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  Container and method for maintaining stability of gas mixturesUSPTO Application #: 20070284395Title: Container and method for maintaining stability of gas mixtures Abstract: A container for holding a single constituent gas or a mixture of constituent gases is disclosed. The container has a wall structure defining an internal space. An opening in the wall structure permits access to the internal space. A closure is positioned over the opening. A seal attaches the closure to the wall structure maintains the single constituent gas or the constituents of the gas mixture at a predetermined concentration over a predetermined time period. A valve is mounted on the closure for permitting controlled release of the mixture. A method of maintaining gas concentration stability by forming a seal between the closure and the wall structure is also disclosed. (end of abstract) Agent: Synnestvedt & Lechner, LLP - Philadelphia, PA, US Inventor: Joseph K. Bernacki USPTO Applicaton #: 20070284395 - Class: 2224021 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20070284395. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001]This invention concerns a method and a container adapted to maintain a single gas or constituent gases of a gas mixture within a predetermined concentration range over a predetermined time period. BACKGROUND OF THE INVENTION [0002]Gas mixtures having precise concentrations of constituent gases are used extensively in many industrial and laboratory processes. For example, mass spectrometers for identifying chemical compounds use a gas mixture comprising bromopentafluorobenzene, trifluoromethyl benzene and VOC-free nitrogen for calibration purposes. These constituent gases are present in the mixture at predetermined concentrations appropriate for the calibration purposes. Proper operation of the mass spectrometer depends upon its accurate calibration, which, in turn, depends upon the accuracy of the relative concentration of the constituent gases in the calibration gas mixture. [0003]When gas mixtures are held under pressure in containers, the relative concentration of the constituent gases may change over time, rendering the mixture useless for its intended purpose. The mixture is said to suffer from "instability", which is defined as the tendency of the constituent gases to change in concentration (either rise or fall) over time. Long term instability of a gas mixture occurs when the concentration of one or more constituent gases in the mixture changes after several (4-6) months, while short term instability is said to occur when the concentration of the constituent gases changes over a period of less than three months. [0004]Instability is an especially acute problem for gas mixtures stored at relatively low pressures and having relatively low concentrations of reactive components. For example, the aforementioned gas mixture for mass spectrometer calibration contains 50 ppmv of bromopentafluorobenzene and 100 ppmv of trifluoromethyl benzene in 5 liters of VOC-free nitrogen at 700 kPa at 21.degree. C. In view of the small amount of the active constituent gases in the mixture, any characteristic of the container that affects the concentration of these constituents will have a significant effect on both the long and short term stability of the mixture. [0005]The container characteristics which have the greatest effect on stability include adsorption, absorption and leakage. Practical containers typically comprise a pressure vessel, for example, a thin-walled aluminum cylinder, with an opening that receives a cap having a valve. For low pressure applications, the cap is crimped or swaged onto an outwardly curled lip that surrounds the opening. An elastomeric gasket, such as an O-ring or washer, is positioned between the cap and the lip to provide a fluid tight seal. The valve may be spring biased, and provides for controlled filling and discharge of the gas mixture. [0006]Gas leakage occurs between the valve cap and the container lip because the swaging or crimping process does not form a continuous, fluid tight connection between the cap and the lip. The use of the gasket between the cap and the lip is intended to provide a fluid tight seal, but leaks can still occur between the gasket and the lip or the valve cap because it is not always possible to control the surface finish of the vessel or the cap, and the gasket cannot conform and block the small surface irregularities that provide unacceptable leakage paths from the container. [0007]A leaky seal will not only allow gas to escape (thereby changing the relative concentration of the constituents), it may also allow contaminant gases, such as oxygen, to diffuse into the vessel against the gas pressure. The oxygen combines with the reactive components, effectively removing them from the mixture and changing the constituent concentration. [0008]Even when the leakage rate is acceptable (i.e., the gas mixture remains stable within acceptable limits), the use of elastomeric gaskets can cause instability by absorbing or adsorbing the more reactive constituent gases, effectively removing them from the mixture and thereby changing the relative concentration of the components. It is clear that the effects of leakage and absorption or adsorption will have a proportionally greater effect for smaller concentrations of the constituent gases. [0009]In addition to containers of gas mixtures, containers having a single gas constituent, for example, pure nitrogen for purging or calibration purposes, may also suffer from instability. In the case of a single gas constituent, the diffusion of contaminants into the container, such as oxygen or moisture, must be mitigated so that the concentration of the impurities does not increase. [0010]There is clearly a need for a container and a method of maintaining the stability of gas mixtures as well as single constituent gases, and especially for gas mixtures stored at relatively low pressures (up to 18 bar) and at relatively small concentrations (in the parts per million by volume range). SUMMARY OF THE INVENTION [0011]The invention concerns a container for holding a mixture of a plurality of constituent gases. The gases have respective predetermined concentrations within the mixture. The container comprises a vessel having a wall structure defining an internal space for containing the gas mixture. The wall structure has an opening therein for access to the internal space. A closure is positioned in overlying relation with the opening. A seal is positioned between the closure and the wall structure for attaching the closure to the wall structure so as to maintain a first one of the gases at a concentration within a range of about +1% to about -5% of the predetermined concentration of the first gas over about a one month period. A valve is mounted on the closure. The valve is openable and closable for permitting controlled release of the gas mixture from the vessel. [0012]The first gas may be maintained at a concentration within a range of about +1% to about -7% of the predetermined concentration of the first gas over about a one month period. The first gas may be maintained at a concentration within a range of about +1% to about -5% of the predetermined concentration of the first gas over about a one month period. A second one of the gases may be maintained at a concentration within a range of about +3% to about -15% of the predetermined concentration of the second gas over about a one month period. The second gas may be maintained at a concentration within a range of about +3% to about -30% of the predetermined concentration of the second gas over about a one month period. A third one of the gases may be maintained at a concentration within a range of about +3% to about -15% of the predetermined concentration of the third gas over about a one month period. The third gas may be maintained at a concentration within a range of about +3% to about -30% of the predetermined concentration of the third gas over about a one month period. [0013]Preferably, the seal comprises a weld integrally joining the closure to the wall structure. The weld is preferably formed by means of an electron beam. [0014]The invention further encompasses a container for holding a mixture of a plurality of constituent gases. The gases have respective predetermined concentrations within the mixture. The container is adapted to maintain the concentrations of the gases within a predetermined concentration range over a predetermined time period. The container comprises a vessel having a wall structure defining an internal space for containing the gas. The wall structure comprises a metal having the property of being inert with respect to the constituent gases and has an opening therein for access to the internal space. A closure is positioned in overlying relation with the opening. A seal for attaching the closure to the wall structure is positioned between the closure and the wall structure. The closure and the seal have the same properties of being inert to the constituent gases as the wall structure. A valve is mounted on the closure. The valve is openable and closable for permitting controlled release of the gas mixture from the vessel. [0015]The invention also includes container for holding a single constituent gas. The single constituent gas has a predetermined concentration of impurities upon filling. The container comprises a vessel having a wall structure defining an internal space for containing the gas. The wall structure has an opening therein for access to said internal space. A closure is positioned in overlying relation with the opening. A seal for attaching the closure to the wall structure so as to maintain the impurities at a concentration within a range of about +1% to about -1% of the predetermined concentration of the impurities upon filling over about a one month period. A valve is mounted on the closure. The valve is openable and closable for permitting controlled release of the gas mixture from the vessel. [0016]The invention also includes a method of maintaining gas concentration stability among constituent gases of a gas mixture within a predetermined concentration variation range and over a predetermined time period. The method comprises the steps of: [0017](a) providing a vessel for holding the gas mixture under pressure, the vessel having an opening therein; [0018](b) providing a closure adapted to engage the vessel in overlying relation with the opening, the closure having a valve mounted thereon for permitting controlled filling and release of the gas mixture to and from the vessel; [0019](c) forming a seal between the closure and the vessel, the seal, the closure and the vessel having the property of being inert with respect to the constituent gases, the seal providing a fluid tight joint between the closure and the vessel that maintains a first one of the gases at a concentration of about +1% to about -5% of a predetermined concentration of the first gas over about a one month period. BRIEF DESCRIPTION OF THE DRAWINGS [0020]FIG. 1 is a longitudinal sectional view of a container for stable storage of gas mixtures and single constituent gases according to the invention; and Continue reading... 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