US3788545A - Gas permeation tube and method for the filling thereof - Google Patents

Abstract

A first permeation tube embodiment has a housing with exterior circular grooves at one end thereof. A gas permeable, Teflon membrane is fitted over the grooved end and an annular retaining ring is then press fitted over the grooved end to create a tortuous seal. An opposite end of the housing contains a needle valve inlet through which a precisely metered volume of gas is introduced by a gas transfer technique. A second permeation tube embodiment includes a container from which liquified gas flows to envelop the exterior surface of a tubular membrane. Air flows through the membrane to dilute gas permeating inwardly through the membrane.

Description

. United States Patent 1 91 Budd et a1. Jan. 29, 1974 [54] GAS PERMEATION TUBE AND METHOD 3,679,133 7/1972 Sekiguchi et a1. 239/34 F THE FILLING THEREOF 3,111,091 11/1963 Hopkinson 239/34 X [75 1 Inventors: fi iz ggg g gag h Primary Examiner-Lloyd L. King {:iathan S. Greenberg, Arlington, ABSTRACT [73] Assignee: Monitor Labs Inc., San Diego, Calif. A first permeation tube embodiment has a housing with exterior circular grooves at one end thereof. A [22] Flled 1971 gas permeable, Teflon membrane is fitted over the [21] Appl. No; 212,272 grooved end and an annular retaining ring is then press fitted over the grooved end to create a tortuous [52] U S Cl 239/34 239/56 seal. An opposite end of the housing contains a needle [51] m0 9/04 valve inlet through which a precisely metered volume [58] Fie'ld 239/34 56 of gas is introduced by a gas transfer technique.

A second permeation tube embodiment includes a [56] References Ci container from which liquified gas flows to envelop UNITED STATES PATENTS the exterior surface of a tubular membrane. Air flows 3 412 935 11/1968 OKeffe- 239/34 through the membrane to dilute gas permeating 3:623:65) 11 1971 Maierwn ei all: 1:: 239/56 mwardly through the membrane 3,283,787 11/1966 Davis; 239/34 6 Claims, 8 Drawing Figures GAS PERMEATION TUBE AND METHOD FOR THE FILLING THEREOF BACKGROUND OF THE INVENTION The present invention relates to a gas dispensing device, and more particularly to a sealed vessel containing a gaseous substance in equilibrium with its liquid phase and having a permeation membrane through which gas can pass and be diluted by another gas.

BRIEF DESCRIPTION OF THE PRIOR ART In the past, permeation tubes have been popularly used as standards for generating gas mixtures. These tubes are usually made by filling a vessel having at least a portion thereof made from FEP Teflon (Du Pont) tubing having known size and characteristics. Permeation of the gas through the Teflon generates a small concentration of this gas into a diluent gas which is generally air. In the area of pollution instrumentation, the permeation tube is employed to generate a small quantity of precisely measured pure pollutant gas where the pollutant gas is stored in the permeation tube.

Typical permeation tube designs are disclosedin U.S. Pat. No. 3,412,935 to OKeeffe. Although'conventional tubes, such as disclosed in this patent, work satisfactorily for certain applications, they suffer from several disadvantages.

Initially, it must be pointed out that for use in pollution instrumentation, permeation tubes must be designed to produce a permeate gas that when diluted with purified air (zero gas), an atmospheric pollutant is simulated. However, the prior art devices are constructed with a relatively large permeating surface so that'a small concentration value cannot be obtained easily. 1

Further, prior art permeation tubes have suffered from inadequate seals. As a result, these tubes not only permeate the gas contained therein, but they also leak.

As to the conventional method of filling permeation temperature to allow transfer of liquid gas from a large storage cylinder to the permeation tube. However, it

has been found that very cold temperatures cause a Teflon membrane to crack due to brittleness.

Because of these and other problems of the prior art permeation tubes, it is highly desirable to improve existing permeation tubes so that smaller concentrations of permeate gas can be obtained. Also, an improved filling method is desirable which would allow the transfer of gas into a permeation tube kept at a temperature sufficiently high to prevent cracking of a Teflon membrane.

BRIEF DESCRIPTION OF THE INVENTION The present invention is a marked improvement of presently existing permeation tubes. Further, there is disclosed a novel method for filling a permeation tube such as the one describedherein.

The present permeation tube includes a permeating membrane that can be made controllably small so that a pollutant gas of small concentration is formed.

The invention includes a tortuous seal between the Teflon membrane and a vessel containing the penneand gas. This seal eliminates leakage of the permeand tubes, a permeation tube is usually maintained at a low which would add an error factor to the permeation rate.

By employing a needle valve within the structure of the permeation tube, the presently disclosed permeation tube can be reliably filled at a high enough temperature to prevent the cracking of the permeation Teflon membrane.

The above-mentioned objects and advantages of the present invention will be more clearly understood when considered in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view illustrating the internal structure of one embodiment of the permeation tube disclosed in the present invention.

FIG. 2 is a diagrammatic illustration of a method for filling the permeation tube of FIG. 1.

FIG. 3a is a partial sectional view illustrating a modification of the end portion of the permeation tube shown in FIG. 1.

FIG. 3b is a partial sectional view illustrating a second modification of the permeation tube structure shown in FIG. 1.

FIG. 4a is a sectional view illustrating a second embodiment of the present invention.

FIG. 4b shows an additional modification of the structure shown in FIG. 4a.

FIGS. 4c and 4d show further modification of the permeation tube structure shown in FIG. 40.

Referring to the figures and more particularly FIG. 1,reference numeral 10 generally indicates a first permeation tube embodiment. The permeation tube includes a hollowedhousing 12 having a central cylindrical body terminating outwardly in anend portion 14 that has circular (non-helical) grooves formed therein. Because of the corrosive nature of the substance contained in thepermeation tube 10, it is preferable that the material from which the tube is manufactured be highly corrosion resistant, for example stainless steel.

Amembrane 16 is slipped over thegrooved end 14 of thehousing 12. By way of example, the membrane is indicated as being cup-shaped. The membrane must be permeable to the enclosed gaseous contents (permeand) of the tube. In a preferred embodiment of the invention, themembrane 16 is made from FEP Teflon (Du Pont). The outward closedend 18 of themembrane 16 forms a membrane closure for the axially formedbore chamber 22 that containsliquid gas 24.

When assembling the permeation tube, themembrane 16 is slipped over thegrooved end 14 and thereafter, aretainer ring 20 is force fitted over the membrane. This causes the Teflon material to cold flow into the grooves to effect a tortuous seal. This seal is far superior to the permeation tube seals of the prior art.

Above theliquid gas column 24 is thegaseous phase 26. The liquid and gaseous phases are in equilibrium.Reference numeral 28 characterizes the permeation path of gas molecules.

The permeation tube has an enlarged, generallycylindrical head 32 containing a needle valve assembly.

Anaxial bore portion 34 is formed in axially spaced relation to theaforementioned bore chamber 22. Threads are formed along the interior surface of thebore 34 to accommodate avalve stem 44 therein.

Anotherbore 36 is formed in perpendicular, intersecting relation with the aforementioned threadedbore 34. Thebore 36 has aninlet port 38 which engages a fill tube during filling of the permeation tube, as discussed hereinafter. The valve stem 44 allows filling of the tube when the stem is unscrewed, and after filling is completed, the valve stem is tightened to shut off the needle valve in thepermeation tube head 32. As will be observed, the inward end of thebore 36 terminates in atapered end 40 which receives the mating end of a fill tube (not shown). A restricted,narrow passage 42 connects confronting ends of thebores 34 and 22. This passage is adapted to receive the inwardtapered end 50 of thevalve stem 44. As will be observed, thetapered end 50 of the valve stem is flat at 52, and a cylindricalintermediate portion 48 connects thetapered end portion 50 and an enlargedcylindrical head section 45 that terminates outwardly in aslot 46, adapted to receive a screwdriver.

An O-ring 54 surrounds the inlet opening 38 inbore 36. This O-ring serves to seal the interface between the permeation tube and a separate fill tube during the filling of the permeation tube. An O-ring 56 is also provided at the outward end ofbore 34 to seal the interface between thevalve stem 44 and thebore 34.

Equivalent means for clamping themembrane 16 tohousing 12 includes the formation of threaded grooves at the transverse outer end of the housing instead of the grooves where illustrated. A retainer ring having mating threads would then clamp the membrane against the outer end of the housing.

FIG. 2 diagrammatically illustrates a method and apparatus for filling thepermeation tube 10. Thetube 68 are closed. When thepermeation tube valve 44 is opened, gas equilibrates in the system and most of the liquid contained in thechamber 76 is transferred to thepermeation tube 10. The transfer of liquid gas fromchamber 76 to the permeation tube incorporates a change to the gaseous phase along ,the path betweenchamber 76 and thepermeation tube 10. However, due to the packing of thepermeation tube 10 in theice bath 60, the gaseous phase is cooled and again changes to the liquid phase once inside the permeation tube. After the gas has been transferred out ofchamber 76, thevalve 44 in thepermeation tube 10 is closed. Theice bath 60 is removed and then thepermeation tube 10 is disconnected from thefiller tube 62. Now, thepermeation tube 10 is readyfor use. When the filled permeation tube is removed from thefiller tube 62, the conduit path betweenvalve 78 and the outlet end offiller tube 62 are filled with air. However, the portion of the system fromvalve 78 back tovalve 82 does not contain is positioned in atemperature bath vessel 58 containing crushed ice. Themembrane 16 is thereby kept at freezing temperature which is not sufficiently low to cause cracking of the Teflon material. However, the freezing temperature is cold enough to maintain gas, in the permeation tube, in the liquified phase. As will be observed from the figure, the enlarged valve portion of thepermeation tube 10 rises above theice pack 60 and is exposed to allow connection of afiller tube 62 into theinlet port 38 of the permeation tube. The opposite end of thefiller tube 62 is connected to a T-connector 64. A second leg of the T-connector is fastened totubing 66 having ashutoff valve 68 along its length. The outward end of thetubing 66 terminates in avacuum pump 70. The third leg of the T-connector 64 is connected to acoupling 72 that is in turn connected to ashutoff valve 74. The opposite end of thevalve 74 communicates with achamber 76 that is bounded at its opposite end by ashutoff valve 78. The volume of thechamber 76 is important. The volume is intentionally designed to be substantially equal to the volume of the liquid column inbore 22 of the permeation tube. Thus,chamber 76 serves as a metering device. As will be explained hereinafter, the metered volume of liquidgas filling chamber 76 is transferred to thepermeation tube 10.

Tubing 80 is connected between theshutoff valve 78 and anothershutoff valve 82. Aninverted fill tank 84 containing liquified gas therein directly communicates with thevalve 82.

In operation of the system illustrated in FIG. 2,valve 82 is closed andvalves 78, 74, 68 and 44 are opened. Then, thevacuum pump 70 is operated to evacuate the entire system, including the permeation tube, of air and water vapor. Next,valve 74 is closed andvalve 82 is opened thereby resulting in the flow of liquid gas fromtank 84 intochamber 76 viatubing 80.Valves 78 and air. Accordingly, when the next permeation tube is positioned in place against the outlet end offiller tube 62, the permeation tube, and the conduit portion betweenvalve 78 and the outlet end offiller tube 62 must be evacuated by thevacuum pump 70. This requires that evacuation take place at the beginning of the next filling cycle. Thereafter,valves 68 and 74 are closed andvalve 78 is opened and liquid gas flows to fill thechamber 76. From this point in the procedure, the aforementioned filling steps are repeated.

As will be appreciated, the filling of the permeation tubes are conducted under relatively high pressure. Also, since themembrane 16 is maintained at a high enough temperature to prevent cracking, a high degree of quality control for the membrane can be realized.

Although the preceding discussion has been with respect to a cup shapedmembrane 16, this particular shape is noncritical and other designs are fully in keeping with the invention.

FIG. 3a illustrates the permeating end of a permeation tube similar to the previously discussedpermeation tube 10. However, the illustration depicts an elongated test tube shaped membrane that is sealed between the retainer ring 86 and thegrooved end 88.

FIG. 3b illustrates a further variation wherein a hollowcylindrical permeating membrane 92 is fastened to thehousing 94 of the permeation tube as in previous embodiments. However, a stainless steel or othernoncorrosive plug 96 is inserted in the outward end of thepermeation membrane 92.

An alternate embodiment of the present invention is shown in FIG. 4a wherein a permeation tube assembly is generally denoted by 98. The assembly includes aninverted container 100 having afill plug 102 therein. It should be understood that any suitable means for filling thecontainer 100 is permissible. Accordingly, the needle valve ofpermeation tube 10 can be used instead of thefill plug 102. Inside thecontainer 100 is a desired permeand in thegaseous phase 106 and theliquid phase 104, that are in equilibrium with one another. The container includes anelongated neck portion 108 that has a centrally formed bore therein which communicates directly with the interior of the main container body. Aleg 110 of a T-connector 112 has abore 114 formed therein for receiving theneck portion 108 of thecontainer 100. A swagelock fitting 116 secures a seal between thecontainer 100 and theleg 110 of the T-connector 112. This leg of the T-connector includes an axially formedpassageway 118 communicating with thecontainer 100. Thepassageway 118 has a lowerannular portion 120 that is positioned in central coaxial relationship to the other legs of the T-connector 112. Acentral bore 122 is formed through these horizontally illustrated legs. Atubular permeation membrane 126 is positioned in thebore 122 andswagelock fittings 128 and 130 seal the exterior ports of the horizontal T- connector legs to thepermeation membrane 126.

Thus, in operation of the device, whenliquid permeand 104 flows through thecontainer neck 108passageway 118,annular passageway 124,-and bore 122, the liquid gas envelops themembrane 126 and is contained between the swagelock seals 128 and 130. Thereafter, the liquid permeand permeates radially inwardly to the inside of thepermeation membrane 126. As zero gas flows axially through thepermeation membrane 126, the zero gas mixes with the generated permeate to form a gas mixture that exists from the outlet end of apermeation member 126.

FIG. 4b illustrates a further variation employing permeation of the liquid permeand in aradially inward direction. In this embodiment, a container 131 has aneck portion 132 that communicates with acylindrical housing 134 having axially projectingannular flanges 136 and 138. Atubular permeation membrane 144 passes through theflanges 136 and 138. O-ring seals 140 and 142 seal the confronting surfaces of theflanges 136, 138 and thepermeation membrane 144. The gas flow is as indicated by arrows.

Still anotherv embodiment is illustrated in FIG. 40 wherein thecontainer 146 has its neck portion 148 extending into acylindrical housing 150 that has transverse flattened ends 152. Axial openings 154 and 156 are formed in the transverse ends 152. O-rings 158 and 160 seal the confronting surfaces between the flattened housing ends 152 and thecircular flanges 170 ofconduit fittings 164 and 162. A short length of tubular permemation membrane 166 is fitted tonipples 168 that extend axially inwardly and are an integral part of thefittings 162 and 164.

A still further variation of the radially inward permeating device is illustrated in FIG. 4d. In this embodiment, acontainer 172 includes gaseous and liquified gas in equilibrium as in-the preceding embodiments. Acontainer neck 174 extends'into acylindrical housing 176. Gastight bulkhead fittings 178 and 180 extend axially through the transverse ends of thecylindrical housing 176. Atubular permeation membrane 188 is axially positioned to pass through thebulkhead fittings 178 and 180 as well as thecylindrical housing 176.Swagelock fittings 186 prevent the escape of gas through the interface between thepermeation membrane 188 and theswagelock fittings 186. As in the 7 case of the lastthree discussed embodiments, air flows into the permeation membrane at the left end thereof and as it passes to the left outlet end of the membrane, the contained permeand permeates radially inwardly into themembrane 188. After the gas enters the membrane, it is diluted by the zero gas flowing through thepermeation membrane 188 and is delivered to the outlet end of the membrane.

With the exception of FIG. 3b, each of the illustrated and described embodiments exclude conventional end plugs for the permeation membrane. Elimination of these plugs is advantageous for several reasons. First, excluding end plugs eliminates the hazard of plug ejection at high temperature. Second, the seals of all embodiments of the present invention provide more positive gas sealing then end plugs.

In the instance of all embodiments presented herein, the permeation tubes eliminate short lifetime due to small fill volumes. The proposed devices can be recharged without adjustment of permeation membrane length. Also, use of a stainless steel housing and container provides more constant heat sinking for contained liquids than a permeation tube alone. As an extension of this advantage, the constructions herein provide rapid temperature change for better control due to the high thermoconductivity of the housing ofpermeation tube 10. In the embodiments of FIGS. 4a-4a', a gas containing container adds to the high thermoconductivity characteristic.

Accordingly, from the aforementioned specification and accompanying claims it can be appreciated that the present invention offers distinct advantages over the prior art permeation tubes.

It should be understood that the invention is not limited to the exact details of construction shown and described herein for obvious modifications will occur to persons skilled in the art.

Wherefore, the following is claimed:

1. A permeation device comprising:

a casing for storing liquid gas therein;

an outer end of the casing having grooves therein;

a permeation membrane positioned over the grooved end of the casing; and

fastener means clamping the membrane against the grooved end of the casing for causing cold flow of membrane material in the grooves and creating a tortuous seal between the membrane and the grooved end of the casing.

2. The structure recited in claim 1 wherein the permeation membrane is cup-shaped and includes a cylindrical wall bounded on one transverse end by a circular integral closure.

3. The structure as set forth in claim 1 wherein the permeation membrane is characterized as test tube shaped with an open end and an oppositely disposed oblong closed end.

4. The structure as defined in claim 1 wherein the permeation membrane has a hollow tubular shape having opened ends, and further wherein a sealing plug is inserted in an outlet end of the membrane.

5. A permeation device comprising:

a container having liquified gas;

a permeation membrane positioned adjacent the container; and

conduit means having a diluent passing therethrough, the conduit means communicating between the container and the permeation membrane to channel flow of the liquified gas around the exterior of the membrane; whereby the gas permeates from the outside of the membrane to the inside where it is mixable with the diluent.

6. The structure of claim 1 wherein the membrane is composed of a polymeric plastic material.

Claims (6)

1. A permeation device comprising: a casing for storing liquid gas therein; an outer end of the casing having grooves therein; a permeation membrane positioned over the grooved end of the casing; and fastener means clamping the membrane against the grooved end of the casing for causing cold flow of membrane material in the grooves and creating a tortuous seal between the membrane and the grooved end of the casing.

2. The structure recited in claim 1 wherein the permeation membrane is cup-shaped and includes a cylindrical wall bounded on one transverse end by a circular integrAl closure.

3. The structure as set forth in claim 1 wherein the permeation membrane is characterized as test tube shaped with an open end and an oppositely disposed oblong closed end.

4. The structure as defined in claim 1 wherein the permeation membrane has a hollow tubular shape having opened ends, and further wherein a sealing plug is inserted in an outlet end of the membrane.

5. A permeation device comprising: a container having liquified gas; a permeation membrane positioned adjacent the container; and conduit means having a diluent passing therethrough, the conduit means communicating between the container and the permeation membrane to channel flow of the liquified gas around the exterior of the membrane; whereby the gas permeates from the outside of the membrane to the inside where it is mixable with the diluent.

6. The structure of claim 1 wherein the membrane is composed of a polymeric plastic material.

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