CN102160456A - Heating element and method for operating such a heating element - Google Patents

Abstract

The temperature of a heating element is commonly regulated to prevent dangerous situations overheating of said heating element. The invention relates to a heating element. The invention also relates to an electrical appliance comprising such a heating element. The invention further relates to a method for operating a heating element, in particular a heating element according to the invention.

Description

Heating element and method for operating such a heating element

technical field

The invention relates to a heating element. The invention also relates to electrical equipment comprising such a heating element. The invention also relates to a method for operating a heating element, in particular a heating element according to the invention.

Background technique

The temperature of the heating element is usually adjusted to prevent a dangerous situation of overheating of said heating element. For this purpose, known heating elements comprise a heating track and an assembly of an analog sensor connected to said heating track and an electronic regulator, wherein the analog sensor can be formed, for example, by a thermocouple or a thermistor (NTC or PTC resistor). The known heating elements have several drawbacks. A first disadvantage of the known heating elements is that the construction of the heating element is relatively complex, wherein the amount of wiring is considerable especially if the heating element comprises a plurality of heating zones. A further disadvantage of the known heating elements is that the heating elements are only suitable for measuring the temperature of the heating track itself, wherein the temperature of usually more thermally critical components directly surrounding the heating element is disregarded. This not only constitutes an important limitation of known safety measures, in which the risk of overheating of surrounding components and/or the medium cannot be accurately monitored, but also complicates efficient and optimal regulation of the heating process to be carried out by the heating element.

Contents of the invention

It is an object of the present invention to provide an improved thermoregulated heating element.

The object of the present invention can be achieved by providing a heating element according to the foregoing, said heating element comprising: at least one electrically conductive heating track made of a material having a temperature variable resistance; at least one sensing element, The at least one sensing element is connected to the heating track for measuring the resistance of the heating track; and a control unit configured to allow the sensing element to measure the heating track at least in a cut-off state of the heating track The resistance. By monitoring the thermoresistance of the heating track at least in its switched-off state, a relatively reliable indication of the actual temperature of the immediate environment of the heating element can be obtained. In the (temporary) cut-off state of the heating track, the heating element will normally cool down and adapt to the temperature of the immediate environment approximately determined by the volume of solid, liquid or gas to be heated by the heating element according to the invention The temperature is determined. By measuring the resistance of the temporarily non-operating heating track, an indication of the actual temperature of the surrounding components and/or medium around the heating element can be obtained. Depending on this ambient temperature, the heating element can be controlled such that a situation of overheating of the heating element and components surrounding the heating element can be prevented. Furthermore, in this way, the properties of the heating element can be controlled in a more precise manner, thus, the heating process of the heating element according to the invention can be optimized. For example, in the case of a heating element incorporated in a grill pan for heating the cooking plate of the grill pan, switching off the heating track of the heating element will produce an equalization of the temperature between the heating track and the (already heated) cooking plate. By measuring the actual resistance of the heating track in this homogenizing state (off state), the actual temperature of the cooking plate can be determined, from which the control of the heating elements can be adjusted. Overheating of the cooking plate can thus be prevented and optimal control of the temperature of the cooking plate and thus of the cooking process can be ensured. A further advantage of the heating element according to the invention is that the heating track itself acts as a thermistor for determining the temperature of the heating track, due to which a relatively simple and effective construction can be provided. Therefore, no additional thermistor and therefore no additional wiring need be applied to determine the temperature of the heating track. Due to the simple construction of the heating element according to the invention, the cost price of the heating element will generally be smaller than that of known thermoregulating heating elements.

Preferably, the control unit is further configured to allow the sensing element to measure the resistance of the heating track in the switched-on state of the heating track. This allows the sensing element to continuously measure the resistance of the heating track in the on and off states of the heating track. An advantage of measuring the resistance of the heating track is to monitor the temperature of the heating track during operation. Due to this safety measure, situations of overheating of the heating element can be anticipated and thus prevented. It should be clear that measuring the resistance of the heating track in the on-state and in the off-state leads to a dual function of the heating element according to the invention: Measuring the resistance of the heating track in the off-state of the heating track is advantageously used to detect the immediate environment of the heating element The temperature of the heating track, while measuring the resistance of the heating track in the switched-on state of the heating track, is advantageously used to detect the temperature of the heating track and thus the temperature of the element itself, thus preventing a dangerous situation of overheating. It is conceivable that the control unit is configured to switch the sensing element on and off during the switch-off phase of the heating track. In this way, multiple temperature samples can be obtained as the component cools. Analysis of the heating element temperature decay function during the interruption enables estimation of specific environmental characteristics such as the amount of solid or liquid being heated and the time required to heat the amount of liquid or solid to a selected temperature.

The control unit is usually not only configured to allow the sensing element to measure the resistance of the heating track, but is also preferably adapted to actively switch the heating track. The control unit is preferably configured to alternately switch the heating channels on and off. To this end, the control unit preferably comprises at least one timer for switching the heating lanes. More preferably, the control unit is adapted to switch on the heating channel for a period of time after the heating channel has been switched off, wherein said period of time is between 1 second and 10 seconds. This latter time is usually sufficient to allow the heating track to cool to ambient temperature and to obtain a reliable and practical determination of the ambient temperature of the heating element. On the other hand, this period of time is intentionally kept short, preferably as short as possible, in order to keep interruptions or disturbances of the heating process carried out by the heating elements as small as possible. The optimum duration of the cut-off state generally depends on the specific application, technical design and size of the heating element. In this context, it is noted that the control of the heating track may depend on the last measured resistance. This provides a self-regulating heating element by means of which the heating process can be optimized and the risk of hazardous situations can be minimized.

The resistance of the heating track as detected by the sensing element will be converted into a temperature value. This switching can be performed remotely, for example by a computer connected to the heating element according to the invention. However, it is generally more preferred that the control unit is adapted to convert the measured resistance of the heating track into a temperature value of the heating track. Assuming that the temperature coefficient of resistance (TCR) of the heating track is known, based on the measured resistance, the actual temperature of the heating track can be calculated by the following formula:

$T_{\mathrm{end}}=T_{\mathrm{begin}}+\frac{R_{\mathrm{end}}-R_{\mathrm{begin}}}{R_{\mathrm{begin}}\·\mathrm{TCR}}$ (Formula 1)

in:

- T _end is the actual elevated temperature of the heating channel;

-T _begin is the initial temperature of the heating channel (usually room temperature);

- R _end is the measured resistance of the heating track at elevated temperature;

-R _begin is the initial resistance of the heating track at the initial temperature of the heating track; and

-TCR is the temperature coefficient of resistance.

It is generally advantageous to calibrate the control unit on first use, and more preferably periodically to ensure accurate conversion of the measured resistance to temperature values. Periodic (re)calibration of the control unit is often advantageous, since in use the resistance of the heating track and thus the TCR can drift. Calibration can be performed in various ways, one of which consists of measuring the resistance of the heating track at at least one predetermined temperature of the heating track, for example by placing the heating element in a temperature-conditioned space such as a conditioning furnace.

In a preferred embodiment, the heating element comprises a thermally conductive substrate for heating. The thermally conductive substrate is usually in contact with the medium (gas, liquid, and/or solid) to be heated. Thermally conductive substrates can be made of metals and ceramics. In case the substrate is electrically conductive, then the heating track is preferably positioned at a distance from said substrate to prevent short circuits within the heating element. The space between the heating track and the substrate to be heated can be filled with an air gap. Preferably, however, the heating element further comprises at least one first dielectric layer arranged on the thermally conductive substrate, wherein at least one electrically conductive heating track is arranged on said first dielectric layer. This latter basic structure of the heating element is also known as thick film element. Generally, the substrate is formed of a stainless steel or ceramic plate on which the first dielectric layer and the resistor paste are sequentially coated. The individual layers are typically applied by screen printing, where each layer is dried and then fired. Thick film elements are preferred over another conventional (sheath) heating element due to their relatively low (thermal) mass, small heat storage, fast response, and ability to spread heat evenly and sent to the heated medium. The following are different preferred embodiments of thick film heating elements.

In a preferred embodiment, the heating element further comprises: at least one electrically conductive sensor track arranged at a distance from the heating track on the first dielectric layer; and at least one second dielectric layer, The at least one second dielectric layer is disposed on the first dielectric layer, wherein the second dielectric layer is connected to at least a portion of the heating track and at least a portion of the sensor track. Preferably, the sensor track is closed between two dielectric layers. More preferably, the resistance of the first dielectric layer is higher than the resistance of the second dielectric layer at almost the same temperature. In case the temperature of the heating element exceeds a predetermined threshold, a leakage current will flow from the heating track to the sensor track via the second dielectric layer. By sensing this leakage current, for example by connecting the sensor track to an ammeter or voltmeter, overheating of the heating element and thus dry operation can be detected in a relatively sensitive and reliable manner. The heating track may be positioned between the first dielectric layer and the second dielectric layer such that the heating track is arranged approximately parallel to the sensor track. Alternatively, at least one heating track is located on the side of the second dielectric layer facing away from the first dielectric layer, which is advantageous in the case of sensor tracks, for example formed by sensor grids. Preferred embodiments are disclosed in more detail in international application WO2006/083162 and non-published international application PCT/NL2008/050360, both of which are hereby incorporated by reference in their entirety for all purposes.

In another preferred embodiment, the heating element further comprises thermal protection means covering at least one heating track part, said thermal protection means being adapted to keep said at least one heating track part covered at a predetermined elevated temperature of the heating track A short circuit is generated, thereby increasing the temperature of the covered at least one heating track portion, such that the covered at least one heating track portion is at least partially destroyed, thereby causing the heating track to be irreversibly interrupted. Due to such a short circuit, the thermal protection device and thus the covered heating track part will reach such a high temperature that the covered heating track part melts and/or evaporates and is thus destroyed. Preferably, the control unit is also connected to the heating element to ensure that said element does not reach an overheating temperature by switching off the heating element preventively at a predetermined critical temperature.

The sensing element can have various characteristics, but preferably comprises an ammeter for (indirect) measuring the resistance of the heating track. Through the ammeter, the current I passing through the heating channel can be detected. The resistance R of the heating channel can be determined by the following formula:

$R=\frac{V}{I}$ (Formula 2)

in:

R = resistance of the heating channel;

V = voltage applied to the heating track; and

I = current measured by ammeter.

Preferably, the heating element comprises a main circuit for connection to the power mains to carry out the heating process and an auxiliary circuit for applying a low voltage to the heating track in case the heating track is disconnected from the power mains. The heating element includes at least one switch for switching between the two circuits. It is obvious that the main circuit and the auxiliary circuit can be at least partially integrated with each other.

In the case where an ammeter is used to determine the actual temperature of the heating track, Equation 1 and Equation 2 combine to yield:

$T_{\mathrm{end}}=T_{\mathrm{begin}}+V\&Center\; Dot;\left(\frac{I_{\mathrm{end}}-I_{\mathrm{begin}}}{I_{\mathrm{begin}}\&Center\; Dot;\mathrm{TCR}}\right)$ (Formula 3)

The invention also relates to an electrical device comprising at least one heating element according to the invention. Preferably, the electrical equipment is formed by an appliance selected from the group consisting of: grill pan, teppanyaki, sandwich maker, griddle, deep frying pan, heat radiator, kettle, (dish) washing machine and hot drink machine . Other appliances which may be fitted with a heating element according to the invention include wallpaper strippers, steam irons, water purifiers, food cookers, dishwashers, floor cleaners, carpet, curtain or furniture cleaners and appliances used in medicine, dentistry or Sterilization equipment for food sanitation applications. The appliance may be portable, or form part of a domestic, industrial, commercial or laboratory treatment unit. In a preferred embodiment, the electrical device comprises multiple heating elements to be able to provide a larger heating surface and ultimately to be able to provide multiple heating zones. The control unit may alternately switch the heating elements to monitor the immediate environment of each heating element in turn. In another preferred embodiment, the electrical device comprises a single heating element, wherein said single heating element comprises a plurality of heating lanes, wherein the control unit can alternately switch the heating lanes so as to monitor the immediate environment of each heating lane in turn. It is desired that the best electrical equipment incorporating one or more heating elements according to the invention be formed by grilling pans, the type and quantity of food being cooked as well as the volume of each and the movement of the food relative to the grilling pan There will be an almost instantaneous response to the temperature of the heating element. Thus, during the state in which the heating track of the heating element is switched off, the temperature of the food product can be measured relatively quickly and accurately.

By analyzing electrical resistance samples and thus indirectly temperature samples (taken during this interrupted off state), the readiness of the food product can be estimated. This information can be provided to the user and/or can be used to further control the grill pan.

The invention also relates to a method for operating a heating element, in particular a method for operating a heating element according to the invention, comprising the steps of: A) switching on the heating track; B) after having connected according to step A) Disconnecting the heating channel for a period of time after the heating channel is switched on; and C) measuring the resistance of the heating channel in the disconnected state of the heating channel. Preferably, the method further comprises a step D) comprising: determining the temperature of the heating track according to the measured resistance of the heating track. The control unit preferably switches the heating element on and off alternately to allow the resistance of the heating track to be measured during the off state of the heating track. Thus, step A) and step B) are preferably repeated at least once. More preferably, all steps A)-D) are repeated at least once. The duration between switching off the heating element according to step B) and switching it on again according to step A) depends on the specific application of the heating element, but is typically between 1 and 10 seconds. The method may also comprise a calibration step for calibrating the heating element prior to step D) in order to ensure an accurate conversion of the measured resistance to an actually determined value of the actual temperature of the heating track. This calibration step may be performed prior to first use and more preferably periodically (eg monthly or annually). It is generally not necessary to calibrate the heating element each time step D) has to be performed.

In a preferred embodiment, the method comprises a step E) comprising: comparing the resistance measured during step C) with a predetermined critical resistance. In cases where the measured actual resistance exceeds (or approaches) the critical resistance, the control of the heating element can be adjusted. Such an adjustment of the control of the heating element can mean, for example, a lengthening of the off state of the heating element, a shortening of the on state and/or an active reduction of the heat capacity of the heating element. By (re)actively adjusting the control of the heating element, excessive heating of the heating element and peripheral equipment around the heating element can be prevented. In this context, it is noted that the critical resistance can be an upper or lower threshold, depending on the nature of the heating track. In case the heating track is made essentially of PTC material, the critical resistance will form the upper threshold; in case the heating track is made essentially of NTC material, the critical resistance will form the lower threshold. Preferably, the method comprises a step F) comprising measuring the resistance of the heating track in the switched-on state of the heating track. Due to this safety measure, an overheating situation on the heating element can be expected and thus prevented. Upon detection of an approaching overheating condition, a timer of the control unit may be adjusted, eg extending the cut-off state of the heating element to allow the heating element to cool down more fully, thereby preventing overheating of the heating element. It is also conceivable that the timer of the control unit is overloaded in this case, wherein the heating element can be switched off in a forced manner.

Description of drawings

The invention is elucidated below on the basis of illustrated non-limiting exemplary embodiments in which:

Figure 1 shows a schematic cross-sectional view of a heating element according to the invention;

Figure 2 shows a schematic view of the method for operating the heating element according to Figure 1; and

Figure 3 shows a kettle fitted with a heating element according to the invention.

Detailed ways

Figure 1 shows a schematic cross-sectional view of a heating element 1 according to the invention. In this embodiment, the heating element 1 forms part of a kettle (not shown). The heating element 1 comprises a heating plate 2 for heating, which is manufactured from ferritic chromium steel with a chromium content of 18% by weight. Another suitable metal or ceramic support such as mild steel, copper, aluminum, titanium, SiN, _{Al2O3 ,} etc. may also be used. The substrate 2 is arranged in direct contact with the water to be heated. A first dielectric glaze layer 3 is arranged on the heating plate 2 . The first glaze layer 3 has a glaze composition substantially as according to column HT of Table 1 below. A conductive sensor layer in the form of a grid 4 is arranged on the first dielectric layer 3 . Grid 4 is for example made of ruthenium oxide (RuO ₂ ) or other suitable conductive (thick film) layer with suitable conductive material (eg silver, palladium, nickel, etc.), and/or said ruthenium oxide combined with said suitable Manufactured in thick film layers of a combination of conductive materials. A second dielectric glaze layer 5 is arranged on the relatively conductive layer 4 . The second glaze layer 5 has some NTC materials, in particular acicular structures of some oxides of nickel, manganese, cobalt and/or iron, in order to be able to enhance the second glaze especially at relatively low temperatures (<300°C). The electrical conductivity of the layer 5 thus makes it easier for leakage currents to flow through the second glaze layer 5 . The glaze composition of the second glaze layer 5 is selected within the range shown in column LT1 of Table 1 below, where the content of the NTC material mixed with the glaze is not considered. Heating tracks 6 are arranged on the second dielectric layer 5 having better conductivity than the first dielectric layer 3 , the heating tracks 6 are used to generate heat to heat the substrate 2 . The heating track 6 is made of a material with temperature variable resistance. The heating element 1 according to the invention has several thermal safety measures. In order to monitor the temperature of the heating element 1 during use, the sensor layer 4 having a better conductivity than both the first layer 3 and the second layer 5 provides the option of determining the leakage current through the second, relatively conductive layer 5 . In order to directly measure the leakage current through the first layer 3 , a first ammeter 7 is connected between the resistive layer 6 and the conductive layer 4 . The magnitude of the measured leakage current is indicative of the magnitude of the highest temperature at a location on the heating element 1 . When a certain temperature is exceeded, the leakage current will increase sharply due to the reduced resistance of the second dielectric layer 5 , which can therefore be easily detected by the first ammeter 7 . Since virtually no leakage current flows through the first dielectric layer 3, it has been found that the measurement of the leakage current by the ammeter 7 becomes more accurate. In addition to the leakage current detection measure, the heating element 1 additionally incorporates a second safety measure described below. The heating element 1 also comprises a second ammeter 8 (shown simplified) for measuring the magnitude of the current through the heating track 6 . From its measured values, the electrical resistance and thus the temperature of the heating track 6 can be determined. A second ammeter 8 is incorporated in the power supply circuit and the measurement circuit. The power circuit is configured to provide electrical power to the heating element 1 to carry out the heating process, wherein the heating track 6 is connected to an electrical mains 9 . By means of a regulating switch 10 to be controlled by a control unit 11 , the actual power applied to the heating track 6 can be regulated. In this embodiment, the regulating switch 10 is also used as a main switch. In this exemplary embodiment the measuring circuit is provided with a separate low voltage source 12 to apply a low voltage to the heating track 6 in case the heating track 6 is disconnected from the mains. The circuit switch 13 is arranged to switch between the power supply circuit and the measurement circuit, wherein the circuit switch is controlled by the control unit 11 . The voltage on the heating track 6 is measured by a voltmeter 14 . In case the heating track 6 is switched off, which means that in this embodiment the heating track 6 is disconnected from the mains 9 and connected to a low voltage source 12, the temperature of the heating element 1 will drop and be compared with the temperature of the heating element 1 immediately surrounding it. The components and/or the medium are homogenized, in particular heated water. In this cut-off state, the temperature of the heating track 6 and thus the temperature of the heated water can be determined via the second ammeter 8 , the voltmeter 14 , and the control unit 11 . Furthermore, since the second ammeter 8 is also integrated in the power supply circuit, the temperature of the heating track 6 is or can also be measured in the switched-on state, although this temperature is only independent of the temperature of surrounding components and/or the medium. In order to monitor the temperature of the immediate surroundings of the heating element 1, in which the water is heated, which gives more relevant information about the potential for overheating and/or the progress of the boiling process, the control unit 11 is adapted to alternately The heating track 6 is switched on and off, allowing the second ammeter 8 to measure (indirectly) the temperature of the heating track 6 in the switched off state. This switched-off state preferably lasts for a time between 1 second and 10 seconds. To this end, the control unit 11 comprises a timer 15 capable of switching the heating track 6 on and off periodically. Depending on the temperature determined by the control unit 9 , a regulating switch 10 can be controlled by the control unit 11 to vary the power delivered to the heating track 6 . In this way, situations of overheating of the heating element 1 and surrounding components and/or the medium can be prevented and an optimum cooking process can be achieved.

Table 1: Preferred glaze compositions in heating elements according to the invention

FIG. 2 shows a schematic view of a method for operating the heating element according to FIG. 1 . The control unit 11 alternately switches the heating tracks 6 of the heating elements 1 on and off. In the switched-off state of the heating element 1 (indicated by the dotted frame), the resistance of the heating track 6 is measured by the second ammeter 8 . The measured value of the resistance is compared with a predetermined threshold resistance by a comparator 16 . The measured values of the electrical resistance and the comparatively relevant information are simultaneously provided to the control unit 11, under the control of which the control of the heating channels 6 is adjusted to prevent overheating situations and/or to enable optimization of the cooking process.

Figure 3 shows a kettle 17 partially filled with water 18, said kettle 17 being equipped with a heating element 1 according to the invention. Details and examples regarding the heating element 1 applied are provided comprehensively above. It will be clear that the heating element 1 according to the invention can also be applied in many other devices, such as, but not limited to, through heaters, steam generators, (dish) washers, humidifiers, dairy and other liquid heaters, Tube heating equipment for liquids, irons, cooking appliances, such as cooking plates and grilling plates, and equipment used in the domestic field, such as cooking, heating and cleaning, and in commerce, industry, public catering, domestic, and Methods and machines in an office environment. The geometry of the heating element is not limited and can have various characteristics, such as flat, rounded or corrugated.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims (21)

1. A heating element comprising: at least one electrically conductive heating track made of a material with temperature variable resistance; at least one sensing element connected to the heating track for measuring the resistance of the heating track; and A control unit configured to allow the sensing element to measure the resistance of the heating track at least in the switched-off state of the heating track. 2. Heating element according to claim 1, characterized in that the control unit is configured to allow the sensing element to measure the resistance of the heating track in the switched-on state of the heating track. 3. Heating element according to claim 1 or 2, characterized in that the control unit comprises at least one timer for switching the heating channels. 4. Heating element according to claim 3, characterized in that the control unit is adapted to switch on the heating track after a period of time after the heating track has been switched off, wherein the period of time is between 1 second and between 10 seconds. 5. Heating element according to any one of the preceding claims, characterized in that the control unit is adapted to convert the measured resistance of the heating track into a temperature value of the heating track. 6. A heating element according to any one of the preceding claims, characterized in that the heating element comprises a thermally conductive substrate for heating. 7. The heating element according to any one of the preceding claims, characterized in that the heating element further comprises at least one first dielectric layer arranged on the heat conducting substrate , wherein the at least one conductive heating track is disposed on the first dielectric layer. 8. The heating element according to claim 7, characterized in that the heating element comprises at least one conductive sensor track, the at least one conductive sensor track is arranged in the first medium at a distance from the heating track. On the electrical layer, and at least one second dielectric layer is disposed on the first dielectric layer, wherein the second dielectric layer is connected to at least a part of the heating track and at least a part of the sensor track. 9. Heating element according to claim 7 or 8, characterized in that the heating element comprises a thermal protection device covering at least one part of the heating channel, the thermal protection device being adapted to The at least one heated track portion that is covered is short-circuited at a predetermined increased temperature of the track, thereby increasing the temperature of the at least one heated track portion that is covered such that the at least one heated track portion that is covered is at least partially Destruction, so that the heating track is irreversibly interrupted. 10. Heating element according to claim 9, characterized in that said thermal protection means are adapted to increase the temperature of said at least one heating track part which is covered, so that said heating track part due to said heating track part Destroyed by melting and/or evaporation. 11. A heating element as claimed in claim 9 or 10, characterized in that at least a part of said thermal protection means is electrically insulating below said predetermined temperature and electrically conductive above said predetermined temperature. 12. A heating element as claimed in any one of the preceding claims, wherein the sensing element comprises an ammeter. 13. Heating element according to any one of the preceding claims, characterized in that the heating element comprises comparison means for comparing the measured resistance of the heating track with at least one predetermined threshold resistance value. 14. An electrical device comprising at least one heating element according to any one of claims 1-13. 15. The electrical device of claim 14, wherein the electrical device is formed from an appliance selected from the group consisting of: grill pan, teppanyaki, sandwich maker, griddle, deep frying pan , radiator, kettle, (dish) washing machine and hot drinks machine. 16. A method for operating a heating element, in particular a method for operating a heating element according to any one of claims 1-13, comprising the steps of: A) switch on the heating channel; B) switching off said heating track after a period of time after said heating track has been switched on according to said step A); and C) Measuring the resistance of the heating track in the off state of the heating track. 17. The method according to claim 16, characterized in that the method further comprises step D): determining the temperature of the heating track according to the measured resistance of the heating track. 18. The method according to claim 16 or 17, characterized in that said step A) and said step B) are repeated at least once. 19. Method according to claim 18, characterized in that the heating element is switched on again according to step A) after a period of time after switching off the heating element according to step B). 20. The method according to any one of claims 16-19, characterized in that: the method comprises step E), and step E) comprises: The resistance measured during said step C) is compared with at least one predetermined critical resistance value. 21. The method according to any one of claims 16-20, characterized in that: the method comprises step F), and step F) comprises: The resistance of the heating track is measured in the switched-on state of the heating track.

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