Patent Publication Number: US-2004040372-A1

Title: Method for determining the permeation of gases into or out of plastic packages and for determination of shelf-life with respect to gas permeation

Description:
TECHNICAL FIELD  
       [0001] This invention relates to testing of gas permeability of plastic packaging and shelf life of substances, particularly food products and especially beverages, packaged in plastic packaging.  
       BACKGROUND OF THE INVENTION  
       [0002] Increasingly, products are packed in plastic packages, because these are light and convenient. This trend is reflected by carbonated beverages, which are frequently packed in PET bottles. Plastic packaging has the disadvantage that it is not as impermeable as glass or metal. Therefore, plastic packaging can permit limited amounts of diffusion, which in turn can affect the “shelf-life” of the product.  
       [0003] “Shelf-life” is the common term, which describes the time during which the product retains its properties. Permeation of gases, particularly oxygen, into the package, and permeation of volatile product components out of the package play a principal role in shelf-life of products packaged in plastic, because other mechanisms leading to product deterioration are normally slower. For the purposes of the present invention, gas permeation is therefore assumed to be the controlling mechanism for determining shelf-life in plastic packages.  
       [0004] Diffusion through the walls of the package can take place inwards and outwards. This involves gaseous components of the product itself, which can diffuse outwards, as well as air and contaminants, which can diffuse inwards. It is therefore important to be able to measure the gas permeation of plastic packaging, so as to determine the barrier property of the plastic against such gas permeation, as well as to determine the shelf-life of the product in the package.  
       [0005] PET bottles and other packages are subjected to barrier treatments, which are frequently coatings. In the development of such treatments, it is important to determine the permeation rates of those gases, which affect shelf-life, and to use this data to evaluate the barrier improvement of said barrier treatments by reference to an untreated package. In production of barrier-treated packages, it is a quality control necessity to be able to measure the permeation and shelf-life of said packages on a regular basis. For quality control, it is desirable to be able to test packages, straight off the production line, without opening the package. Therefore, both for research/development and for production quality-control, a barrier and shelf-life measurement system is needed that gives quick results, can handle many samples daily, with a wide range of package sizes, can be used on un-opened production packages or specially-prepared test-packages, avoids need for highly-trained operators and requires minimum involvement of operators.  
       [0006] Several technologies exist for measuring the barrier property of complete packages, such as bottles, and/or wall-sections taken from these packages and/or materials used in the packages. For example, one known system intermittently flushes one side of a package with an inert gas (eg nitrogen), whilst maintaining an atmosphere of the permeating gas on the other side of the package. Permeation rate is measured by continuously or intermittently exhausting the inert gas at a measured rate and monitoring the concentration of the permeating species within this exhaust, until equilibrium is reached, at which time-point the permeation rate is essentially constant. One example of this basic system is covered by a series of U.S. Pat. Nos. (4,852,389, 5,390,539, 5,265,463, 5,591,898, 5,513,515, 5,081,863, 5,837,888). A further example is covered by Japanese patent (# 110 305 79A). The system suffers from significant inaccuracies when the permeation rate is very low, partly because it compounds the inaccuracy of the gas-concentration measuring device at necessarily low concentrations with the inaccuracy of the gas flow device. In addition, the system has limited capacity, because each package must be installed in the equipment and continuously monitored until equilibrium is measured.  
       [0007] Other systems rely on measuring the loss of permeating gas within a package. An example of this is the common practice of mounting a pressure gauge on a pressurised package, measuring the rate of loss in pressure within the package and equating this to the permeation rate. These systems often find application in the testing of PET beverage bottles. More recently, non-invasive methods of measuring the loss of gas within a package have been developed using IR spectroscopy. U.S. Pat. Nos. 5,614,718 &amp; 5,473,161 provide examples of this basic approach. Such systems, which measure the loss of gas within a package, have the disadvantage of requiring very long test-periods, generally of the order of weeks, in order to provide an accurate measure of permeation rate, because the proportional change within the package is normally very small, when, as in case of PET bottles, the package exhibits only very low permeation.  
       [0008] Many available systems, can only measure permeation rate on gas-filled packages, and this is itself a compromise, when a measurement of shelf-life is required. For shelf-life, it is better to measure the permeation rate of one or several components of the product, whilst the package is filled normally with product and also sealed with its normal closure. The reason for this is that the product, in combination with all the permeating gases, which normally include water vapour, may interact and result in a different permeation effect than that measured in a package which is filled only with gas. Further, the package&#39;s closure can affect gas permeation. It is often desirable to test product-filled packages taken direct from a production line, as this gives a “real life” assessment. Most available systems do not permit this.  
       [0009] Especially when using the permeation results to compute shelf-life, it is often desirable to subject a product-filled package to selected ambient conditions of temperature and pressure, and most available systems either do not have this facility at all, or make its provision complex. The need may be explained by the fact that some products, particularly carbonated beverages, exert a significant in-package pressure, resulting in package expansion. In-package pressure can affect the basic barrier property of a packaging material (eg of a coated or multi-layer bottle), because package-wall expansion can affect the integrity of thin coatings or layers within its structure, whilst the barrier property even of single-material structures can also be affected by stretching.  
       [0010] Additionally, increased ambient temperature increases the vapour pressure of volatile components in the packaged product, and this in itself results in higher permeation rates, even in non-pressurised packages. Furthermore, package expansion can affect the product&#39;s shelf-life by permitting the additional passage of volatile components from the product into the package&#39;s headspace. Finally, package expansion is not only dependent on in-package pressure, because ambient humidity can also affect the degree expansion of a pressurised package, such as a PET-bottle containing a carbonated beverage, because humidity can affect the packaging material&#39;s Young&#39;s Modulus.  
       [0011] For shelf-life computation, it is important to include the effect of absorption of product components within the package&#39;s walls, as well as the effect of package expansion. For example, in the case of carbonated beverages packed in PET-bottles, a significant amount of carbon dioxide is “lost” due to absorption in the bottle walls, and a further amount is “lost” into the headspace. Although theses are not entirely non-reversible losses, they can still affect shelf-life. Existing systems do not adequately cover these effects.  
       [0012] Shelf-life and permeation measurement systems must provide the flexibility needed in development, as well as enable in quality-control monitoring, and most existing systems do not provide this. For package development purposes, it is desirable to be able to measure gas-filled packages, as well as product-filled packages, because it is often necessary to differentiate the permeation effects. In development, it is also sometimes necessary to measure wall-sections or film. Both for development and for production-line quality-control, it is normally necessary to have the ability to measure many samples per working day, and to obtain results quickly, accurately and with minimum operator intervention.  
       [0013] The output of existing systems is often limited, for example because some systems necessitate that the test-package remains connected to the measuring apparatus for the entire permeation period, until a result is obtained. U.S. Pat. Nos. 5,792,940 &amp; 6,116,081 provide an example of this. Additionally, these patents cover a system that can only test bottles filled with a single gas, and cannot test product-filled samples, further restricting flexibility. A further example, with similar disadvantages, is PCT/EP00/13139 (W/O 01/48452), which furthermore needs change-parts and re-establishment for each package size, involving a lot of cost, operator intervention and downtime. Finally, the said system suffers from inaccuracy, because the measurement is affected not only by permeation but also by spurious factors, such as bottle expansion.  
       [0014] It is often necessary, particularly with coatings, to compare the change in permeation properties of a package, either filled with gas or product, through various stages of handling, in the production line or in the market. Many systems (eg the above-mentioned U.S. Pat. Nos. 5,614,718 &amp; 5,473,161, and PCT/EP00/13139:W/O 01/48452) do not permit this, since such systems demand that each permeation measurement must be carried out on a freshly-filled package.  
       [0015] In summary, existing permeation-testing technologies have one or more of the following inherent limitations:  
       [0016] slowness and/or inaccuracy when measuring low permeation rates.  
       [0017] high operator involvement and skill;  
       [0018] inability to measure both gas-filled and product-filled packages.  
       [0019] inability to measure packages, which are under internal pressure.  
       [0020] inability to measure normally-closed packages;  
       [0021] inability to measure un-opened packages direct from production line;  
       [0022] inability to measure many samples per day, without relatively high expense in equipment;  
       [0023] inability to simultaneously measure the permeation of several permeating components;  
       [0024] inability to measure a wide range of package sizes, without change-parts (and accompanying cost, downtime, operator involvement, etc);  
       [0025] inability to relate permeation results to a shelf-life, taking into account all factors, including in-package absorption and headspace expansion; and  
       [0026] inability to measure changes in permeation properties of a package through various stages in handling, without re-filling at each stage.  
       SUMMARY OF THE INVENTION  
       [0027] This invention encompasses a method for the measuring permeation and shelf-life-preserving characteristics of packages, which is quick and applicable to a wide range of package sizes without change-parts. At least one embodiment can test un-opened packages direct from the production line. Preferred embodiments can measure multiple relevant permeating components of the product simultaneously. Preferred embodiments of the method are self-checking, require little operator intervention and training, and can be applied at relatively low cost to give results for a high volume of test-samples per day. In a preferred embodiment, pre-filled packages containing a test-substance, which can be either the product itself or a simulating substance, are inserted in one cell, or a series of cells, and permeating gases, which collect in said cells, are circulated past one, or several, devices for measuring the content of each gas. Preferred embodiments include means of purging air and gases from previous measurements, before each measurement cycle.  
       [0028] In accordance with a preferred embodiment of this invention, a package, filled with a test substance is placed within a test cell. The packaged test substance can be pressurised or un-pressurised, and at choice either closed by the normal closure or by a non-permeable closure, so as to determine the closure&#39;s effect. The test cell is fitted with one or a multiplicity of measuring devices, whereby the measuring device/devices, can accurately measure the quantity of permeating gas in the space between the cell walls and the exterior of the package placed inside the container. For example, in preferred embodiments, the measuring device can be an infra-red (IR) device for measuring CO2 and H2O content, or a surface-active probe (lambda probe) for measuring O2, or a flame-ionisation detector, or a mass-spectroscope, or other means specific and sensitive to the permeating gas. Where the package volume does not change and only one permeating gas is involved, a simple pressure gauge can be used to monitor permeating gas content. In some preferred embodiments, it is normally convenient to install the measuring devices in a piped circuit around the container, as described later herein.  
       [0029] In one preferred optional embodiment, a container is filled, either with a gas, or a mixture of gases or other simulating material, or the product itself, and sealed, before being put into the test cell. In another embodiment, a container can be fixed and sealed against one part of the cell, permitting the permeating gas to be supplied from an external, pressure-regulated gas source, so as to maintain in-container vapour pressure throughout the permeation measurement period. In this second embodiment, the container may be filled with a test substance including a permeating gas, or the product itself. In a third embodiment, one or more measurement devices can measure gas migration into the package (eg oxygen from the gases in the space between the cell walls and the container&#39;s outer skin). The basic principle defined herewith will enable the same apparatus to be simply converted between the above-mentioned embodiments so as to provide flexibility.  
       [0030] The rate of permeation can be measured either by a single measurement of total permeated quantity over a finite time-period, which is the preferred method, or by multiple measurements of permeated quantity at short time-intervals and computing the slope of the quantity/time curve. Where the permeating gas species is significantly absorbed by the package walls, as in case of carbon dioxide gas absorption in PET packages, permeation rate measurement starts after equilibrium absorption has taken place, when the permeation rate stays essentially constant over short periods.  
       [0031] For calculation of shelf-life with respect to gases permeating out of the package, the effect of package expansion, which can lead to losses of permeating gas into the package head-space, and the effect of package-wall saturation must be measured, where these effects are significant. This measurement need only be done once for each package type/design. The measurement is carried out by filling the package with gas at a pressure, which reflects normal package pressure, and then measuring the pressure-loss during the period of package expansion and wall-absorption. This proportional loss must then be deducted from the original package content, giving a net content after the initial post-filling period.  
       [0032] Therefore, in one embodiment of this invention, a method is disclosed for measuring the gas permeation and shelf-life of a packaged product by using of at least one packaged test substance comprising a sealed container and a test substance disposed in the container. The method comprises the steps of:  
       [0033] stabilizing the at least one packaged test substance so that the test substance permeates at a permeation rate out of the sealed container and the permeation rate of the test substance out of the sealed container is substantially free of effects of package expansion and saturation of container walls;  
       [0034] placing the at least one packaged test substance inside a first cell;  
       [0035] closing the first cell and displacing any air or unwanted gases from the first cell with a carrier gas different from the test substance;  
       [0036] filling the first cell with the carrier gas so that the carrier gas contacts the at least one packaged test substance and mixes with any of the test substance that permeates from the sealed container to form a gas mixture comprising the carrier gas and an amount of permeated test substance;  
       [0037] holding the at least one packaged test substance and the carrier gas in the first cell for at least a period of time sufficient for measurable permeation of the test substance out of the sealed container to occur;  
       [0038] thereafter, analysing the gas mixture to determine the amount of permeated test substance; and  
       [0039] determining the permeation rate of the test substance out of the sealed container based on analysis of the gas mixture.  
       [0040] Preferably, in the embodiment described hereinbefore, the step of stabilizing is conducted remotely from the first cell. Also preferably, the method includes the step of calculating the permeation rate of the test substance through the at least one packaged test substance when the sealed container is first filled with the test substance and sealed based on the permeation rate of the test substance as determined in the step of determining the permeation rate of the test substance out of the sealed container based on analysis of the gas mixture. Still more preferably, the method of this embodiment can include calculating shelf life of the at least one packaged test substance based on the permeation rate of the test substance as determined in the step of determining the permeation rate, a calculation of the permeation rate of the test substance through the sealed container when the sealed container is first filled with the test substance and sealed based on the permeation rate of the test substance as determined in the step of determining the permeation rate, and data on expansion and gas absorption characteristics on the at least one packaged test substance.  
       [0041] According to anther embodiment of this invention, a method is disclosed for measuring the gas permeation and shelf-life characteristics of at least one container having an opening, the method comprising the steps of:  
       [0042] placing the at least one container inside a first cell having a gas inlet and a gas outlet so that the at least one container is fixed and sealed to the first cell so as to seal an interior of the container from a space inside the first cell between the first cell and the at least one container;  
       [0043] closing the first cell;  
       [0044] filling the at least one container with a test substance and displacing any air and unwanted gases from the at least one container;  
       [0045] stabilizing the at least one container so that the test substance permeates at a permeation rate out of the at least one container and the permeation rate of the test substance out of the at least one container is substantially free of effects of package expansion and saturation of container walls;  
       [0046] displacing any air or unwanted gases from the first cell with a carrier gas different from the test substance;  
       [0047] filling the space in the first cell with the carrier gas so that the carrier gas contacts the at least one container and mixes with any of the test substance that permeates from the at least one container to form a gas mixture comprising the carrier gas and an amount of permeated test substance and displacing any air and unwanted gases from the first cell;  
       [0048] holding the at least one container and the carrier gas in the first cell for at least a period of time sufficient for measurable permeation of the test substance out of the at least one container to occur;  
       [0049] thereafter, analysing the gas mixture to determine the amount of permeated test substance; and  
       [0050] determining the permeation rate of the test substance out of the at least one container based on analysis of the gas mixture.  
       [0051] According to still another preferred embodiment, a method for measuring the gas permeation and shelf-life characteristics of at least one container having an opening, the method comprising the steps of:  
       [0052] placing the at least one container inside a first cell so that the at least one container is fixed and sealed to the first cell so as to seal an interior of the container from a space inside the first cell between the first cell and the at least one container;  
       [0053] closing the first cell;  
       [0054] filing the at least one container with a carrier gas different from the test substance and displacing any air and unwanted gases from the at least one container;  
       [0055] filling the space in the first cell with a test substance and displacing any air and unwanted gases from the first cell;  
       [0056] stabilizing the at least one container so that the test substance permeates at a permeation rate through the at least one container and the permeation rate of the test substance through the at least one container is substantially free of effects of package expansion and saturation of container walls;  
       [0057] displacing from the at least one container with the carrier gas any test substance that entered the at least one container during the step of stabilizing;  
       [0058] holding the at least one container in the first cell and the carrier gas in the at least one container for at least a period of time sufficient for measurable permeation of the test substance into the at least one container to occur at a permeation rate and form a gas mixture comprising the carrier gas and an amount of permeated test substance;  
       [0059] thereafter, analysing the gas mixture to determine the amount of permeated test substance; and  
       [0060] determining the permeation rate of the test substance into the first container based on analysis of the gas mixture.  
       [0061] Other features of preferred embodiments of this invention will be appreciated from the following detailed description of embodiments and claims. 
     
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
     [0062]FIG. 1 is a schematic representation of one embodiment of this invention, for measuring the permeation out of a package, which is pre-filled with a test substance.  
     [0063]FIG. 2 is a graphical presentation illustrating how results from measurements made using the embodiment in FIG. 1 may be analyzed to give permeation rate and barrier improvement in relation to a reference package.  
     [0064]FIG. 3 is a graphical presentation illustrating how supplementary data, which is needed together with permeation rate to calculate shelf-life, may be obtained.  
     [0065]FIG. 4 is a schematic illustrating another embodiment of this invention for measuring permeation of a package that can be filled within the permeation-measuring equipment.  
     [0066]FIG. 5 is a schematic illustrating still another embodiment of this invention for measuring permeation into a package.  
     [0067]FIG. 6 is a schematic illustrating another embodiment of this invention for measuring permeation of a flat film. This embodiment can be used in conjunction with the methods and equipment described for packages.  
     [0068]FIG. 7 is a schematic illustrating a preferred embodiment of this invention for rapid measurement of multiple package samples, giving permeation rate, barrier improvement against a reference and shelf-life, for a wide range of package sizes, without need to change the basic equipment.  
     [0069]FIG. 8 is a schematic of an extension to the apparatus in FIG. 7, to avoid ingress of atmospheric gases into the circuit of the measuring system.  
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
     [0070]FIG. 1 illustrates one embodiment of the present invention for measuring the permeation rate of gas through a sealed package  10  comprising a container  12  sealed with a closure  14 , or alternatively, without a closure such as with a package that is a sealed pouch. The container is filled with a test substance  16  for testing. The test substance  16  can be a composition for storage in the package, such as a carbonated or non-carbonated beverage or other test substance, or simply a test gas by itself, or a composition that includes a test gas as a component of a more complex mixture such as a beverage. The test gas is a gas or mixture of gases that permeates the package  10  and can be any permeating gas or mixture of gases. For example, the test gas can be a component of a more complex composition such as carbon dioxide in a carbonated beverage, a single gas such as carbon dioxide, oxygen, water vapor or another gas, a gas mixture including combinations of carbon dioxide, oxygen, and water vapor and the like, a gas such as oxygen whose entrance into the package can contaminate or spoil the content of the package, or alternatively a test-gas such as helium or water vapor or the like whose permeation characteristics can be related to the characteristics of shelf-life determining permeating gases. In the latter example, the test gas simulates another gas or composition. Such a simulating test gas has a permeation rate indicative of the shelf-life of a test substance that may include the simulating test gas or may not include the simulating test gas at all.  
     [0071] The package  10  is disposed in a permeation testing device  18  comprising a cell  20  fitted with a purge gas inlet  22  and purge gas outlet  24 . Optionally, an internal displacer  26  may be disposed in the cell  20  for reducing the amount of space between the package  10  and the cell  20 . The permeation testing device is also fitted with a gas measurement device  28  for analysing the gas content.  
     [0072] The container  12  can formed of any material but is preferably a plastic container. The permeation testing device  18  and the other embodiments disclosed herein are particularly suited for testing PET bottles. Most preferable, the embodiments of this invention are useful for testing beverages such as carbonated beverages packaged in PET bottles.  
     [0073] In operation, the permeation tester  18  in FIG. 1 tests the permeation rate of the package  10  when the package is filled with the permeating test substance  16 . After filling, the package container  12  is closed and sealed by closure  14  such as a cap. The test substance  16 , when gaseous, can be filled into the package container  12  by weighing a predetermined quantity of the test substance in its solidified or liquid form (at low temperature), which then forms a gas after closing, when temperature rises to ambient. For example, in case of carbon dioxide, a quantity of dry ice can be weighed into the package container  12  and later allowed to form a gas after the closure  14  has been applied. When measuring the permeation of a gas from the inside of package  10  to the outside, it is important to purge the inside of package container  12  with the same gas, such as carbon dioxide, to ensure that air or other gases in the package can be displaced and expelled.  
     [0074] The package  10  is placed in the cell  20 , which can be opened to admit the package  10  and re-closed. The opening/re-closing and sealing features of cell  20  are conventional and not shown. A gas-space  30 , between the outside of the package  10  and the inside of cell  20 , is filled with a carrier gas, which is normally inert and different than the test substance  16 . For example, when the test substance  16  is carbon dioxide, the gas-space  30  is usually filled with nitrogen. It is important to ensure that no traces of the test substance  16  are present in the gas-space  30  before testing begins, and this is achieved by purging the gas-space  30 , using the inert carrier gas chosen to fill the gas-space  30 , so as to displace all detectable traces of air or other gases. The purge gas inlet  22  and outlet  24  are used for purging. In certain cases, the cell  20  can be fitted with the internal displacer  26 , so as to minimize the volume of the gas-space  30  and enhance the sensitivity of measurement of permeation into the gas-space  30 . However, the internal displacer  26  is not normally needed when sensitive gas-measurement equipment is used.  
     [0075] The gas measurement-device  28  connected to the cell  20  can detect and quantify the test substance  16 . The measurement-device  28  can use any state-of-art method that measures the quantity of the test substance  16  permeating into the gas-space  30 . The simplest of such measurement methods is a pressure gauge, but this is non-specific to the gas species of the test substance  16 , and therefore only applicable when a single, pure test substance is used. When more than one gas species can permeate into the gas-space  30 , whether the presence of more than one gas is intentional, such as when the test substance  16  is a gas mixture, or unintentional, such as when other gases are naturally dissolved in the walls of package container  12 , a measurement device  28 , or a plurality of devices in case of several gases, must be used, whose method is specific to the gas or gases measured.  
     [0076] A state-of-art measurement method, which is specific to many gas species, is IR absorption, and this can analyse the quantity of each of several gas species in a gas mixture. Other state-of-art methods, whose application depends on gas species, include UV-absorption, conductivity/resistivity or paramagnetism probes, flame ionisation devices, or mass spectroscopes. The measurement method used for the measurement device  28  will depend on the test substance  16 . When the test substance  16  is carbon dioxide, or water, or mixture of both, the measurement of IR absorption, using the FTIR method (Fourier Transform IR=FTIR) is effective. For oxygen, a surface-active resistance probe (eg lambda probe) can be used. Where the measurement method for the measurement device  28  is complex/expensive, it is often advantageous to locate the measurement device in an external circuit, rather than mount it directly onto the cell  20  (as described below).  
     [0077] To test the package  10 , the package container  12  is filled with a predetermined quantity of the test substance  16 , then closed and sealed. The package  10  is inserted into the cell  20  and the lid of cell  20  (not shown), which is necessarily opened to admit the package  10 , is closed to commence testing. Normally, the test substance  16  will be absorbed to some degree within the walls of the package container  12 , and it is therefore necessary to delay insertion into the cell  20  until the walls of the container have been saturated with the test substance  16  (as described in further detail below). After inserting the package  10  in the cell  20  and closing/sealing the cell, the gas-space  30  is purged with a suitable gas, such as nitrogen, so as to remove all traces of the test substance  16  in the gas-space  30 . When this purging is complete and the purge-connections  22  and  24  are closed, the concentration of the test substance  16  in the gas-space  30  begins to rise, due to permeation of the test substance into the gas-space. The test method consists basically of measuring the total amount of the test substance  16 , which permeates into the gas-space  30  over a pre-determined period of time. Because the purge connections  22  and  24  are closed, during permeation measurement, the permeation tester  18  is a closed system and the permeated test substance is allowed to accumulate in the gas-space  30  while the amount of inert carrier gas in the gas-space remains constant.  
     [0078]FIGS. 2 and 3 show in principle how the permeation test results may be analyzed. FIG. 2 shows a typical change of permeation-quantity  40  (as would be measured outside the package  10  over the total time shown) against the permeation-time  42 . The change in permeation-quantity  40  is shown both for a reference-package  10 A and for a test-package  10 B, and applies particularly to the permeation of a gas such as carbon dioxide out of a PET-bottle, where the gas absorbs significantly in the bottle walls and where the said walls can also expand under pressure. Permeation-quantity increases slowly until the walls of the package  10  have been saturated by the test substance  16  and, where the package is under internal pressure, also until the expansion of the walls of package has virtually ended.  
     [0079] Time-point a, for reference-package  10 A, and time-point b, for test-package  10 B, indicate the time-point when said wall-absorption and wall-expansion effects are approximately ended and the permeation rate of the test substance out of the package can be measured free of these effects. These are points of maximum absorption of the test substance in the walls of the container (or saturation of the container walls with the test substance) and maximum volumetric expansion of the container  12 . After said time-point a and b, a true measure of permeation-rate (PR) can be obtained by measuring the increase in permeation-quantity  40  with permeation-time  42 , free of package-expansion and wall-absorption effects. Since the time-elapse denoted by 0-a and 0-b in FIG. 2 is very much greater than the time needed to measure PR, one advantage of the present invention is the ability to store the package  10 , after filling, outside the measuring system, for example, in a conventional pre-conditioning cabinet (not shown), and to insert the package  10  in the cell  20  only after time-points a or b. This increases the capacity of the permeation tester  18 , because it eliminates waiting time. A further advantage is that the said conventional pre-conditioning cabinet can be both temperature and humidity-controlled, so that the subsequent permeation-measurement in permeation tester  18  can more closely approximate to real market conditions.  
     [0080] The absolute PR (ie PR measured in terms of volume or weight per unit time) out of the package  10  is highest when the package is full of the test substance  16 , and gradually reduces as the quantity of said test substance in the package drops due to permeation-loss. This is because the driving-force for permeation out of the package  10  reduces as the quantity of test substance  16  in the package is reduced. Therefore the lines of permeation-quantity  40  versus permeation-time  42  in FIG. 2 are approximately linear, and normally, significant signs of non-linearity appear only after a period of at least days, usually weeks, depending on the material of the package  10  and the type of test substance  16 .  
     [0081] In the present invention, measurement of permeation-quantity  40  takes place over a measurement-time  44 , which ranges from less than 5 minutes to less than 3 hours, depending on the material of the package  10  and the type of test substance  16 . During this measurement time, the package  10  is held in the cell  20  for at least a period of time sufficient for measurable permeation of the test substance out of the package to occur. During such a relatively short measurement-time  44 , the lines of permeation-quantity  40  versus permeation-time  42  are linear in practical terms. Therefore, just 2 measurement points are needed, one at the start of measurement-time  44  and one at the end of said measurement-time, as denoted by time-points c and d respectively in FIG. 2. The change in permeation-quantity  40  measured over measurement-time  44  gives the absolute PR at a time-point that is mid-way between time-points c and d.  
     [0082] Although absolute PR out of the package  10  varies with permeation-time  42 , as the quantity of test substance  16  in the package is reduced, the relative PR (ie the permeation rate expressed as a proportion of any quantity of test substance  16  in the package) stays constant. For the reasons explained hereinbefore, the measurement-time  44  must be after time-points a and b for packages  10 A and  10 B respectively. It is convenient to measure packages  10 A and  10 B, both at same time and at a time-point, which is safely past time-points a and b. It is then necessary to correct the absolute PR measured, so that it can be expressed in terms of the known initial quantity of the test substance  16  in the package  10  at time-point 0, or so that it can be expressed in terms of relative PR.  
     [0083] The absolute PR actually measured between time-points c and d (when the permeation-quantity in the package  10  is unknown and less than said quantity known to have been filled at time-point 0) can be used to calculate the absolute PR at time-point 0, or the relative PR, which is constant for all time-points. The permeation characteristic (ie the relative PR) is a constant function of the material of the package  10 , and can be regarded as a fixed, constant resistance to permeation. The actual PR at any time-point varies only because of the varying driving force through said fixed resistance, which is equal to the quantity of test substance  16  in the package  10 , assuming that the quantity of the test substance outside the package is relatively negligible.  
     [0084] Therefore, it can be shown mathematically that:  
     
       P 
       t 
       =P 
       0 
       .e 
       −ct  
     
     [0085] Where:  
     [0086] P t =vapour pressure or quantity of test substance  16  at any time-point t.  
     [0087] P 0 =vapour pressure or quantity of test substance  16  at time-point 0.  
     [0088] c=PR (permeation rate, expressed as proportion of test substance lost per unit time)=a constant.  
     [0089] t=time (eg in days) since filling a known quantity of test substance  16  into the package  10 .  
     [0090] With this relationship, a measurement reasonably close to time-point a or b can be referred to time-point 0, when the quantity of test substance  16  in the package  10  was known. This enables PR to be measured with the main and preferred principles of the present invention, which are demonstrated by FIG. 1. In summary, said principles apply to permeation of the test substance  16  out of the package  10 , whereby the test substance either exerts a vapour pressure above atmospheric (preferable, because this gives most rapid results), or is at atmospheric pressure, and also whereby in-bottle-wall absorption of the test substance, as well as bottle-wall expansion are factors to be considered. In particular embodiments, this applies to carbonated or still beverages packed in PET-bottles, where, even for still beverages, a certain internal vapour pressure from an inert gas is always needed to give bottle stiffness. Although in practice, oxygen permeates into, rather than out of a package, it is convenient and valid to measure all permeations, including oxygen, from inside the package  10  to outside. Where wall-expansion of the package  10 , or in-wall absorption of the test substance  16 , as hereinbefore described, do not apply, the same measurement and calculation principles continue to be valid, but are simplified by the elimination of these effects.  
     [0091] The time-elapse to time-points a and b is the time needed to stabilize the package  10  with respect to secondary factors, such as in-wall absorption and wall-expansion, which influence the permeation measurement outside the package and prevent measurement of the PR of the material package. This said stabilization-time will be denoted ST hereinafter. The ratio calculated by dividing measured PR for the reference-package  10 A with measured PR for the test-package  10 B represents the improvement in permeation-resistance of the test-package  10 B compared with the reference-package  10 A. This ratio is often referred to as the “barrier improvement factor” (BIF), and is of critical interest when assessing the effectiveness of different barrier treatments for the package  10 .  
     [0092] For measuring PR or BIF on a repetitive basis and for many samples, ST must first be measured for both the reference-package  10 A and the representative test-package  10 B. The PR of the package  10  is a major factor affecting ST, and ST is high when the package  10  has a high BIF, low when the package  10  has a low BIF. However, in practice, ST is relatively short and varies only to a limited extent between the highest and lowest BIF characteristics of the package  10 . Therefore, ST need only be measured once for the reference-package  10 A and once for the test-package  10 B, and the starting time-point for all subsequent measurements of BIF can be chosen such as to provide a reasonable margin, so as to be sure that both said packages are well past their ST (ie after allowing for a “safe” ST, which takes into account the whole range of potential differences in BIF).  
     [0093] In order to calculate shelf-life with respect to the test substance  16 , the hereinbefore described effects on permeation-quantity  40  of in-wall absorption of the test substance  16  and wall-expansion of the package  10  must be separately measured for a given basic package design. In practice, these said effects are influenced by the weight, shape, wall-thickness and other dimensions of the package  10 , and the influence of said effects is therefore constant, when—for example—repetitive samples of same design of the package  10 , with different barrier treatments must be measured. Therefore, for a given package design, the said effects can be evaluated on a one-time basis and used to calculate shelf life for different barrier-treated versions of the package  10 , as follows:  
                                                  maximum allowable loss, expressed as fraction           of initial amount of test substance 16 inside           the package 10, from standpoint of acceptable           shelf-life = q           fraction of initial amount of test substance 16           lost from inside of the package 10, during ST due to           package-wall expansion and in-wall absorption = y           net fraction of initial amount of           test substance 16 which can be lost by           permeation to outside the package 10 from           standpoint of acceptable shelf-life = q − y = z           measured permeation PR, expressed           as fraction per day of quantity of the test-           substance 16 in the package 10 at given time-point = p/week           shelf-life with respect to test substance 16 = n days                      
 
     [0094] It can then be shown that:  
       n= log( z+ 1)/log( p+ 1)  
     [0095]FIG. 3 shows in principle how the fraction y of the initial amount of the test substance  16 , which is lost from inside of the package  10  during ST due to package-wall expansion and in-wall absorption, can be measured for a given package design. The other factors in the above equation, giving n, are already known, because z is a function of y and the permissible fraction q (from quality standpoint) and p is equal to PR, as measured with the principles described hereinbefore.  
     [0096] In FIG. 3, the variation of in-package quantity-fraction  46  is shown against permeation time  42 . The quantity-fraction  46  is the quantity of test substance  16  in the package  10  expressed as fraction, where the starting quantity=1. The quantity-fraction  46  can be measured by simple means, for example by installing a pressure-gauge on the package  10  after filling and measuring the quantity-fraction until the ST has been exceeded sufficiently to measure the slope  48 . When the slope  48  is extrapolated to time-point 0, an intercept  50  is obtained. The quantity-fraction  46  at time-point 0 is the start-fraction  18  (=1). The intercept  50  represents the “start-fraction”, which would have applied, if the effects needing ST were not present. Therefore, the quantity-fraction y, due the effects needing ST, is the quantity-fraction denoted y in FIG. 3.  
     [0097]FIG. 4 illustrates a permeation tester  58  that is a modification of the tester  18  shown in principle in FIG. 1. The modified permeation tester  58  comprises a cell  20  including a headpiece  60  at the upper portion of the cell. The headpiece  60  has a fixture  62  for receiving the container  12  of the package  10 . Particularly, the package container  12  has a mouth  64  defining the opening of the container and the fixture  62  of the headpiece  60  receives the mouth of the container. The mouth  64  of the container  12  is sealed to the fixture  62  by an O-ring  66  disposed in the fixture. The container  12  is fixed and sealed to the cell  20  so as to seal an interior of the container from the space  30  inside the cell between the cell and the container.  
     [0098] An external regulated gas supply  68  feeds gas into the container  12  through a dip tube  70  extending through the headpiece  60  of the cell  20  and the mouth  64  of the container  12 . A gas supply valve  72  controls the supply of gas from the external regulated gas supply  68  to the interior of the container  12 . The headpiece of the cell  20  is fitted with a purge  74  for purging gas from the interior of the container  12 . A pressure gauge  76  monitors pressure of gas fed into the container  12  by the external regulated gas supply  68 .  
     [0099] In operation of the modified permeation measuring system  58 , the package  10  can be filled with a gaseous test substance  16  after being inserted in the cell  20  with the external regulated gas supply  68 , whereas in FIG. 1, the package is placed in the cell after filling. Filling of the package  10  within the cell  20  is achieved by fixing the container mouth  64  into the fixture  62  in the head-piece  60  of the cell, and then sealing the mouth with the O-ring  66  or a similar sealing device. The gas-supply  68 , containing test substance  16 , is piped by the dip-tube  70  to base of the container  12 . The purge  74  is provided at top of the package  10 , and purging of air from inside the container is carried out by allowing gas from the gas supply  68  to flow down the dip-tube  70  and flow out of purge, thus displacing air in the package  10 . After purging the inside of the container  12  has been completed, the gas supply valve  72  and purge  74  are shut off, and the gas-space  30  between the cell  20  and the package  10  is purged to displace air in the manner already described under FIG. 1. When purging of the gas space  30  has been completed, measurement of permeation characteristics (PR and BIF and shelf-life) can proceed as already outlined.  
     [0100] The system shown in FIG. 4 can be advantageous, because it enables the test substance  16  to be kept at constant pressure, as monitored by the pressure gauge  76 , for the entire measuring cycle. This constant gas-pressure compensates for permeation losses from the inside of the package  10 , and maintains a constant driving force for permeation. The pressure inside the package can be greater than atmospheric pressure. This constant-pressure mode can be advantageous for certain measurements, since PR remains constant even over extended periods, when the driving force for permeation is constant. When operating in this said mode, the gas supply  68  is fitted with a conventional pressure-regulator and the valve  72  remains open throughout the measurement period.  
     [0101] The disadvantage of this embodiment is that the package  10  must remain connected to the permeation measuring system  58  throughout the period of ST. This reduces the measurement output of the modified measurement system  58  compared with the possible output of a system based on FIG. 1, where the package  10  does not need to be connected to the measuring system until ST is complete. The said disadvantage becomes unimportant for package/test substance combinations with very short or negligible ST.  
     [0102]FIG. 5 illustrates a further modified permeation measuring system  80  using the principles described hereinbefore for measuring permeation characteristics (PR, BIF, shelf-life) when the test substance  16  permeates into the package  10 , whereas the systems described hereinbefore provide means for measuring said permeation characteristics when the test substance  16  permeates out of the package  10 .  
     [0103] The further modified permeation measurement system  80  has a similar structure to the permeation measurement system  58  illustrated in FIG. 4 and like reference numerals indicate like components in the Figures. In the further modified permeation measurement system  80 , a second gas supply  82  is connected to the cell  20  for delivering a gaseous test substance  16  to the gas space  30  between the cell  20  and the container  12 . A second valve  84  regulates the flow of the gaseous test substance  16  from the second gas supply  82  to the gas space  30 . In addition, a second gas measurement device  86  is fitted to the cell  20  for monitoring the gas in the gas space  30  between the cell  20  and the container  12 . This enables use of a second test substance  16   a  from the first gas supply  68 .  
     [0104] In operation of the embodiment  80  illustrated in FIG. 5, the package  10  is filled with an inert carrier gas from the first gas-supply  68 , in the same basic manner as already described for FIG. 4, via the dip-tube  70  and using the purge  74 . The inert carrier gas-supply valve  72  is shut off when air has been purged out of the interior of the package container  12 .  
     [0105] The second gas supply  82  contains the test substance  16 , and therefore, in contrast to the method described in FIG. 1, the test substance  16  is placed in the gas-space  30  between the cell  20  and the container  12  and permeates into the package  10  from outside. Using the second gas-supply  82 , the gas-space  30  is purged via the outlet purge  24  so as to displace all traces of air from the gas-space and thus fill the gas-space entirely with the test substance  16 . Thereafter, either the stop-valve  84  remains open and a constant pressure from the second gas-supply  82  is maintained in the gas-space  30 , or the stop-valve is closed, where the change in pressure during permeation has a negligible effect on the measurement accuracy. The first gas measurement device  28  is used to measure the quantity of gas permeating into the package  10 , and permeation characteristics are measured by applying the same principles as described already in conjunction with FIG. 1.  
     [0106] If permeation of air into the package  10  under atmospheric pressure is to be measured, the base section of the cell can either be provided with apertures for air ingress, or eliminated altogether. However, higher-than-atmospheric pressure of the test substance  16  enables faster achievement of permeation results, and the cell also has the advantage of enabling use of oxygen and other gases, rather than air. If higher-than-atmospheric pressure of the test substance  16  is applied to the gas-space  30 , this must normally be balanced by an equal or greater pressure of inert carrier gas from the first gas-supply  68  inside the package  10 .  
     [0107] In FIG. 5, it is possible to fit the second gas measurement device  86 , which monitors the gas analysis in the gas-space  30 . This enables use of a second test substance  16   a  from the first gas-supply  68  instead of an inert carrier gas, so as to measure permeation into, and out of, the package  10  simultaneously. For example, the first test substance  16  could be oxygen or air whilst the second test substance  16   a  could be carbon dioxide. In common with the principle of the embodiment in FIG. 4, the principle of the embodiment in FIG. 5 requires that the package  10  is connected to the equipment during ST, which the principle of FIG. 1 avoids.  
     [0108]FIG. 6 illustrates a permeation measurement system  90  that applies the measurement principles of the embodiments illustrated in FIGS. 1, 4 and  5  when research work necessitates testing of a film or package wall, or other sheet like material, rather than complete packages. This embodiment  90  comprises a cell  92  including a top section  94  juxtaposed with a base section  96 . A test sample sheet  98  is disposed between the top section  94  and the base section  96  and sealed to the cell with O-rings  100  and  102  at each end of the cell  92 .  
     [0109] A first gas inlet  104  feeds a gaseous test substance  106  into a first compartment  108  between the base section  96  and the test sample sheet  98 . A first gas outlet  110  allows purging of the first compartment  108 .  
     [0110] A second gas inlet  112  feeds inert carrier gas into a second compartment  114  formed between the top section  94  of the cell  92  and the test sample sheet  98 . A second gas outlet  116  provides for purging of the second compartment  114 .  
     [0111] A gas permeation measurement device  118  is operatively associated with the second compartment  114  for measuring the PR of the test substance  106 .  
     [0112] In operation of the permeation measurement system  90  in FIG. 6, the flat test-sample  98  is clamped in the cell  92  between the top section  94  and the base section  96 . Both sides of the test-sample  98  are sealed by the o-rings  100  and  102 . The first gas inlet  104  and first gas outlet  110  enable air-purging and filling of the first compartment  108  with the test substance  106  (as hereinbefore described for a package in FIG. 5), whilst the second gas inlet  112  and second gas outlet  116  enable air-purging and filling of an inert carrier gas (as hereinbefore described for a package in FIG. 1) in the second compartment (or gas-space)  114 . The permeation measurement-device  118  then measures the PR of the test substance  106 , also as already described. Similar devices for flat film exist, but the system  90  in FIG. 6 can be used with the package-testing methods described with regard to the embodiments in FIGS. 1, 4 and  5 . This increases the flexibility of said systems, when applied to research and development.  
     [0113] Cell  92  can also be treated similarly to package  10  by being placed in the cell  20  of the embodiment  18  FIG. 1 or the embodiment  120  in FIG. 7. In this case, a large aperture (not shown) is inserted in the top section  94  of cell  92 , in place of the permeation measurement-device  118 , the second gas inlet  112  and the second gas outlet  116 . The top section  94  and the base section  96  form a flat film holder defining a holding space for the test substance on one side of the flat film and the open aperture defines a free surface on another side of the flat film so that the flat film holder can be placed in the first cell and the test substance can permeate from the holding space, through the flat film, and into the carrier gas in the first cell  20 . This leaves the surface of test sample  98  of flat film free to be measured as already described for the surface of the container  10 . This option constitutes a significant simplification in procedure, especially in conjunction with the embodiment of FIG. 7.  
     [0114]FIG. 7 shows a system  120  based on the principles of the embodiment  18  shown in FIG. 1. Each of a multiplicity of packages  10   a ,  10   b ,  10   c  . . . . to  10   n  are placed in cells  20   a ,  20   b ,  20   c  . . . to  20   n , so that each package is in its own individual cell. Where differentiating between the permeation characteristics of individual packages  10   a - 10   n  is unnecessary, it is also possible that each of the cells  20   a - 20   n  holds a multiplicity of packages. When only one package  10  is to be measured in each cell  20 , the number of cells would normally be at least 12, possibly far more, but for the sake of simple presentation, only 4 are shown in FIG. 7. Each of the packages  10   a - 10   n  contains the test substance  16 , as described above in conjunction with FIG. 1.  
     [0115] Each of the cells  20   a - 20   n  has a top-valve  122   a - n  mounted near top of the respective cell and a base-valve  124   a - n  mounted near the base of the respective cell. It is normally expected that each top-valve  122   a - n  and each base-valve  124   a - n  will be most conveniently connected to the sidewall of the respective cell  20   a - n , with each top-valve  122   a - n  close to the top of the respective cell, and each base-valve  124   a - n  close to the base of the respective cell. Connection of each top-valve  122   a - n  and each base-valve  124   a - n  to the sidewall of respective cells is a practical measure, because it leaves the top of the cells free for a quick-release lid, enabling easy access for inserting and removing packages  10 , and also leaves the base free for mounting the cells onto a back-plate. In FIG. 7, for the sake of simplicity of presentation, the top-valves  122   a - n  and base-valves  124   a - n  are shown mounted on the lid and base of the cells  20   a - n  respectively, since this does not affect the principles described herein. The top-valves  122   a - n  and base-valves  124   a - n  can be conventional 3-way devices (as shown), or consist of a system of more than 1 valve, when this fulfils the operations described hereunder. The top-valves  122   a - n  and base-valves  124   a - n  are conventionally motorized and controlled by a sequence-controller  126  (eg remote-controlled, solenoid-operated valves).  
     [0116] A regulated gas-supply  128 , supplying an inert carrier gas such as nitrogen, is connected via a gas-valve  129  to each top-valve  122   a - n  and to a pump  130 . Each top-valve  122   a - n  is also connected to the pump  130 , and the pump leads to one, or a multiplicity of gas measurement-devices  132   a ,  132   b , etc. The measurement-devices  132   a ,  132   b , etc, can detect and measure the quantity of each component of the test substance  16 . The test substance  16  can be a single gas (eg carbon dioxide) or a mixture of gases (eg carbon dioxide, oxygen, water), depending on the permeation and shelf-life data needed. The reason for using more than 1 measuring device  132   a ,  132   b  is to cover the range of components of the test substance  16 . For example, an FTIR (Fourier Transform Infra-Red) device can measure the quantity of carbon dioxide and water simultaneously, but a different device is needed to measure the quantity of oxygen, if this is also needed.  
     [0117] The measurement-devices  132   a ,  132   b , etc, are connected to the base-valves  124   a - n  so as to form a circuit around each cell  20   a - n . The pump  130  is preferably a metal bellows pump, or similar, so as to avoid possibility either of leaks or of absorption of gases in the pump&#39;s seal or other contact parts.  
     [0118] The base-valves  124   a - n  are connected via an exhaust-manifold  134  to an exhaust-gas flow-meter  136  which is connected to a control valve. A purge-valve  140  is connected to a vacuum-pump  142  for evacuating the gas content of each measurement-circuit, which consists of the pipe-work from each top-valve  122   a - n , through the pump  130  and measurement devices  132   a - b  to each container base-valve  124   a - n  and will be denoted MC. The pressure gain (if any) in the total-test-circuit, over a pre-determined time, is monitored by a pressure-gauge  144 .  
     [0119] A calibration valve  146  in communication with the measurement devices  132   a - b  via the pump  130  is connected to a coupling point  148 . A known-quantity injection-device (eg syringe) can be connected to the coupling point  148  and a calibration amount of each component can be injected by opening the calibration-valve  146 .  
     [0120] A computer  150  regulates the operation of the measurement-devices  132   a  and  132   b , registers the permeation quantities of each component of test substance  16  from each cell  20 , and monitors correct circuit-purging.  
     [0121] The cells  20   a - n  are located in a conditioning cabinet  152 . This cabinet  152  has a temperature-controlled interior, since ambient temperature affects permeation. The means of controlling temperature in the cabinet  152  are state-of-art and will not be discussed further.  
     [0122] In operation of the embodiment  120  shown in FIG. 7, packages  10   a - n  are placed in the respective cells  20   a - n  and the regulated gas-supply  128  supplies a stream of inert carrier gas via the gas-valve  129  and via top-valves  122   a - n  to each of the cells  20   a - n  to purge/displace air out of the gas-spaces  30   a - n  of said containers by exhausting the air, mixed with inert carrier gas, through the exhaust-manifold  134  and flow-meter  136  to the atmosphere. The purpose of said purging/displacement is to eliminate all significant traces of air in gas-spaces  30   a - n  and to replace the air with the inert carrier gas from the gas-supply  128 . The flow-meter  136  enables the flow of displaced gas to be controlled by the control-valve  138 , so as to secure effective purging/displacement within a pre-set time-elapse. The sequence-controller  126  controls the purging operation such that each of the cells  20   a - n  in turn is purged free of air in a predetermined time.  
     [0123] The purging of the gas-spaces  30   a - n  takes a finite time, and this is repeated for the gas-spaces of each cell  20   a - n  in turn. By the time the purge-sequence of all cells  20   a - n  has been completed, the first cell to have completed its purge-cycle has had sufficient time to collect adequate quantities of each component of test substance  16  to enable measurement-devices  132   a - n  to measure the quantity of each said component of test substance  16  that has permeated into the gas-spaces  30   a - n . One factor, which determines the number of cells  20   a - n  in the total system, is the desire to match total purging-time of all cells to the time needed to begin measurement of the first-purged cell.  
     [0124] Before measurement of the quantity of each component of test substance  16  can begin, each measurement-circuit, which consists of the pipe-work from each top-valve  122   a - n , through the pump  45  and measurement devices  132   a - b  to each container base-valve  124   a - n  must be purged completely free of air traces and traces of the gases from the last measurement cycle. This is carried out by opening the purge-valve  140 , which is connected to vacuum-pump  142 , and evacuating the gas content of said MC. Then purge-valve  140  is closed and the gas-valve  129  is opened, filling the said MCs with inert carrier gas from the gas-supply  128 . The cycle of evacuation/refilling of said MC may need to be repeated 2 or more times, till all traces of gas, other than the inert carrier gas from gas-supply  128 , have been expelled. This evacuation and inert carrier gas refilling of the MCs is denoted “circuit-purging” and is repeated before measuring permeation into the gas-space  30  for each cell  20   a - n . The adequacy of circuit-purging is monitored by measurement-device  132   a - n , which should read virtually zero after adequate circuit-purging.  
     [0125] The computer  150  regulates the operation of measurement-devices  132   a - b , registers the permeation quantities of each component of test substance  16  from each cell  20   a - n  and monitors correct circuit-purging. The sequence-controller  126  controls the operation of all valves shown in FIG. 7 (ie top-valves  122   a - n , base-valves  124   a - n , gas-valve  129 , exhaust-valve  138  and purge-valve  140 ), as also the operation of pump  130 , flow-meter  136  (and purge control via exhaust-valve  138 ) and vacuum pump  142 . The sequence-controller  126  regulates the time-elapse between completion of purging (ie start of permeation) and final measurement of permeated quantities, and said time-elapse is relayed to the computer  150 , enabling the computer to calculate PR, BIF and shelf-life, using the basic pre-set data and procedure described in conjunction with FIGS. 2 and 3.  
     [0126] The package  10  must be prepared before placing in a cell  20   a - n  by filling and pre-conditioning. Pre-conditioning involves storing the package  10  at same temperature as the cabinet  152  until ST has been reached or exceeded. Pre-conditioning involves keeping the package  10  in a pre-conditioner (not shown), which is a chamber with similarly controlled temperature as the cabinet  152 . It is desirable that pre-conditioning is in a humidity-controlled environment, because humidity, as well as temperature, can affect permeation. Pre-conditioning chambers with humidity and temperature control are commercially available. The capacity of a pre-conditioning chamber must allow for sufficient packages to be stored for the duration of ST, so as to supply the daily rate of testing by the system  120 .  
     [0127] Filling the package  10  can be optionally with a gas (eg carbon dioxide in a PET package), or a gas mixture (eg carbon dioxide, oxygen and water). The internal pressure of the package  10  during testing should normally reflect the internal pressure of the said package during market distribution at the test temperature. However, since the measurement system  120  can measure permeation characteristics (PR, BIF, shelf-life) at varying conditions, it is also possible to test at higher internal pressure in the package  10  for all or some of the components of the test substance  16 , and relate this to normal market conditions, whilst benefiting from the accelerated testing provided by higher internal pressures.  
     [0128] As partly described above, the filling procedure with gas can be by weighing into the package  10  the cooled solid or liquid form of the gas (eg dry ice for carbon dioxide), or by weighing reacting chemicals (eg oxygen generating chemicals). Alternatively, a stoichiometrically-prepared aqueous mixture of the test substance (eg carbonated water) can be used. Into the carbonated water, weighed-out oxygen-generating chemicals can be added, enabling the permeation characteristics of carbon dioxide, oxygen and water to be measured simultaneously. Mixtures of gas can also be filled into the package  10  by means of a tank (not shown), in which a plurality of packages  10  are placed. Known partial pressures of each test gas are filled into the tank, and the tank is provided with means of sealing the package  10  by applying a closure  3  without releasing pressure. This tank system can be built according to the above description by state-of-art means and will not be described any further.  
     [0129] In all cases, the quantity of each component of test substance  16 , which is filled into the package  10 , must be known with reasonable accuracy, because the measured PR relates to this starting quantity. The measurement system  120  can also be used to measure permeation characteristics (ie PR, BIF, shelf-life) of the package  10 , when this is taken direct from the test substanceion line (eg PET bottles from a carbonated beverage filling line), non-invasively.  
     [0130] Since leaks in the measurement system  120  can be a source of error, a self-checking facility, which can automatically be applied periodically (eg daily before testing begins), is included. Under the automatic control of the sequence-controller  126 , the entire circuit system, consisting of MCs, cells  20   a - n , and all associated interconnecting pipes (“total-test-circuit”), is placed under vacuum by opening the purge-valve  140  to communicate with the vacuum pump  142 . The purge-valve  140  is then closed, and the pressure gain (if any) in the total-test-circuit, over a pre-determined time, is monitored by pressure-gauge  144 . If a pressure-gain is detected, the sequence-controller  126  then proceeds to close off each cell  20   a - n  in turn, whilst repeating the procedure of evacuation and monitoring of pressure-gain, until the specific circuit, which has the leak, has been identified. In addition to leak-testing, the function of measurement-devices  132   a - b  is checked for every measurement batch by keeping one of the cells  20   a - n  empty and using the said measurement-devices to check whether the correct zero-point, based on measuring the empty cell, is maintained.  
     [0131] If a fault is detected, either due to a leak or due to a faulty-reading measurement-device  132   a - b , the computer  150  informs the operator automatically. Operator involvement is reduced to loading filled test-samples of packages  10  into cells a-n, closing the lid of the cells and pressing the “start button” on the sequence-controller  126 , whereupon the system goes through its checks, as described above, and proceeds to measure the PR in relation to each cell  20   a - n  in turn. The computer  150  carries out the calculation of BIF and shelf-life automatically, based on the input data already discussed.  
     [0132] It is important to ensure that each MC has an equal volume. Equal for each MC (ie each MC associated with each cell  20   a - n ) can be achieved in a number of ways, for example, by deliberately adjusting pipeline length from each cell  20   a - n  to/from measuring devices  132   a - b , or by arranging the cells such that said cells are equidistant from said measurement-devices (eg in a circle), etc. Since the measured values of measurement-devices  132   a - b  relate to the MC-volume, these values must be related to absolute permeation rate by calibration. Calibration is carried out by injecting a known amount of each component into one MC, and monitoring the value given by measurement-device  132   a - b . For this purpose, a known-quantity injection-device (eg syringe) can be connected to the coupling point  148  and a calibration amount injected by opening the calibration-valve  146 .  
     [0133] Since measurement-device(s)  132   a - b  are mounted in a circuit outside the cells  20   a - n , and are available to measure numerous containers (in contrast to the embodiment in FIG. 1, where each cell has its own, directly-mounted measurement-device), said measurement-device(s) can be selected for high sensitivity, without prejudicing the stated objective of measuring many packages  10  per day at relatively low cost. Additionally, in practice, it has been shown that measurement-devices, which are not directly mounted to a cell  20 , are sensitive enough to permit a cell to be sized so as to accept the largest package  10  in the package-range of interest, whereby the internal displacer  8 , as shown in FIGS. 1, 4 and  5  is also unnecessary. This enables the stated objective of providing a measurement capability for whole range of sizes of package  10 , without change-parts, or time-consuming adjustment.  
     [0134] According to the objects of the present invention, the system  120  delivers rapid results, within the constraints set by the natural physical parameters of the test substances and the material of package  1 . For example, when package  1  is a 0.5 l PET bottle and filled with carbonated test substance or water, containing 4 volumes of carbon dioxide, a ST of 6/7 days is needed at 38° C. As described hereinbefore, this ST takes place outside the permeation measurement system  120 , in a separate pre-conditioning chamber (state-of-art and not shown). After pre-conditioning (ie after elapse of ST), the package  10  can be installed in the permeation measurement system  120  for permeation measurement.  
     [0135] In the given example of the 0.5 l PET-bottle, permeation-time  13  to provide measurable permeation-quantity  40  is about 60-180 minutes (depending on characteristics of measurement-device  132   a - b ). Therefore, after 60-180 minutes, each cell  20   a - n  can be connected to a measurement-device  132   a - b  for measurement of permeation. Since the actual measurement of permeation-quantity  40  by a measurement-device  132   a - b  takes about 5 minutes, for the example given, the actual measurement-cycle in the permeation measurement system  120  takes between 1 and 3 hours. To said time of 1 to 3 hours must be added a few minutes for purging (ie displacing of air and unwanted gases, as described) of the MC, since said purging must be carried out between each measurement.  
     [0136] The purging of the cells  20   a - n  itself takes place while other cells are being measured, and is therefore not strictly additive to the measurement-cycle time. Therefore, the permeation measurement system  120  can deliver results in, say,  1  to 3 hours, after ST has elapsed (depending on test substance  16  and package materials). ST depends on natural, physical constraints, primarily temperature, pressure and type of permeating gas, and is the main factor, which determines the waiting time for results (although, as explained above, it has no influence on the capacity or the speed of system  120 ).  
     [0137] In most polymers, the permeation of water vapour is relatively fast, because it has a smaller molecule than other shelf-life determining gases, such as carbon dioxide or oxygen. Therefore, water can saturate the walls of the package  10  much more quickly, and reduce ST considerably. For example, when the package  10  is a PET bottle containing a carbonated beverage, the waiting time due to ST is much reduced for water compared with carbon dioxide, so permeation results can be obtained much earlier. Since the PR for water relates to PR for carbon dioxide, said water PR can be used as a quick-test to determine the permeation characteristics (PR, BIF, shelf-life) of carbon dioxide. The method of the permeation measurement system  120  is non-destructive, so one/two out of a large set of test-samples of packages  10  can be retested for carbon dioxide later, to cross-check that results on water can be safely extrapolated to give results for carbon dioxide. The measurement-device  132   a - b  can be the same for water and carbon dioxide, if an FTIR detector is used, further simplifying the option of using water as a quick-test for carbon dioxide.  
     [0138] The methods described with regard to the embodiment  120  in FIG. 7 demonstrate a method for automating and meeting the other objectives of the present invention, using the basic principle described with regard to the embodiment in FIG. 1. The said methods demonstrated by the embodiment  120  in FIG. 7 can also be applied to the basic principles of the embodiments in FIGS. 4 and 5 with similar advantages, if a particular application benefits from the said basic principles of the embodiments in FIGS. 4 and 5. The principal focus of the present invention is measurement of permeation characteristics (PR, BIF, shelf-life) of normally-pressurized PET-bottles, either filled with gas or carbonated or still test substance, so as to enable research or quality-control of barrier enhancing treatments.  
     [0139]FIG. 8 shows a modified version of the embodiment in FIG. 7. This embodiment includes a means of avoiding ingress of atmospheric gases into the test cells  20   a - d , or into the circuit-pipes between the test cells and gas measurement-devices  132   a  and  132   b , or into the said gas measurement-devices themselves. A gas cylinder  160 , complete with a pressure regulator  162  supplies an inert carrier gas  164 . When measuring oxygen in the permeation measurement system  120 , inert carrier gas  164  is normally nitrogen, but if nitrogen is being measured, then another inert carrier gas, which does not interfere with the measurements, must be used. The inert carrier gas  164  passes to an inert gas-valve  166  and then to a gas-distributor  168  within the cabinet or first enclosure  152 . The inert gas inlet valve  166  can also be switched to connect to a gas pressure-controller  170 , whereby this can be a conventional liquid-containing bubbler-tube (as shown), which maintains a gas-pressure equivalent to the bubbler-tube immersion height. The cabinet  152  is first purged to eliminate its air content by opening the inert gas inlet valve  166  and simultaneously opening an inert gas purge-valve  172 . The inert gas inlet valve  166  and the inert gas purge-valve  172  are interlocked, as shown diagrammatically in FIG. 8, so that they function in unison. During the period of purging of the first cabinet  152 , the pressure regulator  162  is opened to provide high gas flow, so as to reduce the purging time.  
     [0140] When air has been purged out of the first cabinet  152 , the inert gas purge-valve  172  is closed and the inert gas inlet valve  166  switched to connect with gas pressure-controller, whilst continuing to maintain the flow of the inert gas  164  to the first cabinet  152 . For this phase, the pressure regulator  162  is turned down to reduce the gas flow to that needed only to replace gas leakage from the first cabinet  152  and to maintain a small positive gas pressure in the first cabinet  152  (eg 10 cm water gauge). This said positive gas pressure helps to reduce ingress of air into the first cabinet  152  during the measurement cycle to a degree, which eliminates possible interference of air with the function of measuring-devices  132   a - b.    
     [0141] Where necessary, a a second cabinet or enclosure  174  can be placed around the measurement-devices  132   a - b , and the associated pipe and valves. The air content in this second enclosure  174  can be purged, using an inert gas inlet valve  176 , the gas distributor  177 , and an inert gas purge valve  178 , in the manner already described for the first cabinet  152 . The inert gas inlet valve  176  can also be switched to connect to a gas pressure-controller  180 , such as a conventional liquid-containing bubbler-tube, which maintains a gas-pressure equivalent to the bubbler-tube immersion height. During the measurement cycle, a small positive pressure of inert gas  164  can be maintained in the second enclosure  174 , again as already described for the first cabinet  152 .  
     [0142] Accordingly, preferred embodiments of this invention address the above-described limitations of existing systems and particularly provide one or more of the following:  
     [0143] accuracy and speed in obtaining results, particularly for low permeation rates;  
     [0144] ability to measure both gas-filled and test substance-filled packages, as well as unopened packages direct from the production line;  
     [0145] ability to measure pressurised or un-pressurised packages;  
     [0146] simple/inexpensive means of handling many samples per working day with minimal operator intervention and low operator skill;  
     [0147] means of measuring the permeation of several components, particularly CO2, O2, H2O, either simultaneously, or separately;  
     [0148] means of relating permeation to a shelf-life;  
     [0149] means of measuring shelf-life under varying ambient conditions;  
     [0150] means of determining the effect on shelf-life of each permeating component, and also the effect of the package&#39;s normal closure;  
     [0151] means of measuring a wide range of package sizes, without change-parts; and  
     [0152] means of investigating permeation changes through several stages of handling, without re-filling the package.  
     [0153] It should be understood that the foregoing relates to particular embodiment of the present invention, and that numerous changes may be made therein without departing from the scope of the invention as defined by the following claims.