GB2207503A

GB2207503A - Boiling detection device

Info

Publication number: GB2207503A
Application number: GB08716986A
Authority: GB
Inventors: Stephen Roland Day
Original assignee: Thorn EMI PLC
Current assignee: Thorn EMI PLC
Priority date: 1987-07-18
Filing date: 1987-07-18
Publication date: 1989-02-01
Also published as: GB8716986D0

Abstract

A device for the detection of the boiling of a liquid in a vessel, such as a domestic kettle 1, comprises: a sound pressure sensor 4 placed within the kettle 1 to detect vibration within the kettle 1, the sound pressure sensor 4 including a transducer to generate electrical signals from the sound impulses of the vibration; means 5,6, to process said electrical signals such that the sound impulses characteristic of the liquid at boiling point are distinguishable from other sound impulses present in the kettle 1, to produce a 'boiling detected' signal; and a control means 14, 15 for a heating element 3 placed within the kettle 1, the control means 14, 15 being responsive to the 'boiling detected' signal, to switch off the heating element 3. <IMAGE>

Description

BOILING DETECTION DEVICE This invention relates to a boiling detection device, more particularly to a boiling detection device for electric kettles.

Various methods of detecting the boiling of a liquid in a vessel, particularly a kettle, are well known. Conventional automatic kettles employ a bimetal strip which is responsive to the steam produced by the boiling water. However, for such a method to work efficiently great care must be taken when placing the strip or else it will not function properly at low water levels; additionally, it will not work at all if the lid is not in place. The lids of modern plastic bodied kettles are designed to force steam past the sensor, and because of this, are not the most convenient shape for the consumer.

A better method for detecting the boiling of water in a kettle is to detect the level of noise or vibration present. GB 2143053A uses the phenonemon of pan vibration to control the level of heat input immediately after boiling has been reached.

This is done by incorporating a vibration sensor into the hotplate. However, the device described in this application is not suitable for use in a kettle, where often already hot water is brought back to the boil, since its operation relies on monitoring the level of vibration as the water gradually heats up. In addition, the device described is not capable of distinguishing between boiling noises and other interfering domestic noises; more particularly, it is not capable of detecting and distinguishing the 'type' of noise in a rolling boil (i.e. continued full boiling of a liquid) from other boiling noises.

It is therefore an object of the present invention to provide a device for the detection of the boiling of a liquid in a vessel, particularly a kettle, which alleviates the aforementioned disadvantages.

According to the present invention there is provided a device for the detection of the boiling of a liquid in a vessel, the device comprising: a sound pressure sensor placed within the vessel to detect vibration within the vessel, said sound pressure sensor including a transducer to generate electrical signals from the sound impulses of the vibration; means to process said electrical signals such that the sound impulses characteristic of the liquid at boiling point are distinguishable from other sound impulses present in the vessel, to produce a 'boiling detected' signal; and a control means for a heating element placed within the vessel said control means being responsive to the 'boiling detected' signal, to switch off the heating element.

Preferably, the signal processing means comprises: a first filter and first amplifier to remove background noise from the signal; a rectifier to rectify the signal; a second filter to obtain a local mean signal amplitude level from the rectified signal; a second amplifier to amplify the local mean signal amplitude level by a predetermined constant amount a first comparator to compare the rectified signal with the amplified local mean signal amplitude level and to produce a pulse when the former exceeds the latter; a third filter to filter out multiple closely spaced pulses produced by the comparator; a fourth filter to test for a significant number of pulses over a predetermined period of time; and a second comparator to compare the output of the fourth filter with a predetermined threshold and produce a 'boiling detected' signal when the former exceeds the latter.

Preferably, the third filter comprises retriggerable monostable.

Preferably, the fourth filter comprises a low pass Bessel filter.

The invention will now be described by way of example only and with reference to the accompanying drawings of which, Figure 1 shows the sound response in the water of a kettle at full boil, Figure 2 shows schematically a practical working embodiment of the invention.

The time domain sound response in the water of a non-resonant kettle body at full boil consists of many discrete impulses, with typically five larger pulses occurring per second; an example of such a sound response is shown in Figure 1. The presence of these repetitive larger pulses in a sound response is indicative of the noise of a so-called rolling boil, which can hence be easily distinguished from other boiling noises and any interfering domestic noises in the kettle's environment - including knocks to the kettle. The detection of a statistically large number of pulses can be carried out by a sound pressure sensor in conjunction with the requisite electronic circuitry.

Figure 2 shows a practical working embodiment of the invention. A kettle 1 contains water 2 which is heated by a heater 3. The vibrations or noises produced in the water 2 are detected by a sound pressure sensor 4 placed in the water close to the heater 3. A suitable sound pressure sensor 4 is a hydrophone which comprises a transducer which generates electrical signals in response to the water-borne sound impulses.

The signal from the sound pressure sensor 4 is amplified by an amplifier 5 and filtered by a filter 6, band pass operating in 1-2kHz band, so as to remove as much background and insignificant signal as possible. The signal is then rectified by a rectifier 7 and used to obtain a local mean signal amplitude level. This is done by a low pass filter 8. The time over which this mean is taken must be fairly short, for example a period of approximately T = owls, to reduce false alarms due to say, sudden rises in the volume of interfering domestic or various pre-boiling noises. This mean signal level is multiplied by a predetermined constant by an amplifier 9 before entering an input A of a comparator 10 which compares the amplified local mean level with the rectified signal from rectifier 7, which enters the comparator 10 through input B.If the rectified signal exceeds the amplified local mean level, the comparator 10 produces a short pulse for the time duration that this condition is present. The signal from the comparator 10 is then fed into a retriggerable monostable 11, with a pulse length t = 0.05s. The monostable 11 acts as a further filter, filtering out multiple, closely-spaced pulses which are 'false alarms' resulting from a sudden increase in background noise level. A low pass Bessel filter 12 is utilized in the final stages of the signal filtering to test for a significant number of detected pulses over a few seconds, typically a period of T = 5s. The signal from the Bessel filter 12 then enters an input A of a comparator 13, which compares it with a predetermined threshold entering in input B. If the water 2 has been boiling for a significant period determined by the Bessel filter 12, then the output signal from the Bessel filter 12 will exceed the threshold and the comparator 13 sends out a 'boiling detected' signal which enters an R input of an SR logic latch 14. A signal is sent from the output Q of the SR logic latch 14 to a triac mains switching device 15 which then switches off the heater 3.

The heater 3 is switched on by a switch 16 which sends a signal to the S input of the SR logic latch which signals to the triac mains switching device 15 to turn the heater 3 on.

Thus the present invention provides a device for detecting the boiling point of water in a kettle, by identifying a characteristic type of noise produced by water at its boiling point. Since the noise in the water close to the heating element is substantially independent of the depth of the water, the boiling point is detected accurately even for low water levels. In addition, the absence or the incorrect placing of a lid does not affect the operation of the detection device.

Claims

1. A device for the detection of the boiling of a liquid in a vessel, the device comprising: a sound pressure sensor placed within the vessel to detect vibration within the vessel, said sound pressure sensor including a transducer to generate electrical signals from the sound impulses of the vibration; means to process said electrical signals such that the sound impulses characteristic of the liquid at boiling point are distinguishable from other sound impulses present in the vessel, to produce a 'boiling detected' signal; and a control means for a heating element placed within the vessel said control means being responsive to the 'boiling detected' signal, to switch off the heating element.

2. A device according to Claim 1, wherein the signal processing means comprises: a first filter and first amplifier to remove background noise from the signal; a rectifier to rectify the signal; a second filter to obtain a local mean signal amplitude level from the rectified signal; a second amplifier to amplify the local mean signal amplitude level by a predetermined constant amount; a first comparator to compare the rectified signal with the amplified local mean signal amplitude level and to produce a pulse when the former exceeds the latter; a third filter to filter out multiple closely spaced pulses produced by the comparator; a fourth filter to test for a significant number of pulses over a predetermined period of time; and a second comparator to compare the output of the fourth filter with a predetermined threshold and produce a 'boiling detected' signal when the former exceeds the latter.

3. A device according to Claim 2, wherein the third filter comprises retriggerable monostable.

4. A device according to Claim 2 or Claim 3, wherein the fourth filter comprises a low pass Bessel filter.

5. A device substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.