WO2016043670A1

WO2016043670A1 - Magnetic induction system, a sensor and a method for measuring air pressure in vacuum insulation panels

Info

Publication number: WO2016043670A1
Application number: PCT/SI2015/000027
Authority: WO
Inventors: Aleš BAGGIA
Original assignee: Baggia Aleš
Priority date: 2014-09-15
Filing date: 2015-08-24
Publication date: 2016-03-24
Also published as: SI24816A

Abstract

The present invention relates to magnetic induction system, a sensor and a method for measurement of pressure in vacuum insulation panels. Measurement of air pressure in vacuum insulation panels according to the present invention is carried out in a way that a vacuum sensor is inserted underneath an enveloping film of the vacuum insulation panel during production. The vacuum sensor comprises an induction heating body and an air chamber. The vacuum sensor is placed in such a way that the air chamber is positioned between the induction body and the enveloping film of the vacuum panel. The air chamber is provided with one or several openings in order to be in pressure communication with the space outside of the vacuum sensor. When the vacuum insulation panel is finished and the enveloping film hermetically closed, pressure measurement may begin. A measuring probe is approached to the vacuum insulation panel from the outside at a spot where the vacuum sensor is located underneath the enveloping film. The measuring probe is provided with an AC electromagnetic field generator and a temperature meter. The AC electromagnetic field generator causes the induction body in the vacuum sensor to heat and the temperature meter is used to measure the temperature on the surface of the vacuum panel where the vacuum sensor is located. In this way the heat flow through the air chamber is measured. As the heat flow through the air chamber depends on the pressure therein, the pressure in the vacuum insulation panel can be deduced from the temperature measured at the spot where the air chamber comes in contact with the enveloping film.

Description

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

DESCRIPTION OF INVENTION

FIELD OF INVENTION

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

TECHNICAL PROBLEM

Vacuum insulation panels are currently considered to be among the best thermal insulating elements because vacuum is an excellent thermal insulator. They are used in construction, shipbuilding, cooling technology, cold chain for use for instance in pharmacy or food industry. In practice, a perfect vacuum cannot be achieved in vacuum insulation panels but only a considerable negative pressure compared to atmospheric pressure; despite this fact, words vacuum and vacuum panels are used for these instances in this area of expertise and in the market.

Vacuum insulation panels are most often composed of a core and an enveloping film. The core is made of a light and extremely air permeable material, such as fibreglass or silica. The core gives the panel a desired shape and maintains the panel structure and negative pressure in the panel when external higher air pressure is exerted on it. The core is enclosed by an enveloping film which is most often a metallic or a metallised film which is hermetically welded and prevents the external air from entering the vacuum insulation panel. The enveloping film may comprise several layers, for instance of an aluminium film and of one or several plastic coatings.

A vacuum insulation panel is typically fabricated as follows: a core is inserted into an enveloping film which is already hermetically welded along three sides and thus forms a sort of a bag. A semifinished product thus prepared is inserted into a machine that exhausts the air from the bag to an adequately low pressure and then welds the fourth side of the metallic bag. The pressure should be as low as possible, however in industrial applications 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 fibreglass. An insulation level of a vacuum insulation panel depends on the quality of the vacuum therein. In other words, to achieve good insulating properties of a panel, an adequately low pressure must be present in it. Therefore, measurement and control of air pressure in vacuum insulation panels is of key importance both during production and in subsequent application.

PRIOR ART

There are several known ways of measuring air pressure: The foil lift-off method

In this method, a manufactured vacuum insulation panel is inserted in a vacuum chamber. Panel thickness is measured by a laser meter. Air pressure within the vacuum chamber is gradually reduced under control. When the pressure in the chamber, which is a known quantity, becomes equal with the pressure within the panel, the panel thickness starts increasing. Therefore, when a laser meter detects an increase in the panel thickness, the pressure in the panel is inferred from the pressure in the chamber. A drawback of this type of measurement lies in the fact that it is time consuming. If the same machine is used both for the measurement and the production of a panel, the machine is not available for fabrication while measurement is under way. This method is therefore not suitable for serial measurement of panels but rather for sample measurement. Of course, such method is not mobile.

Indirect measurement

This method measures thermal conductivity of a vacuum insulation panel and indirectly the air pressure therein. The measurement method is as follows: one side of a panel is heated by a heater of a special measuring device. After a certain time, the increase in temperature at the other side of the panel is measured. The bigger the increase in temperature, the poorer the thermal insulation properties of the panel, which means there is higher air pressure in the panel. A disadvantage of this type of measurement is time consuming procedure, normally lasting at least one hour, so the method is not suitable for measurement in mass production. Of course, this method is not mobile, either. Wireless radio frequency identification (RFID) pressure meter

In this method, an electronic device is inserted into a vacuum insulation panel during production and the device measures the air pressure within the panel and transmits via radio frequency waves the air pressure within the panel to a special reader. This method is disadvantageous for use in serial production, because such electronic device is too expensive and is therefore used only for sampling.

Patent EP 1493007 Al (Caps)

In this method, a solid thermal conductive material, such as a metallic or a ceramic plate, functioning as a heat sink, is inserted into a vacuum insulation panel during production. This plate can also function as a heat source in another variant of the measuring method. The heat sink is placed in such a way that it is separated from an enveloping film of the panel only by a test layer, which is a thin, air permeable, open-cell, nano-structured foil, for instance a polystyrene foil, the thermal conductivity of which changes as a function of air pressure. A measuring probe, which is impressed from the external side of a vacuum insulation panel to a measured part of the panel, is heated to a certain temperature, which is higher than the temperature within the vacuum insulation panel. Thus, a conductive heat flow is achieved through a test layer. The measuring probe measures the temperature and consequently the heat flow, from which the pressure in the vacuum insulation panel is deduced. A part of the heat flow dissipates on all sides through the enveloping film, so the entire measurement must be carried out in a temperature controlled chamber, which makes the measurement method much more difficult.

Patent DE 103 48 169 Al (Caps)

This is a method similar to that in patent EP 1493007 Al (Caps), only that the measurement is not carried out in a temperature controlled chamber. Otherwise, this method also uses a heat sink and a test layer, the thermal conductivity of which depends on air pressure. The pressure is measured on the basis of heat flow measurement due to thermal conductivity through the test layer from a warmer part to a cooler part.

Patent EP 2 069 742 Bl (Caps)

In this example, the heat flow is measured as a consequence of thermal conductivity through a thin test layer. Unlike the patents mentioned earlier, no heat sink or heat source is used here in the form of a thermally conductive plate. A vacuum insulation panel, namely its enveloping film, is produced in a way that a flap is formed at the edge of the panel with a test layer, wherein the flap is linked with the interior of the panel and therefore shares identical pressure. During a measurement, the flap is inserted in a measuring probe so that the bottom and the upper sides of the flap are in contact with different parts of the measuring probe. One part of the measuring probe is hotter while the other is cooler, so a heat flow based on conductivity is achieved through the test layer which is in the flap. An additional drawback of this solution is a need for fabrication of a test flap that is arranged outside of the panel. That is why this method is practically not used in industrial production.

A common feature of the methods described above according to EP 1493007 Al, DE 103 48 169 Al and EP 2 069 742 Bl is that the mentioned test layer is used which is made of an open-cell, nano- porous material, for instance fibreglass, polyurethane foam, polystyrene foam, polyester foam, pebble stone, aerogel, polyurethane mat, polystyrene mat. The essential feature of test layer materials is that their thermal conductivity changes as a function of air pressure. In these methods, the heat flow through the test layer is a result of thermal conductivity, which is transfer of heat from a hotter to a cooler part through a solid substance or through a contact of two solid substances. A drawback of these methods lies in the fact that the pressure-dependent thermal conductivity of the applied materials is non-linear in the range which is important for the measurement in industrially produced vacuum insulation panels, i.e. from 1 mbar to 20 mbar, which results in considerable errors of the measured pressure. Additionally, there are errors that occur as a consequence of irregularity of the test layer, of a different structure of the enveloping film or of different ambient temperatures during the measurement.

The hitherto known ways of measuring air pressure in vacuum insulation panels are relatively expensive due to required equipment or time consuming due to complicated measurements. As such they are not suitable for the measurement of each panel in serial production.

SHORT DESCRIPTION OF THE INVENTION

Measurement of air pressure in vacuum insulation panels according to the present invention is carried out in a way that an induction heating body, hereinafter an induction body, is inserted underneath an enveloping film during the production of vacuum insulation panely. Typically, an induction body will be integrated in a vacuum sensor which comprises said induction body and an air chamber. The vacuum sensor is placed in such a way that the air chamber is positioned between the induction body and the enveloping film of the vacuum panel. The air chamber is provided with one or several openings in order to be in pressure communication with the space outside of the vacuum sensor, thus with the space in the vacuum panel. Consequently, the air chamber and the core or the interior of the vacuum insulation panel share an identical gas mixture and identical pressure when the vacuum sensor is inserted into the vacuum insulation panel. When the vacuum insulation panel is finished and the enveloping film hermetically closed, pressure measurement may begin. A measuring probe is approached to the vacuum insulation panel from the outside at a spot where the vacuum sensor is located underneath the enveloping film. The measuring probe comprises an AC electromagnetic field generator and a temperature meter. The AC electromagnetic field generator causes the induction body in the vacuum sensor to heat and the temperature meter is used to measure the temperature on the surface of the vacuum panel where the vacuum sensor is located. In this way the heat flow through the air chamber is measured, in which there are an identical gas mixture and pressure as in the vacuum panel. The heat flow travels through the air chamber on the basis of thermal convection. As the thermal conductivity of the air, namely the gas mixture, that is present in the vacuum insulation panel and in the air chamber, and consequently the heat flow through the air chamber depend on the pressure, the heat conductivity and consequently the pressure in the vacuum insulation panel can be deduced from the temperature measured at the spot where the air chamber comes in contact with the enveloping film.

The invention will be described in more detail in the continuation and by way of drawings, in which:

Figure 1 and 2 represent various options of inserting a vacuum sensor into a vacuum insulation panel;

Figure 3 represents an embodiment of a vacuum sensor - perspective view;

Figure 4 represents a second embodiment of a vacuum sensor - bottom view;

Figure 5 represents a further embodiment of a vacuum sensor inserted into a vacuum insulation panel;

Figure 6 represents an embodiment of a measuring probe.

DETAILED DESCRIPTION OF THE INVENTION

Measurement of pressure in a vacuum insulation panel of the present invention is carried out as follows: an induction body 5 is inserted during production into a vacuum insulation panel comprising a core 1 and an enveloping film 2. In view of easier implementation, the induction body 5 may be integrated into a prefabricated vacuum sensor 3 which is inserted during production into a vacuum insulation panel under the enveloping film 2, preferably just below the enveloping film 2.

The vacuum sensor 3 comprises an air chamber 4 and the induction body 5. While inserting the vacuum sensor 3 into the vacuum insulation panel, attention should be paid that the air chamber 4 is located between the induction body 5 and the enveloping film 2 where temperature measurement will be carried out.

One of the options of inserting a vacuum sensor 3 is that it is simply put onto the core 1 before the air is extracted and negative pressure created, and before the enveloping film is hermetically closed, for instance welded. This option is shown in Figure 1.

Figure 2 shows a second option of insertion: an indentation 8 is made in the core 1 and a vacuum sensor 3 is fitted therein. It is thus achieved that the vacuum sensor 3 does not move while the air is extracted and while the enveloping film 2 is hermetically closed around the core 1 and that the vacuum insulation panel has no bulge at the spot where the vacuum sensor 3 is embedded.

The induction body 5 is made of a material having ferromagnetic properties, such as iron, to efficiently serve as an induction heating body when placed in an AC electromagnetic field. In a material having ferromagnetic properties, which is exposed to an AC electromagnetic field, heating occurs also due to desired magnetic hysteresis losses, which are converted into Joule heat.

The function of the air chamber 4 is to create a space of certain dimensions above the induction body 5, through which the heat expands. At the other side of the air chamber 4, namely at the side of the enveloping film 2, the temperature is measured. The temperature depends on the heat created by the induction body 5 and on the thermal conductivity of the gas mixture under a certain pressure in the air chamber 4. The air chamber 4 is in pressure communication with the exterior of the vacuum sensor 3 which is the interior of the vacuum insulation panel when the vacuum sensor 3 is inserted in the vacuum insulation panel. The pressure communication is carried out through one or several openings, thereby achieving that the gas mixture and the pressure are identical both in the air chamber 4 and in the core 1, i.e. in the interior of the vacuum insulation panel.

It is important that the air chamber 4 is made in such a way that the interior volume of the air chamber 4 does not get deformed due to pressure in an undesired way so as to lose the necessary volume, through which the heat flow is measured, or the pressure communication with the interior of the vacuum insulation panel. In one of the embodiments the air chamber 4 is made in a way that the bottom of the air chamber 4 is formed by an induction body 5 in the shape of a plate. The plate is preferably of a round shape but can be a polygon as well. The upper part of the air chamber 4 is made of an upper wall 9 which is also in the shape of a plate which has identical or approximately identical surface as the plate of the induction body 5.

The upper wall 9 must be sufficiently firm to withstand the pressure and to prevent deformation of the air chamber 4 while under external pressure, and sufficiently thermally conductive to minimize an error of measuring the heat flow through the air chamber 4.

In this embodiment, the air chamber 4 is made in such a way that one or several spacers 6 are fastened between the induction body 5 and the upper wall 9. The pressure communication between the interior of the vacuum insulation panel and the air chamber 4 is achieved in one of the following ways or a combination thereof:

- one or several holes are made in the induction body; preferably, only one tiny drilled hole 7 is provided in the induction body 5;

- one or several openings are made in the spacer 6;

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

Figure 3 shows an embodiment of a vacuum sensor 3, wherein an air chamber 4 at the bottom side is delimited by an induction body 5 in the shape of a round plate, at the upper side is delimited by an upper wall 9 of substantially the same shape, whereas a spacer 6 is provided with an opening 10, which represents a pressure communication between the air chamber 4 and the interior of the vacuum insulation panel. The interior of the air chamber 4 is not visible in this figure because it is inside of said elements.

Figure 4 shows the bottom side of an embodiment of a vacuum sensor 3 having a different pressure communication than the one in Figure 3. Like in Figure 3, the air chamber 4 is delimited at the bottom side by an induction body 5 in the shape of a round plate and at the upper side by an upper wall 9 of substantially the same shape. A difference is that a spacer 6 is made like a ring so that the gap between the induction body 5 and the upper wall 9 at the edge is closed around the whole circumference. Pressure communication between the air chamber 4 and the interior of the vacuum insulation panel in this embodiment is achieved by a tiny drilled hole 7 in the induction body 5. The Upper wall 9 is not visible in this figure because it is behind the induction body 5 when viewed from the bottom.

To further reduce a measurement error, the upper wall 9 may be made of a material having good thermal conductivity in vertical direction and being a good insulator in horizontal direction, e. g. FR4 which is used in the production of printed circuit boards. On the one hand, such thermal insulating features of the upper wall 9 additionally prevent the influence of heat transfer that would expand through the spacer 6 to the central portion of the upper wall 9 where the temperature is measured through the enveloping film 2; on the other hand, an undisturbed heat transfer is provided in vertical direction from the air chamber 4 through the upper wall 9 to the enveloping film 2 where the temperature is measured.

At its bottom side, i. e. the side facing the air chamber 4 and the induction body 5, the upper wall 9 may be coated with a thin layer of a reflective material, such as aluminium foil, or painted with an aluminium based reflective paint, with which a reflective layer is created that prevents heat transfer as a consequence of radiation from the induction body 5 through the air chamber 4 to the upper wall 9. In the method of measurement according to the present invention only a heat flow through the air chamber 4 due to thermal convection is desired, because the heat flow due to radiation contributes to the error of measurement.

Figure 5 shows a cross-section of an embodiment, in which an opening 19 of preferably a round shape is provided in an upper wall 9. The opening 19 eliminates the influence of thermal inertia of the material of the upper wall 9 on the temperature measurement at the upper part of the air chamber 4. The opening 19 must be sufficiently small so that the difference between the external pressure and the negative pressure in the vacuum insulation panel does not damage an enveloping film 2 when it gets indented into the air chamber 4, and that the enveloping film 2 does not get indented to such an extent that it would reach through the entire air chamber 4 and come in contact with the induction body 5.

To reduce as much as possible the undesired heat transfer from the induction body 5 to the upper wall 9 through the spacer 6, the latter is preferably made of a material having best possible thermal insulation properties.

An AC electromagnetic field generator 11, hereinafter a generator 11, and a temperature meter 12 are further needed to measure pressure according to the present invention. The generator 11 creates an AC electromagnetic field that is suitable for heating the induction body 5. To achieve the heating, the generator 11 must be put sufficiently close to the induction body 5, i.e. the vacuum sensor 3, which was inserted into the vacuum insulation panel prior to the measurement. The temperature meter 12 measures the temperature at the spot on the enveloping film 2 which is closest to the induction body 5. In the embodiments, in which the induction body 5 is incorporated in the vacuum sensor 3 and the latter is positioned between the core 1 and the enveloping film 2, the temperature is measured at the spot where the enveloping film 2 is in contact with the vacuum sensor 3, i. e. with the upper part of the air chamber 4, preferably in the centre of the vacuum sensor 3, thereby achieving the measurement of the heat flow based on thermal convection through the air chamber 4 with as little errors as possible.

In one of the embodiments the generator 11 comprises an electronic assembly 18, which generates AC voltage, and a coil made of a thick multi-core wire, which creates an AC electromagnetic field.

In the second embodiment, the generator 11 comprises an electronic assembly 18 and a coil on a U- shaped ferrite core 17, wherein, while generating AC electromagnetic field, the generator 11 is placed in such a position that the vacuum sensor 3 with the induction body 5 and the air chamber 4 is positioned between the ends of the ferrite core, thereby achieving a better efficiency of the generator 11.

Various temperature sensors can be used as the temperature meter 12, such as PTC resistors, NTC resistors or thermocouples. Desirably, the temperature meter 12 should have as low thermal inertia as possible which contributes to the accuracy of the measurement, shortens the time of the measurement and the time, in which a measurement can be repeated by the same temperature meter.

The generator 11 and the temperature meter 12 are preferably arranged together on a measuring probe 13, which makes the measurement easier as both need to be in proximity to the vacuum sensor 3 during a measurement.

In one of the embodiments, a signal from the temperature meter 12 can be conducted via an analog- to-digital converter 15 to a data processing unit 14, which controls the generator 11 and processes the measured temperature values from the temperature meter 12 at certain times, the time and power of the generator 11. From these data, the pressure in the vacuum insulation panel is then calculated as a function of the heat flow through the air chamber 4 and the thermal conductivity of the air in the air chamber 4. Two out of a plurality of possible measurement methods are described below. A measurement result is shown on a display 16. The analog-to-digital converter 15, the data processing unit 14 and the display 16 are preferably arranged on the measuring probe 13.

Figure 6 shows a measuring probe 13 in an embodiment, in which a generator 11 comprises a coil on a U-shaped ferrite core 17 and an electronic assembly 18. A position of the probe 13 during generating is shown; narhely, the vacuum sensor 3 with the induction body 5 and the air chamber 4 is positioned between the ends of the U-shaped ferrite core 17, thereby achieving a better efficiency of the generator 11. The vacuum sensor 3 was inserted into the vacuum insulation panel during its production onto the core 1 under the enveloping film 2. Figure 6 also shows other constituent parts of the measuring probe 13 in this embodiment, namely a temperature meter 12, the position of which during the measurement is preferably in the centre of the vacuum sensor 3, an analog-to- digital converter 15, a data processing unit 14 and a display 16. As already mentioned, Figure 6 shows the position of the coil on the U-shaped ferrite core 17 and of the temperature meter 12 with respect to the position of the vacuum sensor 3 while the induction body 5 is heated by exposure to AC electromagnetic field and during the measurement; positions of other elements of the probe 13 are shown only symbolically.

The pressure dependence of thermal conductivity of the air is almost linear in the range of up to 20 mbar, which is most relevant in the production and use of vacuum insulation panels. This derives from prior art, e. g. the article of C.C. Minter, 1963, Effect of pressure on the thermal conductivity of a gas, Electrochemistry Branch, Chemistry Division. Due to this linear interdependence, the measurement according the present invention is sufficiently accurate, wherein the heat is transferred through the air chamber 4 due to thermal convention, i. e. transfer of heat from a hotter to a cooler part due to the Brownian motion of gas molecules.

The pressure dependence of thermal conductivity of various materials which are used in prior art as test layers is also known and described for instance on the web site http://www.vip- bau.de/e pages/technologv/vip/howthevwork.htm. If this dependence is compared to the pressure dependence of thermal conductivity of the air, it becomes obvious that the thermal conductivity of air is considerably more linearly dependent on the pressure within a relevant range, i. e. up to 20 mbar, than the thermal conductivity of the materials used for a test layer. Therefore, the results of measurement of the pressure according to the present invention are more accurate.

The induction body 5, inserted in the vacuum insulation panel at a certain distance from the enveloping film 2, the temperature meter 12 and the generator 11 are constituent parts of a magnetic induction measuring system. The induction body 5 is preferably integrated into the above described vacuum sensor 3 which is inserted in the vacuum insulation panel in one of the ways described above. The magnetic induction measuring system may comprise also other elements described above, such as a data processing unit 14, an analog-to-digital converter 15 and a display 16. The following constituent parts of the magnetic induction measuring system may be incorporated together into a measuring probe 13: the temperature meter 12, the generator 11, data processing unit 14, analog-to-digital converter 15 and display 16.

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

1. Measuring the initial temperature with the temperature meter 12 on the enveloping film 2 at the spot which is in direct proximity to the induction body 5, preferably in the centre of the vacuum sensor 3.

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

3. Additional measuring of the temperature with the temperature meter 12 at the same spot or substantially the same spot as the measurement of the initial temperature at one or several time points during or after the heating interval.

According to one of possible embodiments of the described method, the heating interval of the induction body 5 by the generator 11 is determined in advance. An additional temperature measurement is performed only once after the initial measurement, preferably at the very moment when the heating interval is completed. At higher pressures in the vacuum insulation panel the thermal conductivity of the air in the air chamber 4 is higher, consequently the heat flow through the air chamber 4 is higher, and higher is the difference between the initial and the final temperature at a given power and the predetermined duration of the heating interval. At lower pressures, which are desirable in vacuum insulation panels, the difference between the initial and the final temperatures will be smaller.

According to another possible embodiment of the method as described above, a difference between the initial temperature and the final temperature is determined in advance. A heating interval of the induction body 5 is measured and it lasts so long that a predetermined difference between the initial temperature and the final temperature is achieved. This is why the temperature is measured several times within the heating interval of the induction body 5. At higher pressures in the vacuum insulation panel the thermal conductivity of the air in the air chamber 4 will be higher, consequently the heat flow through the air chamber 4 will be higher and the duration of the heating and measuring interval will be shorter in order to reach a predetermined difference between the initial and the final temperatures at a given power of generator 11. At lower pressures, which are desirable in vacuum insulation panels, the duration of a heating interval will be longer.

Claims

CLAIMS:

1. A vacuum sensor (3) for measuring pressure in a vacuum insulation panel, characterized in that it comprises an induction body (5), made of material with ferromagnetic properties, and air chamber (4), whereas the air chamber (4) is in pressure communication with the exterior of the vacuum sensor (3).

2. The vacuum sensor (3) according to claim 1, characterized in that the air chamber (4) is carried out as a space among the induction body (5), an upper wall (9) and one or more spacers (6) placed between the induction body (5) and the upper wall (9).

3. The vacuum sensor (3) according to claims 1 or 2, characterized in that the pressure communication of the air chamber (4) with the exterior of the vacuum sensor (3) is carried out as one or more tiny drilled holes (7) in the induction body (5).

4. The vacuum sensor (3) according to claim 2, characterized in that the pressure communication of the air chamber (4) with the exterior of the vacuum sensor (3) is carried out as one or more openings (10) in the spacer (6).

5. The vacuum sensor (3) according to claims 2 through 4, characterized in that the upper wall (9) is provided with an opening (19).

6. The vacuum sensor (3) according to claims 2 or 3, characterized in that the induction body (5) has a shape of a round plate, the upper wall (9) has s shape of a plate with the same surface as the induction body (5), and the spacer (6) has a shape of a ring.

7. The vacuum sensor (3) according to claims 2 through 6, characterized in that the upper wall (9) is made of a material having good thermal conductivity in vertical direction and being a good insulator in horizontal direction, preferably FR4.

8. The vacuum sensor (3) according to claims 2 through 7, characterized in that the upper wall (9) is, on the side facing the air chamber (4) and the induction body (5), coated with a thin layer of a reflective material, such as aluminium foil, or painted with aluminium based reflective paint.

9. Magnetic induction system for measuring of air pressure in a vacuum insulation panel, characterized in that it comprises the induction body (5), made of material with ferromagnetic properties and inserted into the vacuum insulation panel at a certain distance from the enveloping film (2), the temperature meter (12) and the generator (11).

10. The system according to claim 9, characterized in that the generator (11) comprises a coil on a U-shaped ferrite core (17) and an electronic assembly (18).

11. The system according to claim 9 through 10, characterized in that it comprises also a data processing unit (14), an analog-to-digital converter (15) and a display (16).

12. The system according to claim 9 through 11, characterized in that the induction body (5) is incorporated into the vacuum sensor (3).

13. A method for measuring the air pressure in vacuum insulation panels, characterized in that it comprises the following steps:

measuring the initial temperature with the temperature meter (12) on the enveloping film (2) at the spot which is in proximity to the induction body (5), preferably in the centre of the vacuum sensor (3); heating the induction body (5) by the generator (11) with a certain power, wherein the duration of the heating interval is measured and controlled; additional measuring of the temperature with the temperature meter (12) at the same spot or substantially the same spot as the measurement of the initial temperature at one or several time points during or after the heating interval.

14. The method according to claim 13, characterized in that the heating interval of the induction body (5) by generator (11) is determined in advance and the additional temperature measurement is performed only once after the initial measurement, preferably at the very moment when the heating interval is completed.

15. The method according to claim 13, characterized in that a difference between the initial temperature and the final temperature is determined in advance and a heating interval of the induction body (5) is measured and it lasts so long that a predetermined difference between the initial temperature and the final temperature is achieved.