DEVICE FOR MEASURING THE PERMEABILITY OF BOTTLE CAPS AND CORRESPONDING METHOD

Abstract

A device (1) for measuring the gas permeability of a plug, comprises: - a measuring cell (10) in which the plug can be mounted so as to delimit an upstream chamber (76) to the plug and a downstream chamber (16) to the plug, gas exchanges being possible between the upstream chamber (76) and the downstream chamber (16) only by passing through the plug, - a gas supply (2) making it possible to impose in the upstream chamber (76) an upstream pressure of a first gas, of first predefined pressure value, - a downstream pumping device (3) making it possible to impose in the downstream chamber (16) an initial depression of second predefined value, lower than the first value, then to isolate the downstream chamber (16) from gas exchanges other than those passing through the plug, - a sensor (20) making it possible to carry out pressure measurements in the downstream chamber (16).The downstream chamber (16) is connected by a first valve (11) with a metering chamber (17) capable, when the first valve (11) is closed, of being successively connected to, and isolated from, the atmosphere (18), or a gas supply (2).

Description

Figure 2 is an element of a measuring cell of a device according to the invention; Figure 3 is another element of a measuring cell of a device according to the invention; Figure 4 is a simplified sectional view of a cell of a device according to the invention; Figure 5 is a schematic exploded view of another cell variant of a device according to the invention; Figure 6 is a front view of one of the parts making up the cell of Figure 5; Figure 7 is a schematic sectional view of a third cell of a device according to the invention; Figure 8 is a schematic sectional view of a fourth cell of a device according to the invention. As illustrated in Figure 1, a permeability measuring bench referenced 1 comprises a gas supply 2, a vacuum pump 3, and one or more measuring units referenced here 4, 5, 6, arranged between the gas supply 2 and the vacuum pump 3. In Figure 1, three measuring units have been shown. Depending on the embodiment variants, measuring benches can be constructed comprising only one measuring unit or comprising more than three measuring units, for example five, seven, ten measuring units. The different measuring units 4, 5, 6 are connected in parallel between the gas supply 2 and the vacuum pump 3. A pressure regulating unit 87 comprising an upstream pressure sensor 7 and an upstream pressure regulator 8 is further arranged between the gas supply 2 and the measuring units 4, 5, 6. The pressure regulator 8 may comprise a pressure reducer (not shown) making it possible to evacuate any excess pressure - in relation to a pressure setpoint - which would occur upstream of the regulating unit 87. Each measuring unit 4, 5, 6 comprises respectively a gas pipe respectively 4a, 5a, 6a, connecting the gas supply 2 and the vacuum pump 3. On the path of this gas pipe, a measuring cell 10 is interposed making it possible to interpose, on the path of the gas circulating from the supply 2 to the vacuum pump 3, one or more plugs whose permeability is to be evaluated. In the remainder of the description, we will call the positions close to the gas supply "upstream position" and the positions close to the vacuum pump 3 "downstream position". In the following we describe the architecture of a particular measuring unit, for example measuring unit 6. Each measuring unit 4, 5, 6 comprises an upstream isolation valve 9 allowing it to be isolated from the gas supply 2, and comprises a downstream isolation valve 19 allowing it to be isolated from the vacuum pump 3. These two isolation valves make it possible, for example, to deactivate the associated measuring unit and to continue working on the other measuring units. Downstream of the valve 9 is arranged a pilot-controlled supply valve 12 which, like the valve 9, makes it possible to isolate the measuring unit from the gas supply 2. According to an alternative embodiment, the progressive opening and closing of the valve 12 can be regulated more finely than that of the valve 9, which is an on / off valve. According to another alternative embodiment, the valve 12 can be an on / off valve. The supply valve 12 is connected to an electronic control unit (ECU) (not shown), capable of controlling the valve 12 according to the gas supply requirements of the cell 10 during a permeability measurement cycle. The valve 19 is also assisted by a controlled vacuum valve 14, placed upstream of the valve 19 and which, when the valve 19 is open, can provide the same isolation function with respect to the vacuum pump 3 as the valve 19. The vacuum valve 14 is connected to the electronic control unit (ECU) (not shown), capable of controlling the valve 14 according to the vacuum requirements of the cell 10 during a permeability measurement cycle. The upstream pressure sensor 7 and the upstream pressure regulator 8 are also connected to the electronic control unit. The measuring cell 10 is arranged between the supply valve 12 and the vacuum valve 14. A downstream containment valve 11, also controlled by the ECU, is arranged downstream of the measuring cell 10, between the measuring cell 10 and the vacuum valve 14. The cell 10 contains one or more plugs to be evaluated, arranged so that all gases passing through the cell are forced to pass through one or other of the plugs. The downstream containment valve 11 is arranged close to the cell 10, preferably as close as possible given the assembly constraints of the various connections of the gas pipe (connection of the downstream of the cell 10 with the valve 11 for example, connection of a pressure sensor to the cell or upstream of the valve 11, see the rest of the description...). A downstream chamber 16 is defined by the volume included, inside the pipe 4a, 5a, or 6a and the cell 10, between the downstream face of the plug(s), and the downstream containment valve 11. A pressure sensor 20 is connected to the downstream chamber 16. When the downstream containment valve 11 is closed, the gases passing through the plug(s) arranged in the cell 10 can accumulate in the downstream chamber 16. A second air intake 18 is connected between each pair of downstream containment pilot valves 11 and 14 for vacuuming, of the same measuring unit. This air intake 18 can be isolated by a pilot valve 15, separating the external atmosphere from the portion of pipe between the pilot valve 11 and the pilot valve 14. A pilot valve 15 is preferably provided for each measuring unit 4, 5, 6. A bypass line 42 connects a first portion of line arranged between the upstream of the cell 10 and the downstream of the supply valve 12, and a second portion of line connecting the downstream containment valve 11 and the vacuum valve 14. A bypass valve 13 is arranged on the bypass line 42. When the downstream containment valve 11 and the bypass valve 13 are open, the upstream and downstream of the cell 10 are connected through the bypass line 42, the gas exchanges between the upstream and downstream of the cell 10 then being possible without passing through the plugs contained in the cell. When the bypass valve 13 is closed, all gases passing through the cell 10 are forced to pass through one of the plugs arranged in the cell. An upstream chamber 76 is thus defined by the volume included, inside the pipe 4a, 5a, or 6a and the associated cell 10, between the upstream face of the plug or plugs, the supply valve 12, and the bypass valve 13. When the supply valve 12 and the vent valve 15 of the measuring unit are closed, the valves 11, 14, 19 of the unit being open, the vacuum pump 3 can create a vacuum in the downstream chamber 16 by sucking the gas contained in this chamber through the downstream isolation valve 11 then the regulating valve 14 and finally the downstream isolation valve 19. If the valve 13 is open, the pump 3 can simultaneously create a vacuum in the upstream chamber 76. If the valve 13 is closed, the vacuum is created in the downstream chamber 16 independently of the pressure prevailing upstream of the valve 13, in particular the pressure in the upstream chamber 76. When the valves 11 and 13 of the measuring unit are closed, with the valves 9 and 12 open, gas from the gas supply 2 can enter the measuring unit through the upstream isolation valve 9, pass through the supply valve 12, arrive in the measuring cell 10, pass through the plug(s) interposed in its path, and exit into the downstream chamber 16 equipped with the pressure sensor 20. If the vacuum has previously been created in the downstream chamber 16, all the gas present in the downstream chamber 16 will have arrived there by passing through the plug(s) of the cell 10. The gas arriving through the plugs of the measuring cell 10 then causes an increase in pressure in the downstream chamber 16, which can be measured by the sensor 20, provided that the pressure of the downstream chamber 16 is within the measuring range of the sensor. The portions of pipes between the four valves 11, 13, 14 and 15, which can for example be formed by a cross connection, delimit a dosing volume 17, which can be connected or isolated from the downstream chamber 16 by means of the downstream containment valve 11. This dosing volume 17 can also, as long as the valve 11 is closed, be put under vacuum by the pump 3, through the valves 14 and 19, or brought into contact with the atmosphere through the valve 15, or supplied from the gas supply 2 through the valves 9, 12 and 13, all this without affecting the pressure of the downstream chamber 16. We will now describe the use of the bench to obtain a permeability measurement. The plugs to be tested are placed in the measuring cells 10 whose geometry is described later. When a plug is installed in the cell 10, the upstream face of the plug is subjected to the pressure prevailing in the portion of pipe between the cell 10 and the supply valve 12. The downstream face of the plug is subjected to the gas pressure prevailing in the downstream chamber 16. In order to precondition the plugs, the supply valve 12 is closed, the valves 11, 13, 14, 19 are opened and a vacuum is created simultaneously in the upstream chamber 76 and in the downstream chamber 16 using the vacuum pump 3. This evacuates the gases and moisture trapped in the material of the plug, which could, if this precaution were not taken, increase the pressure in the downstream chamber 16 during subsequent pressure measurements.The downstream containment valve 11 is then closed in order to maintain the vacuum in the downstream chamber 16, the bypass valve 13 is closed, the valve 12 is opened and a predefined gas pressure is applied from the gas supply 2, for example 1200 millibars of oxygen. To do this, the supply valve 12 can be an on / off valve, the pressure upstream of the valve 12 is regulated by the pressure regulating unit 87. At the necessary moment, the supply valve 12 is simply opened to impose the upstream pressure in the upstream chamber 76. A pressure drop valve 44, for example of the needle valve type, is then advantageously interposed between the valve 12 and the measuring cell 10, in order to create a pressure drop and to avoid too sudden an application of the upstream pressure after having created the vacuum upstream and downstream of the plug, which could for example cause a displacement of the plug contained in cell 10. According to another embodiment variant, if the valve 12 is a proportionally controlled valve, the opening of the valve 12 can be regulated by the electronic control unit using values ​​received from the upstream pressure sensor 7. The installation may then not include a valve 44. In order to facilitate pressure regulation by the regulation unit 87, a gas pressure applied upstream of the cell 10 is preferably chosen, which is higher than atmospheric pressure. The plugs enclosed in the cell 10 are therefore subjected to an upstream gas pressure on their upstream face, and to the vacuum created in the downstream chamber 16 on their downstream face. The gas molecules which then pass through the plug gradually increase the pressure in the downstream chamber 16. The electronic control unit connected to the pressure sensor 20 causes a pressure value to be acquired at successive intervals, making it possible to construct a pressure rise curve for the downstream chamber 16. For example, it is possible to record a pressure value every minute, until the pressure in the downstream chamber has reached a threshold pressure of the order of 150 mbar.Instead of recording at predetermined time intervals, it is also possible, using an event detector, to record successive pressure values, each differing from the previous recorded value by a minimum value. This avoids storing "uninteresting" values ​​because they are aligned on a pressure plateau. If the first measurements allow the desired slopes to be determined (see below) even before the pressure exceeds 10 mbar, it may be preferable to limit ourselves to these first values. In the majority of cases, one week is enough to obtain a satisfactory number of measurements. The measurement points obtained are then compared to a theoretical curve, to extract parameters from the experimental curve allowing the permeability of the plug to be quantified. The points on the curve can also be used to determine an oxygen flow rate (e.g. in moles / day) through the plug(s). In order to evaluate the oxygen flow rate corresponding to a particular point on the experimental pressure curve, one can evaluate the slope — of the curve in the vicinity of this point, and use dt the ideal gas equation, i.e. pV=nRT Or : p: the gas pressure in the downstream chamber (in Pa), V: the volume of the downstream chamber (in m3), n: the quantity of gas in moles in the downstream chamber, A: the universal constant of ideal gases (i.e. 8.3 1 4J. K"1. mol'1 ), T the temperature in Kelvin of the room and the measuring bench, i.e. the temperature in C° to which 273.15° has been added, i.e. T = T°C +273.15. By deriving the ideal gas equation with respect to time, we , . , , dn dp PX24X3600 , dp , , , obtains the relation: —x---------, where — is the slope of the dt dt RxT dt of pressure curve in Pa / s, and — is the gas flow rate through the or dt traffic jams, in mol / day. If, for example, we wish to express a gas flow rate in cm3.day'1 (for example, for oxygen, it will be 1"'OTR", in cm3 / day), we know that one mole of gas occupies 22.4 liters, or 22,400 cm3. So dn OTR = — x 22,400. dt If, for example, we wish to express a gas flow rate in mg.day'1 (for example, for oxygen, this will be the "OTR", in mg / day), we know that one mole of CL weighs 32000 mg. So dn OTR = — x 32000. dt In this type of calculation, the pressure difference, thickness and exchange surface are not taken into account. In order to calculate the permeability coefficient Pe of the plugs, we perform a logarithmic transformation of the curve depression by drawing a curve y(t) with j(0 = -ln Pd^-Pu . Pd» -Pu . = -111^(7)- / ^1 + ^ where: Pd(t): is the pressure in the downstream chamber at time t (in Pa), pu: is the gas pressure applied in the upstream chamber (in Pa), k = pao: is for example the pressure in the downstream chamber corresponding to the first measurement point (in Pa). We then linearize this curve y(t) using techniques , , dy , , , . , known to determine its average slope —, and we deduce the dt permeability coefficient Pe of the plug according to the formula: D eVa-Ts dy Pe =---— x A.PsTVm dt Or : Pe: is the desired permeability coefficient (in mol.m'VPa'Vs'1), A: the total section (in m2) of the plugs crossed by the gas, e: 1 thickness (in m) of the plug or the average thickness of the plug, i.e. the thickness of material that the gas must pass through, ps: a standard pressure equal to 101,325 Pa, t: (in s) the abscissa of the times corresponding to the different instants of pressure measurement, T: the temperature (in K) of the measuring bench, Ts: a standard temperature of 273 K, Va: the volume of the downstream chamber in m3, Vm: the molar volume of the gas with which the measurement is made, for example 0.0224 m3 / mole for oxygen. It should be noted that the choice of the initial pressure Pdo does not influence the result because by changing the value of Pdo, we shift simply the curve y(t) of a fixed value along the ordinate axis without modifying the slope of the curve. We can also, as a first approximation, consider that when the pressure pa in the downstream chamber is very low compared to the pressure pu applied in the upstream chamber, y{t} = —, which gives Pu an approximate value for the permeability coefficient Pe q T 1 a substantially equal to — a' s—x—x---—. We can therefore, following a A.Ps.TVm pu dt alternative implementation, simply determine the slope of the pressure versus time directly, instead of performing the logarithmic transformation. It should be noted that the permeability coefficient Pe is intrinsic to the material being measured, with the sample geometry (surface area, thickness) being taken into account in the above formula to reduce the value of Pe to a unit of surface area and a unit of thickness. The gas flow rate, for example the OTR, is an overall value dependent on the sample geometry. The electronic control unit can be configured to automatically process all the pressure measurements obtained. Semi-manual processing of the measurements can also be envisaged, where an operator selects, for example, the portion of the measurement curve that seems most favourable for providing relevant values. A precise estimate of the volume Va of the downstream chamber is of course necessary to obtain a precise estimate of the permeability coefficient Pe. This volume Va being likely to vary depending on the positioning of the downstream face of the plug or plugs in the measuring cell 10, a positioning which is not strictly reproducible of the plugs in the cell can lead to measurement deviations which can distort, for example, comparisons between two plug materials. In order to avoid such artifacts, after each placement of a plug and a series of plugs in the measuring cell 10, an evaluation of the volume of the downstream chamber 16 is carried out. For this To do this, we proceed as follows. The volume of the dosing chamber 17 is invariable and is known by a prior measurement, for example carried out by filling this dosing chamber with water, and by weighing this water to determine its volume. After placing the plug(s) in the measuring cell 10, the valves 13 and 15 are closed, the valves 11, 14 and 19 are opened, the vacuum pump 3 is activated and then the valve 14 is closed. This creates a vacuum simultaneously in the downstream chamber 16 and in the metering chamber 17. The downstream containment valve 11 is then closed, the vent valve 15 is opened, which fills the metering chamber 17 with air at atmospheric pressure. The valve 15 is closed and the valve 11 is opened, which allows the air in the metering chamber 17 to be distributed between the downstream chamber 16 and the metering chamber 17. Valve 11 is then closed. Valves 14 and 19 are opened and the metering chamber 17 is re-evacuated. Valve 14 is then closed. It is checked whether the pressure in the downstream chamber 16 is within the range measurable by the pressure sensor 20. If it is not, the gas in the downstream chamber 16 is split: valve 11 is opened to distribute the gas between the two chambers 16 and 17, valve 11 is closed, the metering chamber 17 is re-evacuated, valve 14 is closed, and valve 11 is reopened to redistribute the gas remaining in the downstream chamber 16. Once the pressure in the downstream chamber 16 is within the measurement range of the sensor 20, the value pi of this pressure is noted, the evacuation is recreated in the metering chamber 17, the valve 14 is closed, the valve 11 is reopened and the pressure p2 of the gas distributed between the two chambers 16 and 17 is measured. As the measurement range of the sensor, a pressure interval recommended by the sensor supplier, or an arbitrary pressure value interval in which the pressure sensor 2 gives reproducible values, can be taken. If we note Va the volume of the downstream chamber 16, and V17 the volume of the dosing chamber 17, the ideal gas equation allows us to write that: let Va = ^17 . Pee The electronic control unit can be configured to automatically perform, after the installation of the measuring cells 10 equipped with the plugs to be characterized, the determination of the volume of the downstream chamber 16 of each measuring unit 4, 5 or 6. To do this, the electronic control unit can be programmed to carry out the fractionations of the gas contained in the downstream chamber 16 until the value of the pressure in the chamber 16 is within a range considered to be a reliable measurement range for the pressure sensor 20. According to an alternative implementation, the electronic control unit can be programmed to carry out the fractionations of the gas contained in the downstream chamber 16 a predetermined number of times. Figures 2 to 6 illustrate some examples of measuring cells allowing the plugs to be tested to be interposed between the upstream chamber and the downstream chamber. Figure 2 illustrates a retaining ring 21 gripping a cap 22 to be tested. We hereinafter call the axial direction the direction of the longitudinal axis of the cap. The ring 21 is shown in partial section. The ring 21 grips the cap 22 in a sealed manner over the entire circumference of the cap. The internal profile of the ring reproduces a typical bottle neck profile in order to place the cap in compression conditions representative of its normal use. The ring can be made of metal alloys that are easy to machine, for example an aluminum alloy. Rings made of synthetic material can also be considered, with an internal coating making it possible to guarantee, between the ring and the cap, a coefficient of friction that remains comparable to that of the coefficient of friction of the cap against glass. The ring 21 is sandwiched between a distribution plate 31 visible for example in Figure 4 and a collection plate 29 visible in perspective in Figure 3. The distribution plate 31 and the plate collection plate 29 have generally similar shapes. They are each provided with a centering hollow 27 for housing and radially holding one of the ends of the ring 21. They are crossed by a set of clamping holes 24 for passing screws, spacers or threaded rods connecting the distribution plate 21 to the collection plate 29 to keep the ends of the ring 21 axially clamped each inside a centering hollow 27. In order to ensure the sealing of the cell 10 with respect to the gases passing through the plug 22, an O-ring 28 is provided inside each centering hollow 27 of the distribution plate 31 and the collection plate 29. The collecting plate 29 is provided in its center with a collecting orifice 25 for recovering the gases having passed through the plug 22. The plug 22 being capable of coming to bear against the bottom of the centering hollow 27, a collecting rosette 26 is provided on this centering hollow in the form of a star-shaped relief delimiting both radial branches 23a in relief serving as an axial stop for the downstream face of the plug 22, and delimiting drainage channels 23b for collecting the gas having passed through the plug over a surface greater than that of the collecting orifice 25, in particular when the plug 22 is in abutment against the centering hollow 27. Figure 4 illustrates a particular form of measuring cell 30, intended to measure the permeability of wire-capped corks, of the champagne cork type. The cell 30 comprises a collection plate 29 identical to that described in Figure 3. It also comprises a distribution plate 31 similar to the collection plate 29, but differing from the collection plate 29 in the relief surrounding a central supply orifice 35 through which the gas is brought to the upstream part of the plug (not shown). The collector rosette 26 surrounding the collector orifice is thus replaced by a frustoconical profile 38 which is less expensive to machine. The collector orifice 25 and the supply orifice 35 may each comprise one or more straight drilling zones 36 as well as a shoulder 37 making it possible to limit the axial displacement of a pipe end (shown) bringing the gas towards the cell or of a pipe end (shown) collecting the gas present in the cell downstream of the plug (shown). As in Figure 3, the collection plate 29 and the distribution plate 31 each comprise a centering hollow 27. The diameters of the centering hollows of the two plates are comparable and can be provided to accommodate either a simple ring 21 as described in Figure 2, or a chamber 40 for an effervescent cap as shown in Figure 4. The chamber 40 for an effervescent cap of Figure 4 comprises an outer ring 32 forming a generally cylindrical chamber, the upper end of which is configured to fit tightly into the centering hollow 27 of the distribution plate 31, and the maximum inner diameter of which is widened relative to the diameter of the centering hollow 27, so as to be able to insert therein, through a lower orifice 151, a shouldered ring 33 and a wire cap (not shown) projecting radially from this ring 33.The outer ring 32 has an axial orifice 43 in the upper part. The truncated cone profile 38 of the distribution plate 31 makes it possible to ensure a transition between the diameter of the supply orifice 35 and the diameter of the upper orifice 43 of the outer ring 32. The shoulder ring 33 comprises a portion of dummy neck 41 reproducing the interior and exterior relief of a bottle neck capable of receiving a wire cap, that is to say a bottle neck provided with a shoulder 34 making it possible to retain the wire cap of the cap. The shoulder ring 33 is crossed by an axial conduit 100 connecting an inlet orifice 150 to an outlet orifice 152 of the shoulder ring 33. The shoulder ring 33 has a lower base whose geometry is complementary to that of the centering hollow 27 of the collection plate 29, so that it can be fitted therein in a sealed manner. The shoulder ring 33 is designed so that it can be fitted inside the outer ring 32 once the dummy neck 41 has been fitted with a wire cap, so that the enclosure 40 is sealed against gases other than those arriving through the orifice 43 of the outer ring 32 or leaving through the center of the dummy neck 41. In order to ensure this seal, inter-ring seals 39 can be arranged around the base of the shoulder ring 33. The cell 30 thus defined makes it possible to test the permeability of a stopper fitted with its wire cage, by applying a gas pressure to the part of the stopper outside the dummy neck 41, and by measuring the pressure of the gases reaching the collector orifice 25. The two ends of the cell having identical outside diameters, the cell can be mounted by interchanging the positions of the two ends relative to the distribution plate 31 and relative to the collection plate 29, to measure a permeability from the base of the stopper, corresponding to the permeability of the CO2 escape towards the outside of the bottle. Figure 8 illustrates a particular form of measuring cell 130, intended to measure the permeability of screw caps, in particular metal screw caps. Figure 8 shows elements common to Figure 4, the same elements being designated by the same references. An enclosure 140 for a screw cap comprises a bell-shaped outer ring 132, defining a generally cylindrical chamber. The upper end of the ring 132 is configured to fit tightly into the centering recess 27 of the distribution plate 31. The maximum internal diameter of the ring 132 is wider than the diameter of the centering recess 27, so as to be able to insert into the ring 132, through a lower orifice, a sample holder 133 comprising a portion 141 in the form of a dummy bottle neck.The dummy neck 141 reproduces on its external part a thread 134 of a bottle capable of being closed by a screw cap (not shown). The . thread 134 of the dummy neck is made so as to be able to assemble a screw cap with existing production means intended for real bottles. A support (not shown), for example in the form of a cylindrical block, can be provided, reproducing the shape of the lower part of a bottle. The assembly constituted by the support with the sample holder assembled on it, can thus be compatible, from a dimension point of view, with the aforementioned production means. A screw cap can thus be crimped onto the sample holder 133 under the same conditions as it would be crimped onto a real bottle. The outer ring 132 has an axial orifice 43 in the upper part. The sample holder 133 is pierced with an axial conduit 100 of diameter smaller than the internal diameter of a glass bottle neck, in order to limit the volume of the downstream chamber of which the axial conduit 100 defines a portion.A collection hollow 101 is formed around the upper end - that is to say the end opposite the collection plate - of the axial conduit 100, so that the capsule (not shown) rests on an annular portion 102 of the dummy neck, the surface of the annular portion being of a surface comparable to the bearing surface of the capsule on a glass bottle neck. It is noted that in the embodiment of Figure 4 as in the embodiment of Figure 8, the inner ring 33 or the sample holder 133, once inserted into the outer ring 32, respectively 132, have a substantially axial free surface portion 103, contained inside the ring 33 or the bell 133. It is on this surface, using the shoulder 34 or the thread 134, that a portion of a stopper or capsule outside the neck can be crimped or fixed. . Figure 5 is an exploded view of a multi-ring cell 50 for applying the upstream gas pressure simultaneously to several plugs, in order to obtain an average permeability value on several plugs. Figure 5 shows elements present in Figures 1 to 4, the same elements then being designated by the same references. A set of eight rings 21 is here interposed between a distribution plate 55 and a collection plate 57. The rings 21 are held between the plates 55 and 57 by spacers 59 interposed axially between the two plates, into which spacers are screwed screws 54 for closing the cell 50. Each screw 54 passes through an unthreaded orifice of one of the plates 55 or 57, and is hooked into an internal thread on a spacer 59. The spacers being slightly shorter than the rings, it is thus possible to ensure the clamping of the rings between the two plates. In order to facilitate the separation of the two plates and the installation of the rings 21, guide bars 51 provided with plate separation springs 52 can be welded at one end to one of the plates and slide inside a guide hole in the other plate. The lengths of springs 52 can be provided to be of the same order as the lengths of rings 21, or to be slightly greater. The distribution plate 55 must this time supply several rings, it is made in two parts, defining between them a distribution volume 60 made watertight by a seal 64. The distribution volume 60 connects a central supply orifice 3 5 to which a pipe bringing the gas to the cell can be assembled, and orifices 61 supplying the center of each of the rings 21. The distribution volume 60 is delimited between a distribution plate base 55a, and a distribution plate cover 55b. The distribution plate base 55a carries the upper centering recesses 27 of each of the rings 21, as well as the orifices 53 for positioning the assembly screws 54 for tightening the rings. The distribution plate cover 55b is crossed by the central distribution orifice 35, and is assembled to the distribution plate base 55a by a group of screws 68. Similarly, the collection plate 57 includes a collection plate base 57a and a collection plate cover 57b, connected by a set of cap screws 67, and defining therebetween a collection volume 77. The collection volume 77 connects collector orifices 25 of each ring 21, drilled in the collection plate base 57a, and two outlet orifices 78 and 79, drilled in the collection plate cover 57b. On the outlet orifices 78 and 79, can be welded respectively, a first connector allowing to connect a pipe going to the vacuum pump 3, and a second connector allowing to connect a pressure sensor 20 (not shown in FIG. 5) capable of measuring the pressure prevailing inside the collector volume 77. It is possible to envisage variant embodiments where the collection plate cover has only one outlet orifice, the pressure sensor 20 then being connected downstream of this outlet orifice, on the pipe going to the vacuum pump 3. Figure 6 illustrates the face of the plate 57a which is masked in Figure 5. It can be seen that the shape of the collection plate base 57a, seen from the side of the collection volume 77, as in Figure 6, is similar to the upper part of the distribution plate base 55a which can be seen in Figure 5. The collection orifices 25 are in particular surrounded by a seal 64, which is substantially star-shaped, in order to be able to bypass the collection orifices 25 from the outside of the plate, and to be able to bypass a group of threaded bores 67a from the center of the plate allowing the assembly screws 67 to be tightened. The collecting volume 77 is however smaller, in volume, than the distribution volume 61, since the collecting volume 77 is limited to a group 63 of star-shaped channels connecting the eight collecting orifices 25 and the two orifices 78 and 79. By comparison, the distribution volume 61 is larger because it includes a central cylindrical portion whose diameter is limited by the internal diameter of the seal 64. The distribution volume 60 is thus less expensive to machine. The collector volume 77, a little more expensive to produce, on the other hand makes it possible to limit the total volume of the downstream chamber 16 of which it is a part, and to improve the precision of the pressure measurements in this downstream chamber. Indeed, for a given volume of gas passing through the plugs, the increase in pressure generated in the downstream chamber 16 is more noticeable if the volume of this chamber is reduced. Figure 7 illustrates another model of measuring cell 70, this time intended to measure the permeability of capsule-type seals. As in the previous embodiment, the cell 70 comprises a distribution plate 31 and a collection plate 29, here connected axially by screws 81 allowing the spacing to be adjusted. Similar to the embodiment of Figure 4, an enclosure 80 is defined by the distributor plate 31, the plate collection 29, and two rings 71 and 72, including an upper ring 71 inserted inside a centering hollow 27 of the plate of distributor provided with an O-ring 28, and a lower ring 72 pressed inside a centering hollow 27 of the plate of collection 29. The lower ring 72 reproduces the geometry of the end of a bottle neck, intended to receive closing capsules covering the end of the neck. It is configured so as to be able to be inserted inside the upper ring while ensuring sealing between the two rings using an inter-ring seal 39. A gap 73 is provided between the lower ring 72 and the upper ring 71, in which it is possible to house a capsule seal 74, so as to reserve an empty peripheral space around the seal 74. The capsule seal is then held on its periphery by pinching between the two rings 71 and 72. A porous support 88 is arranged inside the dummy neck of the lower ring 72 so that a capsule seal 74 pinched between the rings 71 and 72 is not sucked inside the neck when a vacuum is created downstream of this seal 74. The tightening pressure of the cell screws is chosen so that the seal is pinched with a crimping pressure comparable to the crimping pressure of a capsule. It should be noted that a porous insert can also be placed inside the ring under the cap in the assembly described in figures 2 or 4, in order to reduce the effective volume of the downstream chamber. The collection plate 29 has a collector orifice 25 arranged below the porous support 88. The distribution plate 31 is crossed by a central supply orifice 35a making it possible to bring pressurized gas above the central part of the seal 74, that is to say on the surface covering the porous support 88. The distribution plate 31 is also crossed by a peripheral supply orifice 35b, which continues as a lateral channel 82 passing through the upper ring 71 and joining the periphery of the gap 73. Pressurized gas injected into the peripheral supply orifice 35b is thus capable of passing through the seal 74 towards the collector orifice 25, either by diffusing at the interface between the seal 74 and the lower ring 72, or by diffusing inside the material of the seal 74 from the periphery of this seal. This capsule seal measuring cell 70 makes it possible to carry out permeability measurements according to the principle described above if the gas pressure is applied at the central supply orifice 35a. This measuring cell 70 also makes it possible to carry out comparative tests of the permeability of different joints 74, these tests taking into account the effects of interface diffusion and / or diffusion through the edge of the joint. The mechanism of diffusion through the edge of the joint may in fact be different from that of diffusion in the direction of the thickness of the joint. The subject of the invention is not limited to the embodiments described, and can be broken down into numerous variants. The number of measuring units 10 for each measuring bench 1 can vary, as can the number of rings that can be mounted on a given measuring cell 10. It is possible to provide for mounting several cells in parallel for wire-cage caps, or to mount several cells in parallel for capsule seals. It is possible to connect several measuring units to a single pressure regulation unit, as in Figure 1, or to regulate the upstream pressure of each measuring unit by a dedicated pressure regulation unit. The electronic control unit can be connected to all the valves described, which are then piloted valves, i.e. electronically piloted, or some of the valves can be reserved for manual piloting, for example the isolation valves 9 and 19, and / or the bypass valve 42. It is also possible to manually pilot all the valves and have no piloted valves. The valves 11, 12, 13, 14 may be proportionally actuable valves, or may be all or some of them on / off valves. In the case where the supply valve 12 is an on / off valve, the regulation of the upstream pressure is ensured solely by the pressure regulation unit 87. A pressure drop valve 44, for example of the needle valve type, is then advantageously interposed between the valve 12 and the measuring cell 10, in order to create a pressure drop and to avoid too sudden an application of the upstream pressure after having created the vacuum upstream and downstream of the plug, which could for example cause a displacement of the plug. In the example described, the measuring cells 10 are supplied with gas by an upper plate, and the suction to the vacuum pump 3 is from a lower plate, but it is understood that the gas supply could be via the lower plate and the suction via the upper plate. The number of valves equipping the measuring bench 1 or the number of valves in a measuring unit may vary from the example described in Figure 1. For example, there could be no upstream isolation valve 9 or no downstream isolation valve 19. The presence of these isolation valves allows selective maintenance interventions on one of the measuring units, while continuing to use the other measuring units in the device. At the level of the connections of the pipes bringing, or collecting, the gases from the cell 10, it is possible, for example, to provide that the gas pipe arriving at the measuring cell 10 is screwed, which is an inexpensive assembly method, while the connector making it possible to connect the same measuring cell to the pipe forming the downstream of the downstream chamber 16 is instead welded. In this way, leaks of gas arriving downstream of the plug are limited, and the measurement accuracy is increased. Other choices of assembly methods for the pipes arriving and leaving the cell 10 are however possible. It is possible to choose to use only screwed connections or only welded connections. In order to determine the temperature T of the gas present in the downstream chamber 16 and in the metering chamber 17, a temperature enabling the equations described above to be applied, it is possible to work in an air-conditioned room, with an air intake 18 also taken from this air-conditioned room. This solution is of course only applicable if the gases handled are non-toxic. The temperature measured by a sensor in the room can then be used as the value T to be reported in the equations cited above. The temperature T can preferably be measured at the level of the frame supporting the measuring cells 10. It is also possible to provide insulation for a part of the measuring bench comprising the measuring cells 10, the downstream chambers 16, and the dosing chambers 17. It is then possible, for example, to use a temperature sensor taking the temperature inside this reduced enclosure, or an individual temperature sensor taking the temperature inside the downstream chamber 16 associated with each measuring cell. In order for the quantity of gas to be detected to generate a pressure detectable by the sensor 20, an attempt is made to reduce the volume of the downstream chamber 16 as much as possible. Typically for a cell with a single plug, the volume between the downstream of the plug and the valve 11 could thus be of the order of a few cm3. In the case of a cell capable of accommodating several plugs, the overall volume of the downstream chamber 16 will be greater, but we still try to maintain a volume which, once divided by the number of plugs in the cell, remains in this range of a few cm3 per plug. In order for the gas fractionation operations between the downstream chamber 16 and the metering chamber 17 to give usable pressure ratios, it is desirable that the volumes of the two chambers 16 and 17 remain of the same order of magnitude, and in any case do not differ by more than one order of magnitude. For example, dosing chambers with a volume of the order of 2 to 6 cm3, for example between 3 cm3 and 4.5 cm3, may be used. It is advisable to choose ratios between the volume of the downstream chamber 16 and the volume of the dosing chamber 17, such that the volume of the dosing chamber 17 is between 1 / 10 of the volume of the downstream chamber 16 and five times the volume of the downstream chamber 16. The assessment of the volume of the downstream chamber will be further simplified if the two volumes are close to each other, for example if the volume of the dosing chamber is between % and four times the volume of the downstream chamber. The upstream pressure can be slightly higher than atmospheric pressure (1.2 atm for example), to assess the permeability of the plugs to oxygen. The upstream pressure can be even higher to speed up the measurement process, for example, around three atmospheres to measure the permeability to oxygen. This deviates from the actual operating conditions of the plug, as the comparison of permeabilities between different materials remains valid under equal test conditions. The upstream pressure can be significantly higher than atmospheric pressure, for example, around 7 atm, to measure the permeability to gases usually contained under pressure by the plugs, for example CO2. The measuring device according to the invention makes it possible to reliably and reproducibly assess the permeability of thick material samples, such as wine or champagne corks. The measurement can be carried out in a limited time compared to conventional measuring methods measuring diffusion by coulometry. The construction of the measuring device according to the invention requires only standard pipe and valve elements for pressurized gases, as well as sensors widely available on the market such as than pressure sensors or Pirani type vacuum sensors, based on a thermal conductance measurement. The pressure sensor 20 connected to the downstream chamber 16, in particular if it is of the Pirani gauge type, may have a finer resolution than the mbar, for example a measurement accuracy of at least 20% over a measurement range extending between 10*4 mbar and 1 mbar. Slightly less precise sensors, such as capacitive sensors, can also be used. This avoids the need for expensive devices such as coulometric oxygen diffusion measuring devices.

Claims

CLAIMS 1. Device (1) for measuring the gas permeability of a bottle or jar stopper (22), comprising - a measuring cell (10) in which the plug can be mounted so as to delimit an upstream chamber (76) to the plug and a downstream chamber (16) to the plug, gas exchanges being possible between the upstream chamber (76) and the downstream chamber (16) only by passing through the plug, - a gas supply (2) making it possible to impose in the upstream chamber (76) an upstream pressure of a first gas, of first predefined pressure value, - a downstream pumping device (3) making it possible to impose in the downstream chamber (16) an initial depression of a second predefined value, lower than the first value, then to isolate the downstream chamber (16) from gas exchanges other than those passing through the plug (22), - a sensor (20) for taking pressure measurements in the downstream chamber (16), characterized in that the downstream chamber (16) is configured so that its volume remains constant when its internal pressure varies between the applied pressure and the initial depression, and in that the downstream chamber (16) is connected by a first valve (11) with a metering chamber (17) also of constant volume, capable, when the first valve (11) is closed, of being successively connected to, and isolated from, the atmosphere (18) or a gas supply (2).

2. Measuring device according to claim 1, comprising a second valve (12) making it possible to isolate the upstream chamber (76) from the gas supply (2), and comprising a bypass pipe (42) closable by a third valve (13), between the upstream chamber (76) and the metering chamber (17).

3. Measuring device according to the preceding claim, in which the dosing chamber (17) is connected to the pumping device (3) by a pipe provided with a fourth valve (14) separating it from the dosing chamber (17).

4. Measuring device according to the preceding claim, in which the dosing chamber (17) is provided with a fifth valve (15) allowing it to communicate with the atmosphere (18).

5. Measuring device according to one of the preceding claims, comprising an electronic control unit configured to record a series of values ​​delivered by the pressure sensor (20), over a time interval subsequent to a step of supplying gas to the upstream chamber (76), and / or a step of initially applying a vacuum to the downstream chamber (16).

6. Measuring device according to one of the preceding claims, in which the volume of the downstream chamber is less than 10cm3, and preferably less than 5cm3.

7. Measuring device according to one of the preceding claims, in which the volume of the dosing chamber (17) is between 0.5 and 10 times the volume of the downstream chamber (16).

8. Measuring device according to one of the preceding claims, in which the measuring cell (10) is configured to interpose between the upstream chamber (76) and the downstream chamber (16) one or more plugs (22), each arranged inside a generally cylindrical container (21, 32) sandwiched between a gas distribution plate (30, 55) forming part of the enclosure of the upstream chamber (76), and a gas collection plate (29, 57) forming part of the enclosure of the downstream chamber (16).

9. Measuring device according to claim 8, the measuring cell (10) comprising a dummy bottle neck (41, 141) whose base is connected in a sealed manner to the collection plate (29, 57), and an outer ring (32, 132) capable of being assembled over the dummy neck in a sealed manner with the dummy neck (41, 141), and capable of being assembled in a sealed manner with the distribution plate (31, 55), so that the upstream chamber (76) communicates with at least one part outside the dummy neck of a stopper or capsule assembled in or on the dummy neck.

10. Measuring device according to claim 8, the measuring cell (10) comprising a dummy bottle neck (72) whose base is connected in a sealed manner to the collection plate (29, 57), and an outer ring (71) capable of being assembled in a sealed manner with the dummy neck (72) and with the distribution plate (31, 55), by pinching between the outer ring and between the dummy neck a plate or film of seal material(s) (74) arranged over the dummy bottle neck (72), and the distribution plate (31) comprising a first orifice (35a) allowing the upstream chamber to be placed in communication only with a central part of the plate or film (74), and a second orifice (35b) allowing the upstream chamber to be placed in communication only with a peripheral free edge of the plate or film (74).

11. Measuring device according to one of the preceding claims, in which a single pressure sensor (20) is connected to the downstream chamber (16), this sensor (20) having a measurement accuracy of at least 1 mbar over a measurement range extending between zero and 10 mbar. 12 Measuring device according to one of the preceding claims, in which a single pressure sensor (20) is connected to the downstream chamber (16), this sensor (20) having a measurement accuracy of at least 20% over a measurement range extending between 10'4 mbar and 1 mbar.

13. Method for measuring the gas permeability of a bottle stopper, comprising the steps of: -arrange the plug (22) in a measuring cell (10) so as to delimit a chamber upstream of the plug (76) and a chamber downstream of the plug (16), gas exchanges being possible between the upstream chamber (76) and the downstream chamber (16) through the plug, -evaluate the volume of the downstream chamber (16) by comparing its volume with that of a dosing chamber (17), -apply a relative vacuum simultaneously in the upstream chamber (76) and in the downstream chamber (16) for a predefined duration, - create a vacuum in the downstream chamber (16), then isolate the downstream chamber from gas exchanges other than those passing through the plug, -apply an upstream set pressure of gas in the upstream chamber (76), -record several pressure measurements taken in the downstream chamber (16) after having isolated the downstream chamber (16) from gas exchanges other than those passing through the plug (22).

14. Measuring method according to the preceding claim, further comprising a step of calculating a slope proportional to a variation with respect to time of the logarithm of the difference between the upstream set pressure and the pressure measurement in the downstream chamber (16).

15. Measuring method according to claims 13 or 14, further comprising a step of evaluating the volume of the downstream chamber (16), during which the metering chamber (17) is emptied, then a quantity of gas initially contained in the downstream chamber (16) is distributed between the downstream chamber (16) and the metering chamber (17), the last two operations are repeated until the pressure in the downstream chamber (16) is measurable by a sensor (20) connected to the downstream chamber (16), then the downstream chamber (16) is isolated, the metering chamber (17) is further emptied and the quantity of gas contained in the downstream chamber (16) is further distributed between the two chambers, in order to measure the pressure of the gas distributed in the two chambers (16, 17).

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