Abstract:
A heating vessel is described that includes a contact plate having a contact surface configured to be in direct thermal communication with the contents located in a heating chamber of the vessel and a heat distribution plate in thermal communication with the contact plate. The heat distribution plate is configured to be remote from the contact plate in at least one region to define a thermally insulating zone. There is a heat source in thermal communication with the heat distribution plate and an electronic temperature sensor located in the thermally insulating zone, in thermal communication with the contact plate. The electronic temperature sensor is thermally insulated from the heat distribution plate by the thermally insulating zone. The measured temperature may be used to control the heating of the kettle to bring the contents lo the boil or to maintain the measured temperature within a specified range.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to heating vessels and in particular to heating vessels that include temperature sensors for accurately detecting the temperature of the heating vessel&#39;s contents during operation. 
       BACKGROUND OF THE INVENTION  
       [0002]    Heating vessels (such as kettles, percolators, mocha makers, rice cookers, slow cookers and electric fry ware) are commonly used to prepare food and drinks. These heating vessels generally 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&#39;s contents. 
         [0003]    Normally the heating vessel has a temperature sensor to sense the temperature of the vessel&#39;s contents. The temperature detected is used to control the operation of the heating vessel. For instance, a kettle has a temperature sensor to detect when water in the kettle is boiling. In the case of a kettle, the temperature sensor is often a mechanical sensor such as a snap-action bimetallic actuator which turns the kettle off once the water has boiled. 
         [0004]    Usually the temperature sensor is mounted to the heat distribution plate. This mounting location greatly reduces the accuracy of the temperature sensor. The temperature sensor senses the temperature of the heat distribution plate and does not directly sense the temperature of the vessel&#39;s contents. Because of this, discrepancies may arise between the measured temperature and the actual temperature of the contents. For a kettle, this may result in the kettle switching off before the water is actually boiling. 
         [0005]    An inaccurate temperature sensor limits the potential functionality of the heating vessel. Since the temperature of the vessel&#39;s contents is not accurately sensed, only a limited range of functions controlled with reference to an approximate temperature reading are possible. For example, in the case of a kettle, it is only possible to stop the kettle boiling based on an approximate boiling point. 
         [0006]    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. 
       SUMMARY OF THE INVENTION  
       [0007]    According to a first aspect of the invention there is provided a heating vessel for heating contents located in a heating chamber of the heating vessel, the heating vessel including a contact plate having a contact surface configured to be in direct thermal communication with the contents located In the heating chamber of the vessel; a heat distribution plate in thermal communication with the contact plate, the heat distribution plate being shaped so that the heat distribution plate is remote from the contact plate in at least one region to define a thermally insulating zone; a heat source in thermal communication with the heat distribution plate; and 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. 
         [0008]    According to a second aspect of the invention there is provided a method for producing a heating vessel for heating contents located in a heating chamber of the heating vessel, the method including the steps of providing a contact plate with a contact surface configured to be in direct thermal communication with the contents located in the heating chamber of the vessel; attaching a heat distribution plate to an underside of the contact plate so that the heat distribution plate is in thermal communication with the non-contact surface of the contact plate; removing at least one portion of the heat distribution plate to shape the heat distribution plate so that the heat distribution plate is remote to the contact plate in at least one region to form a thermally insulating zone; providing a heat source in thermal communication with the heat distribution plate; and mounting an electronic temperature sensor in the thermally insulating zone, in direct thermal communication with the contact plate, the electronic temperature sensor being thermally insulated from the heat distribution plate by the thermally insulating zone. 
         [0009]    According to a third aspect of the invention there is provided a heating vessel comprising a heating chamber for holding material to be heated; a heat source in thermal communication with the heating chamber; a temperature sensor that generates a temperature signal related to a temperature of the material in the heating chamber; a load sensor that generates a load signal related to a quantity of material held in the heating chamber; and a heat-source controller operable to control the heat source dependent on the load signal and the temperature signal. 
         [0010]    According to a further aspect of the invention there is provided a method for controlling a heat source that heats material held In a heating chamber of a heating vessel, the method comprising the steps of generating a temperature signal related to the temperature of the material in the heating chamber; generating a load signal related to an amount of material held in the heating chamber; selecting a threshold value dependent on the load signal; and switching off the heat source if the temperature signal is greater than or equal to the selected threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0011]    Embodiments of the invention will now be described with reference to the drawings, in which: 
           [0012]      FIG. 1  is a cross-sectional drawing of an electric kettle; 
           [0013]      FIG. 2  is a partially cut-away view of a heater assembly for the kettle of  FIG. 1 ; 
           [0014]      FIG. 3  shows more detail of the heater assembly of  FIG. 2  including an electronic temperature sensor and heat-source controller; 
           [0015]      FIG. 4  shows a cross-sectional view of part of the heater assembly; 
           [0016]      FIG. 5  shows an arrangement in which the heater assembly is positioned on a concave contact plate of the kettle; 
           [0017]      FIG. 6  shows an arrangement in which the heater assembly is positioned on a convex contact plate of the kettle; 
           [0018]      FIG. 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; 
           [0019]      FIG. 8  is a graph illustrating the operation of the kettle in the boil mode; 
           [0020]      FIG. 9  is a graph illustrating the operation of the kettle in the ‘keep warm’ mode; 
           [0021]      FIG. 10  is a graph illustrating the effect of adding water to the kettle during the keep warm mode; 
           [0022]      FIGS. 11 and 12  are graphs comparing the temperature of water in the kettle with the temperature measured by the temperature sensor of  FIG. 2 ; 
           [0023]      FIG. 13  is a graph comparing the performance of the kettle of  FIGS. 1 to 7  with the performance of a standard kettle; 
           [0024]      FIG. 14  shows a bottom view of an alternative heater assembly for use in the kettle of  FIG. 1 ; 
           [0025]      FIG. 15  shows a cross-sectional side view of the heater assembly of  FIG. 14 ; 
           [0026]      FIG. 16  shows a further view of the heater assembly of  FIG. 14 , illustrating the threaded mounting of a temperature sensor; and 
           [0027]      FIG. 17  is a flow diagram illustrating a method of selecting the cut-off temperature dependent on the load in the kettle. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0028]      FIG. 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 . 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 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. 
         [0029]    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  FIGS. 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. 
         [0030]    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. 
         [0031]    The heating element  20  is bonded to a heat distribution plate  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 distribution plate  24 . Many known bonding techniques are suitable, including induction welding, flame or oven welding and impact welding. Alternatively the heating element  20  may be mounted to the heat distribution plate  24  using other known techniques, such as mechanical fasteners. 
         [0032]    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 . Many other known bonding techniques are suitable, including the bonding techniques mentioned above. Alternatively the heat distribution plate  24  may be mounted to the contact plate  16  using other known techniques, such as mechanical fasteners. 
         [0033]    The heat distribution plate  24  may be formed from aluminium, which is a good thermal conductor, and is of sufficient thickness so that heat is evenly distributed over the contact plate  16 . 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. 
         [0034]    The heat distribution plate  24  defines a void  26 . 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 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. 
         [0035]    Mounted in the void  26  is an electronic temperature sensor  28 . The void  26  provides a thermally insulating zone around the electronic temperature sensor  28 . Heat from the 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 . 
         [0036]    Preferably the thermally insulating zone and the temperature sensor  28  are 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 filled, either partially or wholly, with an insulating material, such as silicone or rubber. 
         [0037]    The temperature sensor  28  is mounted 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 contact plate  16 . The thermal coupling may be further improved using known techniques, such as applying a heat transfer paste. 
         [0038]    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 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. 
         [0039]      FIGS. 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 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 mounted in a number of ways which aim to minimise the influence of heat from the heat distribution plate  24 . 
         [0040]    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 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 mounted in a thermally insulating zone in direct thermal communication with the contact plate  16 . 
         [0041]    The heater assembly  18  shown in  FIGS. 2 to 4  has a single void  26  in which the temperature sensor  28  is located, It is also possible to have multiple voids around the temperature sensor. Each void forms 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 mounted in direct thermal contact with the contact plate  16 . 
         [0042]    In one arrangement the contact plate  16  is indent-free. The contact plate  16  shown in  FIGS. 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 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 . 
         [0043]    Alternative arrangements are shown in  FIGS. 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 .  FIG. 5  shows a concave contact plate  33  which is curved towards the heater assembly  18  in the centre of the contact plate.  FIG. 6  shows a convex contact plate  35  which is curved away from the heater assembly  18  in the centre of the contact plate. In the case of  FIG. 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 lower-most 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 . This may improve the accuracy of temperature readings made by the sensor  28 . In further arrangements (not illustrated) 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. 
         [0044]    Referring again to  FIGS. 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 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. 
         [0045]    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 0° C. and 100° C. 
         [0046]    The functions of the kettle  10  are operated by button arrangement  36  which Is shown in  FIG. 7 . The button arrangement  36  consist of 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 translucent. These rings are illuminated by LEDs to provide a user with information regarding the kettle&#39;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 connected to, and provide input to, the controller  34 . The lights are connected to, and are operated by, the controller  34 . 
         [0047]    When the boil button  38  is activated, the controller  34  enters a boil mode, graphically displayed in  FIG. 8 . Before activation, the controller  34  is in a standby mode (indicated by “Area  1 ” in  FIG. 8 ). After activation, the controller  34  enters the boil mode (indicated by “Area  2 ” in  FIG. 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. 
         [0048]    The temperature sensor  28  detects when an upper boiling limit has been reached. The upper boiling limit may be 97° C., though other limits are also suitable. At this point the controller enters a “boiled” mode (indicated by “Area  3 ” in  FIG. 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. 
         [0049]    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  FIG. 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. 
         [0050]    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 example about 85° C. The keep warm mode is graphically illustrated in  FIG. 9 . Prior to activation, the controller is in the standby mode (indicated by “Area  1 ” in  FIG. 9 ) and the water is at ambient temperature. In the keep warm mode (indicated by “Area  2 ” in  FIG. 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 to illuminate the illuminated ring  44  to indicate that the controller  34  is in the keep warm mode. 
         [0051]    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 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 water at an average temperature. 
         [0052]    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 avoid the kettle boiling dry. Alternatively, the keep warm mode may be ended automatically after four hours. 
         [0053]    In one arrangement the kettle  10  may have two or more heating modes dependent on 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 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. 
         [0054]    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. 
         [0055]    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. 
         [0056]    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 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. 
         [0057]    In the keep warm operation, the controller  34  may also select a different upper boiling limit depending on the rate of change of temperature. 
         [0058]    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 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 higher or lower boiling limit dependent on whether the level of fluid is above or below a threshold value. 
         [0059]      FIG. 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 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 greater than or equal to the threshold value. 
         [0060]      FIG. 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 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. 
         [0061]    When the kettle is refilled, the temperature of the water in the kettle drops rapidly, as indicated at reference numeral  56  in  FIG. 10 . When the controller  34  resumes the keep warm 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. 
         [0062]    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 controller  34 . 
         [0063]    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 function properly.  FIGS. 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 temperatures sensed by the temperatures sensor. As can be seen, the two lines are closely matched. 
         [0064]      FIG. 13  shows a comparison between the temperature sensed during boiling by a temperature sensor in a conventional kettle (denoted “STD Kettle” in  FIG. 13 ) and a temperature sensor in a kettle using the arrangements described herein (denoted “Elec Kettle” in  FIG. 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  FIG. 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 electric 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 bimetallic switch used in the standard kettle, 
         [0065]      FIGS. 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  FIGS. 2 to 6 . 
         [0066]    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 screws into the aperture  76  so that the sensor is in direct thermal contact with the contact plate  64 . 
         [0067]    The heating assembly shown in  FIGS. 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 plate. Any one of the bonding methods described above may be used. 
         [0068]    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 mounted in the void in direct thermal communication with the exposed contact plate. Finally the controller is produced and mounted to the heat distribution plate. 
         [0069]    The heater assembly shown in  FIGS. 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 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 direct 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 mounted by screwing a threaded sensor casing into the sensor mount. 
         [0070]    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 sensor  70 . Further, the heat distribution plate does not need to be in direct contact with the contact plate and the heating element does not need to be in direct contact with the heat distribution plate, so long as these components are in thermal communication with each other. 
         [0071]    The principles of the present invention may be applied to other types of heating vessels, such as percolators, mocha makers, rice cookers, slow cookers and electric fry ware. 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 contact (and possibly direct contact) with a contact plate. In the case of fry ware, the heating chamber is the pan. 
         [0072]    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. 
         [0073]    The term “comprises” (or its grammatical variants) is used in this specification as equivalent to the term “includes” and neither term should be taken as excluding the presence of other elements or features.