Vacuum package tester and method

A vacuum package tester is capable of providing data by which the internal pressure and volume of residual gas of a flexible wall vacuum package exposed to atmospheric pressure may be calculated. A container partially filled with liquid is surrounded by a deformation-absorbing enclosure. A vacuum pump reduces the pressure in the head space above the liquid, thereby causing a submerged package to expand. Pressure gauges record the respective pressures in the head space when the package expands by first and second known volumes. From this data, the internal pressure and volume of residual gas of said package, when exposed to ambient pressure and temperature, may be calculated. In use, the invention also performs an improved method.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the field of vacuum package 
testers, and more particularly to improved apparatus capable of providing 
data by which the internal pressure and volume of residual gas of a 
flexible wall vacuum package, exposed to a known external pressure, may be 
calculated. 
2. Description of the Prior Art 
Flexible wall vacuum packages are widely known, and are commonly used to 
package perishible food products such as meat. Bacon, sausage, hot dogs 
and the like are often contained in such packages. 
The shelf life of the food product is inversely related to the amount of 
residual oxygen within the package, which can promote bacteria growth. 
Vacuum packages, in which the flexible package wall assumes the general 
contour of the packaged product, were developed largely in an attempt to 
reduce the amount of oxygen in the package, and thereby increase the shelf 
life of the product. With this known technique, the product is packaged in 
a relative vacuum. The package may be also subjected to a shrink fit 
technique. The end result is that the pressure within the package is 
sub-atmospheric, and the pressure differential across the packaged wall 
causes the wall to assume the contour of the packaged article. 
However, after the product has been packaged, it would be highly desirable 
to determine: (1) the pressure within the package; (2) the percentage of 
total package volume which is occupied by gas; (3) the volumetric 
percentage of residual oxygen within the package; and (4) the actual 
volume occupied by residual gas. Tested samples could indicate defects in 
manufacturing processes, and serve as an indicator of the product's 
estimated shelf life. 
Prior art publications concerning such vacuum packages, sometimes called 
"retort pouches", and testing methods therefor, include: Rizvi, 
"Retortable Pouches: Food Package of the Future?", Agri-Search, S.C. 
Agricultural Research Station, Clemson University (Summer 1980); Lampi, 
"Flexible Packages for Thermoprocessed Foods", Advances in Food Research, 
Academic Press, New York (Vol. 23, 1977); Technical Report 73-4-GP, 
Shappee and Werkowski, "Study of a Nondestructive Test for Determining The 
Volume of Air in Flexible Food Packages", U.S. Army, Natick Laboratories, 
Natick, Mass. 01760 (June 1972); and Ghosh and Rizvi "Nondestructive 
Determination of Residual Volume of Air in Retort Pouches", South Carolina 
Agricultural Experiment Station, Clemson University. 
SUMMARY OF THE INVENTION 
The present invention provides apparatus for, and a method of, providing 
data by which the internal pressure and volume of residual gas of a 
flexible wall vacuum package, exposed to a known external pressure, may be 
calculated. 
The apparatus broadly comprises: a container having a removable closure, 
the container being partially filled with a liquid; a package-to-be-tested 
completely submerged in the liquid, a vacuum pump operatively arranged to 
selectively reduce the pressure in the head space of the container above 
the liquid, and at least one pressure indicator operatively arranged to 
indicate the respective pressures in the head space when the volume 
occupied by the package and the liquid has increased by first and second 
known volumes. 
In use, such apparatus performs an improved method, which broadly comprises 
the steps of: 
Submerging a package-to-be-tested in a liquid; reducing the pressure acting 
on the surface of the liquid to allow the volume of the package to expand; 
indicating the pressure acting on the liquid surface when the volume 
occupied by the package and liquid has expanded by a first known volume; 
and indicating the pressure acting on the liquid surface when the volume 
occupied by the package and liquid has expanded by a second known volume. 
The internal pressure of the package, when exposed to a known external 
pressure, may thereafter be calculated as a function of the known changes 
in volume and the indicated pressures. 
Accordingly, the general object of the present invention is to provide 
improved apparatus for, and a method of, testing a flexible wall vacuum 
package. 
Another object is to provide a method and apparatus for determining the 
volume of gas within such a package. 
Another object is to provide a method and apparatus by which the volume of 
residual oxygen in such a package may be calculated. 
These and other objects and advantages will become apparent from the 
foregoing and ongoing written specification, the drawings, and the 
appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
At the outset, it should be clearly understood that like reference numerals 
used herein are intended to identify the same elements and/or structure 
shown in the accompanying drawing, as such elements and/or structure may 
be further described or explained by the entire written specification of 
which this detailed description is an integral part. 
The invention provides apparatus for, and a method of, providing data by 
which the internal pressure of a flexible wall vacuum package exposed to a 
known external pressure, may be calculated. In FIG. 1, a schematic of the 
presently-preferred embodiment of such apparatus is generally indicated at 
10. The apparatus is shown as broadly including base 11, an inner 
container 12 having a removable closure cover 13, a vacuum pump 14, an 
outer enclosure 15, a float 16, an optical reader 18 operatively 
associated with recording-type first and second pressure gauges 19, 20 and 
a liquid reservoir 21 communicating with the interior of container 12 via 
an intermediate pump 22. 
The base 11 is shown as being a vertically-thickened horizontal plate-like 
member adapted to rest on a suitable support (not shown), such as a table. 
The inner container 12 has a side wall structure which includes an 
upstanding rectangular wall 23, the lower margin of which is suitably 
secured in fluid-tight relation (as by welding, fusing or bonding) to base 
11, an upwardly and inwardly inclined shelf 24, and a rectangular collar 
25 continuing upwardly therefrom to define an uppermost open mouth of the 
container. The closure cover 13 has a pair of parallel bars, one of which 
is indicated at 26 adapted to rest on the lip of the container's mouth, 
and has rectangular solid plug 28 depending therefrom into the neck of the 
container. Thus, the headspace of the inner container above liquid level L 
is continuously in communication with the space within the outer 
enclosure. Cover 13 is also shown as provided with a handle 29. A sight 
gauge 30 penetrates wall 23 and extends upwardly in parallel relation 
thereto. This sight gauge is open at its upper end. 
The outer enclosure 15 surrounds the inner container in spaced relation 
thereto. The outer enclosure has an upstanding cylindrical side wall 31, 
the lower margin of which is suitably secured in fluid-tight relation (as 
by welding, fusing or bonding) to base 11, and has a horizontal circular 
top 32 provided with a central opening 33. A gasket 34 is mounted on the 
upper surface of the enclosure top to encircle opening 33. Opening 33 may 
be selectively closed by placement of a cooperatively-configured 
plate-like cover 35 against gasket 34. Cover 35 is also provided with a 
handle 36. 
Vacuum pump 14 communicates with the interior of outer enclosure 15 via a 
conduit 38. When the vacuum pump is activated with both covers in place, 
pressure within the outer enclosure, as well as in the head space of the 
inner container, will be reduced to sub-atmospheric. 
Reservoir tank 21 is arranged between the outer enclosure and the inner 
container, and communicates with the inner container via pump 22 and 
suitable conduits. 
Float 16 is arranged in the sight gauge. The float has three 
vertically-spaced optically-readable marks thereon. The uppermost mark 
indicates that the level L of liquid in the container is at an initial 
reference level; the intermediate mark indicates that the volume occupied 
by the liquid and a package-to-be-tested has increased by a known first 
volume, and the lowermost mark indicates that the volume occupied by the 
liquid and the package-to-be-tested has increased by a second known 
volume. These three marks may be spaced evenly or unevenly from one 
another. The increase in such occupied volume causes the liquid level to 
rise in the space between the container neck and the plug 28, as well as 
in the sight gauge. The first and second known volumes may be calculated 
or measured by determining the volumes required to cause the liquid to 
rise from the first to the second marks, and from the second to the third 
marks, respectively. 
The optical reader 18 is arranged adjacent the sight gauge and triggers an 
appropriate signal when a float mark is seen. Pressure gauges 19 and 20 
both indicate the pressure within the outer enclosure. However, when the 
optical reader senses the float's middle mark, gauge 19 will stop and 
continue to indicate the pressure (P.sub.1) within the enclosure at which 
such middle mark was sensed. When the optical reader senses the float's 
lower mark, gauge 20 will stop and continue to indicate the pressure 
(P.sub.2) within the enclosure at which such lower mark was sensed. Thus, 
indicators 19,20 are recording-type pressure gauges, which may be reset 
after a test sequence has been completed. Lest the reader be confused, 
these pressure gauges will indicate the extent of vacuum (i.e., 
sub-atmospheric pressure), typically in familiar units of inches of 
mercury. 
OPERATION 
The illustrated structure of the preferred embodiment is deliberately 
schematic to minimize the possibility of obfuscating the salient features 
of the invention with unnecessary detail. 
Assume that the pressure within enclosure 15 is initially at atmospheric 
pressure, and that the liquid level L is low, perhaps somewhere around the 
middle of the height of container wall 23. Such inner and outer pressures 
may be rapidly equalized by opening a vent (not shown), operation of which 
may be either manual or automatic. The reason for having a low initial 
liquid level, is so that when the package-to-be-tested is placed in the 
container, the liquid level will not rise to a point at which the upper 
mark on the float will be sensed. Between tests, pump 22 may be reversely 
operated to pump liquid from the container to the reservoir to reduce the 
liquid to the level desired. Assume also that pressure gauges 19,20 have 
been reset to zero, and that any vent opening has been closed. 
The operator simply removes covers 35 and 13 to gain access to the inner 
container, places a package-to-be-tested (P) in the liquid, and replaces 
the two covers. Package P is of the flexible-wall vacuum type, and simply 
has an object, perhaps a food product, sealed within a container having a 
flexible wall. Such package walls may be made of plastic, paper, metal 
foils, combinations of these, and the like. The expression "vacuum 
package" refers to such packages in which the pressure within the sealed 
package is sub-atmospheric. Because of such relative vacuum, the package 
wall tends to conform to the outward shape of the object contained 
therein. 
With the package P in the container 12, and cover 13 in place, the liquid 
level will rise, albeit preferably to some level still below the point at 
which the float's upper mark will be sensed. If the level is too high, 
liquid may be pumped from the container to the reservoir until the float's 
upper mark falls below the optical reader. Thereafter, pump 22 is operated 
to pump liquid from tank 21 into the container. As this occurs, the liquid 
level in the container, as well as the float, will rise. When the optical 
reader senses the float's upper mark, pump 22 is turned off. The optical 
reader is preferably positioned such that the liquid level will be in the 
neck of the container when the upper float mark is sensed. 
Thereafter, vacuum pump 14 is operated to reduce the pressure within the 
enclosure, as well as in the ullage or head space of the inner container. 
Because plug 28 is held in place by two bars 26, the head space in the 
inner container above the liquid always communicates with the interior of 
the outer enclosure. In short, the pressure in such head space is always 
the same as that within the outer enclosure. Air is then pumped from 
within the outer enclosure, and the pressure acting on the surface of the 
liquid begins to fall. As the pressure continues to fall, the effective 
pressure acting on the outside of the package decreases until, at some 
point, such external pressure is equal to the pressure within the package. 
As vacuum pump 14 continues to pump air from within the enclosure, the 
package P will expand in volume, thereby causing the liquid level L to 
rise. Assuming that the liquid itself does not expand as axiomatic, the 
rise in liquid level must be caused by the increase in volume of the 
package. Indeed, the package's flexible wall will expand until a force 
balance is achieved. In short, during such volumetric expansion of the 
package, the pressure within the package must equal the pressure acting on 
its outer surface, for equilibrium to obtain. So long as the package 
remains at roughly the same place within the inner container, the pressure 
exerted on the package by reason of its being submerged in a liquid (i.e., 
p=.rho.gh) will be substantially constant and may be compensated for. 
Thus, as the pressure within the enclosure falls, the package will expand 
volumetrically, thereby causing the liquid level and float to rise. During 
the initial stages, pressure gauges 19,20 will both simultaneously 
indicate the falling pressure within the enclosure. In due course, the 
liquid level will rise until the optical reader senses the float's middle 
mark. When this happens, gauge 19 ceases to rise, and continues to 
indicate the pressure (P.sub.1) at which the volumetric expansion 
(V.sub.1) of the package caused the liquid level to rise from the float's 
upper mark to its middle mark. However, gauge 20 will continue to indicate 
the pressure within the enclosure until the optical reader senses the 
float's lower mark, after which gauge 20 will continue to indicate the 
pressure (P.sub.2) at which the further volumetric expansion (V.sub.2) of 
the package caused the liquid level to rise from the float's middle mark 
to its lower mark. 
The pressure within the package, when exposed to atmospheric or some other 
known pressure, may be calculated as a function of P.sub.1, P.sub.2, 
V.sub.1 and V.sub.2 as follows: At a constant temperature, Boyle's Law 
provides that the product of pressure and volume will equal a constant. 
Thus, 
EQU P.sub.0 V.sub.0 =P.sub.1 V.sub.1 =P.sub.2 V.sub.2 
Initially, we do not know the volume (V.sub.0) of the gas in the package 
when exposed to atmospheric pressure, nor the pressure (P.sub.0) within 
the package. However, we do known the pressure (P.sub.1) in the enclosure 
when the volume occupied by the water and the package has caused the 
liquid level to rise from the float's upper mark to its middle mark. This 
first change in volume can be either measured or calculated. Similarly, we 
also know the pressure (P.sub.2) which causes the liquid level to rise 
from the float's middle mark to its lower mark. This second change in 
volume may also be calculated or measured. If the cross-sectional area 
between container wall 25 and plug 28 is constant along the height of 
collar 25 and the three float marks are spaced equally from one another, 
then the change in volumes from the upper mark to the middle mark, and 
from the middle mark to the lower mark, will both be equal and constant 
(C). Thus the volume occupied by the residual gas within the package at 
P.sub.1 will be its original volume (V.sub.0) plus a constant (C): 
EQU V.sub.1 =V.sub.0 +C 
Similarly, the volume occupied by the residual gas within the package at 
P.sub.2 will be its volume at P.sub.1 plus a constant: 
EQU V.sub.2 =V.sub.1 +C=V.sub.0 +2C 
Substituting these equations into Boyle's Law: 
##EQU1## 
Once V.sub.0 has been calculated, then the pressure (P.sub.0) within the 
package may be calculated according to the formula: 
##EQU2## 
The volume of the package (V.sub.p) when exposed to atmospheric pressure 
may be readily measured. Once this is known, the percentage of gas in the 
package may be calculated as follows: 
##EQU3## 
Since air is roughly 21% oxygen (O.sub.2): 
##EQU4## 
Thus, the internal pressure and volume occupied by a gas in a flexible 
wall vacuum package may be calculated as a function of P.sub.1, P.sub.2 
and the two changes in volume. 
MODIFICATIONS 
While the disclosed embodiment is presently preferred, many changes and 
modifications may be made. In the preferred embodiment, the function of 
the outer enclosure is to insulate the inner container from deformation 
due to a pressure differential when air is evacuated. In this form, the 
pressure acting on the outer surface of the inner enclosure is equal to 
the pressure in its head space. This design permits the inner container to 
be made of materials having an economic thickness. However, if the inner 
container were designed so as to withstand such pressure differential 
without significant inward deformation as the vacuum is drawn, then it 
would be possible to eliminate the outer enclosure altogether, to provide 
a sealable cover to the inner container, and to communicate the vacuum 
pump with the head space of such container. The tank or liquid reservoir 
is also optional. Moreover, a pressure transducer, such as a suitable 
piezoelectric device capable of converting sensed pressure into a 
proportional electrical signal could be substituted for the disclosed 
pressure gauges. Hence, in the appended claims, the term "pressure 
indicator" is generally intended to cover a visual-indicating pressure 
gauge, or some other transducer capable of transforming sensed pressure 
into some proportional or analog vehicle. While the optical reader is 
preferred because of its ability to reduce the prospect of human error, 
such float positions could be determined visually or by other electrical 
or mechanical means. If desired, the sequence of operations may be 
automated so that, once a package is placed in the container, the operator 
need only push a button and read P.sub.1 and P.sub.2. Indeed, such 
pressure data could be supplied to a microprocessor, or equivalent, 
capable of performing the needed calculations. Materials of construction 
are not deemed critical. Auxiliary apparatus could be provided to maintain 
the liquid at a substantially constant temperature. The size and shape of 
the inner container is not critical so long as the two changes in volume 
may be either measured or calculated. The liquid may be water, to which a 
wetting agent has preferably been added, or some other liquid. The 
apparatus may also be used to provide data by which the internal package 
pressure and gas volume, when such package exposed to a known external 
pressure other than atmospheric, may be calculated. To do this, one need 
only pressurize the interior of the enclosure to the known pressure when 
the float upper mark is aligned with the reader at the beginning of the 
test. 
Also, it may be desirable to continue to evacuate the outer enclosure after 
pressure P.sub.2 has been reached, in order to insure that the package 
walls have completely separated from the object contained therewithin. 
After the package has "ballooned", it may be visually inspected for the 
presence of leaks if the container and enclosure are made of transparent 
materials, such as plexiglass. 
Therefore, while the presently preferred embodiment of the inventive 
apparatus has been shown and described, and several possible modifications 
thereof discussed, persons skilled in this art will readily appreciate 
that various additional modifications and changes may be made without 
departing from the spirit of the invention, as defined and differentiated 
by the following claims.