AU2010224322B2

AU2010224322B2 - Improved temperature sensor for an electric kettle

Info

Publication number: AU2010224322B2
Application number: AU2010224322A
Authority: AU
Inventors: Raymond George Corkin; Nick O'loughlin
Original assignee: Newell Australia Pty Ltd
Current assignee: Newell Australia Pty Ltd
Priority date: 2009-09-17
Filing date: 2010-09-17
Publication date: 2016-03-31
Anticipated expiration: 2027-05-11
Also published as: AU2010224322A1; CN202005623U; AU2009101273A4; AU2009101273B4

Abstract

An electric kettle for heating water located in a heating chamber of the kettle, the kettle including: a contact plate defining a base of the heating chamber and having a contact surface that, in use, is in direct thermal communication with the water when located in 5 the heating chamber of the kettle; a heat distribution plate bonded to an underside of the contact plate, wherein an inlet is defined in a peripheral region of the heat distribution plate, the inlet providing access to the underside of the contact plate and defining at least part of a thermally insulating zone; a heating element in thermal communication with the heat distribution plate; an electronic temperature sensor located 0 in the thermally insulating zone in thermal communication with the heating chamber, the electronic temperature sensor being thermally insulated from the heat distribution plate by the thermally insulating zone; a controller for controlling the heating element; and a user input for a user to select between different heating modes of the controller, wherein the controller controls the heating element responsive to a temperature sensed by the 5 electronic temperature sensor and dependent on a selection made using the user input. C-) CNl

Description

P100/01 1 Regulation 3 2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Improved temperature sensor for an electric kettle The following statement is a full description of this invention, including the best method of performing it known to us: 2 Improved temperature sensor for an electric kettle Field of the invention The present invention relates to electric kettles that include temperature sensors for 5 accurately detecting the temperature of the heating vessel's contents during operation. Background of the invention Electric kettles include an electric heating element which heats a contact plate via a heat distribution plate. The heating surface of the contact plate is in direct contact with the vessel's contents. 10 A kettle generally has a temperature sensor to detect when water in the kettle is boiling. This may be a mechanical sensor such as a snap-action bimetallic actuator which turns the kettle off once the water has boiled. In some arrangements the temperature sensor is mounted to the heat distribution plate. This mounting location greatly reduces the accuracy of the temperature sensor, as the 15 temperature sensor senses the temperature of the heat distribution plate and does not directly sense the temperature of the vessel's contents. Because of this, discrepancies may arise between the measured temperature and the actual temperature of the contents. An inaccurate temperature sensor limits the potential functionality of the heating vessel. 20 Since the temperature of the vessel's contents is not accurately sensed, only a limited range of functions controlled with reference to an approximate temperature reading are possible, in particular for accurately controlling temperature settings below boiling point. Reference to any background art in the specification is not an acknowledgement or any form of suggestion that this background art forms part of the common general 25 knowledge in Australia or any other jurisdiction or that this background art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. Summary of the invention According to a first aspect of the invention there is provided an electric kettle for heating 30 water located in a heating chamber of the kettle, the kettle including: a contact plate defining a base of the heating chamber and having a contact surface that, in use, is in direct thermal communication with the water when located in the heating chamber of the kettle; a heat distribution plate in thermal communication with and welded to an 35 underside of the contact plate, wherein the heat distribution plate is thicker than the contact plate and has a thermal conductivity that is higher than a thermal 3 conductivity of the contact plate, and wherein an inlet is defined in a circumference of the heat distribution plate, the inlet providing access to the underside of the contact plate and defining at least part of a thermally insulating zone; 5 a curved resistive heating element ending in a pair of cold tails, the heating element welded to a peripheral region of the heat distribution plate in thermal communication with the heat distribution plate; an electronic temperature sensor located in the thermally insulating zone in thermal communication with the contact plater, the electronic temperature sensor 10 being thermally insulated from the heat distribution plate by the thermally insulating zone, the electronic temperature sensor sensing temperature in a continuous range; a controller for controlling the heating element; and a button arrangement in communication with the controller for a user to select between different heating 15 modes of the controller, wherein the controller controls the heating element responsive to a temperature sensed by the electronic temperature sensor and the controller activates the different heating modes dependent on a selection made using the button arrangement. Also described herein is a kettle for heating water located in a heating chamber of the 20 kettle, the kettle including: (a) a contact plate defining a base of the heating chamber and having a contact surface that, in use, is in direct thermal communication with the water when located in the heating chamber of the kettle; (b) a heat distribution plate in thermal communication with the contact 25 plate, the heat distribution plate defining a void that provides a thermally insulating zone surrounded by the heat distribution plate; (c) a heating element in thermal communication with the heat distribution plate; and (d) an electronic temperature sensor located within the thermally 30 insulating zone in thermal communication with the contact plate, the electronic temperature sensor being thermally insulated from the heat distribution plate by the thermally insulating zone. The kettle may comprise a heat-source controller for controlling the heating element responsive to a temperature sensed by the electronic temperature sensor. 35 The kettle may have a keep-warm mode in which the heat-source controller acts to maintain the temperature sensed by the electronic temperature sensor substantially at a specified warm temperature.

4 The kettle may also have a boil mode for heating the temperature sensed by the electronic temperature sensor to a specified boil temperature, the kettle having a button arrangement comprising a keep-warm button and a boil button operable for a user to 5 select from the keep warm mode and the boil mode, wherein the heat-source controller controls the heating element responsive to the temperature sensed by the electronic temperature sensor and dependent on a selection made using the button arrangement. The heat distribution plate may be provided with a plurality of ribs and the thermally insulating zone may include a plurality of voids defined by the heat distribution plate and 10 said ribs. The void may be defined by a portion of the heat distribution plate having a reduced thickness relative to a thickness of a region of the heat distribution plate adjacent to said portion. Said portion having reduced thickness may be adjacent said contact plate. 15 Also described herein is an electric kettle for heating water located in a heating chamber of the kettle, the kettle including: (a) a contact plate defining a base of the heating chamber and having a contact surface that, in use, is in direct thermal communication with the water when located in the heating chamber of the kettle; 20 (b) a heat distribution plate bonded to the contact plate, wherein a total area of the heat distribution plate is less than an area of the contact plate to provide one or more void regions in which the contact plate does not abut the heat distribution plate, said void regions providing a thermally-insulating zone; (c) a heating element in thermal communication with the heat 25 distribution plate; (d) an electronic temperature sensor located in the thermally insulating zone in thermal communication with the heating chamber, the electronic temperature sensor being thermally insulated from the heat distribution plate by the thermally insulating zone; 30 (e) a controller for controlling the heating element; and (f) a user input for a user to select between different heating modes of the controller, wherein the controller controls the heating element responsive to a temperature sensed by the electronic temperature sensor and dependent on a selection made using the user input. 35 The contact plate may comprise a temperature sensor housing portion that protrudes into the heating chamber.

5 The temperature sensor housing portion may be formed integrally with the contact plate. Alternatively the temperature sensor housing portion comprises a separately formed housing component that is bonded to the contact plate. 5 The protruding portion may coincide with one of the void regions. Also described herein is an electric kettle for heating water located in a heating chamber of the kettle, the kettle including: (a) a contact plate defining a base of the heating chamber and having a contact surface that, in use, is in direct thermal communication with the water 10 when located in the heating chamber of the kettle; (b) a heat distribution plate bonded to the contact plate, wherein a total area of the heat distribution plate is less than an area of the contact plate to provide one or more void regions in which the contact plate does not abut the heat distribution plate, said void regions providing a thermally-insulating zone; 15 (c) a heating element in thermal communication with the heat distribution plate; (d) an electronic temperature sensor located in the thermally insulating zone in thermal communication with the heating chamber, the electronic temperature sensor being thermally insulated from the heat distribution plate by 20 the thermally insulating zone; (e) a controller for controlling the heating element; and (f) a user input for a user to select between different heating modes of the controller, wherein the controller controls the heating element responsive to a temperature sensed by the electronic temperature sensor and dependent on a 25 selection made using the user input. Preferably, the heating modes include a keep-warm mode in which the controller acts to maintain the temperature sensed by the electronic temperature sensor substantially at a specified warm temperature. The heating modes may include a mode for heating the temperature sensed by the 30 electronic temperature sensor to a specified temperature.

6 Brief description of the drawings Embodiments of the invention will now be described with reference to the drawings, in which: Figure 1 is a cross-sectional drawing of an electric kettle; 5 Figure 2 is a partially cut-away view of a heater assembly for the kettle of Figure 1; 7 Figure 3 shows more detail of the heater assembly of Figure 2 including an electronic temperature sensor and heat-source controller; Figure 4 shows a cross-sectional view of part of the heater assembly; Figure 5 shows an arrangement in which the heater assembly is positioned on a 5 concave contact plate of the kettle; Figure 6 shows an arrangement in which the heater assembly is positioned on a convex contact plate of the kettle; Figure 7 shows a button arrangement for controlling 'boil' and 'keep warm' operating modes for the kettle, the button arrangement including indicators of the state D of the kettle; Figure 8 is a graph illustrating the operation of the kettle in the boil mode; Figure 9 is a graph illustrating the operation of the kettle in the 'keep warm' mode; Figure 10 is a graph illustrating the effect of adding water to the kettle during the 5 keep warm mode; Figures 11 and 12 are graphs comparing the temperature of water in the kettle with the temperature measured by the temperature sensor of Figure 2; Figure 13 is a graph comparing the performance of the kettle of Figures 1 to 7 with the performance of a standard kettle; .0 Figure 14 shows a bottom view of an alternative heater assembly for use in the kettle of Figure 1; Figure 15 shows a cross-sectional side view of the heater assembly of Figure 14; 8 Figure 16 shows a further view of the heater assembly of Figure 14, illustrating the threaded mounting of a temperature sensor; Figure 17 is a flow diagram illustrating a method of selecting the cut-off temperature dependent on the load in the kettle; 5 Figure 18 provides a partial plan schematic of a heat distribution plate with a thermally insulating zone provided by a plurality of voids; Figure 19 provides a perspective view of the heat distribution plate of Figure 18; Figure 20 provides a partial cross-sectional schematic of a heat distribution plate with a further arrangement for a thermally insulating zone; D Figure 21 provides a perspective view of the heat distribution plate of Figure 20; Figure 22 provides an alternative embodiment of the heat distribution plate of Figures 20 and 21; Figure 23 is a plan view of a heater assembly in which the thermally insulating zone includes an inlet projecting into the heat distribution plate; 5 Figure 24 shows a further example of the heater assembly arrangement of Figure 23; Figures 25A and 25B are plan views of heater assemblies in which the temperature sensor is positioned outside an outer periphery of the heat distribution plate; and !0 Figures 26A to 26E show examples of different configurations in which the temperature sensor is positioned in a housing that projects above the contact plate into the heating chamber of the heating vessel.

9 Detailed description of the embodiments Figure 1 shows a cross-sectional view of an electric kettle 10. The electric kettle has a heating chamber 12, which holds the water to be boiled. The water may be poured into the heating chamber 12 of the kettle through the pouring spout 14. A handle is provided 5 for a user to lift the kettle 10 and pour water out of the pouring spout 14. The temperature sensing arrangements described below may also be applied to kettles having a different configuration to that shown in Figure 1, for example a cordless kettle having a base and a removable vessel The base wall of the heating chamber 12 is defined by a contact plate 16. Water stored 0 in the heating chamber 12 is in direct contact with one side of the contact plate 16. The contact plate 16 may be formed from stainless steel. Other materials which are suitable for contacting water and are resistant to high temperatures and oxidation may be used. The contact plate 16 forms part of a heater assembly 18. The heater assembly is generally located underneath the heating chamber 12 on the opposite side of the 5 contact plate to the heating chamber 12. One embodiment of the heater assembly 18 is shown in greater detail in Figures 2 to 4. The heater assembly 18 is powered by a power source (not shown) which is external to the kettle 10. The power may be transmitted to the heater assembly 18 using known techniques, for instance through a plug-in electrical lead. ?0 The heat used to boil the water is generated by a heating element 20, which is curved and terminates in cold tails carrying electrical connections 22. Preferably the heating element 20 is powered by electricity. The heating element 20 shown is a resistance element. Other types of heating elements may be used. For a kettle a 2400W sheathed arcuate heating element is often used. ?5 The arcuate heating element 20 is bonded to a peripheral region at or near an outer edge of a heat distribution plate or billet 24. The bonding achieves a good thermal coupling between the heating element 20 and the heat distribution plate 24 so that heat generated by the heating element 20 is rapidly and efficiently transferred to the heat 10 distribution plate 24. Many known bonding techniques are suitable, including induction welding, flame or oven welding and impact welding. The heat distribution plate 24 is induction brazed to the contact plate 16 so there is a good thermal coupling between the heat distribution plate 24 and the contact plate 16. 5 Many other known bonding techniques are suitable, including the bonding techniques mentioned above. Alternatively the heat distribution plate 24 may be secured to the contact plate 16 using other known techniques, such as mechanical fasteners. The heat distribution plate or billet 24 may be formed from aluminium, which is a good thermal conductor, and is of sufficient thickness to evenly distribute heat from the 0 heating element 20 across the extent of the heat distribution plate 24. Alternative materials for the heat distribution plate 24 include other metals and metal alloys. The heat distribution plate 24 is generally thicker than the contact plate and formed from a material which is a better thermal conductor than the contact plate. The heat distribution plate 24 defines a void 26. In the arrangement of Figure 2 the void 5 is cylindrical with an outer diameter surrounded by the heat distribution plate 24. The void in this arrangement provides access to the contact plate 16 through the heat distribution plate 24. The void 26 forms a thermally insulating zone. This is because heat which is transmitted from the heating element 20 to the heat distribution plate 24 is not as readily transmitted across the void 26. The region of the contact plate 16 located ?0 adjacent the void 26 does not conduct significant amounts of heat when compared to the aluminium heat distribution plate 24 because the contact plate 16 is thin and formed from stainless steel, which is not as good a thermal conductor. An electronic temperature sensor 28 is positioned in the void 26 . The void 26 provides a thermally insulating zone around the electronic temperature sensor 28. Heat from the 25 heat distribution plate 24 is not readily transmitted to the electronic temperature sensor 28. As a result, the electronic temperature sensor 28 is thermally insulated and is not undesirably influenced by the temperature of the heating element 20 and heat distribution plate 24.

11 The thermally insulating zone and the temperature sensor 28 may be located between the cold tails 22 of the heating element 20. The cold tails do not generate significant amounts of heat, so the electronic temperature sensor 28 is further insulated from the heat generated by the heating element 20. Instead of being empty, the void 26 may be 5 filled, either partially or wholly, with a thermally insulating material, such as silicone or rubber. The temperature sensor 28 is positioned in close proximity to the contact plate 16. Optionally, the temperature sensor 28 may be touching the contact plate 16. This improves the thermal coupling between the electronic temperature sensor 28 and the o contact plate 16. The thermal coupling may be further improved using known techniques, such as applying a heat transfer paste. It is an advantage that the temperature sensor 28 is in thermal contact with the contact plate 16 in the region indicated by 29. When water contained in the heating chamber 12 of the kettle 10 heats up, the contact plate 16 will heat to a similar temperature. Due to 5 the void 26, the region of the contact plate 16 located within the void is insulated from the heat distribution plate 24 and will more accurately reflect the temperature of the water. Since the temperature sensor 28 is in thermal communication with the contact plate 16, it senses the water temperature with greater accuracy and responsiveness. Figures 2 to 4 show the temperature sensor 28 being supported by a sensor support 30. 20 The sensor support 30 is formed from silicone, and is held in place by a bracket 32. Other insulating materials are also suitable. The bracket 32 is mechanically fastened to the heat distribution plate 24 and is preferably formed from a relatively rigid material, such as a plastic, metal or metal alloy. The bracket 32 locates the sensor support 30 in the centre of the void 26 so the sensor 28 is insulated and may press the sensor 25 support 30 against the contact plate 16, providing a good thermal connection between the sensor 28 and the contact plate 16. The temperature sensor 28 may be positioned in a number of ways which aim to reduce the influence of heat from the heat distribution plate 24.

12 The temperature sensor 28 is typically a thermistor. NTC thermistors formed from metal oxides are suitable. A thermistor has a number of advantages over other types of temperature sensors. A thermistor senses the temperature of water in the kettle within a continuous range. This provides significantly more information on the temperature of the 5 water than, for example, a bimetallic actuator. A bimetallic actuator is typically activated only when the water reaches a threshold temperature value and is deactivated when the water falls below a threshold temperature value. As a result, a bimetallic actuator only senses whether the water temperature is above or below a threshold value. The thermistor provides responsive and accurate readings because it is positioned in a 0 thermally insulating zone in thermal communication with the contact plate 16. The heater assembly 18 shown in Figures 2 to 4 has a single void 26 in which the temperature sensor 28 is located. As discussed further below (with reference to figures 18 to 22), alternative means of reducing heat transfer between the heat distribution plate 24 and the temperature sensor 28 are possible. For example, it is also possible to 5 have multiple voids around the temperature sensor, each void forming a thermally insulating region. By positioning a number of the thermally insulating regions around the sensor 28, a thermally insulating zone is formed. The sensor 28 is still positioned in thermal contact with the contact plate 16. In other configurations, shown for example in Figures 23-25, the heat distribution plate does not surround the thermally insulating 0 zone. In one arrangement the contact plate 16 is indent-free. The contact plate 16 shown in Figures 2 to 4 is indent-free at least in the region of the temperature sensor 28. This shape may improve the accuracy of the temperature sensor 28. Because the contact plate 16 is indent free, water contained in the heating chamber 12 of the kettle 10 is 25 able to readily and rapidly mix. This means the temperature of water located immediately above the temperature sensor 28 is more likely to accurately reflect the temperature of the remaining water volume contained in the kettle 10. Consequently the temperature sensor 28 gives more accurate readings of the temperature of all of the water in the kettle 10.

13 Alternative arrangements are shown in Figures 5 and 6, in which the contact plate is not uniplanar but is nevertheless free of indents in the region of the temperature sensor 28. Figure 5 shows a concave contact plate 33 which is curved towards the heater assembly 18 in the centre of the contact plate. Figure 6 shows a convex contact plate 5 35 which is curved away from the heater assembly 18 in the centre of the contact plate. In the case of Figure 6, the convex curvature of the contact plate 16 results in the temperature sensor 28 protruding into the heating chamber 12 of the kettle 10 by a greater amount than other regions of the contact plate 16. Since the cold water tends to collect in the lowermost volumes of the heating chamber 12, water located opposite the 0 sensor 28 is more likely to reflect the average temperature of the water contained in the kettle 10. This may improve the accuracy of temperature readings made by the sensor 28. In further arrangements the contact plate 16 has a dome-shaped protrusion in the region adjacent the temperature sensor 28. The dome formed in the contact plate 16 5 may extend into the heating chamber 12 or, alternatively, may extend away from the heating chamber. As illustrated in Figures 26A-E, the electronic temperature sensor may be positioned inside the dome-shaped protrusion. Referring again to Figures 2 to 4, the heater assembly 18 has a heat-source controller 34. The heat-source controller is electronically connected to the temperature sensor 28 20 and the heating element 20. The heat-source controller 34 controls the operation of the heating element 20 with reference to the temperature sensed by the temperature sensor 28. Preferably, the controller 34 consists of an electronic circuit or number of electronic circuits. These circuits may be designed in a number of ways to provide the functionality described below. The controller 34 preferably includes a microprocessor. 25 The heat-source controller may have a number of different functions, such as a boil function and a "keep warm" function, which use feedback from the temperature sensor 28. These functions are made possible because the temperature sensor 28 is able to accurately sense the temperature of the water contained in the kettle 10 within a large range. For example, the temperature sensor 28 may have an operating range between 30 0 *C and 100 *C 14 The functions of the kettle 10 are selected by button arrangement 36 which is shown in Figure 7. The button arrangement 36 includes a a "boil" button 38 and a "keep warm" button 40, both of which are momentary push buttons. The buttons 38, 40 may alternatively be a variety of other button types. A ring 42, 44 around each button is 5 translucent. These rings are illuminated by LEDs to provide a user with information regarding the kettle's operation. The LEDs are optionally LEDs capable of emitting different coloured lights, for example to indicate temperature levels in the kettle. Other types of lights may be used, such as a conventional filament bulb. A "standby" LED 46 is illuminated when the external power is connected to the kettle. The buttons are 0 connected to, and provide input to, the controller 34. The lights are connected to, and are operated by, the controller 34. When the boil button 38 is activated, the controller 34 enters a boil mode, graphically displayed in Figure 8. Before activation, the controller 34 is in a standby mode (indicated by "Area 1" in Figure 8). After activation, the controller 34 enters the boil 5 mode (indicated by "Area 2" in Figure 8). When in the boil mode, the controller 34 turns on the heating element 20, which begins to heat the water in the kettle. The controller 34 additionally causes the illuminated ring 42 to produce, for example, red light, to indicate the controller is in the boil mode and the water is being boiled. The temperature sensor 28 detects when an upper boiling limit has been reached. The .0 upper boiling limit may be set at 97 0 C, though other limits are also suitable. At this point the controller enters a "boiled" mode (indicated by "Area 3" in Figure 8). In the boiled mode, the controller turns off the heating element 20 and the red light in the illuminated ring 42. The controller then turns on, for example, a green light in the illuminated ring 42 to indicate that the water is boiled. ?5 In the boiled mode, the temperature sensor 28 continues to sense the temperature of the water. After the heating element 20 is turned off, the water slowly cools. Once the temperature of the water falls to a lower boiling limit, the controller ends the "boiled" mode and returns to "standby" mode (indicated by "Area 4" in Figure 8). At this stage, the controller turns off the green light in the illuminated ring 42 to indicate the water is 15 no longer at or near boiling temperature. A suitable lower boiling limit is 92 0 C, though other limits are also suitable. When the "keep warm" button 40 is activated, the controller 34 enters a keep warm mode in which the water is first boiled and then maintained at a warm average 5 temperature, for example about 85 0C. Other temperature setpoints may also be used. The keep warm mode is graphically illustrated in Figure 9. Prior to activation, the controller is in the standby mode (indicated by "Area 1" in Figure 9) and the water is at ambient temperature. In the keep warm mode (indicated by "Area 2" in Figure 9), the controller 34 turns on the heating element 20. This heats the water as described 0 previously. The controller 34 also causes a, for example, amber light to illuminate the illuminated ring 44 to indicate that the controller 34 is in the keep warm mode. The heating element 20 continues to heat the water until the temperature sensor 28 detects the water temperature has reached an upper boiling limit, indicated at reference numeral 50. The heating element 20 is switched off and the water in the kettle cools 5 gradually until a lower warm limit is reached, as indicated at reference numeral 52. A suitable lower warm limit is 83 *C, although other values may be used. The controller 34 then switches the heating element 20 back on and the water temperature rises until an upper warm limit is reached (see reference numeral 54). A suitable upper warm limit is 87*C, though other limits are also suitable. When this occurs, the controller 34 turns off ?0 the heating element 20. This process continues so that the water temperature oscillates between the upper warm limit and the lower warm limit, keeping the water at an average temperature. The keep warm mode continues until the keep warm button 40 is pressed to deactivate the keep warm mode. If the kettle is about to boil dry (that is, the water in the kettle has 25 substantially evaporated), the temperature detected by the sensor 28 increases rapidly. If this rapid increase is detected, the controller 34 deactivates the keep warm mode and resumes the standby mode to avoid the kettle boiling dry. Alternatively, the keep warm mode may be ended automatically after a set time, for example four hours.

16 It will be appreciated that a series of buttons may be provided each associated with a particular brewing temperature suitable for different beverages, such as different types of tea. In one arrangement the kettle 10 may have two or more heating modes dependent on 5 the load, i.e. the amount of liquid in the kettle. Low volumes of liquid heat up more rapidly than larger volumes. The controller 34 monitors the measured temperature and determines the rate of change of the measured temperature. The controller 34 selects a heating mode based on the rate of change. If low volumes are deduced (i.e. the rate of change of temperature lies in a specified higher range), then the heating element 20 is 0 switched off at a reduced upper boiling limit. In the boil mode, a reduced upper boiling limit of 93*C is suitable, although other values may be used. If the controller 34 deduces that higher volumes of liquid are present (i.e. the rate of change of temperature lies in a specified lower range), the heating element 20 is switched off at a higher boiling limit, for example 97*C. 5 The controller 34 monitors the rate of change of measured temperature on a regular basis and, if necessary, selects a different upper boiling limit based on the current rate of change. Thus, for example, if cold water is added to the kettle 10, the controller 34 may need to switch to a heating mode that uses a higher cut-out temperature. Two or more heating modes may be established. For the boil mode, the controller 34 ?0 may have a look-up table that lists a suitable upper boiling limit corresponding to different rates of heating. In one arrangement the lower boiling limit may also be reduced for the case of low volumes. For example, the lower boiling limit may be set 40C lower than the selected upper boiling limit. In the keep warm operation, the controller 34 may also select a different upper boiling 25 limit depending on the rate of change of temperature. In alternative arrangements the load may be inferred from measurements other than the rate of change of temperature. Such alternative load measurements include the level of liquid in the kettle or the weight of the filled kettle. For example, a reed switch or 17 capacitive sensor may be used to indicate the level in the kettle. In such an arrangement, the controller 34 may select a higher or lower boiling limit dependent on whether the level of fluid is above or below a threshold value. Figure 17 illustrates a method 200 of selecting the upper boiling limit. In step 202 the 5 temperature sensor 28 generates a temperature signal that is related to the temperature of the water in the kettle 10. In step 204 a load signal is generated that is related to the amount of liquid in the kettle. In the preferred arrangement the controller 34 generates the load signal by monitoring the rate of change of the measured temperature, thereby deducing the load of the kettle. Based on the load signal, in step 206 the controller 34 0 selects a threshold value to use as the upper boiling limit. The threshold value may be read from a look-up table stored in memory. In step 208 the controller 34 acts to switch off the heating element 20 if the temperature signal is greater than or equal to the threshold value. Figure 10 shows the kettle 10 being refilled whilst the keep warm mode is activated. To 5 refill the kettle, it may be disconnected from the external power supply. When this occurs, power to the controller 34 is disrupted. The controller 34 has an electronic memory which stores an indication of whether the controller is in the boil mode and / or the keep warm mode. The memory is preferably EPROM, though other types of memory may be used. Once the kettle is refilled, it is reconnected to the external power 20 supply. The controller 34 then resumes the mode or modes which are stored in the memory. When the kettle is refilled, the temperature of the water in the kettle drops rapidly, as indicated at reference numeral 56 in Figure 10. When the controller 34 resumes the keep warm mode, the water is reheated until the upper warm limit is reached. The keep 25 warm mode then continues as before. A similar process occurs if the kettle is refilled whilst in the boil mode. The kettle may also have a audible indicator (not shown) for providing an audible indication of which mode the controller is in. The controller mode indicator may be one 18 or more buzzers or speakers. The controller mode indicator is connected to, and operated by the controller 34. The additional functionality described above is made possible by the arrangements described herein. These arrangements provide a temperature sensor which is able to 5 accurately and responsively detect the temperature of water contained in the kettle. Without responsive and accurate temperature sensing, the boil mode and keep warm mode described above may not function properly. Figures 11 and 12 graphically show the accuracy and responsiveness of the temperature sensor. In these Figures, the darker line 3 represents the water temperatures and the line 4 represents the 0 temperatures sensed by the temperature sensor. As can be seen, the two lines are closely matched. Figure 13 shows a comparison between the temperature sensed during boiling by a temperature sensor in a conventional kettle (denoted "STD Kettle" in Figure 13) and a temperature sensor in a kettle using the arrangements described herein (denoted "Elec 5 Kettle" in Figure 13). The same volume of water and heating power is used in each case. Once the water has boiled, each of the kettles switches off its respective heating element. However, the time between the water boiling and the heating element switching off is different in the two cases. As seen in Figure 13, for a standard kettle the heating element stays on for a relatively long duration after the water boils, as indicated ?0 at reference numeral 60. In contrast, for the electronic kettle 10, the heating element 20 stays on for a shorter time, as indicated at reference numeral 58. With repeated use, this difference may represent a significant energy saving in the electronic kettle 10 compared with the standard kettle. The improvement in performance is enabled by the greater accuracy of the temperature arrangement described herein compared with the ?5 bimetallic switch used in the standard kettle. Figures 14 to 16 show an alternative heater assembly 60. The heater assembly 60 has a heating element 62, a heat distribution plate 64, a contact plate 66, a controller 68 and an electronic temperature sensor 70 which are similar in description and function to those described in relation to Figures 2 to 6.

19 The heat distribution plate 64 has a toroidal void 72. The void 72 forms a thermally insulating zone around the temperature sensor 70, for the reasons described above. The portion of the heat distribution plate located in the centre of the void 72 is a sensor mount 74 with a threaded aperture 76. The sensor is supported by an internally 5 threaded brass casing which screws into the aperture 76 so that the sensor is in thermal contact with the contact plate 64. The heating assembly shown in Figures 2 to 4 may be produced by the following procedure. Firstly a heat distribution plate is induction welded to the underside of the contact plate. Other bonding methods described above may also be used. At this stage, 0 the heat distribution plate need not have a void. A heating element is then bonded to the heat distribution plate. Any one of the bonding methods described above may be used. If the heat distribution plate is not provided with a void, the void is formed by routing or milling away a region of the heat distribution plate to expose the contact plate underneath. The ease of manufacture is improved by forming the void after the heat 5 distribution plate is bonded to the contact plate. The sensor is then positioned in the void in thermal communication with the exposed contact plate. Finally the controller is produced and mounted to the heat distribution plate. The heater assembly shown in Figures 14 to 16 may be produced using a similar process. In this case, a toroidal void is formed in the heat distribution plate. The centre 20 of the toroidal void is used as a sensor mount. A hole in the centre of the sensor mount is formed to allow the sensor to be in thermal communication with the contact plate and, optionally, to be in direct contact with the contact plate. The hole is tapped and the sensor is positioned by screwing a threaded sensor casing into the sensor mount. In other arrangements the hole in the sensor mount is not threaded but serves to receive 25 and position the temperature sensor. Further embodiments of the invention will now be described with reference to Figures 18 to 22. For clarity of illustration, Figures 18 to 22 are partial schematic views, intended only to depict alternative arrangements of a heat distribution plate 80 (and contact plate 20 78) having one or more thermally insulating zones for minimising heat transfer between the heat distribution plate 80 and a temperature sensor 82. For ease of illustration the heat distribution plate and thermally-insulating void(s) are shown as rectangular regions. In practice, of course, the regions may be rounded or of 5 any other regular or irregular shape. Figures 18 and 19 respectively provide a plan view and perspective view of one alternative embodiment in which the heat distribution plate 80 is provided with a plurality of ribs 86, which abut the sensor support 84. As can be most easily seen in the perspective view of figure 19, the thermally insulating zone in this embodiment includes 0 a plurality of voids 88 defined by the ribs 86, the heat distribution plate 80, and the sensor support 84. While a small amount of heat from the heat distribution plate 80 will, in use, be transmitted along the ribs 86 to the sensor support 84, the voids 88 provide thermal insulation to the arrangement and serve to limit the amount of heat transfer to the sensor support 84 and temperature sensor 82 from the heat distribution plate 80. 5 In the embodiment of figures 18 and 19 four ribs 86 have been shown, however more or fewer ribs may be provided as desired. The heat distribution plate 80 may be integrally formed (e.g. by casting) with ribs 86 and/or the sensor support 84. Alternatively, the heat distribution plate 80 may be provided with a region (either during manufacture of the heat distribution plate 80 or by milling/ routing as described above) in which the ribs 20 86 and sensor support 84 may be positioned and secured. Figures 20 and 21 provide side and perspective views of a further alternative embodiment. In this case the thermally-insulating zone in the heat distribution plate is provided by a void region 90 in the heat distribution plate 80 that does not extend the full depth of the heat insulation plate 80. In this arrangement a thin section 92 of the 25 heat insulation plate 80 remains between the void region 90 and the contact plate 78. This arrangement may be manufactured, for example, by milling or routing void region 90 out of the heat distribution plate 80 as described above, but only to a depth such that section 92 remains.

21 The sensor support 84 may be part of the heat distribution plate 80 that remains after the void region 90 has been removed. Alternatively, the sensor support 84 may be a separate component positioned in a hole in section 92. As shown in figures 21 and 22, a hole 94 may then be drilled or otherwise formed in 5 section 92 for accommodating the temperature sensor 82 and, if necessary, the sensor support 84. By way of alternative, and as shown in figure 22, the hole 94 may be provided so as to directly support the temperature sensor 82, thereby doing away with the need for a sensor support 84. As with the ribs 86 described above, section 92 will allow some heat transfer from the 0 heat distribution plate 80 to the temperature support 84 and/or temperature sensor 82, the amount of heat transfer dependent on the thickness of section 92. By keeping section 92 thin relative to the depth of the recess 90, however, direct heat transfer from the heat distribution plate 80 to the sensor support 84 and/or temperature sensor 82 is limited. 5 In figures 20 to 22 the thin section 92 is adjacent to the contact plate 78. As an alternative, the thin section may remain at the opposed edge of the heat distribution plate 80, leaving a void between the contact plate 78 and the thin section of the heat distribution plate. The void acts to provide a thermally insulating zone. While in the embodiments described with reference to figures 19 to 22 the temperature 20 sensor 82 is shown as being located in the thermally insulating zone by securement (either directly or via the sensor support 84) to the heat distribution plate 80, alternative locating arrangements are possible. For example, the temperature sensor 82 (and/or the sensor support 84 should such be being used) may be located in the thermally insulating zone by a bracket or similar (such as bracket 32 described above in relation 25 to figures 2 to 4). Such a bracket could itself be secured to the heat distribution plate 80, or to any other viable support, for example a printed circuit board located adjacent to the heat distribution plate. The support may include a spring or similar means for biasing the sensor against the contact plate. As a further alternative, the temperature sensor 82 (and/or sensor support 84 if such is being used) may be secured directly to and 22 supported by the contact plate 78. This securement may be via an adhesive or other bond. Figure 23 illustrates a heating assembly in which the heat distribution plate does not entirely surround the thermally insulating zone. As before, a sheathed arcuate heating 5 element 20 is bonded to a peripheral region of a heat distribution plate 102, which is generally circular. The heat distribution plate 102 acts to distribute heat generated by the heating element 20. The heat distribution plate is bonded to contact plate 104, which forms the base of the kettle's heating chamber. The circumference of the heat distribution plate lies within the bounds of the contact plate 104, which is also generally D circular in the illustrated arrangement. A U-shaped inlet 108 is formed at one side of the heat distribution plate 102. The contact plate 104 is accessible through the inlet 108, which provides a thermally insulating zone. A sensor assembly 106 is positioned in the thermally insulating zone to provide a temperature measurement indicative of the water temperature in the heating 5 chamber. The heat-distribution plate may be cast in the shape having the inlet 108. Alternatively, the inlet 108 may be removed from a disc-shaped plate, for example by milling the desired inlet shape. In the illustrated arrangement the inlet 108 is positioned between the cold tails 22. This ?0 configuration further reduces the effect of the element 20 and heat distribution plate 102 on the temperature measured by the sensor assembly 106. The sensor assembly 106 may be similar to the arrangement of Figure 2, in which a sensor support 30 supports the electronic temperature sensor 28. A bracket may hold the sensor support in place. The sensor support 106 may also be similar to the ?5 arrangement of Figure 15, in which a sensor mount is bonded to the contact plate in the thermally insulating zone (ie inlet 108). Further alternative arrangements for the sensor assembly 106 are shown in Figures 26A-E.

23 The inlet 108 may have different shapes. A further example is illustrated in Figure 24, with a larger inlet 108. The cold tails 22 here are positioned over the inlet 108. A larger inlet may increase the insulating effect of the thermally-insulating zone. However, there may be a design trade-off as the reduced area of the heat-distribution plate 102 may 5 reduce its effectiveness in distributing the heat generated by the arcuate element 20. Figure 25A shows a further alternative in which the area of the heat distribution plate 102 is reduced relative to the area of the contact plate 104. In this arrangement there is sufficient clearance for the sensor assembly 106 to be positioned on the contact plate 104 outside the outer bounds of the heat distribution plate 102. In this configuration the D sensor assembly is more remote from the heating effect of the element 20 and heat distribution plate 102 than in the arrangements of Figures 23 and 24. In the arrangement of Figure 25A the sensor assembly 106 is positioned in the vicinity of the cold tails 22. However, the sensor assembly 106 may also be located at other points on the contact plate 104 that lie outside the periphery of the heat distribution plate. An 5 example is shown in Figure 25B in which the sensor assembly 106 is located diametrically opposite the cold tails 22. The electronic temperature sensor 28 may be positioned in a housing that protrudes into the heating chamber 12 of the kettle. Figure 26A shows an example of this configuration, with a sensor assembly 106 located within a void 114 in the heat .0 distribution plate 102. In this example the housing 108 has a dome-shaped portion that protrudes through a hole in the contact plate 104 and an annular flange portion 110,112 that is located against the contact plate outside the heating chamber 12. The flange portion 110,112 may, for example, be brazed to the contact plate. The housing 108 may be stainless steel or some other corrosion-resistant metal. The housing may contain a ?5 thermal paste 113 to help provide good thermal communication between the housing 108 and the temperature sensor 28. In this arrangement the temperature sensor is located in the void 114 and thus the accuracy of the temperature measurement benefits from the thermal insulation provided by the void. The dome-shaped protrusion also helps to distance the sensor 28 from the heating effect of the heat distribution plate.

24 Figure 26B shows another example in which a hole is formed in the contact plate 104 within the void 114. An annular silicon member 118 is positioned in the hole. A groove may be provided around an outer surface of the silicon member 118, sized to accommodate the edge of the contact plate 104. A housing 116 is pressed through the 5 centre of the annular silicon member to protrude into the heating chamber 12. The fit should be sufficiently tight for the silicon member 118 to hold the housing 116 securely in place and limit leakage from the heating chamber 12 through to the void 114. The housing 116 may have an annular shoulder 117 to help position the housing 116 against the silicon member during assembly. The temperature sensor 28 is located 0 inside the housing 116. Figure 26C shows another arrangement in which a housing 120 protrudes through a hole in the contact plate into the heating chamber 12. In the depicted arrangement the void 114 has a smaller diameter than the void 114 of Figures 26A and 26B. The housing 120 may, for example, be silver-soldered to the contact plate 104. As illustrated, the 5 housing 120 does not have an annular flange around its base. Figure 26D shows an arrangement in which a relatively narrow hole 122 (smaller in diameter than the diameter of housing 120) is drilled or otherwise formed in the heat distribution plate 102. A hole is formed in the contact plate 104 sized to accommodate the base of the housing 120. When assembled, the base of housing 120 may rest on the heat distribution plate .0 102. The housing 120 may, for example, be silver-soldered to the contact plate 104. In this arrangement the sensor 28 may experience some heating effect from the heat distribution plate 102. To compensate, the sensor assembly may be located between the cold tails 22. Figures 26E and 26 F show arrangements similar to 26A and 26 B, with the difference 25 that the sensor assembly 106 is located on the contact plate 104 between an outer region 104a of the contact plate 104 and an outer edge 102a of the heat distribution plate 102. The annular portion of the contact plate between the edge of the kettle and the heat distribution plate thus provides a thermally-insulating zone in which the sensor assembly 106 may be positioned (see for example Figures 25A and B for a plan view of 30 such an arrangement) 25 Many alternative embodiments of the present invention are possible without departing from the principles of the present invention. For instance, the void may have any number of different shapes. Likewise, there can be a small portion of thermally conductive material (such as the brass casing) between the contact plate 64 and the 5 sensor 70. The dome-shaped protrusion in the contact plate may be formed integrally with the contact plate, for example by punching or pressing out the dome shape. The convex contact plate 35 shown in Figure 6 and described above provides one non limiting example of such a contact plate with an integrally formed dome. Of course only part of the contact plate need be dome shaped, and not the entire contact plate as 0 shown in Figure 6. It will also be understood that the invention may be applied to kettles having a different configuration to that shown in figure 1, for example a cordless kettle having a base and a removable vessel. In each case, the appliance has an electronic sensor which is insulated from a heating element by a thermally insulating zone and is in thermal 5 contact with a contact plate. It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

1. An electric kettle for heating water located in a heating chamber of the kettle, the kettle including: a contact plate defining a base of the heating chamber and having a contact 5 surface that, in use, is in direct thermal communication with the water located in the heating chamber of the kettle; a heat distribution plate in thermal communication with and welded to an underside of the contact plate, wherein the heat distribution plate is thicker than the contact plate and has a thermal conductivity that is higher than a thermal conductivity of 10 the contact plate, and wherein an inlet is defined in a circumference of the heat distribution plate, the inlet providing access to the underside of the contact plate and defining at least part of a thermally insulating zone; a curved resistive heating element ending in a pair of cold tails, the heating element welded to a peripheral region of the heat distribution plate in thermal 15 communication with the heat distribution plate; an electronic temperature sensor located in the thermally insulating zone in thermal communication with the contact plate, the electronic temperature sensor being thermally insulated from the heat distribution plate by the thermally insulating zone, the electronic temperature sensor sensing temperature in a continuous range; 20 a controller for controlling the heating element; and a button arrangement in communication with the controller for a user to select between different heating modes of the controller, wherein the controller controls the heating element responsive to the temperature sensed by the electronic temperature sensor and the controller activates the different heating modes dependent on a 25 selection made using the button arrangement.

2. The kettle of claim 1, wherein the electronic temperature sensor is positioned in a sensor housing that protrudes into the heating chamber of the kettle. 30

3. The kettle of claim 2, wherein the sensor housing protrudes through an aperture in the contact plate and a flange portion of the sensor housing is located against the underside of the contact plate. 27

4. The kettle of claim 2, wherein the housing is formed integrally with the contact plate.

5 5. The kettle of claim 2, wherein an annular silicon member is positioned in an aperture in the contact plate within the thermally insulating zone, and wherein the housing is pressed through the centre of an annular silicon member to protrude into the heating chamber. 10

6. The kettle of claim 1, wherein the temperature sensor is supported by a sensor support.

7. The kettle of any one of claims 1-6, wherein the inlet is positioned between the cold tails of the heating element. 15

8. The kettle of claim 7, wherein the electronic temperature sensor is positioned in the vicinity of the cold tails.

9. The kettle of any one of claims 1 to 8, wherein an area of the heat distribution 20 plate is less than an area of the contact plate and wherein the electronic temperature sensor is positioned outside an outer bound of the heat distribution plate in the vicinity of the inlet.

10. The kettle of any one of claims 1 to 9, wherein the contact plate is uniplanar. 25

11. The kettle of any one of claims 1 to 10, wherein the heating modes include a keep-warm mode in which the controller acts to maintain the temperature sensed by the electronic temperature sensor substantially at a specified warm temperature. 30

12. The kettle of any one of claims 1 to 11, wherein the heating modes include a mode for heating the temperature sensed by the electronic temperature sensor to a specified temperature. 28

13. The kettle of any one of claims 1 to 12, wherein the heating modes include a boil mode for heating the temperature sensed by the electronic temperature sensor to a specified boil temperature. 5

14. The kettle of any one of claims 1 to 13, wherein the button arrangement comprises: a keep-warm button; and a boil button, wherein a user can select from at least the keep warm mode and the boil mode. 10

15. The kettle of any one of claims 1 to 14, wherein the inlet includes a portion of the heat distribution plate having a reduced thickness relative to a thickness of a region of the heat distribution plate adjacent to said portion. 15

16. A kettle as described herein with reference to any one of the embodiments as illustrated in the accompanying drawings.