AU2012265568A1

AU2012265568A1 - Improved temperature sensor for an electric kettle

Info

Publication number: AU2012265568A1
Application number: AU2012265568A
Authority: AU
Inventors: Raymond George Corkin; Nick O'loughlin; Alex Tofaili
Original assignee: Sunbeam Corp Ltd; Sunbeam Corp
Current assignee: Newell Australia Pty Ltd
Priority date: 2011-12-23
Filing date: 2012-12-14
Publication date: 2013-07-11
Anticipated expiration: 2032-12-14
Also published as: AU2012265568B2

Abstract

An electric kettle (10) for heating water located in a heating chamber (12) of the kettle, the kettle including: a contact plate (16) 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 (12) of the kettle; a heat distribution plate (24) bonded to an underside of the contact plate (16), wherein an inlet is defined in a peripheral region of the heat distribution plate (24), 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 (24); an electronic temperature sensor (28) located in the thermally insulating zone in thermal communication with the heating chamber (12), the electronic temperature sensor (28) being thermally insulated from the heat distribution plate (24) 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 electronic temperature sensor and dependent on a selection made using the user input. Fiqure 1 i04 Figure 47

Description

N100/01 1 Regulation 32 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 accurately detecting the temperature of the heating vessel's contents during operation. 5 Background of the invention Many electric kettles heat water with a sheathed element that is immersed in a heating chamber of the kettle. Other types of electric kettles include an electric heating element located outside the heating chamber which heats a contact plate that is in contact with the vessel's contents. In such arrangements a heat distribution plate may be provided between the heating 10 element and the contact plate to distribute heat evenly across the contact plate. 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. A kettle generally has a temperature sensor to detect when water in the kettle is boiling. 15 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 kettles, steam from boiling water may be directed to the bi-metallic sensor to turn off the kettle. 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 20 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. Since the temperature of the vessel's contents is not accurately sensed, only a limited range of 25 functions controlled with reference to an approximate temperature reading are possible, in particular for accurately controlling temperature settings below boiling point.

3 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 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. 5 Summary of the invention In a first aspect, there is provided an electric heating device comprising: a substrate having a first area and a second area; an electrically resistive heating layer disposed at the first area for heating the substrate, wherein the electrically resistive heating layer is not disposed at the second area; and an electronic temperature sensor located to measure temperature at the 10 second area, and distanced from the electrically resistive heating layer to limit thermal communication between the electronic temperature sensor and the electrically resistive heating layer. In one form, the electric heating device is a kettle. In one form, the electric heating device further comprises; a controller for controlling the 15 electrically resistive heating layer; and a user interface to select between different heating modes of the controller, wherein the controller controls the electrically resistive heating layer responsive to the temperature sensed by the electronic temperature sensor and dependent on a selection made with the user interface. In one form, the heating modes include a specified temperature mode in which the 20 controller acts to maintain the temperature sensed by the electronic temperature sensor substantially at a specified temperature. In one form of the electric heating device, the first area and second area are disposed on a first surface of the substrate. In a further form, the second area is surrounded by the first area. In yet another form, the second area is adjacent the first area. 25 In one form of the electric heating device, the substrate comprises a material with low thermal expansion properties. In one form, the substrate is made of ceramic or a glass-ceramic. A glass-ceramic that may be suitable is SCHOTT CERANTM, offered by SCHOTT AG, 10 Hattenbergstrasse, 55122 Mainz, Germany. This material, in a panel form, has low thermal conductivity. Furthermore, this material has high temperature and mechanical stability, and 4 resistance to thermal shock. The material may also assist in heat transfer by allowing transmittance of infrared radiation. In one form of the electric heating device, the electrically resistive heating layer comprises a printed heating element. In a further form, the printed heating element comprises a 5 printed track with a tortuous path or labyrinth like path. In another form of the electric heating device, the electrically resistive heating layer comprises a film heating element. In another form of the electric heating device, there is provided one or more further electronic temperature sensors located to measure temperature at the second area, and distanced 10 from the electrically resistive heating layer to limit thermal communication between the further electronic temperature sensors and the electrically resistive heating layer. In another form of the electric heating device, the substrate has one or more further areas, wherein the electrically resistive layer is not disposed at the further areas, and wherein one or more additional electronic temperature sensors are located to measure temperature at the further 15 areas, the additional electronic temperature sensors distanced from the electrically resistive heating layer to limit thermal communication between the additional electronic temperature sensors and the electrically resistive heating layer. In another aspect, there is provided a kettle for heating water located in a heating chamber of the kettle, the kettle comprising: a substrate in thermal communication with the 20 heating chamber, the substrate having a first surface outside the heating chamber, and the first surface having a first area and a second area; an electrically resistive heating layer disposed at the first area for heating the substrate, wherein the electrically resistive heating layer is not disposed at the second area; and an electronic temperature sensor located to measure temperature at the second area, and distanced from the electrically resistive heating layer to limit thermal 25 communication between the electronic temperature sensor and the electrically resistive heating layer. In one form, the kettle further comprises: a controller for controlling the electrically resistive heating layer; and a user interface to select between different heating modes of the controller, wherein the controller controls the electrically resistive heating layer responsive to the 5 temperature sensed by the electronic temperature sensor and dependent on a selection made with the user interface. As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to 5 exclude further additives, components, integers or steps. Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings. Brief description of the drawings 10 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; Figure 2 is a partially cut-away view of a heater assembly for the kettle of Figure 1; 15 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 concave contact plate of the kettle; 20 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 of the kettle; 25 Figure 8 is a graph illustrating the operation of the kettle in the boil mode; 6 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 keep warm mode; 5 Figures I I 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; Figure 14 shows a bottom view of an alternative heater assembly for use in the 10 kettle of Figure 1; Figure 15 shows a cross-sectional side view of the heater assembly of Figure 14; 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 15 temperature dependent on the load in the kettle; 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 20 with a further arrangement for a thermally insulating zone; 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 25 zone includes an inlet projecting into the heat distribution plate; 7 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 5 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. Figure 27 is a partially sectioned bottom perspective view of an electric heating device in the form of a kettle; 10 Figure 28 is a bottom perspective view of a substrate with an electrically resistive heating layer for the kettle in Figure 27; Figure 29 is a partially sectioned bottom perspective view of a side wall of the kettle in Figure 27; Figure 30 is a close up view of a portion of the partially sectioned bottom 15 perspective view in Figure 29; Figure 31 is a partially sectioned bottom perspective view of a side wall and substrate of the kettle in Figure 27; Figure 32 is a close up view of a portion of the partially sectioned bottom perspective view in Figure 5; 20 Figure 33 is a partially sectioned top perspective view of a support assembly for the kettle in Figure 27; Figure 34 is a top perspective view of the support assembly of Figure 33 and a control unit; Figure 35 is a top perspective view of the support assembly of Figure 33 with the 25 control unit secured to the support assembly; 8 Figure 36 is an alternative perspective view of the support assembly and control unit in Figure 35; Figure 37 is a perspective view of the substrate for an embodiment of the kettle; Figure 38 is a perspective view of the substrate with the electrically resistive 5 heating layer according to one embodiment; Figure 39 is a perspective view of the substrate with the electrically resistive heating layer according to another embodiment; Figure 40 is a perspective view of the substrate with the electrically resistive heating layer according to yet another embodiment; 10 Figure 41 is a close up perspective view of the electronic temperature sensor located with the substrate of Figure 38; Figure 42 is a close up perspective view of the electronic temperature sensor located with the substrate of Figure 39; Figure 43. is a close up perspective view of the electronic temperature sensor 15 located with the substrate of Figure 40; Figure 44 is a bottom perspective view of the substrate with the electrically resistive heating layer having a labyrinth form in accordance with another embodiment; Figure 45 is a bottom perspective view of the substrate of Figure 44 with an electronic temperature sensor located to measure temperature of the substrate; 20 Figure 46 is a bottom perspective view of the substrate of Figure 45 with the control unit located with the substrate; and Figure 47 is a bottom perspective view of the substrate with the electrically resistive heating layer in another form in accordance with another embodiment.

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 for a user to 5 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 in the heating chamber 12 is in direct contact with one side of the contact plate 16. The contact 10 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 contact plate to the heating chamber 12. One embodiment of the heater assembly 18 is shown in greater detail in 15 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. 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 20 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. 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 25 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 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 30 thermal coupling between the heat distribution plate 24 and the contact plate 16. Many other 10 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 5 thermal conductor, and is of sufficient thickness to evenly distribute heat from the 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. 10 The heat distribution plate 24 defines a void 26. In the arrangement of Figure 2 the void 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 15 26. The region of the contact plate 16 located 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 heat 20 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. 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 25 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 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 30 thermal coupling between the electronic temperature sensor 28 and the contact plate 16. The 11 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 5 kettle 10 heats up, the contact plate 16 will heat to a similar temperature. Due to 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. 10 Figures 2 to 4 show the temperature sensor 28 being supported by a sensor support 30. 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 15 the sensor 28 is insulated and may press the sensor 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. The temperature sensor 28 is typically a thermistor. NTC thermistors formed from metal 20 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 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 25 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 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 30 temperature sensor 28 is located. As discussed further below (with reference to figures 18 to 22), 12 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 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 5 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 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 10 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 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 15 temperature of all of the water in the kettle 10. 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 35 which is curved away from the heater 20 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 sensor 28 is more likely to reflect the average temperature of the water contained in the kettle 10. 25 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 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 30 protrusion.

13 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 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 5 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. 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 10 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 0 *C and 100 *C. 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, 15 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 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 20 filament bulb. A "standby" LED 46 is illuminated when the external power is connected to the kettle. The buttons are 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 25 "Area 1" in Figure 8). After activation, the controller 34 enters the boil 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.

14 The temperature sensor 28 detects when an upper boiling limit has been reached. The upper boiling limit may be set at 97*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 5 controller then turns on, for example, a green light in the illuminated ring 42 to indicate that the water is boiled. 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 10 "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 no longer at or near boiling temperature. A suitable lower boiling limit is 92*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 temperature, for 15 example about 85 'C. 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 l" 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 previously. The controller 34 also causes a, for example, amber light 20 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 gradually until a lower warm limit is reached, as indicated at reference numeral 52. A suitable lower warm limit is 25 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 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 30 water at an average temperature.

15 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 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 5 avoid the kettle boiling dry. Alternatively, the keep warm mode may be ended automatically after a set time, for example four hours. 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 the 10 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 switched off at a reduced upper boiling limit. In the 15 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 0 C. The controller 34 monitors the rate of change of measured temperature on a regular basis 20 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 may have a look-up table that lists a suitable upper boiling limit corresponding to different rates of 25 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 4*C lower than the selected upper boiling limit. In the keep warm operation, the controller 34 may also select a different upper boiling limit depending on the rate of change of temperature.

16 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 capacitive sensor may be used to indicate the level in the kettle. In such an arrangement, the controller 34 may select a 5 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 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 10 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 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 15 greater than or equal to the threshold value. Figure 10 shows the kettle 10 being refilled whilst the keep warm mode is activated. To 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 20 memory is preferably EPROM, though other types of memory may be used. Once the kettle is refilled, it is reconnected to the external power 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 25 mode, the water is reheated until the upper warm limit is reached. The keep 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 or more buzzers or speakers. The controller mode indicator is connected to, and operated by the 30 controller 34.

17 The additional functionality described above is made possible by the arrangements described herein. These arrangements provide a temperature sensor which is able to 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 5 function properly. Figures I1 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 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 10 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 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 15 seen in Figure 13, for a standard kettle the heating element stays on for a relatively long duration after the water boils, as indicated 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 20 accuracy of the temperature arrangement described herein compared with the 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 25 in relation to Figures 2 to 6. 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-threaded brass casing which 30 screws into the aperture 76 so that the sensor is in thermal contact with the contact plate 64.

18 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, the heat distribution plate need not have a void. A heating element is then bonded to the heat distribution 5 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 distribution plate is bonded to the contact plate. The sensor is then positioned in the void in thermal communication with the 10 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 of the toroidal void is used as a sensor mount. A hole in the centre of the sensor mount is formed to allow the 15 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 and position the temperature sensor. Further embodiments of the invention will now be described with reference to Figures 18 20 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 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 25 shown as rectangular regions. In practice, of course, the regions may be rounded or of 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 30 figure 19, the thermally insulating zone in this embodiment includes a plurality of voids 88 19 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 5 heat distribution plate 80. 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 10 80 or by milling/ routing as described above) in which the ribs 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 15 insulation plate 80. In this arrangement a thin section 92 of the 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. The sensor support 84 may be part of the heat distribution plate 80 that remains after the 20 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 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 25 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 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 30 sensor support 84 and/or temperature sensor 82 is limited.

20 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. 5 While in the embodiments described with reference to figures 19 to 22 the temperature 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 10 similar (such as bracket 32 described above in relation 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 15 directly to and 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 element 20 is bonded to a peripheral region of a heat distribution plate 102, which is generally circular. The 20 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 circular in the illustrated arrangement. A U-shaped inlet 108 is formed at one side of the heat distribution plate 102. The contact 25 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 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 30 shape.

21 In the illustrated arrangement the inlet 108 is positioned between the cold tails 22. This 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 5 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 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. The inlet 108 may have different shapes. A further example is illustrated in Figure 24, 10 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 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 15 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 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 20 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 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 25 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 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 30 housing 108 may be stainless steel or some other corrosion-resistant metal. The housing may 22 contain a 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 5 from the heating effect of the heat distribution plate. 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 centre of the annular silicon member 10 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 inside the housing 116. 15 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 housing 120 does not have an annular flange around its base. Figure 26D shows an arrangement in which a relatively narrow 20 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 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 25 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 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 30 portion of the contact plate between the edge of the kettle and the heat distribution plate thus 23 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 such an arrangement) 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 5 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 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 10 dome. Of course only part of the contact plate need be dome shaped, and not the entire contact plate as shown in Figure 6. It will also be understood that in some embodiments, the kettle may have 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 15 heating element by a thermally insulating zone and is in thermal contact with a contact plate. Electric kettle with a thin heating element. Figure 27 shows a partially sectioned view of a heating appliance 501 that is part of an electric kettle 503. The kettle 503 has a side wall 505, and a base wall to define a heating chamber 507 where a volume of water can be heated. The base wall is defined by a substrate 20 509, having two substantial surfaces, a first surface 511 facing away from the heating chamber 507, and a second surface 513 facing the heating chamber 507 and opposite the first surface 511. The substantially planar first surface 511 has a first area 515, and a second area 517. An electrically resistive heating layer 519 is disposed at a first area 515 for heating the substrate 509, which in turn heats the volume of water in the heating chamber 507. An electronic temperature 25 sensor 520 is located to measure temperature at the second area 517. The electronic temperature sensor 520 is distanced from the electrically resistive heating layer 519 to limit thermal communication between the electronic temperature sensor 520 and the electrically resistive heating layer 519. In Figure 27, the electronic temperature sensor 520 and sensor support 521 are in direct 30 contact with the second area 517, although it can be appreciated the electronic temperature 24 sensor 520 may measure temperature without direct contact, such as being in thermal communication with one another by conduction through an intermediate element, or by measuring radiation emitted or reflected from the second area 517 to the electronic temperature sensor 520. 5 Below the side wall 505 and the base wall is a support assembly 523. In addition to supporting the side wall 505 and base wall of the kettle 503, the support assembly 523 bolsters brackets 525 for the sensor support 521. In the illustrated embodiment, a control unit 527 is secured to the support assembly 523. The control unit 527 has a power connector 529 for connecting to, and receiving electrical power 10 from a power base (not shown). The control unit 527 may communicate with a controller (not shown) for controlling electrical current to the electrically resistive heating layer 519. The controller may be located within, or be part of the control unit 527. Alternatively, the controller may be located external to the control unit 527, such as in the support assembly 523, or in a power base. The controller may include a printed circuit board with a microprocessor or 15 microcontroller. In some arrangements the printed circuit board and microprocessor may be located in the power base. In such an arrangement, it may be desirable to use a multi pin power connector (such as a 5 pin power connector), to provide power and electronic communication between the power base and the control unit 527. 20 A user interface (not shown) is provided to select between different heating modes of the controller, whereby the controller controlling the electrically resistive heating layer 519 is responsive to the temperature sensed by the electronic temperature sensor 520 and dependent on a selection made with the user interface. The components of the kettle 503 will now be described in detail. 25 Figure 28 illustrates the substrate 509 with the electrically resistive heating layer 519. The substrate 509 is substantially disc shaped, with the first and second surfaces 511, 513 on opposing sides. In one form, the substrate 509 is made of a material having low thermal expansion properties. This may include SCHOTT CERANTM. The first area 515 of the first surface 511, on which the electrically resistive heating layer 519 is disposed, is substantially 25 annular with the exception of the interruption of the second area 517. Thus an opening 518 is provided through the electrically resistive heating layer 519 at the region of the second area 517. The opening 518 permits the electronic temperature sensor 520 to measure the temperature of the substrate 509 at the second area 517. 5 A further opening 532 through the centre of the annular electrically resistive heating layer 519 defines a central area 531 of the first surface 511. This further opening allows direct contact and thermal conduction to bi-metallic snap actuators 533 that will be discussed below. The electrically resistive heating layer 519 includes a material having an electrical resistance, whereby current passed through the heating layer 519 causes the heating layer 519 to 10 heat up. The heating layer 519 transfers heat to the first area 515 of the substrate 509 including by conduction and/or radiation. The electrically resistive heating layer 519 may be in the form of a film applied to substrate 509. The application of the heating layer 519 may include printing, spraying, gluing, moulding, vulcanizing and other means. In one form, the electrically resistive heating layer 519 15 may be a track applied to the substrate 509. To optimize heat transfer to the substrate 509, the track may be applied in a labyrinth form. This provides an increased track length, and may assist in evenly distributing heat to the substrate 509. It may also optimise a contact surface area between the heating layer 519 and the first surface 511. Figures 29 and 30 illustrate the side wall 505 of the kettle 503. The side wall 505 is in 20 the form of a continuous side wall, forming a substantially cylindrical heating chamber 507. An annular flange 535 extends perpendicular to the side wall 505, in a direction radially inwards to the heating chamber 507. Bosses 537 with internal threading extend downwardly from the annular flange 535. The bosses 537 provide fastening points to secure the side wall 505 to the support assembly 523, which in turn allows the substrate 509 to be sealingly clamped in position. 25 Figures 31 and 32 illustrate the substrate 509 located with the side wall 505. A seal 539, which may be in the form of a gasket or glue, provides a sealed arrangement between the substrate 509 and the flange 535. In the illustrated embodiment, the seal 539 is in the form of an annular band with an inwardly facing C-shaped cross-section that wraps over the edge surfaces of the disc-shaped substrate 509. Referring to Figure 27, the seal 539 is clamped in between the 30 flange 535 and a surface 545 of the support assembly 523, which in turn clamps against the edge 26 of the substrate 509. This provides a sealed arrangement between the substrate 509 and the side wall 505 to define the heating chamber 507. Figures 33 to 36 illustrates the support assembly 523 with and without the control unit 527. The support assembly 523, has fastening protrusions 541, each having apertures 543 for 5 allowing a fastener (not shown) to secure the support assembly 523 to the bosses 537 of the side wall 505. The top surface 545 of the fastening protrusion, as discussed above, provides one half of the clamp against the seal 539, although in other embodiments, the top surface 545 can clamp directly against the substrate 509, or alternatively not at all. Alternatively, the substrate 509 may be glued or otherwise sealed to the side wall 505, and it may not be necessary to clamp the 10 substrate 509. The sensor support 521 is attached to bracket 525, which in turn is secured to the support assembly 523. The bracket 525 may be made of steel, stainless steel or any other suitable material. The sensor support 521, that holds the temperature sensor 520, may be made of a 15 resilient material, such as silicone. The resilience of the sensor support 521 allows the temperature sensor 520 of the assembled kettle 503, to be biased against the second area 517. This maintains good physical contact and thermal communication between the temperature sensor 520 and the exposed area 517 of the substrate 509, even if the assembled components of the kettle 503 are manufactured with loose tolerances, or the assembly becomes loose over time. 20 Furthermore, the silicone material may act as thermal insulation between the temperature sensor 520 and the support assembly 523. The electronic temperature sensor 520 may include a thermocouple to produce a measureable voltage having a known relationship with the temperature of the thermocouple. In another form, the electronic temperature sensor 520 includes a resistance temperature detector 25 (RTD), such as a thermistor, which contains a material with a known electrical resistance at various temperatures, and whereby the resistance is measured to determine the temperature at the measured location. However, it is to be appreciated that any other suitable electronic temperature sensor may be used. The control unit 527 is secured to the support assembly 523, via fasteners (not shown) 30 passing through apertures 547. The apertures 547 may seat a resilient element, such as a silicone 27 washer to bias the control unit 527 towards the substrate 509 when the kettle 503 is assembled. This ensures good contact between the sensor 520 and bi-metallic snap actuators 533 with the substrate 9. Alternatively, springs may be located to bias the control unit 527 to the substrate 509. 5 As best illustrated in Figure 36, the control unit 527 is secured such that the connector 529 is accessible from the bottom of the support assembly 523. This allows the kettle 503 to be placed onto a power base (not shown) to provide power to heat the water, whilst allowing the user to remove the kettle 503 to dispense water from the kettle 503 at another location. Figure 36 shows a (stylised) 5 pin connector 529, although other configurations such as a 10 3 pin connection may be used. The bi-metallic snap actuators 533 secured to the control unit 527, and in contact with the central area 531 of the substrate 509, provide a safety shut-off of the power to the electrically resistive heating layer 519. These snap actuators 533 may operate as part of a thermal fuse to prevent over heating of the heating device 501. As with the temperature sensors 520 and sensor 15 supports 521, the bi-metallic snap actuators and/or the associated components of the thermal fuse, when the kettle is assembled, may be biased towards the central area 531 to maintain contact with the substrate 509. This bias may be due to securing the snap actuators with a resilient material, such as silicone, to ensure the bi-metallic snap actuators 533 are in physical contact and thermal communication with the central area 531. This may also provide thermal 20 insulation between the bi-metallic snap actuators 533/thermal fuse, and the control unit 527/support assembly 523. In one embodiment, the bi-metallic snap actuators in a normal state allow electrical current to pass through an electric circuit. This electric circuit is part of an electric circuit that directly or indirectly provides power, or is an electric circuit that controls power to the 25 electrically resistive heating layer 519. When the bi-metallic snap actuators 533 reach a high temperature threshold (such as when the kettle is switched on without water in the heating chamber 507, or a logic or electronic failure of the controller), the snap actuators distort in shape, thereby breaking the electric circuit. In turn, this stops power to the electrically resistive heating layer 519.

28 Operation The operation of the electronic temperature sensor 520 to measure the temperature of the substrate 509 will now be described with reference to Figure 27. The electronic temperature sensor 520 measures temperature from the second area 517 of the substrate 509. In the 5 illustrated embodiment, the temperature sensor 520 is in direct contact with the second area 517, thereby allowing measurement of the temperature by conduction from the substrate 509. As shown in Figure 27, the electronic temperature sensor 520 does not contact the electrically resistive heating layer 519 that is disposed at the first area 515. The electronic temperature sensor 520 is thus distanced from the electrically resistive heating layer to limit 10 thermal communication, in particular conduction of heat, directly between the electronic temperature sensor 520 and the electrically resistive heating layer 519. It is to be appreciated some indirect conduction from the electrically resistive heating layer 519 to the electronic temperature sensor 520 may occur via the substrate 509. Furthermore, some thermal communication between the electronic temperature sensor 520 and the electrically resistive 15 heating layer 519 may occur through radiation and convection. To minimize this, the electronic temperature sensor support 521 may be provided with a shield (not shown) to limit thermal communication by radiation and convection. To obtain an indication of the heating temperature of the water in the heating chamber 507 (or any other area thermally associated with the substrate 509), the temperature of the 20 substrate 509 is measured at the second area 517. As the substrate 509 is in thermal communication with the heating chamber 507, the temperature at the second area 517 provides a suitable measurement to determine the temperature of the water contained in the heating chamber 507. The temperature at the second area 517 may be taken as being approximately the same as the temperature of the water in the heating chamber 507. Alternatively, the temperature 25 of the second area 517 may be used as an input value in a mathematical function to determine the approximate value of the water temperature in the heating chamber 507. Alternatively, the temperature of the second area 517 may be cross-referenced with, or interpolated with known data. This allows approximation of the water temperature by matching the known correlation between the temperature of the second area 517 and the water temperature in the heating 30 chamber 507.

29 The modes of operation of the kettle will now be described. The user, using the user interface, may select a heating mode as well as optionally providing operating parameters to the controller. In a "boil" mode, the electrically resistive heating layer 519 is heated by providing 5 electrical current via the control unit 527 until the water contained in the heating chamber 507 reaches a boiling temperature of approximately 100*C. When the temperature of the water reaches boiling point as detected by the electronic temperature sensor 520, the controller stops the electrical current to the electrically resistive heating layer 519 that is heating the chamber 507, either immediately or with a time delay. 10 In a "heat to specified temperature" mode, the electrically resistive heating layer 519 is heated until the water contained in the heating chamber 507 reaches the specified temperature. The specified temperature may be a parameter entered by the user through the user interface. In a similar operation to the boil mode, once the specified temperature is reached, the controller stops the electrical current to the electrically resistive heating layer 509, either immediately or 15 with a time delay. In a "keep warm" mode, the controller maintains the temperature of the water contained in the heating chamber 507 approximately at a specified temperature or in a temperature range around the specified temperature. The specified temperature may be a parameter specified by the user through the user interface, or in another form, the temperature at which the water in the 20 kettle 503 is at the time the keep warm mode is initiated. In this mode, the controller may provide electrical current to the electrically resistive heating layer when the temperature of the water is below a lower threshold, and conversely, stop supply of electrical current to the heating layer when the temperature of the water is above an upper threshold. Typically, the specified temperature would be in a range of temperatures between the lower and upper threshold, with the 25 thresholds calculated from the specified temperature. In one form the controller may act to vary electrical current depending on the deviation of the temperature of the water from the specified temperature. A "sleep mode" or "auto-shutoff mode" may be provided to pause or stop the keep warm mode after a time period has elapsed to save power. In a "boil and keep warm" mode, the electrically resistive layer 519 is initially heated by 30 providing electrical current via the control unit 527 until the water in the heating chamber 507 30 reaches a boiling temperature. The water in the vessel is then allowed to cool to a specified temperature, or temperature range, whereby the controller enters a "keep warm" phase, where the controller maintains the temperature of the water in the heating chamber 507 in a similar manner to the "keep warm" mode described above. In the "boil and keep warm" mode, the 5 specified temperature may be a parameter specified by the user through the user interface. It is to be appreciated other modes of operation of the kettle 503 may be implemented. The modes may include input from the temperature sensor and also other parameters, including those set by the user or from the context or environment. Fabrication .0 Fabrication of embodiments of the substrate 509 with the electrically resistive heating layer 519 will now be described with reference to Figures 37 to 43. Figure 37 illustrates a substrate 509 without the heating layer 519. The substrate 509 may be made of SCHOTT CERANTM, having low thermal expansion properties which is advantageous as it will lower the risk of expanding and cracking the heating layer 519. 15 The electrically resistive heating layer 519 is applied to the first surface 511 of the substrate 509. As described above, this application may include printing, spraying, gluing, moulding, electroplating, vacuum lamination, bonding, vulcanizing and other means. In one particular form, the electrically resistive heating layer is bonded with adhesives using a high temperature vacuum lamination process. The second area 517 of the first surface 511 may be 20 masked (i.e. covered) during application of the heating layer to provide the opening 518 through the heating layer 519 to the surface 511. Alternatively, the mask may allow the heating layer 519 to be formed with other configurations, including a labyrinth form. In the embodiments illustrated in Figs 38 to 40, electrical contacts 549 in electrical conduction with the heating layer 519, may be applied, either before, concurrently or after the 25 application of the heating layer 519 to the substrate 509. The electrical contacts 549 provide an interface to electrical connections to the control unit 527 to provide electrical current to the heating layer. The electrical contacts 549 may be constructed of copper, gold or any other electrically conductive material suitable for connection with electrical circuits. In one form, electrical connections (not shown) from the control unit 527 or controller to the electrical 30 contacts 549 may be soldered to the electrical contacts 549. In another form, the electrical 31 connections may be clamped or physically abutted to the electrical contacts 549. It should be appreciated any suitable electrical connecting means may be used. The embodiment in Figures 38 and 41 illustrates a small opening 518 and corresponding second area 517 relative to the size of the sensor support 521. In this embodiment, the heating 5 layer 519 is maximized, however the heating layer 519 is located relatively close to the electronic temperature sensor 520. The embodiment in Figures 39 and 42 illustrates a larger opening 518 and corresponding second area 517 when compared to the embodiment in Figures 38 and 41. Advantageously, a greater distance is provided between the heating layer 519 and the electronic temperature sensor 10 520 when compared to the above embodiment. This may assist in further limiting thermal communication between the heating layer 519 and the electronic temperature sensor 520. The embodiment in Figures 40 and 43 illustrates yet an even larger opening 518 and corresponding second area 517. In this embodiment, the opening 518 extends to the outer edge of the heating layer 519. Advantageously, the large exposed second area 517 of the first surface 15 511 of the substrate 509, allows room to directly mount various components, including the electronic temperature sensor 520 and sensor support 521. This arrangement in mounting components to the substrate 509 may be advantageous as the substrate 509 may be made of a stronger material than the heating layer 519. Furthermore, the heating element 519 may operate at a temperature much greater than the second area 517 of the substrate 509 may reach during 20 normal use. Thereby mounting components to the second area 517 may be more practical than securing to the heating element 519, or the first area 515. Advantages In addition to any other advantages disclosed in this description, embodiments of the present disclosure may have the following advantages. 25 By distancing the electronic temperature sensor from the electrically resistive heating layer, thermal communication between the electronic temperature sensor and the electrically resistive heating layer may be limited. Advantageously, this may allow the electronic temperature sensor to obtain a more accurate reading of the second area of the substrate, with limited or without direct interference from the electrically resistive heating layer. In one 32 embodiment, this arrangement is particularly useful in limiting heat from the electrically resistive layer from being thermally communicated to the electronic temperature sensor by radiation or convection. Furthermore, by sensing the temperature of the substrate at the second area, it may be 5 possible to determine the approximate temperature of another area that is thermally associated with the substrate, such as water in the heating cavity of the kettle defined in part by the substrate. The use of a substrate, heated by an electrically resistive heating layer may provide a smaller and simpler construction of components of a kettle. As discussed above, prior art 0 heating devices may include the use of a heating element, a heat distribution plate and a contact plate. These components are generally stacked one on top of each other, which may increase the size and complexity of the heating device. In contrast, embodiments of the disclosure may require fewer components or layers. In one embodiment, the electrically resistive heating layer is a film, thereby reducing the overall thickness compared to prior art devices. .5 Further variations Figures 44 to 46 illustrates one variation of the electrically resistive heating layer 519 on the substrate 509. In this embodiment, the electrically resistive heating layer 519 is in the form of a printed track on the first surface 511 of the substrate 509. The track is in a labyrinth form, providing a tortuous path of the heating layer across the first surface 511. This track 20 arrangement provides an increased electrical path on the heating layer 519, which may be advantageous in providing increased resistance of the heating layer 519. Such an arrangement may also provide improved and/or consistent heat generation of the heating layer 519 across the entire length of the track of the heating layer 519. This arrangement may also provide a more consistent and/or predictable heat generation and transfer to the substrate 509. 25 As illustrated in Figures 44 to 46, the second area 517 is surrounded by the heating layer 519 on the first surface 511, the second area 517 allowing the electronic temperature sensor 520 to measure the temperature of the substrate 509. Furthermore, the central area 531 is also surrounded by the heating layer 519, to provide a thermal junction on the substrate 509 to contact with the bi-metallic snap actuators 533.

33 Another embodiment of the substrate 509 and electrically resistive heating layer 519 for a kettle 503 is illustrated in Figure 47. In this embodiment, two electronic temperature sensors 520 and respective sensor supports 521 are provided for measuring the temperature of the substrate 509. This may be advantageous as a contingency in case of failure of one of the electronic 5 temperature sensors 520. Alternatively, the temperature of different areas of the substrate 509 may be measured. Having more than one temperatures sensor also allows averaging of the temperature readings to provide a better temperature determination of the substrate 509 and other thermally associated areas. Note in the illustrated embodiment, the second areas 517 associated with the electronic temperature sensors 520 are on the side edges of the electrically resistive 10 heating layer 519. Thus in this embodiment there is no clear "opening 518" through the heating layer 519 as apparent in the above described embodiments. 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 15 aspects of the invention.

Claims

2. An electric heating device according to claim 1, further comprising; 10 - a controller for controlling the electrically resistive heating layer; and - a user interface to select between different heating modes of the controller, wherein the controller controls the electrically resistive heating layer responsive to the temperature sensed by the electronic temperature sensor and dependent on a selection made with the user interface. 5 3. An electric heating device according to claim 2, wherein the heating modes include a specified temperature mode in which the controller acts to maintain the temperature sensed by the electronic temperature sensor substantially at a specified temperature.
4. An electric heating device according to any one of the preceding claims, wherein the first area and second area are disposed on a first surface of the substrate. 20 5. An electric heating device according to claim 4, wherein the second area is surrounded by the first area.
6. An electric heating device according to claim 4, wherein the second area is adjacent the first area.
7. An electric heating device according to any one of the preceding claims, wherein the 25 substrate comprises a material with low thermal expansion properties. 35
8. An electric heating device according to claim 7, wherein the substrate comprises SCHOTT CERAN TM glass-ceramic.
9. An electric heating device according to any one of the preceding claims wherein the electrically resistive heating layer comprises a printed heating element. 5 10. An electric heating device according to claim 9 wherein the printed heating element comprises a printed track with a tortuous (labyrinth) path.
11. An electric heating device according to any one of claims 1 to 8, wherein the electrically resistive heating layer comprises a film heating element.
12. An electric heating device according to any one of the preceding claims, further 0 comprising one or more further electronic temperature sensors located to measure temperature at the second area, and distanced from the electrically resistive heating layer to limit thermal communication between the further electronic temperature sensors and the electrically resistive heating layer.
13. An electric heating device according to any one of the preceding claims, wherein the 5 substrate has one or more further areas, wherein the electrically resistive layer is not disposed at the further areas, and wherein one or more additional electronic temperature sensors are located to measure temperature at the further areas, the electronic temperature sensors distanced from the electrically resistive heating layer to limit thermal communication between the additional electronic temperature sensors and the electrically resistive heating layer. 20 14. A kettle for heating water located in a heating chamber of the kettle, the kettle comprising: - a substrate in thermal communication with the heating chamber, the substrate having a first surface outside the heating chamber, and the first surface having a first area and a second area; 25 - an electrically resistive heating layer disposed at the first area for heating the substrate, wherein the electrically resistive heating layer is not disposed at the second area; and 36 - an electronic temperature sensor located to measure temperature at the second area, and distanced from the electrically resistive heating layer to limit thermal communication between the electronic temperature sensor and the electrically resistive heating layer.
15. A kettle according to claim 14, further comprising 5 - a controller for controlling the electrically resistive heating layer; and - a user interface to select between different heating modes of the controller, wherein the controller controls the electrically resistive heating layer responsive to the temperature sensed by the electronic temperature sensor and dependent on a selection made with the user interface.