WO2010046703A1

WO2010046703A1 - A pressurised container

Info

Publication number: WO2010046703A1
Application number: PCT/GB2009/051420
Authority: WO
Inventors: Mats Bo Ivar Johanson; Ronnie Kjell Anderson
Original assignee: Autoliv Development Ab; Beattie, Alex, T., S.
Priority date: 2008-10-23
Filing date: 2009-10-21
Publication date: 2010-04-29
Also published as: DE112009002424B4; DE112009002424T5

Abstract

A pressurised container comprising an enclosed main interior volume and having a first venting aperture formed therein allowing communication between the main interior volume and the exterior of the container, the container further comprising: a first element which initially seals the first venting aperture; an enclosure formed within the main interior volume which substantially surrounds the first venting aperture, and encloses a first volume around the first venting aperture; a second venting aperture formed through the enclosure; and a second element which initially seals the second venting aperture, wherein the flow path from the main interior volume to the first venting aperture is initially impeded by the second element, so that rupturing, distortion or movement of the second element increases the cross-section of the available flow path from the main interior volume to the first venting aperture.

Description

"A Pressurised Container"

Description of Invention

THIS INVENTION relates to a pressurised container, and in particular concerns a container which may be used for providing pressurised gas to inflate an air-bag.

It has been known for many years to provide air-bags in vehicles to protect occupants of the vehicles in crash situations. For instance, if vehicle sensors detect that a vehicle is involved in, or is likely to be come involved in, a head- on collision, an air-bag may inflate from the dashboard immediately in front of the driver of the vehicle. As the driver is thrown forwardly relative to the vehicle during the impact, the relative motion of the driver compared to the vehicle is slowed by the air-bag, and in the majority of instances injuries to the driver are reduced by the presence of the air-bag.

It will be appreciated that air-bags of this type need to be inflated very quickly, however, and so gas typically needs to be introduced into the interior of an air-bag at high pressure.

It has been proposed to provide air-bag inflators which incorporate one or two bottles or pressure vessels containing gas under pressure. Each bottle may be sealed by a reputable metal foil. The metal foil is initially supported by means of an element which is held in position against the exterior of the foil. When the air-bag is to be inflated, the support member is moved away from the metal foil, enabling the foil to rupture and thus enabling the gas to escape from the gas bottle and flow into the interior of the air-bag to inflate the air-bag. In inflators of this general type one gas bottle may contain a fuel, in the form of an oxidisable gas, and the other bottle may contain an oxidising gas. These gases are mixed when they escape from the respective gas bottles. Subsequently, when the gases are within the air-bag, the gases may be ignited to complete inflation of the air-bag.

An inflator of this type is disclosed in GB 2,417,066. It has been found, however, that the amount of gas delivered by the inflator in the first few milliseconds after the rupturing of the foils can affect the inflation pressure of the air-bag significantly. In many cases, it is desired to reduce the air-bag inflation pressure, by reducing the inflator output during the first few milliseconds after the foils have been ruptured.

The present invention seeks to address the above concerns. Accordingly, one aspect of the present invention provides a pressurised container comprising an enclosed main interior volume and having a first venting aperture formed therein allowing communication between the main interior volume and the exterior of the container, the container further comprising: a first element which initially seals the first venting aperture; an enclosure formed within the main interior volume which substantially surrounds the first venting aperture, and encloses a first volume around the first venting aperture; a second venting aperture formed through the enclosure; and a second element which initially seals the second venting aperture, wherein the flow path from the main interior volume to the first venting aperture is initially impeded by the second element, so that rupturing, distortion or movement of the second element increases the cross-section of the available flow path from the main interior volume to the first venting aperture. Advantageously, a communication passage is formed in the enclosure, allowing communication between the main interior volume and the first volume.

Preferably, a hole is formed through the second element.

Another aspect of the present invention provides an inflator for an air-bag, the inflator comprising a pressurised container according to the above.

Conveniently, the first element is a rupturable element, and a support is initially provided to support the first element, the support being removed upon activation of the inflator to allow the first element to rupture.

Advantageously, the first element is a rupturable element, and a rupturing arrangement is provided to rupture the first element upon activation of the inflator.

Preferably, a rupturing arrangement is provided to rupture the second element in response to a secondary triggering signal.

Conveniently, the rupturing arrangement comprises a rupturing member which, in response to the secondary triggering signal, contacts and ruptures the second element.

Advantageously, the rupturing member comprises part of a solenoid, and the rupturing member is driven towards the second element by an electric current being passed through a coil surrounding the rupturing member.

Conveniently, a pyrotechnic charge is provided to drive the rupturing element towards the second element. Advantageously, the second element is a blocking member which initially blocks the second venting aperture, and may be moved or distorted to allow the flow of gas through the second aperture.

Preferably, the blocking member comprises part of a solenoid, and the blocking member is withdrawn from the second venting aperture by an electric current being passed through a coil surrounding the blocking member.

Conveniently, the second element is configured to distort in response to a secondary triggering signal, thereby allowing gas to flow through the second venting aperture.

Advantageously, the second element comprises one or more piezoelectric elements to cause the second member to distort in response to a flow of current through the piezoelectric elements.

Preferably, the second element comprises one or more magnetic elements which are configured to be attracted to, or repelled from one or more electromagnets which are activated by the secondary triggering signal.

Conveniently, the inflator comprises two pressurised containers according to the above.

A further aspect of the present invention provides an air-bag module comprising an inflator according to the above.

Another aspect of the present invention provides a method of constructing a pressurised container, comprising the steps of: providing a container enclosing a main interior volume and having a first venting aperture formed therein allowing communication between the main interior volume and the exterior of the container: sealing the first venting aperture with a first element; providing an enclosure formed within the main interior volume which substantially surrounds the first venting aperture, and encloses a first volume around the first venting aperture; forming a second venting aperture formed through the enclosure; and sealing the second venting aperture with a second element, the arrangement being such that the flow path from the main interior volume to the first venting aperture is initially impeded by the second element, so that rupturing, distortion or movement of the second element increases the cross-section of the available flow path from the main interior volume to the first venting aperture.

Advantageously, the method further comprises the step of filling the main interior volume with a pressurised gas.

Preferably, a pressure difference threshold at which the second element ruptures is selected so that, following rupturing of the first rupturable element, a delay will occur before the rupturing of the second element.

Conveniently, the delay is at least 5 milliseconds.

Advantageously, the volume of the first volume is selected to control the delay, following rupturing of the first rupturable element, after which the second element will rupture.

Preferably, the method further comprises the step of providing a rupturing arrangement to rupture the second element in response to a secondary triggering signal. Conveniently, the step of sealing the second venting aperture with a second element comprises the step of sealing the second venting aperture with a second element that is configured to distort or move in response to a secondary triggering signal, thus allowing gas to flow through the second venting aperture.

Advantageously, the method further comprises the step of forming a communication passage in the enclosure, allowing communication between the main interior volume and the first volume.

Preferably, the cross-sectional area of the communication passage is selected to control the delay, following rupturing of the first rupturable element, after which the second element will rupture.

In order that the present invention may be more read ily understood, embodiments thereof will now be described, by way of example, with reference to the accompany drawings, in which:

Figures 1 and 2 show elements of a pressurised container into a first embodiment of the present invention;

Figure 3 is a schematic view of components of a pressurised container according to a second embodiment of the present invention; and

Figures 4, 5 and 6 are schematic views of pressurised containers according to further embodiments of the invention.

Referring firstly to figures 1 and 2, cut-away views are shown of an end portion of a pressurised container 1 embodying the present invention. The container 1 is generally cylindrical in shape, and comprises a main outer wall 2, which is formed from a sturdy material such as steel. In figures 1 and 2, only an end portion of the container 1 is shown. However, it should be understood that the outer wall 2 defines a substantially enclosed main interior volume 3.

A first venting aperture 4 is formed in an end surface 5 of the container 1. The first venting aperture 4 is covered by a first rupturable element, in the form of a metal foil 6. In the embodiment shown the metal foil 6 is configured to rupture when the pressure d ifference across the foi l 6 exceeds a predetermined level.

Surrounding the first venting aperture 4 is an enclosure 7, which is preferably generally dish-shaped and substantially circular in shape. The enclosure 7 is attached to an inner surface of the portion 5 of the main wall 2, so that the enclosure 7 entirely surrounds the first venting hole 4. The enclosure 7 protrudes inwardly into the main interior volume 3 of the container 1 , and therefore encloses a first volume 8 around the first venting aperture 4.

A second venting aperture 9 is formed in the enclosure 7. Preferably, the second venting aperture 9 is formed in a central region of the enclosure 7, and is therefore substantially directly opposite, and in line with, the first venting aperture 4.

The second venting aperture 9 is occluded by a second element, in the form of a second rupturable foil 11 , which extends across an inner surface 11 of the enclosure 7. The second foil 11 is also configured to rupture when the pressure difference across the foil 11 exceeds a predetermined value. This value may be the same as the value at which the first foil 6 ruptures, or indeed may be different. A communication passage 12 is also formed through the enclosure 7, the communication passage 12 having a cross-sectional area which is smaller than that of the second venting aperture 9. The communication passage 12 is preferably formed at a side portion of the enclosure 7. In preferred embodiments of the invention, the communication passage 12 is not occluded, and allows gas to flow between the first volume 8 enclosed by the enclosure 7 and the main interior volume 3 of the container 1.

Prior to activation of the air-bag associated with the container 1 , a support (not shown) is provided against the exterior side of the first foil 6. The first foil 6 is therefore prevented from rupturing.

When the air-bag is activated, the support is removed. The difference in pressure between the gas within the first volume 8, which is highly pressurised, and the gas immediately outside the first foil 6 will be greater than the threshold required to rupture this foil 6. The first foil 6 therefore ruptures, allowing gas to flow from the first volume 8 out of the container 1 through the first venting aperture 4.

In alternative embodiments, the first foil is sufficiently robust not to rupture as a result of the difference in pressure between the gas within the container and the surrounding ambient air. In such embodiments a support may be dispensed with, and the first foil can be "actively" ruptured, for instance by a needle or other sharp element, as will be understood by those skilled in the art.

As gas escapes from the first volume 8, the pressure within the first volume 8 will fall. However, for at least the first few milliseconds, the pressure difference between the first volume 8 and the main interior volume 3 will not be great enough to rupture the second foil 1 1 which occludes the second venting aperture 9.

During this first phase of flow, gas may also flow from the main interior volume 3 through the communication passage 12 into the first volume 8, and escape from the container 1 through the first venting aperture 4.

After a certain quantity of gas has escaped from the first volume 8, the pressure in the first volume 8 will drop to a level at which the pressure difference between the first volume 8 and the main interior volume 3 of the container 1 is sufficient to rupture the second foil 11. The second foil 11 then ruptures, allowing gas to flow from the main interior volume 3 directly through the second venting hole 9, and out of the container 1 through the first venting aperture 4. The main "normal" flow of gas from the container 1 is therefore established.

It will be appreciated that, when the first foil 6 is ruptured, the flow of gas from the container 1 will, for a first period, be relatively gentle. Following the rupturing of the second foil 11 , however, the rate of flow will increase. As discussed above, it is often desirable for the rate of flow during the first few milliseconds following activation to be less aggressive than the normal flow of gas from a pressurised container of this type, and it will be understood how the features of the above embodiment allow this to be achieved without requiring separate triggering signals to open first and second venting apertures in the container.

The length of time for wh ich the reduced flow wil l continue may be determined by the size of the first volume 8, the threshold pressure at which the second foil 11 ruptures, and also the diameter of the communication passage 12. The larger the first volume 8 is formed to be, the longer the period of reduced gas flow will continue.

In addition, the higher the rupturing threshold of the second foil 11 , the longer the period of reduced flow will continue.

Final ly, the g reater the cross-sectional area of the communication passage 12, the longer the period of reduced gas flow will continue.

In preferred embodiments, the length of time between the first foil 6 rupturing and the second foil 11 rupturing is at least 5 milliseconds.

Referring to figure 3, a schematic view is shown of components of a second embodiment of the present invention. The portion of the end wall 5 of the container 1 is shown, and once again the container 1 comprises an outer wall 2, which substantially encloses a main interior volume 3. A first venting aperture 4 is formed in the end surface 5 of the main wall 2, with this first venting aperture 4 initially being covered by a first rupturable foil 6.

In common with the first embodiment, an enclosure 13 surrounds the first venting aperture 4, and extends into the interior of the container 1 . The enclosure 13 again comprises a generally dish-shaped portion 14 which is sealed or otherwise attached to the inner surface of the end portion 5 of the wall 2 of the container 1 , and surrounds the first venting aperture 4. A communication passage 15 is formed through a side region of the dish- shaped portion 14, allowing the flow of gas between the main interior volume 3 and the interior of the dish-shaped portion 14, a flow aperture 16 is also formed through the dish-shaped portion 14 of the enclosure 13, and is preferably generally opposite the first venting aperture 4. In this embodiment, the flow aperture 16 is not covered by a burst disc or other rupturable element.

The enclosure 13 also comprises a main body 17, which is attached to the dish-shaped 14 (and may be formed integrally therewith). The main body 17 is substantially of cylindrical form, having a surface of the dish-shaped portion 14 as one end face thereof, and extends away from the dish-shaped portion 14 into the interior of the container 1. The end 18 of the main body 17 which is furthest from the dish-shaped 14 has a second venting aperture 19 formed therethrough, the second venting aperture 19 initially being sealed by a second burst disc in the form of a second rupturable foil 20.

It will be understood that the volume contained within the dish-shaped portion 14 and main body 17 of the enclosure 13 together comprise a first volume 21.

When the air-bag associated with the container 1 is activated, the first foil 6 is ruptured, as described above. Gas will then flow from the first volume 21 out of the enclosure 13, and out of the container 1 through the first venting aperture 4. As this occurs, pressure within the main body 17 will drop, but will not, at least for a short time, reach a point where the pressure difference across the second foil 20 is sufficient to cause the second foil 20 to rupture.

After a sufficient amount of gas has escaped from the enclosure 13, the pressure difference across the second foil 20 will be sufficient to rupture the foil 20, and after this has occurred gas may flow from the main interior volume 3 of the container 1 through the second venting aperture 19, through the flow aperture 16, and out of the container through the first venting aperture 4. Once again, it will be understood that the length of time for which the reduced gas flow will continue may be varied by controlling the size of the first volume 21 , the rupturing threshold of the second foil 20, and the diameter of the communication passage 15. The ways in which altering these parameters will affect the duration of the reduced flow is discussed above.

In either of the above-described embodiments of the invention, a hole may be formed through the second foil 11 , 20. The effect of this would be to prolong the period of reduced gas flow, as gas will be able to flow from the main interior volume 3 into the first volume 8, 21 prior to rupturing of the second foil 1 1 , 20, and thus the difference in pressure between the main interior volume 3 and the first volume 8, 21 will increase at a lesser rate.

Forming a hole through the second foil 11 , 20 may be an alternative to providing the communication passage 12, 15.

It should also be noted that, whether or not a hole is formed through the second foil 11 , 20, the enclosure 7,13 may be formed without a commun ication passage 12, 1 5. Dispensing with a commun ication passage 12,15 may, however, reduce the duration of the reduced gas flow to an undesirably short length. The communication passage 12,15 also proves convenient in filling the container 1 with pressurised gas when initially preparing the inflator.

Referring to figure 4, a schematic view is shown of a further embodiment of the invention. The further embodiment shares many of the features of the first embodiment. However, in this embodiment the second venting aperture 9 is not blocked by a second rupturable element 22. A blocking member 23 is instead provided to block the second venting aperture 9. The blocking member 23 comprises an elongate body, which in the depicted embodiment is generally cylindrical, and which is positioned within the main volume 3, generally aligned with the second venting aperture 9. The blocking member 23 has an engagement end 24 which is rounded, or otherwise shaped to be received snugly in the second venting aperture 9. In embodiments the engagement end may have a coating of rubber or another deformable substance.

The blocking member 23 is held in place within the main volume 3 by a support sleeve 25, which receives the blocking member 23 in a relatively close fit and allows the blocking member 23 to slide in a direction substantially directly towards, and away from, the second venting aperture 9. The support sleeve 25 is held in place by one or more radial spokes 26 which are attached to the outer wall 2. The support sleeve 25 may, alternatively, be attached to any other suitable supporting component, for instance the enclosure 7.

In its initial position, the engagement end 24 of the blocking member is received in the second venting aperture 9, and seals the second venting aperture 9 in a substantially gas-tight manner.

A protruding lip 27 is provided near the pointed end 24 of the blocking member 23. A spring 28 is fixed between the lip 27 and portions of the support sleeve 25 that surround the second venting aperture 9. The spring 28 is under compression, and therefore biases the blocking member 23 towards the enclosure 7.

In the depicted embodiment the blocking member 23 forms part of a solenoid, and has an electrically conductive coil 29 wrapped therearound. It will therefore be understood that the blocking member 23, or a part thereof, is formed from a ferrous material. When an electric current is passed through the coil 29, the blocking member 23 is driven away from the enclosure 7, against the biasing forces provided by the spring 28, so that the engagement end 24 of the blocking element 23 is pulled away from the enclosure 7. It will be understood that this therefore allows pressurised gas in the main volume 3 to flow through the second venting aperture 9, thus establishing the main flow of gas.

It will be understood that this embodiment allows greater control over the flow of gas from the inflator, as the opening of the second venting aperture 9 is reversible, i.e. the second venting aperture 9 can be blocked and unblocked repeatedly, if required. This embodiment also does not rely on gas pressure to unblock the second venting aperture, thereby allowing greater control over the timing of the first and second phases of gas flow.

Turning to figure 5, an alternative embodiment is shown. In this embodiment the second venting aperture 9 is sealed by a second rupturable element 22, and a rupturing member 32 having a sharp engagement end 31 is provided, again forming part of a solenoid arrangement. In the depicted embodiment the solenoid arrangement is operable to drive the rupturing member 32 towards the second venting aperture 9 to rupture the second rupturable element 22. If this technique is used, electric current may flow through the coil 29 of the solenoid for only a relatively brief period of time, so that the rupturing member 32 ruptures the second rupturable element 22, but is then returned to its original position by the biasing force of the spring 28 (which in this embodiment is positioned between the lip 27 and the enclosure 7 and biases the rupturing element 32 away from the second venting aperture 9), and does not impede the subsequent flow of gas through the second venting aperture 9. Other drive arrangements, to drive the rupturing member 32 towards the second rupturable element 22 in response to a secondary triggering signal, are also envisaged. For instance, a pyrotechnic charge may be provided to propel the rupturing member 32.

Turning to figure 6 a further embodiment of the invention is shown. In this embodiment many of the components are similar to those described in relation to the first embodiment. However, the second venting aperture 9 is initially blocked by a second element in the form of a flexible membrane 30, which is, in the depicted embodiment, positioned on the surface of the enclosure 7 that faces into the main volume 3 of the inflator 1. The membrane 30 is attached to the surface of the enclosure 7, preferably only to one side of the second venting aperture 9.

The membrane 30 is sufficiently robust that, during activation of the inflator 1 , the membrane 30 will not be ruptured by forces arising from differences in gas pressure. However, the membrane 30 may be caused to flex and change its shape, in order to open the second venting aperture 9. In preferred embodiments the membrane 30 takes the form of a strip, which may distort so that a central section of the strip lifts away from the second venting aperture 9, thus allowing gas to flow around the sides of the strip and through the second venting aperture 9.

One way in which the membrane 30 may be caused to flex is through the action of one or more piezoelectric crystals, which are connected to sources of current. In one embodiment one or more piezoelectric crystals (not shown) are provided in, or as part of, the membrane 30. When an electric current is passed through the crystals this causes the membrane 30 to distort and flex so that a central region thereof lifts away from the second venting aperture 9. A skilled person will readily appreciate how this may be achieved.

In alternative embodiments, the membrane 30 may be caused to flex through the action of one or more magnets, preferably electromagnets. For instance, a central region of the membrane 30 may be provided with, or made from, a ferromagnetic substance. One or more electromagnets (not shown), which may be positioned, for example, in or near the enclosure 7, may be activated to repel the ferromagnetic material, thus lifting the central region of the membrane 30 away from the second venting aperture 9, and allowing gas to flow from the main interior volume 3 through the second venting aperture 9.

The skilled person will readily appreciate how these two techniques may be applied to the invention. The embodiments shown in figures 4, 5 and 6 allow a secondary triggering signal to be generated which opens the second venting aperture to begin the second phase of gas flow, allowing the duration of the first phase of gas flow to be closely controlled, and the advantages of this will be clear.

It will be appreciated that the present invention provides a simple and robust manner in which the flow of gas from a container 1 may be reduced for an initial period.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS:

1. A pressurised container comprising an enclosed main interior volume and having a first venting aperture formed therein allowing communication between the main interior volume and the exterior of the container, the container further comprising: a first element which initially seals the first venting aperture; an enclosure formed within the main interior volume which substantially surrounds the first venting aperture, and encloses a first volume around the first venting aperture; a second venting aperture formed through the enclosure; and a second element which initially seals the second venting aperture, wherein the flow path from the main interior volume to the first venting aperture is initially impeded by the second element, so that rupturing, distortion or movement of the second element increases the cross-section of the available flow path from the main interior volume to the first venting aperture.

2. A pressurised container according to claim 1 , wherein a communication passage is formed in the enclosure, allowing communication between the main interior volume and the first volume.

3. A pressurised container according to claim 1 or 2, wherein a hole is formed through the second element.

4. An inflator for an air-bag, the inflator comprising a pressurised container according to any preceding claim.

5. An inflator according to claim 4, wherein the first element is a rupturable element, and a support is initially provided to support the first element, the support being removed upon activation of the inflator to allow the first element to rupture.

6. An inflator according to claim 4, wherein the first element is a rupturable element, and a rupturing arrangement is provided to rupture the first element upon activation of the inflator.

7. An inflator according to any preceding claim wherein a rupturing arrangement is provided to rupture the second element in response to a secondary triggering signal.

8. An inflator according to claim 7, wherein the rupturing arrangement comprises a rupturing member which, in response to the secondary triggering signal, contacts and ruptures the second element.

9. An inflator according to claim 8, wherein the rupturing member comprises part of a solenoid, and the rupturing member is driven towards the second element by an electric current being passed through a coil surrounding the rupturing member.

10. An inflator according to claim 9, wherein a pyrotechnic charge is provided to drive the rupturing element towards the second element.

11. An inflator according to any one of claims 1 to 5, wherein the second element is a blocking member which initially blocks the second venting aperture, and may be moved or distorted to allow the flow of gas through the second aperture.

12. An inflator according to claim 11 , wherein the blocking member comprises part of a solenoid, and the blocking member is withdrawn from the second venting aperture by an electric current being passed through a coil surrounding the blocking member.

13. An inflator according to claim 11 in which the second element is configured to distort in response to a secondary triggering signal, thereby allowing gas to flow through the second venting aperture.

14. An inflator according to claim 13, in which the second element comprises one or more piezoelectric elements to cause the second member to distort in response to a flow of current through the piezoelectric elements.

15. An inflator according to claim 13, wherein the second element comprises one or more magnetic elements which are configured to be attracted to, or repelled from one or more electromagnets which are activated by the secondary triggering signal.

16. An inflator according to any one of claims 4 to 15, comprising two pressurised containers according to any one of claims 1 to 3.

17. An air-bag module comprising an inflator according to any one of claims 4 to 16.

18. A method of constructing a pressurised container, comprising the steps of: providing a container enclosing a main interior volume and having a first venting aperture formed therein allowing communication between the main interior volume and the exterior of the container: sealing the first venting aperture with a first element; providing an enclosure formed within the main interior volume which substantially surrounds the first venting aperture, and encloses a first volume around the first venting aperture; forming a second venting aperture formed through the enclosure; and sealing the second venting aperture with a second element, the arrangement being such that the flow path from the main interior volume to the first venting aperture is initially impeded by the second element, so that rupturing, distortion or movement of the second element increases the cross-section of the available flow path from the main interior volume to the first venting aperture.

19. A method according to claim 18, further comprising the step of filling the main interior volume with a pressurised gas.

20. A method according to claim 19 wherein a pressure difference threshold at which the second element ruptures is selected so that, following rupturing of the first rupturable element, a delay will occur before the rupturing of the second element.

21. A method according to claim 20, wherein the delay is at least 5 milliseconds.

22. A method according to any one of claims 18 to 21 , wherein the volume of the first volume is selected to control the delay, following rupturing of the first rupturable element, after which the second element will rupture.

23. A method according to claim 18 or 22, further comprising the step of providing a rupturing arrangement to rupture the second element in response to a secondary triggering signal.

24. A method according to claim 18 or 19, wherein the step of sealing the second venting aperture with a second element comprises the step of sealing the second venting aperture with a second element that is configured to distort or move in response to a secondary triggering signal, thus allowing gas to flow through the second venting aperture.

25. A method according to any one of claims 18 to 24, further comprising the step of forming a communication passage in the enclosure, allowing communication between the main interior volume and the first volume.

26. A method according to claim 25 wherein the cross-sectional area of the communication passage is selected to control the delay, following rupturing of the first rupturable element, after which the second element will rupture.