SI24816A - Magnetic induction system, a sensor and a method for measuring air pressure in vacuum insulation panels - Google Patents

Abstract

The invention relates to a magnetic induction system, a sensor and a method for measuring air pressure in vacuum insulating plates. The measurement of air pressure in vacuum insulation panels is performed in such a way that a vacuum sensor is inserted into the vacuum insulation board during the manufacture under the package, which consists of an air chamber and an induction heating body and the air chamber is located between the inductance body and the packaging of the vacuum plate . The air chamber has one or more openings to be pressure-tight outside the vacuum sensor. When the vacuum insulation board is fully manufactured and the packaging is sealed tightly, pressure measurement can be started. From the outside, the vacuum insulating plate approaches the measuring probe at the point where the vacuum sensor is placed under the package. The probe includes an alternating-field generator and a temperature gauge. With an alternating electromagnetic field generator, the induction body is heated in the vacuum sensor, the temperature gauge measures the temperature on the surface of the vacuum plate where the vacuum sensor is located. In this way, the heat flow is measured through the air chamber. Since the heat flow through the air chamber is dependent on the pressure, the temperature at the point where the air enclosure touches the packaging can be applied to the thermal conductivity and consequently to the pressure level in the vacuum insulating plate.

Description

The present invention relates to the measurement of pressure in vacuum insulation panels.

TECHNICAL PROBLEM

Vacuum insulation panels are currently among the best thermal insulation elements, as vacuum is an excellent thermal insulator. They are used, for example, in construction, shipbuilding, refrigeration, cold chain for use e.g. in the pharmacy or food industry. In vacuum insulation panels, of course, a complete vacuum is not achieved in practice, but only a significant vacuum compared to atmospheric pressure; however, the word vacuum or vacuum plates is used in the profession and in the market for these cases.

Vacuum insulation boards usually consist of a core and a package. The core is formed by a light and extremely air-porous material, such as fiberglass or silica. The function of the core is to give the panel the desired shape and to maintain the construction of the panel and the vacuum in the panel when it is pressed by external higher air pressure. The core is wrapped with a wrapper, which is most often a metal or metallized foil that is airtightly welded and prevents the ingress of outside air into the vacuum insulation board. The package may consist of several layers, for example aluminum foil and one or more plastic coatings.

A typical process for making a vacuum insulation board is as follows. The core is inserted into the packaging, which is already airtightly welded on three sides, thus forming a kind of bag. The semi-finished product prepared in this way is inserted into the machine, which pumps air from the bag to a correspondingly low pressure, then the machine welds the fourth side of the metal bag. It is desirable to keep the pressure as low as possible, but in industrial use the initial pressure value is typically up to 7 mbar if the core material is silica, or up to 1 mbar if the core material is fiberglass.

The degree of insulation of a vacuum insulation board depends on the quality of the vacuum in it. In other words, for a board to have good insulating properties, it must have a correspondingly low pressure. Therefore, the measurement and control of air pressure in vacuum insulation panels is essential, both in the production itself and in subsequent use.

BACKGROUND OF THE INVENTION

There are already some known ways to measure air pressure:

"Lift foil off" method

In this method, the fabricated vacuum insulation board is inserted into a vacuum chamber. The thickness of the plate is measured with a laser meter. The air pressure in the vacuum chamber gradually decreases in a controlled manner. When the pressure in the chamber, which is a known quantity, equals the pressure in the plate, the thickness of the plate begins to increase. So when the laser meter detects an increase in the thickness of the plate, it is inferred from the pressure in the chamber to the pressure in the plate. The disadvantage of this method of measurement is that the measurement is slow. If the same machine is used for measurement as for the production of the plate, the machine is not available for production at the time of measurement. As a result, the method is not suitable for serial control of plates, but only sample measurement is performed with this method. Of course, such a method is also not mobile.

Indirect measurement

In this method, the thermal conductivity of the vacuum insulation board is measured and through this the air pressure in it is also indirectly measured. The measurement procedure is as follows: one side of the plate is heated by a heater of a special measuring device. After a certain time, the temperature rise on the other side of the plate is measured. The higher the temperature rise, the poorer the thermal insulation properties of the panel, so the higher the air pressure. The disadvantage of such a method of measurement is the long time of the measurement process, usually at least one hour, so the method is by no means suitable for control in series production. Also, of course, this method is not mobile.

Wireless radio frequency identification (RFID) pressure gauge

In this method, an electronic device is inserted into the vacuum insulation board already during production, which measures the air pressure in the board after the production and reports the height of the air pressure in the board to a special reader via radio frequency waves. The disadvantage of this method is that it is unsuitable for use in series production because such an electronic device is too expensive and is therefore used only for sampling.

Patent EP 1493007 Al (Caps)

In this method, a solid thermally conductive material is inserted into the vacuum insulation board during fabrication, e.g. metal or ceramic tile that serves as a heat sink. In a different version of the measurement method, this plate can serve as a heat source. The heat sink is installed so that it is separated from the panel envelope only by a test layer, which is a thin, air-porous, open-cell, nano-structured film (thin layer, test layer), for example a polystyrene film whose thermal conductivity varies depending on from air pressure. The measuring probe, which is pressed against the measured part from the outside of the vacuum insulation board, is heated to a certain temperature, which is higher than the temperature in the vacuum insulation board. In this way, a conductive heat flux is achieved through the test layer. The measuring vessel measures the temperature and consequently the heat flow, from which it is inferred the pressure in the vacuum insulation board. Since part of the heat flow is dissipated on all sides also through the packaging, the entire measurement must take place in a controlled, temperature-controlled chamber, which makes the measurement process much more difficult.

Patent DE 103 48 169 Al (Caps)

This is a similar method as in patent EP 1493007 Al (Caps), except that the measurement is not performed in a temperature-controlled chamber. Otherwise, this method also uses a heat sink and a test layer, the thermal conductivity of which depends on the air pressure. The pressure is measured based on the measurement of heat flux due to thermal conduction through the test layer (thin layer, test layer) from the warmer to the colder part.

Patent EP 2,069,742 BI (Caps)

In this case, it is a measurement of heat flux as a consequence of thermal conduction through a thin test layer (thin layer, test layer). The difference from the previously mentioned patents is that no heat sink or heat source in the form of a heat-conducting plate is used. In this case, the vacuum insulation plate or its packaging is made in such a way that it forms a pocket with a test layer at the edge of the plate, whereby the pocket is connected to the inside of the plate and consequently has the same pressure. When measuring, the pocket is inserted into the measuring probe so that the bottom and top of the pocket touch different parts of the measuring probe. One part of the measuring probe is warmer and the other part is colder, thus achieving heat conduction based on conduction through a test layer that is in the pocket. An additional disadvantage of this solution is e.g. the need to make a test pocket located outside the plate. Therefore, this method is practically not used in industrial production.

A common feature of the described methods according to EP 1493007 A1, DE 103 48 169 A1 and EP 2 069 742 BI is that they use a so-called test layer (thin layer, test layer), which is made of open-cell, nanoporous material, such as fiberglass , polyurethane foams, polystyrene foams, polyester foams, quartz, aerogels, polyurethane mats, polystyrene mats. An essential property of the test layer materials is that their thermal conductivity varies depending on the air pressure. The heat flux through the test layer in these methods is due to thermal conduction, i.e. the transfer of heat from the warmer to the colder part via a solid or the contact of two solids. The disadvantage of these methods is that the thermal conductivity of the materials used as a function of pressure is nonlinear precisely in the area important for measurement in industrially produced vacuum insulation boards, ie from 1 mbar to 20mbar, resulting in significant errors of measured pressure. To this must be added the errors that occur as a result of the unevenness of the test layer, the different structure of the packaging or the different ambient temperatures during the measurement.

Until now, the known methods of measuring air pressure in vacuum insulation panels are relatively expensive due to the equipment or time-consuming due to the complexity of the measurement, so they are not suitable for measuring each panel in series production.

BRIEF DESCRIPTION OF THE INVENTION

The measurement of the air pressure in the vacuum insulation plates according to the present invention is carried out by inserting an induction heating body, hereinafter referred to as an induction body, into the vacuum insulation plate already during production. In a typical embodiment, the induction body will be built into a vacuum sensor, which in addition to the induction body also consists of an air chamber. The position of the vacuum sensor is such that the air chamber is located between the induction body and the packaging of the vacuum plate. The air chamber has one or more openings to be pressure-connected to the space outside the vacuum sensor, i.e. to the space in the vacuum plate. As a result, the air chamber has the same gas mixture and the same pressure as in the core or inside the vacuum insulation board when the vacuum sensor is inserted into the vacuum insulation board. Once the vacuum insulation board is fully fabricated and the packaging is airtight, a pressure measurement can be started. From the outside, the measuring probe approaches the vacuum insulation plate at the place where the vacuum sensor is located under the packaging. The measuring probe contains an alternating electromagnetic field generator and a temperature meter. An alternating electromagnetic field generator is used to heat the induction body in the vacuum sensor, and a temperature meter is used to measure the temperature on the surface of the vacuum plate where the vacuum sensor is located. In this way, the heat flow through the air chamber is measured, in which the same mixture of gases and pressure is as in the vacuum plate. The heat flow passes through the air chamber on the basis of thermal convection. Since the thermal conductivity of air or a mixture of gases located in the vacuum insulation plate or in the air chamber, and consequently the heat flow through the air chamber depends on the pressure, the measured temperature at the point where the air chamber touches the packaging can be deduced conductivity and consequently to the pressure level in the vacuum insulation board.

The invention will be described in more detail below and in the figures representing:

Figures 1 and 2 show different options for inserting a vacuum sensor into a vacuum insulation board;

Figure 3 presents an embodiment of a vacuum sensor - perspective view

Figure 4 shows another embodiment of a vacuum sensor - bottom view

Figure 5 shows a further embodiment of a vacuum sensor inserted in a vacuum insulation board

Figure 6 shows an embodiment of a measuring probe

DETAILED DESCRIPTION OF THE INVENTION

The pressure measurement in the vacuum insulation board according to the present invention is carried out in such a way that an induction body 5 is inserted into the vacuum insulation board consisting of a core 1 and a package 2 already during production. For easier implementation, the induction body 5 can be installed in a prefabricated vacuum sensor 3 which is inserted into the vacuum insulation plate under the package 2, preferably just below the package 2, during the manufacture of the vacuum insulation board.

The vacuum sensor 3 consists of an air chamber 4 and an induction body 5. When inserting the vacuum sensor 3 into the vacuum insulation board, it is important that the air chamber 4 is located between the induction body 5 and the package 2, where the temperature measurement will be performed.

One of the possibilities of inserting the vacuum sensor 3 is that it is simply placed on the core 1 before the air is sucked out and a vacuum is created, and the package is sealed, for example welded. This option is shown in Figure 1.

Figure 2 shows another insertion option to make a groove 8 in the core 1, in which the vacuum sensor 3 is installed. In the latter way, the vacuum sensor 3 is not moved between the suction process and the airtight closure of the package 2 around the core. 1, and that the vacuum insulation board does not have a bulge at the place where the vacuum sensor 3 is embedded.

The induction body 5 is made of a material with ferromagnetic properties, such as iron, to effectively serve as an induction heater when placed in an alternating electromagnetic field. In the case of a material with ferromagnetic properties that is exposed to an alternating electromagnetic field, heating also occurs due to the desired hysteresis losses based on magnetic hysteresis, which are converted into Joule heat.

The air chamber 4 serves to create a space of certain dimensions above the induction body 5 through which heat is distributed. On the other side of the air chamber 4, ie on the packaging side 2, we measure the temperature depending on the heat generated by the induction body 5 and the thermal conductivity of the gas mixture under a certain pressure in the air chamber 4. The air chamber 4 is pressure-connected to the outside of the vacuum sensor 3, which represents the interior of the vacuum insulation plate when the vacuum sensor 3 is inserted into the vacuum insulation plate. The pressure connection is made through one or more openings, so that in the air chamber 4 there is the same mixture of gases and the same pressure as in the core 1 or inside the vacuum insulation plate.

When making the air chamber 4, it is necessary to make sure that the inner volume of the air chamber 4 is not deformed in such an undesirable way due to the vacuum in it and consequently the pressure on it, that it loses either the required volume through which heat flow is measured or the pressure connection plates.

In one of the embodiments, the air chamber 4 is designed so that the bottom of the air chamber 4 represents an induction body 5, which is in the form of a plate. The tile is preferably round in shape, but can also be polygonal. The upper part of the air chamber 4 is made of an upper wall 9, which is also in the form of a plate of the same or approximately the same surface as the plate of the induction body 5.

The upper wall 9 must be strong enough to withstand the vacuum pressure and prevent deformation of the air chamber 4 under external pressure, and sufficiently thermally conductive so as not to cause an error in measuring the heat flow through the air chamber 4.

The air chamber 4 in this embodiment is made by attaching one or more spacers 6 between the induction body 5 and the upper wall 9. The pressure connection between the interior of the vacuum insulation plate and the air chamber 4 is achieved in one of the following ways or by a combination thereof :

- to make one or more holes in the induction body 5; preferably there is only one small hole 7 in the induction body;

- to make one or more holes in the spacer 6;

that the spacer 6 or the spacers are arranged in such a way that they do not completely fill the gap around the circumference between the induction body 5 and the upper wall 9.

Figure 3 shows an embodiment of a vacuum sensor 3, wherein the air chamber 4 is bounded on the underside by an induction body 5 in the form of a round plate, on the upper side by an upper wall 9 of substantially the same shape, with an opening 10 made in the spacer 6. which represents the pressure connection between the air chamber 4 and the inside of the vacuum insulation board. The space of the air chamber 4 is not visible in this figure, as it is located inside the said elements.

Figure 4 shows the underside of an embodiment of a vacuum sensor 3 having a different design of the pressure connection than in Figure 3. As in Figure 3, the air chamber 4 is bounded at the bottom by an induction body 5 in the form of a round plate, at the top is bounded by an upper wall 9 of substantially the same shape. The difference is that the spacer 6 is made as a ring, so that the gap between the induction body 5 and the upper wall 9 at the edge is closed around the entire circumference. The pressure connection between the air chamber 4 and the inside of the vacuum insulation plate in this embodiment is achieved by a small hole 7 in the induction body 5. The upper wall 9 is not visible in this figure, as in the bottom view it is behind the induction body 5.

To further reduce the measurement error, we can choose for the upper wall 9 a material that has good thermal conductivity in the vertical direction and a good insulator in the horizontal direction, e.g. FR4 used in the manufacture of printed circuit boards. Such heat-insulating properties of the upper wall 9 further prevent us, on the one hand, from influencing the transfer of heat which would spread through the spacer 6 to the central part of the upper wall 9, where the temperature is measured via the package 2; on the other hand, it allows us to transfer heat as smoothly as possible in the vertical direction from the air chamber 4 through the upper wall 9 to the packaging 2, where the temperature is measured.

The upper wall 9 can be coated with a thin layer of luminous material, such as aluminum foil, on the underside, ie on the side facing the air chamber 4 and the induction body 5, or painted with a luminous paint on an aluminum base, thus a reflex layer is created which prevents heat transfer as a result of radiation from the induction body 5 through the air chamber 4 to the upper wall 9. In the measurement method according to the present invention, only heat flow through the air chamber 4 due to thermal convection is desirable. to the measurement error.

Figure 5 shows a cross-section of an embodiment in which an opening 19 is made in the upper wall 9, preferably round in shape, intended to eliminate the influence of thermal inertia of the upper wall material 9 on the temperature measurement at the upper part of the air chamber 4. This opening must be small enough that the difference between the external pressure and the vacuum in the vacuum insulation plate does not damage the package 2 when it is inserted into the air chamber 4, and that the package 2 does not enter to such an extent that it reaches through the entire air chamber 4 and touches induction body 5.

In order to minimize the undesired transfer of heat from the induction body 5 to the upper wall 9 via the spacer 6, the latter is preferably made of a material having the best thermal insulation properties.

An alternating electromagnetic field generator 11, hereinafter referred to as a generator 11, and a temperature meter 12 are also required to measure the pressure according to the present invention. An alternating electromagnetic field suitable for heating the induction body 5 is generated by the generator 11. close to the induction body 5, ie the vacuum sensor 3, which is already inserted in the vacuum insulation board before the measurement. The temperature gauge 12 measures the temperature at the location of the package 2 closest to the induction body 5. In embodiments where the induction body 5 is integrated in the vacuum sensor 3 and is located between the core 1 and the package 2, the temperature is measured at the location , where the package 2 touches the vacuum sensor 3, namely the upper part of the air chamber 4, preferably in the middle of the vacuum sensor 3, so that the heat flow can be measured based on thermal convection through the air chamber 4 with as few errors as possible.

In one of the embodiments, the generator 11 is implemented with an electronic assembly 18 that generates alternating voltage and a winding of a thicker multicore wire that generates an alternating electromagnetic field.

In another embodiment, the generator 11 is made of an electronic assembly 18 and a winding on a U-shaped ferrite core 17, wherein the generator 11 is preferably positioned during excitation such that the vacuum sensor 3 with the induction body 5 and the air chamber 4 is located between the ends. ferrite core, thus achieving better efficiency of the generator 11.

Different heat sensors, such as PTC resistors, NTC resistors or thermocouples, can be used for the temperature meter 12. It is desirable that the temperature gauge 12 has as little thermal inertia as possible, thereby increasing the accuracy of the measurement, shortening the measurement time, and the time in which the measurement can be repeated with the same temperature gauge.

Preferably, the generator 11 and the temperature meter 12 are mounted together on the measuring probe 13, which facilitates the measurement, as both must be in the immediate vicinity of the vacuum sensor 3 during the measurement.

The signal from the temperature meter 12 can in one embodiment be routed via an analog-to-digital converter 15 to a data processing unit 14 through which the generator 11 is controlled and where the measured temperature values from the temperature meter 12 at individual time intervals and the generator excitation time and intensity 11 are treated accordingly, from which the pressure in the vacuum insulation plate is calculated depending on the heat flow through the air chamber 4 and the thermal conductivity of the air in the air chamber 4. Two of the possible measurement methods are described below. The measurement result is shown on the display 16. The analog-to-digital converter 15, the data processing unit 14 and the display 16 are preferably mounted on the measuring probe 13.

Figure 6 shows a measuring probe 13 in an embodiment using a generator 11 wound on a U-shaped ferrite core 17 and an electronic assembly 18. The position of the probe 13 during excitation is shown, namely that the vacuum sensor 3 with the induction body 5 and the air chamber 4 is located between the ends of the U-shaped ferrite core 17, which achieves better efficiency of the generator 11. The vacuum sensor 3 is already inserted into the core 1 under the package 2 during the production of the vacuum insulation plate. Figure 6 also shows other components of the measuring vessel 13 in in this embodiment, a temperature meter 12, the position of which during the measurement is preferably in the middle of the vacuum sensor 3, an analog-to-digital converter 15, a data processing unit 14 and a display 16. As mentioned, Figure 6 shows the winding position on the ferrite core U of the shape 17 and the temperature gauge 12 with respect to the position of the vacuum sensor 3 during the excitation of the induction body 5 and during the measurement; the position of the other elements of the probe 13 is shown only symbolically.

The dependence of the thermal conductivity of air on air pressure is almost linear in the range up to 20 mbar, which is most relevant in the production and use of vacuum insulation boards, which results from the prior art, e.g. Article C.C. Minter, 1963, Effect of pressure on the thermal conductivity of a gas, Electrochemistry Branch, Chemistry Division. Due to this linear dependence, the measurement according to the present invention is sufficiently accurate, with heat passing through the air chamber 4 due to thermal convection, i.e. the transfer of heat from the warmer to the colder part due to Brownian motion of gas molecules.

The dependence of the thermal conductivity of various materials used as test layers in the prior art on pressure is known and described, for example, at http://www.vipbau.de/e pages / technologv / vip / howthevwork.htm. If this dependence is compared with the dependence of air thermal conductivity on pressure, it is seen that the dependence of air thermal conductivity is significantly more linearly dependent on the pressure in the relevant range, ie up to 20 mbar, than the thermal conductivity of materials used for testing. plastic. Therefore, the results of the pressure measurements according to the present invention are more accurate.

The induction body 5, which is inserted into the vacuum insulation plate at a certain distance from the package 2, the temperature meter 12 and the generator 11 are connected to a magnetic induction measuring system. Preferably, the induction body 5 is integrated in a vacuum sensor 3, which is inserted into the vacuum insulation plate in one of the ways described above.

The method of measuring the pressure in vacuum insulation panels with a magnetic induction measuring system according to the present invention comprises the following steps:

Measuring the initial temperature with a temperature meter 12 at a location on the package 2 located in the immediate vicinity of the induction body 5, preferably in the middle of the vacuum sensor 3.

2. The heating of the induction body 5 with a certain excitation power of the generator 11, wherein the duration of the excitation time interval is measured and controlled.

3. Additional temperature measurement with a temperature meter 12 at the same place or substantially the same place as the measurement of the initial temperature at one or more time points during or after the excitation interval of the generator 11.

According to one of the possible embodiments of the described method, the excitation interval of the induction body 5 with the generator 11 is determined in advance. Additional temperature measurement is performed only once after the initial measurement, preferably at the same time as the excitation interval ends. At higher pressures in the vacuum insulation board, the thermal conductivity of the air in the air chamber 4 is higher, consequently the higher the heat flow through the air chamber 4 and the higher the difference between the initial and final temperature at a given intensity and in a predetermined excitation time interval. At lower pressures, which are desirable for vacuum insulation boards, the difference between the initial and final temperature will be smaller.

According to another of the possible embodiments of the method described above, the difference between the initial temperature and the final temperature is determined in advance. We measure the excitation time interval of the induction body 5, which lasts until a predetermined difference between the initial temperature and the final temperature is reached. Therefore, the temperature is measured several times within the excitation interval of the induction body 5. At higher pressures in the vacuum insulation board, the thermal conductivity of the air in the air chamber 4 will be higher, consequently the higher heat flow through the air chamber 4 and shorter heating and temperature measurement intervals. a predetermined difference between the initial and final temperature at a given excitation strength. However, at lower pressures, which are desirable for vacuum insulation panels, the duration of the heating interval will be longer.

Claims (15)

PATENT APPLICATIONS Vacuum sensor (3) for measuring the pressure in a vacuum insulating plate, characterized in that it consists of an induction body (5) made of a material with ferromagnetic properties and an air chamber (4), the air chamber being (4) in pressure connection with the outside of the vacuum sensor (3). Vacuum sensor 3 according to claim 1, characterized in that the air chamber (4) is designed as a space between the induction body (5), the upper wall (9) and one or more spacers (6) between the induction body (5) and the upper wall (9). Vacuum sensor (3) according to claims 1 or 2, characterized in that the pressure connection of the air chamber (4) with the outside of the vacuum sensor (3) is made as one or more holes (7) in the induction body (5). Vacuum sensor (3) according to Claim 2, characterized in that the pressure connection of the air chamber (4) with the outside of the vacuum sensor (3) is made as one or more openings (10) in the spacer (6). Vacuum sensor (3) according to Claims 2 to 4, characterized in that an opening (19) is provided in the upper wall (9). Vacuum sensor (3) according to claim 2 or 3, characterized in that the induction body (5) is in the form of a round surface plate, that the upper wall (9) is in the form of a plate of the same surface as the induction body (5), and that the spacer (6) is in the shape of a ring. Vacuum sensor (3) according to Claims 2 to 6, characterized in that the upper wall (9) is made of a material having good thermal conductivity in the vertical direction and a good insulator in the horizontal direction, preferably of FR4. Vacuum sensor (3) according to Claims 2 to 7, characterized in that the upper wall (9) on the side facing the induction body (5) is coated with a thin layer of luminous material, preferably aluminum foil or colored with shiny paint on an aluminum base. Magnetic induction system for measuring the pressure in a vacuum insulating plate, characterized in that it is formed by an induction body (5) made of a material with ferromagnetic properties and inserted into the vacuum insulating plate at a certain distance from the package (2), temperature gauge (12) and generator (11). System according to claim 9, characterized in that the generator (11) consists of a winding on a U-shaped ferrite core (17) and an electronic assembly (18). System according to claims 9 to 10, characterized in that it also comprises a data processing unit (14), an analog-to-digital converter (15) and a display (16). System according to Claims 9 to 11, characterized in that the induction body (5) is integrated in the vacuum sensor (3). 13. A method of measuring the pressure in a vacuum insulation board, characterized in that it comprises the following steps: - measuring the initial temperature with a temperature meter (12) at a location on the package (2) located in the immediate vicinity of the induction body (5), preferably in the middle of the vacuum sensor (3); - heating the induction body (5) with a certain excitation power of the generator (11), the excitation time interval being measured and controlled; - additional temperature measurement with a temperature meter (12) at the same place or substantially the same place as the measurement of the initial temperature at one or more time points during or after the generator excitation interval (11). Method according to claim 13, characterized in that the excitation interval of the induction body (5) is determined in advance by the generator (11) and that the additional temperature measurement is performed only once after the initial measurement, preferably at the same time as complete the excitation interval. Method according to claim 13, characterized in that the duration of the time interval of excitation of the induction body (5) is measured, which lasts so long as to achieve a predetermined difference between the initial temperature and the final temperature, the temperature being measured several times within the excitation interval of the induction body (5).