WO2008150172A1

WO2008150172A1 - Heating element and liquid container provided with such a heating element

Info

Publication number: WO2008150172A1
Application number: PCT/NL2008/050360
Authority: WO
Inventors: Simon Kaastra
Original assignee: Otter Controls Limited
Priority date: 2007-06-06
Filing date: 2008-06-06
Publication date: 2008-12-11
Also published as: EP2163130A1; NL2000685C2; CN101772985B; CN101772985A; EP2163130B1; WO2008150171A1

Abstract

The use of enamel as dielectric intermediate layer in the manufacture of heating elements is known. The dielectric enamel layer is herein arranged on a generally metal substrate for heating, after which metal heating tracks are arranged on the dielectric enamel layer by means of silkscreen techniques. The invention relates to an improved heating element. The invention also relates to a liquid container provided with such a heating element.

Description

Heating element and liquid container provided with such a heating element

The invention relates to a heating element. The invention also relates to a liquid container provided with such a heating element.

The use of enamel as dielectric intermediate layer in the manufacture of heating elements is known. The dielectric enamel layer is herein arranged on a generally metal substrate for heating, after which metal heating tracks are arranged on the dielectric enamel layer by means of silkscreen techniques. Such a heating element is for instance described in Netherlands patent application NL 1014601. Described herein is a heating element, for instance for heating liquid in liquid containers or for heating of heating plates, wherein heat is generated by conducting electric current through the at least one heating track. The heating track is herein arranged via a dielectric layer on a substrate for heating. The intermediate layer with dielectric properties not only provides for a good transmission of the generated heat to the substrate for heating, but also for an electric barrier between the - usually metal - substrate for heating and the heating track, whereby short-circuiting in the heating element can be prevented under normal operating conditions. The dielectric can moreover function as protection against overheating. The heating element according to NL 1014601 is provided for this purpose with an ammeter which can detect the leakage current through the dielectric. The leakage current coming from the heating element depends partly on the electrical resistance of the dielectric. Because the electrical resistance of the dielectric, at least in a determined temperature range, in turn depends on the temperature, and this dependence can in principle be predetermined, the detection of the leakage current through the dielectric provides insight into the temperature thereof. The leakage current which can be detected in simple manner with an ammeter therefore forms a measurement value with which the temperature of the dielectric, and thus of the heating element, can be determined. A protection against overheating can be easily built in by coupling the ammeter to a control for the heating element, whereby the supply of current to the heating element can be reduced or even wholly interrupted when a pre-defined minimal leakage current is detected. Although the known heating element provides a simple detection of temperature changes and protection against overheating, separate provisions must generally be made to enable proper detection of the leakage current. It is thus usually necessary on occasions to for instance amplify or, conversely, attenuate the current strength of the leakage current. It has also been found that the leakage current is generally difficult to detect if the heating element is provided with earthing. In that case a galvanically separated transformer system will have to be incorporated in the earth wire, which is time-consuming.

The international patent application WO2006083162 in the name of applicant provides an improved heating element for detecting a temperature change in the heating element with a view to protection against overheating. The improved known heating element comprises a substrate on which are successively arranged a first dielectric layer, an electrically conductive sensor layer, a second dielectric layer and a heating track. The second dielectric layer will generally have a thickness here of about 100 μm. Owing to the particular assembly of the dielectric a leakage current flowing in the second dielectric layer will preferably be diverted to the sensor layer, since in such a case the first dielectric layer acts as electrically more insulating layer (relative to the second dielectric layer). A possible detection of this leakage current by an ammeter or voltmeter coupled electrically to the electrically conductive layer, or connected thereto in other manner, hereby also becomes possible for very low current strengths or voltages, without separate provisions having to be made for this purpose. However, in addition to the particular advantage of the improved known heating element, the improved known heating element also has a number of drawbacks. A significant drawback of the known heating element is that the production process is relatively labour-intensive and time-consuming. Furthermore, research has shown that gas bubbles possibly formed in the second dielectric layer may result relatively quickly in an accelerated breakdown (electric short-circuiting) between the heating track and the sensor layer, which detracts from the ability to accurately and reliably detect a leakage current flowing through the second dielectric. It has also been found that, because the heating track lies substantially uncovered on a side remote from the second dielectric layer, the heating track is relatively vulnerable and can therefore be damaged relatively quickly. The arranging of an additional protective layer over the heating track would make the production process (even) more time-consuming and labour-intensive, and this is undesirable from a financial and logistical viewpoint. The invention has for its object, while retaining the advantage of the prior art, to provide an improved heating element with which at least one of the above stated drawbacks can be obviated.

The invention provides for this purpose a heating element, comprising: a substrate for heating, at least one first dielectric layer arranged on the conductive substrate, at least one electrically conductive heating track arranged on the first dielectric layer, at least one electrically conductive sensor track arranged on the first dielectric layer at a distance from the heating track, and at least one second dielectric layer arranged on the first dielectric layer, which second dielectric layer connects to at least a part of the heating track and to at least a part of the sensor track. By positioning both the at least one heating track and the at least one sensor track between the first dielectric layer and the second dielectric layer the heating track and the sensor track can be arranged on the first dielectric layer in a single pressing run, which considerably simplifies the production process for manufacturing the heating element according to the invention. In this way the distance between the heating track and the sensor track can moreover be kept relatively large (generally about 500 μm) in simple manner, whereby the chance of accelerated breakdown between the heating track and the sensor track as a result of gas bubbles possibly situated between the two tracks can be reduced significantly. An additional advantage is that in this way the heating track is substantially fully protected by the dielectric layers, which enhances the lifespan of the heating element according to the invention. Using the heating element according to the invention leakage currents can thus be measured in relatively efficient manner and at very low current strengths and/or voltages, whereby the (exceeding of a critical) temperature of the heating element according to the invention can be measured relatively quickly and accurately. Although a single first dielectric layer and a single second dielectric layer are in general usually applied in the heating element according to the invention, it is likewise possible to envisage a plurality of first dielectric layers, preferably arranged on each other, and/or a plurality of second dielectric layers, preferably arranged on each other, being applied in the heating element. The different first dielectric layers can herein be of differing composition and thickness. The same applies for the second dielectric layers in the case they are applied. Additional sensor tracks and/or additional heating tracks can optionally be arranged between the different first dielectric layers (and/or second dielectric layers) in order to enable optimizing of the safety and/or the power of the heating element. The heating track and the sensor track are preferably designed such that there is sufficient potential difference between the two tracks in operative mode to enable the forcing of a leakage current at sufficiently high temperature which flows from the heating track with a high potential to an adjacent part of the sensor track with a low potential.

In a preferred embodiment of the heating element according to the invention the electrical resistance of the first dielectric layer is higher than the electrical resistance of the second dielectric layer at substantially the same temperature. Owing to the further increased electrically insulating action of the first dielectric layer relative to the second dielectric layer, an even more sensitive leakage current measurement is found to be possible. It is advantageous here when the first electric layer is situated closer to the surface for heating than the second dielectric layer. During overheating a leakage current will occur which will flow from the heating track to the adjacent sensor track via the second dielectric layer. The leakage current will here not flow via the first dielectric layer, or at least hardly so, which could result in a dangerous situation for a user of the heating element. Owing to measurement or at least detection of the leakage current, combined if desired with a control of the heating element as already described above, in this preferred embodiment a very sensitive and rapidly responding protection against overheating is obtained. This embodiment has the additional advantage here that the protection against overheating gains in reliability and can for instance withstand improper use. The operation of the protection is thus to a large extent insensitive to whether or not the heating element, and in particular the substrate for heating, is earthed.

In the case that a single heating track and a single adjacent sensor track are applied, such a track configuration is usually also referred to as a bifilar track. At least a part of the heating track and at least a part of the sensor track are preferably given a spiral form. In this way the substrate can be heated in relatively complete and efficient manner by the heating track, wherein a possible leakage current can be measured relatively effectively and reliably. The shortest mutual distance between at least a part of the at least one heating track and at least an adjacent part of the at least one sensor track is here more preferably substantially constant, whereby a substantially parallel orientation of the heating track and the sensor track can be realized. In an alternative preferred embodiment it is also possible to envisage reducing the shortest mutual distance between the heating track and the sensor track in position-selective manner in order to be able to predefine, and thereby optimize, the location of the occurrence of a leakage current. In a particular preferred embodiment the shortest mutual distance between at least a part of the at least one heating track and at least an adjacent part of the at least one sensor track lies between 100 μm and 800 μm, preferably between 400 μm and 600 μm, and more preferably amounts to substantially 500 μm. In this way the substrate can on the one hand be heated sufficiently in that the power density per substrate area can in this manner be kept sufficiently high, and reliable detection of a leakage current can on the other hand be guaranteed.

The at least one heating track and/or the at least one sensor track are preferably coupled to a control unit. Using the control unit a leakage current can on the one hand be detected and the heating element can on the other hand be (de)activated, and more preferably regulated. For the purpose of detecting the leakage current it is also advantageous if the sensor track is coupled electrically to an ammeter and/or a voltmeter. A leakage current can be detected in relatively simple and inexpensive manner by applying the ammeter and/or the voltmeter. The ammeter and/or the voltmeter will here generally also take an earthed form in order to be able to detect a potential difference between the fixed world and the sensor track.

In a preferred embodiment at least one electrically conductive element is arranged on the second dielectric layer. The electrically conductive element, such as for instance a layer of silver, may be arranged in position-selective manner on a side of the second dielectric layer remote from the heating track and the sensor track. The electrically conductive element facilitates the flow of a leakage current between the heating track and the sensor track. The electrically conductive element is also adapted to function as additional safety provision in the case the electronic regulation fails at sufficiently high temperature. The electrically conductive element is adapted to be able to facilitate, and thereby guarantee, the flow of a leakage current from the heating track to a heating track with a lower potential when a critical temperature is exceeded. The electrically conductive element, for instance formed by a silver strip, in fact functions here as a bridge for the purpose of being able to facilitate the flow of a leakage current from the heating track to a (heating) track with a lower potential at sufficiently high temperature. The temperature of the second dielectric layer will increase sharply locally due to the leakage current which flows from a heating track via the second dielectric layer, via the conductive element and once again via the second dielectric layer to a track, such as for instance a heating track or a (passive) sensor track, with lower potential. As a result of this local overheating, the heating track will melt locally and the circuit through the heating track will be interrupted. It is also possible to envisage applying a plurality of electrically conductive elements in order to enable the generation of a plurality of leakage current bridges. It is usually advantageous here if the electrically conductive element at least partially crosses the heating tracks with differing potential (in a top view or bottom view of the heating element) in order to be able to limit the resistance between the heating tracks with different potential such that a leakage current will begin to flow between the heating tracks with differing potential when a critical temperature is exceeded. At this increased temperature the resistance of the second dielectric layer and the electrically conductive element will preferably be lower than the resistance of the first dielectric layer, so as to be able to ensure sufficient safety for a user. The leakage current flowing from the heating track to the electrically conductive element could optionally also be detected, wherein the electrically conductive element can for instance be connected to a control unit, an ammeter and/or a voltmeter, whereby the electrically conductive element itself does in fact function as sensor element. In a particular preferred embodiment the shortest distance between the heating track and the electrically conductive element lies between 5 μm and 50 μm, preferably between 12 μm and 20 μm.

In another preferred embodiment the heating track is at least partially protected by a sintered glass layer, preferably a sintered glass layer with a low melting point, more preferably a melting point lower than 450° Celsius. By having the heating track protected on a side remote from the first dielectric layer not only by the second dielectric layer but also partially by a sintered glass layer, an(other) additional safety provision is provided. At a sufficiently high temperature the sintered glass layer will melt. Locally the temperature will increase sharply. The heating track at the position of the melted glass layer will be destroyed and interrupted, whereby the heating element can no longer be operational. In this way overheating of the heating element can also be prevented in the case that - for whatever reason - no leakage current from the heating track to the sensor track is detected when a critical temperature of the heating element according to the invention is exceeded.

If desired, the dielectric can be assembled from a dielectric layer of a polymer and a dielectric layer of enamel. Most preferably however, both dielectric layers are manufactured from enamel. Enamel compositions particularly suitable for this application are marketed under the name Kerdi. The use of an enamel layer as dielectric in the manufacture of, among other products, electrical heating elements is per se known, for instance from NL 1014601. The dielectric herein provides for electrical insulation of the electrical resistance, which generally consists of a metallic track. The manufacture of the dielectric from enamel results here in a mechanically relatively strong dielectric which conducts heat relatively well.

The composition of the enamel for both dielectric layers can be selected within wide limits, this subject to the desired electrical properties, particularly at temperatures occurring during use. The specific electrical resistance of a common enamel composition is generally high at room temperature, usually higher than 1.5*10¹¹ ∑θcm, but can fall drastically as temperatures increase to for instance a typical value of 1.5.10⁷ ∑θcm at 180-400° Celsius. A (relatively small) leakage current through the dielectric becomes possible at such a resistance. The conductivity of an enamel composition can be readily adjusted by for instance making variations in the alkali metal content and/or by adding conducting or, conversely, electrically insulating additives.

In a particular preferred embodiment the dielectric comprises a first and/or a second dielectric layer of an enamel composition and an electrically conductive layer which is assembled from metals and/or semiconductors and/or other conductive materials such as for instance graphite and so forth. A heating element according to the invention which operates particularly well has the feature that the alkali metal content of the enamel composition of the first dielectric layer is lower than that of the second dielectric layer. The manufacture of each layer of the dielectric from an enamel composition which differs only in the alkali metal content has the additional advantage that an optimal adhesion is achieved between the layers. The difference in coefficient of expansion of the layers is moreover relatively small, so that the mechanical stresses in the material are minimized, which results in an improved durability of the dielectric, and therefore also of the heating element.

In addition to the specific resistance of a dielectric layer already described above, the breakdown voltage of such a layer, preferably an enamel layer, is also important. The breakdown voltage is the level of the electrical potential difference over the dielectric layer at which an electric current (with a much greater current strength than a leakage current) begins to flow through the layer. Breakdown can result in undesirable adverse effect on, and even irreparable disintegration of the dielectric layer and also the whole heating element. In order to ensure maximum safety in an electrical heating element, the breakdown voltage of the dielectric must be sufficiently high in accordance with regulations of certifying organizations such as KEMA and ISO, preferably at least 1250 V (alternating voltage) relative to the earth.

By selecting the electrical resistance at a given temperature of the first dielectric layer significantly higher than that of the second dielectric layer, the second dielectric layer will at least partly transmit current at a given moment when the electrical resistance overheats. In such a case the first layer will transmit substantially no current, or in any case less. The heating element according to the invention is therefore resistant to high voltage, even if the element continues heating at too high a temperature due to a failure of the electronic regulation or the switching member/relay connected thereto. During this process the electrical resistance track will then burn through (like a melting fuse) as a result of the above described conductive layer or sintered glass layer, and after this process the first dielectric layer ensures that a sufficient dielectric strength always remains relative to the earth or the consumer. The heating element according to the invention is therefore intrinsically safe.

It is noted that the breakdown voltage of a dielectric is determined by a plurality of factors, including among others the layer thickness of the dielectric, the enamel composition and structural defects such as gas inclusions and the like present in the dielectric. A good adhesion of the dielectric layer, in this case the enamel composition on the surface for heating (generally of steel, aluminium and/or a ceramic material), is also important. A particularly suitable enamel composition for application in a dielectric layer of the heating element, preferably the first dielectric layer, comprises between 0 and 10% by mass OfV₂O₅, between 0 and 10% by mass of PbO, between 5 and 13% by mass of B2O3, between 33 and 53% by mass of SiO₂, between 5 and 15% by mass OfAl₂O₃, between 0-10% by mass of ZrO₂ and between 20 and 30% by mass of CaO. If desired, the preferred composition also comprises between 0 and 10% by mass Of Bi₂O₃. Such a composition results in an enamel layer with an improved durability when used in heating elements. The enamel composition can be melted relatively easily and herein has a favourable viscosity, whereby it can be applied easily to different types of surface. The enamel composition adheres particularly well to metals, in particular to steel, more particularly to ferritic chromium steel, and still more particularly to ferritic chromium steel with numbers 444 and/or 436 according to the American AISI norm. The maximum compressive stress of the enamel layer which can be obtained from the enamel composition lies in the range between 200 - 250 MPa for the new composition. For known enamel compositions the maximum compressive stress generally lies in the range of 70 - 170 MPa. The preferred enamel composition furthermore has a high temperature resistance so that prolonged exposure to temperatures up to about 53O⁰C, with peak loads up to 700⁰C, does not cause problems. A first dielectric layer on the basis of the preferred enamel composition therefore has little risk of breakdown, in other words is less susceptible to degeneration owing to prolonged load at a high voltage than known enamel compositions. The properties of the enamel composition are furthermore such that the chance of crack formation in a dielectric layer manufactured therefrom is reduced in the case of temperature changes. The preferred enamel composition has the additional advantage that dielectric layers with the desired properties can be applied to the surface for heating in small layer thicknesses. This enhances the heat conduction.

A particular preferred embodiment comprises a dielectric in which at least the lithium and/or sodium and/or potassium content of the first and the second dielectric layers differ from each other. It is advantageous herein if the enamel composition of the first dielectric layer is substantially free of lithium and/or sodium ions. In a preferred composition according to the invention the second dielectric layer comprises at least lithium and/or sodium ions. In a preferred embodiment the enamel composition comprises between 0.1 and 6% by weight of potassium. Owing to the addition of potassium the load-bearing capacity of the adhesion of the enamel composition to the substrate surface is less critical. In an assembly of such an enamel composition with a substrate surface there occurs less deformation at increased temperatures, in particular in the case of overheating. This is particularly advantageous when the enamel composition is fired into a heating element. The compressive stress is reduced but is still high enough to prevent the undesired formation of hair cracks. At percentages of potassium higher than 6% by weight the chance of hair crack formation has however been found to increase. In combination with the absence of other alkali metal ions, in particular lithium and sodium, a low leakage current at increased temperatures also remains ensured.

The substrate for heating, on which the dielectric is arranged, can be manufactured from any heat-conducting material. The surface for heating is preferably manufactured substantially from metal, for instance steel and/or aluminium. Particularly advantageous is ferritic chromium steel, preferably with a chromium content of at least 10% by weight.

It is advantageous if the coefficient of expansion of the material from which the surface for heating is manufactured does not differ too much from the coefficient of expansion of the first dielectric layer, for instance no more than 20 to 45%, for instance relative to steel, more preferably no more than 20 to 35%. The coefficient of expansion of the second layer preferably does not differ any more than 0 to 25% relative to that of the first layer. A heating element is thus obtained which has been found to be very well able to withstand temperature changes. Particularly the formation of hair cracks in both the dielectric enamel layers according to the invention has been found to be hereby much less. It has been found that the chance of hair cracks increases again at a difference in coefficient of expansion lower than 20%. It will be apparent that the coefficient of expansion of an enamel composition can be readily adapted to the coefficient of expansion of the surface for heating by for instance adjusting the alkali metal content. Adjusting the potassium content in the enamel composition is recommended here, since the leakage current is hardly influenced hereby at increased temperature. Conversely, it is also possible to choose another material for the substrate for heating. The invention also relates to a liquid container provided with at least one heating element according to the invention. The heating element according to the invention can be applied in many fields. It is thus possible to use the element in a water boiler, wherein electrical safety is provided for the user. The heating element is also particularly suitable for application in steam generators, (dish-)washing machines, humidifiers, milk and other liquid heaters, pipe heating devices for liquids, cooker plates, grill plates and the like.

The invention will be elucidated on the basis of non-limitative exemplary embodiments shown in the following figures. Herein: figure 1 shows a cross-section of a heating element according to the invention, figure 2 shows a bottom view of a part of the heating element according to figure 1 , figure 3 shows the progression of the specific resistance of the first dielectric layer and second dielectric layer forming part of the heating element according to figure 1 as a function of the temperature, and figure 4 shows the progression of the measured current strength as the temperature increases through dielectric layers of different enamel composition.

Figure 1 shows a cross-section of a heating element 1 according to the invention. Heating element 1 comprises a heating plate 2 for heating manufactured from ferritic chromium steel with a content of 18% by weight of chromium. It is also possible to apply another suitable metal or ceramic carrier, such as for instance decarbonized steel, copper, titanium, SiN, Al₂O₃ and so forth. A first dielectric enamel layer 3 is arranged on heating plate 2. The first enamel layer 3 has an enamel composition substantially as according to column HT of Table 1. A heating track 4 and a sensor track 5 running parallel to heating track 4 are arranged on the first, relatively electrically insulating enamel layer 3 in a substantially spiral-shaped pattern, wherein the distance between the tracks amounts to about 500 μm. Heating track 4 and sensor track 5 are preferably manufactured from the same material, more preferably from silver, copper or an alloy of these or other metals, in order to be able to simplify and speed up the production process for the manufacture of heating element 1. A second enamel layer 6 is arranged on top of and between tracks 4, 5, wherein a sintered glass layer 7 with a relatively low melting point is arranged crosswise over two or more of the heating tracks 4. The enamel composition of second enamel layer 6 is selected within the limits indicated in column LTl of Table 1. Arranged on second enamel layer 6 is a metal strip 8, in particular a silver strip, which extends over both heating track 4 and sensor track 5 to a different part of heating track 4 via the second dielectric layer. Heating track 4 and sensor track 5 are electrically connected to a control unit 9. Control unit 9 is adapted here to regulate the current strength through heating track 4. Control unit 9 is coupled to a safety circuit 10 which can for instance be provided with a bimetal. Control unit 9 is also coupled to an ammeter 11 for measuring the leakage current through sensor track 5. Figure 2 shows a bottom view of a part of heating element 1 according to figure 1, in which the second enamel layer 6 is omitted from the figure for the sake of clarity. Figure 2 clearly shows that heating track 4 and sensor track 5 are arranged in substantially spiral shape and substantially parallel to each other on first enamel layer 3. Figure 2 also shows that silver strip 8 overlaps both sensor track 5 and heating track 4, in fact a plurality of heating track sections. The operation of heating element 1 can be described as follows. After activation of heating track 4 by control unit 9 heat will be generated in heating track 4, a substantial part of which heat is transferred to heating plate 2 via enamel layers 3, 6. Heating plate 2 will here generally be in contact with a liquid, to which the heat can then be relinquished. If however the heat developed by heating element 1 can no longer be transferred in adequate manner, the temperature of heating element 1 will rise. In order to be able to prevent overheating of heating element 1 and thereby the occurrence of hazardous situations, the composition of second enamel layer 6 is chosen such that the resistance decreases significantly when a critical temperature is exceeded, such that a leakage current will begin to flow from heating track 4 to sensor track 5 via second enamel layer 6 and possibly also via silver strip 8. A leakage current flowing through sensor track 5 can be detected by ammeter 11. If this leakage current measurement were to fail and further (over)heating of heating element 1 were to occur, the sintered glass layer 7 will then melt and thereby destroy the heating track 4 at that position, whereby the operative mode of heating element 1 will be terminated. Further heating of heating element 1 will then also be no longer possible.

Figure 3 shows the progression of the specific resistance of the first dielectric layer and second dielectric layer forming part of the heating element according to figure 1 as a function of the temperature. As indicated in figure 3, the LTl and HT enamel compositions, among others, ensure that the specific electrical resistance R6 of second enamel layer 6 decreases at a lower temperature than the specific electrical resistance R3 of the first relatively insulating layer 3.

The leakage current characteristic measured with the ammeter for a number of dielectric layers is shown in figure 4 as a function of the temperature T. The leakage current I plotted on the vertical axis remains limited for relatively low temperatures T up to a point close to a determined initiating temperature, above which it suddenly increases rapidly. The initiating temperature greatly depends on the composition of the enamel layer. Figure 4 shows that the composition of the first layer, indicated with HT, has an initiating temperature which amounts to at least 500⁰C. The other four shown leakage current characteristics (designated with LTl) are representative of enamel compositions of the second layer. By adjusting the composition of the enamel compositions to the desired initiating temperature for the first and/or second dielectric layer, a temperature protection for heating element 1 can be realized using a relatively simple electrical circuit.

Table 1: preferred enamel compositions in heating element 1 according to the invention Enamel composition LTl HT

Constituent % by weight % by weight

Li₂O 0-5

K₂O 0-15 0-10

Na₂O 0-10 CaO 20-40

Al₂O₃ 5-15

B₂O₃ 5-13

SiO₂ 33-53

ZrO₂ 0-10

PbO 0-10

V₂O₅ 0-10

Bi₂O₃ 0-10

Total 100 It will be apparent that the invention is not limited to the exemplary embodiments shown and described here, but that numerous variants, which will be self-evident to the skilled person in this field, are possible within the scope of the appended claims.

Claims

1. Heating element, comprising: a substrate for heating, - at least one first dielectric layer arranged on the conductive substrate, at least one electrically conductive heating track arranged on the first dielectric layer, at least one electrically conductive sensor track arranged on the first dielectric layer at a distance from the heating track, and - at least one second dielectric layer arranged on the first dielectric layer, which second dielectric layer connects to at least a part of the heating track and to at least a part of the sensor track.

2. Heating element as claimed in claim 1, characterized in that at substantially the same temperature the electrical resistance of the first dielectric layer is higher than the electrical resistance of the second dielectric layer.

3. Heating element as claimed in claim 1 or 2, characterized in that at least a part of the heating track and at least a part of the sensor track are given a spiral form.

4. Heating element as claimed in any of the foregoing claims, characterized in that the shortest mutual distance between at least a part of the at least one heating track and at least an adjacent part of the at least one sensor track is substantially constant.

5. Heating element as claimed in any of the foregoing claims, characterized in that the shortest mutual distance between at least a part of the at least one heating track and at least an adjacent part of the at least one sensor track lies between 100 μm and 800 μm, preferably between 400 μm and 600 μm.

6. Heating element as claimed in any of the foregoing claims, characterized in that the at least one heating track and the at least one sensor track are coupled to a control unit.

7. Heating element as claimed in any of the foregoing claims, characterized in that an ammeter is coupled electrically to the sensor track.

8. Heating element as claimed in any of the foregoing claims, characterized in that a voltmeter is coupled electrically to the sensor track.

9. Heating element as claimed in any of the foregoing claims, characterized in that at least one electrically conductive element is arranged on the second dielectric layer.

10. Heating element as claimed in claim 9, characterized in that the shortest distance between the heating track and the electrically conductive element lies between 5 μm and 50 μm, preferably between 12 μm and 20 μm.

11. Heating element as claimed in any of the foregoing claims, characterized in that the heating track is at least partially protected by a sintered glass layer.

12. Heating element as claimed in claim 11, characterized in that the melting point of the sintered glass layer is lower than 500° Celsius.

13. Heating element as claimed in any of the foregoing claims, characterized in that the first and/or the second dielectric layer are manufactured from an enamel composition.

14. Heating element as claimed in claim 13, characterized in that the alkali metal content of the enamel composition of the first dielectric layer is lower than that of the second dielectric layer.

15. Heating element as claimed in claim 13 or 14, characterized in that at least the lithium and/or sodium and/or potassium content of the first and the second dielectric layers differ from each other.

16. Heating element as claimed in any of the claims 13-15, characterized in that the first dielectric layer is substantially free of lithium and/or sodium ions.

17. Heating element as claimed in any of the claims 13-16, characterized in that the alkali metal content of the first and the second dielectric layer differ from each other.

18. Heating element as claimed in any of the claims 13-17, characterized in that the enamel composition of the first layer is chosen such that as temperature increases it always has a higher electrical resistance than that of the second layer.

19. Heating element as claimed in claim 18, characterized in that the enamel composition of the first dielectric layer is chosen such that the breakdown voltage is higher than 1250 VAC.

20. Heating element as claimed in any of the foregoing claims, characterized in that the expansion coefficient of the material of which the substrate consists differs by no more than 20 to 45% from the expansion coefficient of the first and/or the second dielectric layer.

21. Enamel composition for application as first dielectric layer in a heating element as claimed in any of the foregoing claims, comprising between 0 and 10% by mass of V2O5, between 0 and 10% by mass of PbO, between 5 and 13% by mass OfB₂O₃, between 33 and 53% by mass of SiO₂, between 5 and 15% by mass OfAl₂O₃ and between 20 and 30% by mass of CaO.

22. Enamel composition for application as second dielectric layer in a heating element as claimed in any of the claims 1-20, comprising between 0 and 5% by mass of Li₂O, between 0 and 15% by mass of K₂O and between 0 and 10% by mass OfNa₂O.

23. Liquid container, provided with a heating element as claimed in any of the claims 1-20.