Self-venting cap for a neck of a dewar vessel

Abstract

A self-venting cap for a neck of a dewar vessel with a lower component having a first plurality of apertures, an upper component with a second plurality of apertures, a seal held between the lower and upper components, and a third component releasably secured to the upper component. The cap forms a compression seal about an inner circumference of the neck of a dewar vessel when the lower and upper components are matingly engaged in a sealing position, creating a plurality of tortuous vapor paths through the cap. When the cap is in the sealing position, a first chamber is formed between the lower and upper components and a second chamber and a vent opening are formed between the upper and third components. In this position, vapor inside the dewar vessel can travel in a plurality of tortuous paths beginning within the neck and then sequentially travelling up through the first plurality of apertures, the first chamber, the second plurality of apertures, the second chamber and then out the vent opening. There can also be a plurality of shorter tortuous paths beginning within the first plurality of apertures, travelling through the first chamber, and then up through at least two of the second plurality of apertures. A semi-permeable membrane, which can be incorporated in the cap, prevents moisture from entering into the dewar vessel while still allowing vaporous cryogen to exit from the dewar vessel.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation of U.S. patent application Ser. No. 09/753,194, filed Dec. 29, 2000, the disclosure of which is specifically incorporated herein by reference, and is otherwise identical to the disclosure of U.S. patent application Ser. No. 09/753,195, now abandoned. The present application is related to U.S. patent application Ser. Nos. 09/753,208 and 09/753,207, both of which were filed on Dec. 29, 2000, the disclosures of which are specifically incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention is in the field of cryogenic shipping containers.

BACKGROUND OF THE INVENTION

[0003] To ensure reproducible results in research and biotechnical processes, today's scientists and clinical practitioners have found it necessary to genetically stabilize living cells and preserve the integrity of complex molecules for storage and transport. This is accomplished by containing these materials in enclosures where cryogenic temperatures are continuously maintained at or near liquid nitrogen or vapor phase liquid nitrogen temperatures (77K and 100K, respectively).

[0004] Advances in cryopreservation technology have led to methods that allow low-temperature maintenance of a variety of cell types and molecules. Techniques are available for the cryopreservation of cultures of viruses and bacteria, isolated tissue cells in tissue culture, small multi-cellular organisms, enzymes, human and animal DNA, pharmaceuticals including vaccines, diagnostic chemical substrates, and more complex organisms such as embryos, unfertilized oocytes, and spermatozoa. These biological products must be transported or shipped in a frozen state at cryogenic temperatures to maintain viability. This requires a shipping enclosure that can maintain a cryogenic environment for up to 10 days and meet other shipping requirements such as being relatively impervious to mechanical shock and effects of directional orientation.

[0005] In addition to the already existing difficulties posed in shipping heat-sensitive biologicals, the International Air Transport Association (IATA) imposed new regulations which became effective in January 1995 pertaining to all shipments that include specimens containing infectious agents or potentially infectious agents. These regulations, endorsed by the U.S. Department of Transportation (DOT) and applicable to all public and private air, sea, and ground carriers, imposed greatly increased requirements upon shipping units to survive extensive physical damage (drop-testing, impalement tests, pressure containment tests, vibration tests, thermal shock, and water damage) without leakage and without fracture of the internal, primary receptacles (vials). Implementation of this regulation further complicated the shipping of frozen biologicals.

[0006] Even though bioshippers are currently available using liquid nitrogen as a refrigerant, little innovation has taken place in the design of packaging for low-temperature transport. Current shippers are generally vulnerable to the physical damage and changes in orientation encountered during routine shipping procedures. Additionally, these shippers rarely comply with the IATA Dangerous Goods Regulation (effective January 1995 or as later amended). Commercial vendors have not developed or certified a cost-effective, standardized shipping unit with the necessary specimen capacity and hold time to meet user demands.

[0007] One of the main criticisms of current shippers is price, which varies from $500.00 to $1,000.00 or more per unit. This substantially limits their use for the transport of many biologicals. Because of the initial cost and limited production of these containers, they are designed to be reusable. However, the cost of return shipping of these heavy containers is significant, particularly in international markets.

[0008] Users also complain about the absorbent filler used in the current dry shippers, which breaks down with continuous use, contaminating the interior of the container. In fact, one large user of these containers has essentially centered their entire shipping operation around cleaning the broken down absorbent material from the inside of these containers after each use.

[0009] Another problem cited by users of currently available dry shippers relates to the functional hold time versus static hold time. Static hold time pertains to a fully charged shipper with no heat load, sitting upright, e.g., essentially not in use. Functional hold time refers to the fully charged shipper in use and containing samples, e.g., in the process of being handled and transported. Even though the static hold time is promoted as being 20 days, if the container is tilted or positioned on its side, the hold time diminishes to hours as opposed to days. This occurs because the liquid nitrogen transitions to the gaseous (vapor) phase more rapidly resulting in outgassing. The liquid nitrogen can also simply leak out of the container when it is positioned on its side.

[0010] The current cryogenic containers are promoted as being durable because they are of metal construction. However, rugged handling frequently results in the puncturing of the outer shell or cracking at the neck, resulting in loss of the high vacuum insulation. This renders them useless. The metal construction also adds to the weight of the container, thereby adding substantially to shipping costs.

[0011] Thus, there is a need for an improved cryogenic container that can be used to ship biologicals safely, reliably, and economically.

[0012] U.S. Pat. No. 6,119,465 seeks to meet this need by using unique, lightweight, low-cost, durable composites and polymers in a semi-disposable vapor phase liquid nitrogen bioshipper. This is accomplished in an inherently simple, reliable, and inexpensive device that will result in reduced shipping costs, enhanced reliability and safety, and fewer service requirements.

[0013] The present invention builds upon the framework laid by U.S. Pat. No. 6,119,465, the disclosure of which is specifically incorporated herein by reference. Creating a self-venting cap that leads to a more reliable, while still economical, cryogenic shipping container does this.

SUMMARY OF THE INVENTION

[0014] The present invention is generally directed to a self-venting cap for a neck of a dewar vessel. The cap has a lower component with a first plurality of apertures, an upper component with a second plurality of apertures and a seal held between the lower and upper components. The cap is capable of forming a compression seal about an inner circumference of the neck of a dewar vessel when the lower and upper components are matingly engaged in a sealing position. When a compression seal is created, there is at least one, and preferably more than one, tortuous vapor path through the cap to allow venting of vaporous cryogen from within the dewar vessel.

[0015] In a first, separate aspect of the present invention, the cap has a third component releasably secured to the upper component. When the cap is in the sealing position, a first chamber is formed between the lower and upper components and a second chamber and a vent opening are formed between the upper and third components. In this position, vapor inside the dewar vessel can travel in a plurality of tortuous paths beginning within the neck and then sequentially travelling up through the first plurality of apertures, the first chamber, the second plurality of apertures, the second chamber and then out the vent opening. There can also be a plurality of shorter tortuous paths beginning within the first plurality of apertures, travelling through the first chamber, and then up through at least two of the second plurality of apertures. The lower component has an outer circumference that is less than the inner circumference of the neck and its first plurality of apertures is located inside of its outer circumference. It is especially preferable that the vent opening and the outer circumference of the upper and third components are located outside of the inner circumference of the neck.

[0016] In still other, separate aspects of the present invention, the cap can be incorporated into a capped dewar vessel assembly, and the compression seal can be strong enough to support the weight of the dewar vessel when it is not charged with a cryogen. A semi-permeable membrane that prevents moisture from entering into the dewar vessel while still allowing vaporous cryogen to exit from the dewar vessel can be incorporated into the dewar vessel, and preferably within the self-venting cap.

[0017] In still further, other separate aspects of the present invention, a male and a female thread matingly engage the lower and upper components. The upper component can contain a positioning device on a lower surface capable of engaging with a second positioning device located in a fixed position relative to the neck of a dewar vessel such that once the first and second positioning devices are engaged, rotation of the third component in a tightening direction will cause the seal to be squeezed between the lower and upper components to form the compression seal. It is especially preferred that the lower component, the upper component and the third component are made of cryogenically compatible material that is non-metallic and non-conductive. It is also especially preferred that the seal is made of a cryogenically compatible material, and more especially a silicone material. In addition, there can be multiple vent openings in the cap.

[0018] Accordingly, it is a primary object of the present invention to provide an improved, self-venting cap for use with a dewar vessel that can be charged with a liquid cryogen.

[0019] This and further objects and advantages will be apparent to those skilled in the art in connection with the drawings and the detailed description of the preferred embodiment set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 is an exploded assembly drawing of a preferred embodiment of a portable, insulated shipping container according to the present invention with a containment system for dangerous materials.

[0021] FIG. 2 is a planar cross section with a partial cutaway view of a preferred embodiment of a portable, insulated shipping container.

[0022] FIG. 3 is an assembly drawing of a preferred embodiment of a dewar vessel assembly.

[0023] FIG. 4 is an exploded assembly drawing of a preferred embodiment of a self-venting cap taken from reverse directions.

[0024] FIGS. 5A-5C are a planar cross section of a preferred embodiment of a portable, insulated shipping container showing connection of a preferred self-venting cap.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The preferred embodiments of the present invention can be used as part of an overall system that utilizes several inventions. Broadly speaking, there is an overall cryogenic shipping container system. Within the shipping container, there is a dewar vessel. Within the dewar vessel, there is a specimen chamber for holding specimens. The self-venting cap of the present invention helps to securely hold specimens within the cryogenic shipping container. Although FIGS. 1-5 are described in greater detail below, the following is a glossary of the elements identified in the Figures:

[0026] 1 portable, insulated shipping container

[0027] 2 dewar vessel

[0028] 3 outer casing of dewar vessel 2

[0029] 3a upper half of outer casing 3

[0030] 3b bottom half of outer casing 3

[0031] 4 opening at top of outer casing 3

[0032] 5 evacuable space between outer casing 3 and inner casing 13

[0033] 6 getter pack

[0034] 7 desiccant

[0035] 8 nipple

[0036] 10 layer of super insulation

[0037] 11 dewar opening into inner vessel 13

[0038] 13 inner vessel of dewar vessel 2

[0039] 13a upper half of inner vessel 13

[0040] 13b lower half of inner vessel 13

[0041] 14 opening at top of inner vessel 13

[0042] 15 inner wall of inner vessel 13

[0043] 21 neck portion of dewar vessel 2

[0044] 30 plastic foam

[0045] 31 foam segment of plastic foam 30

[0046] 32 capillarity separation layer of foam 30

[0047] 40 outer shipping container shell

[0048] 41 base of outer shipping container shell 40

[0049] 42 side wall of outer shipping container shell 40

[0050] 42a top side wall of side wall 42

[0051] 42b top opening formed in top side wall 42a

[0052] 43 top wall of outer shipping container shell 40

[0053] 44 handle molded in outer shipping container shell 40

[0054] 45 pocket for paperwork formed in outer shipping container shell 40

[0055] 46 hinge mechanism

[0056] 47 latch mechanism

[0057] 48 certification plate assembly

[0058] 48a certification plate

[0059] 48b rivet for certification plate assembly 48

[0060] 48c indentation in outer shipping container shell 40 for certification plate

[0061] 50 support assembly for dewar vessel 2

[0062] 51 bottom portion of support assembly 50

[0063] 52 side rib portion of support assembly 50

[0064] 53 top portion of support assembly 50

[0065] 55 safety strap

[0066] 56 adjustable buckle of safety strap 55

[0067] 57 outer bottom of dewar vessel 2

[0068] 60 funnel-shaped vessel plate

[0069] 61 support for plate 60

[0070] 62 spray foam

[0071] 70 specimen chamber

[0072] 71 side wall of specimen chamber 70

[0073] 72 base of specimen chamber 70

[0074] 73 top opening of specimen chamber 70

[0075] 80 containment system

[0076] 81 bag of containment system 80

[0077] 83 porous structural cartridge of containment system 80

[0078] 90 inner plug

[0079] 91 handle of inner plug 90

[0080] 100 self-venting cap

[0081] 101 lower component of self-venting cap 100

[0082] 102 upper component of self-venting cap 100

[0083] 102a lower surface of upper component 102

[0084] 103 seal of self-venting cap 100

[0085] 104 third component of self-venting cap 100

[0086] 105 plate

[0087] 106 screw (threads not shown)

[0088] 107 cover plate

[0089] 108 female thread in lower component 101

[0090] 111 male thread

[0091] 112 female thread

[0092] 113 positioning device

[0093] 114 second positioning device

[0094] 115 rib

[0095] 121 first plurality of apertures in lower component 101

[0096] 122 second plurality of apertures in upper component 102

[0097] 131 first chamber of self-venting cap 100

[0098] 132 second chamber of self-venting cap 100

[0099] 133 vent opening of self-venting cap 100

[0100] FIG. 1 provides an assembly drawing that illustrates all of the components of the cryogenic shipping container, generally designated as 1, in a disassembled state, and FIG. 2 illustrates how all of these components fit together in an assembled state. FIG. 3 is an assembly drawing that illustrates how dewar vessel 2 is assembled. All of these Figures, as well as the assembly of parts illustrated in these Figures, are described in detail in a patent application filed concurrently herewith, Attorney Docket No. JSF35.051, entitled “CRYOGENIC SHIPPING CONTAINER, the disclosure of which is specifically incorporated herein by reference. However, it is worth repeating, for the sake of clarity herein, that a dewar vessel has an outer casing and an inner vessel with each having openings at their tops connected together by a neck portion forming an evacuable space between the outer casing and the inner vessel and a dewar opening into the inner vessel.

[0101] FIGS. 4A and 4B illustrate an especially preferred self-venting cap 100 for use with a dewar vessel 2. The manner in which such a cap functions in an especially preferred application of shipping container 1 is illustrated in FIGS. 5A-5C. Self-venting cap 100 has four primary components—a lower component 101 with a first plurality of apertures 121, an upper component 102 with a second plurality of apertures 122, a seal 103, and a third component 104. It is especially preferred that all of these four primary components be constructed of a cryogenically compatible material that is non-metallic and non-conductive. The first, second and fourth components can be made of an injection moldable material such as Acetyl. The outer circumference of lower component 101 is less than the inner circumference of neck 21 and the first plurality of apertures 121 is located inside of the outer circumference of the lower component as shown in FIGS. 4A and 4B.

[0102] When self-venting cap 100 is assembled, seal 103, which is preferably made of silicone rubber, is attached to lower component 101 by a snap, friction fit. Lower component 101 is secured to upper component 102 and third component 104 by two different means.

[0103] First, a screw 106 (threads not shown) is screwed into a female thread 108 in lower compartment 101 and held in place by plate 105. A cover plate 107 (shown with a trademark of Cryoport, Inc.) covers and seals the chamber in third component 104 in which the top of plate 105 and the head of screw 106 are held. Screw 106 holds all four primary components together in a cap assembly in which the individual primary parts can still move relative to each other. In this assembly, second component 102 is held between first component 101 and third component 104, and seal 103 is held between first component 101 and second component 102.

[0104] Second, lower component 101 has a male thread 111 that screws into female thread 112 of third component 103. When male thread 111 is not fully screwed into female thread 112, seal 103 is held in a taut position (see FIG. 5B) relative to the position it is held when male thread 111 is fully screwed into female thread 112 in a compression seal position (see FIG. 5C). Seal 103 changes position between FIGS. 5B and 5C when third component 104, which functions as a crank top, is rotated in a tightening direction that causes seal 103 to be squeezed between lower and upper components 101 and 102 so as to form a compression seal with neck 21. (FIG. 5B shows cap 100 before it is in a compression seal position while FIG. 5C shows cap 100 once it is in a compression seal position.) Ribs 115 of third component 104 rest against upper component 102, which serve as a stop, and thereby create a plurality of vent openings, in the compression seal position. (The left half of FIG. 5C has been slightly rotated to show a clear vapor path instead of such a top.) A positioning device 113 (shown as indentations in FIG. 4B) on lower surface 102a of upper component 102 engages with a second positioning device 114 (shown as nubs in FIG. 1) to prevent cap 100 from spinning during the tightening process.

[0105] When self-venting cap 100 forms a compression seal with neck 21 of dewar vessel 2 (as shown in FIG. 5C), vapor flow between inner vessel 13 and outside of dewar vessel 2 must flow through vent opening 133. FIG. 5C illustrates one such vapor path. The path includes flow through a first chamber 131 located between lower and upper components 101 and 102, and a second chamber 132 located between second component 102 and third component 104. Vent opening 133 can be a single opening or a plurality of openings. In FIG. 5C, vent opening 133 is located between third component 104 and plate 60, but it could also be located between third component 104 and neck 21 if plate 60 is not used. Vent opening 133 is located outside of the inner circumference of neck 21 because upper component 102 has an upper outer circumference that is located outside of the inner circumference of neck 21.

[0106] Self-venting cap 100 provides many advantages over traditional caps for dewar vessels.

[0107] One advantage of self-venting cap 100 is the strength of the compression seal it forms with neck 21 of dewar vessel 2. The seal can be strong enough to support the weight of dewar vessel 2 when it is not charged with a cryogen, or even stronger. This degree of strength is important when container 1 is subjected to shock or impact because cap 100 restricts access to, and effectively seals off access to, the contents of specimen chamber 70 and specimen containment systems inside of dewar vessel 2.

[0108] Another advantage of self-venting cap 100 is that it creates a plurality of tortuous vapor paths for venting dewar vessel 2. A tortuous vapor path increases the thermal length that gas venting from the dewar vessel must travel. Increasing the thermal length increases the thermal efficiency of the dewar vessel, thereby increasing the hold time for the shipping container. Multiple venting paths increases safety because it eliminates the possibility that a single venting path might become clogged, leading to dangerous build-up of gas.

[0109] In the preferred embodiment of cap 100, each of the first plurality of apertures 121 leads into first chamber 131, and each of the second plurality of apertures 122 leads out of first chamber 131 and into second chamber 132. Thus, vapor inside of dewar vessel 2 can travel in a plurality of tortuous paths when cap 100 is in the compression seal position. The plurality of tortuous paths begin within neck 21 and then sequentially travel up through first plurality of apertures 121, first chamber 131, second plurality of apertures 132, second chamber 132 and then out vent opening 133. In addition, because of the volume of chambers 131 and 132, gas need not always leave the chamber at the same point or points. This means there are also a plurality of shorter tortuous paths beginning within the first plurality of apertures 121, travelling through first chamber 131, and then up through second plurality of apertures 122.

[0110] Because first and second chambers 131 and 132 provide a void space that can be accessed by a plurality of apertures, they create a number of different vapor paths, as already noted. However, they also provide a void space in which gas can accumulate and intermix. This creates an additional benefit because the chambers can have different temperature gradients, and gases entering or leaving these chambers can have different temperature gradients as a result of mixing with gas contained in these chambers. Chambers 131 and 132 also act as buffer zones between gas flowing from outside of cap 100 into inner vessel 13 and gas flowing from inside of inner vessel 13 to outside of cap 100.

[0111] Cap 100 can also include one or more semi-permeable membranes (not shown in the Figures) to prevent moisture (water vapor) from entering from entering into the dewar vessel while still allowing vaporous cryogen to exit from the dewar vessel. For example, such a membrane could be used to cover either or both of the first and second plurality of apertures 121 and 122. (Alternatively, or in addition, a semi-permeable membrane could be placed at any other convenient location in the vapor path shown in FIG. 5C; however, it is preferable that it be conveniently included as part of cap 100 or inner cap 90. Including a semi-permeable membrane in the cap minimizes the portion of the vapor path that is restricted by the membrane and provides the membrane with a convenient structural component for incorporation and structural integrity.)

[0112] Accordingly, it will be apparent to those skilled in the art that still further changes and modifications in the actual concepts described herein can readily be made without departing from the spirit and scope of the disclosed inventions as defined by the following claims.

Claims

1. A self-venting cap for a neck of a dewar vessel, comprising:

a lower component with a first plurality of apertures;

an upper component with a second plurality of apertures, a second chamber and a vent opening;

a seal held between the lower and upper components that can form a compression seal about an inner circumference of the neck when the lower and upper components are matingly engaged in a compression seal position; and

a first chamber located between the lower and upper components;

wherein vapor inside the dewar vessel can travel in a plurality of tortuous paths when the cap is in the compression seal position, the plurality of tortuous paths beginning within the neck and then sequentially travelling up through the first plurality of apertures, the first chamber, the second plurality of apertures, the second chamber and then out the vent opening.

2. A self-venting cap as recited in claim 1, further comprising:

a third component secured to the upper component so as to define the second chamber and vent openings between the upper and third components.

3. A self-venting cap as recited in claim 2, wherein the lower and upper components are matingly engaged by a male and a female thread.

4. A self-venting cap as recited in claim 2, wherein the upper component contains a positioning device on a lower surface capable of engaging with a second positioning device located in a fixed position relative to the neck such that once the first and second positioning devices are engaged, rotation of the third component in a tightening direction will cause the seal to be squeezed between the lower and upper components to form the compression seal.

5. A self-venting cap as recited in claim 2, wherein the lower component, the upper component and the third component are made of cryogenically compatible material that is non-metallic and non-conductive.

6. A self-venting cap as recited in claim 2, wherein the third component is releasably secured to the upper component.

7. A self-venting cap as recited in claim 1, wherein the vent opening is comprised of a plurality of vent openings.

8. A self-venting cap as recited in claim 1, wherein the plurality of tortuous paths include a plurality of shorter tortuous paths beginning within the first plurality of apertures, travelling through the first chamber, and then up through the second plurality of apertures.

9. A self-venting cap as recited in claim 1, wherein there is a plurality of shorter tortuous paths beginning within each of the first plurality of apertures, travelling through the first chamber, and then up through at least two of the second plurality of apertures.

10. A self-venting cap as recited in claim 1, wherein the seal is comprised of a silicone material.

11. A self-venting cap as recited in claim 1, wherein the lower component has an outer circumference that is less than the inner circumference of the neck and the first plurality of apertures is located inside of the outer circumference.

12. A self-venting cap as recited in claim 10, wherein the vent opening is located outside of the inner circumference.

13. A self-venting cap as recited in claim 10, wherein the compression seal is strong enough to support the weight of the dewar vessel when it is not charged with a cryogen.

14. A self-venting cap as recited in claim 10, wherein the upper component has an upper outer circumference that is located outside of the inner circumference.

15. A self-venting cap as recited in claim 14, wherein the vent opening is located outside of the upper outer circumference.

16. A self-venting cap for a neck of a dewar vessel, comprising:

a lower component having an outer circumference that is less than the inner circumference of the neck and a first plurality of apertures that is located inside of the outer circumference;

an upper component having an upper outer circumference and a second plurality of apertures, the upper outer circumference being located outside of the inner circumference;

a seal held between the lower and upper components that forms a compression seal with an inner circumference of the neck when the lower and upper components are matingly engaged in a compression seal position; and

a third component secured to the upper component;

wherein a first chamber is formed between the lower and upper components when the lower and upper components are matingly engaged in the compression seal position;

wherein a second chamber and a vent opening located outside of the upper outer circumference are formed between the upper and third components when the lower and upper components are matingly engaged in the compression seal position; and

wherein vapor inside the dewar vessel can travel in a plurality of tortuous paths when the cap is in the compression seal position, the plurality of tortuous paths beginning within the neck and then sequentially travelling up through the first plurality of apertures, the first chamber, the second plurality of apertures, the second chamber and then out the vent opening.

17. A self-venting cap as recited in claim 16, wherein the lower component, the upper component and the third component are made of cryogenically compatible material that is non-metallic and non-conductive.

18. A self-venting cap as recited in claim 17, wherein there is a plurality of shorter tortuous paths beginning within each of the first plurality of apertures, travelling through the first chamber, and then up through at least two of the second plurality of apertures.

19. A self-venting cap as recited in claim 16, further comprising:

a semi-permeable membrane that prevents moisture from passing through the first plurality of apertures while still allowing vaporous cryogen to pass through the first plurality of apertures.

20. A capped dewar vessel assembly, comprising:

a dewar vessel having an outer casing and an inner vessel with each having openings at their tops connected together by a neck portion forming an evacuable space between the outer casing and the inner vessel and a dewar opening into the inner vessel;

a specimen chamber connected to the inner vessel that extends inside the inner vessel and is accessed through the dewar opening; and

a self-venting cap that restricts access to the specimen chamber when it is in a compression seal position that forms a compression seal with an inner circumference of the neck;

wherein vapor inside the dewar vessel can travel in a plurality of tortuous paths when the cap is in the compression seal position, the plurality of tortuous paths beginning within the dewar opening and then sequentially travelling up through a first plurality of apertures, a first chamber, a second plurality of apertures, a second chamber and then out a vent opening.

21. A capped dewar vessel assembly as recited in claim 20, wherein there is a plurality of shorter tortuous paths beginning within each of the first plurality of apertures, travelling through the first chamber, and then up through at least two of the second plurality of apertures.

22. A capped dewar vessel assembly as recited in claim 20, further comprising:

a semi-permeable membrane that prevents moisture from entering into the dewar vessel while still allowing vaporous cryogen to exit from the dewar vessel.

23. A capped dewar vessel assembly as recited in claim 22, wherein the semi-permeable membrane is incorporated into the self-venting cap.