Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
2003-12-09
2004-11-16
Pelham, Joseph (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S438000, C219S492000, C219S494000, C219S497000, C099S333000
Reexamination Certificate
active
06818867
ABSTRACT:
TECHNICAL FIELD
The present invention relates to methods and apparatuses for heating liquid in an electric kettle.
BACKGROUND
Electric kettles have long been used to heat liquids such as water. Generally, electric kettles include a temperature sensor to measure the temperature of the liquid being heated. Many kettles include a time sensor as well.
It is often possible to determine when the liquid contained in the kettle has reached its boiling point by comparing temperature and time measurements. More specifically, when the temperature of the liquid being heated ceases to change with respect to time, it can be deduced that the liquid's boiling point has been reached. This is because liquid generally cannot be heated to a temperature above its boiling point. Upon reaching the boiling point, a phase transition takes place and all energy is utilized to convert the liquid into gas rather than to heat the liquid. A method of determining when a liquid has reached its boiling point, similar to the method discussed above, was disclosed in European patent application EP 0 380 369 A1, which is incorporated by reference herein.
Many electric kettles have been configured to regulate temperature of the liquids they contain. For example, European patent application EP 0 704 186 A1, incorporated by reference herein, discloses a method for regulating the water temperature in an electric kettle. The water is heated from an initial temperature to a high temperature (less than the boiling point). The remaining heating time required before the water reaches its boiling point is then determined as a function of temperature and time. More particularly, based on the heating time required for the liquid to be heated from its initial temperature to its high temperature, the amount of time required to heat the water from its high temperature to its boiling point can be deduced. Therefore, electric kettles of this sort heat liquid for a calculated amount of time, and then shut off.
In order to prevent overheating of the kettle itself, many electric kettles include overheating protection devices. For example, many kettles include fusible cutouts, bimetal sensors, and/or other mechanical sensors. Other kettles prevent overheating of the kettle by monitoring and regulating temperature of the kettle's heating element.
In European patent application EP 0 380 369 A1, a method is disclosed for detecting overheating of the kettle and preventing dry boiling, i.e., heating of an empty kettle. In order to detect and prevent overheating of the kettle, the temperature of the heating plate of the kettle is measured using a temperature sensor. An excessive increase in temperature of the heating plate is interpreted as a signal that the electric kettle contains too little liquid or no liquid at all. Upon reaching a temperature over a predetermined maximum limit, the kettle is shut off.
The apparatuses and methods discussed above occasionally fail due to defective or inaccurate sensors and/or poor positioning of sensors. Defective temperature sensors generally convey an erroneous temperature measurement. Even properly functioning temperature sensors frequently convey inaccurate temperature readings because they are positioned too far away from the source of heat. For example, according to EP 0 380 369 A1, a defined overtemperature must be reached before the electric kettle switches off. But, heating often takes place above this temperature because the respective temperature sensors are not arranged directly at the heating element. For this reason, an even higher temperature is present at the sensor, until the thermal gradient around the heating element has reached thermal equalization. Similarly, any safety switch-off by means of a fusible cutout only takes place if considerably more thermal energy is supplied to the system than would be supplied during operation at nominal rating.
As noted above, an erroneous temperature measurement may cause the functioning of the apparatuses to be altered earlier or later than desired. For example, upon detecting an erroneously low liquid temperature, the regulated kettle may be shut off too late causing the temperature of the liquid to exceed a desired temperature. Or, upon reading an erroneously low heat plate temperature, the kettle may be shut off too late causing the temperature of the kettle itself to exceed a desired maximum temperature limit. Similarly, erroneously high liquid temperature measurements or heat plate temperature measurements can cause the kettle to be shut off too early resulting in a liquid temperature that is less than the temperature desired.
SUMMARY
According to one aspect of the invention, during heating of a liquid, thermal overshoot resulting from deactivating a heating element too late is prevented. Furthermore, a time delay between the point in time when the heating is switched on and when a temperature increase at a sensor can be detected is taken into consideration. In spite of different fill quantities and different values for a difference between a target temperature and a starting temperature, it is possible to reproducibly achieve a preselected target temperature.
The preset target temperature may be below boiling temperature. This is the case for example if water or mineral water which has been boiled before is used for preparing baby food. Another example includes the preparation of green tea with water or mineral water that has been boiled before.
Precisely when using water which has been boiled before, very slight differences between the target temperature and the starting temperature may occur. Furthermore, this means that the starting temperature can be either above or below the ambient temperature.
If the water fill level is low and, thus, the heat capacity of the system is low, and if the temperature sensor responds relatively slowly, i.e. the delay time is relatively long, the temperature difference between the preselected target temperature and the start temperature that was measured may be insufficient for the heating process to be carried out at full heating output without overshoot occurring. In this case, the temperature difference is the minimum acceptable temperature difference or the reference temperature difference. In this case, a controlled heating process which is based on parameters obtained purely by calculation, at a reduced heating output, is carried out, and the controlled heating process is stopped after a precalculated period of time.
Otherwise, i.e. if the temperature difference between the preselected target temperature and the measured starting temperature is greater than the minimum acceptable temperature differential, a regulated heating process is carried out. This means that the temperature is picked up at the temperature sensor and is compared with the preselected target temperature. In this process, the inertia of the temperature sensor and the heat capacity of the system are taken into account in the calculations. When a final temperature, which is below the preset target temperature of the system, is reached at the temperature sensor, the heating process is ended.
As a result of the above, after temperature equalization in the system, the target temperature is achieved with accuracy in a predefined tolerance band, without overshoot. Consequently, the time required for preparing the water is clearly shortened. Furthermore, heating output and thus electrical energy is saved, which would otherwise unnecessarily be used in the heating process. By dividing the method into two alternative implementation procedures, namely one procedure with a small gap between the target temperature and the starting temperature, and one procedure with a bigger gap between the target temperature and the starting temperature, safe functioning even in boundary states is ensured.
In this aspect, a starting temperature is sensed by the temperature sensor, then a starting temperature differential between the sensed starting temperature and a preselected target temperature is determined. If the starting temperature differentia
Braun GmbH
Fish & Richardson P.C.
Pelham Joseph
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