltage as a boiling reference voltage, and comparing the output voltage of the vibration sensor with the boiling reference voltage. Then, a vibration level determiner that outputs a signal each time the output voltage of the vibration sensor reaches the boiling reference voltage, the detection operation start reference voltage is a voltage above the boiling reference voltage, the output voltage of the vibration sensor is A boiling detection operation start determination means for outputting a signal for starting the boiling detection operation when the detection operation start reference voltage is reached, and an output signal of the boiling detection operation start determination means A liquid boiling detection device is configured with a boiling determination device that starts boiling detection operation and determines boiling based on the signal of the vibration level determination device that is output when the output voltage of the vibration sensor reaches the boiling reference voltage. It was done like this.

In the embodiment of the present invention, as shown in FIG. 3, the boiling detection operation start determination means (7) has a temperature sensor (9) for detecting the temperature of the liquid and a detection operation corresponding to the detection operation start reference temperature. Reference temperature setting means (R7, VR2) for setting the operation start reference voltage (V02) and the detection operation start timing by comparing the output voltage of the temperature sensor (9) with the detection operation start reference voltage When the boiling judging device (8) starts the boiling detecting operation by the output signal of the boiling detecting operation start judging means and the output voltage of the vibration sensor reaches the boiling reference voltage, It is composed of an AND gate for judging boiling based on the output signal of the vibration level judging device.

In another embodiment, the boiling detection operation start determination means, boiling reference voltage setting means (5) that also functions as boiling detection start reference voltage setting means for setting the detection operation start reference voltage (V02), A vibration level determiner (6),
It comprises a series circuit consisting of a capacitor (C3) and a resistor (R8) for gradually increasing the output voltage of the vibration level judging device (6), and the boiling judging device (14) is a boiling judging reference voltage (V03).
Is equipped with a boiling judgment reference voltage setting means (R9, VR3) for starting the boiling detection operation by the first output of the vibration level judging device (6), and by the output again, between both ends of the capacitor (C3). The boiling is judged by comparing the voltage with the reference voltage (V03).

In the embodiment of the boiling detection device according to the present invention, in order to prevent malfunction due to external vibration, the signal from the vibration sensor 3 is detected by utilizing the fact that not only the output voltage but also the frequency band changes as the water temperature rises. A noise filter such as a bandpass filter is used to detect only the frequency components including the frequency band at the time of boiling.

Further, in the embodiment of the present invention, a bandpass filter 20 is inserted between the vibration sensor 3 and the vibration level determiner 6 to pass only an output component of a frequency or frequency band within a certain range. When the liquid is water, it is preferable to employ a bandpass filter that passes only the output component of the frequency or frequency band within the range of 400 to 3000 Hz as the bandpass filter.

In another embodiment, a bandpass filter of 400-3
Instead of detecting boiling with an output component in the frequency range or frequency range of 000 Hz, a piezoelectric vibration sensor having a resonance frequency in the range of 400 to 3000 Hz is adopted.

(Operation) In the liquid boiling detection device according to the present invention, it was found that there is a certain correlation between the temperature of the liquid and the vibration of the liquid or the container that occurs with the temperature rise of the liquid, and the findings have been made. It was completed based on this.

That is, as shown in FIG. 1, an electric heater 2 is attached to the lower part of the side wall of the container 1, a vibration sensor 3 is attached to the bottom of the container 1, and a liquid, for example, water W is put into the container 1 and heated. When the output of the vibration sensor 3 was measured via the amplifier 4, the result shown in FIG. 2 was obtained.

As is clear from FIG. 2, when the liquid contained in the container is heated, vibration of the liquid or the container begins to occur as the temperature rises, and the vibration sensor 3 converts the vibration into an electric signal. When the vibration level is detected, the change in the output of the vibration sensor 3 is extremely small while the water temperature is low (a), but when the water temperature exceeds a certain temperature (60 ° C. in the case of the embodiment), the vibration sensor 3 Output level starts to increase (b), and the output level increases rapidly as the water temperature rises. When the temperature further rises and reaches a constant temperature (90 ° C. in the example described later), the output level reaches the peak value (d), and when the water temperature further rises, the output level of the vibration sensor 3 begins to decrease sharply, Even when water reaches the boiling point (point E), the output level further decreases from the value when it reaches the boiling point (VB), and if boiling is continued as it is, the output level equilibrates at a constant value (VL) (f). Shows the phenomenon.

Although it has not been elucidated for the reason why such a phenomenon occurs,
Observing the change of the output level of the vibration sensor 3 and the state of the liquid in the container 1, the output level is small at a low temperature where the generation of bubbles is hardly observed, and at the same time when the bubbles start to be generated, the output level of the vibration sensor 3 Since the output level of the vibration sensor 3 increases as the bubbles start to increase and the generation of bubbles increases, it is presumed that this is caused by the generation and disappearance of bubbles accompanying the temperature rise of the liquid.

Therefore, while the vibration generated by the temperature rise of the liquid is converted into an electric signal by the vibration sensor 3 to detect the vibration level, the boiling reference voltage setting means 5 outputs the output voltage (VB ) And the output voltage (VL) when reaching the equilibrium level are set as boiling reference voltage V0, and the voltage corresponding to the vibration level detected by the vibration sensor is higher than the reference voltage V0. A boiling point deciding device 8, which will be described later, is used to detect a change point from a high state to a state lower than the reference voltage V0.
Complete boiling can be detected by detecting in 14.

On the other hand, as a result of a more detailed study of the correlation between the liquid temperature or the vibration of the liquid or the container that occurs with the temperature rise of the liquid, the results show that the vibration within the frequency band It has been clarified that the frequency of is closely related to the boiling of the liquid at the temperature immediately before boiling to the boiling (95 ℃ to 100 ℃ when the liquid is water).

That is, in the apparatus shown in FIG.
When the vibration sensor 3 detects the vibration of water or the container 1 while maintaining the temperature at ℃ or 100 ℃, and the relationship between the output voltage of the amplifier 4 and the frequency corresponding to the vibration level is measured, the result shown in FIG. 8 is obtained. Further, when the temperature of the water is raised by extracting only the signal component of the frequency (f) within the range of 100 to 3000 Hz from the output signal of the amplifier 4 through the band pass filter, the result shown in FIG. 9 is obtained. was gotten.

In order to avoid the figure from becoming complicated, the results of the components with frequencies (f) of 100 Hz, 200 Hz, 300 Hz and 400 Hz are shown in Fig. 9a, and the results of the components with frequencies (f) of 500 Hz and 700 Hz are shown in Fig. 9b. 800Hz, 9
The results of the components of 00Hz, 1000Hz, 2000Hz and 3000Hz are shown respectively.

As is clear from the results shown in FIGS. 8 and 9, a large vibration level difference occurs in the frequency range of 400 to 3000 Hz when the temperature of water is 95 to 100 ° C., but it is remarkable at the frequency of 100 to 300 Hz. It does not show any change in vibration level. Although not shown, at a frequency of 3000 Hz or higher, the vibration level is small and the vibration level does not change significantly.

Therefore, the accuracy can be improved by extracting only the output component of the frequency or frequency band in which the output corresponding to the vibration level detected by the vibration sensor is in the range of 400 to 3000 Hz, and detecting the boiling. Further, it is preferable to detect boiling using an output component of a specific frequency or frequency band within the range of 700 to 900 Hz even within the above range of frequencies.

(Example) Hereinafter, an example in which the liquid boiling detection device according to the present invention is applied to an electric pot will be described with reference to the accompanying drawings.

FIG. 3 shows an embodiment of the boiling detection device according to the present invention,
The device basically comprises an inner container 1 as shown in FIG.
Piezoelectric vibration sensor 3 for converting vibration due to temperature rise of liquid inside into electric signal, and boiling reference voltage setting means 5 for setting boiling reference voltage V0 corresponding to boiling temperature of liquid
And the output voltage of the vibration sensor 3 and the boiling reference voltage
It is configured with a vibration level determiner 6 that compares with V0 and detects a transition point at which the output voltage of the vibration sensor 3 changes to the boiling reference voltage V0 or less. In the present embodiment, in addition to these, Boiling detection operation start determination means 7 which outputs a signal for starting the boiling detection operation when the liquid temperature reaches a predetermined temperature.
Is provided, the detection operation is started in response to a signal from the boiling detection operation start determination means 7, and boiling is performed by the output of the vibration level determination device 6 when the output voltage of the vibration sensor 3 reaches the boiling reference voltage V0. A boiling determining device 8 for detecting is provided.

As shown in FIG. 1, the vibration sensor 3 is mounted on the bottom wall of the inner container 1, outputs a signal corresponding to the vibration level, and the output signal is input to the amplifier 4 and the amplifier 4 outputs the signal. After being amplified by the operational amplifier 11, the impedance is converted by the field effect transistor FET, smoothed by the capacitor C2, and output as a voltage (Va) corresponding to the vibration level.

The vibration level determiner 6 is a comparator 12 which is a voltage comparator.
The vibration sensor 3 has an inverting input terminal.
The output voltage (Va) of the amplifier 4 corresponding to the output signal of is input to the non-inverting input terminal of the resistor R5 and the variable resistor VR1.
The boiling reference voltage V0 set by the boiling reference voltage setting means 5 composed of a series circuit is input.

The boiling detection operation start determination means 7 is composed of a temperature sensor 9 which is a thermistor, an operation start reference temperature setting means which is a series circuit of a resistor R7 and a variable resistor VR2, and a comparator 13 which is a voltage comparator. The output of the sensor 9 is input to the inverting input terminal of the comparator 13, and the non-inverting input terminal thereof receives the operation start reference voltage V02 corresponding to the operation start reference temperature set by the resistor R7 and the variable resistor VR2. . The operation start reference voltage V02 is set to a value between the boiling reference voltage V0 and the output peak value of the vibration sensor because the boiling detection operation is started when the liquid temperature reaches a predetermined temperature. In the embodiment, the voltage is set when the liquid temperature reaches 85 degrees.

The boiling judging device 8 is composed of an AND gate, and the boiling detecting operation start judging means 7 is outputted when the liquid temperature reaches a predetermined temperature.
The vibration sensor 3 starts the detection operation in response to the signal from the
The boiling is judged by the output of the vibration level judging device 6 when the output voltage of 1 reaches the boiling reference voltage V0.

The operation of the liquid boiling detection apparatus will be described. When water is put in the inner container 1 and the heater 2 is energized, the output voltage Va from the amplifier 4 is maintained while the water temperature is low as shown in FIG. Is lower than the boiling reference voltage V0 set by the boiling reference voltage setting means 5, the output voltage Vb of the vibration level determiner 6 is
Becomes high level as shown in FIG. 4, but the output voltage Vc from the boiling detection operation start determination means 7 is the temperature sensor 9
Has a large resistance value, and the voltage applied to the inverting input terminal of the comparator 13 is larger than the operation start reference voltage V02 set by the resistor R7 and the variable resistor VR2, the level becomes low and the boiling judgment device 8 detects boiling. Do not do.

When the water temperature rises and reaches around 60 ° C (b), the output voltage Va of the amplifier 4 starts to increase, and the water temperature further rises and the output voltage Va
Reaches the reference voltage V01 (C), the vibration level determiner 6
Output voltage is inverted, the boiling detection operation start determination means 7
Since the output voltage Vc of 1 does not change, the boiling determiner 8 is maintained in the above state. Water temperature rises and operation starts Reference voltage V02
When the temperature corresponding to, for example, 85 ° C. is reached, the output Vc of the boiling detection operation start determination means 7 is inverted to the high level, and the boiling determination device 8 starts the boiling detection operation. When the water temperature further rises, the output voltage Va from the amplifier 4 starts to decrease after reaching the peak value, and when the water reaches the boiling point, the output voltage Va from the amplifier 4 reaches the reference voltage V01 (e) and vibrates. Since the output voltage Vb of the level determiner 6 is inverted to the high level, the boiling determiner 8 determines that the water has boiled and outputs a signal. Therefore, an alarm can be issued by sounding a buzzer or the like by this output signal.

The boiling detection device can be applied, for example, as a power control device for a boiling electric pot.

FIG. 5 shows an embodiment thereof. In this boiling type electric pot, as shown in FIG. 1, a band heater 2 is attached to a lower part of a side wall of an inner container 1, and a boiling water heater 2a of the band heater 2 and A liquid in the inner container 1, for example, water W is heated and boiled by energizing the heat retention heater 2b, the complete boiling of water is detected by the boiling detection device according to the present invention, and the signal is input to the heater control circuit. The relay Ry is operated to stop energizing the heater 2a for boiling water, and thereafter, the thermal reed switch TRS is turned on / off to intermittently energize the warming heater 2b to keep it warm at a predetermined temperature. Reboil switch so that it can be reboiled accordingly
SW is provided.

D1 is a reverse voltage absorbing diode, 15 is a DC power supply circuit, and is composed of a power transformer PT, a bridge rectifier circuit DB consisting of a diode, a smoothing capacitor C4, a current limiting resistor R10 and a stabilizing zener diode ZD. There is. Thermostat TS1, TS2 and thermal reed switch TR2
Is mounted on the inner container 1 as shown in FIG.
The thermostat TS2 is for turning off the power when the container is not heated and is not heated, and is usually set to a temperature of about 110 ° C. and disposed near the band heater 2 of the container. .

In this embodiment, when water having a low temperature is heated to detect its boiling, the temperature at which the output voltage of the vibration sensor 3 becomes equal to the voltage at the time of boiling is detected twice (about 70 ° C. in FIG. 2). And 100 ℃) so I try to use both.

The operation of the electric pot having the above structure will be described with reference to FIG. First, when the water W is put into the inner container 1 to boil water, the thermostats TS1 and TS2 and the thermal reed switch are used because the water temperature is low as shown in part A of FIG.
Both TRSs are on. For this reason, electric power is supplied to the boiling water heater 2a and the warming heater 2b through the thermostats TS1 and TS2 and the thermal reed switch TRS at the same time when power is supplied, and the boiling detection device is supplied with DC power from the power supply circuit 15.

The operation of the boiling detection device is basically the same as that of FIG. 3, but since the capacitor C3 is connected to the output terminal of the comparator 12, the input voltage to the boiling judgment device 14 composed of the comparator is
The rise of Vc is slightly different. That is, at the beginning of the water heater,
Since the output voltage of the vibration sensor 3 is small, the output of the vibration level determiner 6 becomes high level as described above, but since the capacitor C3 is in the discharging state, the input voltage Vc to the non-inverting input terminal of the boiling determiner 14 Does not instantly go high, but gradually increases during the transient period until the capacitor C3 is charged according to the time constant determined by the resistors R8 and C3. Therefore, until the input voltage Vc reaches the reference voltage V03 set by the resistor R9 and the variable resistor VR3, the output Ve of the boiling determiner 14 becomes low level and the relay Ry is turned on. When the input voltage Vc reaches the reference voltage V03, the output Ve of the boiling determiner 14
Is reversed and the relay Ry is turned off. At this time, since the water temperature is not yet high, the thermostat TS1 maintains the ON state, and the electric power is maintained to be supplied to the water heater 2a.

When the water temperature rises and the output from the vibration sensor 3 increases and the output voltage Va of the amplifier 4 exceeds the reference voltage V01,
The output of the comparator 12 is inverted, and as a result, the output voltage Ve of the boiling determiner 14 is also inverted, so that the relay Ry is turned on. When the water temperature further rises and reaches 90 ° C., both the thermostat TS1 and the thermal reed switch TRS are turned off, but since the relay Ry is closed, the power supply to the water heater 2a is maintained.

When the water temperature exceeds 90 ° C., the output voltage of the vibration sensor 3 gradually decreases, so the output voltage Va of the amplifier 4 also gradually decreases, and when the water temperature reaches the boiling point, the output voltage Va becomes the reference voltage.
When it reaches V0, the output of the comparator 12 is inverted from low level to high level. At this time, since the capacitor C3 is discharged, as in the case described above, the input voltage Vc to the boiling determiner 14 gradually increases, reaches the reference voltage V02 after a predetermined time has elapsed,
As a result, the output voltage Ve of the boiling determiner 14 is inverted from the low level to the high level, so that the relay Ry is turned off, the electric power supply to the water heater 2a is stopped, the water heating is completed, and the heat retaining operation is started.

Therefore, in the present embodiment, the boiling detection operation start determination means,
The boiling reference voltage setting means (5) that also functions as boiling detection start reference voltage setting means for setting the detection operation start reference voltage (V02), the vibration level determination device (6), and the vibration level determination device (6). Capacitor (C3) that gradually increases the output voltage
And a resistor (R8) in series circuit, the boiling judging device (14) is equipped with boiling judging reference voltage setting means (R9, VR3) for setting the boiling judging reference voltage (V03), and the vibration level judging device (R9, VR3). Start the boiling detection operation by the first output of 6), compare the voltage across the capacitor (C3) with the reference voltage (V03) by the output again, detect boiling, and stop the boiling detection operation. become.

The thermal reed switch TRS is turned on when the water temperature drops to a predetermined temperature, for example, 87 ° C or lower, and the predetermined temperature, for example,
Since it is turned off when the temperature rises above 90 ° C, the thermal reed switch TRS is turned on / off during the heat retention operation to turn on / off the power supply to the heat retention heater 2b and keep the water temperature within a predetermined range. Be drunk During energization of the heat retention heater 2b, the power of the heat retention heater is small, so that the liquid or the container hardly vibrates, and the output of the vibration sensor 3 is extremely small, so that the boiling detection device does not operate.

If you want to reboil to obtain boiling water while keeping warm, you just need to press the reboil switch SW. That is, the operation at this time is, as shown in the area B in FIG. 6, the reboil switch.
When SW is pushed by hand and turned on temporarily, the electric charge of the capacitor C3 is discharged, so the output voltage of the comparator 12 is inverted from the high level to the low level, and the input voltage Vc to the boiling judging device 14 is changed.
Tries to rise gradually by charging the capacitor C3, but since the temperature of the hot water is already high at around 90 ° C at this time, the output of the vibration sensor 3 increases immediately and the output voltage Va of the amplifier 4 becomes the reference voltage. When V0 is exceeded, the output of the comparator 12 is maintained at low level. Therefore, the input voltage Vc to the boiling determiner 14 drops during the rise, and the output of the boiling determiner 14
Ve remains low. Therefore, the relay Ry is turned on at the same time as the reboil switch SW is turned on, and the electric power to the water heater 2a is maintained until it is completely boiled. When complete boiling, the output voltage Va of the amplifier 4 reaches the reference voltage V0, the relay Ry is turned off by the same operation as before, and the water heater 2a
Is turned off.

When cold water is replenished while keeping warm, the thermostat TS1 is turned on as shown in the area C of FIG. 6, and the boiling detection device performs the same operation as the operation in the area A.

In the embodiment of FIG. 6, instead of setting the output voltage VB when the boiling point is reached as the reference voltage V0, the output voltage VL when the equilibrium level is reached may be set as the reference voltage V0. Alternatively, an arbitrary output voltage between the output voltage VB when the boiling point is reached and the output voltage VL when the equilibrium level is reached may be set as the reference voltage V0. Therefore, in the present invention, the reference voltage corresponding to the boiling of the liquid is the output voltage of the vibration sensor 3 when the liquid reaches the boiling point.
It means a voltage set within a range between VB and the output voltage VL when the equilibrium level is reached.

FIG. 7 shows another embodiment of the present invention, in which the vibration sensor 3
A bandpass filter 20 is inserted between the vibration level determiner 6 and the vibration level determiner 6, and a frequency 40 showing a particularly remarkable level change in the output voltage of the vibration sensor 3 from immediately before boiling to during boiling.
A specific frequency in the range of 0 to 2000 Hz, 900 in this example.
Only the output voltage of Hz is taken out, the output voltage is inputted to the vibration level judging device 6 to detect boiling more, and the controller 21 controls energization to the heater of the electric pot by the boiling detection signal. Is.

In this way, by detecting only the component of the specific frequency, boiling can be detected reliably and accurately.

Further, instead of using the bandpass filter 20, a vibration sensor having a resonance frequency in the range of 400 to 3000 Hz can be used as the vibration sensor 3.

As the vibration sensor, other than the piezoelectric type vibration sensor made of piezoelectric ceramic, composite material, Rochelle salt, plastic material, electrokinetic type vibration sensor, semiconductor gauge type vibration sensor, etc. From the standpoint of realization, a piezoelectric type is preferable.

(Effect of the invention) As is apparent from the above description, the present invention is designed to detect the boiling of the liquid by the vibration of the liquid or the container. Therefore, the vibration sensor can be directly attached to the container to detect the boiling. When applied to electric pots, etc., the structure can be simplified and assembly at the time of production can be facilitated, the electric pots, etc. can be downsized, and boiling can be detected even when a container without a lid is used. It has excellent effects such as being able to. Further, by detecting only the output component of a certain frequency within a specific frequency band, that is, the output component of the frequency band from the output of the vibration sensor to detect boiling, the boiling can be detected reliably and accurately.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram of an electric pot to which a liquid boiling detection device according to the present invention is applied, FIG. 2 is a graph showing the relationship between water temperature and output of a vibration sensor, and FIG. FIG. 4 is an electric circuit diagram of the liquid boiling detection apparatus according to the present invention, FIG. 4 is a time chart showing voltage changes and operations of respective parts during its operation, and FIG.
FIG. 7 is an electric circuit diagram of an electric pot to which the liquid boiling detection device according to the present invention is applied, FIG. 6 is a diagram showing changes in voltage at each part during its operation, and FIG. 7 is another embodiment of the present invention. FIG. 8 is a block diagram showing a configuration of a boiling detection device showing an example, FIG.
The figure which shows the relationship between the vibration level and the frequency in the device of the figure,
FIG. 9 is a diagram showing the relationship between the temperature and the vibration level at a constant frequency. 1 ... container, 2 ... electric heater, 3 ... vibration sensor,
4 ... Amplifier, 5-boiling reference voltage setting means, 6 ... Vibration level judging device, 7 ... Boiling detection operation start judging means, 8,
14 ... Boiling judgment device, 9 ... Temperature sensor, 11 ... Operational amplifier, 12, 13 ... Comparator (voltage comparator), FET ...
… Field effect transistors, C1 to C4 …… Capacitors, R1 ～
R10 ... Resistor, VR1, VR2, VR3 ... Variable resistor.

Claims (5)

[Claims] 1. A vibration sensor (3) for converting a vibration of a liquid or a container caused by a temperature rise of the liquid into an electric signal, and an output voltage (VB) when the boiling point of the vibration sensor (3) is reached. Boiling reference voltage setting means (5) for setting a value between the equilibrium level output voltage (VL) as the boiling reference voltage (V0), the output voltage of the vibration sensor (3) and the boiling reference voltage (V0). And compare the vibration sensor (3)
A vibration level determiner (6) that outputs a signal each time the output voltage of the boiling reference voltage (V0) reaches the boiling reference voltage (V0), and a voltage above the boiling reference voltage (V0) as the detection operation start reference voltage (V02), Boiling detection operation start determination means (7) that outputs a signal for starting the boiling detection operation when the output voltage of the vibration sensor (3) reaches the detection operation start reference voltage (V02).
And a vibration level output when the boiling detection operation is started by the output signal of the boiling detection operation start determination means (7) and the output voltage of the vibration sensor (3) reaches the boiling reference voltage (V0). A boiling detection device for liquid, comprising a boiling judgment device (8, 14) for judging boiling based on a signal from the judgment device (6). 2. The boiling detection operation start determination means (7),
A temperature sensor (9) for detecting the temperature of the liquid and a detection operation start reference voltage (V0 corresponding to the detection operation start reference temperature
Reference temperature setting means (R7, VR2) for setting 2) and a comparator (1 for judging the detection operation start time by comparing the output voltage of the temperature sensor (9) with the detection operation start reference voltage.
3) and the boiling judging device (8) starts the boiling detecting operation by the output signal of the boiling detecting operation start judging means,
The boiling detection device according to claim 1, comprising an AND gate that determines boiling based on a signal from a vibration level determiner that is output when the output voltage of the vibration sensor reaches the boiling reference voltage. 3. Boiling reference voltage setting means (5), wherein the boiling detection operation start judging means also functions as boiling detection start reference voltage setting means for setting a detection operation starting reference voltage (V02), and a vibration level judging device. (6) and a series circuit composed of a capacitor (C3) and a resistor (R8) for gradually increasing the output voltage of the vibration level judging device (6), and the boiling judging device (14) has a boiling judgment reference voltage (V03). ) Is set for boiling judgment reference voltage setting means (R9, VR3), the boiling level detecting operation is started by the first output of the vibration level judging device (6), and the output between the two ends of the capacitor (C3) The boiling detection apparatus according to claim 1, wherein the boiling is determined by comparing the voltage of V.sub.2 with a reference voltage (V03). 4. A filter which passes a signal within a range of 400 to 3000 Hz between the vibration sensor (3) and the vibration level judging device (6) is connected. The boiling detection device according to any one of items 1 to 3. 5. The boiling detection device according to any one of claims 1 to 3, wherein the vibration sensor (3) is a vibration sensor having a resonance frequency in a range of 400 to 3000 Hz.