WO2008046420A1

WO2008046420A1 - A method for determining a volume

Info

Publication number: WO2008046420A1
Application number: PCT/DK2006/050062
Authority: WO
Inventors: Poul Fogh; Thomas Clausen
Original assignee: Pbi-Dansensor A/S
Priority date: 2006-10-18
Filing date: 2006-10-18
Publication date: 2008-04-24

Abstract

A method for determining the volume of an enclosed space is disclosed. The method comprises filling said enclosed space with a first gas and flushing at least partially said enclosed space with a second gas of known composition. The flushing comprises a number of sequential steps of flushing, in which known amounts of the second gas are injected into the unknown enclosed space. The change in concentration in said unknown enclosed space of a measuring gas present in said first gas or present in said second gas before and after one such flushing step is measured. The volume of the unknown enclosed space is calculated, based on change in concentration of said measuring gas.

Description

A method for determining a volume.

The present invention relates to a method for determining a volume, in particular, but not exclusively, the internal volume of a con- tainer.

It is often of interest to determine an internal volume of a container. One such situation, where the determination of the internal volume of a container is of interest, is in connection with the method disclosed in WO-A-03/060485. Here, knowledge about the internal volume of a package, e.g. a plastic bottle, is necessary for determining the penetration rate of a gaseous substance into the package. Moreover, the process step of determining the volume should be one, which is easily handled and implemented in connection with the method.

Traditional methods for measuring volume using a liquid, e.g. to displace the air within the container, are not desirable, as the liquid may adversely affect the properties of the container wall, through which the gaseous substance penetrates, thus flawing the penetration rate measurement.

JP-A-6-258123 discloses a system for measuring the volume of an object without immersing it into a liquid. There the object is placed in a test chamber of known volume. After placing the object in the test chamber and closing the test chamber, the gas concentration of a predetermined gas is measured. A known volume of another gas mixture with a known composition is added, and the resulting gas concentration of the predetermined gas is measured. Based on the change in gas concentration, the residual volume in the test chamber can be calculated, and hence the volume of the object. In order to get a high degree of precision of measurement of different objects, the chamber is flushed between each measurement in order to reduce the concentration of the predetermined gas.

DE-A-19742200 also discloses a system for measuring the volume of an object without immersing it into a liquid. This method describes the calibration of a test apparatus using first a calibration object of known volume, in a test chamber in order to determine the residual volume of conduits etc. to e.g. an oxygen sensor. The system is first flushed with nitrogen. Then the residual volume is isolated from the test chamber. The test chamber is filled with atmospheric air. The atmospheric air in the test chamber and the nitrogen in the residual volume is then mixed in order to determine the residual volume of nitrogen based on the volume of the calibration object, and a measurement of the resulting oxygen concentration in the system. For subsequent measurement of the volume of a test object, the test object is placed in the test chamber instead of the calibration object. The system is again flushed with nitrogen. Then the residual volume is again isolated from the test chamber. The test chamber is again filled with atmospheric air. Again, the atmospheric air in the test chamber and the nitrogen from the residual volume are mixed, and the resulting oxygen concentration measured. As, however, the residual volume is now known, the volume of the test object can be determined from the oxygen concentration measurement.

Other systems for measuring the external volume of an unknown object based on injection of a known volume of a special inert gas in a chamber of a predetermined volume, such as a silo or a test cham- ber, and subsequent measurement of the concentration thereof are known from JP-A-4-134244, JP-A-2000-039349 and JP-A-59-015822.

None of the above apparatuses are suitable for the determination of an internal volume of a package, such as a bottle, as they focus on and are adapted to measurements of external volumes. US-A-4418701 discloses a system for determining residual lung capacity. This method relies on injection and detection of a special detectable gas, such as helium, into a known test volume, and measuring the resulting concentration in the combined volume of the lung residual volume and the known test volume. Using helium is expensive and ne- cessitates the handling of a special gas in connection with the volume measurement.

On this background it is the object of the present invention to provide a method for determining volume, which overcomes the above drawbacks. According to the present invention this object is achieved by a method for determining the volume of an enclosed space, said method comprising, filling said enclosed space with a first gas, exchanging at least partially said first gas in said enclosed space with a second gas of known composition, where said flushing comprises a number of sequential exchange steps, in which known amounts of the second gas are injected into the unknown enclosed space and corresponding amounts of resulting gas mixture are extracted there from, measuring the change in concentration in said unknown enclosed space of a measuring gas pre- sent in said first gas or present in said second gas before and after one such exchange step, calculating from said change in concentration of said measuring gas, the volume of the unknown enclosed space.

The use of sequential exchange steps makes it possible to perform the volume determination of the enclosed space at essentially any convenient time during a flushing process. The method thus obviates the need for any separate, preceding or succeeding, step for volume determination. Moreover, as the method uses the very same gases as needed for the flushing, it is essentially non destructive, and will not influence any subsequent permeability or leak tests, if the enclosed space is a permeability or leak test object.

According to a preferred embodiment of the invention, the steps of measuring the change in concentration in said unknown enclosed space before and after one such flushing step, and of calculating, from said change in concentration of said measuring gas, the volume of the unknown enclosed space, are repeated a number of consecutive times, taking as the volume of the unknown enclosed space, the average value of the volumes calculated in said steps. Taking an average value increases the accuracy of the measurement, because it becomes of less importance that the resulting gas mixture of first and second gas settles in a completely mixed state.

In particular it has been found that it suffices for achieving an acceptable degree of precision in the volume determination, if the number of consecutive times is larger than two, preferably larger than four and most preferred larger than six. In a further preferred embodiment of the invention, the measurement is not performed until the concentration of said measuring gas present in the first gas has passed a predetermined threshold. This allows a further increase in precision of the volume determination, be- cause it allows the volume determination to be performed at a concentration of the measuring gas, where the preferred gas sensor used for the measurement has the highest degree of precision.

In particular it is preferred that the measurement is not performed until the concentration of said measuring gas present in the first gas has decreased below said predetermined threshold. This allows the use of atmospheric air as the first gas and a pure gas such as nitrogen as the second gas in combination with an oxygen sensor, which would allow the method to be implemented directly as a part of the test method performed in accordance with WO-A-03/060485. Thus according to further preferred embodiments, the first gas is atmospheric air, the measuring gas is oxygen, and the second gas is nitrogen. These gasses are all readily available at low costs. Moreover it is mainly oxygen as such, which is of interest to the food or medical industry, so no special trace gas needs to be added. According to yet another preferred embodiment said threshold is selected to have a value of maximum 1% by volume oxygen, preferably maximum 0.1% by volume oxygen. This allows maximum precision of the oxygen sensor used.

As the skilled person will already have understood from the above, the term gas is to be understood in a broad sense as including not only pure gasses but also mixtures of gasses, in particular atmospheric air. This understanding is also to be used when reading the following detailed description of preferred embodiments, which refers to the figures, in which Fig. 1 is a schematic representation of a system allowing the implementation of the method according to the invention for determining the volume of an enclosed space,

Fig. 2 is a plot of several graphs illustrating the results, which can be achieved using the method of the invention. Reference is first made to fig. 1. In fig. 1, a test specimen 1 comprising an enclosed space, the unknown volume of which is to be determined, is shown. The test specimen 1 may be any kind of test specimen comprising an enclosed space. In particular, however, the test specimen 1 could be a food packaging or a packaging for medial supplies, e.g. a sealed plastic bottle or a sealed flexible plastic bag.

In the illustrated example, the sealing of the test specimen 1 is made by a pair of septa 2, 3, through which a pair of syringes 4, 5 may penetrate into the enclosed space. One syringe 4 is for the supply of a flushing gas for flushing the enclosed space within the test specimen 1. In the following description and in particular in the claims, this flushing gas is also referred to as second gas. The flushing gas may in principle be any gas or gas mixture of known composition. In the illustrated example the flushing gas is nitrogen. The nitrogen is supplied from a pres- sure gas cylinder 6 via a flow controller 7, preferably a mass flow controller. The flow controller 7 controls the exchange rate or amount with which the flushing gas is supplied to the enclosed space within the test specimen 1. The other syringe 5 is for extracting the resulting gas mixture from the enclosed space within the test specimen 1. The resulting gas mixture in the enclosed space within the test specimen 1 is extracted through the syringe 5 by means of a pump 8. The extraction takes place at a rate or amount, preferably corresponding to the rate or amount of flushing gas supplied to the enclosed space in the test specimen 1, i.e. at the same exchange rate. The extraction rate is measured by means of a flow meter 9, preferably also a mass flow meter. Here it should be noted that in the preferred embodiment rates are considered in volume, at a preferably constant pressure, which can be calculated from the mass, or simply be included as a calibration of the system. Such control is within the reach of the skilled person, and will not be dis- cussed further. It should however be noted that, if the test specimen 1 is sufficiently rigid, a pressure measurement of the pressure in the enclosed space in the test specimen 1, could replace one of the mass flow measurements, provided the pressure measurement is used to keep the pressure in the enclosed space within the test specimen at a fixed value. Between the syringe 5 and the pump 8 the extracted resulting gas mixture passes a gas sensor 10 for measuring the concentration of a predetermined measuring gas, known to be present in the extracted gas mixture. In the present example the enclosed space within the test specimen 1 would initially have been filled entirely by atmospheric air, typically, the atmospheric air, which was there when the test specimen 1 was sealed. This initial gas, which the skilled person will understand can also be a gas mixture, e.g. atmospheric air, is also referred to as first gas in the present description and in the claims. With the first gas being atmospheric air and the second gas being nitrogen, the gas sensor is preferably an oxygen sensor. This is particularly advantageous in connection with food packaging and packaging of medical supplies because it is most often the oxygen, which is of relevance for the packaging. This is in particular the case in relation to the permeability of the package to oxygen, which is potentially harmful to the packaged goods, such as foodstuffs or medical supplies. For the test of permeability, the package is flushed with nitrogen to remove the atmospheric air. The syringes 4, 5 are removed, and the package is left for an extended period of time, e.g. a month. During this extended period of time the ingress of oxygen is measured at suitable intervals by reinserting the syringe 5 and performing a measurement using the oxygen sensor 10 on a small amount of gas extracted using the pump 8. The permeability test may thus be performed using parts of the very same equipment as was used in the initial flushing, i.e. without the use of any extra equipment.

As to the flushing, it has already been indicated above that the flushing gas, e.g. nitrogen, is supplied from a pressure gas cylinder 6 via a flow controller 7, preferably a mass flow controller. The flow controller 7, which is preferably a mass flow controller, controls the rate or amount with which the flushing gas is supplied to the enclosed space within the test specimen 1. The inventors have realised that by supplying the flushing gas in exchange steps, the volume of the test specimen may be determined at an appropriate time during the flushing thereof. An example of such an appropriate time during the flushing would be the time when the concentration of the measuring has reached the concentration where the measuring gas sensor works optimally in terms of precision. For the oxygen sensor used during the development of this invention, the concentration threshold preferably has a value of less than 1 % per volume, preferably below 0.1 % per volume. The appropriate time does however also depend on the actual context of the measurement, e.g. the degree to which it is necessary to flush the test specimen. In a permeability test of a test specimen 1 known to have a high degree of permeability, it may not be necessary to flush the test specimen 1 from the initial oxy- gen concentration of approximately 20.9 % per volume to the above 1 % per volume. In such case it could be appropriate to perform the volume determination at a higher value, e.g. 10 % per volume.

As to the flushing steps it should be noted that they do not need to all be identical. In particular it may be preferred with one large initial flushing step, bringing the concentration of the measuring gas to the concentration mentioned above, preferred for the volume measurement. This initial flushing step could the be followed by a number of subsequent smaller exchange steps, during which the volume is determined, and possible with one large final flushing step again. Also, the exchange steps do not need to be separated by pauses or waiting time. Rather the steps could simply be delimited by two measurements in an otherwise continuous flow of flushing gas. It is however preferred that the steps are discrete steps separated by appropriate waiting times. Such waiting times allow the resulting gas mixture to settle and equalize to a homogeneous mixture. Thus the measurements will not be flawed by local higher concentrations of measuring gas around the gas sensor in the enclosed space in the test specimen 1.

Moreover the skilled person will understand that it is not of importance whether the exchange step starts with extraction, which is then followed by injection, or vice versa, as the method for volume determination run over a number of such steps any way. In principle the extraction and injection of known volumes could take place simultaneously.

The control of the exchange steps is performed by a combined volume calculation and control device 11. The volume calculation and control device 11, controls the flow controller 7, the pump 8. Moreover it determines the volume of the enclosed space in the test specimen based on data received from the oxygen sensor 10 and flow values received from the flow meter 9 and the flow controller 9. This determination is described below in connection with fig. 2. Though combined in this example, the skilled person will realise that the control and the volume calculation could be performed by separate units.

Fig. 2 shows curves for the volume determination using the method according to the invention, using nitrogen to flush atmospheric air form the enclosed space in a test specimen 1. There are several curves 12, 13, 14, 15 based on the same measurements, but using different averaging intervals, i.e. averaging over different numbers of consecutive steps. Taking an average value increases the accuracy of the measurement, because it becomes of less importance that the resulting gas mixture of first and second gas settles in a completely mixed state. Thus, if necessary, the waiting time between consecutive exchange steps could be reduced, and the volume determination process be speeded up. This would however not be of importance in the context of permeability measurement serving as starting point for the present invention, but as the invention is not limited to the use in this context, it could be of importance. Apart from the curves illustrating the volume determination, fig 2. also shows a curve 16 for the oxygen concentration measured. In fig. 2 the abscissa is the measurement number, against which the volume determinations 12, 13, 14, 15 are plotted with respect to the left- hand ordinate, and against which the oxygen concentration 16 is plotted with respect to the right-hand ordinate.

Curve 12 shows the raw data for the volume determination based on a single exchange step, i.e. two consecutive concentration measurements. This volume determination is derived from the formula:

where ΔV is the amount of gas exchanged, i.e. the amount of flushing gas supplied to the enclosed space in the test specimen 1 or the amount of resulting gas mixture extracted there from, C_n is the concentration of measuring gas before the exchange step, and C_n+i is the concentration of measuring gas after the exchange step. In the test yielding the curves of fig. 2, the amount of flushing gas supplied was 10 ml, in each of the exchange steps. The measurements were taken at 15 s intervals.

As can be seen from fig. 2, the raw measurements of curve 12 do fluctuate quite a bit. This measurement and hence the volume deter- mination may nonetheless be sufficiently precise for some applications. It can however also be seen that more precise volume determinations may be achieved using a running average of several measurements. In the present case the volume determinations based on the average three, five or seven consecutive measurements, represented by the curves 13, 14 and 15, respectively, show less fluctuations indicating a more precise and consistent volume determination. The skilled person will realise that averaging over a larger number of measurements would yield an even more precise value for the volume determination. He would also realise that the average need not necessarily be taken over consecutive meas- urements.

Regarding fig. 2 it should be noted that this is just an example. The fact that the measurement starts out at the initial atmospheric concentration of oxygen and decreases over subsequent exchange steps of 10 ml nitrogen is not restricting to the invention. In fact, as indicated above, it may indeed be advantageous to reduce first the oxygen concentration in the resulting gas mixture to a far lower value.

Also it should be noted that the skilled person will realise that it is not of importance for the principles of the invention whether the flushing gas is a pure gas or a mixture and whether the initial gas in the en- closed space in the test specimen is a pure gas or a mixture, as long as a change in the concentration of the measuring gas can be measured. Moreover it should be noted that though the currently preferred embodiment utilises oxygen as the measuring gas, the measuring gas could just as well be carbon dioxide, or any other readily detectable gas. The important aspect of the invention remains that the volume determination can take place during the flushing procedure. With appropriately programmed control equipment, i.e. the volume calculation and control device 11, the method may moreover be executed automatically during the necessary flushing procedure in connection with the permeability test, as no extra components and/or instruments are needed for the measurements, than those needed for the final check of the atmosphere in the test specimen before starting the subsequent permeability test.

Claims

P A T E N T C L A I M S

1. A method for determining the volume of an enclosed space, said method comprising filling said enclosed space with a first gas, exchanging at least partially said first gas in said enclosed space with a second gas of known composition, where said flushing comprises a number of sequential exchange steps, in which known amounts of the second gas are injected into the unknown enclosed space and corresponding amounts of resulting gas mixture are extracted there from, measuring the change in concentration in said unknown enclosed space of a measuring gas present in said first gas or present in said second gas before and after one such exchange step, calculating from said change in concentration of said measuring gas, the volume of the unknown enclosed space.

2. A method according to claim 1, wherein the steps of measuring the change in concentration in said unknown enclosed space before and after one such exchange step, and of calculating, from said change in concentration of said measuring gas, the volume of the unknown enclosed space, are repeated a number of consecutive times, taking as the volume of the unknown enclosed space, the average value of the volumes calculated in said steps.

3. A method according to claim 2, wherein the number of consecutive times is larger than two, preferably larger than four and most preferred larger than six.

4. A method according to any one of the preceding claims wherein said measurement is not performed until the concentration of said measuring gas present in the first gas has passed a predetermined threshold.

5. A method according to any one of claim 4, wherein said measurement is not performed until the concentration of said measuring gas present in the first gas has decreased below said predetermined threshold.

6. A method according to any one of the preceding claims, wherein the first gas is atmospheric air.

7. A method according to any one of the preceding claims, wherein said measuring gas is oxygen.

8. A method according to any one of the preceding claims, wherein the second gas is nitrogen.

9. A method according to any one of claims 5 to 8, wherein said threshold is selected to have a value of maximum 1% by volume oxygen, preferably maximum 0.1% by volume oxygen.

10. A method according to any one of the preceding claims wherein said exchange steps are separated by a waiting time during which no flushing gas is supplied.