WO2008035159A2

WO2008035159A2 - Method and device for measuring the gas permeability through films and walls of containers

Info

Publication number: WO2008035159A2
Application number: PCT/IB2007/002675
Authority: WO
Inventors: Alessandro Piombini; Mauro Lucchesi
Original assignee: Extrasolution S.R.L.
Priority date: 2006-09-19
Filing date: 2007-09-14
Publication date: 2008-03-27
Also published as: US8117899B2; WO2008035159A3; JP5357758B2; EP2080005A2; RU2447424C2; US20090320564A1; JP2010503850A; EP2080005B1; RU2009114310A; ITPI20060108A1

Abstract

A device for measuring permeability to a gas sample through a thin film, or a wall (2), arranged at membrane between a first and a second chamber (3 and 4) of a measurement cell (1) comprises, upstream from each inlet to the measurement cell (1), a respective regulator of pressure (β and 9) associated with a respective pressure sensor (5 and 8), and can comprise an humifier. The device comprises, furthermore, valves (11 and 12) adapted to switch the flows of gas between a starting wash and a measurement step. Each gas flow is, moreover, adjusted at the exit means of a respective flow regulator (15 and 16). A control unit (23), operates the pressure regulators (6 and 9) and flow regulators (15 and 18) in order to keep at predetermined values the total pressure in the first and in the second chamber (3 and 4), said values being always equal to each other.

Description

TITLE

METHOD and DEVICE for MEASURING the permeability of GAS THROUGH FILM or WALLS of CONTAINERS

TITLE METHOD AND DEVICE FOR MEASURING THE GAS PERMEABILITY THROUGH FILMS AND WALLS OF CONTAINERS

DESCRIPTION

Field of the invention

The present invention relates to a sensing device of the permeability of a gas through the walls of containers, in general containers for industrial products, for example plastic film containers for food, chemical, pharmaceutical, electronic products, etc.

Description of the prior art For preserving products, in particular food in a container, a plastic bag or a thin sheet/film wrapper, it is desirable to minimize the gas migration through the walls of the container. This way it is possible to protect the organoleptic properties of the products by preserving the gaseous mixture created at packaging around the products. To preserve the composition of this mixture with time it is necessary that the wrapper blocks or limits any gas migration, not only through possible closure parts or welding zones, but also through the walls of the container same.

A similar need is felt for other products, such as chemical and pharmaceutical products, electronic and optoelectronic devices and other articles that can be altered if in contact with the gaseous atmosphere present in the environment..

For determining the permeability of a film that can be used to make such containers a permeated gas flow through said film is measured. Systems are known adapted to carry out a gas flow measurement through a thin film or a wall. One of these is described in DE4142064 and comprises a measurement cell consisting of two shells facing at opposite sides with respect to a thin film sample of which permeability has to be determined; the thin film is operatively located between such shells, where such shells are sealed against the sample creating two chambers tightly separated from this film. Both chambers have an inlet and an outlet for a gas or a mixture of gas so that the gas flows in such chambers contacting at opposite sides the above described film. Owing to the permeability of the film, an amount of gas permeates from a chamber to the other so that for measuring the permeability of the film it is enough to measure the concentration of the permeated gas that reaches a sensor conveyed by a gas carrier whose flow rate is known. This permeability, as given by the product of a diffusion coefficient and of a solubility coefficient, typical for the specific material of the film, determined by known systems, is evaluated as the concentration of the gas permeating into the container in the gaseous carrier increases with time, up to reaching an asymptote parallel to the time axis of a concentration/time chart, corresponding to the stabilization of the flow permeating through the film after convergence.

The above described known systems, however, have the drawback that they provide a very slow increasing course of concentration versus time, that gives a rather long stabilization and then measurement time. In addition if different films have to be compared to each concerning the respective permeability features, the measurement time is very long. Another drawback of the known systems is that they provide very low intensity output signals, especially for analysis of low permeability films, thus increasing the measurement time up to obtaining a signal that is intense enough to be detected by common sensors.

Summary of the invention

In the following description, permeability to a gas of a film or a wall is the flow of the permeated gas through the film or wall in steady conditions . It is, therefore, an object of the present invention to provide a method and a device for measuring the permeability of a gas through a fraction of a thin film or of a wall that overcomes the above described drawbacks. A particular object of the present invention is to provide a method and a device adapted to carry out a measurement of permeability of a gas through a film, in a way quicker with respect to the known systems.

Another feature of the invention is to provide a method and a device adapted to carry out a measurement of permeability of a gas through a film, capable of amplifying the detected permeability signal with respect to the known systems.

These and other objects are achieved by a method for measuring the permeability of a gas sample through a thin film or a wall, comprising the steps of:

— arranging said thin film or wall as a membrane between a first and said second chamber in a sealed way, said membrane separating said first and second chamber;

- causing said gas sample to flow into said first chamber and causing said gaseous carrier to flow into said second chamber, an amount of said gas sample permeating into the container in said second chamber through said membrane and being conveyed away by said gaseous carrier; - measuring the fraction of said gas sample permeated into said second chamber and present in the flow of said gas carrier exiting from said second chamber; characterised in that said steps of causing said gas sample and said gas carrier to flow are made at a total pressure of said gas sample and gas carrier in said first and second chamber at a predetermined value substantially larger than the pressure of the environment, the total pressure difference between said first and second chamber remaining substantially equal to zero.

In other words, even if the film is balanced having a same pressure in the first and second chamber, the partial pressure of the gas permeating into the container is increased, in order to control as desired flow thereof through the membrane.

Preferably, said total pressure predetermined value of the gas is set between 2 and 15 bar and preferably between 3 and 7 bar.

Advantageously, a further step is provided of reducing said total pressure in said first and second chamber up to a value less than said predetermined value .

In particular, said lower value is selected from the group comprised of: a value less than said first predetermined value but higher than the atmospheric pressure; a value substantially equal to the atmospheric pressure . According to the invention, by causing the gas sample and gas carrier to flow with a pressure more than atmospheric, the total pressure in the first chamber is always substantially equal to the total pressure of the second chamber, so that the permeated gas flow is higher than that in the prior art. This way, since the permeated gas amount versus time is higher, a quicker transient phase is obtained if a consequent variation of permeated flow is considered. Furthermore, since the permeated gas amount is higher, it is possible to use sensors that in general are less sensitive and then cheaper.

In particular, said step of reducing the total pressure is obtained as desired by means of: - a feedback automatic control capable of working on the concentration of the gas and on the total pressure in said first and second chamber;,

- a open loop automatic variation that is programmed in said first and second chamber. Advantageously, a cleaning step of said first and second chamber and of said membrane are provided, obtaining a controlled flow of gaseous carrier through said first and second chamber, the total pressure difference between said first and second chamber being kept substantially equal to zero, said total pressure being increased up to a predetermined washing value preferably larger than the pressure of the environment .

According to another aspect of the invention, the above described objects are also fulfilled by a device for measuring the permeability of a gas through a membrane of thin film or a wall, comprising:

- a first and a second chamber having respectively a first and a second opening operatively facing to each other and arranged at opposite sides with respect to said membrane, said first and second opening being sealingly coupled with the external surfaces of said membrane, said first and second chamber having at least one respective inlet and a respective outlet, in said first chamber a gas sample flowing and in said second chamber a gaseous carrier flowing, a fraction of said gas test permeating into the container in said second chamber and flowing with said gas carrier towards said outlet of said second chamber;

— means for measuring and adjusting the flow in said first and second chamber; — means for measuring the concentration of the gas exiting from said second chamber; characterised in that means are provided for measuring and adjusting the pressure in said first and second chamber adapted to increase the total pressure of said gas sample and gas carrier in said first and second chamber at a predetermined value substantially larger than the pressure of the environment, the total pressure difference between said first and second chamber remaining substantially equal to zero. Advantageously, said means for measuring and adjusting are adapted also to reduce said total pressure in said first and second chamber up to a further total pressure predetermined value, not higher than said predetermined value. The device according to the invention is then capable of carrying out the method as above described, causing the total pressures of the gas in the first chamber and in the second chamber to increase their value beyond a predetermined value, ensuring however that the membrane arranged as separators of the two chambers is subject to pressures such as on both its surfaces, and then is not forced to deformation. Advantageously, said device comprises a control unit automatic, adapted to controlling said means for adjusting the flows of said gas sample and gas carrier and said means for adjusting the total pressure of the gas in said first chamber and in said second chamber, in order to bring said total pressure beyond a predetermined value and so that said total pressure in said first chamber remains substantially equal to the total pressure in said second chamber.

Advantageously, said first chamber is obtained in a first hollow object and said second chamber is obtained in a second hollow body, said second hollow body creating a base and said first hollow object creating a cover operatively engageable with said base, said membrane being arranged between said base and said cover. In particular, said cover is operatively pressed on said base by means of stopping means preferably selected from the group comprised of:

— at least one bracket;

— at least one screw; - a screw threaded surface between said cover and said base;

— a fixed joint;

— at least one element for engaging said cover to said base, coupled pivotally for rotation or coupled slidingly with respect to said base.

In particular, said first and second chamber, obtained in said first and second hollow body, have cylindrical shape, preferably determined by milling. Advantageously, said means for measuring and adjusting the total pressure of the gas in said first and second chamber are arranged upstream from said first and second chamber. In one form of preferred embodiment, said means for adjusting the flow are arranged downstream of said first and second chamber.

Advantageously, said means for measuring the concentration of the gas exiting from said second chamber are arranged downstream of said second chamber.

Advantageously, said device comprises means for adding humidity to said gas sample as inlet to said first chamber. In particular, said means for adding humidity comprises :

- an outer reservoir containing a liquid comprising water, said reservoir having an inlet and an outlet for a flow of said gas sample through said reservoir;

- at least one inlet duct arranged upstream from said reservoir and exiting into said inlet and at least one delivery duct arranged downstream of said reservoir, said delivery duct exiting into said first chamber.

In particular, said inlet duct of said reservoir comprises a valve adapted to switch the flow of said gas sample between said inlet duct of said reservoir and said inlet of said first chamber. This way it is possible to have in parallel two inputs for gas sample, a for dry gas or in normal conditions and a humidified gas that crosses the reservoir and exits from said first chamber. By combining the fraction of dry gas and humidified gas the desired humidity is obtained.

Advantageously, said device comprises means for measuring the humidity of said gas sample, said means being preferably arranged downstream of said outlet of said first chamber.

In particular, said gas sample, which is adapted to permeate in part from said first to said second .chamber through said membrane is selected from the group comprised of:

- oxygen;

- carbon dioxide.

In particular, said gaseous carrier, adapted to carry towards said outlet of said second chamber said gas sample permeated into said second chamber, is selected from the group comprised of:

- nitrogen;

- hydrogen;

- helium; a mixture of said gases.

Brief description of the drawings The invention will be made clearer with the following description of some its embodiments, exemplifying but not limitative, with reference to the attached drawings wherein:

- Figure 1 shows a diagrammatical view of a device according to the invention having a measurement cell of gas permeability through a thin film;

- Figure 2 shows an exploded view of a measuring device capable of carrying out the method according to the invention;

- Figure 3 shows a cross sectional view of a portion of such a measuring device;

- Figure 4 shows a comparative diagram of the course of gas permeability versus time between the known devices and the device according to the invention, where the measurement permeability is carried out at a high pressure; — Figure 5 shows a comparative diagram of the course of permeability obtained with the known systems with respect to the course obtained with the present invention, where after a first measuring step at a high pressure, the pressure is reduced to a value close to atmospheric pressure;

— Figure 6 shows a comparative diagram of the course of permeability obtained with the known systems with respect to the course obtained with the present invention, wherein, after a first measuring step at a high pressure, the pressure is reduced partially up to a value in any case larger than atmospheric pressure;

- Figure 7 shows a diagram that describes the evolution time of the concentration in the thickness of the film according to the traditional technique; - Figure 8 shows a further chart that describes the evolution time of the concentration in the thickness of the film according to the present invention. Description of the preferred embodiments In the following description an example will be illustrated of a method according to the invention, for measuring the permeability of a gas sample through a membrane of thin film or a wall, for example of containers of food products, of drugs and in general for all the products that, for a correct preservation, need to be preserved under vacuum or in the presence of a predetermined mixture of gas. In such cases it is necessary to know the gas permeability of the container through the film. Such method is obtained with a device shown in figure 1, comprising a measurement cell 1 of known type, consisting of a first hollow object 70 or cover and a second hollow body or body of base 30, in each of which is determined a chamber for milling circular, respectively denominate first chamber or chamber of the cover 3 and second chamber or chamber of base 4. Closing the cover 70 on the body of base 30 with the interposing the membrane 2, the two chambers are separated from the membrane, which is of material thin of which is to be measuring the gas permeability, for example the permeability of a packaging material to oxygen. Each two chambers 3 and 4, as shown in figure 3, has a respective inlet and an outlet for flow of the gas. In particular, the first chamber 3 has an inlet 40 and an outlet 41 for a flow 25 of gas sample, for example oxygen, of which is to be measuring the permeability through the membrane 2, whereas the second chamber 4 has an inlet 31, through which is inserted a flow 26 of gaseous carrier, for example nitrogen, adapted to uguagliare the total pressure of the first chamber for not sollecitare the membrane 2, and adapted to carry, towards the outlet 32, the amount of gas sample permeated into the chamber 4 through the membrane 2 during the measurements, normally a flow 28 of nitrogen more oxygen. In figure 1, upstream from each inlet to the measurement cell 1, the device according to the invention comprises respectively a regulator of pressure β and 9 associated with a respective pressure sensor 5 and 8. For adding humidity to the flow 26 of the gas carrier as inlet can be provided an humidifiedr not shown in figure 1 but indicated with 60 in figure 2, arranged on the flow same, comprising a reservoir of water out of the body of base. The humifier, not shown, is preferably located immediately before the inlet of the gas in the cell on the line upper, and that can be also on that lower not shown. Furthermore, can be provided a catalyst 10 for capturing and eliminating the possible traces of oxygen present on the line inlet of the gaseous carrier. the device comprises the valves to keep 11 and 12 adapted to switch the flows of gas between a step starting pulizia and a measurement of step actual. In fact in the step starting pulizia is caused to flow gaseous carrier, for example nitrogen, in both the chambers 3 and 4, and in this case would be open only the valve 11. During the measurement step actual, instead, the valve 11 is closed and the valve 12 open in order to obtain two flows different of gas sample and gaseous carrier.

Each gas flow is, moreover, adjusted at the exit means of a respective regulator of flow 15 and 16. The flow 28 in outlet is analysed by a sensor 17 specific for permeated gas to analyse. The valves 18 and 19, with the duct 29, allow to excluding the sensor 17 by the gas migration in outlet 28, commuting this passage through the duct 29. A control unit 23, preferably of electronic type, operates the regulators of pressure 6 and 9 and of flow 15 and 18 in order to keep the total pressure in the first and in the second chamber always equal, both during the step of pulizia that during the measurement step step, to avoid the inbending or the voltage excessive of the membrane 2, using the measurement the pressure by the pressure sensors 5 and 8. this control unit 23, furthermore, is capable of registrare the concentration of the gas sample, determined by the sensor 17, permeated into the gas carrier through the film, supplying a value outlet that represents the permeability of the membrane 2. Such a system, thus, increases remarkably the partial pressure of the gas sample, for example oxygen, permeated into the second chamber 4 wherein flows the gaseous carrier, even if maintaining equal to zero the total pressure difference between the two chambers 3 and 4. In particular, carrying the total pressures beyond this predetermined value is obtained a higher permeation of the gas sample in the second chamber than the known systems, obtaining a higher concentration of the gas sample and then a much easier and attendible measurements, as well as time of stabilization and measure definedly lower with respect to those obtained with the known systems.

Figure 2 shows a possible exemplary embodiment of a device according to the invention, as already described above. Comprises a hollow body of base 30, a hollow body or cover 70, means for adding humidity to the flow of a gas carrier, a plurality of ducts for moving the gas. The chamber 4 in the body of base 30 has a hole inlet 31, adapted to move a gas carrier

(nitrogen) , and a hole outlet 32 of the gas carrier more the gas that is riuscito to cross the membrane, normally nitrogen more oxygen. The holes 31 and 32 of the chamber of the body of base 30, communicate with relative inlet ducts 61 and of unloading 62 of the gas made of the body of base 30, along the dashed lies indicated as 42 and 43, in the inlet duct 61 passing N2 and in the discharge duct 62 exiting N₂ + O₂.

The cover 70 has a flange 65 that is superppone to the body of base 30. The chamber of the cover 3 has inlet holes 63 and outlet 64 of the gas of which is to be measuring the permeability through the membrane 2, communicating with relative inlet ducts and of unloading made of the cover 70 sfocianti in the holes 40 and 41 at the flange 65, which meet the respective fittings 39 and 38 on the body of base 30 and communicating with relative further inlet ducts 46 and of unloading 47 that cross the body of base 30, adapted to be crossed by N₂ or O₂. for adding humidity to the gas as inlet to the chamber 3 of the cover is provided a reservoir of water 60 out of the body of base 30 and communicating with a duct 45 that va to immettersi in the inlet duct 46. It is therefore possible, having in parallel two inputs for N₂ or O₂, a for dry gas or in conditions normali, and a humidified gas that crosses the reservoir of water 60, falls within body of base 30 and is ri-immette in the inlet duct 46. Combining the fraction of dry gas and of gas wet it is possible to obtain the humidity desired. a sensor provides humidity, located near the channel outlet 35. in the complex, the body of base 30 is crossed by two ducts respectively inlet 42 and unloading 43 of the gas in the chamber of base 30, and two ducts respectively inlet 48 and unloading 49 of the gas in the chamber 3 of the cover.

It is possible to then on the one hand put gas sample in the chamber 3 of the cover and farlo come out from it, following the path 48 and 49, giving to the gas a desired pressure controlled, for example atmospheric pressure and a measured humidity. Furthermore, on the other hand, it is possible to cause cover the chamber 4 in the body of base by a gas carrier neutral, for example nitrogen, to atmospheric pressure or controlled. The gaseous carrier, following the path 42 and 43, engages withcon se the fraction of gas sample (oxygen) that is permeated into the chamber of base 4 coming from the chamber of the cover 3 through the membrane 2. A detector of gas, not shown, downstream of the duct 43 is capable of giving the data of permeability desirati.

A tools to kit, not shown, controls the flows of gas as inlet and in outlet to provide data amounts of permeability of the membrane to the many gas sample permeated into the seconda chamber.

A device according to the invention, described in figures 1 and 2 and 3, carries out the method according to the invention, this method can comprise a first step of "pulitura" of the membrane 2, during which the regulators of flow 15 and 16 cause to flow an amount controlled of gaseous carrier, typically nitrogen, in both the chambers 3 and 4, at controlled pressure. This way, possible amount of gas inglobate at first in the thickness of the membrane 2, are extracted by the membrane 2 same and transported at the outlet of the gaseous carrier. The first step of pulizia is followed by a measurement of step actual wherein an amount controlled of gas test is caused to flow in the chamber of the cover 3, whereas at the outlet of the chamber of base 4 is record the effects of the permeaction. this measurement of step is done carrying value of total pressure, in both the chambers 3 and 4, above a predetermined value, increasing thus the permeation of gas test in the chamber of base 4 and therefore its partial pressure, allowing a measure more accurate, but especially much quicker with respect to the known systems. after a time predetermined, then after that the permeation has avuto initio, the total pressures in the two chambers 3 and 4 can be again reduced for move to the conditions standard (1 bar) or attesting to a value highest. The reduction of the pressure can be carried out both acting with a control of feedback on the signal produced by the permeated gas or on the pressure, both by a variation time programmed of pressure.

In figure 4 is shown the course time of the permeability determined responsive to the time, then on the axis of the ordinate 80 is responsive to the permeability, whereas on the axis of the abscissas 81 is responsive to the time. In particular, the graphical 82 represents the course obtained from the techniques known, whereas the graphical 83 represents the course of the permeability obtained with the present method. In particular, in figure 4, the course 83 is obtained carrying out the measure with a value of total pressure high, in the first and second chamber, for example a pressure equal to triplo of the pressure maximum used prior art. as is visible in the graphical, the permeability determined assume values larger with respect to those of the prior art and tends to an asintoto horizontal 85. Such an applied on, then can be useful in case of film to low permeability.

Normally, if the film has to be used for preserving of products to atmospheric pressure, is requires to carry out the measurement the permeability to a value of pressure close to atmospheric pressure. In this case, the method according to the invention requires a step of reducing the pressure up to a value close to atmospheric pressure and produce a course of the permeability as described in figure 5 where the curved 82 represents the course obtained with the known systems, whereas the curved 83 represents the course obtained with the present invention, this curved 83 is obtained with a step starting increasing the pressure to a value high and reducingla then to atmospheric starting pressure from the point 86. this method allows to obtain a course 83 of the permeability having a fraction increasing much more ripid of that 82 obtained with the prior art, even if carrying out the measure to pressure close to atmospheric pressure, and then in conditions standard, after a transient phase starting at a high pressure. This is traduce in a quicker stabilization necessary to the measurement the permeability solving the problem of dover await a long time before achieving this stabilization with the known systems . Figure 6 shows a further course 83 of the permeability obtained with the method according to the invention, wherein after a transient phase starting at a high pressure, this pressure is reduced to a further value of pressure less than that of figure 4 but higher than the atmospheric pressure, is observed, thus, a permeability more than of figure 5.

Figure 7 shows the evolution time of the concentration in the thickness of the film, obtained with a method known, whereas figure 8 shows the corresponding evolution time obtained with the method according to the invention. Such graphs have in ordinate (92 or 102) the concentration of gas in the film and in abscissas (91 or 101) the position of detection along the thickness of the film referred to the middle plane and expressed in fractions of the semi-thickness. The curved figures 7 and 8 are parameterssed with respect to the time in both cases. In particular, in figure 7, the curved 98 corresponds to a time of 0.04 hours, the curved 97 to a time of 0.1 hours, the curved 96 to a time of 0.2 hours, the curved 95 to 0.4 hours, the curved 94 to 0.6 hours and the curved 93 to 2.0 hours.

Instead, in figure 8, the curved 107 corresponds to a time of 0.04 hours, the curved 106 to 0.1 hours, the curved 105 to 0.2 hours, the curved 104 to 0.4 hours, the curved 103 to 0.6 hours.

The description of which above one form esecutive specific is capable of show the invention from a viewpoint concettual so that other, using the prior art, can be changing and/or adapting in various applications this shape esecutive specific without further ricerche and without moving away from each other by the concept inventive, and, then is intended that such adaptation and changes will be high as equivalent of the shape esecutive exemplified. The means and the material to provide the various functions described can be changes nature without for this come out from the field of invention, is intended that the expressures or the terminology used have object purely describedvo and for this not limitative.

Claims

1. A method for measuring the permeability of a gas sample through a thin film or a wall, comprising the steps of: - arranging said thin film or wall as a membrane between said first and said second chamber in a sealed way, said membrane separating said first and second chamber; - causing said gas sample to flow into said first chamber and causing said gaseous carrier to flow into said second chamber, an amount of said gas sample permeating into the container in said second chamber through said membrane and being conveyed away by said gaseous carrier; - measuring the fraction of said gas sample permeated into said second chamber and present in the flow of said gas carrier exiting from said second chamber; characterised in that: said steps of causing said gas sample and said gas carrier to flow are made at a total pressure of said gas sample and gas carrier in said first and second chamber at a predetermined value substantially larger than the pressure of the environment, the total pressure difference between said first and second chamber remaining substantially equal to zero.

2. A method, according to claim 1, whereby said total pressure predetermined value of the gas is set between 2 and 15 bar and preferably between 3 and 7 bar.

3. A method, according to claim 1, whereby a further step is provided of reducing said total pressure in said first and second chamber up to a value less than said predetermined value.

4. A method, according to claim 3, wherein said lower value is selected from the group comprised of:

- a value less than said first predetermined value but higher than the atmospheric pressure; - a value substantially equal to the atmospheric pressure.

5. A method, according to claim 3, whereby said step of reducing the total pressure is obtained as desired by means of: - a feedback automatic control capable of working on the concentration of the gas and on the total pressure in said first and second chamber;

- a open loop automatic variation programmed first and second chamber.

6. A method, according to claim 1, whereby a cleaning step of said first and second chamber and of said membrane, obtaining a controlled flow of gaseous carrier through said first and second chamber, the total pressure difference between said first and second chamber being kept substantially equal to zero, said total pressure being increased up to a predetermined washing value preferably larger than the pressure of the environment.

7. A device for measuring the permeability of a gas through a membrane of thin film or a wall, comprising:

- means for measuring and adjusting the flow in said first and second chamber;

- means for measuring the concentration of the gas exiting from said second chambers- characterised in that means are provided for measuring and adjusting the pressure in said first and second chamber adapted to increase the total pressure of said gas sample and gas carrier in said first and second chamber at a predetermined value substantially larger than the pressure of the environment, the total pressure difference between said first and second chamber remaining substantially equal to zero.

8. A device, according to claim 7, wherein said means for measuring and adjusting the pressure are adapted also to reduce said total pressure in said first and second chamber up to a further total pressure predetermined value, not higher than said predetermined value.

9. A device, according to claim 7, wherein said first chamber is obtained in a first hollow object and said second chamber is obtained in a second hollow body, said second hollow body creating a base and said first hollow object creating a cover operatively engageable with said base, said membrane being arranged between said base and said cover.

10. A device, according to claim 9, wherein said cover is operatively pressed on said base by means of stopping means preferably selected from the group comprised of:

- at least one bracket; - at least one screw;

- a screw threaded surface between said cover and said base;

- a fixed joint;

- at least one element of engaging said cover to said base, coupled pivotally for rotation or slidingly with respect to said base.

11. A device, according to claim 9, wherein said first and second chamber, obtained in said first and second hollow body, have cylindrical shape, preferably determined for milling.

12. A device, according to claim 7, wherein said means for measuring and adjusting the total pressure of the gas in said first and second chamber are arranged upstream from said first and second chamber.

13. A device, according to claim 7, wherein said means for measuring and adjusting the flow are arranged downstream of said first and second chamber.

14. A device, according to claim 7, wherein said means for measuring the concentration of the gas exiting from said second chamber are arranged downstream of said second chamber.

15. A device, according to claim 7, comprising means for adding humidity to said gas sample as inlet to said first chamber.

16. A device, according to claim 15, wherein said means for adding humidity comprises: - a outer reservoir containing a liquid comprising water, said reservoir having an inlet and an outlet for a flow of said gas sample through said reservoir;

17. A device, according to claim 16, wherein said inlet duct of said reservoir comprises a valve adapted to switch the flow of said gas sample between said inlet duct of said reservoir and said inlet of said first chamber .

18. A device, according to claim 7, comprising means for measuring the humidity of said gas sample, said means being preferably arranged downstream of said outlet of said first chamber.

19. A device, according to claim 1 or 7 , wherein said gas sample, which is adapted to permeate in part from said first to said second chamber through said membrane is selected from the group comprised of:

- oxygen;

- carbon dioxide

20. A device, according to claim 1 or 7, wherein said gaseous carrier, adapted to carry towards said outlet of said second chamber said gas sample permeated into said second chamber, is selected from the group comprised of:

- nitrogen; - hydrogen

- helium;

- a mixture of said gases.