Abstract:
A valve system that includes (i) first and second longitudinally facing members defining a gap therebetween in fluid communication with transversely offset first and second orifices through the members, (ii) a primary biasing means longitudinally biasing the members away from one another, (iii) an elastic element positioned within the gap between the first and second orifices, and (iv) a repositioning means for longitudinally repositioning one of the members towards the other member against the bias of the primary biasing means for adjusting the longitudinal height of the gap. The members can be repositioned between (a) a first position wherein the elastic element does not seal the gap so as to allow fluid flow from the first orifice to the second orifice through the gap, and (b) a second position wherein the elastic element is compressed between the members and seals the gap so as to prevent fluid flow from the first orifice to the second orifice through the gap.

Description:
BACKGROUND 
     Permeation instruments are used to measure the transmission rate of a target analyte, such as oxygen, carbon dioxide or water vapor, through a film of interest. Typical films subjected to permeation testing are polymeric packaging films such as those constructed from low density polyethylene (LDPE), high density polyethylene (HDPE), oriented polypropylene (OPP), polyethylene terepthalate (PET), polyvinylidene chrloride (PVTDC), etc. Typically, the film to be tested is positioned within a test chamber to sealing separate the chamber into first and second cells. The first cell (commonly referenced as the sensing cell) is flushed with an inert gas to remove any target analyte from the cell and the second cell (commonly referenced as the analyte cell) filled with a gas containing a known concentration of the target analyte. A sensor for the target analyte detects the presence of target analyte that has migrated into the sensing cell from the analyte cell through the film. 
     Permeation instruments typically employ a flow-through method or an accumulation method for sensing the presence of target analyte in the sensing cell. Briefly, the flow-through method uses an inert flushing gas to continuously pick up any target analyte that has migrated into the sensing cell and deliver it to a remote sensor. The accumulation method allows target analyte to build up in the sensing cell for an accumulation period, with the sensor either positioned within the sensing cell or the sensing cell flushed with a flushing gas after the accumulation period for delivery of accumulated target analyte to a remote sensor. 
     Both methods require precision in timing the opening and closing of fluid flows through the instrument, as well as opening and closing access to the analyte sensor. In addition, when sensing cell is sealed to fluid flow during a testing period, such as occurs with the accumulation technique, the instrument is relying solely upon diffusion to move analyte molecules within the sensing cell into sensing contact with the sensor, and the valving system should not seal the sensing cell too far upstream or downstream from the cell as this causes an increase in the effective volume of the sensing cell, thereby reducing the responsiveness and accuracy of the instrument. 
     Accordingly, a substantial need exists for a valving system for permeation testing instruments capable of reliably and consistently opening and closing the various cells within the instrument to fluid flow with a limited increase in the effective volume of the sensing cell of the instrument. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a valve system. One embodiment of the valve system includes (i) first and second longitudinally facing members defining a gap therebetween in fluid communication with transversely offset first and second orifices through the members, (ii) a primary biasing means longitudinally biasing the members away from one another, (iii) an elastic element positioned within the gap between the first and second orifices, and (iv) a repositioning means for longitudinally repositioning one of the members towards the other member against the bias of the primary biasing means for adjusting the longitudinal height of the gap. The members can be repositioned between (a) a first position wherein the elastic element does not seal the gap so as to allow fluid flow from the first orifice to the second orifice through the gap, and (b) a second position wherein the elastic element is compressed between the members and seals the gap so as to prevent fluid flow from the first orifice to the second orifice through the gap. 
     Another embodiment of the valve system includes (i) first and second longitudinally facing members defining a gap therebetween in fluid communication with a first inlet and a first outlet orifice through one member and a second inlet and a second outlet orifice through the other member, wherein (a) the inlet orifices are transversely offset, (b) the outlet orifices are transversely offset, and (c) one of the members is longitudinally repositionable relative to the other member for adjusting the longitudinal height of the gap, (ii) an inlet larger-diameter elastic O-ring positioned within the gap and encompassing both the first and second inlet orifices, (iii) an inlet smaller-diameter elastic O-ring positioned within the gap and surrounding only one of the first or second inlet orifices, (iv) an outlet larger-diameter elastic O-ring positioned within the gap and encompassing both the first and second outlet orifices, (v) an outlet smaller-diameter elastic O-ring positioned within the gap and surrounding only one of the first or second outlet orifices, and (vi) a repositioning means for longitudinally repositioning the repositionable member towards the other member against the bias of the larger-diameter elastic O-rings. The members can be repositioned between (a) a first position wherein the larger-diameter O-rings are compressed between the members and seal the gap so as to prevent fluid flow through the gap from the inlet orifices to the outlet orifices without compressing the smaller-diameter O-rings between the members so as to allow fluid flow from the first inlet orifice to the second inlet orifice through the gap and from the first outlet orifice to the second outlet orifice through the gap, and (b) a second position wherein both larger-diameter O-rings are compressed between the members and seal the gap so as to prevent fluid flow through the gap from the inlet orifices to the outlet orifices, the inlet smaller-diameter O-ring is compressed between the members so as to prevent fluid flow from the first inlet orifice to the second inlet orifice through the gap, and the inlet smaller-diameter O-ring is compressed between the members so as to prevent fluid flow from the first outlet orifice to the second outlet orifice through the gap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic overview of one embodiment of a testing system useful for performing the testing process of the present invention. 
         FIG. 2  is a side view of the measurement unit component of the testing system shown in  FIG. 1 . 
         FIG. 3  is a top view of the measurement unit component of the testing system shown in  FIG. 2 . 
         FIG. 4A  is a cross-sectional side view of the measurement unit shown in  FIG. 3  taken along line  4 - 4  with the upper mounting plate in the open position spaced a distance away from the upper portion of the housing. 
         FIG. 4A   1  is an enlarged cross-sectional side view of the encircled inlet area of the gap in the measurement unit shown in  FIG. 4A . 
         FIG. 4A   2  is an enlarged cross-sectional side view of the encircled outlet area of the gap in the measurement unit shown in  FIG. 4A . 
         FIG. 4A   3  is an enlarged cross-sectional side view of the encircled sensor passageway area of the gap in the measurement unit shown in  FIG. 4A . 
         FIG. 4B  is a cross-sectional side view of the measurement unit shown in  FIG. 3  taken along line  4 - 4  with the upper mounting plate in the closed position immediately adjacent the upper portion of the housing. 
         FIG. 4B   1  is an enlarged cross-sectional side view of the encircled inlet area of the gap in the measurement unit shown in  FIG. 4B . 
         FIG. 4B   2  is an enlarged cross-sectional side view of the encircled outlet area of the gap in the measurement unit shown in  FIG. 4B . 
         FIG. 4B   3  is an enlarged cross-sectional side view of the encircled sensor passageway area of the gap in the measurement unit shown in  FIG. 4B . 
         FIG. 5A  is a cross-sectional side view of the measurement unit shown in  FIG. 3  taken along line  5 - 5  with the upper mounting plate in the open position spaced a distance away from the upper portion of the housing. 
         FIG. 5A   1  is an enlarged cross-sectional side view of the encircled humidity control window in the measurement unit shown in  FIG. 5A . 
         FIG. 5B  is a cross-sectional side view of the measurement unit shown in  FIG. 3  taken along line  5 - 5  with the upper mounting plate in the closed position immediately adjacent the upper portion of the housing. 
         FIG. 5B   1  is an enlarged cross-sectional side view of the encircled humidity control window in the measurement unit shown in  FIG. 5B . 
         FIG. 6A  is a cross-sectional side view of the measurement unit shown in  FIG. 3  taken along line  6 - 6  with the upper mounting plate in the open position spaced a distance away from the upper portion of the housing. 
         FIG. 6B  is a cross-sectional side view of the measurement unit shown in  FIG. 3  taken along line  6 - 6  with the upper mounting plate in the closed position spaced a distance away from the upper portion of the housing. 
         FIG. 7  is a grossly enlarged side view of the encircled portion of the testing chamber shown in  FIG. 3  depicting individual molecules of an analyte of interest on each side of a test film being tested with the measurement unit shown in  FIG. 4B . 
         FIG. 8  is a graph of the O 2  transmission rate over time obtained from the permeation testing conducted in Example 1. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Nomenclature 
     
         
           10  Testing System 
           21  Source of Inert Gas 
           22  Source of Test Gas 
           31   a  Inlet Shutoff Valve for Source of Inert Gas 
           31   b  Outlet Shutoff Valve for Source of Inert Gas 
           32   a  Inlet Shutoff Valve for Source of Test Gas 
           32   b  Outlet Shutoff Valve for Source of Test Gas 
           41   a  Inlet Conduit for Directing Gas From the Source of Inert Gas Into the Upper Cell 
           41   b  Outlet Conduit for Venting Gas From the Upper Cell 
           42   a  Inlet Conduit for Directing Gas From the Source of Test Gas Into the Lower Cell 
           42   b  Outlet Conduit for Venting Gas From the Lower Cell 
           50  Computer or CPU 
           60  Monitor 
           70  Printer 
           80  Electrical Leads from the Sensor to the CPU 
           100  Measurement Unit 
           110  Housing 
           111  Upper Section of Housing 
           111   i  Lower Surface of Upper Section of Housing 
           112  Lower Section of Housing 
           119  Retention Chamber Defined by Housing 
           120  Mounting Plates 
           121  Upper Mounting Plate 
           121   u  Upper Surface of Upper Mounting Plate 
           121   n  Pin On Upper Mounting Plate 
           122  Lower Mounting Plate 
           125  O-ring Between Mounting Plates 
           129  Testing Chamber Defined by Mounting Plates 
           129   1  Upper Cell of Testing Chamber 
           129   2  Lower Cell of Testing Chamber 
           130  Actuator 
           131  Actuator Shaft 
           140  Valve for Passageway to Analyte Sensor 
           141  Valve Body 
           142  Valve Stem 
           151   a  Inlet Channel to Lower Cell Through Upper Section of Housing 
           151   b  Inlet Channel to Lower Cell Through Upper Mounting Plate 
           151   c  Inlet Channel to Lower Cell Through Lower Mounting Plate 
           151   w  Larger O-ring within Gap Encircling Inlet Passageways into the Lower Cell 
           152   a  Outlet Channel from Lower Cell Through Upper Section of Housing 
           152   b  Outlet Channel from Lower Cell Through Upper Mounting Plate 
           152   c  Outlet Channel from Lower Cell Through Lower Mounting Plate 
           152   w  Larger O-ring within Gap Encircling Outlet Passageways from the Lower Cell 
           160  Gap Between Upper Section of Housing and Upper Mounting Plate 
           170  Flow Control Channels and Passageways Through the Upper Section of the Housing and the Upper Mounting Plate 
           171   a  Inlet Channel to Gap Through Upper Section of Housing 
           171   b  Inlet Channel from Gap to Upper Cell Through Upper Mounting Plate 
           172   a  Outlet Channel from Gap Through Upper Section of Housing 
           172   b  Outlet Channel from Upper Cell to Gap Through Upper Mounting Plate 
           173   a  Passageway from Gap to Analyte Sensor Through Upper Section of Housing 
           173   b  Passageway from Upper Cell to Gap Through Upper Mounting Plate 
           180  O-Ring Seals within the Gap 
           181   v  Smaller O-ring within Gap Encircling Inlet Channel through Upper Mounting Plate 
           181   w  Larger O-ring within Gap Encircling Both Inlet Channels 
           182   v  Smaller O-ring within Gap Encircling Outlet Channel through Upper Mounting Plate 
           182   w  Larger O-ring within Gap Encircling Both Outlet Channels 
           183   w  Larger O-ring within Gap Encircling Passageways Leading to the Sensor 
           190  Humidity Control System 
           191   a  Inlet Channel to Humidity Control Chamber Through Upper Section of Housing 
           192   a  Outlet Channel from Humidity Control Chamber Through Upper Section of Housing 
           193  Selectively Permeable Film 
           194  O-ring 
           195  Washer 
           196  Inset Ring 
           197  Locking Ring 
           198   w  Larger O-ring within Gap Encircling Both Inlet and Outlet Channels for a Humidity Control Chamber 
           199  Humidity Control Chambers in the Upper Mounting Plate 
           200  Analyte Sensor 
         A Analyte Molecules 
         F Film Being Tested 
         x Lateral Direction 
         y Longitudinal Direction 
         z Transverse Direction 
       
    
     DESCRIPTION 
     Overview 
     Referring generally to  FIG. 1 , the invention is directed to a valve system for an analyte measurement unit  10 . The valve system ensures that the flow of various fluids through the unit  10  occur in the properly sequence as necessary and appropriate for measuring the effective volume of the sensing cell  129   1  of the measurement unit  10 . 
     The method includes the steps of (i) separating a testing chamber  129  into a first or upper cell  129   1  and a second or lower cell  129   2  with a known area of a film F, (ii) flushing the first cell  129   1  with an inert gas to remove any target analyte A from the first cell  129   1 , (iii) introducing a gas (not shown) containing a known concentration of an analyte A into the lower cell  129   2 , (iv) sealing the upper cell  129   1  to gas flow (not shown) through the upper cell  129   1 , and (v) sensing any analyte A in the upper cell  129   1  with an analyte sensor  200  that consumes the analyte A at a rate greater than the rate at which the analyte A is passing through the film F, until a steady state rate of analyte A consumption is measured by the analyte sensor  200 . The analyte sensor  200  preferably consumes analyte A at least ten times faster than the rate at which the analyte A is transmitted through the film F, more preferably twenty times faster, and most preferably one hundred times faster. 
     Referring to  FIGS. 4A and 4B , the valve system includes (i) an upper section  111  of a housing  110  and an upper mounting plate  121  defining a gap  160  therebetween, (ii) a means for reposition the mounting plate  121  relative to the upper section  111  of the housing  110  as between an open position with a “thicker” gap  160  and a closed position with a “thinner” gap  160 , (iii) flow control channels and passageways  170  through the upper section  111  of the housing  110  and the upper mounting plate  121  and (iv) o-ring seals  180  of different diameters and different thicknesses positioned within a gap  160 , encircling the various channels and passageways  170 . 
     Referring to  FIGS. 4A and 4A   1 , fluid flow into the upper cell  129   1  is provided by laterally x and/or transversely z offset inlet channels  171   a  and  171   b  in the upper section  111  of the housing  110  and the upper mounting plate  121  respectively. In similar fashion, referring now to  FIGS. 4A and 4A   2 , fluid flow out from the upper cell  129   1  is provided by laterally x and/or transversely z offset outlet channels  172   a  and  172   b  in the upper section  111  of the housing  110  and the upper mounting plate  121  respectively. 
     Referring to  FIGS. 4A and 4A   1 , a small diameter o-ring  181   v  is positioned within the gap  160  encircling just one of the inlet channels  171   b , while a large diameter o-ring  181   w , also positioned within the gap  160 , encircles both inlet channels  171   a  and  171   b . In similar fashion, referring now to  FIGS. 4A and 4A   2 , a small diameter o-ring  182   v  is positioned within the gap  160  encircling just one of the outlet channels  172   b , while a large diameter o-ring  182   w , also positioned within the gap  160 , encircles both the outlet channels  172   a  and  172   b.    
     Referring to  FIGS. 4A ,  4 A 1 ,  4 A 2 ,  4 A 3 ,  5 A,  5 A 1 , the thickness or longitudinal y height of the o-rings  181  and  182  are selected so that the large diameter o-rings  181   w  and  182   w  are sealingly engaged within the gap  160  regardless of whether the gap  160  is thicker or thinner, while the smaller diameter o-rings  181   v  and  182   v  are sealingly engaged within the gap  160  only when the gap is thinner. Such positioning of the larger ( 181   w  and  182   w ) and smaller ( 181   v  and  182   v ) o-rings, in combination with the different thicknesses of the larger ( 181   w  and  182   w ) and smaller ( 181   v  and  182   v ) o-rings, permits the inlet ( 171   a  and  171   b ) and outlet ( 172   a  and  172   b ) channels to be simultaneously opened to fluid flow for flushing of the upper cell  129   1  prior to a testing period by longitudinally y moving the mounting plates  120  into the downward or open position as shown in  FIGS. 4A ,  4 A 1  and  4 A 2 , and simultaneously closed to fluid flow for sealing-off the upper cell  129   1  during a testing period by longitudinally y moving the mounting plates  120  into the upward or closed position as shown in  FIGS. 4B ,  4 B 1  and  4 B 2 . 
     The film F can be a perforated or nonperforated film F, and can be porous or nonporous with respect to the target analyte A, so long as the analyte sensor  200  can consume the target analyte A at a rate greater than the rate at which the analyte A is passing through the film F. To ensure that the analyte sensor  200  is consuming all target analyte A that is passing through the film F, the analyte sensor  200  is preferably selected so that it consumes target analyte A at a rate that is at least ten times greater, preferably twenty times greater and most preferably 100 times greater, than the rate at which the target analyte A is likely to be transmitted through the film F. 
     Specific Embodiment 
     Testing System 
     Construction 
     An exemplary embodiment of a testing system  10  capable of measuring the transmission rate of an analyte A through a film F in accordance with the present invention is depicted in  FIG. 1 . A measurement unit  100  defines a testing chamber  129  sealingly divided by a film F to be tested into an upper cell  129   1  and a lower cell  129   2 . A source of an inert gas  21  communicates with the upper cell  129   1  via inlet conduit  41   a  and outlet conduit  41   b  for flushing the upper cell  129   1  prior to testing. Suitable inert gases include specifically, but not exclusively, nitrogen, argon, helium, krypton or a blend of nitrogen and hydrogen, etc. A source of test gas  22  containing a known concentration of an analyte A, communicates with the lower cell  129   2  via inlet conduit  42   a  and outlet conduit  42   b  for continuously providing the lower cell  129   2  with test gas to ensure that the concentration of analyte A within the lower cell  129   2  remains constant throughout a test period. Shutoff valves  31   a  and  31   b  are provided in inlet conduit  41   a  and outlet conduit  41   b  respectively, for controlling the flow of inert gas through the upper cell  129   1 . Similarly, shutoff valves  32   a  and  32   b  are provided in inlet conduit  42   a  and outlet conduit  42   b  respectively, for controlling the flow of gas through the lower cell  129   2 . 
     An analyte sensor  200  for the target analyte A is placed in fluid communication with the upper cell  129   1  for sensing the presence of target analyte A within the upper cell  129   1 . Typical target analytes include oxygen, carbon dioxide, carbon monoxide and water vapor. The analyte sensor  200  may be selected from any of the wide variety of commercially available consuming sensors capable of detecting and consuming the target analyte A, with electrochemical sensors generally preferred based upon the high sensitivity and low cost of such sensors and the fact that such sensors, when employed in the present invention, follow Faraday&#39;s Law—eliminating the need to calibrate the sensor. 
     The analyte sensor  200  communicates via electrical leads  80  with a suitable central processing unit  50  equipped with electronic memory (not shown), and optionally but preferably attached to a monitor  60  and/or printer  70  for storing and reporting analyte A concentrations detected by the analyte sensor  200 . 
     Use 
     A film F to be tested is “loaded” into the testing chamber  129  so as to sealingly separate the testing chamber  129  into an upper cell  129   1  and a lower cell  129   2  with a known area of the film F exposed to both cells  129   1  and  129   2 . Shutoff valves  31   a  and  31   b  are then opened to permit the flow of inert gas through the upper cell  129   1  for flushing analyte A from the upper cell  129   1 . After flushing, the shutoff valves  31   a  and  31   b  are closed to seal-off the upper cell  129   1  from the surrounding environment. Shutoff valves  32   a  and  32   b  are then opened to permit the flow of gas containing a known concentration of analyte A into the lower cell  129   2 . The presence of analyte A within the upper cell  129   1  is then detected and recorded by the analyte sensor  200 . By ensuring that the only route through which analyte A can enter into the upper cell  129   1  is through the “exposed” area of the film F, and by selecting an analyte sensor  200  that consumes analyte A faster than the analyte A is transmitted through the film F, then the rate at which the analyte sensor  200  detects analyte A, once a steady state rate is attained, can be equated directly to the analyte transmission rate for the known “exposed” area of the film F. 
     Measurement Unit Including Valve System 
     Construction 
     An exemplary embodiment of a measurement unit  100  capable of quickly and accurately measuring the transmission rate of an analyte A through a film F in accordance with the present invention is depicted in  FIGS. 2-6 . 
     The measurement unit  100  includes (i) a housing  110 , (ii) mounting plates  120 , (iii) an actuator  130 , (iv) a valve  140  for controlling fluid communication with an analyte sensor  200 , (v) channels  151   a ,  151   b ,  151   c ,  152   a ,  152   b , and  152   c  in the housing  110  and mounting plates  120  for directing test gas (not shown) into a lower cell  129   2  in the mounting plates  120 , and (vi) a flow control system (not collectively numbered) including flow control channels  170  and o-ring seals  180  for selectively opening and sealing closing an upper cell  129   1  in the mounting plates  120  to fluid flow. The measurement unit  100  optionally, but preferably, also includes a humidity control system  190 . 
     The housing  110  includes an upper section  111  and a lower section  112  that cooperatively define a retention chamber  119 . 
     Referring to  FIGS. 4A ,  4 B,  5 A,  5 B,  6 A and  6 B, upper and lower mounting plates  121  and  122  (collectively referenced as mounting plates  120 ) are retained within the retention chamber  119  defined by housing  110  with the upper surface  121   u  of the upper mounting plate  121  longitudinally y offset a distance from the lower surface  111   i  of the upper section  111  of the housing  110  so as to define a gap  160  therebetween. The upper and lower mounting plates  121  and  122  define a testing chamber  129  therebetween. An o-ring  125  encircling the testing chamber  129  is provided between the mounting plates  120 . The testing chamber  129  can be sealingly divided into an upper cell  129   1  and a lower cell  129   2  by placement of a test film F between the mounting plates  120  overlaying the o-ring  125 , and compressing the mounting plates  120  together so as to sealingly compress the entire periphery of the o-ring  125  between the mounting plates  120 . 
     It is generally preferred to configure the testing chamber  129  to provide an upper cell  129   1  of about 1 cm 3  to about 3 cm 3 . An upper cell  129   1  larger than about 3 cm 3  is too slow to respond as molecules of analyte A within the upper cell  129   1  can be consumed and detected by the analyte sensor  200  only when the molecules enter the analyte sensor  200  and the upper cell  129   1  relies solely upon diffusion to move molecules within the upper cell  129   1 . An upper cell  129   1  smaller than about 1 cm 3  tends to cause areas of the film F to contact with the upper surface (not numbered) of the upper mounting plate  121  during the testing period, thereby introducing error into the test results as analyte A cannot readily pass through the film F into the upper cell  129   1  through these “covered” areas. 
     Referring to  FIGS. 4A ,  4 B,  5 A,  5 B,  6 A and  6 B, the lower mounting plate  122  is mounted onto the distal end (unnumbered) of an actuator shaft  131  for longitudinally repositioning of the mounting plates  120  by an actuator  130  as between a lower or open position creating a longitudinally thicker gap  160  between the upper surface  121   u  of the upper mounting plate  121  and the lower surface  111   i  of the upper section  111  of the housing  110 , as shown in  FIG. 4  (collectively  4 A,  4 A 1 ,  4 A 2  and  4 A 3 ), and an upper or closed position creating a longitudinally thinner gap  160  between the upper surface  121   u  of the upper mounting plate  121  and the lower surface  111   i  of the upper section  111  of the housing  110 , as shown in  FIG. 5  (collectively  5 A,  5 A 1 ,  5 A 2  and  5 A 3 ). Suitable actuators  130  include specifically, but not exclusively, pneumatic and hydraulic pistons. 
     Referring to  FIGS. 6A and 6B , fluid flow into the lower cell  129   2  is provided by aligned inlet channels  151   a ,  151   b  and  151   c  in the upper section  111  of the housing  110 , the upper mounting plate  121  and the lower mounting plate  122  respectively. In similar fashion, fluid flow out from the lower cell  129   2  is provided by aligned outlet channels  152   a ,  152   b  and  152   c  in the upper section  111  of the housing  110 , the upper mounting plate  121  and the lower mounting plate  122  respectively. A large diameter o-ring  151   w  is positioned within the gap  160  encircling the inlet channels  151   a  and  151   b  in the upper section  111  of the housing  110  and the upper mounting plate  121  for preventing testing gas from flowing throughout the gap  160 . In similar fashion, a large diameter o-ring  152   w  is positioned within the gap  160  encircling the outlet channels  152   a  and  152   b  in the upper section  111  of the housing  110  and the upper mounting plate  121  for preventing testing gas from flowing throughout the gap  160 . 
     Referring to  FIGS. 4A and 4B , the flow control system (not collectively numbered) includes (i) flow control channels and passageways  170  through the upper section  111  of the housing  110  and the upper mounting plate  121 , and (ii) o-ring seals  180  of different diameters and different thicknesses positioned within the gap  160  and encircling the various channels and passageways  170 . The flow control system provides a quick, simple and reliable method of opening and closing the upper cell  129   1  and the analyte sensor  200  to fluid flow at the appropriate times. 
     Referring to  FIGS. 4A and 4A   1 , fluid flow into the upper cell  129   1  is provided by laterally x and/or transversely z offset inlet channels  171   a  and  171   b  in the upper section  111  of the housing  110  and the upper mounting plate  121  respectively. In similar fashion, referring now to  FIGS. 4A and 4A   2 , fluid flow out from the upper cell  129   1  is provided by laterally x and/or transversely z offset outlet channels  172   a  and  172   b  in the upper section  111  of the housing  110  and the upper mounting plate  121  respectively. 
     Referring to  FIGS. 4A and 4A   1 , a small diameter o-ring  181   v  is positioned within the gap  160  encircling the inlet channel  171   b  in the upper mounting plate  121 . A large diameter o-ring  181   w  is also positioned within the gap  160  for encircling both the inlet channel  171   a  in the upper section  111  of the housing  110  and the inlet channel  171   b  in the upper mounting plate  121  as well as fully encircling the small diameter o-ring  181   v . In similar fashion, referring now to  FIGS. 4A and 4A   2 , a small diameter o-ring  182   v  is positioned within the gap  160  encircling the outlet channel  172   b  in the upper mounting plate  121 , with a large diameter o-ring  182   w  positioned within the gap  160  and encircling both the outlet channel  172   a  in the upper section  111  of the housing  110  and the outlet channel  172   b  in the upper mounting plate  121  as well as encircling the small diameter o-ring  182   v.    
     Referring to  FIGS. 4A ,  4 A 1 ,  4 A 2 ,  4 A 3 ,  5 A,  5 A 1 , the thickness or longitudinal y height of the large diameter o-rings  181   w  and  182   w  is selected so that these o-rings  181   w  and  182   w  are sealingly engaged within the gap  160  regardless of whether the mounting plates  120  are in the open or closed longitudinally y position so as to prevent fluid from flowing freely within the gap  160 . The thickness or longitudinal y height of the smaller diameter o-rings  181   v  and  182   v  is selected so that these o-rings  181   v  and  182   v  are sealingly engaged within the gap  160  only when the mounting plates  120  are in the closed longitudinally y position. Such positioning of the larger ( 181   w  and  182   w ) and smaller ( 181   v  and  182   v ) o-rings, in combination with the different thicknesses of the larger ( 181   w  and  182   w ) and smaller ( 181   v  and  182   v ) o-rings, permits the inlet ( 171   a  and  171   b ) and outlet ( 172   a  and  172   b ) channels to be simultaneously opened to fluid flow for flushing of the upper cell  129   1  prior to a testing period by longitudinally y moving the mounting plates  120  into the downward or open position as shown in  FIGS. 4A ,  4 A 1  and  4 A 2 , and simultaneously closed to fluid flow for sealing-off the upper cell  129   1  during a testing period by longitudinally y moving the mounting plates  120  into the upward or closed position as shown in  FIGS. 4B ,  4 B 1  and  4 B 2 . 
     Referring to  FIGS. 4A ,  4 A 3 ,  5 A, the analyte sensor  200  communicates with the upper cell  129   1  via longitudinally y aligned passageways  173   a  and  173   b  in the upper section  111  of the housing  110  and the upper mounting plate  121  respectively. A large diameter o-ring  183   w  is positioned within the gap  160  encircling both passageways  173   a  and  173   b  for ensuring that fluid diffusing into the analyte sensor  200  from the upper cell  129   1  is not contaminated by fluid from the gap  160 . 
     In order to extend the useful life of the analyte sensor  200 , especially when an electrochemical sensor is employed, the passageway  173   a  should be closed at all times except during testing periods (i.e., only after the upper cell  129   1  has been flushed with an inert gas and sealed so that the only analyte A in the upper cell  129   1  is analyte A that has permeated through a test film F). Referring to  FIGS. 4A ,  4 A 3 ,  5 A, an expedient technique for providing such limited access to the analyte sensor  200  is to position a normally closed tire valve  140  within the passageway  173   a , with the body  141  of the tire valve  140  sealingly wedged into the passageway  173   a  and the stem  142  of the tire valve  140  extending longitudinally y downward towards the gap  160 . An upwardly extending pin  121   n  is provided on the upper mounting plate  121  for pressing longitudinally y upward against the valve stem  142  and thereby opening the valve  140  only when the mounting plates  120  are in the upper or closed position. 
     The transmission rate of analyte A through most plastic films F is sensitive to humidity, with an increase in humidity tending to result in an increase in the transmission rate. Most analyte sensors  200  are also somewhat sensitive to humidity, especially if permitted to “dry out”. Hence, in order to obtain consistent and comparable test results it is important to maintain a constant relative humidity within the testing chamber  129 , especially within the closed upper cell  129   1 . To maintain a constant humidity within the upper cell  129   1 , a humidity control system  190  can be provided. A suitable humidity control system  190  is shown in  FIGS. 5A ,  5 A 1 ,  5 B and  5 B 1 . The humidity control system  190  include a pair of humidity control chambers  199  in the upper mounting plate  121  diametrically positioned relative to the analyte sensor  200  and in fluid communication with both the upper cell  129   1  and the gap  160 . Inlet  191   a  and outlet  192   a  channels are provided in the upper section  111  of the housing  110  for placing each of the humidity control chambers  199  in fluid communication with a source of a gas (not shown) having a known humidity, typically 0% or 100% relative humidity. A large diameter o-ring  198   w  is positioned within the gap  160  encircling each of the humidity control chambers  199  and the corresponding set of inlet  191   a  and outlet  192   a  channels. A film  193  permeable to water vapor and impermeable to the target analyte A, such as a Nafion® film, is provided over the opening of each humidity control chamber  199  into the upper cell  129   1  for purposes of allowing transpiration between the humidity control chamber  199  and the upper cell  129   1  without introducing extraneous analyte A into the upper cell  129   1  or allowing analyte A to escape from the upper cell  129   1  undetected. The selectively permeable film  193  can be sealingly held in position within each humidity control chamber  199  by an o-ring  194 , washer  195 , inset ring  196  and locking ring  197  as shown in  FIGS. 5A   1  and  5 B 1 . 
     Use 
     The mounting plates  120  are removed from the retention chamber  129  by activating the actuator  130  to lower the actuator shaft  131  into a removal position (not shown) where the o-ring seals  180  within the gap  160  no longer contact the upper section  111  of the housing  110 , and sliding the mounting plates  120  out through an open side (not numbered) of the lower section  112  of the housing  110 . 
     The upper mounting plate  121  is then separated from the lower mounting plate  122 , and a sample of the film F to be tested placed atop the lower mounting plate  122  over the test chamber  129  so as to fully engage the entire periphery of the o-ring  125  encircling the test chamber  129 . 
     The upper mounting plate  121  is then placed back atop the lower mounting plate  122  and secured to the lower mounting plate  122  so as to sealingly clamp the film F between the plates  121  and  122 , thereby sealingly separating the testing chamber  129  into an upper cell  129   1  and a lower cell  129   2  with a known area of the film F exposed to both cells  129   1  and  129   2 . The “loaded” mounting plates  120  are then slid back into the retention chamber  119 . 
     Referring to  FIGS. 4A ,  4 A 1 ,  4 A 2  and  4 A 3 , the actuator  130  is activated to move the loaded mounting plates  120  into an “open” position wherein the larger diameter o-rings  181   w ,  182   w ,  183   w  and  198   w  located within the gap  160  sealingly engage the lower surface  111   i  of the upper section  111  of the housing  110  while the smaller diameter o-rings  181   v  and  182   v  within the gap  160  do not. With the mounting plates  120  in the “open” position, the upper cell  129   1  is flushed with an inert gas to remove any target analyte A from the upper cell  129   1  by placing the inlet channel  171   a  in the upper section  111  of the housing  110  in fluid communication with a pressurized source of inert gas  21  and allowing the inert gas to flow sequentially through the inlet channel  171   a  in the upper section  111  of the housing  110 , through that portion of the gap  160  surrounded by the larger diameter o-ring  181   w , through the inlet channel  171   b  in the upper mounting plate  121 , through the upper cell  129   1 , through the outlet channel  172   b  in the upper mounting plate  121 , through that portion of the gap  160  surrounded by the larger diameter o-ring  182   w , and out from the measurement unit  100  through the outlet channel  172   a  in the upper section  111  of the housing  110 . 
     Referring to  FIGS. 4B ,  4 B 1 ,  4 B 2  and  4 B 3 , after flushing, the actuator  130  is activated to move the loaded mounting plates  120  into a “closed” position wherein both the larger diameter o-rings  181   w ,  182   w ,  183   w  and  198   w  and smaller diameter o-rings  181   v  and  182   v  within the gap  160  sealingly engage the lower surface  111   i  of the upper section  111  of the housing  110  so as to seal-off the upper cell  129   1  from the surrounding environment. 
     Referring to  FIG. 4A   3 , movement of the loaded mounting plates  120  into the “closed” position also causes the pin  121   n  on the upper mounting plate  121  to engage the stem  142  on the valve  140  within the passageway  173   a  in the upper section  111  of the housing  110  so as to open the passageway  173   a  and thereby place the analyte sensor  200  in fluid communication with the upper cell  129   1 . 
     With the mounting plates  120  in the “closed” position, the lower cell  129   2  is flushed with a test gas containing a known concentration of target analyte A and continuously supplied with “fresh” test gas throughout the testing period to ensure that the concentration of target analyte A within the lower cell  129   1  remains constant. Test gas is introduced into the lower cell  129   2  by placing the inlet channel  151   a  in the upper section  111  of the housing  110  in fluid communication with a pressurized source of test gas  22  and allowing the test gas to flow sequentially through the inlet channel  151   a  in the upper section  111  of the housing  110 , through that portion of the gap  160  surrounded by the larger diameter o-ring  151   w , through the inlet channel  151   b  in the upper mounting plate  121 , through the inlet channel  151   c  in the lower mounting plate  122 , through the lower cell  129   2 , through the outlet channel  152   c  in the lower mounting plate  122 , through the outlet channel  152   b  in the upper mounting plate  121 , through that portion of the gap  160  surrounded by the larger diameter o-ring  152   w , and out from the measurement unit  100  through the outlet channel  152   a  in the upper section  111  of the housing  110 . 
     Target analyte A will permeate through the film F as the analyte A seeks to diffuse through the film F from a region of higher concentration (i.e., the lower cell  129   2 ) to a region of lower concentration (i.e., the upper cell  129   1 ). Since test gas continuously flows through the lower cell  129   2  the concentration of target analyte A in the region of higher concentration remains constant throughout the relevant test period. Similarly, since the analyte sensor  200  consumes target analyte A within the upper cell  129   1  faster that the target analyte A permeates through the film F, the concentration of target analyte A in the region of lower concentration also remains constant at essentially zero throughout the relevant test period. 
     Eventually, the system will reach a steady state condition where the rate at which analyte A is detected in the upper cell  129   1  by the analyte sensor  200  and reported by the central processing unit  50  remains constant. This steady state rate equates directly to the permeation rate for the film F for the “exposed” area of the film. 
     EXAMPLES 
     Example 1 
     A 1.0 mil thick polyethylene terephthalate mylar film is placed between the mounting plates of the permeation testing system depicted in  FIGS. 1-7  so as to provide a 50 cm 2  area of the film exposed to both the upper and lower cells. Permeation testing is conducted in accordance with ASTM D3985 employing the following testing parameters: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Gas In Upper Cell: 
                   
               
               
                   
                 Type: 
                 100% N 2   
               
               
                   
                 RH: 
                  10% 
               
               
                   
                 Gas In Lower Cell: 
                   
               
               
                   
                 Type: 
                 100% O 2   
               
               
                   
                 RH: 
                  10% 
               
               
                   
                 Testing Chamber Temp:  
                 23° C. 
               
               
                   
                 Barometer: 
                 742.3 mmHg 
               
               
                   
                   
               
             
          
         
       
     
     Oxygen within the upper cell is continuously sensed with a high-sensitivity standard electrochemical oxygen sensor covered with a porous membrane. Utilizing a reporting cycle of five (5) minutes, the transmission rate of oxygen through the film (O2TR) is calculated from the amperes sensed by the sensor each reporting cycle utilizing EQUATION A. The O2TR calculated for each reporting cycle throughout the testing period is depicted in  FIG. 8  and set forth in Table One below. The O2TR for the film, reported after fifty (50) reporting cycles (4 hours and 10 minutes) is 60.975 cm 3 /(m 2 )(day).
 
O2TR=Amperes/(Area)( k   1 )( k   2 )( k   3 )  (EQUATION A)
 
Wherein:
     O2TR=Transmission Rate of Oxygen (cm 3 /(m 2 )(sec))   Amperes=Amperes generated at the sensor (coulombs/second)   Area=Exposed area of the film (m 2 )   k 1 =Molecules of Oxygen per cm 3  at Standard Temperature and Pressure (2.6876*10 19  molecules/cm 3 )   k 2 =Electrons involved in covalent bonding @ the sensor per molecule of Oxygen (4 e − /molecule)   k 3 =Coulombs generated per electron (1.6*10 −19  coulombs/e-)   

     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE ONE 
               
               
                   
                   
               
               
                   
                 Time 
                 O2TR 
               
               
                   
                 (hrs:min) 
                 cm 3 /(m 2 )(day) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                   5 
                 0.1 
               
               
                   
                   10 
                 5.078 
               
               
                   
                   15 
                 15.105 
               
               
                   
                   20 
                 25.023 
               
               
                   
                   25 
                 33.235 
               
               
                   
                   30 
                 39.666 
               
               
                   
                   35 
                 47.96 
               
               
                   
                   40 
                 51.218 
               
               
                   
                   45 
                 53.614 
               
               
                   
                   50 
                 55.399 
               
               
                   
                   55 
                 56.72 
               
               
                   
                 1:00 
                 57.732 
               
               
                   
                 1:05 
                 58.499 
               
               
                   
                 1:10 
                 59.073 
               
               
                   
                 1:15 
                 59.491 
               
               
                   
                 1:20 
                 59.844 
               
               
                   
                 1:25 
                 60.086 
               
               
                   
                 1:30 
                 60.254 
               
               
                   
                 1:35 
                 60.397 
               
               
                   
                 1:40 
                 60.51 
               
               
                   
                 1:45 
                 60.592 
               
               
                   
                 1:50 
                 60.67 
               
               
                   
                 1:55 
                 60.715 
               
               
                   
                 2:00 
                 60.769 
               
               
                   
                 2:05 
                 60.785 
               
               
                   
                 2:10 
                 60.807 
               
               
                   
                 2:15 
                 60.84 
               
               
                   
                 2:20 
                 60.857 
               
               
                   
                 2:25 
                 60.843 
               
               
                   
                 2:30 
                 60.858 
               
               
                   
                 2:35 
                 60.858 
               
               
                   
                 2:40 
                 60.896 
               
               
                   
                 2:45 
                 60.9 
               
               
                   
                 2:50 
                 60.935 
               
               
                   
                 2:55 
                 60.952 
               
               
                   
                 3:00 
                 60.957 
               
               
                   
                 3:05 
                 60.973 
               
               
                   
                 3:10 
                 60.97 
               
               
                   
                 3:15 
                 60.966 
               
               
                   
                 3:20 
                 60.954 
               
               
                   
                 3:25 
                 60.959 
               
               
                   
                 3:30 
                 60.948 
               
               
                   
                 3:35 
                 60.98 
               
               
                   
                 3:40 
                 60.984 
               
               
                   
                 3:45 
                 60.978 
               
               
                   
                 3:50 
                 60.974 
               
               
                   
                 3:55 
                 60.973 
               
               
                   
                 4:00 
                 60.984 
               
               
                   
                 4:05 
                 60.968 
               
               
                   
                 4:10 
                 60.975